Photosynthesis: Answers

What mineral ions are important for the normal functioning of a plant?

Magnesium and nitrates. Magnesium is needed to make the green pigment chlorophyll inside chloroplasts. Without enough magnesium, plants cannot synthesise chlorophyll so won’t be able to carry out photosynthesis. Their leaves will turn yellow and start to die. Nitrates are needed to make amino acids which are used to build proteins. Proteins carry out important functions in cells (e.g. enzymes are proteins) and are used for growth and repair. Without nitrates, plants will fail to grow properly.

Write the word and balanced symbol equations for photosynthesis.

Water + carbon dioxide —> glucose + oxygen

Explain how the structure of a leaf is adapted for carrying out photosynthesis.

The overall shape of a leaf is large and thin. This means there is a large surface area for the absorption of light and a short diffusion distance for carbon dioxide.

The upper epidermis of a leaf is transparent to allow as much light as possible to reach the palisade cells underneath. The palisade cells are close to the top of the leaf to capture light and are packed full of chloroplasts for maximum photosynthesis. The spongy mesophyll later contains air spaces for the diffusion of carbon dioxide. Guard cells open and close stomata to regulate gas exchange and transpiration.

Explain how the rate of photosynthesis is affected by changes to abiotic factors throughout the day.

Abiotic means non-living. Non-living factors include things like temperature, soil pH, the availability of water and carbon dioxide.

Throughout the day, temperature tends to increase around midday and drop during the night. This means the rate of photosynthesis will be highest around midday because the enzymes which control the rate of photosynthesis are moving and working faster. Light intensity is greatest during the daytime hours and very low during the night. Stomata are only open when light is available, so photosynthesis will be greatest during the daytime hours because carbon dioxide can enter the leaf through the stomata.

Explain how very high temperatures might reduce the growth of plants.

Photosynthesis is an enzyme-controlled process. Enzymes are denatured and stop functioning at very high temperatures so the rate of photosynthesis would decrease, reducing plant growth.

High temperatures also increase the rate of transpiration (water loss through the stomata). If too much water is lost, the stomata close to prevent further water loss from the plant. This means less carbon dioxide can enter the leaf and less photosynthesis will be taking place.

How does the number of stomata differ at the top and bottom of a leaf?

There are much more stomata on the bottom of a leaf compared to the top. This minimises the amount of water lost from the leaf through transpiration. The underside of the leaf is in the shade and the cooler temperatures mean there is less evaporation of water on the bottom of the leaf compared to the top.

Give the name of the vessel which transports water.

Xylem is the vessel which transports water from the roots to the rest of the plant.

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