Photosynthesis

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Autotrophs V Heterotrophs

* Autotrophs are self-feeding organisms that can synthesise complex organic molecules from inorganic molecules and an energy source.

- There are two main kinds of autotrophs: chemoautotrophs and photoautotrophs.

Chemoautotrophs are capable of synthesising organic molecules using energy from chemical reactions.

Photoautotrophs use light energy to synthesise complex organic molecules.

* Heterotrophs are organisms that ingest and digest complex organic molecules, releasing the chemical potential energy stored in them.

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Photosynthesis Definition

Photosynthesis - Phototsynthesis is the process where light energy from the sun is transformed into chemical energy and used to synthesise complex organic molecules from inorganic substances.

* Photosynthesis is composed of two main, separate stages: the light dependent stage and the light independent stage.

Light Dependent Stage - uses cyclic and non-cyclic photophosphorylation to use light energy to create ATP - occurs in the granum and intergranum lamellae 

Non-Cyclic photophosphorylation, the more useful form, gives three products:

1 - Oxygen ions 2 - reduced NADP 3 - ATP

Light Independent Stage: the Calvin Cycle - where CO2 and ribulose biphosphate combine in the synthesis of glycerate-3-phosphate or triose phosphate - occurs in the stroma.

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Adaptions of Plants to Maximise Photosynthesis

* Plant leaves demonstrate radial symmetry - orient themselves to maximise sun exposure      * Plants move upwards and towards the light for greater light exposure and greater wind circulation for gas exchange due to hormone helping movement called auxin   Leafs waxy cuticle is transparent to allow maximum light transmission and waterproof to minimise water loss * Upper epidermis is one cell thick to maximise the amount of light passing through to the chloroplasts * palisade cells contain a large number of chloroplasts to absorb as much sunlight as possible * palisade mesophyll cells contain actin filaments to allow for photorelocation of chloroplasts - in high light chloroplasts move towards anticlinal walls, in low light chloroplasts move towards periclinal walls Cylindrical arrangement of palisade mesophyll cells to minimise the effects of membranes from other cells obstructing sunlight.     * large air spaces between spongy mesophyll cells to maximise gaseous exchange.         

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Structure of Chloroplasts

* Outer chloroplast envelope with an outer membrane, an inner membrane and an inner membrane space

* Stroma - the site of the light independent reactions - contains enzymes to catalyse light independent reactions for example Rubisco - also contains circular DNA and ribosomes to make the proteins and enzymes needed in photosynthesis

* Grana - consisting of stacks of thylakoid membranes - giving a large surface area for photosystems and electron transport chain and ATP synthase channels for light-dependent stages. Granal lamellae are the site for Photosystem 2 - Chlorophyll with P680 - involved in photolysis and non-cyclic photophosphorylation. The space inside the thylakoids is used to trap protons for chemiosmosis. Proteins keep photosystems in place in the granal and intergranal lamellae   * Intergranal Lamellae - membranes between the grana - the site of Photosystem 1 - Chloropyll with P700 - involved in cyclic photophosphorylation and non-cyclic photophosphorylation                                                                                                                    * The stroma also contains lipid globules and starch granules for storage that have been produced from the Calvin Cycle.

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Photosynthetic Pigments

* Photosynthetic pigments are chemical compounds which absorb only certain wavelengths of visible light and reflect the rest.

* In chloroplasts they are found in funnel-shaped structures called photosystems embedded in the granal and intergranal lamellae, held in place by proteins.

* Chlorophyll is the most important photosynthetic pigment - there are two main types of chlorophyll - chlorophyll A and chlorophyll B - but they all ahve a similar structure:

- a porphyrin group which is similar to the haem group in haemoglobin except contains a magnesium atom instead of an iron atom

- a long phytol chain (hydrocarbon chain) that is hydrophobic

* There are two different forms of Chlorophyll A, P700 and P680

- P680 - Photosystem 2 - granal lamellae - photolysis and non-cyclic photophosphorylation

- P700 - Photosystem 1 - intergranal lamellae - cyclic and non-cyclic photophosphorylation  

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Photosystems

* A photosystem is funnel-shaped structure found in the granal and intergranal lamellae designed to capture as much light energy as possible to maximise photosynthetic performance

* At the base of the photosystem is the primary pigment reaction centre which contains either chlorophyll P680 or P700.

* Surrounding the primary pigment reaction centre is the antenna complex which contains the accessory pigments which absorb the wavelengths of light that the primary pigment reaction centre reflects and cannot absorb.

* Accessory pigments do not contain a porphyrin group. They absorb wavelengths of light not absorbed by the primary pigment reaction centre and then pass that energy to the primary pigment reaction centre. The most important group of accessory pigments are carotenoids for example Beta carotene and Xanthophylls.  

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Light Dependent Stage of Photosynthesis

* There are two types of light-dependent reaction: cyclic and non-cyclic photophosphorylation

* Cyclic Photophosphorylation: involves only photosystem 1 ( P700) - there is no photolysis involved and only a small amount of ATP is produced. This ATP is often used by the guard cells to pump in potassium ions to lower the water potential and cause water to move in by osmosis, meaning the guard cells swell and  force the stomata open for water and gaseous exchange.

- Sunlight hits the magnesium atom of the P700 chlorophyll porphyrin group in photosystem 1, exciting a pair of electrons which leave the primary pigment reaction centre and pass along an electron tranport chain to an electron acceptor and then pass along an electron transport chain back to the chlorophyll molecule. This synthesises a small amount of ATP by chemiosmosis but forms no reduced NADP or oxygen ion products.

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Light Dependent Stage of Photosynthesis

Non-cyclic Photophosphorylation: involves both Photosystem 2 (P680) and Photosystem 1 (P700)

- Photolysis occurs when light energy hits photosystem 2 and, in the presence of enzymes, is split into protons, electrons and oxygen ions. The electrons released are used to replace those lost from the porphyrin group of the chloropyll P680 in Photosystem 2 and the protons are used in chemiosmosis when the electrochemical energy released from the movement of the electrons down the electron transport chain is used to actively pump the protons across the thylakoid membrane and into the thylakoid space where a proton gradient builds up before the proton motive force forces the protons through ATP synthase channels and synthesises ATP  - As light energy hits the magnesium atom of the porphyrin group of Chlorophyll P680 in the primary pigment reaction centre of Photosystem 2 - this excites a pair of electrons which passes along an electron transport chain to replace the pair of electrons from the porphyrin group of Chloropyll P700 at Photosystem 1 which have also been excited.    - The pair of excited electrons from Photosystem 1 travel to the final electron acceptor, NADP and along with the protons used in chemiosmosis, reduce it to give reduced NADP.  - Thus the products of non-cyclic photophosphorylation are                 * ATP        * reduced NADP       * Oxygen ions       and the ATP and reduced NADP is transported to the stroma for use in the Calvin Cycle.

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The Light-Independent Stage of Photosynthesis

The light-independent stage of photosynthesis occurs in the stroma of chloroplasts. The Calvin Cycle is a series of enzyme-catalysed reactions. Although light is not directly used in the Calvin Cycle, it is reliant on the products of the light-independent stage and therefore soon stops if light is not available.

1) Carbon fixation - Carbon dioxide covalently bonds to the 5-carbon compound           ribulose-1,5- biphosphate in the presence of enzyme Rubisco (ribulose biphosphate carboxylase oxygenase) to give a very unstable 6-carbon compound which quickly breaks into two molecules of glycerate-3-phosphate.

2) Glycerate-3-phosphate can be used to make amino acids or fatty acids or it can be reduced and phosphorylated ( using ATP and reduced NADP from non-cyclic photophosphorylation) to give 2 molecules of triose phosphate.

3) Two molecules of triose phosphate can combine to make a hexose sugar which then can go on to make starch or glycerol, but 5 turns out of every 6 turns of the Calvin Cycle recyles triose phosphate to regenerate 3 molecules of ribulose-1,5-biphosphate using ATP from the light-dependent stage

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The Light-independent Stage of Photosynthesis

* The Calvin Cycle needs 6 turns to make one hexose sugar for example glucose or fructose. Glucose and fructose can combine to make sucrose. Or the sugars can be used to make starch or glycerol.

* Alternatively, glycerate-3-phosphate can be used to produce fatty acids and amino acids. The fatty acids can combine with glycerol produced from triose phosphate to make triglycerides (triacylglycerols)!

* To make a hexose sugar you need:

- 6 turns of the Calvin Cycle, using up 6 molecules of carbon dioxide

- 18 molecules of ATP

- 12 reduced NADP

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Photorespiration and the Compensation Point

As you increase the temperature, at first the rate of photosynthesis will increase because the reacting particles will have more kinetic energy, however, as the temperature increases above about 25 degrees centigrade the oxygenase activity of Rubisco increases over that of its carboxylase activity. This means that the rate of photorespriation exceeds that of photosynthesis. Photorespiration is where oxygen binds to Rubisco not carbon dioxide. As a result of increased photorespiration, the ATP and reduced NADP from the light-dependent stage of photosynthesis are dissipated and wasted. The production of organic molecules slows and stops. The overall rate of photosynthesis, of both light-dependent and light-independent stages decreases. Also, hydrogen peroxide is a product of photorespiration and this is toxic if it builds up inside the plant.

The Compensation Point is the amount of light intensity on the light curve where the rate of photosynthesis exactly matches the rate of respiration.  

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Limiting Factors - Temperature

A limiting factor is the factor that is present at the lowest or least favourable value Temperature - A low temperature limits photosynthesis since the enzymes controlling reactions in the light-independent stage and the light-dependent stage ( such as ATP synthase and those enzymes involved in photolysis) are below their optimum temperature and thus their tertiary structure is compromised and the reacting particles have less kinetic energy. As the temperature increases up to a point the rate of photosynthesis  increases too as a greater proportion of the particles involved have energy greater than or equal to the activation energy. However, when the temperature gets too high:                              * the proteins in the granal and intergranal lamellae holding the photosystems in place become denatured and the photosystems ability to complete cyclic and non-cyclic photophosphorylation becomes impaired, reducing the rate of photosynthesis         * the enzymes involved in photosynthesis, especially with the light-independent Calvin Cycle which is reliant on enzymes such as Rubisco that are more likely to be effected by temperature, become denatured as the high temperature alters their tertiary structure meaning the active site no longer fits the substrate. Also at higher temperatures, photorespiration increases as the oxygenase activity of Rubisco increases over its carboxylase activity, decreasing the rate of photosynthesis * Also, at high temperatures the plant loses an increased amount of water through transpiration , meaning as part of the stress response, stomata are closed, reducing carbon dioxide availability.

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Limiting Factors Continued

* If you limit the carbon dioxide concentration available for use in photosynthesis the Calvin Cycle is limited. If there is less carbon dioxide available for carbon fixation with ribulose-1,5-biphosphate then less glycerate-3-phosphate and less triose phosphate are produced and ribulose-1,5-biphosphate accumulates. This reduces the overall rate of photosynthesis because the ATP and reduced NADP produced from the light-dependent stage is dissipated and wasted meaning less NADP can be recycled so there is no final electron acceptor for non-cyclic respiration.   

* If you increase light intensity then the rate of the light-dependent reaction will increase. There will be more light energy to split water in photolysis and more light energy to excite electrons in the chlorophyll porphyrin group making more ATP and reduced NADP and thus increasing the rate of the Calvin Cycle also. If you reduce light intensity then the light-dependent stage will cease and there will be a reduction the availabilty of ATP and reduced NADP, meaning that less glycerate-3-phosphate can be converted to triose phosphate and less ribulose-biphosphate can be regenerated so the level of glycerate-3-phosphate will increase, the levels of ribulose biphosphate and triose phosphate decreasing untill the reduced levels of ribulose biphosphate mean that the formation of glycerate-3-phosphate stops too, stopping the Calvin Cycle.  

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