How plants make their own food

Ambika Nag

Everybody loves stories, especially children. A science classroom may become interesting for students if the teacher tells them stories about discoveries, about scientists and how gradually our understanding of a concept evolved in the past. During my work with teachers, I realized that they hardly know about historical accounts of discoveries of process and the periodical changes in definitions.

We all know that we need green plants to survive. We show our concern towards the environment. Plants are venerated in all societies across the globe and there are many proverbs that establish their significance and benefits. We find people campaigning to plant more trees. Let’s explore the reasons that they give:

We know that plants are important for us as a source of food, fibre, timber, medicine, oils, rubber, gum, etc., but what is more important is that they are responsible for the production of free oxygen in the atmosphere creating the conditions that allow for more complex forms of life to evolve. Plants check global warming by using carbon dioxide during the process of food-making and oxygen is a waste product of this process.

So the three important benefits of growing plants are – they give out oxygen, they reduce carbon dioxide levels in the atmosphere and they are the primary producers of food. And what makes this possible is the process of food-making called “photosynthesis” that is so central for the survival of not only plants but other organisms on earth as well.

We eat food to survive and grow, but where do the plants get their food from? We know that plants do not grow unless you plant them in soil and water them. It seems, therefore, that either soil or water (or both) have to turn into plant material for plants to grow.

In 1643, a Belgian scientist, Jan Baptista van Hel-mont (1577-1644), thought he would decide the matter by experiment.

This is an extract from van Helmont’s diary…

“I took an earthenware pot in which I put 200 pounds of earth that had dried in a furnace.

At length, after 5 years, the tree did weigh 169 pounds and 3 ounces. I again dried the earth in the vessel and found it weighed almost 200 pounds (less about 2 ounces). Therefore 164 pounds of wood, bark and roots arose out of water only.”

Let’s ponder:

• What was the change in the mass of the tree?
• What was the change in the mass of the soil? Could this have contributed to the growth of the plant? How much?
• Can plants grow without soil?
• What did van Helmont conclude from his experiment?
• Do you agree with his conclusion?
• What other explanations can there be for the results he found?

If you agree with van Helmont that water alone accounted for the growth of the willow tree, then can you survive on water alone? Did Helmont measure how much water was added to the pot over the five years? Helmont didn’t take air into account. Hardly anyone did in those days. You can’t see or feel air, so people usually ignore it. A British scientist, Stephen Hales (1677-1761), studied gases in detail, and, in 1727, he wondered if a gas might be involved in plant growth.

A Dutch scientist, Jan Ingenhousz (1730-1799), kept studying the way in which plants formed oxygen and, in 1779, noticed that this only happened when there was light. Plants did not produce oxygen in the dark. (Asimov, 1989)

Let’s ponder:
• Does sunlight have mass?
• Living things are made of atoms. Are there any atoms in sunlight?
• Can sunlight contribute to the increase in the mass of the plant?
• Is sunlight needed for plants to grow? What role do you think it might have?

Sunlight contains energy and this energy makes it possible for plants to grow and manufacture within themselves the complicated substances that animals use as food.

So, if you are given water, carbon dioxide and sunlight, will you be able to make food, just like plants? What gives plants this unique ability to make food from simpler substances? It must be that plants have something animals don’t have. What about the matter of colour? Plants are green or, at least, have important green parts. Animals are never truly green. Even plants that are not fully green only photosynthesize in those parts that have the green colour. A tree, for instance, does not photosynthesize in its roots, bark, branches, or twigs, but only in its green leaves.

In 1817, two French scientists, Pierre Joseph Pelletier (1788-1842) and Joseph Bienaime Caventou (1795-1877), isolated the green substance from plants. They named it chlorophyll from the Greek words, khloros meaning ‘pale green’ and phyllon meaning ‘a leaf’ which together mean “green leaf”. It is chlorophyll that makes the leaves look green.

Evolution of the term and its definition:

Before 1893, the light-dependent process by which plants reduce carbon dioxide to organic matter was called assimilation. In 1893, Charles Barnes mentioned that assimilation had long been used in animal physiology to designate the appropriation of digested food by the different tissues, and its conversion into the substances of those tissues. He proposed the term photosyntax and defined it as the synthesis of complex carbon compounds out of carbonic acid, in the presence of chlorophyll, under the action of light. He suggested photosyntax as the more preferable than its natural synonym photosynthesis.

Even though Barnes preferred the term ‘photosyntax,’ he was the first to publish the word ‘photosynthesis’ as an alternative. History has shown that, as time went on, an increasing number of investigators chose to use photosynthesis. (Gest, 2002)

After the discovery of an oxygenic bacterial photosynthesis, the general definition of ‘photosynthesis’ became inappropriate. In 1963, in order to include an oxygenic bacterial photosynthesis, Martin Kamen suggested a revised definition by
(a) avoiding any specifi cation of the carbon source for growth, and
(b) Not indicating oxygen as a photosynthetic product.

Kamen’s definition is: ‘Photosynthesis is a series of processes in which electromagnetic energy is converted to chemical free energy which can be used for biosynthesis.’

Thirty years later, adding the essential character of the phototrophic life mode in Kamen’s definition, Gest¹ defined photosynthesis as “Photosynthesis is a series of processes in which electromagnetic energy is converted to chemical energy used for biosynthesis of organic cell materials; a photosynthetic organism is one in which a major fraction of the energy required for cellular syntheses is supplied by light.”

Process of science in the classroom

Teachers may help students construct the concept of photosynthesis and simultaneously address some related misconceptions by including the following activities in their lesson plan. There are a number of hypotheses and students have the opportunity to test them through experimentation, collecting evidence, analyzing, questioning their findings and then reaching a conclusion.

Activity 1
Material required: Paper cups-10 (at least 150 ml capacity), dry garden soil, gravel, wood scraps, fine coal, paper cuttings, cloth cuttings, small pieces of sponge, plantlets (preferably local/seasonal plants), weighing balance and water.

Hypothesis: To grow, plants get their food from soil. Process: Make small groups of 3 to 5 students.
Ask each group to take a paper cup and fill it with different substrates as per availability and then weigh it. Then take a plant sapling, weigh it and plant it in the cup and cover the upper surface of the cup nicely with newspaper, so that nothing else goes in and nothing goes out. Then water the plant (limited quantity, to avoid clogging) for the next 15 days and keep it in an open place.

Observe the growth in the plantlet. After a period of 15 days, stop watering the plants. Gently take out the plantlet and weigh it, note the change in weight (day 1 and day 10). Let the material in the cup dry and then weigh it, note the change in weight.

Analyze on the following pointers:

• Whether there is any change in the weight of the plantlet? Does the change in weight indicate growth?
• Whether there is any change in the weight of material used as substrate –
  soil/paper or cloth cuttings/gravel/coal/wood scraps, etc.? Does the change indicate this material being used up?
• Do you see growth in plantlets using substrate other than soil?
• What might be the role of soil in the life of these plants?

This experiment rejects the hypothesis and we find that plants grow on other substrates as well, and there is no significant change in the weight of the soil as compared to the change in the weight of the plantlet – thus plants do not get their food from the soil. Although plants take minerals from the soil, the amount of these minerals is very small compared to the proteins, carbohydrates, lipids, and nucleic acids that make up the plant’s body.

By trying a next level of this activity, the teacher may discuss the role of other factors such as water, sunlight and air by keeping some samples without water, in dark and in a closed chamber respectively.

Question: If plants do not take carbon as a source of food from soil, why do we add organic manure to get better crop?
Answer: Dead and decaying organic material is an important part of the carbon cycle. The organic manure when added to soil acts as a substrate for saprophytic microbes present in the soil. Organic-bound nitrogen, phosphorus, and sulfur upon decomposition by these microbes, provide slow release of nutrients and thus enhance plant nutrient availability and retention. Some of the mycorrhizal fungi penetrate the roots of plants and facilitate movement of nutrients (minerals and nitrates) from soil to plants thus resulting in better growth of plants. Secondly, soil carbon improves aggregation of soil particles, resulting in better soil structure, allowing for movement of air and water through the soil as well as better root growth thus providing an improved rooting environment. It also improves water retention in soil, which is an important attribute for plant growth in arid and mesic environments².



Activity 2
Material required: Hydrilla plant (aquatic plant), transparent glass tumblers, transparent funnels that fit into the tumbler, test-tubes, baking soda, KOH (Potassium Hydroxide solution), black sheet, different sources of light – an LED, an incandescent bulb, halogen light, cellophane paper of red, blue, yellow and green colour.

Hydrilla is an aquatic plant that occurs as a weed in ponds. It lives submerged in the water and thus uses gases dissolved in the water for respiration and photosynthesis. We may use any other such plant for this activity.

Question: Why do we use aquatic plant?
Answer: Plants use six moles of carbon dioxide and give out six moles of oxygen during photosynthesis, so the volume of gas remains unchanged. Thus with terrestrial plants, we can hardly make an assembly to collect gas released by plants. Since we know that solubility of carbon dioxide is much higher than oxygen, it is possible that oxygen released by plants, saturates water and is then released as gas bubbles. So, we can make an assembly to collect the gas thus released and check whether it is oxygen.

Prepare an assembly:
Fill a glass tumbler with tap water. Put some branches of Hydrilla plant in a funnel and reverse it in the tumbler as shown in this picture. Now fill a test tube with water and place it upside down on the funnel. Mark the initial water level with marker pen. One assembly without the plant can be used as control.

Following hypotheses may be tested through this assembly:

1. Plants use carbon dioxide during the process of photosynthesis.
   Three kind of set ups are designed. One assembly with tap water, the second where we cut the availability of carbon dioxide by adding a few drops of 1% KOH and the third where we improve the availability of carbon dioxide by adding a pinch of baking soda.

   Place this assembly in sunlight and observe the results after two hours. The change in the level of water is seen as a result of the gas released by the plant during photosynthesis. We can also count the bubbles per unit time. By comparing the three results, we can infer the effect of the availability of carbon dioxide as substrate on rate of photosynthesis.

2. Plants give out oxygen during photosynthesis. We can collect the gas from the above assembly and test it for both carbon dioxide and oxygen.
   ^*The flame will be brighter when exposed to this gas if it is oxygen and will turn off if it is carbon dioxide.
3. Photosynthesis takes place in the presence of sunlight.
   By placing the above assembly in the dark, sunlight, in artifi cial light – such as LED, CFL, bulb, halogen light, coloured lights, we can compare the changes in the level of water in tubes. If we see any such change in the presence of artificial light, we will be able to say that the process takes place in artificial lights as well. That is why, in plant tissue culture labs, test-tube plants are grown under artificial light and a devise called lux-meter is used to measure and control intensity of such light.

Activity 3
Hypothesis:

1. Plants produce carbohydrates as a product of photosynthesis.
2. Plants have pigments that help in photosynthesis.

Material required: Potted plants (of different colour: white/green, red, purple leaves – croton, beetroot, etc.) alcohol, hot plate, a vessel to boil water, iodine solution.
Process: Take some potted plants, keep them in the dark for 48 hours.

On a green leaf, cover some portion with a strip of black sheet and fix it with a clip. Put this plant in the sunlight for one day. On the next day, pluck that leaf and boil it in water till it softens. Now boil it in alcohol (in a water-bath) till it becomes colourless. The pigments are released in alcohol and it turns greenish. Wash the leaf with water and place a few drops of iodine solution on it, the part where starch is formed turns blue-black.

Analyze: What part of the leaf turned blue-black and showed positive result for starch? Also observe the result in the white part versus green part of a croton leaf.

When we test purple and red leaves, what colour do they give in alcohol? Try to prepare a chromatograph to distinguish these pigments. Put a dot of alcoholic extract of leaf on a blotting paper strip. Dip that end in clear alcohol. Alcohol will rise on paper due to capillary action and various pigments will separate on column depending on their size. We can compare and detect the presence of green and yellow pigment that correspond to chlorophyll a and b in these leaves.

Through this experiment we can see that starch is formed as a result of photosynthesis and not only green, but red and purple leaves also have chlorophyll that act as a site of photosynthesis.

Observe site of photosynthesis – Chloroplasts:
Simply take a leaf of a Hydrilla plant and mount it directly on a slide and observe under microscope. You will be able to see dark coloured lines of the cell wall and many green coloured disc like strictures, these are chloroplasts.

Note
1. Howard Gest (2002) Defi nition of photosynthesis History of the word photosynthesis and evolution of its defi nition, Photosynthesis Research 73: 7-10.

2. C.W. Rice (2005) CARBON CYCLE IN SOILS | Dynamics and Management, in Encyclopedia of Soils in the Environment.

*Leaves are the primary site of photosynthesis in plants. Food prepared in the leaves in the form of carbohydrate is transported via phloem and stored in other plant parts. An important carbohydrate found in plants is starch. If we keep the leaves in the dark for a long time, the starch is not present since leaves cannot store starch except the fleshy ones.



See also: The photosynthesis story by Vijayalakshmi Nandakumar, Teacher Plus, August 2013.

The author has done her doctorate in Botany from Mohanlal Sukhadia University, Udaipur. She has over 20 years’ experience of teaching Botany and Biotechnology. She is currently a Resource Person at Azim Premji Foundation and is based at Jaipur. She can be reached at ambika.nag@azimpremjifoundation.org.

April 2018, Classroom Update