S3 BGE BIOLOGY Part 2 PHOTOSYNTHESIS2C: Investigating Photosynthesis

You will study the following topic at Level’s 3, 4 and 5. 2a. Photosynthesis intro/Leaf Structure & chloroplasts in plant cells (L3,4)2b. Uses of sugar (L3)/ Conversion of glucose into plant products (L5) 2c. Investigating photosynthesis

Testing a leaf for starch (the product glucose is stored as starch) (L3,4) Light and water are essential for photosynthesis as raw materials (L3,4) CO2 and chlorophyll are required as ideal conditions for photosynthesis (L3,4) Oxygen is produced as a by-product during photosynthesis (L3,4)

2d. The importance of photosynthesis and Life on Earth (link with producer + food chains) (L4)

2e. Factors affecting photosynthetic rate/Limiting factors introduction (L4,5)2f. Carbon dioxide and temperature as a limiting factor + Graphs (L4,5)2g. Impact of limiting factors on crop growth (L5)/How farmers overcome limiting factors

(L4) 2h. Biochemistry of photosynthesis (L5)

Photosynthesis intro/Leaf Structure & chloroplasts in plant cell Learning OutcomesL3 Learning Outcomes: You should be able to…

o Investigate the production of starch on leaves subjected to different conditions.L4 Learning Outcomes: as well as L3 outcomes: you should be able to…

o Describe the process of testing a leaf for starch.o Prove that water, carbon dioxide, light and chlorophyll are essential for

photosynthesis.o Explain how to set up the elodea bubbler experiment to show that oxygen a by-

product.

DEFINITIONS TO LEARN – MAKE ME A FLASH CARDLevel

Biological term

Definition

3-5 Iodine Solution

A stain which is the test for starch. Also used to rest a leaf for starch. It turns from brown to blue/black in the presence of starch

3-5 Variegated leaf

A leaf which is two colours e.g. white and green. Useful for investigating if chlorophyll is essential for photosynthesis to occur

3-5 Ethanol A type of alcohol used in the testing leaf for starch experiment to burst open the chloroplasts. Highly flammable.

3-5 Elodea Type of pond weed. Gives off oxygen as bubbles under water during photosynthesis

3-5 Elodea Bubbler

An experiment which shows oxygen been given off during photosynthesis.

Level 3 Planet Earth: Biodiversity & interdependence SCN 3-02a; Topical Science SCN 3-20bNational 3: Unit 1: Key Area 1.4 – The process of photosynthesisInvestigate the production of starch in leaves under different conditions,such as starch (storage) and cellulose (structural)

Level 4 Planet Earth: Biodiversity and interdependence SCN 4-02a; Topical Science SCN 4-20aNational 4: Unit 1: Key Area 1.6 The effect of limiting factors on photosynthesis. 1.6a Carry out starch tests on plants in various conditions1.6b Use Elodea to find out about limiting factors

This experiment can also be used to measure the rate of photosynthesis.

(L3) Proving the equation by experimentation

It is possible to prove each part of the photosynthesis word equation. This is possible because there is an experiment for each part.

IT IS EASIER TO TEST A LEAF FOR STARCH THAN GLUCOSE When plants store their food, they join glucose molecules together to

form long chains called starch.  Plant leaves don’t normally contain much glucose because they use it straight

away (that is why it’s more difficult to test for glucose).  Any leftover they store as starch. We can test a leaf for starch using iodine

solution.

IODINE SOLUTION IS THE TEST FOR STARCH If there is NO starch present. The iodine will

not change it will stay brown. If there IS starch present the starch will turn

blue/black

TESTING A LEAF FOR STARCH EXPERIMENTTHINGS TO NOTE…. Boiling the leafsoftens it and

bursts the cell membranes. The Bunsen must be

switched off before ethanol is used. This is because ethanol is extremely flammable.

Boiling the leaf again, this time in ethanol bursts open the chloroplasts to remove the green pigment chlorophyll.

It is important to rinse the leaf (by placing the leaf back into the beaker of pre-boiled water). This removes the ethanol and further softens the leaf ready for testing with iodine solution.

1. Proving that glucose is stored as starch during photosynthesis. This can be investigated by carrying out the basic testing a leaf for starch experiment as described above.CONCLUSIONOnce carried out the conclusion for the experiment should be as follows: “Since iodine solution had a positive result for starch (it changed from brown to blue/black), it can be concluded that starch was stored during photosynthesis”

2. Proving that light is essential for photosynthesis to take place This can be investigated by carrying out the basic testing a leaf for starch experiment with a slight variation.The variation is either 

Take a potted plant and de-starch it by keeping it in darkness for 24 hours or……

Attach a piece of metal foil or paper on both sides of a leaf and leave it for 24-48hrs).

USING A CONTROLIt is important to test a leaf given normal conditions (this is called a control) alongside the leaf deprived of light. That way wee can be certain that it is the presence or absence of light that is causing the results and not some other factor.

CONCLUSIONOnce carried out the conclusion for the experiment should be as follows: “As iodine solution had a negative result for starch only in the leaf kept in the dark. It can be concluded that light is essential for glucose to be stored as starch during photosynthesis. We know the result is true as the control (the plant that was given light) produced starch as expected; it had a positive result for starch (iodine turned blue/black).

3. Proving that carbon dioxide is essential for photosynthesis to take place

This can be investigated by carrying out the basic testing a leaf for starch experiment with a slight variation.The variation is…. 

Take a potted plant and de-starch it by covering it with a Bell Jar containing sodium hydroxide and leave it for 24-48hrs (sodium hydroxide absorbs all carbon dioxide)

CONCLUSIONOnce carried out the conclusion for the experiment should be as follows: “As iodine solution has had a negative result for starch deprived of carbon dioxide. It can be concluded that carbon dioxide is essential for glucose to be stored as starch during photosynthesis. We know the result is true as the control (the plant exposed to normal air containing carbon dioxide) produced starch as expected, it had a positive result for starch (iodine turned blue/black)”.

4. Proving that water is essential for photosynthesis to take placeThis can be investigated by carrying out the basic testing a leaf for starch experiment with a slight variation.The variation is…. 

Take a potted plant and de-starch it is depriving it of water for 48-72hrs

CONCLUSIONOnce carried out the conclusion for the experiment should be as follows: “As iodine solution had a negative result for starch in the plants deprived of water. It can be concluded that water is essential for glucose to be stored as starch during photosynthesis.We know the result is true as the control (the plant given water) produced starch as expected, it had a positive result for starch (iodine turned blue/black)”.

5. Proving that water is essential for photosynthesis to take place This can be investigated by carrying out the basic testing a leaf for starch experiment with a slight variation.The variation is…. 

Use a variegated leaf. This means it is two colours (here the colours are white and green).

This is useful because we know that chlorophyll the green pigment found in chloroplasts gives a plant the green colour.

CONCLUSIONOnce carried out the conclusion for the experiment should be as follows: “As iodine solution has had a negative result for starch on the white parts of the leaf, and a positive result for starch on the green parts of the plant. It can be concluded that chloroplasts (containing chlorophyll) are essential to produce glucose to be stored as starch during photosynthesis”.

6. Proving that oxygen is produced as a by-product of photosynthesis. This time a different experiment is used. It is called the ELODEA BUBBLER EXPERIMENT, shown in the diagram opposite.

Elodea is a pond weed; it is useful because the oxygen is given off as bubbles which are easily seen under the water.   As the oxygen gas is released from the plant it floats up and displaces the water (shoves it aside) at the top of the tube. We can test the gas to prove it is oxygen.

THE TEST FOR OXYGEN GAS: RELIGHTING A GLOWING SPLINT.

When enough gas is collected the test-tube is removed and a glowing splint is inserted into the tube to test for oxygen

If the splint does not relight the gas is not oxygen.

Because oxygen is very flammable it can easily relight a glowing splint when present.

Tying it all together.