0610 · 6.1Photosynthesis

Photosynthesis equation and chlorophyll

0:00A huge oak tree can weigh many tonnes, and it grew from a single small acorn. So where did all that mass come from? Most people guess that it came from the soil. But the soil around a tree hardly gets any lighter as the tree grows. In fact, most of the dry mass of a tree came from the air, from carbon dioxide, built into sugar using energy from sunlight.

0:19That process is photosynthesis, and almost every food chain on Earth depends on it. By the end of this video you will be able to describe photosynthesis and write its equation, explain what chlorophyll does, describe how plants use the sugar they make and why they need mineral ions, describe the classic photosynthesis experiments, and, for the extended syllabus, explain limiting factors. You need two ideas from earlier videos. Chloroplasts are the green structures in plant cells, mostly in the leaves.

0:47And glucose is a simple sugar, a carbohydrate, which can be joined into larger molecules such as starch and cellulose. The syllabus describes photosynthesis as the process by which plants synthesise carbohydrates from raw materials using energy from light. Synthesise means make. So plants make carbohydrates, starting with glucose, from simple raw materials, using light as their energy source. The raw materials are carbon dioxide and water. Carbon dioxide enters the leaves from the air, and water is taken up from the soil by the roots.

1:18Using energy from light, the plant turns them into glucose, and oxygen is released as a waste product. So the word equation for photosynthesis is carbon dioxide plus water gives glucose plus oxygen, in the presence of light and chlorophyll. Light and chlorophyll are written above the arrow, because they are conditions the reaction needs, not raw materials. For the extended syllabus, you need the balanced chemical equation. Six molecules of carbon dioxide plus six molecules of water give one molecule of glucose plus six molecules of oxygen.

1:49The formula of glucose is C, six, H, twelve, O, six. Check that it balances. There are six carbon atoms on each side, twelve hydrogen atoms on each side, and eighteen oxygen atoms on each side. What does chlorophyll do? The syllabus wants you to state that chlorophyll is a green pigment found in chloroplasts. A pigment is a coloured substance that absorbs light. You also need to state that chlorophyll transfers energy from light into energy in chemicals, for the synthesis of carbohydrates.

2:19In other words, chlorophyll captures light energy and the plant stores it as chemical energy in glucose. That stored energy is what you get when you eat a plant, and what is released again in respiration. A common mistake is to say that chlorophyll makes energy, or that it turns light into glucose. It transfers energy. It does not make it.

Uses of glucose and mineral ions

2:38Next, what happens to the glucose a plant makes? The syllabus lists five uses. First, glucose is turned into starch, as an energy store. Starch does not dissolve, so it can be stored in cells without upsetting their water balance. Potatoes and seeds are full of starch. Second, glucose is turned into cellulose, to build cell walls. Third, glucose is used in respiration, to provide energy for the plant's cells.

3:04Fourth, glucose is turned into sucrose, which is carried around the plant in the phloem. And fifth, glucose is turned into sugars in nectar, which attracts insects to flowers for pollination. Plants also need mineral ions from the soil. Two are named in the syllabus. Nitrate ions are needed for making amino acids, which are joined to make proteins. Remember that proteins contain nitrogen, and carbohydrates do not, so a plant cannot make proteins from glucose alone.

3:30Without nitrate, a plant grows poorly, with weak stems and yellowing older leaves. Magnesium ions are needed for making chlorophyll. Each chlorophyll molecule contains a magnesium atom. Without magnesium, a plant cannot make enough chlorophyll, and its leaves turn yellow, so it cannot photosynthesise well.

Testing photosynthesis requirements

3:47Now to the experiments. The syllabus asks you to investigate the need for chlorophyll, light and carbon dioxide for photosynthesis, using appropriate controls. All three experiments test leaves for starch, because starch is made from the glucose produced in photosynthesis. If a leaf contains starch, it has been photosynthesising. First, the plant must be destarched. You leave it in the dark for about two days. With no light, the plant cannot photosynthesise, so it uses up its stored starch.

4:14This makes sure that any starch found later was made during the experiment, not before it. Then you test a leaf for starch, in four steps. Step one, put the leaf in boiling water for about a minute. This kills the cells and stops all reactions. Step two, put the leaf in hot ethanol. Ethanol catches fire easily, so the tube of ethanol is heated in a beaker of hot water, with the Bunsen burner turned off.

4:36The ethanol removes the chlorophyll, so the leaf turns white, and the iodine colour will be easy to see. Step three, rinse the leaf in water to soften it. Step four, spread it on a white tile and add iodine solution. Blue-black means starch is present. Orange-brown means there is no starch. To show that chlorophyll is needed, use a variegated leaf. A variegated leaf has green parts, which contain chlorophyll, and white parts, which do not.

5:01After the plant has been in the light, test the leaf for starch. The parts that were green turn blue-black, and the parts that were white stay orange-brown. So only the parts with chlorophyll made starch. The green parts, which have chlorophyll, act as the control, so we can compare them with the white parts. To show that light is needed, take a destarched plant and cover part of a leaf, on both sides, with black paper or foil, so that no light reaches it.

5:25Leave the plant in the light, then test the leaf. The uncovered parts turn blue-black, and the covered part stays orange-brown. The uncovered part of the same leaf is the control. So only the parts that received light made starch. To show that carbon dioxide is needed, put a destarched plant inside a sealed clear plastic bag with a dish of soda lime, which absorbs carbon dioxide from the air.

5:46As a control, put a second destarched plant in a sealed bag with a dish of water instead. Leave both in the light, then test a leaf from each. The leaf from the soda lime bag stays orange-brown, and the control leaf turns blue-black. So without carbon dioxide, no starch was made. In each of these experiments, the control is what makes the result convincing. It is set up exactly the same, except for the one factor being tested, so you can be sure that factor caused the difference.

Investigating photosynthesis rate

6:11The syllabus also asks you to investigate the effects of light intensity, carbon dioxide concentration and temperature on the rate of photosynthesis. The classic method uses pondweed in water. As the pondweed photosynthesises, it gives off bubbles of oxygen. You count the bubbles per minute, or collect the gas and measure its volume. More bubbles per minute means a faster rate. To change light intensity, move a lamp closer to or further from the pondweed.

6:36The closer the lamp, the brighter the light. A lamp also gives off heat, so use an LED lamp, or put a beaker of water between the lamp and the pondweed, to keep the temperature the same. And do it in a dark room, so the lamp is the only light. To change carbon dioxide concentration, add different amounts of sodium hydrogencarbonate to the water, which releases carbon dioxide. To change temperature, stand the beaker in water baths at different temperatures.

7:00Each time, change only one factor and keep the others the same. What do the results show? As light intensity increases, the rate of photosynthesis increases, and then it levels off. Carbon dioxide concentration gives the same shape. The rate increases, then levels off. Temperature is different. The rate increases up to an optimum, and then it falls, because the enzymes that control photosynthesis are denatured at high temperatures, just as you saw in the enzymes video.

7:26The last investigation uses hydrogencarbonate indicator solution, which changes colour with the amount of carbon dioxide in the water. At normal air levels it is orange-red. When carbon dioxide increases, it turns yellow. When carbon dioxide decreases, it turns purple. You set up three stoppered tubes, each with the same volume of indicator. One has pondweed and is left in the light. One has pondweed and is covered with foil, so it is in the dark.

7:51And one has no pondweed at all, and is left in the light, as a control. After a few hours, the tube in the light turns purple. The plant is photosynthesising faster than it respires, so it takes carbon dioxide out of the water. The tube in the dark turns yellow. With no light, the plant cannot photosynthesise, but it still respires, so it adds carbon dioxide to the water.

8:12The control stays orange-red, showing that the plant caused the changes. A common mistake here is to say that plants only respire in the dark. Plants respire all the time, day and night. In the light, photosynthesis is simply faster than respiration, so there is an overall uptake of carbon dioxide.

Limiting factors and graph questions

8:30For the extended syllabus, you need to identify and explain limiting factors. A limiting factor is the factor in shortest supply, which stops the rate of photosynthesis from going any higher. The three main limiting factors are light intensity, carbon dioxide concentration and temperature. Look at the graph of rate against light intensity. On the rising part, light is the limiting factor. More light gives a faster rate. But on the flat part, adding more light makes no difference.

8:56Something else is now limiting the rate, such as carbon dioxide concentration or temperature. If you increased that factor, the rate would rise again, to a new, higher plateau. The limiting factor depends on the conditions. On a cold, sunny winter morning, there is plenty of light, but the low temperature limits the rate. At night, light is the limiting factor. On a warm, bright summer afternoon, carbon dioxide concentration is often the limiting factor.

9:20That is why some farmers grow crops in greenhouses, where they can add heat, extra light and extra carbon dioxide, so that no single factor holds the rate back. Let us try an exam-style question. A graph shows the rate of photosynthesis against light intensity at two carbon dioxide concentrations. Both curves rise and then level off, but the high carbon dioxide curve levels off at a higher rate. Explain why the low carbon dioxide curve levels off.

9:46At low light intensity, both curves rise, because light is limiting. But the low carbon dioxide curve levels off sooner, at a lower rate. On its flat part, more light makes no difference, so light is no longer the limiting factor. Carbon dioxide concentration is now limiting. We know this, because at the same light intensity, the higher carbon dioxide concentration gives a faster rate.

Common mistakes and recap

10:07Before the recap, let us collect the common mistakes. The first is saying that plants get their food from the soil. They make it by photosynthesis. The soil supplies water and mineral ions. The second is forgetting light and chlorophyll in the word equation. The third is saying that chlorophyll makes energy. It transfers energy. The fourth is saying that plants only respire at night. And the fifth is forgetting to destarch the plant, or forgetting the control.

10:33Let us recap. Photosynthesis is the process by which plants synthesise carbohydrates from raw materials using energy from light. Carbon dioxide plus water gives glucose plus oxygen, in the presence of light and chlorophyll. Chlorophyll is a green pigment in chloroplasts that transfers energy from light into energy in chemicals. The glucose is used for starch as an energy store, cellulose for cell walls, respiration for energy, sucrose for transport in the phloem, and nectar to attract insects.

11:02Nitrate ions are needed to make amino acids, and magnesium ions to make chlorophyll. Starch tests on destarched plants show that chlorophyll, light and carbon dioxide are all needed. Light intensity, carbon dioxide concentration and temperature all affect the rate, and hydrogencarbonate indicator shows the plant taking in carbon dioxide in the light and giving it out in the dark. Whichever factor is in shortest supply is the limiting factor.

11:23In the next video, we will look at how the structure of a leaf is adapted for photosynthesis.