PhotosynthesisPhotosynthesis

20.1 The Primary Food Source

• Some examples of food chains:

Grass Rabbit Snake Hawk

20.1 The Primary Food Source

• Some examples of food chains:

Tree Aphid Ladybird Small bird

20.1 The Primary Food Source

• Some examples of food chains:

Grass Zebra Tsetse fly (sucks the blood of zebras)

20.1 The Primary Food Source

• In the food chains you observed previously, did you notice that plants are always found at the beginning of a food chain?

• Plants provide all living organisms with food and energy to live.

• Plants are the only living things that can produce their own food from raw materials and energy.

• Thus, plants are called producers.• Animals are called consumers.

20.1 The Primary Food Source

• Food for plants is energy-rich substances called carbohydrates (e.g. starch and sugar).

• When broken down, carbohydrates release a lot of energy for plants to grow and survive.

20.2 Sources of Raw Materials and Energy

• Plants obtain water and dissolved mineral salts from the soil.

• However, these substances are not rich in energy like the carbohydrates found in the plant body.

20.2 Sources of Raw Materials and Energy

Experiment 1: Do energy-rich compounds in the plant’s body come from soil?

Experiments have shown that plants can grow well with their roots in a solution of mineral salts, without carbon.

1. Plant grown in a solution of mineral salts.

2. The weight gained by this plant is much more than the weight lost by the mineral salts solution.

3. Plant matter is also made of other substances like carbohydrates. Where do the carbon and energy present in these carbohydrates come from?

20.2 Sources of Raw Materials and Energy

• Two hypotheses (scientific guesses) on the importance of carbon dioxide and light for a plant to make carbohydrates:o Hypothesis 1: Sunlight is needed for

carbohydrate formation in plants.o Hypothesis 2: Carbon dioxide is needed for

carbohydrate formation in plants.

• Experiments are needed to test these hypotheses.

20.2 Sources of Raw Materials and Energy

• Starch is an important storage compound of energy and carbon in plants.

• We have to find out whether carbon dioxide and sunlight are necessary for its formation.

20.2 Sources of Raw Materials and Energy

Experiment 2: Starch test

The details of the starch test are given in your practical book. If the leaf contains starch, it will turn blue-black when tested with iodine solution. If starch is absent, the leaf will turn light brown.

a) Starch is present b) Starch is absent

leaf turns blue-black

leaf turns light brown

Experiment 3: Is sunlight necessary for starch formation?

20.2 Sources of Raw Materials and Energy

a) Plant kept in the dark b) Plant kept in the light

leaf turns light brown

leaf turns blue-black

This experiment shows that sunlight is essential for starch formation.Grow a potted plant in the absence of light. Compare this potted plant with a plant grown under normal conditions in sunlight.

Experiment 4: Is carbon dioxide in the air necessary for starch formation?

20.2 Sources of Raw Materials and Energy

a) Plant grown in air without carbon dioxide

b) Plant grown in air with carbon dioxide

Sodium or potassium hydroxide

Water

From this experiment, you will learn that starch is formed in plants only when supplied with carbon dioxide.

Grow a potted plant in a bell-jar in the absence of carbon dioxide. Use sodium or potassium hydroxide solution to absorb carbon dioxide from the air in the jar. Compare this with a plant grown in normal air containing carbon dioxide.

Experiment 5: Does the green pigment, chlorophyll play an important part in starch formation?

20.2 Sources of Raw Materials and Energy

a) A variegated leaf b) A variegated leaf tested for starch

These results tell you that starch is formed only in those parts of the leaf that contain chlorophyll. Some plants (e.g. coleus) have pigments that give the leaves a colour other than green. In these plants, chlorophyll is hidden under these pigments to help in starch formation.

For this investigation, we need to use plants with variegated leaves. In a variegated leaf, some parts are green and some parts are without chlorophyll. • Step 1: Grow a plant under normal conditions.• Step 2: Remove a variegated leaf.• Step 3: Make a drawing of the leaf to show the distribution of chlorophyll.• Step 4: Test the leaf for starch. See practical details in

your workbook for the test for starch.

20.3 Changing Raw Materials into Sugar – Photosynthesis

• Sunlight, carbon dioxide and chlorophyll are essential for starch formation.

• This process is called photosynthesis.• Photosynthesis is the process by which

green plants, in the presence of chlorophyll and light energy, manufacture carbohydrates (glucose) from carbon dioxide and water. During this process, oxygen is given off.

20.3 Changing Raw Materials into Sugar – Photosynthesis

• Photosynthesis can be summarised in this equation:

Chlorophyll

Light energyCarbon dioxide + Water Glucose + Oxygen

source of energy

carbon dioxide

oxygen glucose

water

20.3 Changing Raw Materials into Sugar – Photosynthesis

• Glucose, a sugar, is formed during photosynthesis.

• However, we test for starch instead. This is because excess glucose produced is changed into starch for storage.

• If starch is present in a leaf, excess glucose has been produced.

• This will indicate that photosynthesis has occurred.

Conditions essential for photosynthesis

Sunlight (light energy)Light energy is converted to chemical energy stored in glucose.

Suitable temperaturePhotosynthesis depends on the reactions of enzymes in the chloroplasts. Enzyme reactions are fastest at a suitable temperature.

ChlorophyllThis green pigment absorbs light energy and changes it into chemical energy needed to manufacture glucose.

Carbon dioxide and waterThese are the raw materials needed to form carbohydrates (glucose).

water

glucose

20.4 Ornamental Plants and Large-scale Crops

• There are two types of commercial plants – ornamental plants and large-scale crops.

The healthy growth of plants

• Both ornamental plants (e.g. orchids and roses) and crops (e.g. sugarcane and wheat) need favourable conditions to be healthy. These include:

o Sunlight, chlorophyll, water, carbon dioxide and suitable temperature

o Warm and humid conditions (plants generally grow faster)

o Fertilisers to provide nutrients (e.g. nitrogen and magnesium)

o Suitable spacing between plants to prevent overcrowdingo Suitable soil for the growth of ornamental plants and

crops

Ornamental plantsOrnamental plants1. The temperature and the

amount of light in the greenhouse or nursery can be controlled to suit different plants.

2. Ornamental plants are normally planted in pots, in a greenhouse or nursery.

3. The amount of water can be controlled for plants grown in greenhouses and nurseries.

5. Fertilisers are used to provide plants with adequate nutrients.

4. Adequate spacing between pots of plants prevents them from competing for sunlight.

Large-scale cropsLarge-scale crops1. The temperature and the

amount of light cannot be controlled. Both depend on the weather. A shade might be used if the weather is too hot.

2. Large-scale crop plants are grown in large quantities and require large plots of land.

5. Besides providing plants with nutrients, fertilisers also improve the soil quality and increase crop yields.

4. Adequate spacing allows healthy growth and prevents overcrowding.3. Water comes from rain and

irrigation canals. It cannot be controlled. Drainage systems are used to prevent the land from flooding during heavy rainfalls.

Miscellaneous

• http://www.emc.maricopa.edu/faculty/farabee/BIOBK/BioBookPS.html

• http://www.sciencemadesimple.com/leaves.html• http://www.newtonsapple.tv/video.php?id=915• http://www.accessexcellence.org/AE/AEC/AEF/1

996/morishita_pictures.html• http://www.ks.uiuc.edu/Research/psu/psu.html• http://www.ipmcenters.org/cropprofiles/docs/ncor

namentals.html• http://www.epa.gov/agriculture/ag101/crop.html