photosynthesis Flashcards
Photosynthesis definition
process by which energy in the form of light from the sun is used to build complex organic molecules (e.g glucose)
carbon dioxide + water –> glucose + oxygen
Energy transfer
light energy is transferred to chemical energy trapped in the bonds of the complex organic molecules produced
structure + function of chloroplasts
the network of internal membranes provides a large surface area to maximise the absorption of light
membranes form flattened sacs (thylakoids) which are stacked to form grana.
Grana are joined by membranous channels (lamellae)
light is absorbed by complexes of pigments which are embedded in the thylakoid membrane
fluid in the chloroplast (stroma) - the site of many chemical reactions resulting in the formation of complex organic molecules - it contains enzymes, sugars and organic acids
carbohydrates produced by photosynthesis and not used straight away are stored as starch grains in the stroma
Photosynthetic pigments including chlorophyll
pigment molecules absorb specific wavelengths of light and reflect others - different pigments absorb + reflect different wavelengths which is why they are different colours
Chlorophyll - usually absorbs red + blue light and reflects green
Pigments include chlorophyll a, chlorophyll b, xanthophylls and carotenoids
pigments are found in the thylakoid membranes (attached to proteins) - the protein + pigment is called a photosystem
proteins + pigments form a light harvesting system (antennae complex) which absorbs light of different wavelengths and transfers this energy to the reaction centre
chlorophyll a is located in the reaction centre - where the reactions involved in photosynthesis take place
photosystem - light harvesting system + reaction centre
primary pigments
they are reaction centres where electrons are excited during the light dependent reaction
accessory pigments
make up light harvesting systems - surround reactions centres + transfer light energy to them to boost the energy for electron excitement to take place
Photosystem 1 optimum wavelength for absorbing light
700 nm
Photosystem 2 optimum wavelength for absorbing light
680 nm
what are the 2 stages of photosynthesis? (Overview)
Light dependent stage:
- energy from sunlight is absorbed and used to make ATP
- Hydrogen from water is used to reduce coenzyme NADP to reduced NADP
Light independent stage:
- H from reduced NADP and CO2 are used to build organic molecules
- ATP is needed to provide energy for the reaction
use of coenzymes in photosynthesis
Coenzyme - a molecule that aids the function of an enzyme - usually by transferring a chemical group from 1 molecule to another
In photosynthesis:
- NADP transfers hydrogen from one molecule to another
photolysis definition
the splitting of a molecule using light
Photophosphorylation definition
adding phosphate to a molecule using light
where does the light dependent reaction take place?
thylakoid membrane
Photosystems are linked by…
electron carriers
electron carriers definition
proteins that transfer electrons
electron transport chain definition
a chain of proteins through which electrons flow
- formed from photosystems and electron carriers
Non- Cyclic photophosphorylation produces …
ATP
Reduced NADP
O2
Stages of the light dependent reaction
1) LIGHT ENERGY EXCITES ELECTRONS IN CHLOROPHYLL:
- light energy is absorbed by PS II
- Light energy excites electrons in chlorophyll
- the electrons move to a higher energy level
- these high-energy electrons move along the electron transport chain to PS I
2) PHOTOLYSIS OF WATER PRODUCED H+ ions, electrons and O2
- as the excited electrons from chlorophyll leave PS II they have to be replaced
- Light energy splits water into H+, electrons and O2
REACTION EQUATION:
H2O –> 2H+ + 1/2 O2
3) ENERGY FROM THE EXCITED ELECTRONS MAKES ATP:
- the excited electrons lose energy as they move along the electron transport chain
- this energy is used to transport protons into the thylakoid, via membrane proteins (proton pumps) so the thylakoid has a higher proton concentration than the stroma (forms a proton gradient across the membrane)
- protons move down their concentration gradient into the stroma via an enzyme (ATP synthase). The energy from this movement combines ADP and Pi to form ATP
4) Generates reduced NADP
- Light energy is absorbed by PS I which excites the electrons to a higher energy level
- The electrons are transferred to NADP along with a proton from the stroma forming reduced NADP
oxygen evolving complex
forms part of PS II
- an enzyme that catalyses the breakdown of water
cyclic photophosphorylation
- only uses PS I
- the electrons from chlorophyll aren’t passed onto NADP - instead they are passed back to PS I via electron carriers
- doesn’t produce O2 or reduced NADP - only small amounts of ATP
where does the light independent reaction take place?
stroma
Stages of the light independent reaction (Calvin cycle)
- doesn’t rely on light energy directly but does rely on the products of the light-dependent reaction
1) CO2 IS COMBINED WITH RIBULOSE BISPHOSPHATE TO FORM 2 MOLECULES OF GLYCERATE 3-PHOSPHATE
- CO2 enters the leaf via the stomata and diffuses into the stroma of the chloroplast
- CO2 is combined with RuBP (a 5-C compound), giving an unstable 6-C compound (carbon fixation). This breaks down into 2 molecules of a 3-C compound - Glycerate-3-phosphate (GP)
- RuBisCo catalyses this reaction between CO2 and RuBP
2) ATP AND REDUCED NADP ARE REQUIRED FOR THE REDUCTION OF GP TO TP
- ATP from the LDR provides the energy to turn GP into a different 3-C compound - triode phosphate (TP)
- This reaction required H+ ions which come from reduced NADP. Reduced NADP is recycled to NADP for use in the LDR
- TP can be converted into many useful compounds e.g glucose
3) RuBP IS REGENERATED
- 5 out of every 6 molecules of TP are used to regenerate RuBP
- this requires the rest of the ATP produced by the LDR
Summary of the Calvin cycle
FIXATION:
CO2 is fixed in the first step
REDUCTION
GP is reduced to TP by the addition of H from reduced NADP
REGENERATION:
- RuBP is regenerated from the recycled TP
Why are the issues with the enzyme RuBisCo
- it is inefficient as it is competitively inhibited by O2 and lots of it is needed to carry out photosynthesis successfully
Regeneration of RuBP
For 1 glucose molecule to be made, 6 CO2 molecules need to enter the Calvin cycle - this results in the production of 12 TP molecules
2 TP molecules are removed to make glucose
10 TP molecules (10 x 3 carbons) are recycled to regenerate 6 RuBP molecules (6 x 5 carbons)
The cycle has to turn 6 times to make 1 hexose sugar
6 turns of the Calvin cycle require 18 ATP molecules and 12 reduced NADP
What are GP and TP used to make
Carbohydrates
- hexose sugars are made by joining 2 TP molecules
- larger carbohydrates are made by joining hexose sugars together in different ways
Lipids:
- glycerol is synthesised from TP
- fatty acids are synthesised from GP
Amino acids:
- some are made from GP
Optimum conditions for photosynthesis
1) High light intensity of a certain wavelength
- the higher the light intensity, the more energy it provides
- photosynthetic pigments only absorb red and blue light
2) Temp - around 25 degrees C
- if temp is too low (below 10 degrees) RuBisCo becomes inactive but if it is too high (above 45) it may denature
- chlorophyll could be damaged - this can reduce the amount of pigment that can absorb light energy
- thylakoid membranes may be damaged - this could reduce the L-D stage reactions by reducing the number of sites for electron transfer
- the membranes around the chloroplast could be damaged - this could cause enzymes used in the Calvin cycle to be released which would reduce the light-independent stage reactions
3) CO2 ar 0.4%
- increasing CO2 increases rate of photosynthesis but any higher conc means the stomata will start to close
How does light intensity affect the rate of reaction?
As light intensity increases, ATP and reduced NADP are produced at a higher rate
In low light intensities:
- products of the L-D reaction (ATP + reduced NADP) will be in short supply
- the conversion of GP to TP and RuBP is slow
- the level of GP increases and levels of TP and RuBP fall (as they’re used to make GP)
- there is less TP available to regenerate RuBP
How does CO2 concentration affect the rate
Increasing the CO2 concentration:
- increases the rate of carbon fixation in the Calvin cycle
- increases the rate of TP production
At low CO2 concentrations:
- conversion of RuBP to GP is slow (as there is less CO2 to be fixed)
- levels of RuBP rise
- levels of GP and TP fall ( as they’re used to make RuBP)
how does temperature affect the rate
As temperature increases:
- the rate of enzyme controlled reactions increases until the enzymes denature
- increased rate of carbon fixation
- when the enzymes denature, levels of RuBP, GP and TP fall
At low temperatures:
- reactions are slower as enzymes work more slowly
- levels of RuBP, GP and TP fall
how does water stress affect rate
- if plants don’t have enough water, their stomata will close to preserve water
- less CO2 will enter the leaves
- slows down rate
Using TLC to separate photosynthetic pigments
Stationary phase - thin layer of silica gel applied to gas
Mobile phase - liquid solvent
- Different solubilities result in of pigments in the mobile phase and differing interactions with the stationary phase lead to them moving at different rates
Method:
1) Grind up several leaves with anhydrous sodium sulfate and some propane
2) Transfer the liquid to a test tube and add some petroleum ether. Gently shake the tube - 2 distinct layers will form in the liquid - the top layer is the pigments mixed with the petroleum ether
3) transfer some liquid from the top layer into a second test tube with some anhydrous sodium sulfate.
4) Draw a horizontal pencil line near the bottom of the chromatography plate. Build up a single concentrated spot of the solution from step 3 on the line (point of origin)
5) Once the point of origin is dry, put the plate into a glass beaker with some prepared solvent (a mixture of propanone, cyclohexane and petroleum ether). The point of origin should be above the solvent.
6) Put a lid of the beaker and leave the plate to develop - as the solvent spreads up the plate, the different pigments move with it but at different rates
7) When the solvent has nearly reached the top, take the plate out and mark the solvent front with a pencil and leave it to dry in. well-ventilated plate
8) Calculate the Rf value and look them up in a database to identify what the pigments are
chemiosmosis definition
the process of electrons flowing down the electron transport chain and creating a proton gradient to drive ATP synthesis
