Photosynthesis

Photosynthesis is the process whereby light energy from the sun is transformed into chemicak energy and used to synthesise large organic molecules from inorganic substances.

Light energy is used to produce complex organic molecules.

• Autotroph: organisms that use light energy or chemical energy and inorganic molecules to synthesise complex organic molecules.
• Heterotroph: organisms that ingest and digest complex organic molecules releasing the chemical potential energy stored in them.

• Both autotrophs and heterotrophs respire. Respiration depends on photosynthesis as chemical potential energy is released from the complex organic molecules made in photosynthesis.

• Oxygen is a product of photosynthesis, and is used in respiration. Carbon dioxide is a product of respiration, and is used in photosynthesis. The two processes are very similar.

• Photosynthesis is a two-stage process that takes place in the chloroplasts.

Structure:

• vary in shape and size but are mainly disc shaped between 2-10um long.
• Each chloroplast is surrounded by a double membrane (envelope)
• There is an intermembrane space (10-20nm wide)
• The outer membrane is permeable to small ions
• The inner membrane is less permeable and has transport proteins embedded in it. It is folded into lamellae, which are stacked. Each stack of lamellae is called a granum.
• Between grana are intergranal lamellae.
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Regions:

• There are two distinct regions in the cholorplasts: Stroma and Grana.

• The Stroma is the fluid filled matrix where the light independant stage takes place as it contains all the necessary enzymes. The stroma also contains ribosomes, DNA, starch grains and lipid droplets.

• The Grana are flattened stacks called thylakoids. These are the sites of light absorption and ATP synthesis during the light dependant stage.

• Inner membrane with its transport proteins control the entry and exit of substances between the cytoplasm and stroma.
• The many grana provides a large surface area for photosynthetic pigments, electron carriers and ATP synthase enzymes for the light dependant reaction.
• Photosynthetic pigments are arranged into Photosystems allowing maximum absorption of light.
• Proteins in the grana hold these photosystems in place.
• The stroma contains the enzymes necessary to catalyse the reactions of the light independant stage.
• The grana are surrounded by the stroma so that the products of the light dependant reaction which are needed for the light independant reaction can be readily transferred.

Photosynthetic pigments: molecules that absorb light energy . Each pigment absorbs different wavelengths and other wavelengths are reflected.

• There are many different pigments that act together to capture as much light energy as possible. They are in the thylakoid membranes arranged in funnel shaped structures called photosystems.
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Chlorophyll: is a mixture of pigments. All have a similar structure consisting of a hydrocarbon chain and a magesium atom. Light hits the chlorophyll causing the elcetrons associated with the magneisum to become excited. There are two forms of chlorophyll: P680 and P700 which both appear yellow-green. Both are found at the centre of photosystems and are known as the primary pigment reaction centre.

Accessory Pigments (Cartenoids): they absorb light wavelengths that are not well absorbed by chlorohylls and pass the energy to the chlorophyll at the base of the photosystem. They do not have a magnesium group and absorb blue light, reflecting orange and yellow.

The Light Independent stage takes place on the thylakoid membranes, where the photosystems (with the photosynthetic pigments) are embedded. The pigments trap light energy to be converted into chemical energy (ATP)

Photosphosphorylation

A photon hits a chlorophyll molecule and the energy is transferred to two electrons which become excited. These electrons are captured by electron acceptors and passed along a series of electron carriers embedded in the thylakoid membranes.

Energy is released when the electrons pass along the elcetron carriers, pumping protons across the thylakoid membrane to create a gradient. The protons flow down their gradient through channels associated with ATP synthase enzymes (chemiosmosis). This produces a force that combines Pi with ADP to form ATP.

Cyclic

• Uses only photosystem 1.
• The excited electrons pass to an electron acceptor and back to the chlorophyll molecule from which they were lost.
• There is no photolysis of water and no generation of reduced NADP but small amounts of ATP are made.

• Involves photosystem 1 and 2
• Light strikes PS2 exciting a pair of electrons that leave the chlorophyll from the primary pigment reaction centre.
• These electrons pass along a chain of electron carriers and the energy released is used to synthesise ATP
• Light has also struck photosystem 1 and a pair of electrons has been lost.
• These lectrons along with protons join NADP to create reduced NADP
• The electrons from oxidised PS2 replace the electrons lost from PS1
• Electrons from photolysed water take part in chemiosmosis to make ATP and are then captured by NADP in the stroma. They will be used in the light-independant stage.

Photosystem 2 contains an enzyme that in the presence of light can split water into protons electrons and oxygen. The splitting of water is called photolysis.

Some of the oxygen produced in this way in used by the plant for respiration but most diffuses out of the leaves.

Water is a source of:

• Hydrogen ions which are used in chemiosmosis to produce ATP. These protons are accepted by NADP which becomes reduced NADP to be used in the calvin cycel to reduce CO2 and create organic molecules.
• Electrons which replace those lost by the oxidised chlorophyll.

The light-independant stage takes place in the stroma of the cholorplasts.

The role of Carbon Dioxide:

• Is the source of carbon and oxygen for the production of large organic molecules.
• These molecules are used as structures or act as energy stores/ sources for all the carbon based life forms on the planet.

• Carbon Dioxide diffuses into the leaf via the stomata, then through the spongy mesophyll, the cellulose cell walls, the cytoplasm and through the chloroplast envelope into the stroma.
• Carbon Dioxide combines with RuBP (ribulose bisphosphate) a carbon dioxide acceptor. The reaction is catalysed by the enzyme rubisco.
• RuBP becomes carboxylated.
• An intermediate compound is formed.
• Two molecules of a 3-carbon compound are formed: GP (glycerate phosphate)
• GP is reduced and phosphorylated to TP (triose phosphate) ATP and reduced NADP are used in this process.
• 5 out of every 6 molecules of TP are recycled by phosphorylation using ATP from the light-dependant reaction to 3 RuBP.

How the products of the calvin cycle are used:

• GP is used to make amnio acids and fatty acids.
• pairs of TP form hexose sugars which can be polymerised to other sugars
• TP also can be made into glycerol to combine with fatty acids to make lipids.

• Light intensity provides the energy needed in the light dependent stage, though it is linked to the light independent stage also. Light is trapped by cholorphyll molecules to excite electrons and splits water molecules to provide protons both used in photophosphorylation to create ATP and reduce NADP

Increasing Light Intensity

Decreasing Light Intensity

Carbon Dioxide is essential for the functioning of the light independant stage.If the levels of CO2 rise, so will the rate of photosynthesis as long as there are no other limiting factors.

Increasing CO2

 More carbon dioxide fixation leads to more molecules of GP being created. Thus, more GP can be converted to TP and more RuBP can be regenerated. This is because if levels of CO2 are high, rubisco has a higher affinity to bind with CO2

Decreasing CO2

If Carbon Dioxide concentration drops, the levels of RuBP will rise. GP and TP will decrease. This is because if CO2 is limited, rubisco will fix oxygen instaed (photorespiration)

Tempterature doesn't really impact on the photochemical reactions of the light dependent stage, but does impact the enzyme catalysed reactions of the Calvin cycle.

• Between 0 and 25 degrees, the rate of photosynthesis doubles for every 10 degree increase as enzyme activity increases (more enz\yme-substrate complexes/ kinetic energy/ collisions)
• Once the temperature rises above optimum the rate of photosynthesis plateaus and eventually falls as enzyme activity decreases and they begin to denature.
• Oxygen begins to compete for the active site of rubisco, preventing it from combining with CO2, causing photorespiration to exceed photosynthesis, thus ATP and reduced NADP are wasted.
• High temperatures cause water loss from the stomata, leading to a stress response in which the stomata close and CO2 is prevented from entering the leaf to be used in the calvin cycle (effects RuBP, GP and TP)

If the amount of water increases, more NADP can be reduced and more electrons can replace those lost. The rate of photosynthesis will increase.

If water decreases, less NADP is resuced and the electrons lost cannot be replaced. The rate of photosyntehsis will decrease.

If the plant does not have enough water, the stomata will close preventing CO2 from entering the leaf, effecting the calvin cycle and therefore the production of GP, TP and the regeneration of RuBP.