Photosynthesis occurs in chloroplasts．Students investigated the effect of light intensity on the rate of photosynthesis in a chloroplast suspension． The blue dye DCPIP can be used to investigate photosynthesis．During photosynthesis，the dark－blue colour of the DCPIP changes and gradually disappears so that it becomes colourless． A chloroplast suspension that is made from spinach leaves has a green colour． The students used this method： Step 1 Prepare a chloroplast suspension from fresh spinach leaves． Step 2 Label one test－tube $$$\mathbf{A}$$$ and another test－tube $$$\mathbf{B}$$$ ． Step 3 Put $$$1 \mathrm{~cm}^{3}$$$ of the green chloroplast suspension into test－tube $$$\mathbf{A}$$$ and into test－tube $$$\mathbf{B}$$$ ． Step 4 Put test－tubes $$$\mathbf{A}$$$ and $$$\mathbf{B}$$$ into an ice－cold water－bath for two minutes． Step 5 Put $$$5 \mathrm{~cm}^{3}$$$ of the blue DCPIP solution into test－tube A．Put a test－tube rack 10 cm from a bright light．Put test－tube $$$\mathbf{A}$$$ into the test－tube rack． Step 6 Turn on the light and start the stop－clock． Step 7 Measure the time taken for the blue liquid in test－tube $$$\mathbf{A}$$$ to return to a green colour． Step 8 Reset the stop－clock to zero． Step 9 Move the test－tube rack so that it is 30 cm from the bright light． Step 10 Put $$$5 \mathrm{~cm}^{3}$$$ of the blue DCPIP solution into test－tube B． Put test－tube B into the test－tube rack and start the stop－clock． Step 11 Measure the time taken for the blue liquid in test－tube $$$\mathbf{B}$$$ to return to a green colour． The times measured in step 7 and step 11 are shown in Fig．1．1． time recorded for test－tube $$$\mathbf{A}$$$ time recorded for test－tube B Fig． 1.1 * 0000800000003 * State which substance present in food can be identified using DCPIP． ......................................................................................................................................... ［1］

Biology

IGCSE&ALevel

CAIE

Exam No：0610_w24_qp_62 Year：2024 Question No：1(b)

Answer:

Knowledge points:

4.1.1 List the chemical elements that make up: – carbohydrates – fats – proteins

4.1.10 Describe the roles of water as a solvent in organisms with respect to digestion, excretion and transport

4.1.2 State that large molecules are made from smaller molecules, limited to: – starch and glycogen from glucose – cellulose from glucose – proteins from amino acids – fats and oils from fatty acids and glycerol

4.1.3 Describe the use of: – iodine solution to test for starch – Benedict’s solution to test for reducing sugars – biuret test for proteins – ethanol emulsion test for fats and oils – DCPIP test for vitamin C

4.1.4 Describe the use of: – iodine solution to test for starch – Benedict’s solution to test for reducing sugars – biuret test for proteins – ethanol emulsion test for fats and oils – DCPIP test for vitamin C

4.1.5 Describe the use of: – iodine solution to test for starch – Benedict’s solution to test for reducing sugars – biuret test for proteins – ethanol emulsion test for fats and oils – DCPIP test for vitamin C

4.1.6 State that water is important as a solvent

4.1.7 Explain that different sequences of amino acids give different shapes to protein molecules

4.1.8 Relate the shape and structure of protein molecules to their function, limited to the active site of enzymes and the binding site of antibodies

4.1.9 Describe the structure of DNA as: – two strands coiled together to form a double helix – each strand contains chemicals called bases – cross-links between the strands are formed by pairs of bases – the bases always pair up in the same way: A with T, and C with G (full names are not required)

Answer:

Knowledge points:

Solution:

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