FACTFILE: GCSE DAS BIOLOGY: UNIT 1 File... · 2019. 10. 9. · 1.2.2 recall the word equation for photosynthesis and the balanced chemical equation: carbon dioxide + water light chlorophyll

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Learning outcomesStudents should be able to:1.2.1 demonstrate knowledge and understanding of photosynthesis as an endothermic process that takes place in chloroplasts, where chlorophyll absorbs light energy, producing sugars and starch;

1.2.2 recall the word equation for photosynthesis and the balanced chemical equation:

carbon dioxide + water light

chlorophyll glucose + oxygen

6CO2 + 6H2O light

chlorophyll C6H12O6 + 6O2

1.2.3 explain investigations into how photosynthesis requires light, carbon dioxide and chlorophyll to show that biology is an evidence-based discipline, including:• how and why a leaf is destarched;• testing a leaf for starch by boiling in water,

boiling in ethanol, softening in water and testing with iodine solution;

• the production of oxygen;• using sodium hydroxide to absorb carbon dioxide;

and• using a variegated leaf to illustrate the role of

chlorophyll;

• Investigate the need for light and chlorophyll in photosynthesis by testing a leaf for starch (Prescribed Practical B1); and

1.2.4 explain how temperature, light intensity and carbon dioxide concentration affect the rate of photosynthesis and interpret data on the limiting factors on the rate of photosynthesis.

1.2.5 explain the relationship between photosynthesis and respiration in plants, using hydrogencarbonate indicator, to include: • demonstrating knowledge of the colour

changes of hydrogencarbonate indicator (high CO2 – yellow, normal CO2 – red and low CO2 – purple);

• demonstrating compensation point.

1.2.6 describe the structure and shape of the mesophytic leaf and identify its adaptations for gas exchange and light absorption including:• the epidermis, with cell walls as a physical

defence that are transparent to allow light through;

• the waxy cuticle, which is a physical defence that is transparent to allow light through and waterproof to reduce water loss;

• the palisade mesophyll cells, which are tightly packed, end on to upper surface with many chloroplasts to increase photosynthesis;

• the spongy mesophyll cells, with a few chloroplasts and a large surface area for gas exchange;

• intercellular spaces, which allow diffusion of gases through the leaf; and

• guard cells and stomata, which allow gases to diffuse into and out of the leaf.

Photosynthesis and plants

FACTFILE: GCSEDAS BIOLOGY:UNIT 1.2

FACTFILE: GCE HISTORY OF ART / ARCHITECTUREFACTFILE: GCSE DAS BIOLOGY : PHOTOSYNTHESIS

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What is Photosynthesis?Photosynthesis is the process a plant uses to make its own food.

Energy is required for this process to happen, therefore it is an endothermic reaction.

Word equation for photosynthesis:

carbon dioxide + water chlorophyll

sunlight food + oxygen

The chlorophyll absorbs the light energy and this energy ensures that the raw materials of carbon dioxide and water combine to form food and oxygen.

The food is glucose and is stored as starch in the vacuole of each plant cell.

Oxygen is a waste product of this reaction, and is released into the atmosphere.

Symbol equation for photosynthesis:

CO2 +H2O C6H1206 +6O2

To prove that photosynthesis has taken place, we must prove that either food or oxygen has been made.

• To prove that food has been made, the presence of starch is tested for in the green leaf. This is because the food made is glucose which is stored as starch in the vacuole.

Iodine solution will change colour in the presence of starch from yellow/ brown to blue/black.

This colour change can be difficult to see if the leaf is green so before testing we must remove the chlorophyll from the leaf. The colour change is then easier to see in a pale leaf. This is detailed in the Practical Manual found on the CCEA website.

• To prove that oxygen has been made, pondweed (Elodea) can be used to collect the bubbles of gas released over water. When a test-tube of gas has been collected it can be shown to be oxygen by placing a glowing splint into the neck of the test-tube and it will re-light.

Oxygen bubbles can be seen being released into the water from the pondweed.

Set up a test-tube over a funnel and place them in a basin of water containing pond weed.

As the oxygen collects at the top of the test-tube it will push the water out, until eventually there will be a test-tube filled with oxygen, which can be tested.

To prove that light is neededUse the same equipment in the previous diagram but include a lamp. Shine the lamp at the pond weed and record the bubbles of oxygen being released every minute. Move the lamp progressively closer and compare the number of bubbles per minute.

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Burning food held on mounted needle

Thermometer

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Oxygen collecting in the test-tube over water

Bubble of oxygen being released

Pondweed in water


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Now that photosynthesis has been explained, the conditions needed for it to occur and the products made, this can be related to nature to get a clearer picture of the importance of this process.

Importance of PhotosynthesisPhotosynthesis produces uses carbon dioxide and produces oxygen.

Carbon dioxide(CO2) is produced and released into the atmosphere during respiration and combustion of fossil fuels. If the level of CO2 in the atmosphere increases, more heat energy will be trapped as it is a greenhouse gas. This will lead to global warming.

The usual amount of CO2 in the atmosphere is about 0.03%, but this has risen due to increased burning of fossil fuels and the removal of trees (deforestation). Photosynthesis removes CO2 from the atmosphere and returns O2 during daylight. In this it maintains the level of CO2; if the amount of CO2 put into the atmosphere and by respiration equals the amount used by photosynthesis, then the overall amount does not change.

We can illustrate the changing levels of CO2 using hydrogen carbonate indicator. (HIGHER LEVEL ONLY)

Level of CO2

Colour of hydrogen carbonate indicator

Normal Red

Low Purple

High Yellow

REMEMBER:Photosynthesis uses CO2, reduces levels in the surrounding air or water:

6CO2 + 6H2O light

chlorophyll C6H12O6 + 6O2

Respiration produces CO2, increases levels in the surrounding air or water:

C6H12O6 + 6O2 6CO2 + 6H2O

To show these processes:Place pondweed into 4 test-tubes of hydrogen carbonate indicator. Set each test-tube in different conditions:

TEST-TUBE 1: Leave in full light to allow photosynthesis to occur.

TEST-TUBE 2: Cover in muslin to reduce the light entering and therefore reduce the photosynthesis

TEST-TUBE 3: Cover in tin-foil to block the light completely and prevent photosynthesis

TEST-TUBE 4: Place a water beetle in the test-tube with the pond weed, to increase the rate of respiration.

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RESULTS1. More photosynthesis than respiration. More CO2 used up than produced Overall the level of CO2 goes down Hydrogen carbonate indicator is PURPLE

2. Same photosynthesis as respiration. Same CO2 used as produced Overall the level of CO2 stays the same Hydrogen carbonate indicator is RED

3. Respiration ONLY CO2 produced Overall the level of CO2 goes UP Hydrogen carbonate indicator is YELLOW

4. More respiration than photosynthesis (beetle is respiring too)

More CO2 produced than used by photosynthesising pond weed

Overall the level of CO2 goes up Hydrogen carbonate indicator is YELLOW

The main organ of photosynthesis is the leaf. In order to understand how and where the process takes place, it is important to understand the structure of the leaf and how that structure aids the process.

The leaf can be described in layers.

The top outer layer: consists of a waxy cuticle, which helps prevent excess water loss. In drier climates, the waxy cuticle will be very thick to prevent the plant drying out in hot weather.

The inside middle layer: contains the palisade layer and the spongy mesophyll.

The palisade cells are rich in chloroplasts and tightly packed together to maximise the amount of sunlight absorbed.

The spongy mesophyll cells are more loosely arranged with air spaces between them. This allows gases (carbon dioxide and oxygen) to move easily in and out of the cells and the leaf.

The lower outer layer or underneath of the leaf: contains stomata with their guard cells. The stoma is an opening which allows carbon dioxide into the leaf and oxygen and excess water out. When the guard cells fill with water they expand and close the stoma, preventing water loss. But when the guard cells deflate the stoma opens, and gases can pass through.

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Waxy Cuticle

Palisade cell, tightly packed

Spongy mesophyll with air spaces

Lower Epidermis


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FOUNDATION LEVEL SAMPLE QUESTIONS

Double Award Science: Unit B1 and Separate Biology: Unit 1

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© CCEA 2017


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FACTFILE: GCSE DAS BIOLOGY: UNIT 1 File... · 2019. 10. 9. · 1.2.2 recall the word equation for photosynthesis and the balanced chemical equation: carbon dioxide + water light chlorophyll