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  • The energy we used has been captured by photosynthesis from sunlight. It also produces the oxygen we breathe by releasing it from water molecules. 
  • Photosynthesis is a process in which light energy is used in the synthesis of organic molecules, such as glucose. 
  • Photosynthesis is a series of enzyme catalysed reactions, and therefore any factor that affects enzyme activity will affect the rate of photosynthesis. 
  • Photosynthesis takes place in the chloroplasts of cells in the leaves and other green parts of plants.
  • The reactions involve the absorption of light by the pigment chlorophyll.
  • Algae and cyanobacteria are also able to photosynthesise to make food. 
  • The organisms then respire some of the food they make to provide them with energy for metabolic processes. 
  • Excess food can be stored or used to make other substances such as cellulose, proteins, lipids etc, which are used for growth. 
  • Photosynthesis: 6CO2 + 6H20 ------------------------> 6O2 + C6H12O6
  • The reactions of photosynthesis occur in two distinct stages (both in chloroplasts): light dependent reactions (light) & light-independent reactions (dark-can occur in light)
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1. The light dependent reactions

The light dependent reactions involve the capture of light energy which is used for 3 purposes:

  • To add inorganic phosphate molecule (Pi) to ADP, thereby making ATP. ADP+Pi -> ATP
  • To split water into H+ ions (proton) and electrons. As the splitting is caused by light it is known as PHOTOLYSIS.
  • To excite electrons in chlorophyll. 

The reactions occur on the thylakoid membranes of the chloroplasts, which contain tightly-packed chlorophyll molecules.

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light dependent reactions

  • When light falls on the chlorophyll molecules, a pair of electrons in the molecules gain energy, i.e. they become excited. The electrons become so energy that they leave the chlorophyll molecule and are taken up by a molecule called an electron carrier. 
  • The electrons are now passed along a series of electron carriers in a series of REDOX reactions. The electron carriers form an electron transport chain located in the membranes of the thylakoids. 
  • Each new carrier is at a slightly lower energy level than the previous one so that the electrons lose the energy they gained from light at each stage. 
  • This energy is used to combine an inorganic phosphate molecule with ADP to make ATP, in a process called photophosphorylation. 
  • At the same time a molecule of water is split into hydrogen ions (protons), electrons and oxygen in a process known as photolysis: 

H2O ------->2e- + 2H+   + 1/2 O2 


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light dependent reactions

  • The electrons produced by photolysis are used to replace those lost from chlorophyll, allowing it to continually absorb light energy. 
  • The protons produced by photolysis react with the oxidized coenzyme NADP, along with the electrons released at the end of the electron transfer chain to form reduced NADP.
  • The oxygen produced by the photolysis of water is either used in respiration or diffuses out of the leaf as a waste product of photosynthesis. 

The ATP and reduced NADP formed by the light-dependent reactions provide the energy and the hydrogen necessary to form carbohydrate in the light-independent reactions.

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2. the light-independent reactions

  • Unlike the first stage of photosynthesis, this stage does not require light directly. 
  • In practice however, it soon stops in the absence of light as it requires the products of the light dependent reactions, ATP and reduced NADP.
  • The ATP and reduced NADP are now used to reduce carbon dioxide from the atmosphere, building it into carbohydrates. 
  • The light independent reactions occur in the STROMA of the chloroplast, and are catalysed by enzymes. 
  • They are often referred to as the Calvin cycle:
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light independent reactions

  • In these reactions CO2 combines with a 5 carbon molecule called ribulose biphosphate (RuBP) to form 2 molecules of the 3 carbon compounds, glycerate-3-phosphate (GP).
  •  In the next stage the ATP and the reduced NADP produced in the LDR's is used to reduce the glycerate-3-phosphate (GP) to triose phosphate (TP).
  • The ATP provides the necessary energy for this reaction (i.e. it activates GP) and the reduced NADP provides the hydrogen for the reduction reaction.
  • The NADP is reformed and goes back to the LDP's to be reduced again by accepting more hydrogen. 
  • Some (about one-sixth) of the triose phosphate is converted into carbohydrates such as glucose, sucrose, starch or cellulose. 
  • The rest of the triose phosphate (about five-sixths) is used to regenerate RUBP, using ATP from the LDR. ATP is required as it supplies the phosphate ncessary to convert ribulose phosphate into rublose biphosphate. 

CALVIN CYCLE = part of photosynthesis

KREBS CYCLE = part of respiration

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role of chloroplasts in photosynthesis


  • Occur in thylakoids 
  • Chlorophyll in the thylakoids absorb light energy
  • Grana increase efficiency of LDR by capurting as much light as possible


  • Stroma contains most enzymes for LIR
  • Outer membrane control movement in and out of chloroplast
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factors limiting the rate of photosynthesis

Environmental factors that affect rate of PS:

  • light intensity
  • temp
  • CO2 conc. 

If one of these factors falls below a certain level, it will start to limit the rate of photosynthesis. Although temp, co2 and light may all affect rates of photosynthesis, only the one that is in shortest supply will limit the rate at any particular point in time. This factor is called the limiting factor. The rate of photosynthesis can be increased by increasing that factor.

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factors affecting PS

Light intensity

  • As light increases, rate of photosynthesis also increases. 


  • When light intensity is high, increasing temperature will have a significant effect on the rate of photosynthesis. 
  • Between 10 - 35 degrees, a 10 degree rise in temp will double the rate of photosynthesis. 
  • Increasing temp increases the rate of photosynthesis as enzymes work faster - more kinetic energy, more collisoons and so more enezyme substrate complexes form. 
  • If photosynthesis starts to decline it would be due to the temp going above the optimum temp of the enzymes. Enzymes denature, hydrogen bonds break and this changes the tertiary structure.
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factors affecting PS - co2 conc

CO2 usually makes up only 0.04% of the volume of the air. This is lower than the optimum value (0.1%) for photosynthesis. This means that co2 conc. will often be the limiting factor, particularly in tropical areas where temp and light intensity are high. 

co2 is needed for photosynthesis.

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commercial glasshouses

Knowledge of limiting factors can enable growers to increase the yield of crops grown in glasshouses. The costs of maintaining optimum temp, light intensity and CO2 conc. need to be outweighed by the greater income from the crop. 

Benefits of producing crops in glasshouses in UK:

  • Can control temp/light intensity/co2 conc.
  • Soil replaced with mineral solution - removing soil means no risk of soil organisms causing disease.

Growers need to control possible limiting factors inside commercial glasshouses. The faster the rates of photosynthesis, the more carbohydrates the plants can make. The more carbohydrates made. the more energy and materials are available for growth and fruit formation.

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Commercial glasshouses

How each factor could be controlled:

1) Light intensity 

  • Artificial lighting
  • Specific wavelengths used

2) Carbon dioxide conc.

  • Pump CO2 into glasshouse
  • Use paraffin heaters
  • Ventilation

3) Temperature

  • Glass - stops heat escaping
  • Optimal temp achieved - heating/cooling mechanisms
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commercial glasshouses

At ideal temps for photosynthesis, water loss by transpiration is likely to be high. Excessive water loss can lead to plants closing their stomata to limit this loss. This means that carbon dioxide would not be able to enter the leaves and rates of photosynthesis would be dramatically reduced. Growers need to ensure plants have plenty of water and many glasshouses have automatic watering systems with sprinklers and humidifiers. 

How will a humidifer ensure plants have plenty of water?

  • Increase amount of water vapour in air (in glasshouse)
  • Increases water potential gradient (between leaves and air)

All of the above factors can be controlled by computers. Sensors are used to monitor the level of each factor.

There are costs involved in controlling the environment and these are only worthwhile if the increased yield produced enough profit to exceed these costs. A grower will try to achieve an optimum tield to balance out these factors.

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