IGCSE NOTES : Biology - Plant nutrition and Photosynthesis

Photosynthesis

Photosynthesis is the process by which plants manufacture carbohydrates from raw materials using energy from light.

All living organisms need food. They need it as a source of raw materials to build new cells and tissues as they grow. They also need food as a source of energy. Food is a kind of ‘fuel’ that drives essential living processes and brings about chemical changes.

Animals take in food, digest it and use the digested products to build their tissues or to produce energy. Plants also need energy and raw materials but, apart from a few insect-eating species, plants do not appear to take in food. The most likely source of their raw materials would appear to be the soil. However, experiments show that the weight gained by a growing plant is far greater than the weight lost by the soil it is growing in. So there must be additional sources of raw materials. Jean-Baptiste van Helmont was a Dutch scientist working in the 17th century. At that time very little was known about the process of photosynthesis. He carried out an experiment using a willow shoot. He planted the shoot in a container with 90.8 kg of dry soil and placed a metal grill over the soil to prevent any accidental gain or loss of mass. He left the shoot for 5 years in an open yard, providing it with only rainwater and distilled water for growth. After 5 years he reweighed the tree and the soil. and came to the conclusion that the increase in mass of the tree (74.7 kg) was due entirely to the water it had received. However, he was unaware that plants also take in mineral salts and carbon dioxide, or that they use light as a source of energy.

A hypothesis to explain the source of food in a plant is that it makes it from air, water and soil salts. Carbohydrates contain the elements carbon, hydrogen and oxygen, as in glucose (C6H12O6). The carbon and oxygen could be supplied by carbon dioxide (CO2) from the air, and the hydrogen could come from the water (H2O) in the soil. The nitrogen and sulfur needed for making proteins could come from nitrates and sulfates in the soil.

This building-up of complex food molecules from simpler substances is called synthesis and it needs enzymes and energy to make it happen. The enzymes are present in the plant’s cells and the energy for the
fi rst stages in the synthesis comes from sunlight. The process is, therefore, called photosynthesis (‘photo’green substance, chlorophyll, in the chloroplasts of plant cells, plays a part in photosynthesis.

Chlorophyll

Absorbs sunlight and makes the energy from sunlight available for chemical reactions. Thus, in effect, the function of chlorophyll is to convert light energy to chemical energy. A chemical equation for photosynthesis would be carbon dioxide + water → glucose + oxygen

In order to keep the equation simple, glucose is shown as the food compound produced. In reality, the glucose is rapidly converted to sucrose for transport around the plant, then stored as starch or converted into other molecules.

Experiments to investigate photosynthesis

A hypothesis is an attempt to explain certain observations. In this case the hypothesis is that plants make their food by photosynthesis. The equation shown above is one way of stating the hypothesis and is used here to show how it might be tested.

If photosynthesis is occurring in a plant, then the leaves should be producing sugars. In many leaves, as fast as sugar is produced it is turned into starch. Since it is easier to test for starch than for sugar, we regard the production of starch in a leaf as evidence that photosynthesis has taken place.

The first three experiments described below are designed to see if the leaf can make starch without chlorophyll, sunlight or carbon dioxide, in turn. If the photosynthesis hypothesis is sound, then the lack of any one of these three conditions should stop photosynthesis, and so stop the production of starch. But, if starch production continues, then the hypothesis is no good and must be altered or rejected.

In designing the experiments, it is very important to make sure that only one variable is altered. If, for example, the method of keeping light from a leaf also cuts off its carbon dioxide supply, it would be impossible to decide whether it was the lack of light or lack of carbon dioxide that stopped the production of starch. To make sure that the experimental design has not altered more than one variable, a control is set up in each case. This is an identical situation, except that the condition missing from the experiment, e.g. light, carbon dioxide or chlorophyll, is present in the control.

Destarching a plant

If the production of starch is your evidence that photosynthesis is taking place, then you must make sure that the leaf does not contain any starch at the beginning of the experiment. This is done by destarching the leaves. It is not possible to remove the starch chemically, without damaging the leaves, so a plant is destarched simply by leaving it in darkness for 2 or 3 days. Potted plants are destarched by leaving them in a dark cupboard for a few days. In the darkness, any starch in the leaves will be changed to sugar and carried away from the leaves to other parts of the plant. For plants in the open, the experiment is set up on the day before the test. During the night, most of the starch will be removed from the leaves. Better still, wrap
the leaves in aluminium foil for 2 days while they are still on the plant. Then test one of the leaves to see that no starch is present.

Testing a leaf for starch

Iodine solution (yellow/brown) and starch (white) form a deep blue colour when they mix. The test for starch, therefore, is to add iodine solution to a leaf to see if it goes blue. However, a living leaf is impermeable to iodine and the chlorophyll in the leaf masks any colour change. So, the leaf has to be treated as follows:

• Heat some water to boiling point in a beaker and then turn off the Bunsen flame.
• Use forceps to dip a leaf in the hot water for about 30 seconds. This kills the cytoplasm, denatures the enzymes and makes the leaf more permeable to iodine solution.
• Note: make sure the Bunsen flame is extinguished before starting the next part of the procedure, as ethanol is flammable.
• Push the leaf to the bottom of a test-tube and cover it with ethanol (alcohol). Place the tube in the hot water. The alcohol will boil and dissolve out most of the chlorophyll. This makes colour changes with iodine easier to see.
• Pour the green alcohol into a spare beaker, remove the leaf and dip it once more into the hot water to soften it.
• Spread the decolourised leaf flat on a white tile and drop iodine solution on to it. The parts containing starch will turn blue; parts without starch will stain brown or yellow with iodine.

1 Is chlorophyll necessary for photosynthesis?

It is not possible to remove chlorophyll from a leaf without killing it, and so a variegated leaf, which has chlorophyll only in patches, is used. The white part of the leaf serves as the experiment, because it lacks chlorophyll, while the green part with chlorophyll is the control. After being destarched, the leaf – still on the plant – is exposed to daylight for a few hours. Remove a leaf from the plant; draw it carefully to show where the chlorophyll is (i.e. the green parts) and test it for starch as described above.

Result

Only the parts that were previously green turn blue with iodine. The parts that were white stain brown.

Interpretation

Since starch is present only in the parts that originally contained chlorophyll, it seems reasonable to suppose that chlorophyll is needed for photosynthesis. It must be remembered, however, that there are other possible
interpretations that this experiment has not ruled out; for example, starch could be made in the green parts and sugar in the white parts. Such alternative explanations could be tested by further experiments.

2 Is light necessary for photosynthesis?

• Cut a simple shape from a piece of aluminium foil to make a stencil and attach it to a destarched leaf.
• After 4 to 6 hours of daylight, remove the leaf and test it for starch.

Result

Only the areas which had received light go blue with iodine.

Interpretation

As starch has not formed in the areas that received no light, it seems that light is needed for starch formation and thus for photosynthesis.

You could argue that the aluminium foil had stopped carbon dioxide from entering the leaf and that it was shortage of carbon dioxide rather than absence of light which prevented photosynthesis taking place. A further control could be designed, using transparent material instead of aluminium foil for the stencil.

3 Is carbon dioxide needed for photosynthesis?

• Water two destarched potted plants and enclose their shoots in polythene bags.
• In one pot place a dish of soda-lime to absorb the carbon dioxide from the air (the experiment). In the other place a dish of sodium hydrogencarbonate solution to produce carbon dioxide (the control).
• Place both plants in the light for several hours and then test a leaf from each for starch.

Result

The leaf that had no carbon dioxide does not turn blue. The one from the polythene bag containing carbon dioxide does turn blue.

Interpretation

The fact that starch was made in the leaves that had carbon dioxide, but not in the leaves that had no carbon dioxide, suggests that this gas must be necessary for photosynthesis. The control rules out the possibility that high humidity or high temperature in the plastic bag prevents normal photosynthesis.

4 Is oxygen produced during photosynthesis?

• Place a short-stemmed funnel over some Canadian pondweed in a beaker of water.
• Fill a test-tube with water and place it upside-down over the funnel stem. (The funnel is raised above the bottom of the beaker to allow the water to circulate.)
• Place the apparatus in sunlight. Bubbles of gas should appear from the cut stems and collect in the test-tube.
• Set up a control in a similar way but place it in a dark cupboard.
• When sufficient gas has collected from the plant in the light, remove the test-tube and insert a glowing splint.

Result

The glowing splint bursts into flames.

Interpretation

The relighting of a glowing splint does not prove that the gas collected in the test-tube is pure oxygen, but it does show that it contains extra oxygen and this must have come from the plant. The oxygen is given off only in the light. Note that water contains dissolved oxygen, carbon dioxide and nitrogen. These gases may diffuse in or out of the bubbles as they pass through the water and collect in the test-tube. The composition of the gas in the test-tube may not be the same as that in the bubbles leaving the plant.