IGCSE Biology 0610: Photosynthesis Word Equation and Uses of Glucose

A gardener is growing a pumpkin plant from seed, and its vine is now spreading across the ground with several developing pumpkin fruits.

(a) Complete the word equation for photosynthesis, as it takes place in the leaves of the pumpkin plant, by filling in the two raw materials and the two products in the correct spaces:

_______________ + _______________ → _______________ + _______________

(this reaction takes place in the presence of light and chlorophyll) [2]

(b) State where in the pumpkin plant's leaf cells chlorophyll is found, and describe its role in photosynthesis. [2]

(c) As the pumpkin plant grows, the glucose made by photosynthesis in its leaves is used in several different ways. Other than being used immediately in respiration to release energy, state two different fates of this glucose, and for each one explain briefly why it is useful to the growing, fruiting pumpkin plant. [4]

Part (a): The word equation

Photosynthesis takes the two raw materials carbon dioxide and water, and, using light energy and chlorophyll, turns them into the two products glucose and oxygen:

carbon dioxide + water → glucose + oxygen

Part (b): Where chlorophyll is found and its role

Chlorophyll is found inside the chloroplasts of the pumpkin plant’s leaf cells. It is a green pigment that absorbs light energy and transfers this energy into chemical energy. This chemical energy is what drives the reaction that builds simple raw materials (carbon dioxide and water) into carbohydrates such as glucose.

Part (c): Fates of the glucose made in the leaves

Once glucose is made in a leaf cell, the pumpkin plant can use it in several ways besides respiring it straight away for energy:

Starch: glucose molecules can be joined together and stored as starch, for example in the developing fruit or in the roots, giving the plant a compact reserve of energy it can call on later.
Cellulose: glucose can be built into cellulose, which is used to construct new cell walls as the vine’s stem and leaves continue to grow and lengthen across the ground.
Sucrose: glucose can be converted to sucrose, a sugar that is small and soluble enough to be transported in the phloem to the swelling pumpkin fruit, supplying it with the sugar it needs as it develops.
Nectar: glucose can also be used to make nectar in the plant’s flowers, which attracts insects. These insects pollinate the flowers, which is necessary for the pumpkin fruit to form in the first place.

Any two of these fates, each with its explanation, would earn full marks in part (c).

Final answers

(a) carbon dioxide + water → glucose + oxygen
(b) Chlorophyll is found in chloroplasts; it absorbs light energy and transfers it into chemical energy used to make carbohydrates.
(c) Any two of: starch (energy store), cellulose (cell walls), sucrose (transport to the fruit in the phloem), nectar (attracts pollinating insects).