Photosynthesis

• Adaptations of the leaf for photosynthesis


  • Leaf stalk
  • Thin, broad leaf blade
  • Waxy cuticle on the upper and lower epidermis
  • Stomata in the epidermal layers
  • Chloroplasts containing chlorophyll in all mesophyll cells
  • More chloroplasts in upper palisade tissue
  • Interconnecting air spaces in the spongy mesophyll
  • Veins containing xylem and phloem close to mesophyll cells
• Leaf stalk

  It holds the leaf in position to absorb maximum light energy
• Thin, broad leaf blade

  • A thin leaf blade provides a short diffusion distance for gases and enables light to reach all mesophyll cells easily
  • A broad leaf blade provides a large surface area for maximum absorption of light
• Functions of the waxy cuticle

  • It reduces water loss through evaporation from the leaf
  • It is transparent for light to enter the leaf
• Stomata in the epidermal layers

  Stomata open in the presence of light, allowing carbon dioxide to diffuse into and oxygen to diffuse out of the leaf
• Chloroplasts containing chlorophyll in all mesophyll cells

  Chlorophyll absorbs and transforms light energy to chemical energy used in the manufacture of glucose
• More chloroplasts in upper palisade tissue


  More light energy can be absorbed near the leaf surface
• Interconnecting air spaces in the spongy mesophyll

  The air spaces allow rapid diffusion of carbon dioxide and oxygen into and out of mesophyll cells
• xylem & phloem

  • Xylem transports water and mineral ions to mesophyll cells
  • Phloem transports sugars away from the leaf
• Glucose is first formed from carbon dioxide during photosynthesis
• Presence of starch in the leaves suggests that photosynthesis has taken place
• Destarching (removal of starch) must be carried out on the plants before the experiments
• How guard cells control the size of stomata


  1. Guard cells swell (become turgid), stoma opens

  2. Guard cells become limp (flaccid), stoma closes
• Guard cells


  • They contain chloroplasts and manufacture glucose by photosynthesis in daylight
  • They absorb water from nearby epidermal cells and become turgid, causing the stoma to open
  • When the leaf loses too much water in strong sunlight, the stomata close to prevent excessive water loss
• How carbon dioxide enters the leaf

  1. Carbon dioxide diffuses from the surrounding air through the stomata into the air spaces in the leaf

  2. The dissolved carbon dioxide then diffuses into the mesophyll cells
• How water enters the leaf

  1. Xylem transports water and dissolved mineral ions to the leaf from the roots

  2. The water and mineral ions move from cell to cell right through the mesophyll of the leaf
• Once the green cells receive the essential raw materials such as carbon dioxide and water, they can manufacture food by photosynthesis
• The phloem transports the sugars made in the leaves to other parts of the plant
• Word equation for photosynthesis

  Carbon dioxide + water + light energy -> glucose + oxygen
• Chemical equation for photosynthesis

  6CO2 + 6H2O -> C6H12O6 + 6O2
• Conditions necessary for photosynthesis

  • Sunlight
  • Carbon dioxide
  • Chlorophyll
  • A suitable temperature
  • Water
• Photosynthesis provides food, stores energy from the Sun as chemical energy, and maintains the balance of oxygen and carbon dioxide in the atmosphere
• What happens to the glucose formed during photosynthesis

  1. Glucose is either used immediately or stored in the plant as starch

  2. Glucose is broken down during respiration to release energy for vital activities

  3. Glucose is converted into amino acids and proteins

  4. Glucose is converted into fats