Photosynthesis

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

Heterotrophs- organisms that ingest or digest complex organic molecules, releasing the chemical potential energy stored in them.

Phototrophs- organisms that uses energy from sunlight to synthesise organic compounds for nutrition.

Chemoautotrophs– organisms which synthesise complex organic molecules using energy derived from exergonic chemical reactions.

The importance of photosynthesis

• Photosynthesis is the process whereby light energy from the Sun is transformed into chemical energy and used to synthesis large/complex organic molecules from inorganic substances. 

• Both photoautotrophs and heterotrophs can release the chemical potential energy in complex organic molecules (made during photosynthesis) - this is respiration. They can also use the oxygen for aerobic respiration.

• This equation summarises the process of photosynthesis: 6CO2 + 6H2O (+ light energy) → C6H12O6 + 6O2

• Once the Earth's atmosphere contained free oxygen, organisms evolved that could use the oxygen for aerobic respiration. This releases carbon dioxide back into the atmosphere and produces water. 

• C6H12O6 + 6O2 → 6CO2 + 6H2O (+ energy, some as ATP)

• In plants photosynthesis is a two-stage process (light-dependent and light-independent) taking place in chloroplasts.

The compensation point

• Plants respires all the time, but only photosynthesise in daylight. The intensity of light has to be sufficient to allow for photosynthesis at a rate that replenishes the carbohydrate stores used up in respiration. 

• When photosynthesis and respiration proceed at the same rate, so there is no net gain or loss of carbohydrate, the plant is at its compensation point. The time taken to reach this point is the compensation period, and is different for different species of plants. For example, shade plants have a shorter compensation period than sun plants, which require a higher light intensity to reach their optimum rate of photosynthesis

Chloroplast structure

• Most are disc-shaped and between 2-10µm long. 

• The intermembrane space is 10-20nm wide. The outer membrane is permeable to small ions. The inner membrane is less permeable and has transport proteins embedded in it.

• The inner membrane, with its transport proteins, can control entry and exit of substances between the cytoplasm and the stroma.

• The many grana, consisting of stacks of up to 100 thylakoid membranes, provide a large surface area for the photosynthetic pigments, electron carriers and ATP synthase enzymes (light-dependent reaction).

• Photosynthetic pigments are arranged into photosystems, which allows maximum absorption of light energy. Proteins embedded in the grana hold the photosystems in place.

• The fluid-filled stroma contains the enzymes needed to catalyse the reactions of the light-independent stage of photosynthesis.

• The grana are surrounded by the stroma so the products of the light-dependent reactions, which is needed for the light-dependent reaction, can readily pass into the stroma.

• Chloroplasts can make some of the proteins they need for photosynthesis, using genetic instructions in the chloroplast DNA, and the chloroplast ribosomes to assemble the proteins.

Photosynthetic pigments

• Photosynthetic pigments- molecules that absorb light energy. Each pigment absorbs a range of wavelengths in the visible region and has its own distinct peak of absorption. Other wavelengths are reflected. They appear to us as the colour of the light wavelengths that they are reflecting. 

• There are many different pigments that act together, to capture as much light energy as possible. They are in thylakoid membranes, arranged in funnel-shaped structures called photosystems, held in place by proteins.

• Photosystem- a funnel-shaped light-harvesting cluster of photosynthetic pigments, held in place in the thylakoid membrane of a chloroplast. The primary pigment reaction centre is a molecule of chlorophyll a. The accessory pigments consist of molecules of chlorophyll b and carotenoids. 

• Light harvested in the chloroplast membranes via primary and accessory pigments form photosystem/antenna complex. Photon/light energy absorbed by pigment molecules. 

• Electron becomes excited and moves to a higher energy level.

• Energy is passed from one pigment to another.

• Energy is passed to reaction centre/chlorophyll a/ primary pigment.

• A range of accessory pigments allow range of wavelengths to be absorbed.

The light-dependent stage of photosynthesis takes place on the thylakoid membranes of the chloroplasts.

The light-independent stage of photosynthesis takes place in the stroma of chloroplasts.

The Light-dependent stage of photosynthesis

Non-cyclic Photophosphorylation

1.  Light energy hits photosystem I and II.

2. The electrons become excited in chlorophyll a and moves to a higher energy level.

3. The electrons are accepted by electron acceptors and the move along the electron transport chain. The electrons lost in photosystem II are replaced by the electrons made in the photolysis of water (forms hydrogen ions and oxygen). The electrons lost in photosystem I are replaced by the electrons from photosystem II.

4. Protons are pumped across the thylakoid membrane by the flow of the electrons from the stroma to the thylakoid space, down the proton gradient.

5. The hydrogen ions made in the photolysis of water combines with electrons to form hydrogen atoms. The NADP combines with hydrogen to form reduced NADP.

6. As the protons move across the thylakoid membrane by chemiosmosis through ATP synthase, causing photophosphorylation producing ATP.

Cyclic photophosphorylation

1. Only photosystem I is used (P700). The excited electrons pass to an electron acceptor and back to the chlorophyll molecule from which they were lost.

2. There is no photolysis of water and no generation of reduced NADP, but small amounts of ATP are made. This may be used in the light-independent reaction of photosynthesis or it may be used in guard cells (their chloroplasts only contain only photosystem I) to bring in potassium ions, lowering the water potential and causing water to follow by osmosis. This causes the guard cells to swell and opens the stomata. 

• Photosystem II contains an enzyme that, in the presence of light, can split water into H+ ions (protons), electrons and oxygen. This splitting of water is called photolysis.

• 2H20 -> 4H+ + 4e- +O2

Water is a source of:

• Hydrogen ions – used in chemiosmosis to produce ATP. These protons are then accepted by a coenzyme NADP, which becomes reduced NADP, to be used into the light-independent stage to reduce carbon dioxide and produce organic molecules. 

• Electrons – replace those lost by the oxidised chlorophyll and to reduce chlorophyll a.

• Oxygen – used by the plant in aerobic respiration but much of it diffuses out of the leaves, through stomata, into the air.

The light-independent stage of photosynthesis

1. In the stroma, carbon dioxide combines with ribulose bisphosphate (5C), catalysed by the enzyme rubisco. RuBP becomes carboxylated to make a 6C compound.

2. The 6C compound is unstable so breaks down into 2 molecules of glycerate 3-phosphate – carbon fixation.

3. The products from the light-dependent stage are used to reduce and phosphorylate GP into another 3C compound, triose phosphate. 

4. The TP is used to regenerate RuBP using ATP, so that the cycle can start again. 

• Carbon dioxide is the source of carbon for the production of all large organic molecules. These molecules are used as structures, or act as energy stores or sources, for all the (carbon-based) life forms on this planet. 

• Most triose phosphate is recycled to ribulose bisphosphate. 5 out of 6 molecules of TP (3C) are recycled by phosphorylation, using ATP from the light-dependent reaction, to 3 molecules of RuBP (5C). 

Limiting factor- a factor that prevents a process from increasing any further at its lowest value.

Measuring photosynthesis

• A photosynthometer is used to measure the rate of photosynthesis by collecting and measuring the volume of oxygen produced in a certain amount of time

• The gas given off by the plant is collected in the flared end of the capillary tube forming a gas bubble and the length of this bubble can be used to calculate the volume of gas collected.

Volume of gas collected = length of bubble x πr2

• The water bath keeps the temperature constant. Sodium hydrogen carbonate solution is added to the water in the tube to provide carbon dioxide. The investigation has to be carried out in a darkened room, so that the only light available to the plant is form the light source. If the same apparatus is used throughout the investigation, the diameter (and therefore the radius) is constant and we can compare the rates of photosynthesis by using just the length of gas bubble evolved per unit time. 

To use a photosynthometer:

1. Fill the apparatus with water by removing the plunger from the syringe. Replace the syringe plunger and gently push water out of the flared end of the capillary tube, until the plunger is nearly at the end of the syringe and there are no air bubbles.

2. Place a cut shoot, end upwards, into a test tube containing the same water that the plant has been kept in and add 2 drops of sodium hydrogen carbonate solution. Stand the test tube in a beaker of water and use a thermometer to measure the temperature of the beaker.

3. Place a light source as close to the beaker as possible. Measure the distance (d) from the plant to the light source. Allow the plant to acclimatise. 

4. Position the capillary tube over the cut end of the plant and pull the syringe plunger so that the bubble of gas collected is in the capillary tube near the scale. Measure the length of the bubble and note it down. 

5. Gently push the plunger back so that the bubble is expelled and reposition the capillary tube to repeat the experiment.

Factors to change and measure:

• Light Intensity- move the light source further from the plant. Measure the distance and calculate the light intensity (or use a light meter to measure light intensity). Allow the plant to acclimatise and repeat steps 4 and 5.

• Temperature- keep all other factors constant and use a light intensity that produced a high rate of photosynthesis. Alter the temperature of the water bath and measure the volume of gas produced, in a known period of time, at each temperature. 

• Carbon Dioxide Concentration- keep all other factors constant and vary the number of drops of sodium hydrogencarbonate solution. Measure the volume of gas produced, in a known period of time, at each temperature.

Using Changes in Density of Leaf Discs:

1. Use a drinking straw to cut several leaf discs.

2. Place 5/6 discs in a 10cm3 syringe and half-fill the syringe with dilute sodium hydrogencarbonate solution.

3. Hold the syringe upright, place your finger over the end of the syringe and gently pull on the plunger - pulls the air out of the spaces of spongy mesophyll in the leaf disks. As the density of the discs increase they sink to the bottom of the syringe.

4. Once the discs have sunk, transfer the contents of the syringe to a small beaker. Shine a light from above and time how long it takes for one leaf disc to float to the top of the solution - the reciprocal of the time taken (1/t) is a measure of the rate of photosynthesis.

5. Repeat the procedure twice more at this light intensity and find the mean rate of photosynthesis. Repeat at other light intensities.

• This method can also be adapted to investigate the effect of temperature, light wavelength or carbon dioxide concentration on the rate of photosynthesis.