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GROWTH
Seed germination
Seed germination is the process by which plant grows from the seed.
Parts of the seed
The seed consists of the following parts:
1. An embryo: this part consists of a radicle that develops into a shoot after germination
2. A seed coat(testa): it is a protective layer surrounding a seed
3. Cotyledons: this part grows to form the first embryonic leaves after germination. It also stores and provides food for young plants.
4. Micropyle: This is a pore through which water enters the seed during germination
5. Hilum: This is a scar that shows the seed’s point of attachment to the ovary wall.
Changes which occur during seed germination
During germination, physical and chemical changes occur in a seed. The physical changes include
1. Swelling of the seed after absorption of water through the micropyle.
2. The water absorbed by the seed softens the seed coat and activates enzymes in the cotyledons.
3. This process is followed by the bursting of the seed coat and the emergence of the radicle and plumule.
4. The radicle is the one that forms a primary root and the plumule gives rise to a shoot.
5. The radicle continues to elongate and give rise to many side roots.
Conditions necessary for seed germination
For a seed to germinate, it requires certain conditions. These conditions include the following:
Water
Germination occurs when a seed is supplied with water. A seed absorbs water through the micropyle. Water plays an important role during seed germination. When a seed absorbs water, the embryo cells enlarge due to pressure.
Such action leads to the rapture of the seed coat. Water softens the seed coat, hence increases seed permeability. Water also activates enzymes necessary for seed germination and growth.
Oxygen gas
Oxygen gas is necessary for aerobic respiration to generate energy that is required for the growth of the embryo of a seed.
Temperature
Seeds need the optimum temperature to germinate. The seed of each plant species has its optimum temperature for germination. Temperature is essential for the activation of enzymatic reactions during respiration.
Enzymes and hormones
The viability of seeds depends on the presence of the relevant enzymes and hormones in the required amounts. These help to break seed dormancy and catalyze all metabolic activities associated with the process of germination
Types of seed germination
There are two types of seed germination
- Epigeal germination
- Hypogeal germination
Epigeal germination
This is the type of germination in which the cotyledons of the seed are pushed above the ground.
This type of germination is seen in many dicotyledonous plants, such as pea, beans, sunflower, castor, and bean.
Hypogeal germination
Hypogeal germination is a type of germination in which the cotyledons remain underground. It occurs in plants such as maize, pigeon peas, wheat, etc.
TRANSPORTATION OF MATERIALS IN LIVING THINGS
Osmosis
This is the movement of water molecules from a region of higher concentration to a region of lower concentration through a semi-permeable membrane.
A partially-permeable membrane is a membrane that allows small particles such as water molecules to pass through it, but not larger particles such as sugar molecules and ions from salts. Examples of semi-permeable membranes are cell membranes and a pig’s bladder.
For osmosis to take place there must be:
1. Two solutions with different concentrations; and
2. A partially permeable membrane to separate them.
Aim of the experiment:
To demonstrate the process of osmosis using potato cubes
Materials: Three raw potatoes, sugar solution, water and beakers or dishes
Procedures
1. Boil one of the potatoes (boiling kills the cells).
2. Cut each potato in half and make a shallow hole in each half
3. Place each potato half in a beaker or dish that has water. The water should not cover the potato.
4. Put some sugar solution in the shallow hole in the one raw potato and the boiled potato. Leave the other potato empty. Label the set-ups (a),(b) and (c)
5. Mark the level of water in each beaker or dishes
6. Leave the setups undisturbed overnight
Observation
Effects of osmosis in living organisms
Osmosis and animal cells
1. When animal cell is placed in hypotonic solution, it absorbs water. If it remains in the solution for a long time, it absorbs excess water and swell and finally bursts (haemolysis) due to the absence of cell wall that exert pressure of the excess water
2. When animal cell is placed in hypertonic solution, it loses water. If it remains in the solution for a long time, it will lose more water and finally shrink and shrive (crenation).
Osmosis is important for the reabsorption of water in the colon and the kidneys. This helps to maintain the body’s water balance.
Osmosis in plant cell
1. In an isotonic solution, plant cells neither lose nor gain water.
2. In a hypotonic solution, plant cells absorb water, causing the cell membrane to push against the cell wall. The cell is then said to be turgid. It does not burst because the cell membrane exerts pressure on the cell wall which restricts additional intake of water. Turgidity helps plants to maintain their shape.
3. In a hypertonic solution, plant cells lose water. This causes the vacuole to shrink and the surface of the cell membrane to pull away from the cell wall, making the cell flaccid. Such a cell is said to be plasmolyzed and the process is called plasmolysis
If a plasmolyzed cell is placed in a hypotonic solution, it absorbs water and becomes turgid
Osmosis is important for the absorption of water by plant roots. Opening and closing of stomata also depend on osmosis. When guard cells absorb water the stomata open and when they lose water the stomata close.
Osmosis in unicellular organisms
Unicellular organisms that live in fresh water, for example amoeba and euglena, are hypertonic to their surrounding so water enters the organisms by osmosis.
These organisms have a contractile vacuole. The contractile vacuole collects the excess water and removes it from the cell. This prevents the cell from bursting.
Diffusion
Experiment: Demonstrating Diffusion using Potassium Permanganate Crystals
Materials:
1. Glass beaker or clear glass container
2. Potassium permanganate crystals
3. Water
4. Stirring rod or glass rod
5. Stopwatch or timer
6. Paper or cardboard
Procedure:
1. Fill the glass beaker or container with water, about two- thirds full.
2. Add a few potassium permanganate crystals to the water. The crystals will slowly dissolve, turning the water purple.
3. Use the stirring rod to gently stir the water and potassium permanganate mixture.
4. After stirring, observe the initial distribution of the purple color in the water.
Questions to Explore:
1. What happens when potassium permanganate crystals are added to water?
2. Why does the water turn purple when potassium permanganate is added?
3. Describe the initial distribution of the purple color in the water after stirring.
4. What do you think will happen to the purple color in the water over time? Why?
5. How does the purple color spread throughout the water? What process is responsible for this movement?
6. What do you notice about the movement of the purple color particles in the water? Is it uniform, or does it show any specific pattern?
7. How can you explain the process responsible for the spreading of the purple color in the water?
8. Does the movement of the purple color particles depend on the presence of an external force, like stirring? Why or why not?
9. What factors do you think can affect the rate of color spreading in the water? How can you design an experiment to test this?
10. Can you compare the diffusion process observed here to any natural processes you know of? How are they similar or different?
Factors affecting the rate of Diffusion
1. Concentration gradient; A huge difference between the concentration of the molecules in two regions leads to a faster rate of diffusion
2. Surface area to volume ration; the higher the ratio the greater the rate of diffusion
3. Temperature; increase in temperature increases the rate of diffusion and decrease in temperature decreases the rate of diffusion
4. Thickness of the membrane; thin membrane enhances the higher rate of diffusion than the thick membrane
5. Size of molecules; small and light molecules diffuses faster than large and heavy molecules
Some important processes that involve diffusion are;
1. Gaseous exchange in the lungs of animals and in the leaves of plants,
2. Absorption of digested food in the ileum,
3. Remove of water materials from cells
4. Absorption of nutrients and oxygen into cells.