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Flashcards · H420 · Topic 3
Exchange and transport flashcards
18 cards on Topic 3, subtopics 3.1.1 to 3.1.3. Pick a subtopic or study them all.
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3.1.1 Exchange surfaces
- Why do larger, more active organisms need specialised exchange surfaces?
- Surface area to volume ratio falls as size increases, while a higher metabolic rate raises demand, so diffusion across the body surface is too slow. They need a large, thin exchange surface with a maintained gradient.
- Mechanism of inspiration in mammals
- External intercostal muscles and the diaphragm contract: the ribs move up and out and the diaphragm flattens. Thorax volume increases, pressure falls below atmospheric, and air moves in.
- Tidal volume, vital capacity and ventilation rate
- Tidal volume: volume per resting breath. Vital capacity: maximum volume exhaled after a maximal inhalation. Ventilation rate = tidal volume × breathing rate.
- Countercurrent flow in fish gills
- Blood in the lamellae flows in the opposite direction to water, so water always meets blood with less oxygen. A diffusion gradient is kept along the whole lamella; with parallel flow, net uptake would stop partway along.
- Gas exchange in insects
- Air enters through spiracles into tracheae and tracheoles. Exchange happens at the tracheal fluid at the ends of the tracheoles. Movements of the thorax and abdomen change body volume to ventilate.
3.1.2 Transport in animals
- Open vs closed, single vs double circulation
- Insect: open (fluid bathes tissues at low pressure). Fish: closed, single (blood passes the heart once per circuit). Mammal: closed, double (twice per circuit, so pressure is restored after the lungs).
- How tissue fluid forms and returns (hydrostatic and oncotic pressure)
- At the arteriole end hydrostatic pressure exceeds oncotic pressure, forcing fluid out. Hydrostatic pressure falls along the capillary, so at the venule end oncotic pressure from plasma proteins draws water back in. Excess drains into lymph.
- Pressures and valves during the cardiac cycle
- Atrial systole: AV valves open; atrial pressure above ventricular. Ventricular systole: AV valves shut, semilunar valves open; ventricle pressure above aorta. Diastole: semilunar valves shut, AV valves open; atria fill.
- Cardiac output
- Cardiac output = heart rate × stroke volume.
- How is the heartbeat coordinated?
- Cardiac muscle is myogenic. The SAN starts a wave of excitation across the atria; the AVN delays it; Purkyne tissue carries it through the ventricles.
- ECG terms: tachycardia, bradycardia, fibrillation, ectopic beat
- Tachycardia: faster than normal. Bradycardia: slower. Fibrillation: irregular, uncoordinated. Ectopic beat: an extra beat that comes early. Heart rate = 60 ÷ time between R waves (s).
- Fetal haemoglobin and the Bohr effect
- Fetal haemoglobin has a higher affinity for oxygen, so its dissociation curve lies to the left of adult haemoglobin. More carbon dioxide shifts the curve to the right (Bohr effect), so more oxygen is unloaded.
3.1.3 Transport in plants
- How does water move up the xylem?
- Water evaporates from leaf cells and diffuses out through the stomata, lowering water potential. Cohesion between water molecules pulls the column up under tension (the transpiration stream); adhesion to the walls helps.
- Translocation: active loading of sucrose
- Companion cells pump H⁺ out using ATP; H⁺ returns with sucrose by co-transport. Sieve tube water potential falls, water enters, hydrostatic pressure rises, and sap flows from source to sink, where sucrose is removed.
- Xylem vessels vs phloem sieve tubes
- Xylem: dead, hollow, lignified cells joined end to end with no end walls, with pits; carry water and mineral ions up the plant. Phloem: living sieve tube elements joined by perforated sieve plates, with few organelles, supported by companion cells; carry assimilates up and down.
- Symplast vs apoplast pathway
- Symplast: water moves through the cytoplasm of cells, linked by plasmodesmata. Apoplast: water moves through cell walls and the spaces between cells. At the endodermis the Casparian strip blocks the apoplast, so water must enter the symplast.
- Factors that affect the rate of transpiration
- Light intensity (stomata open in light), temperature, humidity, air movement and the water available in the soil. A potometer measures water uptake, which is close to the rate of transpiration.
- Adaptations of xerophytes
- Thick waxy cuticle; sunken stomata; rolled leaves and hairs that trap moist air; fewer stomata; leaves reduced to spines; long or spreading roots. Example: marram grass. Hydrophytes such as water lilies instead have large air spaces and stomata on the upper leaf surface.