Photosynthesis

What is photosynthesis?

• The process by which light from the sun is transformed into chemical energy and used to synthesise large organic molecule from inorganic substances
• The process by which light energy is transformed into chemical potential energy which is then available to consumers and decomposers
• It releases oxygen from water into the atmosphere allowing aerobes to respire

Autotrophs are organisms that use light or chemical energy and inorganic molecules (carbon dioxide and water) to synthesise complex organic molecules. 

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

Autotrophs: 

• Organisms that can synthesise complex organic molecules e.g. Carbohydrates, lipids, nucleic acids from inorganic molecules and an energy source 
• Autotrophs that gain their energy source from light energy are called photoautotrophs
• The majority of food chains on earth have a producer that is a photoautotroph 

Heterotrophs:

• Animals, fungi and some bacteria are heterotrophs
• They cannot make their own food but digest complex organic molecules into simpler, organic molecules from which they can synthesis complex molecules. 

Why does respiration in autotrophs and heterotrophs depend on photosynthesis?

• Both autotrophs and heterotrophs release the chemical energy from complex organic molecules made in photosynthesis during respiration 
• Aerobic organisms can use the oxygen produced in photosynthesis to respire aerobically

The Structure of Chloroplasts 

• Between 2-10 micrometers in length 
• Surrounded by a double membrane 
  • The outer membrane is permeable to many small ions 
  • The inner membrane is less permeable and is embedded with transport proteins
  • It is folded into lamella 
  • Each lamella stack is called a granum 
  • In between each grana are integranal lamellae 

Structure of a chloroplast can be divided into the stroma and the grana 

Stroma: 

• Fluid filled matrix 
• Where the light-independent reactions of photosynthesis occur 
• The enzymes necessary for the light-independent reactions are located in the stroma 

• Also in the stroma are starch grains, oil droplets, DNA and ribosomes 

Grana 

• Stacks of flattened membrane compartments called Thylakoids 
  • The thylakoid membranes are the site of light absorption
  • The membranes contain photosystems and photopigments 
  • They are the site of the light-dependent stages of photosynthesis 

How are chloroplasts adapted for their role?

1. Grana  

• Consist of many thylakoid membranes providing a large surface area (photosystems, ATP synthase enzymes and electron carriers) for the light-dependent stages of photosynthesis 
• The photosynthetic pigments are arranged into photosystems which allow for maximum light absorption 

2. Stroma 

• The stroma contains all the necessary enzymes for the light-independent stages of photosynthesis 
• The grana are surrounded by the stroma so the products of the light-dependent stages of photosynthesis can diffuse into the stroma to be used in the light-independent stages
• Chloroplasts can make some of the proteins they need for photosynthesis using the DNA and ribosomes in the stroma 

Photosynthetic Pigments are molecules that absorb light energy. Each pigment absorbs a range of wavelengths in the region of visible light and has its own peak of absorption. Other wavelengths not absorbed are reflected. 

N.B. When talking about photosynthetic pigments and light absorption remember that pigments absorb a range of wavelengths. Refer to the wavelengths of light, not just the colour. 

• Photosynthetic pigments are substances that absorb certain wavelengths of light and reflect others
• They are arranged into photosystems in the thylakoid membrane in a way that captures as much light energy as possible
• Many different photosynthetic pigments act together to capture light over a range of wavelengths

Photosystems are funnel shaped, light harvesting clusters of photosynthetic pigments. The primary pigment reaction centre allows the energy harvested from light to be transferred to the electron transport chain. 

Photosynthetic pigments fall into two categories:

• Primary Pigments 
  • Two types of Chlorophyll a 
  • Each absorbs red light at slightly different wavelengths (absorption peak)
• Accessory Pigments
  • Carotenoids 
  • Absorb blue light 
  • They absorb wavelengths which are not well absorbed by chlorophylls and pass the energy onto the primary pigment reaction centre
  • Examples include Carotene and Xanthophyll 

Why are Photosynthetic Pigments important? 

Photosynthetic pigments are what allows plants to capture energy in the form of light energy and use it in the process of forming complex organic molecules. Photosynthetic pigments are highly specialised to absorb certain wavelengths of light. The combination of pigments within a plant ensures that light across a broad range of wavelengths can be captured and the light dependent stages of photosynthesis can occur at a high rate. 

The light dependent stages of photosynthesis take place in the thylakoid membranes. Photosystems containing photosynthetic pigments are embedded in the membrane. These pigments trap light energy that can be converted into chemical energy in the form of ATP. 

1. When a photon of light strikes PS II it excites an electron. A water molecule binds to an enzyme on PS II. 

2. The excited electrons leave the PS and are passed onto the electron chain. The water molecule is split. This is called photolysis. 

3. The oxygen produced from photolysis diffuses out of the plant through the stroma. The electrons produced from photolysis are used to replace those lost by PS II. 

4. As each protein in the electron chain is reduced, protons are pumped across the thylakoid membrane from the stroma into the thylakoid space. This creates a proton gradient across the membrane. 

5. The thylakoid membrane is impermeable to protons. Therefore the protons diffuse back into the stroma via ATP synthase enzymes. 

6. Molecules of ADP are phosphorylated to form ATP. This is photophosphorylation. 

7. When electrons reach the end of the electron transport chain they reduce photosystem I

8. When a photon of light strikes PS I the enzyme NADP reductase then donates a pair of electrons from PS I to a molecule of NADP to form NADPH. 

This process is called photophosphorylation. There are two types of photophosphorylation: cyclic and non-cyclic. 

Cyclic Phosphorylation 

• This uses photosystem I only 
• Excited electrons move from the photosystem to a protein in the electron transport chain then back to the chlorophyll molecule from which they were lost
• No photolysis of water occurs 

Non-Cyclic Phosphorylation 

• Uses photosystem I and II 
• Electrons lost from PS I are replaced by electrons lost by PS II (i.e. electrons leaves PS I and PS II) 
• An electron from a water molecule replaces the electron lost from photosystem II 

The Calvin Cycle is a series of reactions that results in the conversion of carbon dioxide into organic products. It occurs in the stroma. 

1. Carbon dioxide diffuses into the leaf via open stomata, through the chloroplast envelope and into the stroma. 

2. Here it reacts with a 5-carbon compound called ribulose bisphosphate (RuBP). This reaction is catalysed by the enzyme Rubisco. 

3. The product is unstable and quickly reacts to form two molecules of 3-carbon compound called glycerate 3-phosphate (GP). The carbon dioxide has now been 'fixed'. 

4. GP is reduced and phosphorylated using ATP and NADPH produced from the light dependent stages of photosynthesis. The product is a a 3-carbon compound called Triose Phosphate (TP). 

5. Molecules of TP are then converted into the 5-carbon compound Ribulose Bisphosphate (RuBP) using ATP from the light dependent stages of photosynthesis. 

6. The cycle can start again 

How are the products of the Calvin Cycle used?

1. Glycerate 3-Phosphate (3C) 

• Amino Acids 
• Fatty Acids/Lipids

2. Triose Phosphate (3C)  

• Hexose Sugars e.g. glucose 
  • Starch 
  • Cellulose
  • Fructose 
  • Sucrose (2 molecules of glucose)
• Glycerol
  • Fatty acids 

How are the light dependent stages and light independent stages linked?

The ATP and NADPH produced from the light dependent stages are used to phosphorylate and convert carbon substances in the Calvin Cycle. 

Photosynthesis is a complicated process and consequently many factors have to be considered in it's progress. The factor which is in the least favourable supply is the factor that limits the process of photosynthesis - the limiting factor. 

Limiting factor for a metabolic process if the factor that is present at the lowest or least favourable level. 

What is light responsible for in photosynthesis?

• Opening of the stomata, allowing Carbon Dioxide into the cells of the leaf 
• It is a form of energy which can be trapped by photosynthetic pigments to provide energy for photophosphorylation
• It splits water molecules in order to provide electrons to replace those lost in the photosystems 

The rate of photosynthesis is proportional to the light intensity-the more light is available the higher the rate of photosynthesis will be. This is because all of the processes listed above can occur at a higher rate. 

The process involved in the light dependent stages of photosynthesis are not much influenced by temperatures. However the enzyme catalysed reactions of the Calvin Cycle are. 

Between the temperatures of 0-25 degrees the rate of photosynthesis doubles for every 10 degree rise in temperature. At temperatures above 25 degrees the rate of photosynthesis levels off. This is because:

• Enzymes become denatured and work less efficiently 
• Oxygen begins to outcompete Carbon Dioxide for the active site of Rubisco
• The increased temperature causes the stomata to close to prevent water stress meaning less CO2 can diffuse into the leaf

Enhanced levels of CO2 will increase the rate of photosynthesis as more molecules of CO2 collide with the active site of Rubisco per second. This is only the case if Carbon Dioxide concentration is the only limiting factor. 

Light Intensity: An increase in light intensity increases the rate of the light dependent reactions. 

• More ATP and NADPH is produced in the light dependent reactions 
• ATP and NADPH are used in the light independent stages, consequently the rate of these reactions increase 
• Glycerate 3-Phosphate is reduced to Triose Phosphate more quickly 

If the light intensity dramatically decreases

• GP will not be reduced to TP 
• GP will accumulate and levels of TP will decrease 
• Eventually the levels of RuBP will also fall (RuBP is formed from TP) and levels of new GP will fall 

An increase in carbon dioxide concentration may lead to an increase in carbon fixation in the Calvin Cycle. 

• A higher concentration of carbon dioxide means that more RuBP will be converted into Glycerate 3-Phosphate and consequently more molecules of Triose Phosphate will be formed
• Consequently all of the products formed from GP (fatty acids and amino acids) and TP (hexose sugars and glycerol) will also increase in concentration 
• If concentration of Carbon Dioxide falls then RuBP will accumulate and levels of GP and TP will be reduced

Increasing the temperature will only increase the rate of the light independent reactions as they are the only reactions catalysed by enzymes. 

However as the temperature rises above 25 degrees oxygen begins to outcompete carbon dioxide for the active site of Rubisco and consequently the concentration of GP will fall. 

It also means that the the rate of photorespiration exceeds the rate of photosynthesis. Consequently CO2 is released at a faster rate than it is being used up.