Photosynthesis (OCR A Level Biology)

Plants are capable of synthesising a variety of molecules from the products of the light-independent stage of photosynthesis.

Fig 22.1 summarises these processes.

Fig. 22.1

Identify the molecules represented by the letters A, B, C and D in Fig. 22.1
A .........................................................................................................
B ..........................................................................................................
C ...........................................................................................................
D ...........................................................................................................

A scientist investigated the rate of photosynthesis in lesser pondweed, Potamogeton pusillus.

The method used is outlined below:

• Add 200cm³ of distilled water to a 300cm³ glass beaker.
• Dissolve 5g of NaHCO₃ in the water to provide an excess of CO₂.
• Place the beaker in a water bath at 10°C and leave for 10 min to equilibrate.
• Insert an oxygen sensor into the water in the beaker and measure the baseline O₂ concentration.
• Place 100g of P. pusillus into the beaker.
• Remove all other light sources from the room and place an LED light source 20cm above the top of the beaker.
• Use a light intensity meter to ensure the light intensity above the beaker is 5000 lux.
• Measure the concentration of oxygen dissolved in the water using a data logger every 10min for 200 min.
• Carry out four more repeats at 10°C.
• Repeat all the above steps in water baths at 15°C, 20°C, 25°C and 30°C.
  
  

   independent variable
   ...........................................................................................................................................
   dependent variable
   ...........................................................................................................................................
   one control variable
   ...........................................................................................................................................

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Fig. 22.2 is a graph of the scientist’s results.

Fig. 22.2

Describe and explain what these results show about photosynthesis in P. pusillus.

The light-independent stage of photosynthesis used to be referred to as the ‘dark reaction’.

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The scientist then investigated the effect of auxin on P. pusillus stems.

The growing tips of stems were removed and the stems were placed in solutions containing different concentrations of auxin.

The scientist analysed the results and determined the following relationship:

The higher the concentration of auxin in the solution, the fewer side shoots grew on the P. pusillus stems.

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Chromista are photosynthetic protoctists that live in water.

Chromista are different from other photosynthetic organisms because they contain the pigment chlorophyll c.

Chlorophyll c is not found in plants.

Fig. 17.1 is a diagram of the chloroplast found in a Chromista cell.

Fig. 17.1

Outline the structural differences between the Chromista chloroplast in Fig. 17.1 and the chloroplasts found in flowering plants.

Fig. 17.2 is a diagram of part of the plasma membrane of a Chromista cell.

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A scientist used a respirometer to investigate the rate of respiration and photosynthesis of maize in different light intensities.

• The scientist placed ten maize seedlings in a respirometer and kept it in the dark for three hours. 
• The respirometer contained soda-lime to remove any CO₂ produced by the seedlings.
• The scientist placed ten maize seedlings in a separate respirometer without soda-lime and placed it in different light intensities for three hours at a time.

Table 22.1

Answer = ............................................mm³h^-1 [3]

Answer = ...................................................... % [2]

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The scientist made the following claim:

These results suggest that, in maize seedlings, the rate of photosynthesis only exceeds the rate of respiration when the light intensity is above 1510 lux.

Use the data in Table 22.1 to explain why the scientist made this claim.

A student investigated photosynthetic pigments in spinach leaves using thin layer chromatography (TLC).

Fig. 17.1 shows the student’s plate at the end of the investigation. A cm-ruler has been added to the plate.

Rf = ...............................................................
pigment = ...............................................................
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The student used the following method for the investigation:

   Step 1: Extraction of pigments

• Take 0.5g of fresh spinach and add 1g of sand
• Grind the mixture until it becomes a fine, light green powder
• Transfer the powder to a test tube and add 2cm3 of propanone
•  Stir for about 1 minute then allow to stand for another minute
• Transfer the dark green upper layer with a pipette to a clean test tube and seal with film when not in use.

   Step 2: TLC analysis

• Hold the TLC plate carefully by the edges and avoid damaging the surface of the plate
• Draw a pencil line across the width of the TLC plate 1cm from the bottom edge
• Spot the extract on the pencil line using a pipette, one drop at a time, allowing the spot to dry before adding the next drop
• Place chromatography solvent in a jar so that it is no more than 0.5cm deep
• Lower the TLC plate into the jar and lean the top against the side of the jar. Make sure the plate does not touch the sides of the jar anywhere else
• Place a cap on the jar and allow the solvent to soak up the plate
• When the solvent has reached a few mm from the top of the plate, remove the plate from the jar and mark the position of the solvent front with a pencil

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Algae are single-celled photosynthetic organisms.

An investigation was carried out into the effect of CO₂ concentration on the concentrations of ribulose bisphosphate (RuBP) and glycerate 3-phosphate (GP) present in algae.

Algae were grown in a solution containing a high concentration of CO₂. The concentration of CO₂ was then reduced.

RuBP and GP concentration were measured for 2 minutes before and 3 minutes after the reduction in CO₂ concentration.

The results are shown in Fig. 17.2.

Describe and explain the effect of reducing the CO₂ concentration on the concentration of RuBP and GP in the algae.

Light intensity, carbon dioxide concentration and temperature are all limiting factors in photosynthesis.

Explain what is meant by a limiting factor.

Plants photosynthesise and respire. Fig. 18.1 shows the rate of production of carbohydrate in photosynthesis and the rate of use of carbohydrate by respiration.

Fig. 18.1

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Plants grow successfully in temperatures that are suited to their metabolism. Some plants are adapted for growth in cool climates while others can grow well in warm climates.

Plants also vary in their photosynthetic metabolism. Many plants produce a 3-carbon compound as the first product of carbon fixation and so are referred to as C3 plants. Another group of plants produces a 4-carbon compound as the first product and so are referred to as C4 plants.
C3 plants include barley, lentil, rice, soya, sunflower and wheat.
C4 plants include maize, millet, sorghum and sugar cane.

Fig. 18.2 shows the assimilation of carbon dioxide by four different crops at different temperatures.

Fig. 18.2

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C3 .............................................

C4 .............................................

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Sugar cane, which grows in warm climates.

.......................................................................................................

Barley, which grows in cool climates.

.......................................................................................................

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Temperature is very important in determining a plant’s ability to photosynthesise effectively.
Temperature stress is becoming of great concern to plant physiologists because of climate change.

• High temperature (HT) stress is defined as the rise in temperature that is sufficient to cause irreversible damage to plant growth and development.

Some of the stress effects of temperature have been recorded in various plants and are outlined in Table 18.1.

Table 18.1

Suggestion ................................................................................................

Explanation ..............................................................................................

Suggestion ................................................................................................

Explanation ..............................................................................................

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