A LEVEL: Biology (F214), Photosynthesis
BIOLOGY: UNIT 1
PHOTOSYNTHES
The process by which light energy from the sun is transformed into chemical energy and used to synthesise large organic molecules from inorganic substances.
Photoautotrophs have organelles called chloroplasts that help with photosynthesis. Similar to eukaryotic cells in terms of them having prokaryote ribosomes, similar pigments, same size and circular DNA.
- Chloroplasts vary in shape but are generally 2-10 micrometers
- Double membrane envelope
- Outer membrane is permeable to small ions
- Inner one is less permeable and has transport proteins embedded in it.
- Granum is a stack of thylakoids
- Stroma (light independent) & Grana (light dependent) are distinct.
photosynthetic pigments absorb light at certain wavelengths although they may reflect others.
Chlorophyll
(mixture of different pigments)
- P680 (PS11) & P700 (PS1) Chlorophyll A
- Appears yellow green, absorbs red light at different wavelengths : 680nm
and 700 nm respectively.
- Chlorophyll a - absorbs red and blue light around 450nm
- Chlorophyll b - absorbs wavelength around 500 - 640 nm, appears blue-green
Accessory pigments: carotenoids, absorbs light wavelengths that are not wee absorbed by chlorophylls and pass the energy on into chlorophyll a at the bottom of the photosystem.
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THE LIGHT INDEPENDENT STAGE:
Takes place on the thylakoid membranes of the chloroplasts.
Photosystems are embedded in these membranes.
Photosystem II contains an enzyme that can split water in a process called photolysis, as it is in the presence of light.
2H2O ——> 4H+ + O2
Water is used for:
- Chemisosmosis
- Create reduced NADP for the light independent stage of the reaction
Oxygen is mainly lost through the stomata although some is used for respiration.
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Photophosphorylation:
Is the process that makes ADP from inorganic Phosphate, in the presence of light.
Photon of light hits chlorophyll a, at photosystem II, this provides the energy for photolysis to occur and the water gets broken down into oxygen, two electrons and two hydrogens. The electrons replace the pair of excited electrons which are captured by electron acceptors and then carried across the electron carrier system.
As the electrons pass along the electron carriers energy is released and protons are pumped across the thylakoid membrane and into the thylakoid space where they accumulate and create a proton gradient. Flow of electrons is called chemiosmosis, and this drives the protons back across the membrane and through ATP Synthase and creates ATP which is used in the light independent reaction.
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THE LIGHT INDEPENDENT STAGE
- CO2 diffuses through the air in the chloroplast, where it then binds with RuBP and the rubisco enzyme. The enzyme catalyses the reaction to form.
- Product of this reaction is two molecules of 3C compound. glycerate-3-phophate. (GP)
- GP is then reduced and phosphorylated to another 3 carbon compound called triose phosphate. ATP and reduced NADP are used in this process. ATP becomes ADP and reduced NADP becomes NADP.
- Only 5/6 of the TP are recycled by phosphorylation, using ATP from the light dependent reaction, to form three molecules of RuBp.
LIMITING FACTORS:
The limiting factor for a metabolic process is the factor that is present at the lowest or least favourable value.
Constant temperature: Rate of photosynthesis varies with light intensity. Zero light intensity = no photosynthesis
Low light intensity: As light intensity intensifies so does the increase of rate of photosynthesis. So light is limiting the rate of photosynthesis.
As light intensity readily increases the rate of photosynthesis begins to plateau and light intensity is no longer the limiting factor. Changing light intensity does not change the rate of photosynthesis.
Another factor such as concentration of CO2 may be the new limiting factor.
Increasing CO2 may increase the rate of photosynthesis, but the graph will once again begin to plateau and incasing the CO2 will not affect the rate of photosynthesis and so temperature is the new limiting factor.
However even increasing temperature will eventually reach the plateau again and even so if the temperature reaches too high, proteins and enzymes required for photosynthesis will begin to denature.
Law of Limiting Factor:
at any given moment, the rate of metabolic processes is limited by the factor that is present in the least favourable value.
Effect of Carbon Dioxide Concentration on the rate of photosynthesis
In a green house, carbon dioxide concentration can be increased by burning methane or oil fired heaters. This increases the rate of photosynthesis if there are no other limiting factors.
Oceans act as carbon sinks and absorbs 1/3 of the carbon dioxide produced from human activities. Growing forests can absorb the carbon dioxide but mature forests take in almost as much CO2 from photosynthesis that they release, due to decomposition and respiration.
Effect of Light Intensity of the rate of photosynthesis
The rate of photosynthesis is directly proportional to the list intensity. As light intensity increases so does the rate of photosynthesis.
- It causes stomata to open and CO2 enters the leaves and cells
- It is trapped by chlorophyll where it excites electrons
- It spirits water molecules to produce protons
The electrons and protons are then used for photophosphorylation in which ATP is produced. Rate of photosynthesis varies throughout the day along with the light intensity.
Effect of Temperature on the rate of photosynthesis
Affects the enzyme catalysed reaction of the Calvin Cycle.
0ºC - 25ºC —> Rate of photosynthesis doubles for each 10ºC rise in temperature.
High temperatures —>
more water loss from the stomata, leading to stress response in which the stomata close and this limits the carbon dioxide availability.
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LIMITING FACTORS AND THE CALVIN CYCLE
LIGHT INTENSITY
/ = 1/d2
More light energy = more excited electrons
Electrons take part in photophosphorylation, increased light means more ATP and reduced NADP is produced.
Both are used in the light-independent reaction.
The hydrogen and energy help to reduce glycerate-phosphate (GP) to triose phosphate (TP)
The hydrogen and energy help to reduce glycerate-phosphate (GP) to triose phosphate (TP)
ATP is used to phosphorylate five out of every six molecules of TP to regenerate RuBP.
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LOW LIGHT INTENSITY
GP increases in low light intensity because there is less ATP and Reduced NADP produced to reduce GP into TP.
GP begins to accumulate.
Levels of TP falls, and so will RuBP as there is less fixation of CO2 and TP is not phosphorylated into RuBP.
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CARBON DIOXIDE CONCENTRATION
More CO2 means more CO2 fixations in the Calvin Cycle and more GP is produced. If light intensity is not a limiting factor.
More GP means more TP and amino acids/fatty acids.
More TP converted to hexose sugars, and
disaccharides/polysaccharides.
More regeneration of RuBP.
Stomata which are open to allow gaseous exchange lead to increased transpiration so plant may wilt.
If water uptake does not exceed water loss by transpiration, a stress response follows:
Plant releases: abscisic acid, stomata close.
This reduces CO2 uptake and rate of photosynthesis decreases.
If carbon dioxide concentration drops then RuBP accumulates, and TP and GP concentrations fall.
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TEMPERATURE
Increasing temperature has little effect on the light dependent reaction, as only the photolysis of water requires the use of enzymes.
It will alter the rate of light-independent reactions as it has many enzyme catalysing reactions.
- 25°C <
(the oxygenase activity of rubisco exceeds the rate of carboxylase reactions)
- So photorespiration is exceeding photosynthesis.
- More ATP and Reduced NADP from the light dependent reactions are being wasted.
- Overall rate of photosynthesis decreases.
- High temperatures will increase rate of transpiration and water loss. So stomata may close, and this reduces rate of photosynthesis again.
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