Photosynthesis and plants

• Photosynthesis is an endothermic chemical reaction by which green plants make glucose using carbon dioxide, water and sunlight. It is a reaction which absorbs energy
• Photosynthesis equation: Carbon dioxide + water (light, chlorophyll)→ glucose + oxygen
• Photosynthesis equation: 6CO2 + 6H2O(light, chlorophyll)→ C6H12O6 + 6O2
• Carbon dioxide enters the leaf through the stomata by diffusion from the atmosphere. it is in a high concentration in the atmosphere and a low concentration inside the leaf air spaces
• Water is absorbed from the soil by the root hair cells. the root hair cell has a long extension to increase the surface area of the cell to increase uptake of water
• Photosynthesis occurs within the chloroplasts which are found in leaf cells. chloroplasts contain chlorophyll, the green pigment absorbs light energy
• The majority of photosynthesis in a plant occurs in the palisade cells in the leaf. These cells are adapted by having numerous chloroplasts containing chlorophyll to maximise absorption of light.
• The oxygen produced from photosynthesis is a waste product which is either used in the palisade cells for respiration or diffuses back out the leaf through the stomata.
• The glucose produced from photosynthesis can:

  1. be used in respiration to release energy for the plant
  2. The glucose can be stored in the form of starch in the leaf and converted back to glucose for respiration in the dark. Starch is a large molecule made of many glucose molecules bonded together. It is good for storage as it is large and insoluble and doesn't diffuse out of the cell
  3. The glucose is converted into other compounds that the plant needs for growth and reproduction e.g. cellulose, nectar, fructose, amino acids and fats(oils)
• The cross-section of a mesophytic leaf consists of the waxy cuticle, upper epidermis, mesophyll(palisade mesophyll, spongy mesophyll), lower epidermis, the stomata and guard cells.
• In a mesophytic leaf, the leaves are thin so that the short distance from the top to bottom means CO2 can diffuse quickly to all cells
• In a mesophytic leaf, There is a waxy cuticle on the upper surface of the leaf since the waxy cuticle is water-proof so reduces water loss by evaporation. It is transparent to allow light through.
• In a mesophytic leaf, the cell walls of the upper epidermal cells protect the lower tissue layers and are transparent to allow light to penetrate to the palisade cell
• In a mesophytic leaf, palisade cells are vertically arranged and tightly packed at the top of the leaf. This maximises the amount of light reaching the chloroplasts of these cells
• In a mesophytic leaf, palisade cells have many chloroplasts to maximise the amount of light absorbed and therefore photosynthesis in each cell.
• In a mesophytic leaf, there are large air spaces in the spongy mesophyll layer, this speeds up diffusion of CO2 from the atmosphere to the palisade cells
• In a mesophytic leaf, there are spongy mesophyll cells which contain a few chloroplasts and have a large surface area for gas exchange.
• In a mesophytic leaf, there are numerous stomata and guard cells on the underside of the leaf. the stomata opens during the day to allow gas exchange
• Compensation point = the light intensity at which the rate of photosynthesis exactly matches the rate of respiration
• • Plants carry out photosynthesis when light is available and this process uses CO2
  • Plants can carry out respiration constantly and this process produces CO2
  • Consequently the balance of CO2 used/ produced by a plant varies over a 24 hour period
• During night time/ when it is dark:

  • There is no light to carry out photosynthesis, therefore the plant is respiring but not photosynthesising
  • Rate of respiration > rate of photosynthesis
  • CO2 made > CO2 used
  • Therefore CO2 concentration increases
• During noon:

  • Light intensity is high so photosynthetic rate is high
  • Rate of respiration < Rate of photosynthesis
  • CO2 made > CO2 used
  • Therefore CO2 concentration decreases
• During dawn and dusk:

  • Light is available but intensity is low
  • Rate of respiration = Rate of photosynthesis
  • CO2 made = CO2 used
  • Therefore CO2 concentration remains at normal atmospheric levels
  • = Compensation point
• Hydrogencarbonate indicator is red at a normal CO2 concentration, Purple at a low CO2 concentration, and yellow at a high CO2 concentration.
• The following affect rate of photosynthesis:

  1. Temperature
  2. Light intensity
  3. Carbon dioxide concentration
• Photosynthesis and temperature: Photosynthesis is an enzyme catalysed reaction. If temperatures are too low enzymes have little kinetic energy so reaction rate decreases. If temperature are too high enzymes become natured and catalysis of the photosynthetic reaction stops. Enzymes have an optimum temperature at which they work best, so it is important for commercial plant growers to maintain this optimum temperature in their greenhouses
• Photosynthesis and light intensity: As light increases the rate of photosynthesis increases. Photosynthesis cannot happen in the dark. Therefore it is a waste of money for commercial growers to provide extra CO2 or warmth in the dark. Artificial lighting can be used to maximise photosynthetic rate.
• Photosynthesis and [CO2]: An adequate supply of CO2 is necessary for photosynthesis. Commercial growers may enrich the CO2 content in an enclosed environment by using gas burners. These have the dual function of increasing the temperature and producing CO2
• It is important that producers monitor the conditions in a greenhouse using sensors, in order to maximise plant growth in the most efficient way. A cost-benefit analysis should be carried out before spending money on these features to ensure additional expenditure results in increased crop yield and therefore profit for the grower
• Light intensity is the limiting factor for photosynthesis in plants, to a point. After that point, the limiting factor becomes carbon dioxide concentration or temperature