Biology and Health Sciences

Key unit competency

To be able to explain the process of photosynthesis and how various environmental factors affect the rate at which photosynthesis occurs.

Learning objectives

After studying this topic, I should be able to:

• Define and state the word equation for photosynthesis.

• Recall the location of plastids and chloroplasts in a plant cell.

• Identify the products of photosynthesis.

• Explain the importance of nitrates and magnesium in chlorophyll synthesis.

• Explain how the internal and external structures of a leaf are adapted for photosynthesis.

• Describe the uses and dangers of fertilisers.

• Carry out experiments to test for starch in green leaves.

• Appreciate the process of photosynthesis.

Introduction

By now, you must have heard about photosynthesis. What does the word photosynthesis mean? How does it occur? What does it require in order for it to take place?

Look at the plant leaves below. Which one do you think is best suited to carry out the process of photosynthesis? Why?

Discuss the above questions with your friend. From your discussion, what do you think you will learn in this unit?

Plants and certain types of bacteria are autotrophs. This means that they can synthesise their own food. Plants use energy from the sun, carbon dioxide and water as raw materials to make food in a process known as photosynthesis. They have chlorophyll which traps light energy from the sun. In the process, oxygen is given off as a by-product. The process of photosynthesis can be summarised as follows:

The chlorophyll are found in chloroplasts. Chloroplast is an example of a plastid. It  is the organelle in a plant cell where photosynthesis takes place. Chloroplasts are found in the cytoplasm of the cells found in either palisade cells  mesophyll, spongy mesophyll and guard cells in a leaf. Cells that have chloroplasts are called photosynthetic cells.

To find out whether the leaf is the site for photosynthesis, we test for the presence of starch in the leaf.

Activity 7.1: Testing for the presence of starch in leaves

Requirements

Procedure

1.  Half-fill a beaker with water.

2.  Boil the water in the beaker.

3.  Dip a leaf into the boiling water for 2 to 3 minutes.

4.  Put a boiling tube half-filled with methylated spirit into the boiling water (water bath).

     Caution: Do not expose methylated spirit to a naked flame.

5.  Take the leaf and dip it into the boiling methylated spirit.

6.  Leave it in the hot methylated spirit until all the chlorophyll is removed.

7.  Take the leaf, which is now white in colour, and dip it once again in the boiling water. 

8. Take the leaf using a pair of forceps and spread it out carefully onto a white tile.

9. Using a dropper, place a few drops of iodine solution onto the leaf and note the colour of iodine.

Study questions

(a) Give reasons why the leaf was boiled in water?

(b) What is the significance of boiling the leaf in ethanol using a water bath rather than directly heating with an open flame.

Change in colour of iodine solution from brown to blue-black shows presence of starch. This means that the part of the leaf tested has starch and therefore photosynthesis must have taken place in that part.

For photosynthesis to take place certain conditions are necessary. These conditions include the presence of:

               • Light energy

               • Carbon dioxide

               • Water

               • Chlorophyll

7.1 Necessity of chlorophyll, light energy and carbon dioxide for photosynthesis

Activity 7.2: To investigate necessity of chlorophyll for photosynthesis

Requirements

Procedure

1.  Expose the variegated potted plant (that had previously been put in darkness for 48 hours - a process called destarching) to sunlight for about 3 hours.

2.  Detach a variegated leaf from the plant and draw it.

3.  Label the green part and the white part.

4.  Test the leaf for starch. (Refer to activity 7.1)

5.  Draw the leaf after the test and label the brown parts and the blue-black parts.

Study questions

(a) In the test for starch, what is the colour of the part of the leaf that was: 

(b) Explain the results that you get on testing the variegated leaf for starch.

(c) Which part of the leaf acts as a control?

Health Check!

• Put on eye protection.

• Ethanol is highly flammable; therefore remember to turn out the flame before putting the boiling tube with the ethanol into the hot water.

A variegated leaf is one whose surface shows two colours, for example, green on some parts and white on others.

The green part of the leaf has cells with chlorophyll. Therefore photosynthesis will take place and form starch. Starch  present in this part will turn iodine from brown to blue-black colour. The yellow part has cells that do not contain chlorophyll. These cells will not carry out photosynthesis; therefore no starch will be formed. The starch test will be negative. The green part of the leaf acts as a control experiment because it has all the conditions required for photosynthesis.

Activity 7.3: To investigate necessity of light in photosynthesis

Requirements

Procedure

1. Cut out a star shape or circle in the middle of an aluminium foil or black carbon paper that is big enough to cover a big portion of a leaf.

2.  Use a paper clip to attach this paper securely round one of the green leaves of a potted plant as shown in figure 7.4. Cover the leaf with aluminium foil only leaving out a star shaped space that exposes part of the leaf.

3.  Expose the plant to sufficient light for about 3 hours.

4.  Pick the leaf that had been covered and one other uncovered leaf and carry out a starch test on the leaves.

5.  Record your results.

6.  Account for the observations.

7.  Discuss with a classmate your findings.   

Study questions

(a) What do you observe when you test the leaf for starch?

(b)  Draw the different parts of the leaf as observed in the experiment.

(c) Which part of the leaf acts as a control?

(d)  Explain your answer in (c) above.

The part of the plant not covered with the foil had all conditions needed for photosynthesis. Therefore, on testing with iodine, it appeared blue-black. This is because starch was present. This part of the leaf acts as a control set-up.

When a test for starch was carried on the part covered with aluminium foil, the colour of iodine persisted. This shows that starch was absent. The foil prevented sunlight from reaching this part of the leaf and hence photosynthesis did not take place.

Activity 7.4: To investigate the necessity of carbon dioxide in photosynthesis

Requirements

Procedure

Set up the two potted plants as follows:

Plant A

1.  Put some sodium hydroxide or potassium hydroxide into the small plastic container. Place the container carefully on the soil holding the plant.

2.  Take the transparent polythene bag and cover the whole plant with it. Secure the bottom by tying it with one or two elastic bands.

Plant B

3. Repeat the procedure but place sodium hydrogen carbonate (sodium bicarbonate) in the plastic container.

4.  Leave the set-up in a well-lit part of the laboratory or in sunlight for several hours or 1 to 2 days.

5.  Detach leaves from each setup and test for the presence of starch.

Study questions

(a) What is the role of the sodium hydroxide (potassium hydroxide) in set-up A?

(b) What did you find out when you tested for starch in leaves from set-up A and B?

(c) What conclusion can you make from the results of set-up A and B?

(d) Between set-up A and B which one acts as the control experiment?

(e) What is the role of sodium bicarbonate in set-up B?

As mentioned earlier, photosynthesis involves synthesis of organic food substances by plants. The overall photosynthesis equation is:

End products of photosynthesis

Glucose: This is the main product. It is used in respiration to release energy. Excess glucose is stored as starch or oil.

Oxygen: Some of the oxygen is used during respiration while the rest is released to the atmosphere during gaseous exchange.

Activity 7.5: To show that oxygen is produced during photosynthesis

Requirements

 Procedure

1.  Prepare two set-ups of apparatus as shown below.

Note: Set up A placed in a dark cupboard Set up B placed in a bright sunshine.

3. Observe the set-up in the dark cupboard. 

     • What did you notice?

4. Observe the set-up in the bright sunshine. 

     • What do you notice?

5. Test any gas produced using a glowing splint.

Study questions

(a) What is the role of the sodium hydrogen carbonate (sodium bicarbonate) dissolved in the water?

(b) What happens to the glowing splint when it is exposed to the gas in the test tubes? • What is your conclusion from the observation?

(c) What was the role of the setup that was placed in the dark cupboard?

In presence of sunlight, photosynthesis takes place producing oxygen as a  by-product. Oxygen relights a glowing splint. In darkness no photosynthesis takes place. The set up in the dark cupboard therefore does not produce any oxygen.

Self-evaluation Test 7.1

1.  Name the raw materials required for photosynthesis to occur.

2.  Below is an incomplete symbolic equation of photosynthesis.

     (a) What does Y represent?                  (b) What is product X?

3.  Plants convert glucose into _______ and store it in _______, _________ and ________.

4.  The figure below shows an experiment that was carried out on two green leaves. The mid-rib of leaf M was severed at point x while still attached to the tree in an evening of a sunny day. The mid-rib of leaf N was left intact. The two leaves were detached from the plant and tested for starch.
                

         (a)  Account for the distribution of starch in part of leaf M.

         (b)  There was complete absence of starch in leaf N. Account for this.

7.2 Limiting factors of photosynthesis

A limiting factor is a variable which limits the rate of photosynthesis.

The process of photosynthesis does not always take place efficiently. Sometimes there are some factors that hinder its progress. Photosynthesis may be limited by a shortage in supply of one or more raw materials or other factors necessary for photosynthesis.

Discussion corner

1. Discuss with your classmates the effects of the following on the process of photosynthesis under the following conditions:

         (i) Very low temperatures

         (ii) Cloudy conditions

         (iii) Very high light intensity

         (iv) Low carbon dioxide

2. What does this tell you about the process of photosynthesis

    Some of these factors are given below.

a. Light intensity

The intensity of sunlight varies with time of the day, season and position of the plants on the earth’s surface. Light is a necessity for photosynthesis to occur. In darkness, plants cannot photosynthesise at all. In low light intensity, the rate of photosynthesis is low. But as light intensity increases, the rate of photosynthesis also increases. There reaches a point where the plant cannot photosynthesise any faster even with further increase in light intensity. At this point, any other factor affecting the rate of photosynthesis hinders the rate of photosynthesis.

• Between points A and B; rate of photosynthesis is affected by the amount of sunlight. Increase in light intensity leads to an increase in the rate of photosynthesis. Light is therefore a limiting factor.

• Between B and C however, the plant cannot photosynthesise any faster even with increase in light intensity. At this point, light is no longer a limiting factor. Instead, other factors limit the rate.

b. Carbon dioxide concentration

The amount of carbon dioxide in the atmosphere is quite low (0.03%).  Therefore it can also be a limiting factor to photosynthesis. Increase in carbon dioxide increases the rate of photosynthesis. But this continues only to a certain point where rate of photosynthesis does not increase further with more carbon dioxide since other factors affecting photosynthesis become limiting.

AB: Increase in concentration of carbon dioxide causes a rise in rate of photosynthesis.

BC: Limiting factors set in and a further rise in carbon dioxide concentration does not cause a corresponding increase in rate of photosynthesis.

c. Temperature

Photosynthesis is an enzyme- catalysed reaction and is therefore affected by temperature. If temperatures are low, plants photosynthesise very slowly; but as temperature increases, the rate of photosynthesis also increases. Rate of photosynthesis is highest at optimum temperature. Further increase in temperature above optimum results to a decrease in rate of photosynthesis since enzymes are denatured.

AB – Increase in temperature causes a corresponding increase in the rate of photosynthesis. The rate of photosynthesis is highest at B.

BC – Any further increase in temperature denatures the enzymes.  Therefore the rate of photosynthesis declines.

d. Water

Only about 1% of the water taken in by plants is used for photosynthesis. Water shortage only indirectly affects the rate of photosynthesis, for instance, when water is in short supply, the stomata close. This lowers the exchange of gases between the leaf and the atmosphere. As a result, less carbon dioxide diffuses via the stomata into the leaf. This reduces the rate of photosynthesis.

Self-evaluation Test 7.2

1.  Which of the following is not a limiting factor in photosynthesis?

      A. Amount of oxygen

      B. Concentration of chlorophyll

      C. Light intensity

      D. Temperature

      E. Carbon dioxide concentration

2.  The graph below shows the effect of increasing carbon dioxide on the rate of photosynthesis.

Explain why the rate of photosynthesis does not continue to increase with increase in carbon dioxide concentration.

3.  Temperature is a factor that limits the rate of photosynthesis. Explain why the rate of photosynthesis starts to decrease as temperature increases beyond 40oC and eventually stops beyond 60oC.

4.  The atmospheric air contains 0.03% carbon dioxide. What will happen to the rate of photosynthesis if carbon dioxide level rises to 4%?

7.3  Internal structure of the leaf and its adaptaions to photosynthesis  

The leaf is the main organ for photosynthesis. However, other green parts of the plant carry out photosynthesis as well. The structure of the leaf is well suited for photosynthesis. Look at the diagram below. It shows the internal parts of the leaf.

The leaf has cuticle which covers both upper and lower surfaces of the leaf. Immediately after the cuticle is the upper and lower epidermis respectively. The palisade mesophyll which contains palisade cells that  carry out photosynthesis is located below the upper epidermis. Its is followed by the spongy mesophyll layer which has cells with large air spaces between them. On the lower epidermis are tiny openings called stomata. They are surrounded by special cells known as guard cells.

Research work

1.  Using text books and the internet research on the adaptations of the leaf for photosynthesis.

2.  Write down short notes regarding  your findings.

3.  Discuss your findings with your class members

The following are some of the adaptations of the leaf to photosynthesis :

• The leaf blade is broad and flat to provide a large surface area for absorption of sunlight and carbon dioxide.

• Most leaves are thin. This reduces the distance across which carbon dioxide has to diffuse from the stomata to reach the photosynthesising cells. 

• Leaves have vascular bundles that contain structures (xylem and phloem)  which supply the cells with water and mineral salts. The vascular bundles also transport manufactured food to the other parts of the plant.

• The leaf cuticle and epidermis are transparent and thin to allow easy penetration of light.

• Presence of stomata on the leaves allows easy diffusion of carbon dioxide.

• The leaves are well arranged to avoid overlapping and overshadowing. This ensures maximum exposure to light. • The spongy mesophyll layer has cells that are irregular in shape and are loosely arranged hence have large air spaces between them. This allows gases to circulate freely thereby enhancing gaseous exchange between the cells and the air surrounding them.

• Palisade cells are closely packed. Further, they are  elongated and lie at right angles to each other. They also contain many chloroplasts hence absorb maximum sunlight required for photosynthesis.

Self-evaluation Test 7.3

1.  Use the diagram of the cross-section through the leaf to answer the questions that follow.

2. Differentiate between the following.

3.  Explain the role of vascular bundles in photosynthesis.

4.  Describe how each of the following helps a leaf to photosynthesise.

7.4 Importance of photosynthesis

Discussion corner

1. Discuss with a friend the following questions:

2.  Share your findings with the class.

All life on earth depends on photosynthesis directly or indirectly for the following reasons.

a. As a source of energy

We are already aware that plants make their own food through photosynthesis. Animals too depend directly or indirectly on plants for their food. Food contains energy from the sun stored as chemical energy. This energy is necessary for the normal life processes to take place.

b. Provides oxygen in air

Oxygen is a by-product of photosynthesis. This oxygen replaces oxygen in air which is continuously used up by all living things during respiration.

c. Makes carbon available to plants and animals

Carbon is a major and important component of chemicals in cells. During photosynthesis, the carbon part of carbon dioxide from the air is incorporated into synthesised food. This way the carbon is made available to living things.

d. Prevents accumulation of carbon dioxide in the air

Some of the carbon dioxide in the air is used up during photosynthesis. This reduces the amount of carbon dioxide in the atmosphere. A reduced level of carbon dioxide in the atmosphere prevents global warming. Global warming is the increase in global temperature caused by increasing levels of carbon dioxide in the atmosphere among other factors.

e. It is responsible for the energy stored in coal and petroleum

Plants and animals that existed on earth millions of years ago were converted into fossils. The energy they contained, which they obtained as a result of photosynthesis is stored in their fossil. Their fossils contain energy inform of fuels such as petroleum and coal.

Self-evaluation Test 7.4

1.  Describe the role of plants in an ecosystem.

2.  Suggest conservation methods you can initiate in your village to prevent environmental degradation.

7.5  Mineral requirements for plant growth

Plants require essential mineral ions such as magnesium and nitrate ions for proper growth.

Research work

1.  Using soilless cultures, devise experiments to investigate the effect of nitrate and magnesium ion deficiency on plant growth.

2.  Tabulate your results using the following guidelines:

     • For magnesium ions, record number of leaves that are green, green with yellow patches and yellow.

     • For nitrate ions, record the height of plant, stem diameter, size of leaves and number of leaves with yellowish colour.

3.  Account for the observations you make.

     • What are the dangers of applying excess fertilisers to crops?

4.  Present your work to the rest of the class.

a. Magnesium ions

Magnesium ions are required for the synthesis of chlorophyll, the pigment required in photosynthesis. They are also needed for activation of many enzymes involved in the energy transfer processes.
Deficiency of magnesium ions affects plant growth and development as

photosynthesis is directly affected.  Leaves start to change colour from green to yellow. The result of this is less photosynthesis occurs hence poor growth.

(b) Nitrate ions

Nitrate ions are required by plants for protein synthesis. Nitrogen is a constituent of amino acids, proteins, coenzymes, and chlorophyll.

Deficiency of nitrate ions results to poor synthesis of chlorophyll hence leaves become yellowish. This causes less photosynthesis to occur resulting to stunted growth.

Uses and dangers of nitrogen fertilisers

Research Activity

1. Using text books and internet, research on the following:

    (a)  The effects of increased soil acidity due to high nitrate content.

    (b) Meaning of term eutrophication and its effects.

2.  Present your findings to the rest of the class.

Soils naturally contain many nutrients like nitrogen, phosphorous, calcium, and potassium. These nutrients enable plants to grow healthy. When soil nutrients are missing or in short supply, plants suffer from nutrient deficiency. Their growth and development is affected hence the plant cannot function properly to produce food.

In nutrient deficient soils, fertilisers are applied. Fertilisers add plant nutrients such as nitrogen, phosphorus, and potassium that are lacking in the soil. Fertilisers are simply nutrients applied to agricultural fields to supplement required elements missing in the soil. Nitrogen, naturally comes from the atmosphere, and enters the soil through rainfall and lightning. It can also be found in beneficial microorganisms (some types of bacteria), which live in the soil.

It is the misuse and overuse of nitrogen fertilisers that has severely affected the Earth’s natural ecosystem. It has also depleted minerals from the soils.

Dangerous levels of nitrates in the environment; due to overuse of nitrogen fertiliser is linked to diseases such as diabetes, cancer, and Parkinson’s disease.

Consumption of ground water containing high amounts of nitrate may lead to health problems in young children.

Harmful effects of nitrogen fertilisers to the environment

(a)  Nitrogen fertilisers aid plants in their growth.  However, weeds and non-native plants tend to grow more readily with additional nitrogen supplies.

(b)  Excess nitrogen in the soil creates an imbalance of nutrients. This causes a depletion of other important minerals such as calcium, phosphorus and magnesium.

(c)  High amounts of nitrates in the soil results to increased acidity. This affects plants that require alkaline conditions to grow.

(d) Nitrogen-polluted air, caused by nitrates from automobiles and industrial plants, results in acidification of the soil when acid rain falls.

(e)  Surface run-offs containing high amounts of nitrates enter into water bodies resulting to eutrophication. When nitrogen levels in rivers and streams increase, they aid in algae overgrowth. As algae dies and decompose, organic matter in water increases. This process uses up the available oxygen, causing levels to drop. Without oxygen, fish, crabs and other aquatic organisms die.

(f)  Nitrogen is a soluble substance. It soaks deeply into the soil after a rainstorm or irrigation. It may reach ground water and nearby wells hence contaminating them.

My environment, my life!

All of us have a role to play in ensuring that the environment is conducive for humans and other living organisms. This preserves biodiversity for future generations.

Self-evaluation Test 7.5

1.  Describe the symptoms shown by plants due to deficiency of:

2.   A student carried out an investigation to show how magnesium affected the growth of a water plant, Elodea. The student prepared five dishes containing 20 plants of the water plant with different concentration of magnesium salt. After 30 days, the student counted the number of plants that had grown and recorded their appearance. The results are shown in the following table.

(a) Describe the effect of decreasing the concentration of magnesium salt on the growth of Elodea.

(b) How does magnesium deficiency affect the growth and appearance of Elodea?

Unit summary

• Photosynthesis is a process used by plants and other organisms to convert light energy, normally from the sun, into chemical energy that can be later released to fuel the organisms’ activities.

• Plants use sunlight energy, carbon dioxide and water as raw materials to make glucose as product and oxygen as a by-product.

• Chlorophyll is a green pigment, present in all green plants and in cyanobacteria, responsible for the absorption of light to provide energy for photosynthesis. Its molecule contains a magnesium atom.

• The limiting factors of photosynthesis include carbon dioxide concentration, light intensity and temperature.

• Plant leaf is structurally modified to carry out the process of photosynthesis efficiently.

• Photosynthesis is important to organisms as it helps to cycle oxygen and carbon dioxide in the environment.

• For plants to grow healthy they require mineral salts.

• Lack of mineral salts in the soil especially nitrates and magnesium leads to poor plant growth.

• Mineral salts can be replaced in deficient soils through application of fertilisers.

• Overuse of fertilisers have adverse effects to organisms and the environment.