QCAA Biology Alternative Sequence Cellular energy, gas exchange and plant physiology

Two features of gaseous exchange surfaces that are the same in lungs and gills are that they have a large surface area and they are moist.

Low-oxygen blood in capillaries flows in the opposite direction to the high-oxygen water flowing past the gills. Therefore, a concentration gradient is always maintained. Oxygen continues to be transferred into the blood as it flows past.

Guard cells regulate the opening and closing of the stomata by swelling or shrinking in response to osmotic changes.

Palisade mesophyll cells have large amounts of chloroplasts and are located below the cuticle (or epidermis) in the top layer of the leaf to maximise sunlight reaching them. Vascular bundles are below palisade mesophyll cells. Their role is to transport water to photosynthetic cells via xylem tubes, and transport products of photosynthesis to other parts of the plant via phloem tubes.

The stomata regulate the exchange of gases and water vapour between a leaf and its surrounding environment. When the stomata are closed, water molecules cannot escape the leaf, so the conversion of solar energy into chemical energy (in the form of NADPH and ATP) continues in the grana of chloroplasts.
But as the leaf also cannot acquire new carbon dioxide molecules from the environment, this limits the light-independent reactions to continuing only until the carbon dioxide stores in the leaf are depleted.

Comparing the control with Test 1, the increase in light brightness increased transpiration, i.e. 125 mL to 142 mL.
Comparing the control with Test 3, a higher temperature increased transpiration, i.e. 125 mL to 296 mL.

Plants transpire more rapidly in the light than in the dark. Light stimulates photosynthesis and therefore increases gas exchange (carbon dioxide). Stomata open to increase carbon dioxide exchange, and open stomata result in increased transpiration.
As temperature increases, transpiration rate increases due to higher temperatures, causing increased evaporation of water vapour from leaves.

glucose + oxygen $\rightarrow$ carbon dioxide + water + energy

Rate of oxygen uptake
$= (1 - 0.91) / 30$
$= 0.003 \text{ mL min}^{-1}$

Cohesion is the force of attraction between water molecules. This enables water to form a continuous water column in xylem and as one water molecule moves, it attracts other water molecules to move with it.
As the plant transpires, water molecules leave the leaf through stomata, leaving the remaining water under tension and the column of water molecules is pulled further up the stem to replace the water lost from the leaf.

The products of photosynthesis in chloroplasts (oxygen, glucose) are the reactants for cellular respiration in the mitochondria and vice versa. Plants need photosynthesis to produce the glucose necessary for cellular respiration, as this is their only mechanism for gaining the necessary requirements for life.

Xylem tubes are comprised of two types of dead cells: tracheids and vessels, whereas phloem is comprised of two types of living cells: sieve tubes and companion cells. Phloem is not continuous, but sectioned by sieve plates, but xylem tubes are continuous.

The energy from cellular respiration is needed to support all cellular functions, including repair, growth and reproduction. These cell processes are critical for the survival of organisms.

Both prokaryotes and eukaryotes are cells containing genetic information and have a cell membrane. Eukaryotic cells possess membrane-bound organelles, but prokaryotes do not. As eukaryotic cells are more complex, these similarities and differences reflect the concept that eukaryotes evolved from prokaryotic cells.