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  • Created on: 24-12-17 12:37

Site of Photosynthesis & structure of the Leaf:

The leaf is thte main photosynthetic structure in eukaryotice plants, chloroplasts are cellular organelles in the leaf where photosynthesis takes place. The leaf has many adaptations so that photosynthesis is as efficient as possible:

  • Large surface area -> absorbs as much light as possible
  • Leaves arranged for little overlapping -> Avoids one lea shadowing another
  • Thin -> Ligh absobed within first few micrometers of leaf & diffusion distance kept short
  • Transparrent cuticle & epidermis -> let light through to photosynthetic mesophyll cells
  • Long, narrow upper mesophyll cells, pack with choloplasts -> collect sunligt
  • Stomata -> open and close in response to change in light intensity
  • Numerous stomata -> for gas exchange so all mesophyll cells have short diffusion pathway
  • Many air spaces -> in lower mesophyll layer to allow rapid diffusion
  • Network of xylem -> brig water into leaf cells
  • Network of phloem -> carries away sugars produced during photosynthesis
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Outline of Photosynthesis:

The equation for photosynthesis is:

6CO2 + 6H2O --light--> C6H12O6 + 6O2

Carbon Dioxide + Water --Light--> Glucose + Oxygen

The equation is highly simplified, photosythesis is a complex metabolic pathway involving many intermediate reactions. It's a process of energy transferral  in which some energy in light is conserved for chemical bonds, there are 3 main stages:

1) Capturing of Light Energy: By chloroplast pigments, e.g. chlorophyll

2) Light-Dependant Reaction: Some of the light energy absorbed is converted into chemical bonds. An electron flow is created by the effect of light on chlorophyll, causing photolysis into protons, electrons and oxygen. Produced products are reduced NADP, ATP and oxygen

3) Light-Independant Reaction: Protons (hydrogen ions) are used to produce sugars and other organicmolecules

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The Structure and Role of Chloroplasts in Photosyn

Photosynthesis takes place within cell organelles called chloroplasts. They vary in shape and size, but are typically dicsshaped, 2-10 um long, and 1 um in diameter. They're surrounded by a double membrane, inside the chloroplast membranes are 2 regions:

  • The Grana: Satcks of up to 100 disc-like structures, thylakoids, where the light-dependant stage of photosynthesis takes place. Within thylakoids is chlorophyll. Some thylakoids have tubular extentions that join up with thylakoids in adacent grana - intergranal lamellae
  • The stroma: A fluid filled membrane where the light-independant stage of photosynthesis takes place. Other structures, e.g. starch grains, are found in the stroma
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Summary of the Light-Dependant Reaction:

The light-dependant reactios involves the capture of light, the energy is used for 2 purposes:

1) Making ATP -> adding a phosphate molecule to ADP 

2) Photolysis -> Splitting water into H+ ions (protons) and OH- ions <- the reaction is caused by light

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Oxidation and Reduction:


  • When a substance gains oxygen or loses hydrogen, so when a substance loses electrons
  • The substance which oxygen has been added to or hydrogen has been lost by is said to be oxidised
  • Oxidation results in energy being given out


  • When a substance loses oxygen or gains hydrogen, so when a substance gains electrons
  • When energy is being taken in


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The making of ATP

When chlorohyll absorbs light energy it boosts energy in a pair of electrons in the chlorophyll, these electrons are in an excited state. They're so energetic that due to photoionisation, they leave the chlorophyll, ionising the chlorophyll. The electrons are taken up by an electron carrier, the chlorophyll is oxidised and the electron carrier is reduced. 

The electrons are passed along electron carriers, that have formed a chain, in a series of oxidation-reduction reactions in the thylakoid membrane. Each carrier has a lower energy level that the previous one in the chain, so the elecrons lose enrgy as they go down the chain. Some of the energy is used to combine a phosphate molecule to ADP to make ATP. ATP production is explained by the chemiosmotic theory:

  • Each thylakoid is protons (H+) are dumped from the stroma using proton pumps
  • The energy for the process comes from electrons being released in the photolysis of water
  • Water photolysis also produces protons, further increasing their conc  inside the thylakoid
  • Creating & maining the conc  gradient of protons across the thylakoid membrane, high conc inside thylakoid, low conc in the stroma
  • Protons only cross the thylakoid membrane through ATP synthase channel proteins
  • As protons pass through ATP synthase channels, they change the shape of the enzyme which catalyses the combination of ADP + Pi
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Photolysis of Water:

When light strikes chlorophyll and electrons leave, the chlorophyll is short of electrons, to continue absorbing light energy, the electrons must be replaced. These are provided during photolysis of water. The equation is:

2H2O ----> 4H+   +   4e-    + O2

Water ----> protons  +  Electrons  +  Oxygen

Protons pass out the thylakoid through ATP synthase channels & are taken up by an electron carrier - NADP. Having taken up the protons, the NADP is reduced. Reduced NADP is the main product of the light-dependant stage of photosynthesis & it enters the light-independant stage along with the electrons. 

Reduced NADP is important as it is a source of chemical energy in the plant. Oxygen is a by-product of the reaction and is either used in respiration or diffuses out of the leaf as a waste product

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Site of the Light-Dependant Reaction:

The light-dependant reaction takes place in the thylakoids of the chloroplasts. Chloroplasts are structurally adapted to their function in the following ways:

  • Thylakoid membrane provides a large surface area for attachment of chlorophyll, electron carriers and enzymes
  • A network of proteins in the grana hold the chlorophyll in a very precise manner, allowing maximum light absorption
  • Granal membranes have ATP synthase channels within them which catalyse ATP production. They're also selectively permeable, so establish a proton gradient
  • Chloroplasts contain DNA and ribosomes, so can quickly manufacture some of the proteins involved in the light-dependant reaction. 
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Summary of the Light-Independent Reaction:

The products of the light-dependant reaction are ATP and reduced NADP, they're used to reduce glycerate triosphosphate in the light-independant reaction. 

This stage doesn't require light, it occurs whether light it available or not. It requires the products of the light-dependant stage and so raplidly stops when there is no light. The process takes place in the stroma of the chloroplasts, and is known as the calvin cycle

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The Calvin Cycle:

1) CO2 from the atmosphere diffuses into leaf, through stomata & dissolves in water around walls of mesophyll cells. Then diffuses through the cell-surface membranes into stroma of the chloroplasts

2) In the stroma, CO2 reacts with (5-carbon molecule) ribulose bisphosphate (RuBP), the reaction is catalysed by rubisco

3) The reaction between CO2 and RuBP produces two molecules of (3-carbon) glycerate 3-phosphate (GP)

4) Reduced NADP from the light-dependant reaction is used to reduce glycerte 3-phosphate to triose phosphate (TP) using energy supplied by ATP

5) NADP is reformed and goes back to the light-dependant reaction to be reduced again

6) Some triose phosphate molecules are converted into organic substances the plant requires, e.g. starch, cellulose, lipids, amino acids

7) Most triose phopsate molecules are used to regenerate ribulose bisphophate using ATP

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Site of the Light-Independent Reaction:

The light-independant reaction of photosynthesis takes place in the stroma of the chloroplasts. The chloroplast is adapted to its function because:

  • The fluid of the stroma contains all the enzymes needed to carry out the light-independant reaction. Stromal fluid is membrane bound in the chloroplast meaning a chemical environment which has a high conc of enzymes and substrates can be maintained within it
  • Stroma fluid surrounds the grana, so the products of the light-dependant reaction in the grana can readily diffuse into the stroma
  • It contains both DNA and ribosomes, so it can quickly manufacture some proteins involved in the light independant reaction
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Measuring Photosynthesis:

The rate of photosynthesis in aquatic plants (e.g. Canadian pondweed) can be found by measuring the volume of oxygen produced by usinga photosynthometer.

  • A waterbath is used to maintain a constant temperature throughout the experiment
  • Potassium hydrogencarbonate is used around the plant to provide a source of carbon dioxide 
  • A light source is arranged close to the apparatus, otherwise the environment is dark
  • The apparatus is kept in the dark for 2 hours prior to the experiment
  • Light source is switched on and the plant left for 30 mins to allow air spaces in the leaves to fill with oxygen
  • Oxygen released by the plant during photosynthesis collects in the funnel end of the capillary tube above the plant
  • After 30 mins the oxygen is drawn up the capillary tube by gently withdrawing the syringe until its volume can be measure on the scale in mm3
  • Gas is drawn up into the syringe and then depressed again before the process is repeated at the same time for light intensity four or five times, a mean volume of oxygen produced per hour is calculated
  • Apparatus is left in the dark for 2 hours before procedure is repeated with light source set at a different intensity
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