Gas exchange in plants

• Label this diagram of a dicotyledonous leaf

  A) upper epidermis

  B) palisade mesophyll

  C) air spaces

  D) spongy mesophyll

  E) stoma

  F) lower epidermis

  G) bundle sheath parenchyma

  
• Where are the xylem and phloem in a leaf?

  In the vascular bundle

  The xylem is on top of the phloem
• How are the waxy cuticle and upper epidermis adapted for photosynthesis?

  They are transparent, so light penetrates through them to the mesophyll layers
• How is the palisade mesophyll layer adapted for photosynthesis?

  -The cells are elongated, meaning it can accommodate a large number of them, increasing the surface area

  -they are packed with chloroplasts to capture as much light as possible
• How is the spongy mesophyll layer adapted for photosynthesis?

  Chloroplasts can rotate and move, so they can move into the best positions for maximum absorption of light
• How are air spaces in the spongy mesophyll significant for both gas exchange and photosynthesis?

  Gas exchange: allows oxygen and carbon dioxide to diffuse between the stomata and the cells

  Photosynthesis: allows carbon dioxide to diffuse to photosynthesising cells
• The stomata are used for gas exchange in and out of the leaf
• Why is a large surface area significant for both gas exchange and photosynthesis?

  Gas exchange: room for many stomata

  Photosynthesis: captures as much light as possible
• Why is a thin leaf a good adaptation for both gas exchange and photosynthesis?

  Gas exchange: short diffusion pathway for gases entering and leaving

  Photosynthesis: light can penetrate through the leaf
• What causes the stomata to open?

  -when turgid, they expand unevenly

  -the thick inner cell wall is inelastic and only stretches a little

  -the thin outer cell wall is much more elastic stretches a lot

  -this changes the shape of the guard cell and opens the stoma
• Outline the Malate hypothesis?

  1. photosynthesis occurs in the chloroplasts -> light energy is used to make ATP
  2. Potassium ions move into the guard cell by active transport (using the ATP from photosynthesis)
  3. Insoluble starch is converted into soluble malate (due to potassium ions being present)
  4. Water potential decreases due to potassium ions and malate being present
  5. Water moves into the guard cell down a water potential gradient by osmosis
  6. This makes it turgid, causing the thin outer cell wall to expand