Section 5 - Energy transfer in and between organisms: 11. Photosynthesis Flashcards
What is the site of photosynthesis
The leaf is the main photosynthetic structure, containing chloroplast.
What are the main components within the structure of a leaf
- Waxy cuticle
- Upper epidermis
- Palisade mesophyll (contains chloroplast)
- Spongey mesophyll
(xylem and phloem) - Lower epidermis
(contains stomata surrounded by guard cells)
How is the leaf adapted for it’s function
- 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
What are are the main structural components of chloroplast
Disc shaped organelle containing:
- Double membrane
- Grana (Made up of thylakoids)
- Stroma
- Ribosomes
- Chloroplast DNA
What is a thylakoid within chloroplast
Disc-like structures, containing chlorophyll, acting as the site of the LDR in photosynthesis
What are grana within chloroplast
Stacks of roughly 100 thylakoids, joint together by intergranular lamella
What is the stroma within chloroplast
Fluid filled surrounding the grana, acting as the site of the LIR in photosynthesis
What is the equation of photosynthesis
6CO(2) + 6H(2)O → C(6)H(12)O(6) + 6O(2)
Carbon Dioxide + Water →(Light)→ Glucose + Oxygen
What are the main stages of photosynthesis
- Capturing light energy
- The Light dependent reaction (LDR)
- The Light independent reaction (LIR)
What is the role of the LDR of photosynthesis
Uses captured light energy for the synthesis ATP and the photolysis of water, releasing electrons
What are the main processes in the LDR of photosynthesis
- Photolysis of water
- Electron transport chain (REDOX reactions)
- Movement of H+ ions into the thylakoid
- Synthesis of ATP (Chemiosmosis)
- Production of reduced NADP
How does the photolysis of water occur during the LDR
- 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
What happens along the electron transport chain during the LDR of photosynthesis
- 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
What is photoionisation
When light energy is absorbed (by the chlorophyll) and excites the electrons to a higher energy level
How are H+ ions moved into the thylakoid during the LDR of photosynthesis
- 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
How is ATP synthesised in the LDR of photosynthesis (Chemiosmotic theory)
- 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
How is reduced NADP produced during the LDR of photosynthesis
- 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
What are the overall products of the LDR of photosynthesis
ATP and Reduced NADP are released to be used in the LIR, and oxygen is released as a by product
How is the thylakoid adapted to be the site of the LDR of photosynthesis
- 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.
What is the role of the LIR of photosynthesis (Calvin cycle)
Uses the products of the LDR to reduce ‘glycerate 3-phosphate’, leading to the production of glucose
What are the main processes in the LIR of photosynthesis (Calvin cycle)
- Carbon fixation (RuBP → GP)
- Reduction (GP → TP)
- Regeneration (TP → RuBP)
What happens during the carbon fixation stage of the LIR of photosynthesis (Calvin cycle)
- 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)
What happens during the reduction stage of the LIR of photosynthesis (Calvin cycle)
- 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
How is glucose produced during the Calvin cycle
- 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
What happens during the regeneration stage of the LIR of photosynthesis (Calvin cycle)
- 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
What are the overall products of the LIR of photosynthesis
Produces 1 molecule of glucose every 6 cycles, but releases ADP + Pi and NADP to be reused in the LDR
How is the stroma adapted to be the site of the LIR of photosynthesis
- 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.
‘What is the law of limiting factors’ for a reaction
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)
What is the ‘saturation point’ for a factor effecting the rate of a reaction
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.
What is the ‘light compensation point’ for photosynthesis
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)
How do you set up a Photosynthometer to measure the rate of photosynthesis in an aquatic plant
- 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
How would you measure the rate of photosynthesis in an aquatic plant using a Photosynthometer
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)
What is the set up for the ‘Lollipop experiment’ used to determine the process of the LIR of photosynthesis
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
How does the ‘Lollipop experiment’ determine the process of the LIR of photosynthesis
- 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.
