Photosynthesis and Uses of Glucose

Photosynthesis is the process plants use to make their own food: they convert carbon dioxide and water into glucose, using energy from light.

This matters because plants are producers — they build the glucose that nearly every other organism eventually relies on for energy. A plant does not "eat"; it manufactures sugar inside its cells and uses or stores it.

Three things go into the reaction and two things come out:

Goes in (reactants)	Comes out (products)
Carbon dioxide (CO₂)	Glucose (C₆H₁₂O₆)
Water (H₂O)	Oxygen (O₂)
Light energy

The oxygen released is a waste product of the reaction — it diffuses out of the leaf. The glucose is the useful product that the plant keeps.

Photosynthesis: the process by which plants use light energy to convert carbon dioxide and water into glucose, releasing oxygen as a by-product.

Photosynthesis takes place in the chloroplasts — small structures found mainly in the cells of the leaf.

Chloroplasts contain a green pigment called chlorophyll. Chlorophyll absorbs light energy and makes that energy available to drive the reaction. The green colour of leaves comes directly from chlorophyll.

Most photosynthesis happens in the palisade mesophyll cells near the top of the leaf, because they are packed with chloroplasts and sit where light reaches first.

Because the reaction depends on light reaching chlorophyll, leaves are broad and thin — a large surface area to capture as much light as possible.

The reaction is summarised by a word equation. Light is written above the arrow because it supplies the energy but is not a chemical substance that gets used up into a product.

$\text{carbon dioxide}+\text{water}\stackrel{\text{light}}{\to }\text{glucose}+\text{oxygen}$

Read it as: carbon dioxide plus water react, using light energy, to produce glucose plus oxygen.

Key points to get right in an exam:

• The reactants (left of the arrow) are carbon dioxide and water — never glucose.
• The products (right of the arrow) are glucose and oxygen.
• Light sits on the arrow. It is a condition for the reaction, not a reactant you add to the list.

Learn the equation in this exact order: carbon dioxide + water → glucose + oxygen. Swapping reactants and products is one of the most common ways marks are lost on this topic.

For higher-tier answers you should be able to write the balanced symbol equation. The chemical formulae you must recognise are CO₂, H₂O, C₆H₁₂O₆ and O₂.

$6C{O}_2+6H_{2}O\to C_6{H}_{12}{O}_6+6O_2$

Worked example — confirming the equation is balanced. A balanced equation has the same number of each type of atom on both sides. Count atom-by-atom:

Atom	Left side	Right side
Carbon (C)	6 (from 6CO₂)	6 (from C₆H₁₂O₆)
Hydrogen (H)	12 (from 6H₂O, each has 2 H)	12 (from C₆H₁₂O₆)
Oxygen (O)	18 (12 from 6CO₂ + 6 from 6H₂O)	18 (6 from glucose + 12 from 6O₂)

Every atom balances: 6 C, 12 H and 18 O on each side, so the equation is correct. The large numbers in front (the 6s) are needed — without them the atoms would not balance.

Photosynthesis is an endothermic reaction: it takes in energy from the surroundings rather than releasing it.

The energy is transferred from the environment to the chloroplasts, usually by light. Without that energy input, the reaction does not happen — which is why plants do not photosynthesise in the dark.

This is the exact opposite of respiration, which is exothermic (it releases energy):

Feature	Photosynthesis	Respiration
Energy	Takes in (endothermic)	Releases (exothermic)
Energy source	Light, from the environment	Glucose
Glucose	Produced	Used up
Oxygen	Released	Used up

Endothermic: a reaction that transfers energy in from the surroundings. Photosynthesis is endothermic because energy is transferred from the environment to the chloroplasts by light.

A common slip is to call photosynthesis exothermic because it "makes energy-rich glucose." The reaction stores energy in glucose, but to do that it must first take energy in — so it is endothermic.

The glucose made in photosynthesis does not just sit in the leaf. The plant uses it in five ways. Some glucose is used straight away; the rest is converted into other substances, several of which are insoluble so they can be stored without affecting the cell's water balance.

Use of glucose	Why the plant needs it
Used in respiration	Releases energy for all of the plant's life processes
Converted to insoluble starch	A compact, insoluble store of glucose for later use
Used to make fats or oils	An energy store, especially concentrated in seeds
Used to make cellulose	Strengthens the cell walls, giving the plant structure
Used to make amino acids	Building blocks for proteins (also needs nitrate ions)

Worked example — why convert glucose to starch? Glucose is soluble in water. If a plant stored large amounts of glucose dissolved in its cells, water would be drawn into those cells by osmosis and the cell could be damaged. Starch is insoluble, so it can be stored in large amounts without pulling water in — making it the ideal storage molecule. When energy is needed, starch is broken back down into glucose.

Glucose contains only carbon, hydrogen and oxygen — but proteins also contain nitrogen. So glucose alone is not enough to build proteins.

To make amino acids (which join together to form proteins), the plant combines glucose with nitrate ions absorbed from the soil. The nitrate ions supply the nitrogen that glucose cannot.

Worked example — why a plant short of nitrate grows poorly. Proteins are needed for growth, for enzymes, and for repairing tissues. If the soil is low in nitrate ions, the plant cannot make enough amino acids, so it cannot make enough protein. The result is stunted growth and often yellowing older leaves — which is why farmers add nitrate-containing fertilisers to crops.

Glucose → amino acids needs nitrate ions from the soil. If an exam question asks what extra substance a plant needs to make protein, the answer is nitrate ions (a source of nitrogen).

(Extra context — not required by AQA 8461.) The nitrogen in nitrate ions originally enters the soil through the nitrogen cycle, including nitrogen-fixing bacteria, but you are not assessed on that mechanism here.

1. Writing the equation back to front

Reactants are carbon dioxide + water; products are glucose + oxygen. Writing glucose on the left, or putting oxygen with the reactants, loses marks. Memorise the direction.

2. Calling photosynthesis exothermic

Photosynthesis is endothermic — it transfers energy in from the environment to the chloroplasts by light. Respiration is the exothermic one. Do not confuse the two.

3. Listing light as a reactant

Light is an energy source, written on the arrow. It is not a chemical reactant, so it does not belong in the list of substances that combine.

4. Forgetting nitrate ions for protein synthesis

Glucose supplies carbon, hydrogen and oxygen, but proteins need nitrogen. The plant must also absorb nitrate ions from the soil to make amino acids and therefore proteins.

5. Saying glucose is stored directly

Glucose is soluble, so it is converted to insoluble starch for storage, not stored as glucose. Mentioning starch (and that it is insoluble) is what gains the mark.

6. Confusing where it happens

Photosynthesis occurs in the chloroplasts, using the pigment chlorophyll to absorb light — not in the mitochondria (that is respiration).