Photosynthesis
The plant cell:
Word List -
Cell Wall:
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Cell Membrane:
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Nucleus:
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Vacuole:
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Chloroplast:
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Cytoplasm:
Let’s take a look at how those cells are arranged to form a leaf. A cross section of a leaf:
1- Waxy cuticle:
2- Upper Epidermis:
3- Palisade mesophyll:
4- Spongy mesophyll:
5- Vascular bundle:
6- Stoma (Plural: Stomata):
7- Guard Cells:
A closer look at stomata: -
Stomata are microscopic openings found at the bottom of the leaf which control the entrance and exit of oxygen, carbon dioxide, and water vapor.
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Look closely at the figure below to see how they are found on the bottom of the leaf.
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Each stoma (microscopic opening) is surrounded by two guard cells. Those cells control the size of the stoma.
Photosynthesis Photosynthesis is the way plants make food and eat, but to be more specific and scientific, Photosynthesis is the process by which plants chemically combine water and carbon dioxide (in the presence of sunlight) to PRODUCE ďƒ glucose and oxygen
1- Carbon dioxide enters the leaf from the stomata and travels to the chloroplast within the plant cells. 2- Water travels from the roots up the stem and into the chloroplast of the cells of the leaf. 3- The exposure of leaves to sun light helps the chloroplast to chemically combine the water and carbon dioxide. 4- This results in the production of glucose and oxygen. -
Oxygen is released to the atmosphere through the stomata Glucose is found in the form of fruits.
The FATE of glucose Plants spend their time during the day (in the presence of sunlight) producing glucose. But why is glucose so important? In what way does it help plants to survive?
1- Glucose is used - As a sugar in fruits - for respiration
2- Glucose is converted into fats which - Is used as a source of energy in the seed. - Helps in the formation of the cell membrane.
3- Glucose is converted into proteins (by the addition of nitrogen and sulphur). This helps in producing enzymes.
4- Excess glucose is converted into starch for storage.
5- Glucose molecules are connected together to form cellulose which makes up the cell wall of plant cells.
Lab Work In this section of the chapter, we will be working with plants in the lab to prove that carbon dioxide + water --(sunlight)ďƒ glucose + oxygen is correct. The experiments we will be doing are 1- De-starch/ Starch test (testing for the presence of starch, which is a complex form of glucose). 2- Proving that oxygen gas is a product of photosynthesis. 3- Proving that carbon dioxide is needed for photosynthesis. 4- Proving that sunlight is needed for photosynthesis to take place.
1- De-starch/starch test In this experiment we will be testing for the presence of starch within a leaf of a plant (because that is where photosynthesis occurs). a- We must keep the plant in a dark room for a few hours. Due to the lack of photosynthesis, the plant uses up all of its stored starch. This is called destarching b- The plant is placed under sunlight to start making food by photosynthesis. c- After a couple of hours, remove a leaf from the plant and start testing it for starch. But to do so, the leaf must undergo few reactions to allow starch to be exposed. Starch Test i-
Boil the leaf Boiling the leaf for 5 minutes helps breaking down the waxy layer and the cell walls of the cells.
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Heating the leaf in ALCOHOL (or ethanol) Alcohol removes all the chlorophyll from the leaf.
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Dipping the leaf in water Water removes the alcohol from the leaf.
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Add few drops of Iodine Iodine will convert starch into a blue/black color which indicates a positive result.
2- Is oxygen a product of photosynthesis? To find out, we must devise an apparatus to collect the produced gas and test whether it’s oxygen or not.
This apparatus has a plant in a beaker under water, and it is allowed to photosynthesize. The produced gas is collected into an inverted test tube.
Once a sufficient amount of gas is collected, we will expose it to a glowing splint. The glowing splint will relight which indicates the presence of OXYGEN.
3- Is Carbon dioxide required in the process of photosynthesis? To find out, we must destarch a plant to make sure it runs out of stored starch. Then we will allow the plant to photosynthesize by exposing the plant to sunlight, but this time it will be deprived from carbon dioxide.
In order to deprive a plant form carbon dioxide, we simply cover with a glass jar and add a chemical called soda lime. The soda lime is placed next to the plant inside the jar. Its main function is absorbing carbon dioxide from within the jar. The jar is then kept under the sun to for a whole day. To check wether the plant did or did not photosynthesize, we must do the starch test. As a result, the leaf will have a yellow brown result indicating the absence of starch. This is caused by the absence of carbon dioxide.
4- Is light needed for photosynthesis to be completed? To find out, we must supply a plant with water and carbon dioxide, and deprive it from light. This test is done as follow.
1- Cover half a leaf with a black paper to prevent light from passing to the leaf. 2- Supply the plant with carbon dioxide and water. 3- Place the plant under the sun to allow it to photosynthesize. 4- After a few hours, take the covered leaf and do a starch test on it. 5- After adding iodine the resulting color will appear as follow:
Interpretation: The covered section of the plant showed a yellow / brown color which proves the absence of starch. Starch was not made because there wasn’t any sunlight reaching the leaf The uncovered section shoed a dark blue color which indicates the presence of starch. Starch was made in the leaf since it was exposed to sunlight (along with water and carbon dioxide).