Photosynthesis

  • Created by: amyquince
  • Created on: 04-06-19 20:23

KEY TERMS

- metabolic pathway - a series of small reactions controlled by enzymes

- phosphorylation - adding phosphate to a molecule

- photophosphorylation - adding phosphate to a molecule using light

- photolysis - the splitting of molecule (lysis) using light (photo) energy

- hydrolysis - the splitting of a molecules using water

- decarboxylation - the removal of carbon dioxide from a molecule

- dehydrogenation - the removal of hydrogen from a molecule

- redox reactions - reactions that involve oxidation (removal of electrons or hydrogen or gain of oxygen) and reduction (gained electrons or hydrogen or loss of oxygen)

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INVOLVEMENT OF COENZYMES

- coenyzme aids function of enzyme

- usually work by transferring chemical goup from one molecule to another

- NADP used in photosynthesis - transfers hydrogen from one molecule to another

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CHLOROPLASTS

- small, flattened organelles found in plant cell - location for photosynthesis

-double membrane called chloroplast envelope

- thylakoids are stacked in grana, grana linked by bits of thylakoids called lamallea

- contain photosynthetic pigments (chlorophll a, chlorophyll b and carotene) - coloured substances that absorb light energy needed for photosynthesis - found in thylakoid membranes, attached to proteins

- protein and pigment called photosystem

- contains 2 types of photosynthetic pigments - primary and accessory

- primary pigments are reaction centres where electrons are excited during light-dependant reaction

- accessory pigments make up light-harvesting systems that surround reaction centres and transfer light energy to them

- 2 photosystems to capture light energy - photosystem I (absorbs light best at 700nm and photosystem II (absorbs light best at 680nm)

-stroms is in inner membrane of chloroplast and surroundin thylakoids - contains enzymes, sugats and organic acids

-circular DNA of chloroplast found in stroma, carbohydrates stored as starch grains also found in stroma. 

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STAGES OF PHOTOSYNTHESIS

LIGHT-DEPENDANT REACTION

- needs light

- takes place in thylakoid membranes

- light energy absorbed by photosynthetis pigments

- light energy used to add phosphate group to ADP to produce ATP and to reduce NADP to NADPH. ATP transfers energy and NADPH transfers hydrogen during light dependant reaction.

- H20 oxidised to O2

LIGHT-INDEPENDANT REACTION

- calvin cycle - doesnt use light energy

- takes place in stroma

- ATP and NADPH supply energy and hydrogen to make glucose from CO2

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SEPARATING PHOTOSYNTHETIC PIGMENTS

THIN LAYER CHROMATOGRAPHY

1. grind uo leaves with anhydrous sodium sulfate and propanone

2. transfer liquid to test tube, add petroleum ether and shake tube. 2 distinct layers will form in liquid - top layer is the pigments mixed in with the petorleum ether

3. transfer some of liquid from top layer into second test tube with some anhydrous sodium sulfate

4. draw horizontal pencil line near bottom of chromatography plate. build up a single concentrated spot of liquid on line by applying several drops - this is point of origin

5. once point of origin in completely dry, place plate into glass beaker with some prepared solvent. put lid on beaker and leave to develop. as solvent spreads up plate, the different pigments move with it but at different rates so they separate.

6. when solvent has nearly reached top, mark the solvent front with pencil and leave the plate to dry in well-ventilated plate

7. should be several spots between point of origin and solvent front. 

Rf value = distance travelled by spot / distance travelled by solvent

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LIGHT-DEPENDENT REACTION

1. light absorbed by photosystem II, excites electrons in chlorophyll, electrons move to higher energy level, high energy electrons move along electron transport chain to PSI.

2. as excited electrons from chlorophyll leaves PSII to move along electron transport chain, they must be replace, light enegy splits water into protos, electrons and oxygen

3. excited electrons move along the electron transport chain, energy used to transprt protons into the thylakoid, via membrane proteins called proton pumps, so that the thylakoid has a higher concentration of protons in the stroma, forms proton gradient across the membrane, protons move down concentration gradient into stroma via enzyme ATP synthase, energy combines ADP and inorganic phosphate to form ATP.

4. light energy is absorbed by PSI which excites electron again to an even higher energy level, electrons are transferred to NADP along with proton h+ from stroma to form NADPH

CYCLIC PHOSPHORYLATION

- PSII is non-cyclic, PSI is cyclic- electrons from chlorophyll molecule arent passed onto NADP but are passed back to PSI via electron carriers (electrons recycled, only produces small amount of ATP)

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LIGHT-INDEPENDENT REACTION

-the calvin cycle takes place in stroma

1. co2 enters leaf through stomata and diffuses into stroma, co2 combined with rubp which gives an unstable 6c molecule which qucily breaks into 2 3GP, rubisco catalyses reaction between co2 and rubp

2. ATP from light dependent reaction provides energy to turn 3GP into 3TP, this reaction also requires h+ ions which come from NADPH, reduced NADPH recycled to NADP and used in light dependent again.

3. TP then converted into many useful organic compounds eg: glucose.

4. 5 out of every 6 moleculesof TP produced in cycle arent usedto hexose sugars but to regenerated rubp

5. regenerating rubp uses the rest of ATP produced by light-dependent reaction

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TP AND GP

-used to make carbohydrates, lipids and amino acids

- carbohydrates - hexose sugars are made by joining together 2 TP molecules and larger carbohydrate molecules made by joining hexose sugars in different ways

- Lipids - made using glycerol which is synthesised from TP, and fatty acids which are synthesised by GP. 

- amino acids - some made from 3GP

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MAKING HEXOSE SUGARS

- calvin cycle needs to turn 6 times to make 1 hexose sugar

- 3 turns produces 6 molecules of TP, because 2 molecules of TP are made for every one co2 molecule used

- 5 out of 6 of these TP molecules are used to regenerate rubp

- means that 3 turns of cycle only produces 1 TP molecule

- hexose sugar has 6 carbons so 2 molecules of TP are needed to form one hexose sugar

- this means cycle must turn 6 times to produce 2 molecules of TP

- 6 turns needs 18 ATP and 12 NADPH from light-dependent reaction. 

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LIMITING FACTORS IN PHOTOSYNTHESIS

- LIGHT INTENSITY - light needed to provide energy or light-dependent reaction, the higher the intensity the more energy it provides, only certain wavelengths are used (chlorophyll a, chlorophyll b and carotene only absorb red and blue light in sunlight, green is reflected - which is why they look green)

- TEMPERATURE - photosynthesis involves enzymes so if temp drops below 10 enzymes become inactive but if above 45 enzymes may start to denature, stomata close to avoid losing water which causes photosynthesis to slow down as less co2 is entering the leaf, thylakoid membranes may be damaged which reduces rate of light dependent reaction by reducing number of sites available for electron transfer. membranes around chloroplasts could be damaged which could cause enymes to be released into cell which reduces rate of reactions. chlorophyll could be damaged which would reduce amount of pigment that can absorb energy

- CARBON DIOXIDE - make up 0.04% of gases in atmosphere, increasing this to 0.4% gives a higher rate of photosynthesis, but at any higher and stomata start to close.

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LIMITING FACTORS AFFECT ON GP, RUBP AND TP

- LIGHT INTENSITY - in low light intensity NADPH and ATP will be in short supply. means conversion of GP to TP and RUBP is slow. so levels of GP will rise (as its still being made) but levels of TP and RUBP will fall (as theyre being used to make GP)

- TEMPERATURE - reactions in calvin cycle catalysed by enzymes. at low temps all reactions will be slower as enzymes work slower. means levels of RUBP, GP and TP will fall. GP, TP and RUBP are affected in same way in high temps because enzymes denature.

- CARBON DIOXIDE - at low co2 conc, conversion of RUBP to GP is slow (less co2 to combine with RUBP to make GP). so level of RUBP will rise and levels of GP and TP will fall (as theyre used up to make RUBP)

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INVESTIGATING LIMITING FACTORS

- pondweed can be used to measure effect of light intensity, rate at which o2 is produced can be easily measured and corresponds to the rate of photosynthesis

1. test tube containing pondweed and water connected to capillary tube full of water

2. tube of water connected to syringe

3. source of white light is placed at specific distance from the pondweed

4. pondweed left to photosynthesise for set amount of time. as it photosynthesises, oxygen released will be collected in the capillary tube

5. at end of experiment, syringe is used to draw the gas bubble in the tube up alongside a ruler and length of gas bubble is measured. this is proportional to volume of co2 produced

6. any variables that could affect results should be controlled eg: temp, time the pondweed is left

7. experiment is repeated and average length of gas bubble is calculated to make results more precise

8. whole experiment repeated with light source placed at different distances from pondweed

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