Chemistry

• The site of photosynthesis

  • In most plants, photosynthesis takes place in the leaves
  • Leaves come in many shapes and sizes but are adapted to allow photosynthesis to take place efficiently
• Adaptations of leaves for efficient photosynthesis

  • Large surface area
  • Thin, transparent, watery leaf cuticle
  • Presence of chloroplasts rich in chlorophyll
  • Palisade cells tightly packed together near the upper surface of the leaf
• Light absorption


  • Maximised by:
  • The short distance from top to bottom, allowing all cells to receive light
  • The large surface area
  • The thin, transparent, waxy cuticle prevents evaporation and allows light to reach the cells
  • The presence of chloroplasts rich in chlorophyll
  • The palisade cells tightly packed together near the upper surface of the leaf
• All of these adaptations allow for maximum light absorption and efficient photosynthesis
• Gaseous exchange


  Maximised by air spaces between cells in the spongy mesophyll which allow carbon dioxide to later enter the palisade cells. Gaseous exchange is also needed for respiration.
• Gaseous exchange


  1. Gases can enter and leave through the Stomata

  2. Stoma is surrounded by the guard cells that regulate opening and closing to prevent dehydration by excess evaporation
• Photosynthesis


  The process where plants take inorganic chemicals, carbon dioxide and water, to make the organic chemical glucose (a carbohydrate). This simple food can be used as an energy source in the form of starch or converted into other useful organic molecules like proteins and fats.
• Photosynthesis requires an input of energy, making it an endothermic reaction. Plants have found a way to capture the energy of sunlight using a pigment called chlorophyll.
• Once the light energy has been captured it can be used to create glucose, converting the light energy into chemical energy. Oxygen gas is released as a waste product.
• The word equation and balanced chemical equation for photosynthesis is: Carbon dioxide + Water -> Glucose + Oxygen
• Fate of glucose produced in photosynthesis

  • Stored as starch
  • Used to make other substances like fats and proteins
• Starch test


  1. Put plant in dark for 24 hours to destarch

  2. Place plant in bright light for several hours

  3. Test leaf using iodine solution

  4. Areas with chlorophyll will test positive for starch
• Light test


  1. Destarch a plant

  2. Cover part of leaf with lightproof material

  3. Place plant in bright light for several hours

  4. Test leaf using iodine solution

  5. Areas exposed to light will test positive for starch
• Measuring rate of photosynthesis

  Measure the volume of oxygen gas bubbles released over time
• Factors affecting rate of photosynthesis

  • Light intensity
  • Carbon dioxide concentration
  • Temperature
• The law of limiting factors states that the rate of photosynthesis is limited by the factor present in the minimum amount
• Food components


  • Simple carbohydrates (glucose)
  • Complex carbohydrates (starch, cellulose)
  • Fats/lipids
  • Proteins
• Food tests


  1. Starch test: Add iodine, turns blue-black

  2. Reducing sugar test: Add Benedict's solution, turns brick-red precipitate

  3. Protein test: Add biuret solution, turns lilac/purple

  4. Fat test: Add ethanol, forms white emulsion
• Enzymes


  Biological catalysts that speed up the rate of reactions without being used up themselves
• Types of enzymes


  • Intracellular (inside cells)
  • Extracellular (outside cells)
• Enzyme mechanism


  • Substrate binds to active site of enzyme
  • Enzyme-substrate complex forms
  • Reaction occurs
  • Products are released
• Enzyme properties


  • Unchanged by reactions
  • Active site shape is complementary to substrate
  • Specific to one substrate
  • Affected by factors like temperature and pH
• Denaturation of enzymes occurs when factors like high temperature or extreme pH alter the shape of the active site, preventing substrate binding
• Enzymes are not changed by a reaction and can be reused
• Active site


  The shape of the enzyme and the substrate are complementary. They fit together
• "The action of enzymes is like a lock and key": 'The analogy used to describe how enzymes and substrates fit together'
• Enzymes


  • They are proteins and biological catalysts
  • Each enzyme is specific and will only work with its own substrate
• Enzyme specificity


  • Amylase can only digest starch, it cannot digest protein
• Factors that alter enzyme shape

  High temperature and extremes of pH
• Denaturation of enzymes results in the active site being affected, reducing enzyme activity
• Denatured enzymes are called "lazy"
• Enzyme-catalysed reactions


  1. Binding of enzyme and substrate

  2. Transition state facilitation

  3. Catalysis

  4. Release of products
• At very low temperatures


  Enzymes are inactive and have little energy, resulting in few collisions between enzyme and substrate
• As temperature increases


  Enzyme activity increases as molecules move faster, collide and react at an increased rate
• Optimum temperature


  The specific temperature at which an enzyme is most active and has the maximum rate of reaction
• At temperatures that are too high, the active sites of enzymes change, preventing the substrate from binding, so activity slows and stops as the enzyme is denatured
• It is impossible to kill an enzyme, they are not living but are found in living cells. Denaturation means the enzymes may die, not the chemicals!
• Inhibitors


  Molecules that slow down the rate of enzyme-catalysed reactions
• Competitive inhibitors


  Inhibitor molecules that have a similar structure to the normal substrate and can temporarily bind to the active site, preventing the substrate from binding
• Adding more substrate will reverse the effects of competitive inhibition