Photosynthesis Flashcards
What are autotrophs?
organisms that sustain themselves without feeding on other organisms
opposite. heterotrophs (consumers, humans)
eg. plants
- use light as a source of energy to synthesise organic compounds -> known as photoautotrophs
plants undergo photosynthesis where light energy is used to synthesise carbohydrates from carbon dioxide and water. oxygen is given out in the process as a by-product.
2 main stages in photosynthesis
- light-dependent
- light-independent (Calvin cycle)
Structures of chloroplasts
- thylakoids of the grana
- stroma (Calvin)
- Excess carbohydrate from photosynthesis is stored as starch grains in the chloroplasts.
Thylakoids properties which allow the light-dependent reactions to be carried out
1) Large surface area of thylakoid membrane
* Allows numerous 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.
Stroma
a dense fluid which contains the enzymes and dissolved substrates required for the light-independent reactions of photosynthesis, also known as Calvin cycle
photosynthetic pigments
- chlorophylls
- carotenoids
these pigments are found in the photosystems (PS) which are embedded on the thylakoid membranes. the role of the pigments is to absorb light energy and convert it into chemical
energy.
Chlorophyll
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.
Structure of chlorphyll
- 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.
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.
what is Absorption spectrum?
A graph showing the relative absorbance of different wavelengths of light by a
pigment.
What is 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.
correlation between absorption spectrum and action spectrum
The close similarity/correlation between the absorption spectrum and action spectrum
indicates that the photosynthetic pigments are responsible for absorption of light in
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) for the light-independent reactions (Calvin cycle).
- As the energy for the synthesis of ATP comes from light, it is called photophosphorylation.
There are 2 types of photophosphorylation – cyclic and non-cyclic.
Photosynthetic units
There are 2 types of photosynthetic units: photosystem I (PSI) and photosystem II (PSII).
Each photosystem comprises:
i. Light-harvesting complexes
- Consists of accessory pigments (chlorophylls and carotenoids).
- Accessory pigments funnel the energy absorbed from light to the reaction
centre in either PSI or PSII.
- The light-harvesting complexes surround the reaction centre.
ii. A reaction centre
- A protein complex that includes two special chlorophyll a molecules (primary
pigments) and a molecule called the primary electron acceptor.
- Two different forms of special chlorophyll a, P680 and P700 (P stands for primary
pigment of chlorophyll a while 680 and 700 are their absorption peaks’ wavelength
in nm).
- The two special chlorophyll a molecules each emit one electron and the two
electrons are accepted by the primary electron acceptor.
Excitation of Primary Pigments by Light (Absorption of Light Energy)
Primary pigments (i.e. special chlorophyll a in the reaction centres) are molecules that
absorb energy from accessory pigments. When the energy levels of the electrons found in
the primary pigments are boosted, they reach an excited state. The excited pigments will
then emit their electrons, leaving positive ‘holes’ in their molecules.
chlorophyll -> (light) chlorophyll+ + e-
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 -> (enzyme) 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 back into the
stroma, down the electrochemical proton gradient. This provides enough energy for
ATP synthase to catalyze the synthesis of ATP from ADP and Pi.
* 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+ -> (NADP reductase) NADPH + H+
Establishment of Electrochemical Proton Gradient (Proton Motive Force)
Across the Thylakoid Membrane
(a) Higher concentration of protons in thylakoid space
1. Water undergoes photolysis in the thylakoid space, generating protons in the process.
2. As electrons are passed from one electron carrier to the next, energy is released to actively
pump protons across the membrane into the thylakoid space.
(b) Lower concentration of protons in stroma
3. Protons are removed from the stroma when they are taken up by oxidised NADP.
(c) Impermeable nature of thylakoid membrane to protons allows the gradient to be established
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.
stages of light-dependent reactions (Calvin cycle)
- CO2 fixation
- Carbon reduction
- Regeneration of RuBP
Light-dependent reactions: CO2 fixation
RuBP + CO2 + H2O -> (RuBP carboxylase-oxygenase) (Rubisco) unstables 6C intermediate -> 2 GP
RuBP = ribulose bisphosphate, 5C sugar
GP = glycerate-3-phosphate previously known as Phosphoglyceric acid (PGA)
- 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). - Rubsico thus regulates the rate of photosynthesis.
Light-dependent reactions: carbon reduction
GALP = glyceraldehyde-3-phosphate (3C sugar)
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.
ATP -> ADP + Pi
Reduced NADP -> oxidised NADP
Light-dependent reactions: 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.
This regeneration of RuBP makes this process a cycle.
In order to generate 1 molecule of GALP (3C sugar) from the Calvin cycle, we require:
* 3 CO2
* 9 ATP
* 6 reduced NADP
Therefore, in order to generate 1 molecule of glucose (6C sugar), we require:
* 6 CO2
* 18 ATP
* 12 reduced NADP
What is a limiting factor
A limiting factor is the one factor that is in the shortest supply and thus determines the
rate of the overall reaction.
limiting factors
- 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. - 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. - 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). - 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.
