1.4.1Cell Level Systems

Photosynthetic Organisms as Producers — OCR Gateway GCSE Combined Science A J250 - Biology Revision Notes & Topical Questions

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📚 OCR Gateway GCSE Combined Science A J250 - Biology Hub / Topical Revision

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Part 1 of 7Photosynthesis and producers

Photosynthesis is the process by which plants, algae and some bacteria use light energy to make glucose from carbon dioxide and water, releasing oxygen as a by-product.

Producers

Because they can make their own organic molecules, these organisms are called producers and begin almost every food chain.

All the energy and biomass in an ecosystem begin with them.

Endothermic

Photosynthesis is an endothermic reaction,

as energy is taken in from the surroundings: light energy is transferred to chemical energy stored in glucose.

Word equationcarbon dioxide + water → glucose + oxygen (in the presence of light and chlorophyll)

Symbol equation6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂

Part 2 of 7Where photosynthesis happens

Photosynthesis takes place in the chloroplasts, which contain the green pigment chlorophyll. Chlorophyll absorbs light, mainly red and blue wavelengths, and reflects green, which is why leaves look green. Chloroplasts are most abundant in the palisade cells near the upper surface of the leaf, where the light is strongest.

A leaf is adapted for photosynthesisit is broad and thin, giving a large surface area to absorb light, and a short diffusion distance for gases.

Where photosynthesis happens

The upper epidermis is transparent to let light through.

Air spaces in the spongy layer allow carbon dioxide to diffuse to the cells,

and stomata let gases in and out and are opened and closed by guard cells.
- (pores, mainly on the underside)

Veins contain xylem, which brings water, and phloem, which removes sugars.

Part 3 of 7The raw materials

Carbon dioxide

Carbon dioxide enters through the stomata by diffusion from the air.

Water

Water is absorbed from the soil by root hairs through osmosis and carried up the xylem to the leaves.

Light energy

Light energy comes from the Sun, and is trapped by chlorophyll.

The oxygen released diffuses out of the leaf through the stomata and is used by organisms for respiration.

Part 4 of 7Uses of glucose in the plant

Use	What happens
Respiration	Glucose is broken down to release energy for the plant's processes
Making cellulose	Glucose is built into cellulose for cell walls
Storage as starch	Glucose is converted to insoluble starch in leaves, roots, stems and seeds, which can be converted back to glucose when needed
Making fats and oils	Glucose is converted to lipids for storage in seeds, as in sunflower seeds
Making proteins	Glucose combines with nitrate ions absorbed from the soil to make amino acids, and then proteins

Sugars are carried in the phloem to wherever they are needed, such as growing tips and storage organs. Plants therefore need minerals as well as carbon dioxide and water: nitrate for protein and magnesium for chlorophyll. A lack of magnesium leads to yellow leaves (chlorosis).

Part 5 of 7Testing a leaf for starch

The presence of starch in a leaf shows that photosynthesis has taken place.

Testing a leaf for starch

The method is: (1) dip the leaf in boiling water for about a minute to kill it and stop reactions,

and soften the leaf; (2) heat it in hot ethanol in a water bath (never over a flame,
as ethanol is flammable) to remove the chlorophyll; (3) rinse the leaf in warm water to soften it again;
and (4) spread it on a white tile and add iodine solution.

Areas containing starch turn blue-black.

In a variegated leaf only the green parts turn blue-black,

and a leaf that was kept in the dark or covered with foil is negative, showing that chlorophyll and light are needed.

Part 6 of 7Importance of producers

Because they transfer light energy into a form usable by other organisms, producers support all the consumers in the ecosystem.

Importance of producers

Photosynthesis also removes carbon dioxide from the atmosphere and gives out the oxygen that most organisms need,

and the plants and algae of the oceans do about half of this globally.

Fossil fuels are the remains of ancient producers.

The loss of forests and algae therefore reduces food, oxygen production and the ability to absorb carbon dioxide.

Producers in different ecosystems

On land the main producers are plants such as grasses and trees, whereas in lakes and oceans they are algae and phytoplankton, microscopic organisms that support the whole marine food web.

Importance of producers

In a food chain such as phytoplankton, krill, penguin, seal, the energy that reaches the seal started as sunlight absorbed by the phytoplankton.

Because only a small part of the light is converted and energy is lost at each trophic level, there must be far more producer biomass than top predators.

This is why the producers form the wide base of a pyramid of biomass.

Part 7 of 7Exam-style questions

Write the word equation for photosynthesis and state where in a leaf cell it takes place. [3]3 marks
A plant leaf that was partly covered with black paper for 24 hours is tested for starch. Describe and explain the results. [4]4 marks

Key terms

Term	Meaning
Photosynthesis	The process in which producers use light energy to make glucose from carbon dioxide and water.
Producer	An organism that makes its own food.
Chlorophyll	The green pigment that absorbs light for photosynthesis.
Chloroplast	The organelle that contains chlorophyll.
Stomata	Pores in the leaf surface for gas exchange.
Endothermic	A reaction that takes in energy from the surroundings.
Xylem	Tissue that carries water and minerals up the plant.
Phloem	Tissue that carries sugars around the plant.

Avoid these mistakes

Saying plants take in oxygen and give out carbon dioxide in photosynthesis. That is respiration.

Saying chlorophyll makes glucose. It absorbs the light energy.

Heating ethanol directly with a flame in the starch test.

Thinking plants get their mass from the soil. Most of their dry mass comes from carbon dioxide and water.

Last updated: 20 October 2026