Nutrition in plants

Plants obtain their food by the process of photosynthesis, which is undoubtedly the most important chemical reaction on Earth. It produces the oxygen we breathe and the sugars that we eat, by simply combining carbon dioxide and water together with light energy. Without photosynthesis, our planet would be a much more challenging place to live.

Photosynthesis

Plants obtain their food by photosynthesis, which converts light energy into chemical energy. They convert carbon dioxide and water into glucose and oxygen. The glucose is used by the plant to release energy during respiration and oxygen is removed through the stomata and forms part of the air we breathe in. Photosynthesis takes place inside organelles called chloroplasts in the leaf which contain the colourful pigment chlorophyll. Chlorophyll reflects green wavelengths of light, which is why leaves appear green.

Mineral ions

Aside from glucose, plants also need to obtain certain mineral ions for growth. Plants absorb minerals such as magnesium and nitrates from the soil. Magnesium is important because it is used by the plant in the production of chlorophyll, whereas nitrates are needed for the production of amino acids which are used to build proteins for growth. A lack of magnesium will cause the leaves of the plant to turn yellow whereas a lack of nitrate will result in stunted growth.

Factors which affect photosynthesis

Many factors affect the rate of photosynthesis, including the availability of carbon dioxide, water and light. Factors such as temperature and pH will also influence photosynthesis, since these factors can alter enzyme activity.

Investigating photosynthesis by testing for starch

You can investigate whether photosynthesis has occurred in a leaf by testing the leaf for starch using iodine solution. The glucose molecules produced by photosynthesis are joined together into a long carbohydrate chain, called starch. If starch is present in the leaf, the iodine solution will change colour from brown to blue/black. In order to carry out the iodine test on a leaf you will need to perform the following steps:

1. Place the leaf in boiling water to stop any chemical reactions.

2. Place the leaf in boiling ethanol to remove the chlorophyll (and make the blue/black colour change easier to see).

3. Place leaf on a white tile and add a few drops of iodine solution.

4. A blue/black colour change indicates the presence of starch, therefore photosynthesis has occurred.

To investigate whether light, chlorophyll and carbon dioxide are required for photosynthesis, the following experiments can be performed:

Light: place a strip of aluminium foil over a leaf of a plant. After a period of 24 hours, test the leaf for starch. Iodine solution will turn blue/black only in the regions which were exposed to light.

Chlorophyll: use a variegated leaf, which has both green areas (containing chlorophyll) and non-green areas (containing no chlorophyll). Only the green parts of the leaf will test positive for starch.

Carbon dioxide: place a transparent plastic bag over the plant and place a substance such as calcium oxide (quicklime) inside. The calcium oxide reacts with and removes all of the carbon dioxide from the air. After 24 hours, test a leaf for starch. Less starch should be present in these leaves due to less photosynthesis taking place.

Investigating rates of photosynthesis and respiration

We can determine the relative rates of photosynthesis and respiration by testing for carbon dioxide using hydrogencarbonate indicator.

• Hydrogencarbonate turns purple when carbon dioxide levels are low, indicating that more carbon dioxide is being absorbed by the plant than released, which means more photosynthesis is taking place than respiration.

• Hydrogencarbonate turns yellow when carbon dioxide levels are high, which tells us that more carbon dioxide is being released from respiration than is being used in photosynthesis.

• Hydrogencarbonate stays red if the rate of photosynthesis and respiration is equal. Just as much carbon dioxide is absorbed by the leaf for photosynthesis as is released from respiration, so the overall carbon dioxide concentration remains the same.

Leaf adaptations

Leaves are adapted to carry out photosynthesis:

• The epidermis is thin and transparent to allow more light to reach the palisade cells

• The waxy cuticle prevents water loss

• The palisade layer contains lots of chloroplasts and is at the top of the leaf to maximise light absorption

• Spongy layer contains air spaces to allow carbon dioxide to diffuse through the leaf

• Fewer stomata at the top of the leaf to reduce water loss by transpiration.

The overall shape of a leaf also facilitates photosynthesis. For instance, leaves tend to be thin (so short diffusion distance for carbon dioxide) and have a large surface area to volume ratio (for maximum light absorption).

Stomata

The stomata control gas exchange in the leaf. Each stoma can be open or closed depending on how turgid its guard cells are. Stomata open and close to:

• regulate transpiration

• allow gas exchange

Xylem and phloem

Xylem and phloem form continuous tubes and are like blood vessels for the plant. Xylem transports water and mineral ions from the roots to other parts of the plant. Phloem transports sugars and amino acids from the leaves to the rest of the plant. The movement of sugars around the plant in the phloem is called translocation. Water is moved by a process called transpiration.

Transpiration

Water moves into root hair cells by osmosis down a water potential gradient and is transported to different parts of the plants in the xylem. Water near the surface of the leaf evaporates, becoming water vapour and exiting the leaf through the stomata. When this happens water is drawn up from the xylem to replace the water lost from the leaves. This is known as a '‘transpiration stream’ or ‘transpiration pull’.

Factors which affect the rate of transpiration:

• Humidity: humid conditions means the air is full of water vapour, which reduces the concentration gradient of water vapour between the inside and the outside of the leaf. This reduces the rate of transpiration as water vapour diffuses out of stomata more slowly.

• Wind speed: the faster the wind speed, the faster the rate of transpiration since windy conditions will move any water molecules hanging around outside of the leaf. This increases the concentration gradient so increases the rate of transpiration.

• Temperature: higher temperature increases the rate of transpiration as the water molecules have more kinetic energy so move faster out of the stomata. Higher temperatures also increase evaporation of water from a liquid to gaseous state.

• Light intensity: higher light intensity causes the stomata to be open for longer time periods, therefore the amount of transpiration will increase.

Download worksheet: Photosynthesis / Answers