Photosynthesis Flashcards
(5 cards)
(a) identify structure of chloroplasts in drawings, photomicrographs and electronmicrographs.
The thylakoids possess the following properties which allow the light-dependent reactions of photosynthesis to be carried out:
1) Large surface area of thylakoid membrane
• Allows photosystems and electron carriers to be embedded.
• Allows stalked particles containing ATP synthase to be embedded.
2) Thylakoid membrane is impermeable to protons
• Allows electrochemical proton gradient to be set up between the thylakoid space and stroma.
The stroma is a dense fluid which contains the enzymes and dissolved substrates required for the light-independent reactions of photosynthesis, also known as the Calvin cycle.
Excess carbohydrate from photosynthesis is stored as starch grains in the chloroplasts. Lipid droplets, which are associated with the breakdown of membranes and lipids, also accumulate within the chloroplasts.
(b) explain the absorption and action spectra of photosynthetic pigments.
Absorption and Action Spectra
a) Absorption spectrum
• A graph showing the relative absorbance of different wavelengths of light by a pigment.
b) Action spectrum
• A graph showing the effectiveness of different wavelengths of light in stimulating photosynthesis.
• It is a record of the amount of photosynthesis occurring at each wavelength of light.
The close similarity/correlation between the absorption spectrum and action spectrum indicates that the photosynthetic pigments are responsible for absorption of light in photosynthesis.
Chlorophylls
There are 2 main types of chlorophyll – chlorophyll a and chlorophyll b. They absorb mainly red and blue-violet light, reflecting green light and therefore giving plants their characteristic green colour.
Their chemical structure consists of:
• a head made up of a porphyrin ring with a magnesium ion in the centre, and
• a long hydrocarbon tail which is joined to its head by an ester linkage. Different chlorophylls have different side-chains on the head and these modify their absorption spectra.
(b) Carotenoids
There are 2 main types of carotenoids – carotenes and xanthophylls. They are yellow, orange, red or brown pigments that absorb strongly in the blue-violet light spectrum. They are usually masked by the green chlorophylls.
Carotenoids protect chlorophylls from excess light and oxidation by oxygen produced during photosynthesis.
(c) with reference to the chloroplast structure, describe and explain how light energy is harnessed and converted into chemical energy during the light-dependent reactions of photosynthesis.
Light-dependent Reactions
Light-dependent reactions occur in the grana. The objective of these reactions is to provide ATP and reduced nicotinamide adenine dinucleotide phosphate (reduced NADP / NADPH + H+) for the light-independent reactions (Calvin cycle).
Non-cyclic Photophosphorylation
Step 1:
• Light of particular wavelengths strikes an accessory pigment molecule in the light harvesting complex of PSII and PSI.
• This energy is relayed to neighbouring accessory pigment molecules until it accumulates and reaches one of the two P680 chlorophyll a molecules in the reaction centre of PSII.
• The same occurs for the P700 chlorophyll a molecules in the reaction centre of PSI.
Step 2:
• This excites one of the P680 electrons and one of the P700 electrons to a higher energy state, which subsequently gets emitted and captured by the primary electron acceptor within each PS.
• A positive ‘hole’ is left behind in each P680 and P700 chlorophyll a molecule in PSII and PSI respectively.
Step 3 (occurs concurrently with Step 4): • Photolysis of water occurs when an enzyme catalyses the splitting of a water molecule into protons, electrons and molecular oxygen. H2O--> 2H+ + 2e- + ½ O2 • Electrons from the photolysis of water are used to fill up positive ‘holes’ in the reaction centre of PSII to return P680+ to ground state.
Step 4 (occurs concurrently with Step 3): • The photoexcited electron (that was emitted by P680 in PSII previously) passes from the primary electron acceptor of PSII to P700+ in PSI, to fill the positive ‘hole’ in P700+. • This occurs via an electron transport chain made up of electron carriers, each with an energy level lower than the one preceding it.
Step 5:
• Energy from the electron transfer down the chain of electron carriers is used to actively pump protons from the stroma into the thylakoid space.
• This generates an electrochemical proton gradient for the synthesis of ATP.
• Protons diffuse through the stalked particle containing ATP synthase which catalyzes the synthesis of ATP from ADP and Pi into the stroma.
• This process by which protons (H+) diffuse through a stalked particle for the synthesis of ATP is known as chemiosmosis.
Step 6:
• Electrons are subsequently passed from the primary electron acceptor of PSI to the protein ferredoxin (the last electron carrier).
Step 7:
• The enzyme NADP reductase catalyses the transfer of electrons from ferredoxin to oxidised NADP (the final electron and proton acceptor) to form reduced NADP. NADP+ + 2e- + 2H+ –> NADPH + H+
Cyclic Photophosphorylation
In cyclic photophosphorylation, the electrons follow a different route.
PSI is now both a donor and acceptor of electrons. The excited electrons in the primary electron acceptor of PSI pass to ferredoxin and back to the cytochrome complex in the electron transport chain. The electrons eventually return to the PSI reaction centre.
Cyclic photophosphorylation does not involve photolysis of water.
The energy released during the cycle of electrons down the chain of electron carriers allows protons to be actively pumped from the stroma into the thylakoid space, generating an electrochemical proton gradient across the thylakoid membrane, just like in non-cyclic photophosphorylation.
The electrochemical proton gradient allows for the synthesis of ATP by the stalked particles embedded on the thylakoid membrane.
Cyclic photophosphorylation yields only ATP whereas non-cyclic photophosphorylation yields O2, ATP and reduced NADP.
(d) outline the three phases of the Calvin cycle in C3 plants: (i) CO2 fixation (ii) PGA reduction and (iii) ribulose bisphosphate (RuBP) regeneration, including the roles of rubisco, ATP and NADP in these processes that ultimately allow synthesis of sugars.
The light-independent reaction / Calvin cycle occurs in the stroma of the chloroplast. There are 3 main stages:
(a) CO2 fixation (CO2 uptake)
RuBP + CO2 + H2O —-RuBP carboxylase-oxygenase (Rubisco)–> unstable 6C intermediate–> 2 GP
RuBP = ribulose bisphosphate,
5C sugar GP = glycerate-3-phosphate
RuBP carboxylase-oxygenase (Rubisco) is present in large amounts in the stroma of the chloroplast.
It catalyses the fixation of CO2 by a 5C sugar known as ribulose bisphosphate (RuBP), which gives an unstable 6C intermediate that immediately breaks down to 2 molecules of 3C compound known as glycerate-3-phosphate (GP).
GP can be converted to pyruvate which is used to synthesize fatty acids. The pyruvate that is formed can also be converted to acetyl coenzyme A which undergoes the Krebs Cycle (covered in ‘Respiration’) to form α-ketoglutarate. α-ketoglutarate can undergo further reactions to form amino acids in plants.
(b) Carbon Reduction
The reducing power of reduced NADP and energy from the hydrolysis of ATP are used to convert GP to GALP. GALP contains more chemical energy than GP, and is the first carbohydrate made in photosynthesis. About 1/6 of the total amount of GALP is used to synthesize glucose, other carbohydrates (e.g. sucrose and starch) and glycerol.
GALP = glyceraldehyde-3-phosphate (3C sugar)
(c) Regeneration of RuBP
About 5/6 of the total amount of GALP has to be used to regenerate the RuBP consumed in the first reaction. This process requires energy from the hydrolysis of ATP.
(e) discuss limiting factors in photosynthesis and carry out investigations on the effect of limiting factors such as temperature, light intensity and carbon dioxide concentration on the rate of photosynthesis.
A limiting factor is the one factor that is in the shortest supply and thus determines the rate of the overall reaction
(a) Light
(i) Light intensity
Light intensity is an important limiting factor in the light-dependent stage to excite the special chlorophyll a molecules for photophosphorylation to occur. However, it is seldom the limiting factor during daylight hours (except in the case of shaded plants).
Photosynthesis results in uptake of carbon dioxide and evolution of oxygen. At the same time respiration uses oxygen and produces carbon dioxide. There will come a point when the light intensity causes photosynthesis and respiration to exactly balance each other. This is called the light compensation point (i.e. light intensity at which net gas exchange is zero).
The light compensation points of plants grown in abundant sunlight is higher than those grown in shade
(ii) Wavelength of light
Wavelength of light is also a limiting factor as demonstrated by comparing the action and absorption spectra for photosynthesis. The rate of photosynthesis is highest at the red and blue-violet regions of the action spectrum and lowest at the green region.
(b) Temperature
Temperature is an important limiting factor as it affects the rate of enzyme reactions during light-dependent (e.g. NADP reductase) and light-independent stages (e.g. Rubisco).
(c) Carbon Dioxide
Carbon dioxide is a major limiting factor as its concentration in the atmosphere is low (0.03 – 0.04%). It is the raw material for the Calvin cycle and its increased concentration will increase the rate of photosynthesis.
