6.1 Photosynthesis

Takes place in the leaves of the plant. Water is absorbed by the root hair cells from the soil and brought up to the leaves by the xylem vessels. Carbon dioxide is diffused into the stomata of the leaves. Light energy from the sun is absorbed by the chlorophyll and converted into chemical energy.
Photosynthesis occurs in 2 stages - the dependent stage and the light dependent stage.
In the light-dependent stage, chemical energy from the sun is used to break down water into hydrogen and oxygen [photolysis of water]. Oxygen gases escape from the stomata of the leaves, whereas the hydrogen molecules are used to reduce carbon dioxide to carbohydrate[glucose]. This process of reduction does not require direct energy from the sun, hence, it is known as the light-independent stage. Enzymes play a part in this stage

2. State that chlorophyll is a green pigment that is found in chloroplasts

3. State that chlorophyll transfers light energy into chemical energy for the formation of glucose and other carbohydrates

4. Outline the subsequent use and storage of the carbohydrates made in photosynthesis, limited to:

(a) Starch as an energy store

During the day, the rate of photosynthesis is so great that more glucose is produced than it can be used up by the plant. Hence, it is converted into starch. At night, when the plant is no longer photosynthesising, starch is converted back to glucose to provide energy for the plant. Glucose is stored in the form of starch because if it were stored in the form of glucose, excess glucose would affect the osmotic balance between the cells. Starch is an insoluble compound, hence, it does not alter the concentration of the cell contents

(b) Cellulose to build cell walls

Cellulose molecules are long chains of glucose and make a tough meshwork outside the cell. The cellulose cell wall is freely permeable

(c) Glucose is used in respiration to provide energy for cellular activities.

(d) Sucrose for transport through the plant

Excess glucose is converted into sucrose, which is then transported throughout the plant by the phloem vessels to areas like underground storage organs, growing buds and roots.

5. State the word equation and balanced chemical equation for photosynthesis

6. Investigate the need for chlorophyll, lightand carbon dioxide for photosynthesis, using appropriate controls

Test for starch

A leaf is dropped in boiling water to kill and break down the cell walls
The leaf is left for 5-10 minutes in hot ethanol in a boiling tube. This removes the chlorophyll so that colour changes from iodine can be seen more clearly.
The leaf is dipped in boiling water to soften it.
The leaf is spread out on a white tile and covered with iodine solution.
In a green leaf, the entire leaf will turn blue-black as photosynthesis is occurring in all areas of the leaf.
This method can also be used to test whether chlorophyll is needed for photosynthesis by using a variegated leaf (one that is partially green and partially white).
The white areas of the leaf contain no chlorophyll when the leaf is tested, only the areas that contain chlorophyll stain blue-black

Test for light

Before starting the experiment, the plant needs to be destarched by placing it in a dark cupboard for 24 hours.
This ensures that any starch already present in the leaves will be used up and will not affect the results of the experiment.
Following destarching, a leaf of the plant can be partially covered with aluminium foil and the plant placed in sunlight for a day.
The leaf can then be removed and tested for starch using iodine.
The area of the leaf that was covered with aluminium foil will remain orange-brown as it did not receive any sunlight and could not photosynthesise, while the area exposed to sunlight will turn black. This proves that light is necessary for photosynthesis and the production of starch

Test for carbon dioxide

Destarch a plant. A clear bag containing sodium hydroxide, which will absorb carbon dioxide from the surrounding air, around one leaf.
A clear bag containing water(control experiment), which will not absorb carbon dioxide from the surrounding air, is placed around another leaf.
Place the plant in bright light for several hours. Test both leaves for starch using iodine.
The leaf from the bag containing sodium hydroxide will remain orange brown as it cannot photosynthesise due to a lack of carbon dioxide.
The leaf from the control bag containing water should turn black as it had all requirements for photosynthesis

7. Describe and explain the effect of varying light intensity, carbon dioxide concentration and temperature on the rate of photosynthesis

Light intensity

Effect: Light intensity directly affects the rate of photosynthesis because it provides the energy needed to drive the process. As light intensity increases, so does the rate of photosynthesis, up to a certain point.
Explanation: Initially, when light intensity is low, the rate of photosynthesis is limited by the availability of light energy. As light intensity increases, more photons are available for the light-dependent reactions of photosynthesis, leading to an increase in the rate of the light-dependent reactions, which in turn drives the overall rate of photosynthesis. However, once a certain threshold is reached, the rate of photosynthesis plateaus because other factors, such as carbon dioxide concentration or temperature, become limiting.

Carbon dioxide concentration

Carbon dioxide concentration also directly affects the rate of photosynthesis. An increase in carbon dioxide concentration generally leads to an increase in the rate of photosynthesis until another factor becomes limiting.

Temperature

As temperature increases, it leads to more frequent collisions between enzymes and substrates, thus an increase in the rate of photosynthesis. However, beyond the optimal temperature range, enzymes may denature, leading to a decrease in photosynthetic activity. Extremely high temperatures can lead to stomatal closure, limiting the availability of carbon dioxide and inhibiting photosynthesis.

8. Investigate the effect of varying light intensity, carbon dioxide concentration and temperature on the rate of photosynthesis using submerged aquatic plants and hydrogencarbonate indicator solution

The plants usually used are Elodea or Cabomba, types of pondweed.
As photosynthesis occurs, oxygen gas is released.
As the plant is in water, the oxygen released can be seen as bubbles leaving the end of the pond weed.
The number of bubbles produced over a minute can be counted to record the rate.
The more bubbles produced per minute, the faster the rate of photosynthesis.
A more accurate version of this experiment is to collect the oxygen released in a test tube inverted over the top of the pondweed over a longer period and then measure the volume of oxygen collected.
This practical can be used in the following ways

9. Identify and explain the limiting factors of photosynthesis in different environmental conditions

A limiting factor is something present in the environment in such short supply that it restricts life processes. There are three main factors which limit the rate of photosynthesis:

Temperature

As temperature increases, the rate of photosynthesis increases as the reaction is controlled by enzymes. However, as the reaction is controlled by enzymes, this trend only continues up to a certain temperature, beyond which the enzymes begin to denature and the rate of reaction decreases

Light intensity

The more light a plant receives, the faster the rate of photosynthesis. This trend will continue until some other factor required for photosynthesis prevents the rate from increasing further because it is now in short supply

Carbon dioxide concentration

Carbon dioxide is one of the raw materials required for photosynthesis. This means the more carbon dioxide that is present, the faster the reaction can occur. This trend will continue until some other factor required for photosynthesis prevents the rate from increasing further because it is now in short supply

Credits: Notes compiled by Manahil Naeem of Karachi Grammar School

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