Biology · Lesson 6

Photosynthesis

JAMBWAECNECO

What the syllabus asks

JAMB: Nutrition: modes of nutrition, photosynthesis, chemosynthesis, mineral requirements of plants

WAEC: Nutrition in plants: autotrophic nutrition, photosynthesis (raw materials, conditions, light and dark stages, products), structure of the leaf in relation to photosynthesis, experiments on photosynthesis, factors affecting the rate, mineral nutrition (macro- and micro-elements, deficiency symptoms)

NECO: Nutrition in plants: autotrophic nutrition, photosynthesis (raw materials, conditions, light and dark stages, products), structure of the leaf in relation to photosynthesis, experiments on photosynthesis, factors affecting the rate, mineral nutrition (macro- and micro-elements, deficiency symptoms)

What you will learn

Distinguish autotrophic from heterotrophic nutrition.
State the equation, raw materials, conditions and products of photosynthesis.
Describe the light and dark stages and where they occur.
Relate the structure of the leaf to its function in photosynthesis.
Describe experiments to show the need for light, CO₂ and chlorophyll and the production of oxygen.
Explain limiting factors and interpret rate graphs.
State the functions of key mineral elements and their deficiency symptoms.

Key points

Autotrophs make their own food; heterotrophs take in ready-made food.
6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂ with light and chlorophyll as conditions.
Light stage (grana): photolysis of water, O₂ released, ATP and NADPH formed.
Dark stage (stroma): CO₂ fixed and reduced to glucose using ATP and NADPH.
The oxygen released comes from water.
Glucose is used for respiration, stored as starch, transported as sucrose, or built into cellulose, proteins and oils.
Leaf adaptations: broad, thin, palisade chloroplasts, air spaces, stomata, veins.
Starch test: boil, ethanol (water bath), warm water, iodine; blue-black means starch. Always destarch first.
Rate is set by the limiting factor: light, CO₂ or temperature.
N, P, K, Mg, Ca, S are macro-elements; Mg deficiency causes interveinal chlorosis.

Start here: How living things get food

All living things need food for energy and for building their bodies. There are two broad ways of getting it.

Autotrophic nutrition: the organism makes its own organic food from simple inorganic substances (CO₂, water, minerals).
Heterotrophic nutrition: the organism takes in ready-made organic food from other organisms.

A mango tree in your compound is an autotroph. It takes carbon dioxide from the air and water from the soil and builds sugar, starch, oils and proteins. A goat browsing on the mango leaves is a heterotroph; so are you when you eat the mango.

Two kinds of autotrophs

Photoautotrophs use light energy. This is photosynthesis: green plants, algae such as Spirogyra, and some bacteria.
Chemoautotrophs use energy released from chemical reactions. This is chemosynthesis: for example nitrifying bacteria (Nitrosomonas, Nitrobacter) in the soil, which oxidise ammonia and nitrites.

Heterotrophs include holozoic feeders (animals that ingest and digest food), saprophytes (fungi such as mushrooms and Rhizopus that feed on dead matter), and parasites (such as Loranthus on trees, tapeworm in humans). Some plants, such as the pitcher plant and sundew, are insectivorous: they photosynthesise but trap insects to obtain extra nitrogen.

Worked example 6.1

Classify: (a) maize plant (b) bread mould (c) Nitrosomonas (d) cow.

Maize: photoautotroph (green, uses light).
Bread mould: heterotroph, saprophytic.
Nitrosomonas: chemoautotroph.
Cow: heterotroph, holozoic (herbivore).

See steps

⚠️ Common mistake: calling mushrooms plants that photosynthesise. Fungi have no chlorophyll; they are saprophytes.

Try it yourself. Why are insectivorous plants still called autotrophs?

Show solution

They make their own carbohydrate by photosynthesis; insects only supply extra nitrogen.

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Practice questions

1. The oxygen released during photosynthesis comes from

water
carbon dioxide
glucose
chlorophyll

Show answer

A: water
Photolysis of water in the light stage releases O₂.

2. The reagent used to test for starch is

Benedict's solution
Millon's reagent
Sudan III
iodine solution

Show answer

D: iodine solution
Iodine turns blue-black with starch. Benedict's = reducing sugar; Millon's = protein; Sudan III = fats.

3. The dark stage of photosynthesis takes place in the

stomata
mitochondria
stroma
grana

Show answer

C: stroma
The light stage is in the grana; CO₂ fixation (dark stage) is in the stroma.

4. Yellowing of leaves (chlorosis) is commonly caused by deficiency of

iodine
magnesium
calcium
sodium

Show answer

B: magnesium
Magnesium is part of the chlorophyll molecule (also nitrogen deficiency).

5. Which of these is NOT a raw material for photosynthesis?

Oxygen
Carbon(IV) oxide
Water
Light energy

Show answer

A: Oxygen
Oxygen is a product, not a raw material.

Key words

Autotroph: An organism that makes its own food from simple inorganic substances.
Heterotroph: An organism that obtains ready-made food from others.
Photosynthesis: The making of carbohydrates from CO₂ and water using light energy absorbed by chlorophyll.
Chlorophyll: Green pigment that absorbs light energy, mainly red and blue light.
Chloroplast: Organelle in which photosynthesis takes place.
Grana: Stacks of thylakoid membranes where the light stage occurs.
Stroma: Fluid around the grana where the dark stage occurs.
Photolysis: Splitting of water by light energy.
Stomata: Pores, mainly on the lower leaf surface, for gas exchange.
Limiting factor: The factor in shortest supply that controls the rate of a process.
Destarching: Removing starch from a plant by keeping it in darkness for 24 to 48 hours.
Chlorosis: Yellowing of leaves due to lack of chlorophyll.

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