Topic 1: Transportation - Biology Notes Form 6

TRANSPORTATION OF MATERIALS IN LIVING ORGANISM

Transportation is the act or process of moving molecules, ions, or substrates across a biological membrane, such as the plasma membrane.

Plants have two transport systems – xylem and phloem . Xylem transports water and minerals. Phloem transports sugars and amino acids dissolved in water.

Why is there a need of special transport system?

Answers:

Over short distances, means of transport are rapid and efficient ie: osmosis, diffusion, endocytosis etc. But in multicellular organisms which have a large surface area to volume ratio; these means are sufficient as cells may be too widely separated from each other. Therefore for these practices to be adequate, specialised long distance transport systems are necessary.

Materials are generally moved by a mass flow system, being the bulk transport of materials from one point to another as a result of pressure difference between the two points.

Example of flow systems in plants and animals are;-

• In plants- Vascular system.
• In animals – Alimentary canal, respiratory system, etc.

Definition:

A vascular system is one which has tubes which are full of fluid being transported from one place to another.

In animals the blood system is a vascular system.

In plants the xylem and phloem form vascular systems.

Definitions:

1. Osmosis: This is a movement of water molecules from a region of higher water potential or lower solute potential to a region of lower water potential or higher solute potential through a deferentially permeable membrane.
2. Diffusion: This is a movement of materials from a region of higher concentration to a region of lower concentration.
3. Active transport: This is a transportation of materials against concentration gradient. Due to this, the process involves the consumption of energy. Any part of the body where active transport occurs is characterized by;

a) Presence of numerous mitochondria.

b) High rate of metabolism.

c) High concentration of ATP.

• Since active transport involves the use of energy, the materials transported actively move faster than those transported passively.

Significance of transportation system

The system of transportation of materials is important for:-

1. Distribution of food materials in the body.
2. Carriage of excretory wastes from their sites of synthesis.
3. Carriage of hormones from their respective glands to their target organs.
4. Distribution of antibodies.
5. Carriage of respiratory gases.

(I) TRANSPORT IN PLANTS

• The movement of substances through the conducting or vascular tissues of plants is called Translocation.

Important application of the study of Translocation:

• It is useful to know how herbicides, fungicides, growth regulators and nutrients enter plants and the routes that they take through plants, in order to know how best to apply them and to judge possible effects that they might have.
• Plant pathogens are sometimes translocated, and such knowledge could influence treatment or preventive measures.

Terms used:

(i) Water potential, symbol Ψ, Greek latter psi.

• The term is used to describe water movement through membranes. It can be described as the tendency of water molecules to move from one place to another. The higher (less negative) the water potential, the greater tendency to leave a system.

Factors affecting water potential of plant cells are:-

1. Solute concentration and
2. Pressure generated when water enters and inflates plant cells.

They are expressed in terms of Solute and Pressure Potentials respectively.

NOTE

• Pure water has maximum water potential (zero).
• Water always moves from a region of higher Ψ to a region of lower Ψ.
• All solutions have lower Ψ than pure water, therefore negative values of Ψ

(ii) Solute potential, Ψs

• The effect of dissolving solute molecules in pure water is to reduce the concentration of water molecules and hence to lower the water potential.
• Solute potential is a measure of the change in water potential of a system due to the presence of solute molecules.

(iii) Pressure potential, Ψp

If pressure is applied to pure water or a solution, its water potential increases. This is because the pressure tends to force water from one place to another.

Ψ = Ψs + Ψp

(iv) Plasmolysis and Turgidity

If a plant cell is in contact with a solution of lower water potential than its own contents, then water leaves the cell by osmosis through the cell surface membrane. Consequently, the protoplast shrinks and eventually pulls away from the cell wall. The process is called plasmolysis and the cell is said to be plasmolysed.

The point at which plasmolysis is just to happen is called Incipient plasmolysis. At this point, the protoplast has just ceased to exert any pressure against the cell wall, so the cell is Flaccid. Water will continue to leave the protoplast until its contents have the same Ψ as the external solution. No further shrinkage then occurs.

If a plasmolysed cell is placed in pure water or a solution of lower solute potential or higher water potential than the contents of the cell, water enters the cell by osmosis. As the volume of protoplast increases, it begins to exert pressure against the cell wall and stretches it.

The pressure inside the cell rises rapidly, the pressure is called the Ψ_p. As the Ψ_p of the cell increases due to water entering by osmosis, the cell becomes turgid.

Animal cells have no cell wall and the cell surface membrane is too delicate to prevent the cell expanding and bursting in a solution of higher Ψ. They are therefore protected by Osmoregulation.

Question.

1. What occupies a space between the cell wall and the shrunken protoplasts in plasmolysed cells?
2. What is the Ψ_p of a flaccid cell?

Answer;

1. The external solution, since the cell wall is freely permeable to solutions.
2. Zero. The protoplast is not exerting pressure against the cell wall.

• In higher plants, the materials are transported by the vascular tissues. Which are of two types:-

1. The xylem and
2. The phloem.

(i) The xylem tissue

This is a plant vascular tissue which is mainly concerned with transportation of water and dissolved mineral salts through the plant.

Structure of the xylem

The histology of the xylem tissue reveals the presence of four types of cells.

Note: The only conducting cells are the vessels and tracheids.

1. TRACHEIDS

Structural features:

-They are more or less elongated cells with tapering ends.

-They have secondary thickened or lignified walls with a variety of pits (simple or bordered).

-They are not perforated.

-They are dead at maturity ie: they lose all the protoplasmic contents leaving an empty lumen.

NOTE:

Tracheids are present in all vascular plants, but in the coniferophytes they are only xylem conducting cells

Diagram

2. VESSEL MEMBERS

– These are perforated elements that aggregate into files of cells connected to one another by means of perforations.

-The vessel members are more specialised than the tracheids and they are characterized by the following features:-

1. They have secondary thickened wall.
2. They are dead at maturity.
3. They are shorter and wider.
4. They have bordered pits along their sides.
5. They have perforated plates.

Diagram:

Role of vessels and tracheids

• The vessels and tracheids conduct water and dissolved mineral salts through the plant ie: from the roots to the shoots.

Adaptations of the Xylem (vessels and tracheids) to transport

1. Both have long cells joined end to end. This allows the flow of water and dissolved mineral salts in a continuous column.
2. The end walls of the xylem vessels have been broken down forming uninterrupted flow of water from the roots to the leaves. Even in the tracheids where the end walls are present, larger bordered pits reduce the resistance of flow due to the presence of end walls. Absence of end walls in the vessels and presence of bordered pits in tracheids facilitate easy flow of water as resistance to flow is reduced.
3. There are pits at particular parts in the lignified walls. These allow lateral movement of water and mineral salts where this is necessary.
4. Narrowness of the lumen of vessels and tracheids increases the capillarity force.
5. The walls are lignified (for strength) making them especially rigid to prevent them from collapsing due to large tension force set up by the transpiration pull.
6. Impregnation of the walls with lignin material increases the adhesion of water molecules which helps the water to rise up the plant by capillarity.
7. Loss of protoplast in the vessels and tracheids leaves an empty lumen which forms a continuous tube as one cell rests on top of the other.
8. Since the conducting cells of the xylem tissues are dead, their materials are transported through them passively and this minimizes energy consumption.

Question: Describe the histology of the xylem conducting cells and show how they structurally relate to their function.

3. THE XYLEM FIBRES

-These are elongated, slender, thick walled and non conducting cells.

-They are thought to have been evolved together with the tracheids and they function as supporting elements.

-They also facilitate lateral movement of materials and they sometimes store food.

4. THE XYLEM PARENCHYMA

-These consist of simple undifferentiated living cells.

-They have lignified pitted walls and they are frequently arranged in radial sheets to form rays.

-They function as pathways for lateral movement of materials and they sometime store food.

(ii) The phloem

tissue

• It is chiefly concerned with translocation of photosynthetically manufactured food from the autotropic parts (sources) to the heterotropic parts (sinks) of the plant.

Structure of phloem

The histology of the phloem tissue reveals the presence of the following structural cells:-

1. Sieve element.
2. Companion cells.
3. Phloem parenchyma.
4. Phloem fibres and sclereids

1. SIEVE ELEMENTS

-These include the sieve cells and sieve tube members.

2. COMPANION CELLS

• These are associated with the sieve tube members only in the Angiospermophytes. They are highly specialized parenchyma cells.
• They arise from the same meristematic initial with the sieve tube cells.
• They contain nucleated dense cytoplasm in communication with the cytoplasm of sieve tube member by means of plasmodesmata in the pitted areas of the thin dividing walls.

3 .PHLOEM PARENCYMA

• These contain stored carbohydrates and accumulation of tannins and resins.
• The phloem parenchyma is always in communication with the sieve elements and companion cells by the adjacent sieve areas.

4. PHLOEM FIBRES AND SCLEREIDS

• These occur in both primary and secondary phloems.
• The walls may be lignified or not but generally pitted with simple or bordered pits thus facilitating lateral movements of food substances.

Phloem:

Summary:

1. The sieve tube elements when mature do not have the nucleus, no ribosomes, no golgi bodies, no tonoplast. There are no mitochondria and there is a very little peripheral cytoplasm. However, the cells remain living since they

are connected to the companion cells

2. The companion cells have a dense cytoplasm with a nucleus, mitochondria and ribosome. The cells are very metabolically active.

3. The sieve tube members and the companion cells are in communication with one another by means of a large plasmodesmata.

Function of the phloem tissue:

The role of the phloem (sieve tube) is to carry food substances from the leaves to the other parts of the plant.

Adaptations of the phloem:

1. The sieve elements are tubular to allow the passage of food substances.
2. There are sieve plates with various pits to facilitate the passage of food from one cell to another.
3. The pits in the sides together with plasmodesmata facilitate lateral movements of food.
4. The mitochondria in the companion cells provide energy necessary for active transport of food.
5. The tubes in the sieve elements are very narrow. This increases the pressure which in turn facilitates rapid transportation of food.

Questions:

1. Describe the structure of the phloem tissue and show how its structure relates to its function.
2. Describe the histology of the plant vascular tissue.

NECTA 2001 P1 12

(b) How is the structure of xylem tissue suited to its function of transporting water.

Movement of materials across the root

• Water and dissolved mineral salts from the soil, enter the root by osmosis across the epidermis of the root hair cells.
• In the root, these materials move through three different routes/pathway namely:-

1. Apoplastic pathway.
2. Symplastic pathway.
3. Vacular pathway.

I. Apoplastic pathway

Definition:

• The apoplastic pathway is found throughout the plant. However, in the root endodermis, it is prevented by the water proof substances called Suberin or Casparian strips.
• Due to the presence of casparian strips, the moving water and dissolved mineral salts are forced into the living protoplast of the endodermis as the only available route to the xylem. This in turn, causes active secretion of salts into the vascular tissues from the endodermal cells. This makes water potential in the xylem lower (more negative) thereby increasing more chances for water to move into the xylem.

Significance of the casparian strips:

1. They increase the chances of water moving into the xylem. This is because as they force water into the living protoplast of the endodermal cells, they cause salts to be actively secreted into the vascular tissue (xylem)from the endodermal cells. This makes water potential in the xylem lower (more negative), the result of which water moves into the xylem following a water potential gradient.
2. They prevent an apoplast movement of water and dissolved mineral salts and therefore water and salts must pass through the cell membrane under the cytoplasmic control of the endodermal cells i.e They ensure the control of moving water by the living cells.
3. They regulate salts movement and may be protective measures against entry of toxic substances example; Fungal pathogens.
4. They serve the life of the cell when it plasmolyses as the cell surface membrane remains held in position at the strips although other parts detach.
5. Since they are water proof bands, they normally regulate the amount of water to be admitted into the root.
6. They are connected for maintenance of root pressure since they cause active secretion of ions/salts into the xylem vessel.
7. It acts as an air tight dam in that it prevents water from being clogged with air.
8. Symplastic pathway

Definition:

1. Symplast:-Is a system of interconnected protoplasts in the plant in which the cytoplasm are connected by the plasmodesmata, the cytoplasmic strands that extend from one cell to another through the pores in the cell wall.

• The system in which the plasmodesmata link to ensure a living connection between the two neighbouring cells is called a Symplasm.

(ii)

Symplastic pathway:- It is a pathway in which water moves from the cytoplasm of one cell to that of another through the pits by the aid of the plasmodesmata.

1. Vacuolar pathway

In the vascular pathway, water moves from vacuole to vacuole. In this process, water moves across the cell surface membrane and tonoplast by the process of osmosis.

Diagram: Routes for water transport across cells

Questions:

1. Give an illustrated description of the pathway through which water and mineral salts pass from the root hair to the xylem vessel.
2. Describe the significance of the casparian strips in the root endodermal cells.

Uptake of mineral salts and their transport across the root and through

the plant

The plants take in the necessary mineral salts from the soil and their absorption is greatest in the region of root hairs. These are taken in either solution form or ions.

To explain the absorption of mineral salts, the following facts should be adhered to;

• The cell membranes including the cell surface membrane and the tonoplast are not true semi permeable, but rather are differentially (selectively) permeable allowing some minerals to pass.
• Minerals may be absorbed either passively or actively.

I. Passive absorption

If the concentration of a mineral in the soil solution is greater than its concentration in a root hair cell, the mineral may enter the root hair by diffusion.

II. Active absorption

If the concentration of a mineral in a soil solution is less than that in a root hair cell, it may be absorbed by active transport. Most minerals are absorbed in this way. The process is selective because active absorption requires energy; the rate of absorption is dependent upon respiration.

• There is a continuous system of interconnected cell walls, the apoplast in which water and any solute it contains enters by Mass flow and to a lesser extent by diffusion.
• Although leaves can also absorb them if sprayed with a suitable solution, such sprays are called foliar feeds.
• Water moves through the apoplast as a part transpiration stream. Transpiration stream is a movement (flow) of water from a root to the stomata.

Question:

Concisely but precisely, describe the process of uptake of mineral salts by the root.

Ans:

• Generally the concentration of mineral salts in the root hair cells is greater than that in the surrounding environment. This implies that the mineral salt enter the plants against the concentration gradient.
• However, some ions are more concentrated than others. This suggests that ions are selectively absorbed by active transport involving the expenditure of energy from ATP.
• Basically, ions (minerals salts) are absorbed from the soil by the root hair cells, where prime role is to increase the surface area for absorption. They are taken in together with water in order to form solutions after dissolving.
• After absorption, they move to the xylem tissue through the apoplast pathway.
• In the endodermis, there are water proof bands called casparian strips which concentrate the ions before passing to the xylem vessels.
• In the xylem tissue, the ions together with water are carried upwards to the upper parts of the plant. 

Upward movement of water and mineral salts

Once the mineral salts have been taken up from the soil, they enter the xylem together with water for their further transportation through the entire plant. This upward movement of water and mineral salts take place in the xylem vessels.

Forces governing upward movement of water and mineral salts

The forces that govern the upward movement of water and mineral salts through the plant include the following:-

1. Capillarity.
2. Root pressure.
3. Transpiration pull.

1. Capillarity

• Capillarity (capillary forces) is the upward force that draws water up the plant against gravitational pull.
• By this force, water moves up the plant through the narrow tubes of the xylem under the influence of pressure. The latter is due to narrowness of the xylem vessel tubes.
• Capillarity is an important force as it causes water to rise high up in the tall trees.

The cohesion –

Tension theory:

• The cohesion theory describes the upward movement of water by capillarity through the plant.
• According to this theory, the rising of water from the root is caused by evaporation of water from the lenticels.
• Evaporation results in reduced water potential in the cells next to the xylem. Water therefore enters the cell from the xylem sap where it has a higher potential.
• The xylem vessels are full of water and as water leaves the xylem, a tension is set up in the column of water.
• This is transmitted back down the roots by cohesion of water molecule.
• The latter have high cohesion due to their polarity and therefore tend to stick together being held by the hydrogen bonds.
• Water molecules also tend to stick to the vessel walls by a force called adhesion. Thus, the tension in the xylem vessels builds up a force capable of pulling the whole column of water upwards by mass flow.
• Water thus, rises in the fine capillary tubes due to high surface tension.

2. Root pressure

• Water initially enters the root cells by osmosis from the soil solution and in that way it lowers the solute potential of root epidermal cells.
• An active secretion of salts and other solutes into the xylem sap, tends to lower its water potential and for this reason water moves from one root cell by osmosis and then into the xylem.
• The overall result is the creation of the root pressure which generates a hydrostatic pressure which causes a continuous upward movement of water.
• However, this force alone is not sufficient to draw up water except in the slowly transpiring herbaceous plants where it causes guttation.

3. Transpiration pull

During transpiration; water is lost from the epidermal cells, as a result the water potential is lowered in the respective cells. Consequently, water is drawn from the xylem vessels whose sap has a higher water potential into the epidermal cells of the leaf.

• This creates a continuous stream of water flow called a TRANSPIRATION STREAM   which is a main route of upward movement of water and dissolved mineral salts. The force that pulls water upwards in favour of transpiration is called TRANSPIRATION PULL.

TRANSPIRATION

Definition:

Transpiration is a process whereby a plant loses water from the epidermal cells of the leaves in the vapour form.

TYPES OF TRANSPIRATION

• There are three types of transpiration:-

(a) Stomatal transpiration

• This is a major way by which water evaporates from the plant leaves. It is a type of transpiration where by the plant loses water through the stomatal pores.

(b)Cuticular transpiration

• This involves loss of water through the cuticle. In this way a very little amount of water is lost from the plant because the cuticle among other functions restricts water loss from the plant.

(c)Lenticular transpiration

• This involves loss of water by the lenticels.
• The latter are small slits in the stems and bark of trees for gas exchange.

Significance of transpiration in plants:

Transpiration is considered to be “Necessary evil”. This is because it is an inevitable but potentially harzadous process. It thus, has beneficial and harzadous effects.

Beneficial effects

Transpiration is necessary in that;

1. It is a means of transportation of water and dissolved mineral salts through a plant.
2. It is a means of cooling the plant ie: evaporation of water from the surface of the plant eg: leaves has a cooling effect.
3. It is a means of removal of excess water as waste product.
4. Aids uptake of water and mineral ions.

Hazardous effects

• Transpiration is an “evil” process because excessive transpiration (loss of water) leads to dehydration of cells.
• It also interferes with the process of photosynthesis, excretion, respiration etc. all of which require water.
• As a consequence of excessive water loss the plant wilts and finally dies.

STRUCTURE OF A STOMATA

• Structurally, the stomata pore is bordered by two sausage shaped guard cells. The latter have their inner walls being thick and less elastic whereas, the outer wall is thin and elastic (extensible).
• The guard cells have chloroplast capable of photosynthesis. Around the guard cells are the epidermal and subsidiary cells.

Diagram:

Mechanism of stomata closure and opening:

The closure and opening of the stomatal pore is caused by change in turgor pressure of the guard cells. If water is drawn into catalyses the hydrolysis of ATP into ADP and Pi and energy is released.

• The released energy used to pump K⁺ ions into the guard cell and H⁺ ions out of the guard cells. This also causes the inside of the guard cell to be alkaline.
• The accumulation of K⁺ ion and glucose into the guard cells results into increased osmotic pressure in there.
• The result of this increased osmotic pressure is the osmotic movement of water in to the guard cells from the epidermal cells.
• Turgidity of the guard cells result into opening of the stomata aperture.
• On contrary during the night, K⁺ ions are pumped out of the guard cells and H⁺ ions are pumped in. There is also an accumulation of CO₂ in the intercellular spaces.
• This result into increased acidity of the guard cells ie: Fall in the pH value. This fall in pH value favours the association of glucose forming starch in the guard cells while in the surrounding epidermal cell K⁺ ions (allophone) causes the accumulation of glucose.
• The net effect is the osmotic movement of water from the guard cells to the epidermal cells. Thus loss of water from the guard cells causes them flaccid and hence closure of the stomatal pore.

Illustration:

• Stoma is closed in the dark, but in the presence of light ATPase is stimulated to convert ATP to ADP and so provide the energy to pump out H⁺ from the guard cells. These protons return on a carrier, which also bring Cl^– with it. At the same time K⁺ enter guard cells.
• As a result of this influx of ions, the water potential of the guard cells becomes more negative (lower) causing H₂O to pass in by osmosis. The resultant increase in pressure potential causes the stoma to open.
• In the dark, the movement of ions and H₂O is reserved.

Question:

Describe the mechanism of stomatal closures and stomatal opening based on the osmotic pressure (Pressure flow) hypothesis.

• The guard cells, the latter become turgid and stomatal pore opens. And when the cells are flaccid, the stomatal pore closes. The guard cells, the latter become turgid and stomatal pore opens. And when the cells are flaccid, the stomatal pore closes.
• The guard cells have thicker inner inelastic walls and thinner elastic outer walls. During expansion they do not expand uniformly in all directions.
• The thick and less elastic inner walls are less pulled out wards leaving an open between them.

How is the mechanism explained?

1. A traditional hypothesis

The starch-sugar hypothesis suggested that an increase in sugar concentration in guard cells during the day led to their solute potential becoming more negative, resulting in entry of water by osmosis.

However, sugar has never been shown to build up in guard cells to the extent necessary to cause the observed changes in solute potential.

K⁺ ion and osmotic pressure theories:-

It has now been shown that potassium ions and associated negative ions accumulate in the guard cell during the day in response to light and are sufficient to account for the observed changes.

In darkness, potassium (K⁺) ions move out of the guard cell into surrounding epidermal cells. The water potential of the guard cells increases as a result and water moves out of the cells. The loss of pressure makes the guard cells change shape and stoma closes.

What causes K⁺ to enter the guard cells in the light?

Ans: K⁺ may enter in response to the switching on of an ATPase which is located into the cell surface membrane which pumps out H⁺ and K⁺ may then enter to balance the charge.

More explanations:-

• During the day, the plant photosynthesizes by consuming CO₂.
• This reduces the concentration of CO₂ in the intercellular spaces of the leaf.
• This lowers the level of Carbonic acid and hence a rise in pH value ie: The cells become more alkaline.
• This favours the conversion of starch into glucose which accumulates in the guard cells. At the same time the enzyme ATPase.

2. Using carbon – 14 isotope

• If a plant with a ringed stem is supplied with CO₂ containing C-14 isotope ie: ¹⁴CO₂, the food substances accumulated above the ring appear to contain C-14. This suggested that the synthesized food is translocated through the phloem.

3. Using mouth parts of a feeding aphid

• An aphid is an insect that uses its tubular needle – like mouth part to feed on the sugary solutions from the phloem sieve tubes.
• If the feeding insect is anaesthesized with CO₂ and the mouth parts are carefully cut so that the tube remains inserted into the phloem vessel, the food substances continue to move through the tubular needle of aphid.
• Analysis of this solution reveals the presence of sugary substances and amino acids all of which are the products of photosynthesis.

1. There are diunal variations in the concentrations of the glucose which are in turn reflected in the phloem sieve tubes.

Mechanism of Translocation by the phloem:

There is no one agreed mechanism by which food substances are translocated through the phloem. However there are various hypotheses that try to describe the mechanism of phloem translocation. They include:

A. Mass flow hypothesis (Münch 1930)                                    

This is also called Münch’s hypothesis or pressure flow hypothesis. According to this hypothesis, food substances are translocated through the phloem in a mass flow mechanism.

Consider the munch model bellow:

• Could the ions reach the xylem entirely by means of the apoplast pathway?

Ans: No, the endodermis is a barrier to the movement of water and solutes through the apoplast pathway. This is due to the presence of casparian strips which prevents further progress

• To cross the endodermis, ions must pass by diffusion or active transport through the cell surface membranes of endodermal cells, entering their cytoplasm and possibly there vacuoles. Thus the plant monitors and controls which type of ions eventually reach the xylem.

Ions can also move through the symplast pathway. The final stage in the movement of mineral salts across the root is the release of ions into the xylem.

Once in the xylem, they move by mass flow throughout the plant in the transpiration stream.

The chief sinks, ie: Regions of use, for mineral elements are the growing regions of the plant, such as the apical and lateral meristerms, young leaves, developing fruits and flowers and storage organs.

Translocation of the manufactured food:

• In higher vascular plants, food substances are translocated through the phloem.

Evidence to show that phloem translocates food:

(i) Ringing experiment.

A ring of tissue containing phloem was removed from the outer region of the stem, leaving the xylem intact. It was found that the leaves did not wilt, but growth below the ring was greatly reduced. This is because, movement of sugars down the plant were stopped without affecting passage of water upwards.

Description of the model

• In the model, there is an initial tendency of water passing by osmosis into A and C. However, the tendency is greater for A than for C because the solution in A is more concentrated than that in C.
• As water passes into A, a pressure potential (hydrostatic pressure) builds up in the closed system A-B-C forcing water out of C.
• Mass flow of solution occurs through B along the hydrostatic pressure so generated.
• There is also an osmotic gradient from A to C and eventually the system comes into equilibrium as water dilutes the contents of A and solutes accumulate at C.

Application of the model to the living plant

• The leaves which make sugars during photosynthesis are represented by A. The synthesized sugars, lower the water potential of the leaf cells and consequently this fuses the flow of water into the leaves by osmosis through the xylem (D).
• Due to hydrostatic pressure generated into the phloem (B), food from the source (A) to the sinks such as roots and storage organs (C) are transported in a mass flow system.
• In the plants, equilibrium is not reached because sugars are constantly being made at sources (A) and constantly being used at sinks (C).

Critique (weakness) to the hypothesis

1. It is purely physical explanation and so does not explain why sieve tubes must be living and metabolically active.
2. It does not explain the observation that the leaf cells are capable of loading sieve tube against the concentration gradient. ie: The fact that the Ψ_s of sieve tubes is more negative than that of the leaf cells. The hypothesis

has therefore been modified to include an active loading mechanism of solutes into the sieve tubes. The osmotic and hydrostatic pressure gradient therefore starts in the tubes rather than in the photosynthetic cells. It is

also believed that unloading at the sinks is an active process.

1. It ignores the membrane barriers between the sieve tubes and the plastids.
2. It assumes an empty sieve lumen and fully open sieve plate pores.

(B) Transcellular strands hypothesis (THAINE).

• The hypothesis was described by Thaine. It explains the role of phloem proteins in the translocation of food.
• According to Thaine, the protein fibrils that run from one end of the sieve tube to the other are the ones that carry food substances.
• The food substances pass along these fibrils due to the peristatic action of the protein sheath in a manner resembling cytoplasmic streaming.
• This is an active transport and it accounts for transportation of materials in both directions in the same sieve tube.

Ideas of the hypothesis are summarized as;

(i) Food is transported by phloem protein, due to peristatytic action the food flows along the fibres.

(ii) Food is transported in both directions.

(iii) Food is transported actively.

(C) ELECTRO – OSMOSIS HYPOTHEIS (SPANNER)

• According to Spanner, the flow of food is produced and maintained by electro-osmotic force set up across the sieve plates.
• According to this hypothesis, K⁺ ions are actively transported across the sieve plates, carrying with them water and dissolved mineral salts.
• This means that K⁺ ions create an electric potential gradient as a result of which water molecules flow through the sieve plates carrying the sucrose molecules with them.

(D) SURFACE SPREADING HYPOTHESIS

In this hypothesis, the idea is that the solute molecules might spread over the interface between two cytoplasmic materials as oil spreads over an air-water interface the form of bands called Casparian strips. Therefore water and solutes particularly salts in the form of ions, must pass through the cell surface membrane and into the living part (cytoplasm) of the cells of endodermis. In this way the cells of the endodermis can control and regulate the movement of solutes through the xylem. Such control is necessary as a protective measure against the entry of toxic substances, harmful disease-causing bacteria, fungi etc.

As roots get older, the extent of suberin in the endodermis often increases. This blocks the normal exit of water and mineral salts from the cell     

Uptake of mineral salts and their transport across roots.

In plants, minerals are taken up from the soil or surrounding water by roots. Uptake is greatest in the region of the root hairs.

Note:

• Mineral elements exist in the form of salts which are made up of ions, and in solution the ions can separate (dissociate) and move freely.
• Ions can cross membranes in a number of ways, including:-

1. Active uptake (transport)- In which ions are taken up into cells against a concentration gradient using energy from respiration.
2. Passive uptake– Where ions move by mass flow and diffusion through the apoplast.

The figure below shows the uptake of K⁺ ions by young cereal roots which had previously been thoroughly washed in pure water. After 90 minutes respiratory inhibitor potassium cyanide was added to the solutions.

                                                                                         

Figure above shows that two distinct phases of uptake. The 1^st phase lasts for about 10 -20 minutes. Uptake during this phase is relatively rapid. K⁺ ions come into contact with the epidermis of the root and start to move through the cell walls of the apoplast pathway, it is shown that this phase is more or less independent of temperature since it occurs just rapidly at 0^oC. It is passive process.

• The 2^nd phase is temperature dependent and does not occur at 0^oC when the rate of metabolism and respiration is very low. Its inhibition by KCN shows that it is dependent on respiration and the uptake at this time

probably by active transport across all membrane into cells.

• Why the roots were thoroughly washed before placing them in a solution containing K⁺ ions?

Answer:

To flush out any existing K⁺ ions from the root

• It is shown that if the carrot discs are transferred from pure water to KCl solution, the rate of respiration increases. Why?

Answer:

Rise in respiratory rate is accompanied by a rise in KCl uptake. Once KCl is available, it is therefore apparently taken up by active transport, the energy being supplied by an increased respiratory rate.

• If KCN is added, the rise in KCl stops, this is because KCN inhibits respiration and therefore inhibits active transport of KCl into the carrot discs.

GUTTATION

• Guttation is a physiological process of the plants in which water is lost in form of liquid / droplets.
• The process occurs in members of the grass family, and in species found at the leaf margin and apex.
• Guttation is favoured by those factors that favour low rate of transpiration eg: High humidity, low temperature, absence of light etc.

Question. Summarize differences between guttation and transipiration.

Ans:

Transipiration	Guttation
(1)  Water is lost in form of vapour	-Water is lost in form of liquid.
(2)  It occurs all the time	-It occurs only at night.
(3)  It occurs in stomata, cuticle and lenticels	-It occurs in the hydathodes.
(4)  It is favoured by high temperature, low humidity and light	-Favoured by high humidity, low temperature and darkness.
(5)  Occurs in all plants	-Occurs only in members of the grass family.

Evidence to show that xylem transports water

The evidence for water transportation in the xylem comes from the following observations:-

1. If a red dye such as cosin is dissolved in water and a cut end of the stem is immersed in that water, the plant takes water. After a time lag, the red dye is traced in the xylem vessels. That shows that xylem transports water.

2. If molten fats are added into water having a plant example; Potted plant, as the plant absorbs water, it takes some fats too. The latter block the xylem vessels resulting into wilting of the plant.

3. Ringing experiment

If a part of stem is ringed to remove the phloem, the plant does not wilt. However, if the tissues beneath the phloem are removed, the plant wilts showing that the removed tissues are xylem.

Question. Summarize the properties of xylem which make it suitable for the long distance transport of water and solutes.

Answer

1. Long tubes formed by fusion of neighboring cells, with breakdown of cross walls between them.
2. No living contents, so less sensitive to flow.
3. Tubes are rigid, so do not collapse.
4. Fine tubes are necessary to prevent water columns from collapsing.

Uptake of water by roots

Water moves across the root by pathways similar to those in the leaf namely apoplast, symplast and vacuolar pathways.

Symplast and vacuolar pathways

As water moves up the xylem in the root, it is replaced by water from neighboring parenchyma cells. As water leaves cell A, the water potential of cell A decreases and water enter it from cell B by osmosis or through the symplast. Similarly, the water potential of cell B then decreases and water enters it from cell C and so on across the root to the epidermis.

The soil solution has a higher water potential than cells of the epidermis including the root hairs. Water therefore enters the root from the soil by osmosis.

Apoplast pathway

The apoplast pathway operates in much the same way as in the leaf. However, there is one important difference. When water moving through spaces in the cell walls reaches the endodermis, its progress is stopped by a water

proof substance called Suberin which is deposited in.

Question. Why does transpiration occur mainly through leaves and not so much through the cuticle and lenticels?

Answer:

• Leaves contain a very large number of stomata for gaseous exchange and there is little resistance to movement of water vapour through these pores.
• Leaves have a large surface area (for trapping sunlight and exchanging gases). The greater the surface area, the greater will be the loss of water by transpiration.

FACTORS AFFECTING THE RATE OF TRANSPIRATION

• The factors that affect the rate of transpiration are of two main categories;
• External (Environmental) factors.

1. Internal (plant) factors.

A: External factors

(i) Light

The rate of transpiration is high during the day. This is because the stomata pores get open due to turgidity of the guard cells. Thus, to night when the stomatal pores are closed, only lenticular and cuticular transpiration occur.

(ii) Temperature

High temperature favours the rate of water loss from the mesophyll cells. High temperatures do also lower humidity of air around the leaf. All these favour loss of water from the leaf to the surrounding area.

(iii) Humidity and vapour pressure

Low humidity around the leaf favours transpiration, because it results into a steeper diffusion gradient of water from the leaf atmospherer to external atmosphere.

(iv) Wind (Air currents)

If the air is still, the rate of transpiration becomes low. This is because the humidity of the atmosphere is high. If air is in motion (in windy situation) the rate of transpiration is high. This is because the blowing wind sweeps away the water vapour concentrated around the leaf surface thereby lowering the humidity and hence favoring high rate of transpiration.

(v) Availability of soil water

The rate at which the plant loses water by transpiration depends in the amount of water available in the soil. If the soil has insufficient amount of water, the rate of transpiration gets reduced as in decidious trees that shed their leaves in the dry season.

B: Internal factors

The plant factors include the following;

1. Surface areas to volume ratio – The greater the surface area to volume ratio, the greater is the rate of transpiration, since broad leaves have high transpiration rate than narrow leaves.
2.  Cuticle (water proof material)-The thinner the cuticle, the higher the rate of transpiration and vice versa.
3. Stomata

(a) Size of the stomatal pore – The larger the size of the stomatal pore, the higher the rate of transpiration and vice versa.

(b) Number of the stomatal pore – The greater the number of stomata, the higher the rate of transpiration.

(c) Density of stomata

• The higher rate of transpiration occurs at the upper side of the leaf because it is at this side where stomata are directly exposed to light energy.

Question: Describe the factors that affect the rate of transpiration.

The molecular film so formed could be kept moving by molecules being added at one end removed at the other.

(E) ACTIVE TRANSPORT HYPOTHESIS

• This suggests that, the translocation of food through the phloem involves some sort of active mechanisms. This is supported by the facts that;

1. The phloem tissue has a high rate of respiration and there is a close correlation between the speed of transduction and metabolic rate.
2. Lowering temperature and treating the phloem with metabolic poisons, also lower the rate of translocation. This means that the enzymes involved in the production of energy are affected.

Question:Describe the various hypotheses of the phloem translocation.

Xerophytic adaptations

• Xerophytes are plants which have adapted to conditions of unfavourable water balance. This is the condition where the rate of loss is potentially greater than the availability of water.

• Mesophytes are plants which have adapted to conditions where water is available.

• Halophytes are plants that live in salt marshes where the concentration of salts in the soil makes it difficult to obtain water. Halophytes also exhibit Xeromophic features.
• Xerophytes plants have evolved a wide range of features designed to reduce the rate of transpiration. These are known as Xeromophic features.

Xeromophic adaptations take three general forms:

1. Reduction in the transpiration rate – clearly anything which lowers the rate of transpiration helps to conserve water when in short supply.

2. Storage of water – Plants living in areas where water supply is intermitted, store water for use during periods of drought. Plants which store water are termed Succulents.

3. Resistance to desiccation – Some species exhibit a remarkable tolerance to water loss and resistance to wilting.

Xeromophic adaptations of plants

1. Features for reduction of the transpiration rate:-

1. Thick cuticle – Reduces cuticular transpiration by forming a waxy barrier preventing water loss.
2. Rolling of leaves – Preventing water diffusing out through stomata which are confined to the inner surface.
3. Layer of protective hairs on leaf – Moist air is trapped in the hair layer, so reduce transpiration rate.
4. Absence of leaves – Reduces the rate of transpiration.
5. Orientation of leaves – The positions of leaves are constantly changed so that the sun strikes them obliquely. This reduce their temperature hence rate of transpiration.
6. More negative water – This makes it more difficult for water to potential of the cell sap, be drawn from them.

2. Futures for succulence (water storage)

1. They have succulent leaves which stores water.
2. They have succulent stems which stores water.
3. Closing of stomata during day light, so reducing transpiration rate.
4. Shallow but extensive root systems – This allows efficient absorption of water over a wide area.

3. Features for resistance to desication

(a) Reduction of transpiration surface through loss or adaption of leaves.

(b) Lignification of leaves – Preventing it from wilting in times of drought.

(c) Reduction in cell size – Making the plants less liable to wilt.

Hydrophytes adaptations

• Plants living in wholly or partly submerged in water are called hydrophytes.
• The greatest problem for hydrophytes is to obtain oxygen for respiration.

Adaptations

(i)Plants have aeration tissue (Aerenchyma) which comprises large air spaces called Lucunae between the cells of the stem and leaves. These stores oxygen produced by photosynthesis which can be used for respiration.

(ii) Plants can tolerate high level of ethanol which is a product of anaerobic respiration.

(iii) Aerating tissue confers buoyancy, raising leaves to the surface where they can take  maximum advantage of the light.

(iv) They lack supporting tissue (water provides support) which would make the plant more rigid, rendering it liable to breakage by water currents.

Evidence supporting the role of xylem in transporting minerals

1. The presence of mineral ions in xylem sap.
2. A similarity between the rate of mineral transport and the rate of transpiration.
3. Experiments using radioactive tracers show that where lateral transfer of minerals can take place, minerals pass from the xylem to the phloem.

NOTE:

• The xylem transports water and dissolved mineral salts from the roots to the leaves, and phloem transports sugars and other products of photosynthesis from leaves to other parts of the plants.
• The fascinating thing is that two systems employ quite different principle. Xylem transport is essentially a passive process, depending mainly on water potential gradients within the plant. Indeed, the xylem tissue in which it takes place is composed of dead cells. Phloem transport on the other hand is an active energy requiring process which takes place in living tissue.

Factors affecting the rate of translocation

1. Temperature – Increase in temperature up to a maximum of about 35^o C, increases the rate of translocation probably by affecting the enzymes involved in the secretion and removal of sucrose from the tubes.
2. Light– Translocation to the roots is greatly enhanced in the dark.
3. Metabolic inhibitors– Hydrogen cyanide and dimtrophenol inhibit carbohydrate translocation.
4. Concentration gradients – Carbohydrate seems to move from regions of higher concentration to regions of lower concentration.
5. Mineral deficiencies- Boron seems to be important in forming an ionisable complex with sucrose which then passes more easily through the cell membranes. It also appears to slow down the enzymic conversion of glucose-1-phosphate to starch thus keeping more sugar available for translocation.
6. Hormones – Cytokinins, IAA and gibberellins appear to at best control translocation, probably by their effects on metabolic rates at the source and sink.

(II) TRANSPORT IN ANIMALS

As organisms increase in size and complexity so the quantity of materials moving in and out of the body increases. The distance that materials have to travel within the body also increases, so that diffusion becomes inadequate as a means for their distribution.

There are two circulatory systems which rely on mass flow in animals, names:-

– Vascular system and

– Lymphatic system.

In animals, materials are transported through blood vascular system which is characterized by the following features;

1. Presence of the circulatory fluid eg: blood.
2. Presence of the tubes in which the blood flows eg: blood vessels.
3. Presence of a pumping device such as heart or modified blood vessels.

The cardiac muscle

The cardiac muscle is that muscle that forms the walls of the heart.

Structure of the cardiac muscle:

• Structurally, the cardiac muscle has fibers, each of which consists of cylindrical short cells arranged in columns. Each cell has a central nucleus, myofibrils and faint transverse striations.
• Adjacent columns are joined by oblique cross connections.
• The cells have abundant sarcoplasm and they are well supplied with blood and mitochondria.

Diagram:

Adaptations of the cardiac muscle

1. It is striated to confer strength so that it withstands the pumping pressure of the blood.
2. It is highly vascularized to ensure adequate supply of food and oxygen.
3. The numerous mitochondria supply the necessary energy required to pump the blood.
4. It is myogenic ie: Contraction and relaxation are initiated within itself.
5. It is capable of contracting and relaxing throughout its life without any fatigue.
6. Cells can tolerate high levels of lactate (a product of anaerobic respiration).

Composition of blood

(i) Plasma:- It consists of 90% water and 10% of a variety of substances in solution and suspension. ie: proteins, hormones, mineral salts, gases, wastes, water soluble vitamins B and C.

(ii) Blood cells:- They include;

(a) Red bood cells (RBC)- Which are packed with haemoglobin, the oxygen carrying compound which gives blood its red colour. They lack nucleus therefore makes more room for haemoglobin. RBC (erythrocytes) also lacks mitochondria which means they have to respire anaerobically. Therefore they do not use up any of the oxygen they carry.

RBCs also contain the enzyme carbonic anhydrase which plays an important role in carbondioxide transport.

(b) White blood cells (leucocytes)- They play an important role in the body’s defence mechanisms against diseases.

(c) Platelets (Thrombocytes) – They are responsible for starting the process of blood clotting.

Types of circulatory systems in animals

Two distinct types of blood systems are found in animals;

1. Open blood system and
2. Closed blood system.

1. The open blood system

• This is a type of blood vascular system in which blood mixes with the body tissues.
• In this case, blood is pumped by the heart into an aorta which branches into a number of arteries.
• These open into a series of blood species collectively called Haemocoel.
• The blood then goes back through the open ended vein. In this system, the flow of blood cannot be adjusted and, the blood flows at a low pressure.
• This system is found in arthropods and molluscs.

2. Closed circulatory system

• This is a type of circulatory system in which the blood is confined to vessels.
• Blood never mixes up with the body tissues nor does it bath the organs directly.
• In this case, the speed of the blood can be adjucted, the blood is at high pressure and therefore goes around the body very quickly.
• This system is found in Annelids and vertebrates.

Question: Summarize the differences between open and closed circulatory systems.

Subdivisions of closed circulatory system

• The closed circulatory system is subdivided into:

(a) Single circulatory system

• This is a type of circulatory system in which blood passes the heart only once in a single complete circulatory turn.
• In fish for example blood from the heart first goes to the gills to collect oxygen, but then continues round the whole body before returning to the heart.

The deoxygeneted blood from various parts of the body passes direct to the heart which pumps it to the gills for being oxygenated for the circulation to begin once more.

(a) Diagram: Single circulation of blood in fish

(b) General plan of the mammalian circulatory system (Double circulatory system)

 

(b) Double circulatory system

• In this type of circulation, blood passed the heart twice in a single complete circulatory turn.
• Only birds and mammals have true double circulations. It is probably no coincidence that only birds and mammals are warm blooded.
• Warm-bloodedness requires a high metabollic rate and this is only possible if a good supply of oxygen is available for high levels of aerobic respiration. Animals with a high metabollic rate can maintain higher levels of activity than other animals.

Advantage of double circulation system

• Blood can be sent to the lungs to pick up oxygen and then be returned to the heart to be pumped again before travelling around the body.

Double circulatory system has;

(i) Pulmonary circulation and

(ii) Systemic circulation.

(iii) Coronary blood circulation.

(i) Pulmonary circulation

• This is a circulation between the heart and the lungs. Deoxygeneted blood from the heart is carried by the pulmonary artery to the lungs where as oxygenated blood from the lungs to the heart is carried by pulmonary vein.

(ii) Systematic circulation

The circulation between the heart and all other body parts except the lungs. Deoxygenated blood from various parts of the body is brought to the heart by the vena cava where as oxygenated blood from the heart is pumped to various body parts through the aorta.

(iii) Coronary circulation:- This is the circulation within the walls of the heart.

Features of a human circulation

1. It is a double circulation.
2. The organs are arranged in parallel rather than in series. If they were arranged in series, blood would pass from organ. A to B to C and so on, losing pressure, oxygen and nutrients in each stage. This would be extremely inefficient. Also, any damage done to a blood vessel linking two organs would interrupt the whole circulation.
3. A portal vessel (vessel linking two organs neither of which is the heart) links the gut to the liver ie: Gut and liver are linked in series not in parallel.

Advantage of this series linkage is that blood from the gut is variable in composition and it contains other substances such as alcohol. Liver monitors blood passing through it and maintains a constant composition. Eg: Liver removes excess glucose from the blood and stores it as glycogen.

NOTE:

Vessels conveying blood away from the heart are called Arteries. These divide into smaller arteries called arterioles. The arterioles divide many times into capillaries where exchange of materials between blood and tissue takes place. Within the organ or tissue the capillaries reunite to form Venules which begin the process of returning blood to the heart. The venules join to from Veins. Veins carry blood back to the heart.

Section through heart, simplified diagram:

The cardiac cycle

• Refers to the sequence of events which takes place during the completion of one heart beat. It involves repeated contraction and relaxation of the heart muscle.

Contraction is called Systole and relaxation is called Diastole.

It occurs is follows;

1. Atrial diastole – During the time when the atria and the ventricles are both relaxed, blood returning to the heart enters the two atria. Oxygenated blood enters the left atrium and deoxygenated blood enters the right atrium. At first the bicuspid and tricuspid valve are closed but as the atria are filled with blood the valve are pushed open.

2. Atrial systole – When the atriole distole ends, the two atria contract simultaneously. This is termed as atrial systole and results in blood being pumped into the ventricles.

3. Ventricular systole. The ventricles contract and pressure rises in them and forces open the semi-lunar valve of the aorta and pulmonary artery and blood enters these vessels. During ventricular systole the first heart sound described as “Lub” is produced.

4. Ventricular diastole – Ventricular systole end and is followed by ventricular diastole. The higher pressure developed in the aorta and pulmonary artery tends to force some blood back towards the ventricles and this closes the semi-lunar valves of the aorta and pulmonary artery. Hence back flow into the heart is prevented. The closing of the valves causes the second heart sound “dub.” The two heart sounds are therefore:

Ventricular systole = lub.

Ventricular diastole = dub.

Myogenic contraction of heart rate

When a heart is removed from a mammal and placed in a well oxygenated salts solution at 37^oCit will continue to beat rhythmically for a considerable time without stimuli from the nervous system or hormones. This demonstrates the myogenic nature of the stimulation of the heart, ie heart muscle has its own ‘build in’ mechanism for bringing about its contraction.

The stimulus for contraction of the heart originates in a specific region of the right atrium called the SINO-atrial node (SAN). This is located near the opening of the vena cavae. It consists of a small number of cardiac muscle fibers and a nerve ending from the automatic nervous system. The SAN can stimulate the heart beat on its own but the rate at which it beats can be varied by stimulation from the automatic nervous system.

The cells of the SAN slowly become depolarized during atrial diastole. This means that the charge across the membrane is gradually reduced. At a certain point an action potential is set up in the cell. A wave of excitation similar to a nerve impulse passes across muscle fibres of the heart as the action potential spreads from the SAN. It causes the muscle fibres to contract. The SAN is the PACEMAKER because each wave of excitation begins here and acts as the stimulus for the next wave of excitation.

Once contraction has begun, it spreads through the walls of atria through the network of cardiac muscle fibres and both atria contract more or less simultaneously.

The atrial musle fibres are completely separated from those of the ventricles except for a region in the right atrium called the atrio-ventricular node (AVN)

The structure of the AVN is similar to that of the SAN and is connected to a bundle of specialized muscle fibres, the AV bundle which provides the only route for the transmission of the wave of excitation from the atria to the ventricles. There is a delay of approximately 0.15s in conduction from the SAN to AVN, which means that atrial systol is completed before ventricular systole begins.

The AV bundle is connected to the bundle of His (strand of modified cardiac fibres) which gives rise to finer branches known as Purkyne.

Impulses are conducted rapidly along the bundle and spread out from there to all parts of the ventricles. Both ventricles are stimulated to contract simultaneously. The wave of ventricular contraction begins at the bottom of the heart and spread upwards squeezing blood out of the ventricles towards the arteries which pass vertically upwards out of the heart.

NOTE: The period during which cardiac fibres do not respond is called Absolute

refractory

period. This period is longer in cardiac muscle than in other types of muscles and enables it to recover fully with becoming fatigued, even when contracting vigorously and rapidly. As muscle recover it passes through a relative refractory period when it will respond only to a strong stimulus.

Functions of mammalian Blood

1. Transport of digested food from the small intestine to various parts of the body where they are stored or assimilated and transport from storage areas to places where they are used.
2. Transport of soluble excretory materials to organs of excretion.
3. Transport of hormones from the glands where they are produced to target organs. This allows communication within the body.
4. Distribution of excess heat from the deeply seated organs. This helps to maintain a constant body temperature.
5. Transport of respiratory gases (ie: oxygen and CO₂).
6. Defence against diseases. This is achieved in three ways:-.

i. Clotting of blood which prevents excessive blood loss and entry of pathogens

ii. Phagocytosis which engulf and digest bacteria.

iii. Immunity achieved by antibodies and lymphocytes   

  7.  Maintenance of constant blood solute potential and pH as a result of plasma protein Activity

Transplantation

• Refers to the replacement of diseased tissue or organs by healthy ones.
• A technique is used increasingly in surgery today.
• However, when foreign tissue is inserted into another individual it is usually rejected by the recipient because it acts as an antigen, stimulating the immune response in the recipient.

Types of transplant:-

• Isograft – Grafting within the same individual.
• Autograft – Grafting between two individuals who are genetically identical.
• Allograft – Two individuals of the same species.
• Xenographt – Two individuals of different species.
• The details of these are beyond the scope of your level

Comparison of the structure and function of:-

Artery	Vein	Capillary
–         Transports blood away from the heart.	–         Transport blood towards the heart.	–  Link arteries to vein. Site of exchange of materials.
–         Tunica media thick and composed of elastic & smooth tissues.	–         Tunica media relatively thin and only slightly muscular. Few elastic fibres.	– No tunica media.

–  No elastic fibres.

–         No semilunar valves (except where leaves heart).–         Semilunar valves present so as to prevent back flow of blood.–  No semilunar valves.–         Blood plow rapid.–         Blood flow slow.–  Blood flow slowing.–         Low blood volume.–         Much higher blood volume than capillaries or arteries.–  High blood volume.–         Blood oxygenated except in pulmonary artery.–         Blood deoxygenated except in pulmonary vein.– Mixed oxygenated and deoxygenated blood.

Changes in fetal circulation at birth (The foetal circulation)

• Throughout the development in the uterus the fetal lungs do not function since gaseous exchange and nutrition are provided by the mother via the placenta.
• Most of the oxygenated blood returning to the fetus via the umbilical vein by-passes its liver in a vessel, the ductus venosus which shunts blood into the inferior vena cava and passes it to the right atrium.
• Some blood from the umbilical vein flows directly to the liver, blood entering the right atrium therefore contains a mixture of oxygenated and deoxygenated blood. From here most of the blood passes through an opening. Some blood passes from the right atrium into the right ventrical and into the pulmonary artery but does not pass to the lungs. Instead it pass through the ductus arteriosus directly to the aorta, so by-passing the lungs, pulmonary vein and the atrium and ventricle of the left side of the heart. Blood from the left atrium passes into the left ventricle and into the aorta which supplies blood to the body and the umbilical artery.
• Pressure into the fetal circulatory system is greatest in the pulmonary artery and this determines the fetus and placenta.

The baby finally acquires an adult’s circulatory system and independent physiology 

Changes at birth

At birth the sudden inflation of the lungs reduce the resistance to blood flow through the pulmonary capillaries and blood flows through them in preference to the ductus arteriosus; this reduces the pressure in the pulmonary artery. At the same time the typing of the umbilical cord prevents blood from flowing through the placenta, and this increases the volume of blood flowing through the body of the baby and leads to a sudden increase in blood pressure in the aorta, left ventricle and left atrium. This pressure change causes the small valves guarding the foramen ovale which open to the left atrium to close, preventing the short circuiting of blood from right to left atrium. Within a few months these valves fuse to the wall between the atria and close the foramen ovale completely. If this does not occur, the baby is left with a “hole in the heart” and will require surgery to correct the defect.

The increased pressure in the aorta and decreased pressure in the pulmonary artery force blood backwards along the ductus arteriosus into the pulmonary artery and hence to the lungs, thereby boosting its supply. After a few hours, muscles in the walls of the ductus arteriosus constrincts under the influence of the rising of oxygen in the blood and close off this blood vessel. A similar mechanism of muscular contraction closes of the ductus venosus and increasing blood flow through the liver. The mechanism of closing down the ductus venosus is not known but is essential in transporting the ante-natal (before birth) circulation into the post-natal (after birth) condition.

Note:

Failure of the foramen ovule to close up, results into continued mixing up of the oxygenated and deoxygenated blood. Due to high concentration of CO₂ in the blood, the baby develops the blue tinks on its skin and it is described as a “Blues baby”.