Photosynthesis
· It is the process by which green plants manufacture their own food
· They trap sunlight using chlorophyll and convert it to glucose using water & oxygen
· Oxygen is released as a by product
Equation
Chlorophyll
Carbon dioxide + water Carbohydrates + Oxygen
Sunlight
Raw materials of Photosynthesis
1. Carbon dioxide
2. Water
Products of Photosynthesis
1. Carbohydrates
2. Oxygen
Conditions necessary
1. Chlorophyll
2. Sunlight
Sources of raw materials for photosynthesis
Water – it is sucked from the soil by the roots (osmosis) & transported up the stem to the leaf where it is used
Carbon dioxide– it moves into the leaf from the air by the process of diffusion through the stomata (tinny holes under the leaf)
The Process:- How photosynthesis occurs
Ø Carbon dioxide and water enter leaf cells
Ø Chloroplasts in the chlorophyll traps sunlight energy.
Ø The light energy is s used to split water into hydrogen and oxygen.
Ø The oxygen escapes from the leaf to the atmosphere as a by-product.
Ø The hydrogen reacts with carbon dioxide to form carbohydrates.
Structure of the leaf
1. Cuticle - It is a thin waxy layer at the top of leaf which results in evaporation
2. Upper epidermis - It is a layer of cells which protects the leaf
3. Palisade mesophyll layer - These cells contain most of the chloroplasts & carries out most of the photosynthesis
4. Spongy mesophyll layer - It is a layer of cells with spaces in between them which allows diffusion of carbon dioxide & water. The layer carries out photosynthesis &stores nutrients
5. Vascular bundle - It contains xylem vessels which carry water and phloem vessels which carry nutrients to plant cells for respiration and to seeds and other storage organs.
6. Lower epidermis - It is like the upper epidermis but it contains small holes called stomata. These holes control the movement of carbon dioxide into the leaf and oxygen out of leaf. Special cells called guards cells controls the opening and closing of the stoma allowing the gaseous exchange.
Factors needed for photosynthesis
1. Carbon dioxide
2. Water
3. Sunlight
4. Chlorophyll
Factors affecting the rate of photosynthesis
There are 4 factors affecting the rate of photosynthesis,
1. Amount of carbon dioxide – increased carbon dioxide increases rate of glucose production which speeds up photosynthesis
2. Light intensity- light energy is trapped by chlorophyll to split water molecules & therefore increasing light energy will speed up the rate of photosynthesis
3. Temperature – temperature affect rates of chemical reactions. Low temp makes enzymes inactive & slows down photosynthesis while very high temp will kill the enzymes stopping photosynthesis
4. Amount of water available – adequate amounts of water are needed & any shortage will slow down photosynthesis
Adaptations of leaves to photosynthesis
1. Most leaves are broad to maximise the area which absorbs sunlight.
2. The leaves are thin to allow the penetration of sunlight and easy diffusion of carbon dioxide.
3. The palisade cells contain many chloroplasts closely packed at the upper surface of the leaf so that they absorb sunlight more efficiently
4. Spongy cells have spaces between them & the lower epidermis has holes which allow easy gaseous exchange. (gaseous exchange refers to the entry of carbon dioxide from the leaf and the exit of oxygen from the leaf)
5. The leaves have an extensive network of veins to supply water to the leaf
Importance of photosynthesis
1. It produces carbohydrates which are food for animals. The plants themselves also manufacture their own food. Therefore, plants are called Producers when looking at food chains and food webs.
2. Photosynthesis produces oxygen which is used by plants and animals during respiration.
3. Plants use carbon dioxide during photosynthesis which helps to reduce global warming.
4. Photosynthesis converts light energy into chemical energy. This chemical energy can be used for many processes in our everyday life e.g. we burn firewood to get heat
What happens to the products of photosynthesis.
What happens to oxygen produced
1. Most of the oxygen diffuses into the air where it is used by animals in respiration.
2. Some of the oxygen is used by plants during respiration.
What happens to the carbohydrates.
1. Some of the carbohydrates produced are used by the plant during respiration
2. The balance is stored by the plant in seeds, fruits, bulbs or tubers.
Experiments
Experiment 1: Testing a leaf for starch
Materials.
A green leaf from a potted plant which was kept in the dark 3 days, a green leaf of a potted plant which was in the sunlight, a test tube, a beaker, a burner, a stand, water, methylated spirit, iodine solution, a dropper, & a white tile.
Process diagram.
N.B: the methylated spirit is heated in a water bath because it is highly flammable (can easily catch fire easily)
Method
1. Boil the leaf in water to destroy the enzymes in the leaf, to prevent any chemical reactions
2. Boil the leaf in alcohol to remove chlorophyll so that results are easy to see
3. Dip the leaf in hot water to soften it as boiling it in alcohol makes brittle
4. Place the leaf on a white tile & use a dropper to put the iodine solution on the leaf, iodine solution is used to test for starch & changes colour from brown to blue-black if starch is present
Observation
1. For the leaf that was kept in the dark for 3 days the iodine solution did not change colour, this means there was no starch. Putting a plant in the dark is called de-starching because darkness stops all photosynthesis & all the starch that had been manufactured is used up
2. For the plant that was in the sunlight the iodine changed colour from brown to blue black meaning starch was present in the leaf. In science we say the test was positive
Experiment 2: to test if carbon dioxide is necessary for photosynthesis
Materials
1. 2 similar potted plants
2. 10cm3 of soda lime
3. 10cm3 of sodium hydroxide solution
4. 2 bell jars
5. Starch testing kit
Method
1. 2 similar potted plants are kept in the dark for 3-4 days in order to de-starch them. The starch from their leaves is used up when plant respires in the dark & is not replaced as no photosynthesis can take place without light
2. Before the experiment ensure that the plants are completely de-starched by testing for starch
3. Set up the apparatus as shown in the diagram below. Place soda lime and sodium hydroxide as shown by the diagram as these remove co2 from the air
4. Expose both plants to sunlight for 6 hrs & then test a leaf for starch from each plant
Diagram
Observations & Conclusions
1. When tested for starch the leaf from jar B turned blue – black to show the presence of starch this is because photosynthesis was taking place as all necessary conditions were present, jar B is the control of the experiment
2. When tested for starch, the leaf from jar A remained brown to prove absence of starch. No photosynthesis was taking place as the soda lime prevented carbon dioxide from the air entering & sodium hydroxide absorbed any carbon dioxide inside the jar. Without carbon dioxide photosynthesis cannot take place
N.B. The stopper on the bell jar is sealed with Vaseline to prevent entrance of carbon dioxide
Experiment 2: testing if light is necessary for photosynthesis
Materials
1. Potted plant
2. Aluminium foil
3. Cello tape
4. Scissors
5. Starch testing kit
Method
1. De- starch the potted plant before the experiment
2. Take a small piece of foil and cut a simple shade in the middle & cover the leaf with the foil ensuring that the cut shade is on top of the leaf
3. Leave the potted plant for 6 hrs
4. Make a sketch of the leaf, taking note of the areas which are completely covered
5. Remove a leaf from the plant& test it for starch ( use the covered leaf)
Diagram
Observations & Conclusions
1. The areas which were exposed to sunlight turned blue-black during the starch test to prove that photosynthesis was taking place
2. The portion which was covered did not have any starch & iodine remained brown this proved that sunlight is necessary for photosynthesis
Experiment 4: testing if chlorophyll is necessary for photosynthesis
Materials
1. A variegated leaf and a starch testing kit
Method
1. Make a sketch of the leaf taking note of those areas without chlorophyll
2. Test the leaf for starch
Diagram
Results & Conclusions
1. The portions which had chlorophyll had a positive result when tested for starch (iodine turn blue- black)
2. Those portions which were not green had a negative result when tested for starch. This proves that chlorophyll is necessary for photosynthesis to take place
N.B. If you do not make a sketch first it will not be possible to remember which parts were green & which parts were white since the whole leaf will be white when boiled in alcohol during the starch test
Experiment 5: To prove that oxygen is produced during photosynthesis
Materials
1. A water weed
2. Beaker
3. Funnel
4. Test hole
5. Glowing splint
Method
1. Pour water into a glass beaker
2. Place the water weed in the water & invert a short stemmed funnel over it making sure that the stem of the funnel is completely submerged into the water
3. Fill the test tube with water & and invert it over the stem of the funnel in the water
4. Place the apparatus in the sunlight for 3 hrs observing what happens
Diagram
Observations
1. Gas bubbles were seen coming from the weed and collecting in the test tube
2. The collected gas increased with time pushing the water in the test tube
3. When the test tube was removed without turning it, a glowing splint was put inside, the splint re-ignited
Conclusions
· Because the splint ignited, it proves that the gas that was collected was oxygen
N.B: This experiment can be adapted & used to observe the effect of different light intensity levels on photosynthesis.
The apparatus as set up will be put closer or further away from a light source & count the number of gas bubbles produced in each case. More bubbles will be produced if it is put closer to a light source while less bubbles are produced the further the light source will be.
Another variation will be to use bulbs of different power.
In this case the greater the power (the light intensity) the more the number of bubbles produced per minute.
