yr 8 biology

• Photosynthesis

  One of the most important processes on the planet, where light energy from the Sun is harnessed and enters our food chains
• Without photosynthesis, there would not be life as we know it on Earth
• Importance of photosynthesis

  It is how energy enters a food chain. Plants harness the energy from the Sun and store it in their tissues. When these plants are eaten by animals, that chemical energy is used and stored by the animals
• Photosynthesis


  1. Converts carbon dioxide and water into oxygen and glucose

  2. Sunlight provides the energy source for this process
• Waxy cuticle

  • The waterproof top layer of the leaf
  • Prevents water from escaping from the uppermost surface
• Parts of the leaves

  • Stoma
  • Waxy cuticle
  • Spongy mesophyll
  • Palisade cells
• The equation for photosynthesis is: carbon dioxide + water → oxygen + glucose
• Mesophyll


  • A layer of cells with air spaces so that gasses can diffuse in and around the leaf
• Palisade layer


  • A layer of tightly packed cells that are filled with chloroplasts, ensuring almost all light is absorbed and little can pass through
• Stomata


  • Open pores which allow gas exchange with the environment
  • Can close when water is low to minimize water loss, to ensure photosynthesis can still occur
• Increasing the carbon dioxide concentration

  Increases the rate of photosynthesis
• Carbon dioxide is a reactant in photosynthesis
• Above a certain threshold, further increases in the carbon dioxide concentration


  Do not increase the rate of photosynthesis
• Another factor (such as light intensity) is limiting the rate of reaction
• Increasing carbon dioxide concentration

  1. Initially increases the rate of photosynthesis

  2. Above a certain threshold, further increases do not increase the rate of photosynthesis as it is no longer the limiting factor
• Increasing light intensity

  Increases the rate of photosynthesis
• More energy is provided
• Above a certain threshold, further increases in light intensity


  Do not increase the rate of photosynthesis
• Another factor (such as temperature) is limiting the rate of the reaction
• Increasing light intensity

  1. Initially increases the rate of photosynthesis because more energy is provided

  2. Above a certain threshold, further increases do not increase the rate of photosynthesis as light intensity is no longer the limiting factor
• Increasing the temperature

  Increases the rate of photosynthesis
• If the temperature is increased to above about 45°C


  The enzymes that catalyse (speed-up) the reaction begin to denature (not work anymore)
• The rate of the reaction

  Drops sharply until it stops altogether
• Increasing temperature

  1. Initially increases the rate of photosynthesis because more energy is provided

  2. Above about 45°C, the enzymes involved in the reaction begin to denature
• High chlorophyll concentration gives a high rate of photosynthesis
• What is the word equation for photosynthesis?

  Carbon dioxide + water + sunlight -> glucose + oxygen (sunlight should be on top of arrow)
• Charles Darwin's theory of natural selection

  All species have evolved from simple life forms that first appeared on Earth more than three billion years ago
• Genetic variation

  • Different individual animals in a population (or species) have different combinations of genes
• Survival of the fittest

  Individuals that are best adapted to the environment will have a greater chance of survival
• Successful breeding

  Individuals with the best chances of survival are the most likely to breed successfully (because they survive)
• Best characteristics survive

  The characteristics responsible for the greater chance of survival are more likely to be passed on to the next generation