5.6 - Photosynthesis

Question 1

Describe the interrelationship between photosynthesis and respiration.

Answer 1

All organisms respire all of the time. Respiration removes oxygen from the atmosphere and returns carbon dioxide. Photosynthetic organisms only photosynthesis when light intensity is high enough. Photosynthesis removes carbon dioxide from the atmosphere and adds oxygen.

Question 2

What is the compensation point?

Answer 2

When then the rate of photosynthesis and respiration proceed at the same rate. There is no net gain or loss of carbohydrates.

Question 3

How do compensation points differ between light and shade plants?

Answer 3

Shade plants reach their compensation point sooner than sun plants. Shade plants require a lower light intensity to reach their optimum rate of photosynthesis. Converse points apply for sun plants.

Question 4

What is the overall equation for photosynthesis?

Answer 4

6H2O + 6CO2 + (light energy) → C6H12O6 + 6CO2

Question 5

What is the overall equation for respiration?

Answer 5

C6H12O6 + 6O2 → 6H2O + 6CO2 + energy (ATP)

Question 6

Why is it important for plants to respire as well as photosynthesise?

Answer 6

In the dark there is no ATP production in photosynthesis. Some tissues are unable to photosynthesise. ATP cannot be moved between cells or stored. ATP is needed for active transport. ATP is needed for growth, synthesis of proteins and DNA

Question 7

Relate the ultrastructure of a chloroplast to its function.

Answer 7

Outer membrane: Permeable to ions and gases.
Inner membrane: Impermeable to ions.
Grana/Granum: Site of LDR, thylakoids (flattened sacs) stacked into grana, increase surface area for reactions, photosystems held in membrane.
Photosystems: PSI an PSII, funnel shaped, contain photosynthetic pigments, each absorb light of different wavelengths, primary reaction centre at base contains chlorophyll a.
Stroma / Matrix: Site of LIR, contains enzymes, RuBP, RUBISCO, lipid droplets, starch granules, plasmid DNA for synthesis of chloroplast proteins and enzymes, small prokaryote like ribosomes.

Question 8

Describe chlorophyll a.

Answer 8

Blue-green. PSII absorbs light at peak 680nm. PSI absorbs light at peak 700nm. Some of blue light absorbance at 440nm.

Question 9

Describe chlorophyll b.

Answer 9

Yellow-green. Absorbs light 400-500 nm and 640nm.

Question 10

What are accessory pigments?

Answer 10

Carotenoids, appear orange, absorb blue light 400-500nm.

Xanthophylls, appear yellow, absorb blue and green light, 375-550nm.

Question 11

Describe the stages of the light-dependent reaction.

Answer 11

Chlorophyll absorbs photons (light energy).
This excites electrons and they leave/ emitted from the chlorophyll.
Electrons move along an electron transport chain in a series of oxidation-reduction reactions releasing energy.
Energy used to pump protons across thylakoid membrane from stroma into thylakoid space.
Proton gradient forms across membrane, protons diffuse down concentration gradient through ATPsynthase, chemiosmosis.
Energy released is used to combine ADP and Pi to form ATP - hydrolysis reaction, photophosphorylation.
Electrons from PSII replace those lost from PSI.
Photolysis of water produces protons, electrons and oxygen.
Electrons replace those lost from PSII.
NADP is reduced by electrons and protons, forms NADPH, catalysed by NADPH reductase.

Question 12

What is cyclic phosphorylation?

Answer 12

Uses only PSI I. Electrons excited by photons passed back down ETC. Small amount of ATP synthesised. No photolysis so no protons, no oxygen, no NADPH. Used in guard cells to regulate opening/closing of stomata.

Question 13

Describe the stages of the light-independent reaction/the Calvin cycle.

Answer 13

Occurs in the stroma, uses products of LDR.
Carbon dioxide combines with 5C ribulose bisphosphate (RuBP) (carboxylation). 6C intermediate is unstable.
Produces 2 molecules of 3C glycerate-3-phosphate (GP), carbon fixation.
This is reduced to 3C triose phosphate (TP) using reduced NADP (reduction) and energy from ATP (phosphorylation).
Triose phosphate converted to other organic substances. The majority is used to regenerate RuBP.

Question 14

What is a limiting factor?

Answer 14

The factor that limits the rate at which the process takes place.

Question 15

What are the limiting factors of photosynthesis?

Answer 15

Light intensity; carbon dioxide concentration; temperature; water stress.

Question 16

Describe the effect of light intensity on the rate of photosynthesis.

Answer 16

As the light intensity increases so does the rate of photosynthesis.

Question 17

Describe the effect of carbon dioxide concentration on the rate of photosynthesis.

Answer 17

As the carbon dioxide concentration increases so does rate of photosynthesis.

Question 18

Describe the effect of temperature on the rate of photosynthesis.

Answer 18

As temperature increases so does rate of photosynthesis. Between 0 and 25°C the rate of photosynthesis is doubled for each 10°C rise in temperature.
Above 30°C oxygen competes with carbon dioxide for the active site of RuBISCO (photorespiration). This means there is less carbon dioxide to carboxylate RuBP, less GP, less TP, and therefore less regeneration of RuBP. Rate slows.
Above 45°C enzymes denature, and the rate stops.

Question 19

Describe the effects of water stress.

Answer 19

Roots unable to take up enough water to replace that lost through transpiration. Cells become plasmolysed. Roots release abscisic acid, which causes stomata close, and reduces gas exchange. Leaves and tissues wilt. Rate of photosynthesis reduced.