A brief history on photosynthesis:

At first all plant needs were thought to come from the soil and nothing from the atmosphere. It was in1667 that a Belgian scientist Jean Baptista van Helmont started with 100kg of soil in which he planted a 2kg branch of a tree.

He watered the soil for five years till the branch grew into a small tree weighing 85kg. When he weighed the soil it had only lost less than 0.5kg. So van Helmont concluded that the extra weight did not come from the soil alone but also from the water that was added.

Little was known that even gases in the atmosphere contribute to plant protoplasm till 1772 when an English chemist Joseph Priestley performed an experiment to show so.

When he placed a burning candle under a jar, it soon went off meaning that it had used up all oxygen in the jar and had released a lot of carbon dioxide.

When however, a candle was burned under a jar with a potted plant beside it, the candle continued burning for as long as there was sunlight. Priestley concluded that the green plant was using up carbon dioxide from the burning candle and releasing oxygen that supports burning.

That green plants produce oxygen was first performed by Theodor Engelman, a German, in 1880. He placed a filament of green algae in a drop containing large numbers of aerobic bacteria which require oxygen. When he illuminated the algal filament with sunlight, the aerobic bacteria distributed themselves evenly along the filament meaning that it was giving out oxygen.

When he illuminated the filament with white light that had been split into a spectrum by a prism, the aerobic bacteria clustered themselves around the red band and some around the blue band.

So Engelman concluded that red light stimulated photosynthesis most followed by blue light.

Light can only be effective in stimulating photosynthesis if chlorophyll absorbs it. The type of chlorophyll that absorbs red light is called chlorophyll a and that which absorbs blue light is called chlorophyll b

The chemical structure of chlorophyll shows that it has a magnesium atom in its molecule (C₅₄H₃₃O₅N₄Mg).

So lack of magnesium in the soil may lead to the plants whose leaves lack chlorophyll (yellow leaves) and that situation is called chlorosis.

Definition of Photosynthesis:

This is the process by which green plants make carbohydrates from carbon dioxide and water using sunlight energy which is absorbed by chlorophyll and oxygen is given off as a biproduct.

The water is got by land plants from the soil by the root system and carbon dioxide is got from the atmosphere through the stomata in the leaves.

Photosynthesis goes on in the leaves but any green part of a plant may photosynthesize.

The process is often summarised by this equation:

6CO₂ + 6H₂O + sunlight energy absorbed by chlorophyll à C₆H₁₂O₆ + 6O_2.

But it must be realised that this equation shows only the beginning and end of a very complex chain of chemical reactions involving many enzymes.

It indicates correctly what raw materials enter the process (carbon dioxide and water) the conditions for the process (sunlight and chlorophyll) and what finished products come out (carbohydrate and oxygen) but it does not explain what happens in between.

The Chemical Reaction

The overall chemical reaction for photosynthesis is 6 molecules of carbon dioxide (CO₂) and 6 molecules of water (H₂0), with the addition of solar energy. This produces 1 molecule of glucose (C₆H₁₂O₆) and 6 molecules of oxygen (O₂) (Figure 4.8 ). Using chemical symbols the equation is represented as follows:

6CO₂ + 6H₂O              C₆H₁₂O₆+ 6O₂.

 The two phases of photosynthesis:

If chlorophyll from a green plant is given water without carbon dioxide and then shone with light, bubbles of oxygen will come out and this is in the absence of carbon dioxide. If the light is switched off, oxygen production ceases.

But we know oxygen is produced in the process of photosynthesis when carbon dioxide and water combine.

There are two logical conclusions:

-  Since there was complete absence of carbon dioxide the bubbles of oxygen must have come from water. (The important thing is that water is split up)

-  There must be two phases in photosynthesis; one which requires sunlight but not carbon dioxide and the other which requires carbon dioxide but not sunlight.

Thus photosynthesis occurs in two steps:

The first step requires light and is called a light reaction or the photophase.

The second step does not require light and so it is called the dark reaction or the synthesis phase for manufacture of carbohydrate.

So photosynthesis can be considered to be of phases: photo and synthesis.

* In the photophase, light is absorbed by chlorophyll and the energy is used to split the water (H₂O) molecules to H₂ and O₂

            2H₂O + sunlight  à  2H₂ + O_2.

The oxygen is given off as waste product but the H₂ is sent to the next phase.

* In the synthesis phase, the H₂ from photophase converts inorganic CO₂ from the atmosphere to organic carbohydrate containing C, H and O.

Light plays no role in the synthesis phase of photosynthesis.

To show that all oxygen O₂ given off during photosynthesis comes from water, an isotope of oxygen of mass number 18 is used in the water but not in the carbon dioxide, and this isotopic oxygen will appear in the oxygen given out.

So oxygen produced during photosynthesis results from splitting of water alone during the photophase.

The epidermis:

This is a layer of cells fitting together with no air spaces between them. There are two epidermal layers. The one on the top surface is called the upper epidermis and the one on the lower surface is called the lower epidermis. The epidermis secretes a waxy layer called the cuticle which reduces evaporation.

So the epidermis:

a) maintains leaf shape;

b) protects the inner cells from bacteria, fungi and mechanical damage; and

c) reduces evaporation.

The epidermal cells, except the guard cells of the lower epidermis do not contain any chloroplasts; so they do not photosynthesize. They are also transparent so that sunlight can pass through the epidermis to the inner cells which have chloroplasts.

 

 

The palisade layer:

This is one or more rows of a tall, cylindrical cells with air spaces between them. In the palisade cells, most photosynthesis occurs.

Adaptations of the palisade cells for photosynthesis:

-  They lie just below the epidermis so they receive most sunlight.

-  They have many chloroplasts.

- They are elongated resulting in less sunlight being absorbed by the horizontal cross walls before it reaches the chloroplasts.

-  The chloroplasts can move up down according to light intensity.

-  They have air spaces for supply of carbon dioxide.

-  The chloroplasts are arranged along the side walls not far from the carbon dioxide supplies in the air spaces.

Chloroplasts:

These are made of proteins and they contain chlorophyll, a green pigment which absorbs light for photosynthesis.

FIGURE 4.6: The chloroplast is the photosynthesis factory of the plant.

The spongy layer:

The cells in this layer do not fit closely together and large air spaces are left between them. The air spaces communicate with each other and with the atmosphere through the stomata. So they allow free circulation of air to all internal cells of the leaf.

The spongy cells can photosynthesize but at a lower rate than the palisade cells. This is because the spongy cells receive less light than the palisade cells and also they contain fewer chloroplasts.

The palisade layer and the spongy layer are together called the mesophyll.

The stomata:

These are openings in the epidermis. They are more abundant in the lower epidermis than in the upper. Each stoma is formed between two guard cells which control the opening and closure of the stomata. Stomata normally open when there is light.

Guard cells:

These have got chloroplasts so they can photosynthesize.

Their cell walls are of uneven thickness; the outer wall is thicker than the inner wall.

They control the opening and closing of the stomata.

Process of stomatal opening:

N.B. Rapid use of carbon dioxide during light leads to a high pH which favours conversion of starch to sugars. This could also lead to a decrease in the osmotic potential of guard cells so that they absorb water and swell to finally open the stomata.

Process of stomatal closure:

 The vascular bundles:

These have phloem tissue for conducting manufactured food away from the leaves to the storage organs.

They also have xylem tissue for conducting water from the soil in the leaf. Water passes from the nearest vein to the palisade cells by osmosis.

Adaptation of a leaf for photosynthesis:

• They have a broad surface area for absorption of carbon dioxide and sunlight
• They have a thin structure for easy carbon dioxide diffusion and sunlight penetration.
• They have intercellular spaces for carbon dioxide diffusion.
• They have stomata which allow gaseous exchange with the atmosphere.
• They have palisade cells with chloroplasts near sunlight.
• They have a network of veins which contain the xylem for water supply and the phloem for food manufactured by photosynthesis to other parts of the plant.
• They have a permeable cuticle on the epidermis for carbon dioxide absorption.
• They have a transparent epidermis to allow in sunlight.

Photosynthesis in a palisade cell:

In the palisade cell water enters by osmosis from the nearest vein and carbon dioxide diffuses from the air spaces.

Oxygen is released and it diffuses to the air spaces and out through the stomata to the atmosphere.

During rapid photosynthesis, sugars like sucrose are the immediate product of photosynthesis. They are however immediately changed to starch which is osmotically inactive. This is advantageous in that it prevents disturbances that would result if sugars accumulated in the leaf.

So the presence of starch in a leaf is proof of on-going photosynthesis.

In darkness, all starch is converted to sugars and is removed.

So if a leaf is kept in darkness for 24hours, if will be destarched. Also leaves are destarched early in the morning after a dark night.

When starch is converted by enzymes to soluble sugars, they will be carried off in the phloem tubes.

These manufactured sugars may be:

But some plants like monocotyledons only form sugars in their leaves and never starch.

Gaseous exchange and compensation point:

Generally, photosynthesis is the reverse respiration.

In the green plants and in all living things respiration goes on all the time. But photosynthesis takes place only in daylight.

So only respiration occurs in darkness but in daylight both photosynthesis and respiration occur.

In darkness only respiration can take place and no photosynthesis goes on.

So the net gaseous exchange with the atmosphere will be such that oxygen will leave the atmosphere and enter the leaf and carbon dioxide will leave the leaf to the atmosphere.

In bright light the rate of photosynthesis greatly exceeds the rate of respiration.

So the carbon dioxide produced by respiration can not be enough for photosynthesis and the oxygen produced by photosynthesis is more than enough for respiration.

So the net gaseous exchange is such that carbon dioxide will enter the leaf from the atmosphere to supplement for photosynthesis and the excess oxygen will leave the leaf to the atmosphere.

In dim light like at dusk and at dawn, the rate of respiration (carbohydrate break down) is equal to the rate of photosynthesis (carbohydrates build up).

So the carbon dioxide produced during respiration is just enough for photosynthesis and the oxygen produced by photosynthesis is just enough for respiration. So there is no net gaseous exchange with the atmosphere. Such a point is called the compensation point.

EXPERIMENTS ON PHOTOSYNTHESIS:

Destarching: This is the removal of starch from a plant's leaves before experiments are carried out.

Since the presence of starch is regarded as a proof of photosynthesis, it is important that the experimental plants should have no starch in them at the beginning of the experiment.

Methods of destarching:

-  If they are potted plants, they are destarched by leaving them in a dark cupboard for two days.

-  If it is a big plant like a mango tree, the experiment is carried out early in the morning after being setup the day before since during the night most of the starch will removed from the leaves.

-  Preferably, the selected leaves are destarched by wrapping them in aluminum foil for two days. One of the leaves should be tested before the experiment to ensure that no starch is present.

 

Testing a leaf for starch:

Materials required: Cupboard, water, heat, alcohol, beaker, test tube, white tile, iodine solution.

Method:

-  The leaf is detached and dipped in boiling water for a minute. This kills the protoplasm by destroying the enzymes in it and so prevents any further chemical changes. It also makes the cell more permeable to iodine solution by bursting the starch grains.

-  The leaf is then boiled in methylated spirit using a water bath until the chlorophyll is dissolved out. This leaves a white colour on the leaf so that any colour changes caused by interaction of starch and iodine are much easier to see.

-  The leaf is then spread flat on a white surface such as a glazed tile.

-  Iodine solution is placed on the leaf.

-  Excess iodine is rinsed off the leaf by using water from a wash bottle.

Observation: Parts of the leaf turn dark blue; others are stained brown by iodine.

Conclusion: The parts that turn dark blue have starch present in them and the parts that are stained brown have no starch in them.

 

To show that chlorophyll is necessary for photosynthesis:

Materials required: A plant with variegated leaves since it is impossible to remove chlorophyll from a leaf without killing it, a cupboard and all materials used when testing for starch.

A variegated leaf has chlorophyll in some patches only.

Method:

*  Destarch the plant and then expose the leaf to be tested to daylight for three hours.

Detach the leaf and test it for starch.

Observation: Only the parts that were previously green turn dark blue with iodine. The parts that were without chlorophyll are stained brown.

Conclusion: Since starch is present only in the parts that were green it is obvious that photosynthesis occurs in the presence of chlorophyll.

Experiment to show that light is necessary for photosynthesis:

Materials required: Cupboard, razor blade, aluminium foil, strings, polythene paper and materials used to test for starch.

Method:

-  Destarch a leaf.

-  Cut a simple shape out from a piece of aluminium foil to make an L shaped stencil.

-  Attach the stencil to the destarched leaf.

-  Leave in day light for four hours.

-  Detach the leaf and test it for starch.

-  Make a control to the experiment using polythene paper for the stencil.

 

Observation: Only the region which received light turned blue with iodine and in the control the whole leaf turns blue with iodine.

Conclusion: As starch has not accumulated in the areas without light and yet the whole leaf in the control turned blue it can be concluded that light is necessary for photosynthesis.

 

Experiment to show that carbon dioxide is necessary for photosynthesis:

Materials required: Cupboard, soda lime, potted plants, sodium bicarbonate, polythene bags, strings, materials for testing for starch.

Method:

*  Destarch two potted plants and after watering, cover with polythene bags.

*  Include soda lime in one to absorb all carbon dioxide and sodium bicarbonate in the other to produce more carbon dioxide.

*  Tie tightly using the strings.

*  Place the plants in sunlight for four hours.

*  Detach some leaves from the plants and test them for starch.

Observation: The leaf from the plant derived of carbon dioxide is stained brown with iodine and the one from the plant provided with carbon dioxide turns dark blue with iodine.

Conclusion: Carbon dioxide is necessary for photosynthesis.

Experiment to show that oxygen is given out during photosynthesis:

Materials required: Pond weed, beaker, pond water, funnel, test tube, small blocks, glowing splint.

Method:

- Place a short-stemmed funnel over pond weed in a beaker of water.

-  Invert a test tube filled with water over the funnel stem.

-  Use the small blocks to raise the funnel above the bottom of the beaker to allow free circulation of water.

-  Place the apparatus in sunlight for thirty minutes.

-  When sufficient gas has collected in the test tube, remove the test tube and test the gas by inserting a glowing splint.

-  Set up a control experiment in a similar way but place in a dark cupboard.

Observation:  Bubbles of gas rise and collect in the test tube and on testing using a glowing splint it burns into flame.

Conclusion: Oxygen is given out during photosynthesis.

N.B. Sodium bicarbonate could be used instead of water so that it adds carbon dioxide to the plant.

PROTEIN SYNTHESIS AND MINERAL SALTS:

From the sun's energy carbohydrates are made and these may combine with other carbohydrates to form cellulose walls or they may combine with nitrogen and sulphur to form proteins.

There are still other elements required by plants for various activities.

Magnesium is a component of chlorophyll, phosphorous is for enzyme systems and root growth; calcium forms the middle lamella; potassium controls rates of photosynthesis and respiration and also helps in leaf growth; and iron is important in chlorophyll formation.

These elements are important to the plants and are called essential elements or macro elements.

Other elements not needed in large quantities like copper; manganese, boron, etc are called trace elements or micro-elements.

These elements are got in the form of mineral salts.

Examples of the mineral salts include.

*  Potassium nitrate provides potassium and nitrogen.

* Magnesium sulphate provides magnesium and sulphur.

* Calcium nitrate provides calcium and nitrogen; and

*  ferric (iron) chloride provides iron.

Water culture solutions:

Since plants need these salts then it is possible to make plants grows in an aqueous solution of these salts. Such a solution is called a water culture solution.

To make a culture solution;

- dissolve 2g of calcium nitrate;

- 0.5 of each of the salts KNO₃, MgSO₄, and K₂PO₄

- the add a pinch of FeCl₂ and

- dissolve all these in two litres of distilled water.

- Put the germinated seedlings in cotton wool and put in the beaker at the solution level.

This is now a complete solution.

                                               

To make a control lacking sulphur, use MgCl₂ instead of MgSO₄.

To make a control lacking calcium, use KNO₃ instead of Ca(NO3)₂.

To make a control lacking magnesium, use K₂SO₄ instead of  MgSO₄.

To make a control  lacking nitrogen, use chlorides instead of nitrates.

To make a control lacking potassium, use calcium salts instead of potassium salts.

To make a control lacking iron, omit ferric chloride altogether.

To make a control lacking every essential element, use distilled water.

After three weeks, count and measure the size of leaves, stem and roots.

Observation:

Plants that grew in distilled water are stunted and weak while those in the complete culture are healthy and strong.

Those that lacked magnesium have yellow leaves (chlorosis); plants without nitrogen have few and short leaves; those without phosphorus have few and short roots; and those without potassium have short stems.

N.B. Dry weight gives better results.

 

Precautions to take:

• Exclude light from the culture solutions in order to prevent growth of algae which could affect mineral content.
• Use seeds with very limited supply of food in the cotyledons or endosperms for example sorghum. This is because the food reserves are exhausted quickly so the plant depends on the culture solutions immediately.
• Keep topping up with water.

N.B. Nitrogen of the atmosphere cannot be utilised so all nitrogen comes from the water cultures.

Lesson Summary

 Review Questions

 Recall

1. What are the reactants required for photosynthesis?

2. What are the products of photosynthesis?

Apply Concepts

3. What happens to the glucose produced from photosynthesis?

4. Why is it important to animals that oxygen is released during photosynthesis?

5. Describe the structures of the chloroplast where photosynthesis takes place.