IGCSE NOTES : Biology - The process of photosynthesis

The process takes place mainly in the cells of the leaves and is summarised in

In land plants water is absorbed from the soil by the roots and carried in the water vessels of the veins, up the stem to the leaf. Carbon dioxide is absorbed from the air through the stomata (pores in the leaf, see ‘Leaf
structure’ later in this chapter). In the leaf cells, the carbon dioxide and water are combined to make sugar. The energy for this reaction comes from sunlight that has been absorbed by the green pigment chlorophyll. The chlorophyll is present in the chloroplasts of the leaf cells and it is inside the chloroplasts that the reaction takes place.

Chloroplasts are small, green structures present in the cytoplasm of the leaf cells. Chlorophyll is the substance that gives leaves and stems their green colour. It is able to absorb energy from light and use it to split water molecules into hydrogen and oxygen (the ‘light’ or ‘light-dependent’ reaction). The oxygen escapes from the leaf and the hydrogen molecules are added to carbon dioxide molecules to form sugar (the ‘dark’ or ‘light-independent’ reaction). In this way the light energy has been transferred into the chemical energy of carbohydrates as they are synthesised.

There are four types of chlorophyll that may be present in various proportions in different species. There are also a number of photosynthetic pigments, other than chlorophyll, which may mask the colour of chlorophyll even when it is present, e.g. the brown and red pigments that occur in certain seaweeds.

The plant’s use of photosynthetic products

The glucose molecules produced by photosynthesis are quickly built up into starch molecules and added to the growing starch granules in the chloroplast. If the glucose concentration was allowed to increase in the mesophyll cells of the leaf, it could disturb the osmotic balance between the cells.

Starch is a relatively insoluble compound and so does not alter the osmotic potential of the cell contents. The starch, however, is steadily broken down to sucrose and this soluble sugar is transported out of the cell into the food-carrying cells of the leaf veins. These veins will distribute the sucrose to all parts of the plant that do not photosynthesise, e.g. the growing buds, the ripening fruits, the roots and the underground storage organs. The cells in these regions will use the sucrose in a variety of ways.

Respiration

The sugar can be used to provide energy. It is oxidised by respiration to carbon dioxide and water, and the energy released is used to drive other chemical reactions such as the buildingup of proteins described below.

Storage

Sugar that is not needed for respiration is turned into starch and stored. Some plants store it as starch grains
in the cells of their stems or roots. Other plants, such as the potato or parsnip, have special storage organs (tubers) for holding the reserves of starch. Sugar may be stored in the fruits of some plants; grapes, for example, contain a large amount of glucose.

Synthesis of other substances

As well as sugars for energy and starch for storage, the plant needs cellulose for its cell walls, lipids for its cell membranes, proteins for its cytoplasm and pigments for its flower petals, etc. All these substances are built up (synthesised) from the sugar molecules and other molecules produced in photosynthesis.

By joining hundreds of glucose molecules together, the long-chain molecules of cellulose are built up and added to the cell walls. Amino acids  are made by combining nitrogen with sugar molecules or smaller carbohydrate molecules. These amino acids are then joined together to make the proteins that form the enzymes and the cytoplasm of the cell. The nitrogen for this synthesis comes from nitrates which are absorbed from the soil by the roots. Some proteins also need sulfur molecules and these are absorbed from the soil in the form of sulfates (SO4). Phosphorus is needed for DNA  and for reactions involving energy release. It is taken up as phosphates (PO4).

The chlorophyll molecule needs magnesium (Mg).
This metallic element is also obtained from salts in the soil. Many other elements, e.g. iron, manganese, boron, are also needed in very small quantities for healthy growth. These are often referred to as trace elements.

The metallic and non-metallic elements are all taken up in the form of their ions by the plant roots. All these chemical processes, such as the uptake of salts and the building-up of proteins, need energy from respiration to make them happen.

Gaseous exchange in plants

Air contains the gases nitrogen, oxygen, carbon dioxide and water vapour. Plants and animals take in or give out these last three gases and this process is called gaseous exchange. You can see from the equation for photosynthesis that one of its products is oxygen. Therefore, in daylight, when photosynthesis is going on in green plants, they will be taking in carbon dioxide and giving out oxygen. This exchange of gases is the opposite of that resulting from respiration but it must not be thought that green plants do not respire. The energy they need for all their living processes – apart from photosynthesis – comes from respiration, and this is going on all the time, using up oxygen and producing carbon dioxide.

During the daylight hours, plants are photosynthesising as well as respiring, so that all the carbon dioxide produced by respiration is used up by photosynthesis. At the same time, all the oxygen needed by respiration is provided by photosynthesis. Only when the rate of photosynthesis is faster than the rate of respiration will carbon dioxide be taken in and the excess oxygen given out.

The metallic and non-metallic elements are all taken up in the form of their ions by the plant roots. All these chemical processes, such as the uptake of salts and the building-up of proteins, need energy from respiration to make them happen. Gaseous exchange in plants Air contains the gases nitrogen, oxygen, carbon dioxide and water vapour. Plants and animals take in or give out these last three gases and this process is called gaseous exchange. You can see from the equation for photosynthesis that one of its products is oxygen. Therefore, in daylight, when photosynthesis is going on in green plants, they will be taking in carbon dioxide and giving out oxygen. This exchange of gases is the opposite of that resulting from respiration but it must not be thought that green plants do not respire. The energy they need for all their living processes – apart from photosynthesis – comes from respiration, and this is going on all the time, using up oxygen and producing carbon dioxide.

During the daylight hours, plants are photosynthesising as well as respiring, so that all the carbon dioxide produced by respiration is used up by photosynthesis. At the same time, all the oxygen needed by respiration is provided by photosynthesis. Only when the rate of photosynthesis is faster than the rate of respiration will carbon dioxide be taken in and the excess oxygen given out.

How will the gas exchange of a plant be affected by being kept in the dark and in the light?

This investigation makes use of hydrogencarbonate indicator, which is a test for the presence of carbon dioxide. A build-up of carbon dioxide turns it from pink/red to yellow. A decrease in carbon dioxide levels causes the indicator to turn purple. n Wash three boiling tubes first with tap water, then with distilled water and finally with hydrogencarbonate indicator (the indicator will change colour if the boiling tube is not clean).

• Then fill the three boiling tubes to about two thirds full with hydrogencarbonate indicator solution.
• Add equal-sized pieces of Canadian pondweed to tubes 1 and 2 and seal all the tubes with stoppers.
• Expose tubes 1 and 3 to light using a bench lamp and place tube 2 in a black box, or a dark cupboard, or wrap it in aluminium foil (Figure 6.15). After 24 hours note the colour of the hydrogencarbonate indicator in each tube.

Result

The indicator in tube 3 (the control) which was originally pink/red should not change colour; that in tube 2 (plant in the dark) should turn yellow; and in tube 1 (plant in the light) the indicator should be purple.

Interpretation

Hydrogencarbonate indicator is a mixture of dilute sodium hydrogencarbonate solution with the dyes cresol red and thymol blue. It is a pH indicator in equilibrium with the carbon dioxide, i.e. its original colour represents the acidity produced by the carbon dioxide in the air. An increase in carbon dioxide makes it more acidic and it changes colour from orange/red to yellow. A decrease in carbon dioxide makes it less acid and causes a colour change to purple. The results, therefore, provide evidence that in the light (tube 1) aquatic plants use up more carbon dioxide in photosynthesis than they produce in respiration. In darkness (tube 2) the plant produces carbon dioxide (from respiration). Tube 3 is the control, showing that it is the presence of the plant that causes a change in the solution in the boiling tube. The experiment can be criticised on the grounds that the hydrogencarbonate indicator is not a specific test for carbon dioxide but will respond to any change in acidity or alkalinity. In tube 1 there would be the same change in colour if the leaf produced an alkaline gas such as ammonia, and in tube 2 any acid gas produced by the leaf would turn the indicator yellow. However, knowledge of the metabolism of the leaf suggests that these are less likely events than changes in the carbon dioxide concentration.

Effects of external factors on rate of photosynthesis

The rate of the light reaction will depend on the light intensity. The brighter the light, the faster will water molecules be split in the chloroplasts. The ‘dark’ reaction will be affected by temperature. A rise in temperature will increase the rate at which carbon dioxide is combined with hydrogen to make carbohydrate.
Limiting factors Key definition A limiting factor is something present in the environment in such short supply that it restricts life processes. Beyond that point, any further increase in light intensity has only a small effect. This limit on the rate of increase could be because all available chloroplasts are fully occupied in light absorption. So, no matter how much the light intensity increases, no more light can be absorbed and used. Alternatively, the limit could be imposed by the fact that there is not enough carbon dioxide in the air to cope with the increased supply of hydrogen atoms produced by the light reaction. Or, it may be that low temperature is restricting the rate of the ‘dark’ reaction.

Any one of the external factors – temperature, light intensity or carbon dioxide concentration – may limit the
effects of the other two. A temperature rise may cause photosynthesis to speed up, but only to the point where the light intensity limits further increase. In such conditions, the external factor that restricts the effect of the others is called the limiting factor. Since there is only 0.03% of carbon dioxide in the air, it might seem that a shortage of carbon dioxide could be an important limiting factor. Indeed, experiments do show that an increase in carbon dioxide concentration does allow a faster rate of photosynthesis. However, recent work in plant physiology has shown that the extra carbon dioxide affects reactions other than photosynthesis.

The main effect of extra carbon dioxide is to slow down the rate of oxidation of sugar by a process called photorespiration and this produces the same effect as an increase in photosynthesis. Although carbon dioxide concentration limits photosynthesis only indirectly, artificially high levels of carbon dioxide in greenhouses do effectively increase yields of crops.

Greenhouses

Greenhouses also maintain a higher temperature and so reduce the effect of low temperature as a limiting factor, and they clearly optimise the light reaching the plants. Parts of the world such as tropical countries often benefit from optimum temperatures and rainfall for crop production. However, greenhouses are still often used because they allow the growers to control how much water and nutrients the plants receive and they can also reduce crop damage by insect pests and disease. Sometimes rainfall is too great to benefit the
plants. In an experiment in the Seychelles in the wet season of 1997, tomato crops in an open field yielded 2.9 kg m−2. In a greenhouse, they yielded 6.5 kg m−2.

The concept of limiting factors does not apply only to photosynthesis. Adding fertiliser to the soil, for example, may increase crop yields, but only up to the point where the roots can take up all the nutrients and the plant can build them into proteins, etc. The uptake of mineral ions is limited by the absorbing area of the roots, rates of respiration, aeration of the soil and availability of carbohydrates from photosynthesis.

Carrot plants grown in increasing concentrations of carbon dioxide from left to right Currently there is debate about whether athletic performance is limited by the ability of the heart and lungs to supply oxygenated blood to muscles, or by the ability of the muscles to take up and use the oxygen.

The role of the stomata

The stomata in a leaf may affect the rate of photosynthesis according to whether they are open or closed. When photosynthesis is taking place, carbon dioxide in the leaf is being used up and its concentration falls. At low concentrations of carbon dioxide, the stomata will open. Thus, when photosynthesis is most rapid, the stomata are likely to be open, allowing carbon dioxide to diffuse into the leaf. When the light intensity falls, photosynthesis will slow down and the buildup of carbon dioxide from respiration will make the stomata close. In this way, the stomata are normally regulated by the rate of photosynthesis rather than photosynthesis being limited by the stomata. However, if the stomata close during the daytime as a result of excessive water loss from the leaf, their closure will restrict photosynthesis by preventing the inward diffusion of atmospheric carbon dioxide.

Normally the stomata are open in the daytime and closed at night. Their closure at night, when intake of carbon dioxide is not necessary, reduces the loss of water vapour from the leaf.