Photosynthesis

Plants use energy from light to make carbohydrates from carbon dioxide and water. This process is called photosynthesis. Glucose is a carbohydrate produced by photosynthesis; oxygen is also produced.

The word equation for photosynthesis is:

Photosynthesis takes place in structures in plant cells called chloroplasts. Chloroplasts contain chlorophyll, a green pigment that absorbs light.

Chlorophyll transfers energy from light into chemical energy. The plant uses this energy to synthesise carbohydrates from carbon dioxide and water.

Uses of photosynthetic carbohydrates

Photosynthesis makes carbohydrates that a plant can use immediately, store, transport or turn into other substances. Glucose is one of the carbohydrates available for these uses.

Energy and storage

Cells use glucose in respiration to release energy for their activities. When glucose is not needed immediately, the plant can convert it into starch for energy storage. The stored carbohydrate can later be used in respiration.

Building, transport and pollination

The plant uses carbohydrate to make cellulose, which forms its cell walls. It also converts carbohydrate into sucrose, a sugar transported in the phloem to other parts of the plant.

Some sugars are present in nectar in flowers. Nectar attracts insects, which may carry pollen between flowers and so pollinate them.

Nitrate and magnesium ions in plant nutrition

Plants absorb mineral ions dissolved in soil water through their roots. Nitrate and magnesium ions supply different materials that plants need to grow and produce carbohydrates.

Nitrate ions and proteins

Plants use nitrate ions ($$\ce{NO3-}$$) to make amino acids. Amino acids are joined together to build proteins, which are needed to build and maintain plant cells. A plant therefore needs nitrate ions to make the proteins required for growth.

Magnesium ions and chlorophyll

Plants use magnesium ions ($$\ce{Mg^2+}$$) to make chlorophyll. Chlorophyll is the pigment that absorbs light energy, allowing the plant to use that energy in photosynthesis to produce carbohydrates.

Nitrate and magnesium ions have different roles: nitrate is needed to make amino acids and proteins, while magnesium is needed to make chlorophyll. Neither ion can replace the other.

Testing the requirements for photosynthesis

To test whether a plant needs a particular condition for photosynthesis, remove that condition while keeping the other requirements available. Compare the result with a control, in which all the requirements are available. If starch is found in the control but not where the condition was absent, the result supports the conclusion that the condition is needed.

Plants can convert carbohydrate made during photosynthesis into starch. A starch test therefore provides evidence that photosynthetic carbohydrate has been made. Before an investigation, plants are usually kept in darkness long enough to use up stored starch. This is called destarching and makes it less likely that starch found afterwards was already present before the test.

Testing a leaf for starch

After the plant has been exposed to the test conditions, remove a leaf and:

1. Heat it in hot water, then heat it in ethanol using a hot water bath. The ethanol removes chlorophyll so the test colour can be seen.
2. Rinse the leaf in water to soften it, then spread it out and add iodine solution.
3. Look for a blue-black colour, which indicates starch. A yellow-brown colour indicates that starch has not been detected.

Comparing the requirements

Light: Cover part of a destarched leaf with opaque material and leave the rest exposed to light. Keep the plant supplied with carbon dioxide. After a suitable period, test the leaf for starch. The exposed part acts as the control: it should turn blue-black, while the covered part should remain yellow-brown.

Chlorophyll: Expose a destarched leaf with both green and non-green areas to light, with carbon dioxide available. Record which areas are green before testing the leaf, because the ethanol removes the colour. The green areas contain chlorophyll and act as the comparison. If they turn blue-black while the non-green areas remain yellow-brown, the result supports the need for chlorophyll.

Carbon dioxide: Place one destarched plant in conditions where carbon dioxide is available and a comparable plant in an enclosure from which carbon dioxide is removed. Give both plants light and ensure they have chlorophyll. After a suitable period, test a leaf from each plant. Starch in the plant supplied with carbon dioxide, but not in the plant without it, supports the need for carbon dioxide.

In each comparison, other conditions should be kept as similar as possible. For example, the plants or leaf areas should receive comparable treatment and be tested after the same length of time. This makes the requirement being tested the relevant difference between the results.

Factors affecting the rate of photosynthesis

The rate of photosynthesis is how quickly a plant carries out photosynthesis. It depends on conditions including light intensity, carbon dioxide concentration and temperature. A factor that restricts the rate under the current conditions is called a limiting factor.

Light intensity and carbon dioxide concentration

Photosynthesis requires light and carbon dioxide. If light intensity is limiting, increasing it raises the rate of photosynthesis. If carbon dioxide concentration is limiting, increasing it raises the rate.

The rate does not keep rising indefinitely as either factor increases. Eventually, another factor becomes limiting, so increasing the first factor further has little or no effect. For example, increasing light intensity may initially increase the rate, but the rate may then level off because carbon dioxide is limiting. Increasing the carbon dioxide concentration could allow the rate to rise again.

Temperature and photosynthesis

Photosynthesis involves enzyme-controlled reactions. As temperature rises, these reactions generally become faster, so the rate of photosynthesis increases towards an optimum temperature.

Above the optimum, the rate falls. High temperatures can cause enzymes to lose their working shape, so they can no longer carry out their reactions effectively.

Investigating a factor affecting photosynthesis

To investigate one factor, change that factor and measure the rate in the same way at each value. Keep other relevant conditions controlled so that they do not cause the observed change.

For example, to investigate light intensity, a water plant can be exposed to different light intensities while its temperature and carbon dioxide supply are kept constant. The volume of oxygen produced in a fixed time can be used as an indicator of the photosynthetic rate. Comparing the measurements shows how the rate changes as light intensity changes.

Light–dark gas exchange in aquatic plants

An aquatic plant releases carbon dioxide into the surrounding water when it respires. Respiration continues in both light and darkness. In light, the plant also takes in carbon dioxide for photosynthesis. The observed change in carbon dioxide depends on the balance between the two processes.

Using hydrogencarbonate indicator

Hydrogencarbonate indicator shows the level of dissolved carbon dioxide in the water. It is red-orange at the carbon dioxide level associated with ordinary atmospheric conditions. It becomes purple when the carbon dioxide level falls below this reference level and yellow when it rises above it.

Comparing light and darkness

In sufficient light, photosynthesis can take in carbon dioxide faster than respiration releases it. The amount of dissolved carbon dioxide then decreases, so indicator starting at red-orange can turn purple. If the two processes occur at equal rates, there is no net change in carbon dioxide. If respiration releases carbon dioxide faster than photosynthesis takes it in, the amount increases instead.

In darkness, photosynthesis stops but respiration continues. Carbon dioxide therefore builds up in the water, and indicator starting at red-orange turns yellow.

Comparing the indicator changes in light and darkness shows the plant’s net gas exchange: the overall gain or loss of carbon dioxide after both processes are taken into account.

Balanced chemical equation for photosynthesis

Photosynthesis uses carbon dioxide and water to produce glucose and oxygen. Its balanced chemical equation is:

The numbers in front of the formulae are coefficients. They show that six molecules of carbon dioxide react with six molecules of water to produce one molecule of glucose and six molecules of oxygen. No coefficient is written before glucose, so its coefficient is one.

Why the photosynthesis equation is balanced

An equation is balanced when the same number of atoms of each element appears on both sides. A coefficient multiplies every atom in the formula that follows it.

• Carbon: Six $$\ce{CO2}$$ molecules contain six carbon atoms; one $$\ce{C6H12O6}$$ molecule also contains six.
• Hydrogen: Six $$\ce{H2O}$$ molecules contain twelve hydrogen atoms; one $$\ce{C6H12O6}$$ molecule also contains twelve.
• Oxygen: The reactants contain twelve oxygen atoms in $$\ce{6CO2}$$ and six in $$\ce{6H2O}$$, giving eighteen. The products contain six oxygen atoms in $$\ce{C6H12O6}$$ and twelve in $$\ce{6O2}$$, also giving eighteen.

Light supplies energy for photosynthesis, and chlorophyll enables the transfer of that energy. Neither light nor chlorophyll is a reactant in the chemical equation.