Required Practical 7: Investigation of the effect of a specific limiting factor such as light intensity on the rate of photosynthesis

Describe how temperature effects the rate of photosynthesis.

• Rate of photosynthesis increases as temp increases up to an optimum, decreases after

• Limits light independent reaction as it’s enzyme controlled (rubisco)

• Increasing temp up to optimum…

• More Ek.

• More E-S complexes (rubisco).

• Above optimum…

• H bonds in tertiary structure break active site changes shape /

enzyme denatured (rubisco)

• Fewer E-S complexes

• Rate decreases

• Above 30 degrees, O2 competes with CO2 for the active site of rubisco, reducing the amount of CO2

accepted by RuBP, so GP decreases and TP decreases.

(High temp can cause stomata to close → decrease in CO2 therefore decrease in calvin cycle)

Describe how light intensity effects the rate of photosynthesis.

• Rate of photosynthesis increases as light intensity increases (then plateaus, so no longer limiting factor), as more energy is provided for the LDR

• If light intensity was dramatically reduced …

• Levels of ATP and reduced NADP would fall, because…

• Light dependent reaction limited as less photoionisation of chlorophyll as less electrons excited (and less photolysis)

• So, the light independent reaction would also slow/stop…

• GP can’t be reduced to TP (requires ATP and reduced NADP)

• TP can’t regenerate RuBP (requires ATP)

• Light causes stomata to open so decreased light means less gas exchange so less CO2 can enter

What is the light compensation point?

When all CO2 being produced by respiration is being utilised in photosynthesis, so no net gas exchange, rate of photosynthesis = rate of respiration

Describe how CO2 concentration effects the rate of photosynthesis.

• Rate of photosynthesis increases as CO2 conc increases (then plateaus)

• If carbon dioxide concentration dramatically decreased…

• Limits light independent reaction

• Less CO2 to combine with RuBP to form GP

• Less GP reduced to TP

• Less TP (and GP) converted to organic substances e.g. hexose and to regenerate RuBP

What is a common agricultural practice to overcome the effect of light intensity (limiting factor of photosynthesis)?

- Use lightning at night (current wavelength)

- Growing plants under artificial lighting to maximise light intensity

- Greenhouse allows light to pass in

What is a common agricultural practice to overcome the effect of CO2 concentration (limiting factor of photosynthesis)?

Burn fossil fuel in greenhouse → increases CO2 concentration (e.g. propane, paraffin burner)

What is a common agricultural practice to overcome the effect of temperature (limiting factor of photosynthesis)?

- Burner increases temperature
- Greenhouse traps warm air, increasing temperature
- Heating / cooling system to maintain optimum temperature (thermostat)

If limiting factors are minimal, rate of photosynthesis will increase, so...

Faster production of glucose allowing faster respiration. → More ATP to provide energy for growth e.g. cell division, protein synthesis. → Higher yield so more profit.

Describe the effect of water availability on the rate of photosynthesis.

- Sufficient water → plant is turgid: turgid guard cells open stomata for gas exchange
- Limited water → plant is flaccid; flaccid guard cels close stomata so CO2 can no longer enter through stomata, reducing rate of photosynthesis (light-independent reaction)
- Limited water also decreases rate of photosynthesis as photolysis cant occur

Required practical 7: Investigation of the effect of a specific limiting factor such as light intensity on the rate of photosynthesis: TYPE 1; Counting bubbles: METHOD

1. Set up the apparatus as shown in the figure.

2. Put boiling tube 15cm away from LED light source using a ruler; led and not normal lamp as normal lamp release, heat, LED insures only 1 variable is being changed. If using normal light bulb, place beaker of water between boiling tube and light source so it absorbs the heat being given out.

3. Fill boiling tube with sodium hydrogencarbonate (1%) solution to provide source of CO2 for photosynthesis

4. Place boiling tube in water bath to maintain constant temperature

5. Take a piece of aquatic plant/pondweed [camboda or elodea] (cut to 5cm) and push it into boiling tube until end is just below surface if potassium hydrogencarbonate solution. Use scissors to cut the stem undre solution at an angle/slant to increase surface area of oxygen leaving plant. Make sure stem stays below the surface of the solution.

6. Light source is switched on and 5 minute timer begins to allow the plant to acclimatise to the light, then count the number of O2 bubbles rising from the end of the stem in one minute.

7. Repeat the experiement at this distance 4 more times and calculate a mean + remove anomalies.

8. Repeat all the previous steps at additional readings at distance 30cm away form the lamp, 45, 60 and 75.

9. The distances from the lamp can be conevrted into a measure of light intensity by calculation 1/d2, where d = distance from plant to lamp

10. Record results of investiaton in table.

11. Process your data in a suitable statistic test to investigate teh correlation between light intensity and rate of photosynthesis

Investigation of the effect of a specific limiting factor such as light intensity on the rate of photosynthesis: TYPE 1; Counting bubbles, what are the problems with this experiment?

- The number of bubbles produced may be too quick to count accurately
- The bubbles are not always the same size; different volumes of O2

Under some conditions oxygen reacts with rublose bisphosphate to give glycerate 3-phosphate and phosphoglycolate. This reaction is summarised by the equation:

RuBP + oxygen → glycerate 3-phosphate + phosphoglycolate

Phosphoglycoate takes no part in the light-independent reaction.

An investigation was carried out on the effect of temperature and oxygen concentration on the rate of photosynthesis in leaves. The results are shown in the graph. Describe and explain the effect of oxygen on the rate of photosynthesis. (2 MARKS)

- High levels of oxygen reduce photosynthetic rate, effect greater at higher temperatures;
- Higher concentration more effective competitor / more RuBP combines with oxygen (instead of carbon dioxide) / greater chance of binding with the active site / colliding with the enzyme

EQ: Heat stress is a condition that often occurs in plants exposed to high temperatures for a prolonged period of time. Heat stress is a major factor in limiting the rate of photosynthesis.

Another effect of heat stress is a decrease in the activity of the enzyme rubisco.

A decrease in the activity of an enzyme means that the rate of the reaction it catalyses becomes slower. A decrease in the activity of the enzyme rubisco would limit the rate of photosynthesis.

Explain why. (2 MARKS)

1. (Less/no) carbon dioxide (reacts) with RuBP;
2. (Less/no) GP;

EQ: Pondweed is an aquatic plant.

A student investigates the effect of light intensity on the rate of photosynthesis in pondweed.

The student:

• cuts two pieces of pondweed, each 10 cm long

• puts each piece of pondweed in a separate test tube

• adds 0.1% sodium hydrogen carbonate solution to each tube

• adds buffer solution to each tube

• measures the oxygen concentration of the solutions in each tube

• seals each tube with a bung

• puts each tube in a different location:

tube 1 – 20 cm away from a lamp

tube 2 – 10 cm away from a lamp

• measures the oxygen concentration of the solutions in each tube every 5 minutes.

The student added buffer solution to each tube to keep the pH constant.

Suggest why the pH would increase during photosynthesis without this buffer solution. (1 MARK)

Removal of (sodium) hydrogen carbonate / removal of CO2;

A student investigates the effect of different wavelengths of light on the rate of photosynthesis.

The student:

• uses photosynthesising algal cells

• sets up the apparatus shown in the diagram

• shines different wavelengths of light onto the flask

• takes measurements to determine the rate of photosynthesis.

What two measurements should the student take to determine the rate of photosynthesis? (1 MARK)

oxygen concentration/reading on oxygen meter and time;

A student investigates the effect of different wavelengths of light on the rate of photosynthesis.

The student:

• uses photosynthesising algal cells

• sets up the apparatus shown in the diagram

• shines different wavelengths of light onto the flask

• takes measurements to determine the rate of photosynthesis.

The student controls the temperature, pH and carbon dioxide concentration.

The student adds a reagent to the suspension of photosynthesising algal cells. The reagent is blue when oxidised and is colourless when reduced. The blue colour disappears when the suspension of algal cells is exposed to light.

Explain why. Use your knowledge of the light-dependent reaction of photosynthesis. (2 MARKS)

- Electrons/e− (from chlorophyll) excited/reduced NADP formed;
- hydrogen/electrons (from water/chlorophyll) change dye colour;

A student investigates the effect of different wavelengths of light on the rate of photosynthesis.

The student:

• uses photosynthesising algal cells

• sets up the apparatus shown in the diagram

• shines different wavelengths of light onto the flask

• takes measurements to determine the rate of photosynthesis.

The student controls the temperature, pH and carbon dioxide concentration.

Some purple bacteria can photosynthesise, but use hydrogen sulfide instead of water. The hydrogen sulfide has a similar role to water in photosynthesis.

Suggest how the bacteria use hydrogen sulfide. (3 MARKS)

Any three from four

1. (hydrogen sulfide is) source of hydrogen/protons/H+ and electrons/e−;

2. (hydrogen/protons/H+ used for) reduction of NADP/electron carrier;

Reject NAD

Accept production of NADPH/NADPH2/reduced NADP

3. (electrons) replace electrons lost from chlorophyll/pigment;

4. (protons) generate ATP;