Photosynthesis
Photosynthesis
- The process by which light energy is used to build complex organic molecules.
- Photosynthetic organisms are said to be autotrophic.
- Can be summarised by the equation; Carbon Dioxide + Water → Glucose + Oxygen
- Photosynthesis occurs in chloroplasts.
- Network of membranes present within chloroplasts provides a large surface area to maximise absorption of sunlight.
Chlorophyll
- The pigment which absorbs specific wavelengths of light.
- The primary pigment is chlorphyll a, alhtough there are other pigments such as chlorophyll b, xanthophylls and corotenoids which absorb different wavelengths.
- Chlorophyll b, xanthophyll and caroteinods are embedded in the thylakoid membrane in the chloroplast.
- With other proteins, they form a 'light harvesting system' known as an antennae complex.
- The complex's role is to absorb various wavelengths of light and transfer this energy quickly and efficiently to the reaction centre.
- Chlorophyll a is found in the reaction centre, which is where most photosynthetic reactions occur.
- The antennae complex and reaction centre are collectively known as a photosystem.
Light Dependant Phase; Non-Cyclic Photophopshorylation
- Sunlight strikes both photosystems 1 and 2, causing 2 eletrons to be excited and released from each photosystem's primary pigment reaction centre.
- These electrons are passed along an electron transport chain, by the carrier protein ferredoxin.
- As they move along the electron transport chain, ATP is formed.
- They electrons from PS2 replace those lost from PS1.
- The electrons from PS1 synthesise NADPH at NADH Reductase coenzyme (with H+ and NADP)
- To replace the electrons lost from PS2, a water molecule is split into 2e-, 2h+ and ½O2
- This is catalysed by the oxygen-evolving complex at PS2
- The hydrogen ions chemiosmose down a proton gradient to NADP Reductase coenzyme.
- On the way, the travel through ATP synthase helping to phosphorylate ADP to ATP.
- The oxygen is released as a gaseous waste product.
- Electrons leaving PS1 can be returned to PS1 again, instead of going to the NADP Reductase coenzyme.
- This allows for the production of ATP molecules, without a supply from PS2.
- When this happens, NADPH is not produced.
Light Independent Phase; Calvin Cycle
- The calvin cycle has 3 basic phases; fixation > reduction > regeneration
- RuBisCo, the carbon fixing enzyme, catalyses the formation of a 6 carbon unstable intermediate from CO2 and RuBP.
- This unstable intermediate immediately splits into two Glucose 3-phosphate molecules.
- GP is reduced to TP by H+ ions from NADPH, with the energy released from ATP.
- 5 out of 6 of these TP molecules are regenerated back into RuBP, with 1 in 6 being used to form a product.
- 2 GP molecules can bond to form amino acids or fatty acids.
- 2 TP molecules can bond to form a hexose sugar, such as glucose.
- TP can also be used in the formation of glycerol, which can then join with fatty acids (formed from GP) to form lipids.
Regeneration of RuBP
- As only 1 in 6 TP molecules is used to form a hexose sugar, the cycle undergoes 6 full cycles, requiring 6 CO2 molecules forming 12 TP molecules, two of which combine to form 1 hexose sugar.
- This means 10 TP molecules are recycled to form 6 RuBP molecules.
- This is because 10 TP molecules have a total of 30 shared carbons, and 30 ÷ 5 gives 6 RuBP (5C) molecules.
- This means 10 TP molecules are recycled to form 6 RuBP molecules.
Photorespiration
- Oxygen is a competitive inhibitor of the RuBisCo enzyme, as not only does it catalyse carbon fixation is photosynthesis, but it also catalyses the photorespiration reaction.
- This occurs when the CO2 concentration is very low, and results in phosphoglycogate; a toxic 2 carbon molecule which requires ATP to remove it.
- RuBisCo does however have a higher affinity for CO2 than O2, but still 25% of the products of the calvin cycle are lost to photorespiration.
- This makes it very difficult to measure the rate of photosynthesis accurately, as neither the CO2 taken in or O2 given off is a true reflection of the photosynthetic process alone.
Photosynthetic Pigment Chromatography and Retention Values
- Chromatography can be used to separate photosynthetic pigments because of differing solubility of pigments means they can travel further than others through silica gel.
- The retention value (Rf) can be calculated using the formula:
- Rf = Distance traveled by the pigment ÷ distance travelled by the solvent
- If light is too intense, photosynthetic pigments can be destroyed - in the summer chlorophyll is continuosly synthesised to maintain required levels for photosynthesis.
- If there is little or no light, chlorophyll is not produced.
- Carotenoids are responsible for the yellow/orange colouration on leaves, which are resistant to stong sunlight so continue to photosynthesise through the summer months.
- Leaves turn yellow during the autumn and winter, as green chlorophyll a is no longer produced and the carotenoids are not masked.
- Anthocyanin is a red/purple formed from a reaction of sugars and proteins in cell sap, when sugar concetration is very high or there is strong light intensity.
- The hue of the anthocyanin is pH dependant, ranging from red apple skins to purple grape skins.
- They act as a sunscreen by absorbing blue-green UV light, stopping the destruction of chlorophyll.
- They help maximise prodcution in the final stages of the growing season as leaves turn red.
- They also disguise the plant from herbivorous species who cannot see red wavelengths of light.