Photosynthesis

• Photosynthesis:

  • photosynthesis is the process by which plants produce their own food in the form of glucose
  • algae and cyanobacteria also photosynthesis to make organic compounds, such as glucose
  • this glucose is then used for respiration in the plant to produce ATP for growth. The glucose can also be stored as starch or used to build cellulose for cell walls

  photosynthesis can be summarised by the simplified equation;

  • carbon dioxide + water $\rightarrow$ glucose + oxygen
  • 6$CO_2$ + 6$H_2O$ $\rightarrow$ $C_6H_{12}O_6$ + 6$O_2$
• Photosynthesis:

  • photosynthesis is actually a complex metabolic pathway involving many immediate reactions
  • it is a process of energy transferral, in which some of the energy in light is transferred into chemical bonds
  • 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
• Chloroplast structure:

  • chloroplasts are surrounded by a double membrane
  • they contain their own 70S ribosomes and circular DNA
  • the chloroplast contains stacks of thylakoid membranes called grana (connected by lamella) and a fluid called the stroma. Chloroplasts may also contain starch grains
  • the thylakoid membrane contains chlorophyll (and other photosynthetic pigments e.g. carotene). The chlorophyll is found attached to proteins in the thylakoid membranes, forming structures called photosystems

  different photosystems absorb different wavelengths of light;

  • PSI - absorbs light best at a wavelength of 700nm
  • PSII - absorbs light best at a wavelength of 680nm

  the thylakoid membrane also contains ATP synthase enzyme
• Stages of photosynthesis:

  there are two stages to photosynthesis which occur in different parts of the chloroplast;

  • the light dependent reaction occurs across a thylakoid membrane
  • the light independent reaction occurs in the stroma
• The light dependent reaction:

  • this involves the absorption of light energy by chlorophyll in the thylakoid which is used to drive the 4 stages of the reaction. The main products of this are NADPH and ATP. Oxygen is also a waste product from this. The stages include;
  • photoionisation
  • making ATP (by photophosphorylation)
  • making NADPH
  • photolysis of water
• Photoionisation:

  • when a chlorophyll molecule (in PSII) absorbs light energy, electrons become excited
  • the electrons leave the chlorophyll molecule. As a result, the chlorophyll molecule becomes oxidised
  • the electrons that leave the chlorophyll are received by an electron carrier and is passed along the electron transport chain. The electron carrier, which has gained electrons, has been reduced
• Making ATP (by photophosphorylation):

  • as the electrons pass along the electron transport chain, they lose energy
  • this energy lost by the electrons is used to pump protons (H$^+$) into the thylakoid from the stroma
  • this increase the concentration of H$^+$in the thylakoid, so proteins diffuse back into the stroma via the channel in the enzyme ATP synthase
  • the energy from this movement is used to combine ADP with Pi to form ATP
  • chemiosmosis - the process of electrons flowing down the electron transport chain and creating a proton gradient across a membrane to drive ATP synthesis
• Making NADPH:

  • light energy is absorbed by another chlorophyll (in PSI). This excites the electrons again
  • electrons are then transferred to the coenzyme NADP along with H$^+$(proton) from the stroma, forming reduced NAPD (NADPH)
• Photolysis of water:

  • photoionisation of chlorophyll leaves it short of electrons. If the chlorophyll molecule is to continue absorbing light energy, these electrons must be replaced
  • the replacement electrons are provided from water molecules that are split using light energy. This photolysis of water also yields protons (H+) and oxygen
  • 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 photosynthesis
• Light dependent reaction:

  • light dependent reaction is known as non cyclic photophosphorylation - the products of which are ATP reduced NADPH and oxygen
  • cyclic photophosphorylation can also occur in which only ATP is produced, and PSI is the only photosystem used. It is called cyclic because the electrons from the chlorophyll molecule are not passed on to NADP - they pass back to PSI via electron carriers. In this process, no reduced NADP or oxygen are produced
• Chloroplasts in the LDR:

  chloroplasts are structurally adapted to their function of capturing sunlight and carrying out the light dependent reaction of photosynthesis;

  • the thylakoid membranes provide a large surface area for the attachment of chlorophyll, electron carriers and enzymes that carry out the light dependent reaction
  • chloroplasts contain both DNA and ribosomes so they can quickly and easily manufacture some of the proteins involved in the light-dependent reaction
• The light independent reaction (the Calvin cycle):

  • this stage does not require light directly; however, it will stop in the absence of light as of requires the products of the light dependent reactions, ATP and reduced NADP
  • the light independent reactions occur in the stroma of the chloroplast
• The light independent reaction (the Calvin cycle):

  • carbon dioxide combines with the 5-carbon compound ribulose biphosphate (RuBP) a reaction catalysed by an enzyme called rubisco
  • the reaction between carbon dioxide and RuBP produces two molecules of GP (glycerate 3-phosphate)
  • ATP produced in the LDR is hydrolysed to provide energy to reduce GP to TP (triose phosphate)
  • reduced NADP from the LDR is used to reduce GP to TP as it provides hydrogen
  • some TP molecules (1/6) are converted to organic substances that the plant requires such as glucose, sucrose and amino acids
  • most TP molecules (5/6) are used to regenerate RuBP, using ATP from the LDR, which supplies the necessary phosphate
• The light independent reaction:

  the light independent reaction of photosynthesis takes place in the stroma of the chloroplasts. The chloroplast is adapted to carrying out the LIR of photosynthesis in the following ways;

  • the fluid of the stroma contains all the enzymes needed to carry out the LIR
  • the stroma fluid surrounds the grana and so the products of the LDR 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 involved in the LIR
• The Calvin cycle:

  the calvin cycle is the starting point for making all the organic substances a plant needs. TP and GP molecules are used to make carbohydrates, lipids and amino acids;

  • carbohydrates - hexose sugars are made from 2 TP molecules. Starch and cellulose are made from joining hexose sugars together in different ways
  • lipids - made using glycerol synthesised from TP and fatty acids synthesised from GP
  • amino acids - some made from GP
• The Calvin cycle:

  the calvin cycle needs to turn 6 times to make one molecule of glucose;

  • 2 molecules of TP are produced for every molecule of carbon dioxide used/turn of the calvin cycle
  • 6 turns of the cycle would produce 12 molecules of TP
  • 5/6 = 10/12 of these TP molecules will be used to regenerate RuBP
  • 1/6 = 2/12 of these TP molecules will be used to produce glucose
  • 6 turns of the cycle would need 18 ATP and 12 reduced NADP from the LDR

  this seems inefficient but the cycle keeps going, ensuring that there is always enough RuBP to combine with carbon dioxide. It is very important that RuBP is generated. If it were not, then GP would not be formed, the calvin would stop and photosynthesis would be unable to continue
• RP7: Use of chromatography to investigate the pigments isolated from leaves or different plants:

  chromatography can be used to separate plant pigments from a sample from a plant leaf. Paper chromatography could be used. Both involve 2 phases;

  • mobile phase - the molecules can move. In chromatography, the mobile phase is a liquid solvent
  • stationary phase - the molecules cannot move. In paper chromatography, this is a piece of chromatography paper. In thin-layer chromatography, this is a thin layer of solid e.g. silica gel or a TLC plate
• Chromatography:

  both types of chromatography work in the following principle;

  • the mobile phase moves over the stationary phase
  • the components in the mixture spend different amounts of time in the mobile and the stationary phase
  • the components that spend longer in the mobile phase travel faster/further. The time spent in the different phases is what separates the components of the mixture

  the solvent used for this experiment is toxic and highly flammable. It is best to use the solvent in a fume cupboard as the chemicals used are volatile and vapours are hazardous
• Chromatography:

  • it is important that the correct solvent is used for an investigation - different plant pigments have different solubilities in different solvents
  • in plants, each different pigment absorbs a different wavelength of light, so having more than one type of pigment increases the range of wavelengths of light that a plant leaf can absorb and use for photosynthesis
  • different species of plant contain different proportions and mixtures of photosynthetic pigments
  • a sample of pigments can be extracted from the leaves and separated using chromatography, and you can identify the pigments present by calculating their Rf values
• Rf values:

  • Rf value = distance a substance has moved through the stationary phase in relation to the solvent
  • each pigment has a specific Rf value under specific conditions (e.g. the Rf value when using one particular solvent will be different to the Rf value when using a different solvent), and these can be looked up in a data base to compare your sample against to identify the pigments present in your sample
  • the Rf value should always be a number between 0 and 1
  • Rf value = distance travelled by spot ÷ distance travelled by solvent
• Chromatography graphs:

  • there are graphs that look at different lead pigments and the wavelengths of light they absorbed
  • absorption and reflections shouldn't be confused when describing photosynthetic pigments; pigments absorb the light wavelengths that they use, and reflect the ones that they do not e.g. chlorophyll absorbs red and blue light most efficiently, while it reflects green light
• RP8: investigation into the effect of a named factor on the rate of dehydrogenase activity in extracts of chloroplasts:

  • in the LDR, NADP acts as an electron acceptor and is reduced
  • the reaction is catalysed by a dehydrogenase enzyme
  • the activity of this enzyme can be investigated by adding a redox indicator dye to extracts of chloroplasts
  • like NADP, the dye acts as an electron acceptor and gets reduced by dehydrogenase in the chloroplasts
  • as the dye gets reduced, you will see a colour change
  • for example, DCPIP changes from blue to colourless when reduced
  • the rate of dehydrogenase activity can be measured by measuring the rate at which DCPIP loses its blue colour
  • a colorimeter is used to measure how much light a solution absorbs when light is shone directly through it. A coloured solution absorbs more light than a colourless solution
• Measuring the rate of photosynthesis:

  the rate of photosynthesis is usually measured by;

  • the volume of oxygen released by a plant
  • the volume of carbon dioxide taken up a plant
  • measuring the production of carbohydrates
  • measuring the increase in dry mass

  as the equation for respiration is the reverse of the one for photosynthesis, these methods do not measure photosynthesis alone - they are actually measuring the balance between photosynthesis and respiration
• Photosynthometer:

  • a photosynthomerter measures the rate of photosynthesis
  • the apparatus is completely airtight to prevent any gas escaping or entering
  • a water bath is used to maintain a constant temperature - optimum for enzyme activity
  • potassium hydrogencarbonate solution is used around the plant to provide a source of carbon dioxide
  • a source of light, whose intensity can be adjusted, is close to the apparatus, which is kept in an otherwise dark room
• Photosynthometer:

  • the apparatus is kept in the dark two hours before the experiment begins to allow the rate of photosynthesis to equilibrate. The light source is then switched on and the plant is left for 30 minutes to allow the air spaces in the leaves to fill with oxygen
  • oxygen released by the plant during photosynthesis collects in the capillary tube above the plant
  • after 30 minutes, the volume of oxygen collected can be measured in mm³
  • the gas is drawn up into the syringe, which is then depressed again before the process is repeated at the same light intensity 4 or 5 times, and the mean volume of oxygen produced per hour is calculated
  • the apparatus is left in the dark for 2 hours before the procedure is repeated with the light source set at a different intensity
• Compensation point:

  • the point where there is no net exchange of gases into or out of the plant
  • the rate of photosynthesis equals the rate of respiration
  • the amount of carbon dioxide used is the amount of carbon dioxide produced (this is the same as oxygen)
• Factors affecting the rate of photosynthesis:

  the optimum conditions for plants are;

  • high light intensity - the higher, the more energy there is provided for the LDR, so more glucose is produced and the rate of photosynthesis is faster. If too high, chlorophyll could become damaged
  • using the optimum wavelength of light - the photosynthetic pigments chlorophyll a & b and carotene only absorb red and blue light from sunlight - so using the optimum wavelength, the rate of photosynthesis is increased
  • carbon dioxide levels should be around 0.4% - if they are much higher, the stomata start to close (the atmosphere only contains around 0.04%)
• Factors affecting the rate of photosynthesis:

  the optimum conditions for plants are;

  • temperature should be around 25°C - this is the optimum temperature for the enzymes involved in photosynthesis (rubisco, ATP synthase). If the temperature falls below 10°C, the enzymes become inactive. If the temperature rises above 45°C, the enzymes denature. Stomata can also start to close in high temperatures to avoid water loss and so less carbon dioxide can diffuse into the plant
  • water - plants need a constant supply for the LDR - too little and photosynthesis would stop. If too much, the plant becomes waterlogged, and the lack of oxygen reaching roots results in less aerobic respiration and so there would be less ATP for the active transport of minerals e.g. magnesium needed for chlorophyll
• Limiting factors of photosynthesis:

  limiting factors include light intensity, temperature and carbon dioxide;

  • if one of these factors falls below a certain level, it will start to limit the rate of photosynthesis
  • although temperature, carbon dioxide and light intensity/wavelength may all affect rates of photosynthesis, only the one that is shortest in 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
• Commercial glasshouses:

  • knowledge of limiting factors can enable growers to increase the yield of crops grown in glasshouses as well as enabling countries to produce more crops they would not normally be able to grow
  • 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
• Commercial glasshouses:

  factors many commercial glasshouses would have:

  • artificial lighting with specific wavelengths
  • pump carbon dioxide into glasshouse or use propane/paraffin burners (these have a dual purpose - carbon dioxide is a product of combustion)
  • ventilation
  • glass panels - stops heat from escaping as well as allowing light through
  • thermostat/thermometers
  • humidifier and automatic watering system

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