BIO CH&12&13 PLANT SYSTEMS

Explain how carbon dioxide, oxygen and water vapour move between a leaf and the atmosphere.

• Carbon dioxide (CO₂): CO₂ concentration is usually higher in the atmosphere than inside the leaf, so CO₂ diffuses into the leaf through the stomata. It is used in photosynthesis.

• Oxygen (O₂): Photosynthesis produces oxygen inside the leaf, creating a higher O₂ concentration inside than outside. O₂ therefore diffuses out through the stomata.

• Water vapour: Water evaporates from the moist surfaces of mesophyll cells into the air spaces inside the leaf. Water vapour then diffuses out through the stomata into the atmosphere. This loss of water is called transpiration.

Explain how guard cells regulate stomatal opening and why stomatal opening creates a trade-off for the plant.

Guard cells are specialised cells surrounding each stoma. They control whether the stoma is open or closed by changing their turgor pressure.

• When guard cells take up ions, water enters them by osmosis.

• The guard cells become turgid, causing them to curve apart and open the stoma.

• When ions leave the guard cells, water leaves by osmosis.

• The guard cells become flaccid, causing the stoma to close.

The trade-off

Opening stomata allows CO₂ to enter the leaf for photosynthesis. However, it also allows water vapour to escape, increasing transpiration and the risk of water loss.

Explain why plant gas exchange does not involve the xylem or phloem transport systems

Plant gas exchange does not directly involve the xylem or phloem because gases move by diffusion, not by long-distance transport.

• CO₂ and O₂ move between the atmosphere and the leaf through the stomata and internal air spaces, following concentration gradients.

• Water vapour also diffuses out through the stomata during transpiration.

• The xylem transports water and mineral ions from the roots to the leaves.

• The phloem transports sugars and other organic nutrients around the plant.

Compare gas exchange at stomata with gas exchange at an animal gas-exchange surface.

Stomata connect the leaf's internal air spaces directly with the atmosphere, whereas animal gas-exchange surfaces are closely associated with circulatory systems that transport gases around the body.

. State the materials transported by xylem and the usual direction in which they move.

Xylem transports water and minerals upward from the roots to the shoots.

Explain the roles of root pressure, capillary action, cohesion, adhesion and transpiration in xylem transport.

1. ranspiration creates the pull, while cohesion maintains the water column and adhesion helps keep it against the xylem walls. Root pressure and capillary forces can also contribute, but they are generally less important than transpiration in tall plants.

State the products transported by phloem and explain the terms source, sink and translocation.

• Products transported by phloem: Mainly sucrose (a soluble sugar) and amino acids. These are transported to parts of the plant that need or store them.

• Source: A region where sugars are produced or released, such as leaves during photosynthesis.

• Sink: A region where sugars are used or stored, such as roots, fruits, seeds, growing shoots, or developing flowers.

• Translocation: The transport of dissolved organic substances, mainly sucrose, through the phloem from sources to sinks.

. Compare xylem and phloem in terms of materials transported, direction of movement and transport mechanism.

Xylem mainly carries water and minerals upward, while phloem carries sugars and other organic nutrients between sources and sinks.

Explain how water loss at the leaf can cause water to move from the roots to the leaves.
 

Water evaporates from leaves → tension develops in xylem → cohesion transmits the pull → water moves upward from roots to leaves.

Explain how each of the following can reduce water loss: a thick waxy cuticle, sunken stomata, leaf hairs, small or reduced leaves, rolled leaves and vertically hanging leaves.

. Thick waxy cuticle: The waterproof waxy layer on the leaf surface reduces evaporation of water from the leaf.

2. Sunken stomata: The stomata are recessed into pits, trapping moist air around them. This reduces the water vapour concentration gradient between the leaf and the outside air, slowing diffusion of water vapour out of the leaf.

3. Leaf hairs: Hairs trap a layer of still, moist air around the leaf surface. This reduces air movement and slows the diffusion of water vapour away from the leaf.

4. Small or reduced leaves: They have a smaller surface area, so less water is lost through evaporation from the leaf surface and through transpiration.

5. Rolled leaves: Rolling traps moist air inside the leaf, around the stomata, reducing the water vapour concentration gradient and therefore reducing water loss.

6. Vertically hanging leaves: They receive less direct sunlight, reducing heating and evaporation. They may also experience less exposure to strong sunlight during the hottest part of the day, helping to reduce transpiration.

Explain why closing stomata conserves water but may reduce photosynthesis.

• Water conservation: When stomata close, less water vapour can escape from the leaf, so transpiration and water loss decrease.

• Reduced photosynthesis: Carbon dioxide normally enters the leaf through stomata. When they are closed, less CO₂ enters, so there is less CO₂ available for photosynthesis and the rate of photosynthesis decreases.

Choose one Australian plant adapted to an arid environment. Describe at least three relevant adaptations and explain how each improves survival.

These adaptations help Spinifex conserve water and continue surviving when rainfall is low and temperatures are high.