Section 5 - Energy transfer in and between organisms: 11. Photosynthesis

Question 1

What is the site of photosynthesis

Answer 1

The leaf is the main photosynthetic structure, containing chloroplast.

Question 2

What are the main components within the structure of a leaf

Answer 2

• Waxy cuticle
• Upper epidermis
• Palisade mesophyll (contains chloroplast)
• Spongey mesophyll
  (xylem and phloem)
• Lower epidermis
  (contains stomata surrounded by guard cells)

Question 3

How is the leaf adapted for it’s function

Answer 3

• Large SA to absorb sunlight
• Arranged to minimise overlapping (shadows)
• Transparent cuticle and epidermis allows light to reach the photosynthetic mesophyll
• Cells in the upper mesophyll are long, narrow and contain lots of chloroplast
• Thin, providing short diffusion path for gas exchange
• Numerous stomata on underside
• Guard cells regulate gas exchange and water loss through the stomata in response to light intensity and water levels
• Air spaces in spongy mesophyll allow for efficient gas exchange
• Xylem/Phloem transport water/sugar to/from photosynthetic cells

Question 4

What are are the main structural components of chloroplast

Answer 4

Disc shaped organelle containing:
- Double membrane
- Grana (Made up of thylakoids)
- Stroma
- Ribosomes
- Chloroplast DNA

Question 5

What is a thylakoid within chloroplast

Answer 5

Disc-like structures, containing chlorophyll, acting as the site of the LDR in photosynthesis

Question 6

What are grana within chloroplast

Answer 6

Stacks of roughly 100 thylakoids, joint together by intergranular lamella

Question 7

What is the stroma within chloroplast

Answer 7

Fluid filled surrounding the grana, acting as the site of the LIR in photosynthesis

Question 8

What is the equation of photosynthesis

Answer 8

6CO(2) + 6H(2)O → C(6)H(12)O(6) + 6O(2)

Carbon Dioxide + Water →(Light)→ Glucose + Oxygen

Question 9

What are the main stages of photosynthesis

Answer 9

• Capturing light energy
• The Light dependent reaction (LDR)
• The Light independent reaction (LIR)

Question 10

What is the role of the LDR of photosynthesis

Answer 10

Uses captured light energy for the synthesis ATP and the photolysis of water, releasing electrons

Question 11

What are the main processes in the LDR of photosynthesis

Answer 11

• Photolysis of water
• Electron transport chain (REDOX reactions)
• Movement of H+ ions into the thylakoid
• Synthesis of ATP (Chemiosmosis)
• Production of reduced NADP

Question 12

How does the photolysis of water occur during the LDR

Answer 12

• Water molecules are ‘split’ using the absorbed light energy
  H(2)O →(Light)→ 4H(+) + 4e(-) + O(2)
• Electrons enter a transport chain within the thylakoid membrane (accepted by the chlorophyll molecules)
• The oxygen is a by-product of photosynthesis
• The H+ ions (protons) remain inside the thylakoid for later use in chemiosmosis

Question 13

What happens along the electron transport chain during the LDR of photosynthesis

Answer 13

• Chlorophyll molecules in the Thylakoid membrane (Photosystem II) take in electrons released by the photolysis of water
• Absorbed light energy excites the electrons (photoionization), increasing their energy levels
• High energy electrons pass along electron carrier proteins within the thylakoid membrane through REDOX reactions
• The electrons lose energy as they move along the chain, until they reach another chlorophyll molecule (Photosystem I) where photoionization reoccurs
• Electrons then continue along the transport chain until they reach the last protein

Question 14

What is photoionisation

Answer 14

When light energy is absorbed (by the chlorophyll) and excites the electrons to a higher energy level

Question 15

How are H+ ions moved into the thylakoid during the LDR of photosynthesis

Answer 15

• As electrons pass along the electron transport chain, they lose energy
• Some of this energy can be used to transport H+ ions into the thylakoid, against their concentration gradient (proton pump)
• This increases the conc. of H+ ions within the thylakoid

Question 16

How is ATP synthesised in the LDR of photosynthesis (Chemiosmotic theory)

Answer 16

• There is a high conc. of H+ ions maintained inside the Thylakoid (From photolysis of water and proton pump)
• The protons can only leave via the ‘ATP synthase protein channels’ (Form granules on the membrane surface called stalked granules)
• As the protons diffuse through the channel, they case the ATP synthase enzyme to change shape, allowing it to produce ATP from ADP

Question 17

How is reduced NADP produced during the LDR of photosynthesis

Answer 17

• At the end of the transport chain, the electrons must be removed to allow the constant movement (and the overall reaction) to continue
• The coenzyme NADP takes up the electrons at the end of the chain
• This NADP then combines with a H+ ion that has just passed out of the thylakoid, to form reduced NADP

Question 18

What are the overall products of the LDR of photosynthesis

Answer 18

ATP and Reduced NADP are released to be used in the LIR, and oxygen is released as a by product

Question 19

How is the thylakoid adapted to be the site of the LDR of photosynthesis

Answer 19

• The membrane provides a large SA for the reaction to occur along
• Proteins in the grana position the chlorophyll to allow for maximum light absorption
• Proteins in the membrane allow an electron transport chain to occur
• ATP synthase channels are present in the membrane to allow for the production of ATP
• The membranes are selectively permeable, allowing for the establishment of a proton gradient
• Chloroplast contain DNA and ribosomes near the thylakoids, to easily produce proteins for use in the LDR.

Question 20

What is the role of the LIR of photosynthesis (Calvin cycle)

Answer 20

Uses the products of the LDR to reduce ‘glycerate 3-phosphate’, leading to the production of glucose

Question 21

What are the main processes in the LIR of photosynthesis (Calvin cycle)

Answer 21

• Carbon fixation (RuBP → GP)
• Reduction (GP → TP)
• Regeneration (TP → RuBP)

Question 22

What happens during the carbon fixation stage of the LIR of photosynthesis (Calvin cycle)

Answer 22

• CO(2) reacts with a 5-carbon molecule called ‘RuBP’ (ribulose bisphosphate)
• Reaction is catalysed by the enzyme ‘Rubisco’ (ribulose bisphosphate carboxylase)
• Produces an unstable 6-carbon molecule that immediately splits into 2 3-carbon molecules called ‘GP’ (glycerate 3-phosphate)

Question 23

What happens during the reduction stage of the LIR of photosynthesis (Calvin cycle)

Answer 23

• The reduced NADP from the LDR is used to reduce the GP forming a 3-carbon molecule called ‘TP’ (triose phosphate)
• This requires energy, released from the hydrolysis of the ATP produced in the LDR
• NADP is then released to be used again as an electron acceptor in the LDR

Question 24

How is glucose produced during the Calvin cycle

Answer 24

• Some of the produced TP is used to produce organic substances (eg. glucose)
• 2 molecules of TP are required to make glucose, but only 1/6 of the total produced used for this.
• ∴ 6 cycles of the LIR produce 1 glucose molecule, with 5/6 of the produced TP molecules being used to regenerate RuBP so the cycle can continue

Question 25

What happens during the regeneration stage of the LIR of photosynthesis (Calvin cycle)

Answer 25

• 5/6 of all the TP molecules produced are used to regenerate RuBP so the cycle can continue
• This uses energy released by the hydrolysis of the ATP from the LDR
• The ratio of TP used for regeneration is 5:1, as 10TP (30 carbon molecules) are required to make 6RuBP (6 x 5-carbon molecules), and 2TP are required to make glucose
  ∴ The cycle occurs 6 times simultaneously, producing 1 glucose molecule before it repeats

Question 26

What are the overall products of the LIR of photosynthesis

Answer 26

Produces 1 molecule of glucose every 6 cycles, but releases ADP + Pi and NADP to be reused in the LDR

Question 27

How is the stroma adapted to be the site of the LIR of photosynthesis

Answer 27

• Fluid of the stroma contains the enzymes required for the LIR (eg. Rubisco)
• Fluid is membrane bound, maintaining an environment with a constantly high enzyme concentration
• Stroma surrounds the grana so the products of the LDR diffuse out of the thylakoids and into the fluid, making them readily available for the LIR
• Contain DNA and ribosomes to easily produce proteins needed for the LIR.

Question 28

‘What is the law of limiting factors’ for a reaction

Answer 28

At any given moment, the rate of a physiological process is limited by the factor that is at it’s least favourable value (limiting factor)

Question 29

What is the ‘saturation point’ for a factor effecting the rate of a reaction

Answer 29

The point after which the rate is no longer proportional to this factor, as some other factor is now limiting the rate.

eg. Light saturation point : After this, increasing the light intensity won’t increase the rate of photosynthesis, as CO(2) conc. is now the limiting factor.

Question 30

What is the ‘light compensation point’ for photosynthesis

Answer 30

Point where the light intensity allows for a rate of photosynthesis that produces CO(2) at the same rate it is used up by respiration
(Rate of photosynthesis = Rate of respiration)

Question 31

How do you set up a Photosynthometer to measure the rate of photosynthesis in an aquatic plant

Answer 31

• Place the plant shoot in a large beaker of pond water, and remove any bubbles on the leaf surface by gently running your finger and thumb over them
• Fill the capillary tubing of the Photosynthometer with water.
• Place the funnel end of the tubing into the beaker of water and position the shoot so the cut end is in the funnel
• The beaker is placed in a water bath to maintain a constant temperature.
• Leave the set up in the dark for 2 hours, to stop all photosynthesis
• The set up is now ready for the investigation

Question 32

How would you measure the rate of photosynthesis in an aquatic plant using a Photosynthometer

Answer 32

Measure the rate of O(2) production:
- After the photosynthometer has been left in the dark for 2 hours, a light is switched on for 30 mins
- In this time, O(2) will be released as photosynthesis occurs, and will be collected in the capillary tube
- After 30 mins, the O(2) is drawn up the tube using the syringe on the other end, until the volume can be measured on the scale
- The rate of photosynthesis = rate of O(2) production
- The syringe can be used to empty the collected O(2), so the experiment can be repeated for different light intensities/wavelengths to compare the rate
(allow for 2hrs in darkness between each repeat)

Question 33

What is the set up for the ‘Lollipop experiment’ used to determine the process of the LIR of photosynthesis

Answer 33

Large ‘lollipop’ vessel containing algae in nutrient medium
- Valve at the top, so the algae can be added
- Tube near the base to allow air/CO(2) in
- Tube near the top to allow air/O(2) out
- Syringe on the side for injecting radioactive C-14
- Valve at the base, above a beaker of hot methanol, to kill the algae

Question 34

How does the ‘Lollipop experiment’ determine the process of the LIR of photosynthesis

Answer 34

• Radioactive C-14 is added to the algae in the vessel
• Light is shone on the vessel to induce photosynthesis
• After different time periods (eg. repeats, at 5s intervals), the algae is killed by passing it into the heated methanol, to stop the metabolism (photosynthesis)
• Dead algae samples are analysed with 2D chromatography, separating out the different carbon compounds
• Radioactive compounds on the chromatogram are identified using autoradiography (x-ray film exposure)
• By comparing the compounds present after different periods of light exposure, the order in which the compounds are produced was determined.