National 5 Biology | Unit 3 | Key Area 3: Photosynthesis

Where does photosynthesis only occur in?

Green plants, because they will contain the green pigment chlorophyll, which is used in photosynthesis.

State the equation for photosynthesis.

carbon dioxide + water → sugar + oxygen

Photosynthesis occurs in two stages. Name these stages.

Light reactions, then carbon fixation

How is sunlight (light energy) trapped in a plant?

Chlorophyll in chloroplasts trap light energy and convert it into chemical energy. ATP is produced in this way.

Describe how the first stage of photosynthesis (light reactions) is carried out by a plant.

When chlorophyll is struck by light:

- A series of events is initiated which result in the production of molecules of ATP (from ADP and phosphate).

- Water molecules are split into hydrogen and oxygen (photolysis of water).

In the first stage of photosynthesis (light reactions), name and describe what happens to the three products produced.

- ATP produced is passed to the second stage. It provides chemical energy for the reactions of carbon fixation.

- Hydrogen broken down from water molecules combine with hydrogen acceptor molecules, which carries it to carbon fixation.

- Oxygen is released as a byproduct and is allowed to diffuse out the plant.

Describe how the second stage of photosynthesis (carbon fixation) is carried out by a plant.

- Carbon fixation occurs as a cycle of enzyme-controlled reactions in a different part of the chloroplasts.

- ATP provides chemical energy to join hydrogen to carbon dioxide to produce sugar molecules.

Carbon is absorbed in the carbon fixation stage of photosynthesis. Where is this taken from, and what does it do?

Carbon is absorbed in the form of carbon dioxide from the surrounding air (or water for aquatic plants). Carbon is used to build up larger organic molecules.

The sugars made in photosynthesis can be converted into what?

Glucose (type of sugar), carbohydrate starch, cellulose, proteins, fats

Describe the uses of the sugar produced in photosynthesis.

- Breakdown of glucose by respiration releases chemical energy. Source of energy used to power the metabolism of the cell.

- Plants store carbohydrates by converting sugar into the carbohydrate starch.

- Cellulose is used to make cell walls in plant cells.

- Sugar may enter metabolic pathways that convert it into proteins or fats.

In experiments, how is a leaf prepared for a starch test using iodine solution?

- The leaf is boiled in water to remove the waterproof cuticle and break open membranes.

- The leaf is then boiled in alcohol (ethanol) to remove chlorophyll as it would mask the colour of iodine solution.

Name the four main limiting factors of photosynthesis.

Light intensity, temperature, carbon dioxide concentration, water availability

Limiting factor

The factor in shortest supply, therefore preventing a process from being as efficient as it could be. eg. low temperature will limit the rate of photosynthesis.

How do you determine which factor is the limiting factor?

At any given moment, a plant will have its overall rate of photosynthesis limited by the factor in shortest supply. This is the one that is rate-limiting.