9 min readPathway sheet1-mark MCQ, a 2 or 3-mark question on stomata or the events of photosynthesis, and sometimes a 5-mark answer

Life processes and photosynthesis

Life processes keep a body working. Green plants make their own food by photosynthesis, using sunlight, chlorophyll, carbon dioxide and water, and exchange gases through stomata.

Life processesNutrition, respiration, transportation and excretion

Photosynthesis6CO₂ + 12H₂O → C₆H₁₂O₆ + 6O₂ + 6H₂O (sunlight, chlorophyll)

StomataTiny pores in the leaf surface, each opened and closed by two guard cells

Test for starchIodine solution turns blue-black

What are life processes?

Life processes are the jobs a living thing must keep doing to stay alive and repair itself, even while it sleeps. The four main ones are nutrition (getting food), respiration (breaking down food to release energy), transportation (moving materials around the body) and excretion (removing wastes). A single-celled organism does all this through its surface. In a large, many-celled body, most cells are far from the surface, so simple diffusion is not enough and special organs do these jobs.

Photosynthesis: the three events

**Autotrophs** (green plants and some bacteria) make their own food from simple substances: carbon dioxide and water. This is **autotrophic nutrition**. Carbohydrate that is not used at once is stored as **starch**.

1
Chlorophyll in the chloroplasts of leaf cells absorbs light energy.
2
Light energy is changed to chemical energy, and water splits into hydrogen and oxygen.
Oxygen is given out
3
Carbon dioxide is reduced to carbohydrate (glucose).
Desert plants take in CO₂ at night, store it as an intermediate, and use it in the day
4★
Overall: 6CO₂ + 12H₂O → C₆H₁₂O₆ + 6O₂ + 6H₂O
Write sunlight and chlorophyll over the arrow

★ key step

Stomata: open and closed

Open stomaWater flows into the guard cells. They swell and curve apart, so the pore opens. CO₂ enters, O₂ leaves and water vapour escapes.

Closed stomaWater flows out of the guard cells. They shrink and come together, so the pore closes. The plant closes stomata when it does not need CO₂, to save water.

Most gas exchange for photosynthesis happens through stomata, but some also happens through the surface of stems, roots and leaves.

Activity: temporary mount of a leaf peel to show stomata

1
Take a fresh leaf of a plant such as Bryophyllum, lily or money plant. Tear it gently at a slant to pull off a thin, transparent peel from the lower surface.
2
Put the peel in a watch glass of water so it does not dry.
3
Add a drop of safranin stain for about a minute, then wash off the extra stain in water.
4
Place the peel on a clean slide, add a drop of glycerine, and lower a coverslip slowly with a needle so no air bubbles form.
5
Wipe off extra glycerine with blotting paper. Observe under low power, then high power.
6★
Observe: many stomata, each with two kidney-shaped guard cells around a pore; the guard cells contain chloroplasts.

★ key step

How to draw open and closed stomata

Draw two bean-shaped guard cells facing each other, with their inner walls curved apart.
Label the gap between them stomatal pore.
Draw a nucleus and small oval chloroplasts inside each guard cell.
Draw a few irregular epidermal cells around the guard cells.
Draw a second figure with the guard cells straighter and touching, with no gap: this is the closed stoma.
Write the titles: (a) Open stomatal pore, (b) Closed stomatal pore.

In real life

Every grain of rice, every dal and every vegetable on your plate began as sugar made by photosynthesis in a leaf. The neem, mango and tulsi plants around Indian homes also refill the oxygen we breathe.

Exam angle

2 marks: “What are stomata? Write two of their functions.” Tiny pores in the leaf epidermis, each bordered by two guard cells. Functions: exchange of gases (CO₂ in, O₂ out) and loss of water vapour in transpiration.

3 marks: “List the three events that take place during photosynthesis.” Absorption of light energy by chlorophyll; conversion of light energy to chemical energy and splitting of water into hydrogen and oxygen; reduction of carbon dioxide to carbohydrate.

5 marks: “Explain photosynthesis with a balanced equation. How do guard cells open and close the stomatal pore? Draw an open stoma.” Definition; equation with conditions; three events; starch storage. Guard cells swell when water enters (pore opens) and shrink when it leaves (pore closes). Labelled diagram.

Remember it

Photosynthesis needs CLAW: Chlorophyll, Light, Air (CO₂), Water.