Biology- Photosynthesis


Overview of Photosynthesis

The leaf is the main photosynthetic structure. The chloroplasts are the cellular organelles within the leaf where photosynthesis takes place.

Structure of the leaf: Adaptations

  • A large surface area
  • Arrangement of the leaves which minimises overlapping 
  • Thin, so that the diffusion distance is kept short
  • A transparent cuticle
  • Long, narrow upper mesophyll cells packed with chloroplasts
  • Numerous stomata for gaseous exchange 
  • Stomata that open and close in response to changes in light intensity
  • Many air spaces in the lower mesophyll layer to allow diffusion of carbon dioxide and oxygen 
  • A network pf xylem that brings water to the leaf cells, and phloem that carries away the sugars produced by photosynthesis.
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Overview of Photosythesis 2

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

Photosynthesis is a complex metabolic pathway involving many intermediate reactions. There are three main stages:

  1. Capturing of light energy - by chloroplast pigments such as chlorophyll
  2. The light-dependent reactions - in which light energy is converted into chemical energy. During the process an electron flow is created by the effect of light on chlorophyll and this causes the water to split (photolysis) into protons, electrons and oxygen. The products are rNADP, ATP and oxygen.
  3. The light-independent reaction - in which these protons (hydrogen ions) are used to reduce CO2 to produce sugars and other organic molecules.

Photosynthesis takes place within cell organelles called chloroplasts. They are surrounded by a double membrane. There are two distinct regions:

  • The grana - are stacks of up to 100 disc-like structures called thylakoids where the light-dependent stage of photosynthesis take place. Some thylakoids have tubular extensions that join up with thylakoids in adjacent grana. These are called inter-granal lamellae.
  • The stroma - is a fluid-filled matrix where the light-independent stage of photosynthesis take place. Within the stroma are a number of other structures such as starch grains.
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The Light-dependent Reaction

The light-dependent reaction involves the capture of light whose energy is used for two purposes:

  1. To add an inorganic phosphate molecule to ADP, therby making ATP.
  2. To split water into H+ ions and OH- ions. As the splitting is caused by light, it is known as photolysis.

When a substrate combines with oxygen the process is called oxidation. The substrate that is losing the oxygen is said to be reduced, the process being know as reduction.

The making of ATP

When a chlorophyll molecule absorbs light, it boosts the energy of a pair of electrons (excited state), within the chlorophyll molecule. The electrons leave the chlorophyll and are taken up by an electron carrier molecule. The electrons are passed down a number of electron carriers through a series of oxidation-reduction reactions. These chains are located in the membranes of the thylakoids. Each new carrier is at a slightly lower energy level than the previous, and so the electrons lose energy at each stage. This is used to combine a Pi molecule and ADP molecule in order to make ATP.  

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The Light-dependent Reaction 2

The photolysis of water.

The loss of electrons when light strikes a chlorophyll leaves it short of electrons. The replacement of these electrons are provided from water molecules that are split using light energy. The equation for this process is:

                                         2H2O    ---->    4H+    +     4e-     +     O2

These hydorgen ions are taken up by an ecletron carrier called NADP. The NADP then becomes reduced. The rNADP then enters the light-independent reaction along with the electrons from the chlorophyll molecules. The oxygen by-product from the photolysis of water is either used in respiration or diffuses out of the leaf as a waste product of photosyntheis.

The site of light-dependent reaction.

Chloroplast are strucurally adapted to their function in the following ways:

  • Large surface area of the thylakoid membrane
  • A network of proteins in the grana hold the chlorophyll in a very precise way
  • The granal membranes have enzymes attached to them, which help manufacture ATP
  • Chloroplasts contain both DNA and ribosomes so they can quickly and easily maufacture the proteins needed for light-dependent reactions
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The Light-independent Reaction

The products of the light-dependent reaction of photosynthesis, namely ATP and rNADP, are used to reduce CO2 in the second stage of photosynthesis.

The Calvin cycle.

  1. CO2 from the atmosphere diffuses into the leaf through stomata and dissolves in water around the walls of the mesophyll cells. It then diffuses through the plasma membrane, cytoplasm and chloroplast membranes into the stroma of the chloroplast.
  2. In the stroma, the CO2 combines wuth the 5-carbon combound RuBP using an ezyme Rubisco.
  3. The combination of CO2 and RuBP produces two molecules of the 3-carbon glycerate 3-phosphate (GP) 
  4. ATP and reduced NADP from the light-independent reaction are used to reduce the activated glycerate 3 phosphate to triose phosphate (TP) 
  5. The NADP is re-formed and goes back to the light-dependent reaction to be reduced again by accepting more hydrogen. 
  6. Some TP molecules are converted to useful organic substances, such as glucose. 
  7. Most TP molecules are used to regenerate ribulose bisphosphate using ATP from the light-dependent reaction. 
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The Light-independent Reaction 2

Site of the light-independent reaction

The light-independent reaction of photosynthesis takes place in the stroma of the chloroplast. 

The chloroplast is adapted to carrying out the light-independent reaction of photosynthesis in the following ways:

  • The fluid of the stroma contains all the enzymes needed to carry out the light-independent reaction.
  • The stroma fluid surrounds the grana and so the products of the light-dependent reaction in the grana can readily diffuse into the stroma.  
  • It contains both DNA and ribosomes so it can quickly and easily manufacture some of the proteins needed for the light-independent reaction.
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Factors Affecting Photosynthesis

Limiting Factors. 

  • In complete darkness, it is the absence of light alone that prevents photosynthesis occurring. 
  • If we provide light, however, the rate of photosynthesis will increase. 
  • As we add more light, the rate increases further. This does not continue indefinitely, however, because there comes a point at which further increases in light intensity have no effect on the rate of photosynthesis. 
  • At this point some other factor, such as CO2 concentration becomes the limiting factor. 
  • As with light, more CO2 will lead to more photosynthesis. 
  • Further increases in CO2 level will have no effect on the rate of photosynthesis. 
  • At this point a different factor, such as temperature, is the limiting factor and only an alteration in its level will affect the rate of photosynthesis. 

Photosynthesis is made up of a series of small reactions. It is the slowest of these reactions that determines the overall rate of photosynthesis. In turn, it is the level of factors such as temperature and the supply of raw materials tht determines that speed of each step.

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Factors Affecting Photosynthesis 2

Effect of light intensity on the rate of photosynthesis. 

The rate of photosynthesis is usually measured in one of two ways: 

  • The volume of oxygen released by the plant.
  • The volume of carbon dioxide taken up by a plant.

As light intensity increased, the volume of oxygen and carbondioxide absorbed due to photosynthesis will increase to a point at which it is exactly balanced by the oxygen absorbed and the carbon dioxide produced by cellular respiration. This is known as the compensation point. 

Effect of carbon dioxide concentration on the rate of photosynthesis.

Carbon dioxide is present in the atmosphere at a concentration of about 0.04%

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