4.2 - Gas Exchange in Plants

Question 1

What is the internal structure of a leaf?

Answer 1

• waxy cuticle
  water-proof, protective layer
• upper epidermis
  transparent layer, only one cell thick, allows maximum light can enter
• palisade mesophyll layer
  layer of elongated cells containing chloroplast and so where most of the photosynthesis occurs
• spongy mesophyll layer
  contains many air spaces sp gases can diffuse efficiently and have a large surface area
• lower epidermal cell
  where epidermal cells and guards cells are

Question 2

explain the structure and function of stomata

Answer 2

• pores located primarily on the underside of leaves, surrounded by
  pairs of specialized guard cells.
• regulate gas exchange (Open stomata allow carbon dioxide (CO2) to diffuse into the leaf for
  photosynthesis and enable the release of oxygen (O2),) and water loss by opening and
  closing in response to environmental and internal plant signals.

Question 3

how do the stomata work

Answer 3

Guard cells control stomatal aperture through changes
in turgor pressure. When turgid (due to water influx driven by ion accumulation, particularly
potassium), the stomata open. When flaccid (due to water loss), they close.

Question 4

structure and function of guard cells

Answer 4

• each stoma is surrounded by two guard cells
• Thick cell walls facing the air outside the leaf and
  the stoma.
• Thin cell walls facing adjacent epidermal cells.
• Cellulose microfibrils arranged in bands around the
  cell.
• regulate the opening and closing of stomata, tiny pores on the surface of leaves and stems, which control gas exchange and water loss

Question 5

structure and function of lenticels

Answer 5

• Lenticels are small, spongy regions in the bark of
  woody plants that facilitate gas exchange between
  the internal tissues and the external environment.
• Structure: Lenticels consist of loosely packed cells
  with large intercellular spaces, allowing gases to
  diffuse through them.
• Function: They permit the exchange of O2 and CO2
  in regions where the bark or epidermis would
  otherwise restrict gaseous diffusion. Lenticels are
  particularly important for respiration in woody
  stems and roots.

Question 6

explain the mechanism of gas exchange in leaves

Answer 6

When the guard cells are turgid (full of water) the stoma remains open allowing air to enter the leaf.

• The air spaces within the spongy mesophyll layer allow carbon dioxide to rapidly diffuse into cells.
• The carbon dioxide is quickly used up in photosynthesis by cells containing chloroplasts -
  maintaining the concentration gradient.
• No active ventilation is required as the thinness of the plant tissues and the presence of stomata help to create a short diffusion pathway.

Question 7

adaptations

Answer 7

• large SA which allows for diffusion to occur at a faster rate
• leaf has a thin structure which provides a short diffusion pathway
• Air spaces: The spongy mesophyll layer contains intercellular air spaces that facilitate the
  movement of CO2 towards photosynthetic cells and O2 away from them.
• Cuticle: Although waxy and waterproof, it helps limit excessive water loss while allowing
  stomata to function as the primary gas exchange sites.