Topic 5: Nutrition - Biology Notes Form 5

Topic 5: Nutrition - Biology Notes Form 5

NUTRITION

Nutrition is the process of acquiring energy and materials such as proteins, glucose, minerals, fats etc.

The taking in and use of food and other nourishing material by the body.

Living organisms can be grouped on the basis of their source of energy or source of carbon.

Autotrophic

Use of inorganic sources of carbon i.e. CO_2, for example plant, Algae and Bacteria e.g. cyanobacteria.

Heterotrophic

The source of carbon is organic for example animals and protoctista.

PHOTOSYNTHESIS.

Photosynthesis is the process whereby green plants, algae makes their own food in which complex organic molecules, glucose is formed by the use of simple inorganic materials such as CO₂, H₂O and minerals in the presence of light absorbed by chlorophyll.

IMPORTANCE OF PHOTOSYNTHESIS.

1. It converts light energy into chemical energy.
2. Maintains life in the ecosystem.
3. Convert the inorganic forms in the world i.e. reduce CO₂ from the air.
4. Almost life on the earth depends on photosynthesis either directly or indirectly.
5. Release of oxygen
6. Sources of fossil fuel

THE LEAF STRUCTURE

The leaf is the main photosynthetic organ of the plant although other parts like stems, sepals, roots and other parts may also photosynthesize.

Adaptations of leaf for photosynthesis

1. They grow positively to phototropism
2. Rapid elongations of shoots in dark to ensure leaves are brought into light as soon as possible.
3. The leaves are arranged in mosaic form (avoiding or minimizing overlapping) in order to allow maximum absorption of light.
4. Leaves have large surface area to volume ratio to capture as more sunlight as possible.

Example: in cold areas where sunlight is scarce leaves are

(Very small). This is an adaptation so that each leaf will receive sun light

1. The cuticle and epidermis (protective cells) are transparent. They can allow passage of light.
2. Mesophyll cell are packed with chloroplast.
3. Chloroplast holds chlorophyll
4. Chloroplast is surrounded by two membranes which forms envelope.
5. Chloroplasts contain green pigments i.e. chlorophyll and photosynthetic pigments like carotenoids
6. Inside the envelope fluid is stroma.
7. Membrane system of chloroplast is a site for light depending reactions.
8. Stroma is a site for light independent reaction it contain enzymes particularly for Calvin cycle and sugar and inorganic materials.

BASIC PHYSICS OF LIGHT

Light is a form of radiant energy; part of electromagnetic spectrum

Photosynthesis makes use of visible spectrum.

Properties of light

Light behaves as a particle and wave.

According to the quantum theory light is transferred in the form of discrete units known as quanta. One quantum of light is known as photon.

According to wave theory light is transferred in form of waves. The properties of waves include wavelength, velocity, frequency etc.

• These entities are related to plank’s theory.

Wavelengths that correspond to colors of visible lights are 400 – 750 nm.

There are three important properties of light to photosynthesis.

1. Spectral quality (colour of light) e.g. chlorophyll is sensitive to blue and red.
2. Time (Duration).
3. Light intensity (amount of light) i.e. the amount of lights which falls on a photosynthetic pigments within a certain period of time.
4. Light intensity is measured in photons cm²/sec

PHOTOSYNTHETIC PIGMENTS

The types of photosynthetic pigments in high plants are.

(a) Chlorophyll

(b) carotenoid

Their role is to absorb light energy and convert it to chemical energy.

A. Chlorophyll

The types of chlorophyll include

1. Chlorophyll a and b
2. Chlorophyll b and c
3. Chlorophyll a and c

The most important ones for photosynthesis are chlorophyll a and b.

Carotenoids.

• They are yellow, red or brown, mostly present in carrots, red, pepper responsible for flower colour.
• Absorb strongly in blue – violet range
• They are necessary pigments because they pass absorbed light to the chlorophyll.

The types of carotenoids include

1. Carotenes
2. Xanthophylls (divided from carotenoids)

EXCITATION OF CHLOROPHYL BY LIGHT MECHANISM

The important parts of reactions are:

1. Antennal chlorophyll
2. Reaction center
3. Electron acceptor

 EXCITATION OF CHLOROPHYLL BY LIGHT

Chlorophyll replaces electrons from electron donor

PHOTOSYSTEM

The photosynthetic pigments molecules are isolated in two types of photosynthesis.

Photosystem I

• P₇₀₀   reaction centre i.e. has ability to absorb light for a wavelength 700nm.
• They have about 200 chlorophyll molecules and 50 carotenoids molecules
• There is Ferodoxin ( protein ) which acts as a w-Ractor electron acceptor

Photosystem II

• P₆₈₀ reaction centre i.e. have ability to absorb light of wavelength 680nm.
• They have about 200 chlorophyll molecules and 50 carotenoids molecules.
• Surrounded by Mg atom i.e. The light harvesting complex (antennae molecule) is surrounded by magnesium atom.

BIOCHEMISTRY OF PHOTOSYNTHESIS.

• In photosynthesis light energy to first converted into electrical energy (e – moving) and finally to chemical energy (ATP).

This involves three phases.

1. Light harvesting

Light energy is captured by chlorophyll pigments. This is done by antennae molecules.

2. Light dependent stage.

A flow of electrons results from the effects of light on chlorophyll and so causes the splitting of water into hydrogen on electrons and oxygen.

3. light independent stage

The use of hydrogen ions and chemical energy ATP is used in reducing CO₂ to form sugar

PHOTOPHOSPORYLATIONS

This is the synthesis of ATP from ADP and phosphate when the chloroplast is exposed to the light. The NADP is reduced to NADPH₂ and Oxygen is evolved. In dark reaction however when ATP and NADPH₂ and provided Carbon dioxide is reduced to Carbohydrate.

If energy comes from light the process will be called photophosphorylation. When Oxygen is used which comes from oxidation of food substances usually glucose the process is Oxidative phosphorylation (conversion of ADP + Pi to ATP using chemical energy obtained from food by respiration).

Photophosphorylation is the conversion of ADP + Pi to ATP using light energy from photosynthesis.

ADP + Pi           ATP

NADP^– + H⁺        NADPH

LIGHT DEPENDENT REACTION.

These are sequence of reactions which depends on the light directly for boosting of electrons, leading to the formation of ATP and NADP.

ADP + P_i              ATP

NADP ^– + H ⁺         NADPH

Light dependent reactions divided into two pathways

1. cyclic electron pathways (cyclic phosphorylation)
2. Non cyclic election pathway (Non cyclic Phosphorylation) directly.

Non cyclic electron pathway (z scheme)

• The electrons removed are not returned back to their sources directly.
• The electrons are supplied by photolysis   of water.

Roles of photons of light

1. Break down of H₂0 molecules (photolysis)
2. Boosting of electrons
3. Activate enzymes

Factors for recurrence of cyclic pathways.

1. Mostly occur in bacteria and algae since they need immediate energy.
2. When the ratio of NADPH and NADP⁺ is high.
3. Availability of NADPH⁺ from dark reactions.
4. When there is a need of more ATP than NADPH
5.  Cyclic electron pathway.
   

Events on the non-cyclic electron pathway.

Light energy is trapped in the reactive chlorophyll a molecule of photo system II boost electrons to higher energy level and electrons are accepted by primary electron acceptor quinone.

These electrons are replaced by electrons coming from photolysis of water.

The electrons are passed from the primary electron acceptor along an electron transport chain to a lower energy level; the reaction of photo system I.

As they pass along this electron transport chain the energy contains in them is used to pump the protons from stroma to the lumen. (Thylakoid space) creating thylakoid protein gradient. An electrochemical energy contained in protons is used to drive photophosphorylation.

Light energy absorbed by photo system I boost electrons to another primary acceptor (ferrodoxin). They are passed via other electron carries to NADPH (NADPH⁺)

The electrons removed from photo system I are replaced by these from photo system II.

ATP and NADPH replaced the net gain from the energy capturing.

NB: This pathway is called non-cyclic because the isoelectrons do not come back to their original position P₆₈₀ or P₇₀₀ reaction center.

To generate one molecule of NADPH; two electrons must be boosted from photo system II and two from photo system I.

Two molecules of water are split into protons and oxygen gas also making available the two replacement electrons needed by photo system II.

To generate one molecule of NADPH; few protons must be absorbed; two by photo system II and two by photo system I.

Events in the cyclic electron pathway.

This event is the alternative pathway which is used to occur when the ratio of NADPH /NADP⁺ is high or when the cell needed more ATP than NADPH.

Light strikes chlorophyll molecules at photo system 1 (P₇₀₀) and electrons are boasted to higher energy level.

They are accepted by the primary electron accepted (Ferredoxin) then they are transferred back to photo system I via cytochrome complex to plastocyanin to P₇₀₀ reaction centre; the energy contained in them is used to pump the protons from stroma to the thylakoids space. (Lumen) creating the proton gradient. The latter drives phosphorylation i.e. ADP + P_i     ATP. This is called cyclic pathway because the boosted electron return back to their original position i.e. P₇₀₀ reaction center.

QUESTION

How is electron flow along the thylakoid membrane related to ATP synthesis in the chloroplast?

ANSWER

This can be explained by the concept of photosynthetic phosphorylation chemiosmotic hypothesis.

• In this process; Electrons from chlorophyll are boosted to a high energy level by sunlight. Slow down an electron transport chain in the thylakoid membrane.
• The energy released as they move to a lower energy level is used to pump protons from the stroma into the thylakoids space; creating proton electrochemical gradient of potential energy.

As the protons flow down the gradient from the thylakoids space back into the stroma (passively by diffusion) ADP is phosphorylated to ATP through ATP syntheses.

NB

The potential energy gradient possessed by protons catalyses phosphorylation as proton diffuses passively from the thylakoid space to the stroma.

                                        LIGHT INDEPENDENT REACTION (DARK REACTIONS)

It is a sequence of events which was discovered by a man called Calvin Benson in 1946 – 53. Therefore is called Calvin Benson cycle.

• Takes place in stroma of chloroplast does not depend on light but uses ATP and NADPH/ The reduced Nicotinamide. Adenine Diphosphate) to reduce CO₂ to from sugar.
• The Calvin cycle functions as a sugar factory within the chloroplast uses inputs like CO₂, ATP, and NADPH to construct out part energy by rich sugar molecule.

Stages of Calvin cycle

1. Carbon dioxide fixation (Acceptance of carbon dioxide).
2. Energy consumption.
3. Release of one molecule of G 3 P ( Glycetaldehyde- 3-phosphate)
4. Regeneration of RUBP (Ribulose Biphosphate).

RUBP – Ribulose Bisphosphate (6 – C)

Rubisco – Ribulose Biphosphate carboxylase/ oxyginase.

PGA – Phosphoglyceric Acid (3c)

DiPGA – Diphosphoglycene Acid

3 – PGAL – glyceralaldehyde – 3-phosphate.

RUMP – Ribulose monophosphate.

1. Acceptance of CO₂ (carbon dioxide fixation)

The CO₂ acceptor is a 5C sugar (pentose) is RUBP. Addition of CO₂ to a compound is called carboxylation; the enzyme involved in is carboxylase. The 6C products is unstable and breakdown immediately to two molecules of glycerate phosphate (GP). This is the first products of photosynthesis.

The enzyme Ribulose biphosphate is present in large amounts in the chloroplast stoma and is in fact the world’s common protein.

2. Energy consuption.

The product is a 3C sugar, phosphate (a triose phosphate i.e. a sugar with a phosphate ground attached. This contains more chemical energy than the 3 – phosphoglyceric acid, and is the first carbohydrate made in photosynthesis.

3. Release of two molecule of 3-PGAL.

One molecule of 3 – PGAL (3- phosphoglyceraldehyde) for the synthesis of organic compound is isomerizes to DHAP (Dihydroxyacetone phosphate) accordingly.

Secondly; the 3 – PGAL combine with DHAP

What is the role of 3-PGAL?

4. Regeneration of RuBP

Uses energy to regenerate RuBP

3 – PGAL            RUMP

• Some of the triose phosphate TP has to be used to regenerate ribulose biphosphate consumed in the first reaction
• This process involves a complex cycle containing 3,4,5,6 and 7c sugar phosphates.
• It’s here that the remaining ATP is used to convert RUMP to RuBP.
• There are six turns of Calvin cycle so as to generate a 6c compound such as glucose from one molecule CO₂. This is according to philosophy.

GENERAL EQUATIONS

C₃ PLANTS

• C₃ plants are the plants which after fixing CO₂ the first product has three carbons.
• Most of C₃ plants are found in temperate and most cold regions so they don’t need any modifications since the environment support them.
• Enzyme for fixing CO₂ is Rubisco.
• Under high light intensity and high concentration of O₂ the C₃ plants can fix O₂ instead of CO₂. This condition is called photorespiration this reveals that the C₃ plant are not efficient for photosynthesis
• Photorespiration is wasteful oxidation process since in the normal Calvin cycle, the oxygen is used instead of CO₂ forming nothing (No food formed)

When O₂ is fixed instead of CO₂ the enzyme is Ribulose   Biphosphate Oxygenase. This shows that RUBISCO has high affinity to O₂ than CO₂.

Example Potato, Tobacco, Beans, Wheat etc

(Have only fixing enzyme)

C₄ plants

The scientists krantz and Hatch slack discovered the c₄ plants and so the name krantz and Hatch pathway.

• C₄ plants are plants in which the first compound to formed after mixing CO₂ has 4 carbon atoms (Oxaloacetate )

E.g. Maize, sorghum etc.

There is distinct arrangement of chloroplast in mesophyll cells and bundle such each one has its chloroplast, the mesophyll cells has grain but few starch grain compared to the bundle sheath.

• This way of arrangement of chloroplast is called kranz anatomy
• The role of this is to fix CO₂ as twice as much as C₃ plants.
• This is an adaptation since the C₄ live in drought areas ( with no water)
• There is a rapid opening and closing of stomata so as to conserve water.
• ( C₄ pathway has two fixing enzyme i.e. PEP(phosphenol pyruvate in mesophyll cells and RUBISCO in bundle sheath (normal calvin

Cycle)

• PEP has high affinity to carbon dioxide since it can fix it at low concentration and high temperature. This is an adaptation.

NB; Carbon dioxide fixation and Calvin cycle are separated in space.

• The role of this is to conserve water. C₄ plants have PEP which can fix carbon dioxide 120 times the C₃ plants.
• Another adaptation is that it can fix carbon dioxide even when stomach is closed. This occurs in all mesophyll cells.
• The compound oxaloacetate acts as a compound for fixing CO₂
• Uses a lot of water.
• PEP can work above 25⁰C which RUBISCO is affected by high temperature.

Significance of C₄ Plants

1. They are maximum rate of CO₂ fixation at high light intensity and high temperatures. C₄ plant increase in dry mans more rapidly than C₃ plants.
2. More tolerant to dry conditions in order to reduce water loss. C₄ plants can adapt drought condition.

DIFFERENCE BETWEEN C₃ AND C₄ PLANTS

	C3 PLANTS	C4 PLANTS
Examples	Most crop plants e.g.	Maize and sugar
	Cereals, Tobacco, Beans
Amounting light intensity	10000 – 30000 fact candles	Not saturated at 105
CO2 Fixation	Occurs once	Occurs twice 1st in mesophyll 2nd in Bundle sheath cells.
Effects on Temperature rise from 250C – 350C	No change in rate or low rate	50% greater   at 350C
CO2 ACCEPTOR	RuBP, a 5 C compound	Mesophyll cell – PEP, a 3C compound, Bundle sheath cell – RUBP.
CO2 fixation Enzyme	Only one type of chloroplast no bundle sheath	Krantz anatomy i.e. two type of cells each with its chloroplast (own type).
Leaf anatomy	RUBP carboxylase which is very efficient	Mesophyll cell – PEP carboxylase which is efficient
Amount of energy used	Low energy	A lot of energy.
Efficiency	Less efficient in photosynthesis	More efficient in photosynthesis

CAMPLANT

• Live in very dry environment (almost desert)
• Fix CO₂ during the night.
• This physiciological mechanism was discovered in plants genus crassulacea and therefore called CAM (crassulacea Acid Metabolic )
• They experience different PH forms i.e. during day time they are alkaline while during night they are acidic.
• They open stomata during night because the temperature is low and humidity is high. The vice versa happen during the day.
• Closes during the day to conserve water, by the time already it has CO₂ from its night time. So fixed CO₂ stored in vacuole.
• CO₂ is fixed in stem
• E.g. cacti, Pineapple, spinach, ferns, (Plants that live in dry environment fix CO₂ at night).
• Use enzyme PEP carboxylase.
• The only difference between C₄ and CAM plant is in anatomy.

FACTORS EFFECTING PHOTOSYNTHESIS

The rate of photosynthesis is affected by a number of factors; the level of which determines the yield of material by a plant. Before reviewing these factors, it is necessary to understand the principle of limiting factors.

The concept of limiting factors.

In 1905; F.F black man; a British plant physiologist; measured the rate of photosynthesis under varying condition of light and carbon dioxide supply. As a result of his works, he formulated the principle of limiting factors; it states that.

“At any given moment; the rate of physiological process is limited by the one factor which is in the shortest supply and that factor alone.

In another words it is the factor which is nearest to its minimum value which will determine the rate of the reaction.

Any changes in the level of this factor; called a limiting factor will affect the rate of reaction. Changes in the level of other factors have no effects.

E.g. Photosynthesis cannot proceed in the dark because the absence of light limits the process.

The supply of light will alter the rate of photosynthesis i.e. more light, more photosynthesis). However if more CO₂ or high temperature is supplied to a plant in the dark; there will be no change in the rate of photosynthesis. Light is the limiting factor therefore only a change can affect the rate.

If the amount of light given to a plant is increased the rate of photosynthesis increases up to a point and then fails off.

At this point, some other factors such as the CO₂ concentration is in short supply and so limit the rate/ increase in CO₂ concentration increases the amount of photosynthesis until some other factors

E.g. temperature limits the process.

Factors affecting photosynthesis.

The rate of photosynthesis increases as controlled by limited external and internal factors.

EXTERNAL FACTORS

1. Light intensity
2. Carbon dioxide
3. Temperature
4. Water

1. Light intensity

The rate of photosynthesis increases as light intensity increases and vice versa.

The rate of photosynthesis is proportional to the light intensity provided that other conditions are suitable.

2. Temperature

This affects the activity of enzymes below the optimum temperature. The rate of photosynthesis increases as the temperature is raised; beyond the optimum temperature i.e. 60⁰c. The enzymes are destroyed.

The rate of photosynthesis therefore decreases.

3. CO₂ concentration

Provided other conditions are suitable for this process the rate of photosynthesis increases as CO₂ concentration increases especially for C₃ plants.

4. Water.

Water is important in reaction i.e. photolysis.

 

The rate of photosynthesis depends on how much water is present in the plant body. The water present in plant body depends on the water present in the surroundings.

When water is so scarce; the plant will wilt and the rate of photosynthesis decreases.

INTERNAL FACTORS.

These are factors which are related to the plant structure.

1. Leaf surface area.
2. Number of chloroplast
3. Number of stomata
4. Orientation of plants body to the direction of sunlight

HETEROTROPHIC NUTRITION

Hetero – other different

Trophic – feeding

Heterotrophic means depends on other feeding.

Meaning

Heterotrophic nutrition is the type of nutrition which involves the organisms that are not capable of manufacturing their own food by the process of photosynthesis.

The organisms involved in this mode of feeding are called heterotrophs. They comprise of animals, fungi and majority of bacteria and few flowering plants (insectivorous plants)

In heterotrophic type of nutrition organisms depend on the organic sources of carbon, carbohydrates, lipids and proteins.

FORMS/TYPES OF HETEROTROPHIC NUTRITION

There are three types of heterotrophic nutrition.

1. Holozoic nutrition
2. Saprotrophic nutrition
3. Parasitic nutrition

1. HOLOZOIC NUTRITION

Means feeding on solid organic materials from bodies of living or dead organisms which may either be plants or animals.

This method is usually seen in animals and carnivorous plants and some protoctists

2. PARASITIC NUTRITION

A parasite feeds on organic materials often but not always soluble from the body of another living organism known as hosts.

3. SAPROTROPHITIC NUTRITION.

A means of feeding on soluble organic materials dead plants and animals.

Saprophytic nutrition can be grouped into:-

1. Saprophitic nutrition i.e. feed on dead plants.
2. Saprozoic nutrition i.e. feed on dead animals.

Saprotrophs secrete enzymes into food when it is digested.

• It occurs mainly in protoctists, bacteria and fungi although there are some saprophytic animal e.g. Hyena.
• Saprotrophic nutrition is of biological great importance because it plays a role in decomposition of biological materials and retaining nutrients to the soil and at the atmosphere and hence nutrient circulation in the ecosystem.

NB; in saprotrophic nutrition materials should first decay.

HOLOZOIC NUTRITION

The process involved in Holozoic nutrition is:-

1. Ingestion: The intake of food.
2. Digestion:  The breakdown of food into simple molecules
3. Absorption:  The uptake of those ample molecules into living cells.
4. Assimilation:   The use to which absorbed molecules are put.
5. Egestion: Expulsion of undigested food materials from the body.

DIGESTION

Digestion is the breakdown of large organic molecules into smaller simple soluble which can be absorbed.

There are two types of digestion.

a)Physical digestion

This involves mechanical breakdown of fossils by teeth and abdominal muscles.

b). Chemical digestion.

This involves activities of enzymes in the digestive canal

DIGESTION IN MAMMAL

Digestion in the mouth.

Mechanical breakdown of food begins in the mouth or buccal cavity.

Humans are omnivores and hence have an unspecialized diet of mixed animals and plants origin. The teeth reflect the lack of specialization. All types being present and developed to a similar extent.

Apart from assisting speech, the tongue also manipulate all the food during chewing and ensures it is well mixed saliva produced from three pairs of salivary gland/about 1.0 – 1.5dm³ saliva are produced daily.

SALIVA CONTAINS

1. Water – Over 99% of saliva in water.
2. Salivary amylase. A digestive enzyme which hydrolyses starch to maltose.
3. Mineral salt e.g. NaHCO₃ this helps to maintain a PH of around 6.5 – 7.5 which is the optimum of salivary amylase.
4. Mucin – a sticky material which helps to bind food particles together and lubricate them to assist swallowing the thoroughly chewed food is rolled into bolus and passed to the back of the mouth for swallowing. The process of digestion starts in the mouth (buccal cavity).

• After ingestion of food there is physical/ mechanical digestion
• Physical digestion begins as the process of food are bitten off by teeth and then chewed.

AIMS OF PHYSICAL DIGESTION.

Breaking down food into smaller pieces making it easier for swallowing and also increasing the surface area for the enzyme to act on. Chemical digestion is brought about by saliva.

ROLES OF SALIVA

1. It is slightly alkaline neutralized the effects of acidic foods or acids produced by the bacteria in the mouth.

2. Contains an enzyme salivary amylase which hydrolysis polysaccharide starch to maltose.

3. It contains mucus which moistens and lubricates make swallowing easier.

4. Defensive mechanism: It contains ant bacterial enzyme for killing bacterial for preventing the growth of bacteria.

DIGESTION IN THE STOMACH.

Stomach is roughly U shaped situated below the diaphragm. It is a muscular sac with a folded minor layer called the gastric mucosa. Embedded in this is a series of gastric pits which are lined with secretory cells. These produce

gastric juice which contains.

1. Water :

The bulk of secretories is water in which are dissolved other constituents.

2. Hydrochloric acid

This is produced by the oxyntic cells and with the water forms dilute solution giving gastric juice it PH of around 2.

It helps to kill bacteria brought in with food particles and activates the enzymes pepsinogen and prorenin.

It also initiates the hydrolysis of glucose and nucleic protein (nucleic acid)

3. Pepsinogen

This is produced by the zymogen chief cells in inactive form to prevent it from hydrolyzing the proteins and the cells producing it once in the stomach it activates the pepsin by hydrochloric acid. Pepsin is an endopepsidal which hydrolyses protein into polypeptides.

4. Prorennin

This not produced by zymogen cells and is an inactive form of renin an enzyme which coagulates milk converting it into the soluble caseinogens to insoluble casein. It is therefore especially important in young mammals.

• Prorenin is too activated by HCL

5. Mucus

This is produced by goblet cells and forms a protective layer on the stomach wall thus preventing pepsin and HCL from breaking down gastric mucosa. (Preventing autolysis). If protection is not effective and the gastric juice attacks the mucosa an ulcer results.

Mucus also helps to lubricate movement of food within the stomach.

During its stay in the stomach food is thoroughly churned and mixed with gastric juice by periodic contraction of the muscular stomach wall. In this way a creamy fluid called chyme and contraction of the stomach walls as the chyme to enter duodenum

The chyme from any one meal is released gradually over a period of 3-4 hours. This enables the small intestine to work on little material of time and provides a continuous supply of food and for absorption thoroughly the period between meals.

FUNCTIONS OF HCL

1. It has a PH of 1- 2.5 which makes the stomach content ideal for the optimum activity of the stomach enzyme.
2. It kills bacteria which are associated food defensive mechanism.
3. It denatures many protein their tertiary structure is altered making them unfold and easy to digest.
4. It converts

DIGESTION IN SMALL INTENSTINE

In humans the small intestine is over 6 m in length and its coils fill much of the abdominal cavity.

It consists of two main parts.

i) In humans the small duodenum where most digestion occurs and the pancreatic and bile ducts into about 25cm

ii) The longer ileum which is largely concerned with absorption.

The duodenum leads in the ileum which is 3m long in a living body.

The walls of the illeum are folded and possess finger like projections called villi, the villi contains fibres of smooth muscles and regularly contracts and relax. This helps to mix the food in it. The enzyme secretions and keep press supplies in contact which the villi for absorption.

The digestive juice which appears in the small intestine comes from three sources.

• Liver
• Pancreas
• Intestinal wall

THE LIVER

Produce bile juice which is complex green fluid.

It contains no enzyme but possesses two other substrates important in digestion.

1. Mineral salts.

They help to neutralize the acidic chyme from the stomach and secrete more neutral PH for the enzyme of the small intestine to work.

2. Bile salt: sodium and glycocholate and teurochorate.

They emulsify lipids, breaking them down into minute droplets. This is the physical not chemical change which provides a greater surface area for pancreatic lipase to work on.

The liver performs other functions some associated with digestion. These are

• Carbohydrate metabolism i.e. Glucose glycogen
• Lipid metabolism
• Protein metabolism
• Production of bile
• Detoxification
• Production of heat during its physiological activities.

2. Pancreatic Juice

The pancrease is situated below the stomach and is unusual in that it produces both on exocrine secretion, the pancreatic juice and endocrine secretion, the hormone insulin and glucagon. The endocrine is not concerted directly with digestion and pancreatic juice in addition to water, contains.

1. Mineral salt (NaHCO₃)

Helps to neutralize acidic enzyme from the stomach and so provide a more neutral PH in which the intestine enzymes can operate.

2. Protease

These include trypsinogen which when activated by enterokinase from the intestinal wall forms the endopeptidase called trypsin which hydrolyses proteins to peptides.

Trypsin also activates another protease in the secretion chymotrytripsnogen into chymotrypsin this is to convert proteins into peptides.

Also present in the exopeptidase called carboxypeptidase which convert peptides into smaller peptides and some amino acids.

3. Pancreatic amylase– Completes the hydrolysis of starch to maltose which began in the mouth.

4. Lipase – Breaks down fats into fatty acids and monoglycerides (Glycerol + one fatty acid) by hydrolysis.

5. Nuclease – Converts nucleic acids into their constituent nucleotides

Intestine Juice (Succus entericus)

The mucus and sodium hydrogen carbonate in intestinal juice are made by coiled Brunner’s glands whereas the enzymes are produced by the breakdown (lysis) of cells at the tips of the villi.

1. Mucus – Helps to lubricate the intestine walls and prevents autolysis.

2. Mineral salts (e.g. sodium hydrogen carbonate)

Produced by the Brunner’s glands in order to neutralize the acid chyme from the stomach and so provide a more suitable pH for the action of enzymes in the intestine.

3. Protease (erepsin)

These include the exopeptidase called aminopepeptidase, which converts peptides into smaller peptides and amino acids, and dipeptidase, which activates the trypsinogen produced by the pancrease.

4. Enterokinase – A non- digestive enzyme which activates the trpsinogen produced by the pancrease.

5. Nucleotidase – Converts peptides into pentose sugars, phosphoric acid and organic bases.

6.Carbohydrates- These include amylase, which helps complete the hydrolysis of starch to maltose; maltase, which hydrolyses maltose to glucose; lactase, which hydrolyses the milk sugar lactose into glucose and galactose; and sucrase, which hydrolyses sucrose into glucose and fructose.

HORMONAL CONTROL OF SECRETION OF THE DIGESTIVE JUICE

The production of digestive secretion must be timed to coincide with the presence of food in the appropriate region of the gut. The secretion of such juices in mammals are under both nervous and hormonal control. The sight, smell or even thought of food may cause salivary glands to secrete saliva. This is a conditional reflex response.

Hormonal control of secretions begins with the presence of food in stomach. This stimulates the stomach to secrete hormone called gastrin which passes through blood stream to stimulate the production of gastric juice.

–        When food leaves the stomach and enters the duodenum it stimulates the production of two hormones from duodenal walls. These are

Secretin – This pass through blood stream to the liver where it stimulate the production of bile and to the pancreases where it stimulate the secretion of the mineral salts.

Cholecystokinin- pancreozymin– This causes the gall bladder to contract (releasing the bile juice into the duodenum) and stimulate the pancreases to secrete its enzymes