Term Paper on Photosynthesis | Chemical Process | Plants | Biology

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Term Paper # 1. Introduction to Photosynthesis:

Green plants are described as autotrophic (‘auto’ = self, ‘trophic’ = feeding). They use small molecules around them (carbon dioxide from the air, and water from the soil) to build large organic molecules of glucose (a carbohydrate). This process occurs during the chemical reaction known as photosynthesis.

Energy is needed to link the carbon dioxide and water molecules which are used to make carbohydrate. This energy is provided by sunlight. Sunlight is trapped in a plant by chlorophyll. Chlorophyll is a green-coloured chemical that contains magnesium. It is found within the chloroplasts of cells, where photosynthesis takes place.

Through the process of photosynthesis, light energy becomes ‘locked away’ within the carbohydrate molecule as chemical energy.

This process is represented by the following equation:

Photosynthesis is therefore a process in which light energy is converted into chemical energy.

If a living organism breaks down the carbohydrate molecule during a metabolic process, the energy is released. Before being broken down, the molecule is often stored by the organism.

The first carbohydrate made during photosynthesis is glucose. Glucose is a simple sugar and is soluble. It increases the concentration of the cytoplasm in the cell, which slows down the rate at which enzymes in the cell work. Glucose is therefore usually converted to the insoluble carbohydrate, starch.

Starch is first stored in the chloroplasts within the photosynthesising cells. It is then converted to sucrose to be carried to the storage organs of a plant (see the section on translocation below). Here, sucrose is converted back to starch. The tuber of a potato is an example of a plant’s storage organ.

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Term Paper # 2. The Importance of Photosynthesis to the Living Universe:

Almost all forms of life rely on the chemical energy found in carbohydrate, the product of photosynthesis. Plants may convert this carbohydrate into protein or fat before it is passed on.

The oxygen produced by photosynthesis is essential for the respiration of most life forms. Photosynthesis also uses up the carbon dioxide released by respiration, converting it into carbohydrate (see the section on the carbon cycle below).

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Term Paper # 3. Process of Photosynthesis:

When it comes to plants, the process of creating new plants and feeding themselves is very dependent on other organisms. The very creation of a plant itself is dependent on sunshine, water and the nutrients found in the soil. But first, a seed needs to be fertilized. Here is a perfect example of the need to use other organisms to complete this process.

Step One – Fertilization:

A flower will produce either the egg or sperm depending on the sex of the plant. Some plants will produce both, depending on their species. Yet in order to achieve conception, the plant needs to rely on other species to provide the connection between the egg and sperm.

Bugs and insects are often called upon to provide that connection. Bees are known for their critical role in plant fertilization. They often pick up the pollen necessary to fertilize the eggs. As they move from flower to flower, they pick up and deposit pollen dozens of times. To repay that kindness, the flowers produce a nectar that the bees use to feed themselves. Thus, plants play a role in the larger process of biodiversity.

These fertilized eggs are found in the form of seeds, which fall to the ground. Once in the soil, the seeds begin to grow, placing roots and then sprouting leaves and stems. At this point, all the tools for photosynthesis are in place.

Step Two – A Little Water and Sunshine:

The plants collect sunshine by means of their leaves. They use this collected sunshine as the energy to produce sugars, lipids and other proteins. But how does this occur? The first stage involves taking the sunshine and turning it into chemical energy, which is stored in the form of sugar. But how does this occur? The plant takes in sunshine, carbon dioxide and water. These are combined to produce oxygen, water and glucose aka sugar.

The leaves are both the collection point for the materials needed to produce the sugar, oxygen and water, but they are also the point where the results of photosynthesis are released. Stomatas release the oxygen, while gathering the required carbon dioxide.

Here the roots step in and gather the only thing the leaves cannot, which is water. It is piped up through the stem to the leaves, which are the factory where the magic happens. Once the process is complete, the materials are released and then the process begins again.

So what are the parts of the leaves? The chloroplasts are the factory where the process of photosynthesis. Within the chloroplasts, there are several important structures. Chlorophyll is the green pigment that absorbs the necessary sunshine. The stroma is a dense fluid within the chloroplast that is the actual site of the conversion of carbon dioxide to the essential sugar. Grana is a stack of thylakoid sacs, which are flattened and are the site of the conversion of solar energy to chemical energy.

The end result of this process is that organic compounds are created, which are used by both the plant and other life forms.

Light reactions produce the energy and the dark reactions, which occur in the stroma, produce the sugar. The process is necessary to help the plant survive, but to also produce the oxygen necessary for other organisms, such as animals, insects, fish and humans.

It is this essential process that leads us into the larger discussion of the biosphere. From the cellular to the biggest picture of the lives on earth, all these processes show how they are interconnected. One organism cannot survive without the functions provided by other organisms.

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Term Paper # 4. What does a Plant Need to Photosynthesise?

We can use experiments to show that carbon dioxide, light energy and chlorophyll are necessary for the process of photosynthesis. It is difficult to show that water is necessary for photosynthesis, because the removal of water from a plant may have serious effects on other functions of the plant, not just on photosynthesis.

Term Paper # 5. Things to Remember When Performing Experiments:

The following experiments use potted plants; varieties such as Pelargonium, a member of the geranium family, or Coleus may be used, depending on what plants are available.

i. The Importance of a Control:

In the first three experiments given below, a comparison is made between a leaf (or part of a leaf) which is able to function with all the requirements for photosynthesis, and a leaf (or part of one) where one of the requirements is missing. In this way, the results of the experiment are shown to be valid.

The apparatus and materials which provide this comparison make up the control to the experiment. Wherever possible, all biological experiments should have a control.

ii. Experimental Error:

To reduce error, each of the experiments below should be repeated several times, if possible. An average of the results should then be taken.

Experiment 1:

To Show that Carbon Dioxide is Necessary for Photosynthesis:

Apparatus:

i. Two well-watered de-starched potted plants (e.g. Pelargonium or Coleus)

ii. Polythene bag to fit over one of the pots

iii. Cotton (to tie the polythene bag over the pot and around the stem)

iv. A large piece of flat glass

v. Two bell jars

vi. Petroleum jelly (Vaseline)

vii. A small beaker containing concentrated sodium hydroxide (follow the safety precautions when handling)

Before the experiment, the plants should be kept in a dark place (e.g. a cupboard) for 24-48 hours, so that any starch present in their leaves has been used up. This is important because these types of plants immediately turn any glucose they make during photosynthesis into starch, and we will be testing the leaves after the experiment to see if any starch is present. We must therefore make sure there is no starch present before we begin the experiment.

Method:

Set up the experiment as follows:

The two plants are left side-by-side in sunlight. After about eight hours, a leaf is taken from each plant and tested for the presence of starch as shown below.

The Starch Test: To Show the Presence of Starch in a Leaf:

i. Transfer the leaf to a beaker of boiling water for about one minute.

ii. Turn off the Bunsen burner.

iii. Transfer the leaf to a large test-tube which is half-full of methylated spirits (meths).

iv. Place the test-tube (with the leaf in the meths) into the recently-boiled water, and allow the heat of the water to bring the meths to the boil. The chlorophyll is removed from the leaf, but the leaf becomes brittle.

v. Remove the leaf from the meths, and rinse it in the hot water. This will soften the leaf.

vi. Spread the leaf on a white tile and add iodine solution.

vii. If starch is present, the leaf will turn blue/black. If not, it will stain brown.

Results:

If this procedure is carried out on the two leaves from Jar A and Jar B, the results should be as follows:

Leaf from Jar A stains brown. There is no starch present.

Leaf from Jar B stains blue/black. Starch is present.

Conclusion:

Conditions in the two jars were identical, except that only Jar B contained carbon dioxide. Carbon dioxide is necessary for photosynthesis.

Experiment 2:

To Show that Light is Necessary for Photosynthesis:

Apparatus:

i. A well-watered, de-starched, potted plant (e.g. Pelargonium or Coleus)

ii. A cork cut into two pieces

iii. A pin

Method:

The apparatus is set up as shown in Fig. 20.

The experiment is left in sunlight for eight hours.

The cork is removed from the leaf, and the starch test is carried out on the leaf (as shown in experiment 1).

Results:

Where the cork covered the leaf, the leaf stains brown. The rest of the leaf stains blue/black.

Conclusion:

Starch is produced only in areas of the leaf where light is able to reach. Light is necessary for photosynthesis.

Experiment 3:

To Show that Chlorophyll is Necessary for Photosynthesis:

Apparatus:

A potted plant (e.g. Pelargonium or Coleus) that is well-watered, de-starched and variegated

Note:

A ‘variegated’ plant has leaves one part of which are green (where chlorophyll is present) and the rest contains no chlorophyll (and is, therefore, often white).

Method:

Leave the plant in sunlight for eight hours. Remove one leaf and carry out the starch test (as shown in experiment 1).

Results:

Conclusion:

Starch is made only in areas of the leaf where chlorophyll is present. Chlorophyll is necessary for photosynthesis.

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Term Paper # 6. Waste Product of Photosynthesis:

After carrying out the three experiments described above, we can deduce that carbohydrate (starch) is produced when a plant has access to carbon dioxide, sunlight and chlorophyll.

However, there is also a waste product of the process. This can be shown if a water plant is allowed to photosynthesise in the laboratory. Water plants such as Elodea or Hydrilla may be used for these investigations.

Experiment 4:

To Show that Oxygen is given Off during Photosynthesis:

Apparatus:

i. Large beaker

ii. Short-stemmed funnel

iii. Two thick coins (to act as funnel supports)

iv. Sodium hydrogen-carbonate powder (to supply carbon dioxide to the plant)

v. Test-tube

vi. Water plant (e.g. Elodea or Hydrilla)

Method:

Set up the apparatus as shown in the diagram below. Leave it for two days in a place where it will receive sunlight.

Results:

A gas collects at the top of the test tube which is found to re-light a glowing splint.

Conclusion:

Only oxygen has the power to re-light a glowing splint, so oxygen has been released during photosynthesis.

The Control for this experiment is to set up the same experiment in a dark cupboard, for the same length of time. No gas collects in the test tube.

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Term Paper # 7. The Rate of Photosynthesis:

We can now look at a plant’s rate of photosynthesis, that is, how quickly it photosynthesises. This rate changes if we vary the levels of certain elements-light, carbon dioxide and temperature. We can demonstrate this in experiments by measuring the release of oxygen by a water plant.

Experiment 5:

To show the effect of varying light intensity on the rate of photosynthesis:

Apparatus:

i. A large test-tube

ii. A bench lamp

iii. A length of stem from a water plant (e.g. Elodea or Hydrilla)

iv. A timer

Method:

The apparatus is set up as shown in Fig. 22. It is better to carry out this experiment in a darkened room so that the only source of light reaching the plant is the bench lamp.

As the water plant photosynthesises, bubbles are released from the cut end of the stern. Place the bench lamp at position A. Leave the plant for about 10 minutes – this allows the plant time to adjust to the conditions. Count the number of bubbles released by the plant for a measured period of time (e.g. 3 minutes). Record the results.

Move the lamp to position B. Again leave the plant for 10 minutes, then count the bubbles released by the plant for the same length of time and record the results.

Move the lamp to position C, and repeat the procedure.

Results:

As the lamp is moved further away from the plant, the intensity of light reaching the plant decreases. As the light intensity decreases, so does the number of bubbles released by the cut stem over the same period of time.

Conclusion:

The rate of photosynthesis decreases with decreased light intensity.

Experiment 6:

To Show the Effect of Varying the Carbon Dioxide Concentration on the Rate of Photosynthesis:

Apparatus:

i. A large test-tube

ii. A bench lamp

iii. A length of stem from a water plant (e.g. Elodea or Hydrilla)

iv. A timer

v. Sodium hydrogen-carbonate powder

Method:

When sodium hydrogen-carbonate dissolves in water, it supplies the water with carbon dioxide:

NaHCO₃ → NaOH + CO₂

Immerse the length of stem in a test-tube filled with tap water. The water should be at room temperature. Leave it for about 20 minutes so the plant stem can adjust to the conditions. Place a bench lamp about 25 cm away and direct it at the plant. Count the number of bubbles released by the stem over a period of 3 minutes, and records the results.

Carefully weigh out 0.25 g of sodium hydrogen-carbonate and add it to the water in the test-tube. Gently shake the tube to dissolve the powder, and then leave for 20 minutes. Again, place the bench lamp about 25 cm away from the plant, and count the number of bubbles released over 3 minutes. Repeat this process twice, adding a further 0.25 g of hydrogen-carbonate each time.

Results:

More carbon dioxide becomes available to the plant as more sodium hydrogen-carbonate is added to the water. As the amount of carbon dioxide increases, so does the number of bubbles released by the stem, measured over the same period of time.

Conclusion:

As the amount of carbon dioxide available to the plant increases, so does the amount of carbon dioxide released from it. The increased amounts of available carbon dioxide therefore increase the rate of photosynthesis.

Experiment 7:

To Show the Effect of Varying the Temperature on the Rate of Photosynthesis:

Apparatus:

i. A large test-tube

ii. A bench lamp

iii. A length of stem from a water plant (e.g. Elodea or Hydrilla)

iv. A large beaker

v. A timer

vi. A thermometer

Method:

Set up the apparatus as in Investigation 6.6, but with the test-tube in a large beaker of water (a water bath). The temperature of the water bath is the only factor in this experiment that is altered. Lower the temperature of the water bath by adding ice, and then leave the plant for 20-30 minutes to adjust to the conditions. Count the number of bubbles released by the cut stem over a period of 3 minutes and records the results.

Raise the temperature of the water bath above room temperature by adding warm water. Again, give the plant time to adjust (20-30 minutes). Check the thermometer regularly to make sure the temperature is maintained, adding warm water if necessary. Count the number of bubbles released over 3 minutes and records the results.

Results:

The warmer the temperature (up to around 45°C) the more bubbles are released over the same period of time.

Conclusion:

The warmer the temperature (up to about 45°) the faster the rate of photosynthesis.

Limiting Factors:

The availability of light, carbon dioxide, water and a suitable temperature all affect the rate of photosynthesis. However, the rate of photosynthesis in a plant depends on which one of these factors is in shortest supply. That particular factor will limit the rate of photosynthesis, even when all others may be ideal (or ‘optimum’). Light, carbon dioxide, water and temperature are therefore limiting factors.

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Term Paper # 8. How Plants Obtain Raw Materials?

Roots:

Roots absorb the water necessary for photosynthesis from the soil. Water is carried from the soil to the leaves.

A few millimetres behind the tip of every root, and extending also for a few millimetres, lies the region of root hairs. The many root hair cells provide a very large surface area for the uptake of water (and of ions) from the soil.

Leaves:

In most plants, the leaves:

(i) Absorb carbon dioxide from the air

(ii) Absorb sunlight energy

(iii) Manufacture carbohydrate

(iv) Release the waste product, oxygen.

Most of a plant’s photosynthesis takes place in the leaves. Leaves are organs containing several different tissues. Cells within these tissues are adapted to perform a particular function as efficiently as possible.

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Term Paper # 9. Leaf Structure:

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Term Paper # 10. How a Leaf is Adapted for Photosynthesis?

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Term Paper # 11. How a Leaf is Involved in the Process of Photosynthesis?  

1. Carbon dioxide diffuses down a concentration gradient from the atmosphere, through the stomata, into the leaf.

2. Carbon dioxide diffuses freely throughout the leaf in the intercellular spaces.

3. Carbon dioxide dissolves in the film of water which surrounds the mesophyll cells. The water is delivered to the leaf in the xylem of the vascular bundles.

4. Carbon dioxide diffuses in solution into the mesophyll cells and passes to the chloroplasts, where photosynthesis occurs.

5. Sugar made by photosynthesis is carried away (‘translocated’) from the leaf in the phloem of the vascular bundles.

6. Oxygen diffuses from the mesophyll cells into the intercellular spaces and out through the stomata, down a concentration gradient into the atmosphere.