Photosynthesis

• Photosynthesis

  The process whereby green plants use the radiant energy of the sun to manufacture food in the form of glucose
• Photosynthesis equation

  Carbon dioxide + water + Light energy → Glucose + oxygen
• Chloroplast

  • Surrounded by a double membrane
  • Membrane allows carbon dioxide and water to enter
  • Contains stroma (ground substance)
  • Contains stacks of coin-like structures called grana (made up of lamella)
  • Grana contain thylakoids with chlorophyll to trap sunlight
  • Contains starch granules, ribosomes and enzymes (used for dark phase)
• Chlorophyll is used to trap sunlight
• Ribosomes are the site of protein synthesis, where enzymes are manufactured
• Enzymes are used for the dark phase of photosynthesis
• The light phase of photosynthesis

  1. dependent on light

  2.Takes place in grana of chloroplast 3.Chlorophyll absorbs radiant energy

  4. Used to form ATPAdenosine Triphosphate

  5. Used to split water into hydrogen (high energy atoms) and oxygen
• Oxygen is released into the atmosphere as a by-product
• The dark phase of photosynthesis

  1. Light independent (occurs in stroma)

  2. Carbon dioxide provides carbon and oxygen

  2. Combines with hydrogen atoms from light phase to form high energy carbohydrates like glucose

  3. Requires energy from ATP and high energy hydrogen atoms
• The reactions of the dark phase are controlled by enzymes
• Factors affecting rate of photosynthesis

  • Internal factors: Leaf features that allow more light and CO2 entry
  • External factors: CO2 concentration, light intensity, temperature
• As CO2 concentration increases

  Rate of photosynthesis increases, but becomes constant at high levels due to pH changes affecting enzymes
• As light intensity increases

  Rate of photosynthesis increases, but becomes constant due to lack of CO2 and water
• Optimum temperature

  Temperature at which rate of photosynthesis is maximum
• Photosynthesis in mesophytic plants has an optimum temperature
• pH

  Measure of acidity or alkalinity
• Enzymes denature

  Enzymes lose their structure and function when exposed to high or low pH
• Light intensity

  • If light intensity increases, rate of photosynthesis increases
  • If light intensity decreases, rate of photosynthesis decreases
• Light is one of the requirements of photosynthesis, therefore any change in light intensity would affect the rate of photosynthesis
• The rate of photosynthesis does not increase indefinitely with increasing light intensity
• At a certain light intensity, the rate of photosynthesis becomes constant
• This occurs because there might not be enough carbon dioxide and water
• Optimum temperature

  The temperature at which the rate of photosynthesis is at its maximum
• The optimum temperature for photosynthesis in a mesophytic plant is 25°C
• As temperature increases


  The rate of photosynthesis increases
• Temperature higher than optimum temperature


  The rate of photosynthesis decreases and finally stops
• This happens because once the temperature becomes higher than optimum, the enzymes start to denature
• Biological importance of photosynthesis

  • Provision of food/energy
  • Control of carbon dioxide/oxygen levels
• Green plants absorb radiant energy and use it to make food through the process of photosynthesis
• All living organisms on Earth possess energy from the sun
• All plants and animals take in oxygen for the process of cellular respiration, therefore the levels of O2 in the atmosphere drops
• All plants and animals give off CO2 during cellular respiration, and CO2 is also given off during decomposition and the burning of fuels, therefore the levels of CO2 in the atmosphere increase and become harmful
• During photosynthesis, CO2 is required and the plant absorbs the excess CO2, causing the level of CO2 in the atmosphere to drop to normal
• During photosynthesis, O2 is given off, increasing the levels of O2 in the atmosphere back to normal
• Photosynthesis ensures that the level of CO2 and O2 in the atmosphere is always constant
• Greenhouse

  A building in which plants are grown, usually made of glass or layers of polyethylene
• How a greenhouse works

  1. The glass walls and ceiling allow light to enter

  2. Cold air and pests are kept out

  3. Providing the plant with water, fertilizer, good air flow and heat allows the plant to grow in any season and climate
• In large commercial greenhouses, special equipment and trained staff are available to ensure the correct amount of water, fertilizer, good air flow and heat are available to the plant at all times
• Light, temperature and carbon dioxide concentration affect the rate of photosynthesis and cellular respiration, and therefore the growth of the plant
• Controlling factors in a greenhouse

  1. Shade cloths may be used to reduce light entering if it is too bright

  2. Artificial light may be added if there is not enough light

  3. Temperature can be altered by using heaters to increase or fans to decrease temperature

  4. Carbon dioxide concentration can be increased by pumping in compressed CO2