Leaves and Photosysthesis

• Stomata


  Small openings on leaf surface for gas exchange, CO2 absorption for photosynthesis, and water vapor regulation
• Leaves


  Site of photosynthesis, food storage, water absorption, temperature regulation, and structural support
• Functions of leaves


  • Site of photosynthesis
  • Gas exchange
  • Storage
  • Protection from predators
• Photosynthesis


  The plant cell uses solar energy to convert carbon dioxide and water into glucose and oxygen
• Leaf cells


  • Absorb energy from sunlight through organelles called chloroplasts
• Gas exchange


  Gas exchange between the interior of the plant and its environment
• Epidermis


  • Contains pores through which gases can pass in and out
• In photosynthesis, the plant uses CO2 and releases O2 and H2O vapours through these pores
• Storage


  The sugars (glucose) that are produced during photosynthesis are stored in the leaves as well as other parts of the plants
• Protection from predators


  Ex: cacti leaves are reduced to sharp spines; some leaves have surface hairs or toxins that prevent herbivores from eating it
• Vascular tissue in leaves

  • Xylem carries water, minerals and nutrients upwards from the roots to the top of the plant
  • Phloem carries food (sugars) from one part of a plant to another
• Epidermal cells


  • Tightly packed in a single layer and covered by a waxy coating called a cuticle
  • The cuticle prevents water loss and provides a physical barrier against bacteria, molds and insects
  • Epidermal cells do not contain chloroplasts, so they do not perform photosynthesis
  • Epidermal cells are transparent, so light can pass through them to the cell within the leaf
• Mesophyll


  • Specialized for photosynthesis
  • Palisade mesophyll (region directly under the upper epidermis) - tightly packed, elongated arrangement maximizes the amount of light the plant can collect for photosynthesis
  • Spongy mesophyll (region directly beneath the palisade) - loosely packed layer with lots of air spaces allow for gas exchange between the mesophyll cells and the atmosphere through stomata
• Stomata


  Openings in the epidermis of a leaf, through which gases pass in and out
• Guard cells


  • Two kidney-shaped cells that control the opening and closing of a stoma
  • Expand and bend apart when there is a good supply of water within the leaf, opening the stomata
  • Become soft and collapse when there is a shortage of water, closing the stomata
  • Also have a mechanism that responds to light levels, closing the stomata at night when carbon dioxide is not needed because there is no light for photosynthesis
• When stomata are open

  Carbon dioxide can enter the leaf and oxygen can escape
• When stomata are open


  Water Vapour can also escape
• Water loss


  A very thin leaf could dry out and die very quickly on a sunny or windy day. Preventing too much water loss is therefore a major concern for many plants
• Photosynthesis


  Light is used with Carbon dioxide from the air and water from the root system to produce glucose and oxygen. This occurs during the day
• Cellular Respiration


  The glucose is broken down (with oxygen from the air) to create ATP with carbon dioxide and water as by products