Flashcards · 7402 · AS Topic 3

Organisms exchange substances with their environment (AS Level) flashcards

17 cards on AS Topic 3, subtopics 3.3.1 to 3.3.4. Pick a subtopic or study them all.

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Why do large organisms need specialised exchange surfaces and mass transport?: Surface area to volume ratio falls as size increases, because volume grows faster than surface area. Diffusion across the body surface can no longer meet demand.
Surface area to volume ratio of a 3 cm cube: SA = 6 × 3 × 3 = 54 cm²; V = 3 × 3 × 3 = 27 cm³; SA:V = 54 ÷ 27 = 2 : 1.
Why do small mammals have a high metabolic rate?: A small animal has a large surface area to volume ratio, so it loses heat quickly. It needs a high rate of respiration to release enough heat to keep its body temperature constant.

Counter-current principle in fish gills: Blood in the lamellae flows in the opposite direction to water. Water always meets blood with a lower oxygen concentration, so a diffusion gradient is kept along the whole lamella and more oxygen is absorbed than with parallel flow.
Gas exchange in insects: Air enters through spiracles into tracheae and then tracheoles, which reach the muscle cells; gases diffuse along these air-filled tubes. Spiracles can close and a waterproof cuticle limits water loss.
How is the alveolar epithelium adapted for gas exchange?: One flattened cell thick (short diffusion distance), very many alveoli (large surface area), a dense capillary network keeping the gradient steep, and a moist lining in which gases dissolve.
Inspiration: muscles and pressure: Diaphragm contracts and flattens; external intercostal muscles contract, moving the ribs up and out. Thoracic volume increases, pressure falls below atmospheric, and air flows in.
Pulmonary ventilation rate: PVR = tidal volume × breathing rate.

How are lipids absorbed in the ileum?: Bile salts help form micelles holding monoglycerides and fatty acids. Micelles carry them to the epithelial cell surface, where these lipid-soluble molecules diffuse across the membrane.
Digestion of carbohydrates in mammals: Amylases (in saliva and from the pancreas) hydrolyse starch to maltose. Membrane-bound disaccharidases on the ileum epithelium (maltase, sucrase, lactase) hydrolyse disaccharides to monosaccharides.
Endopeptidases, exopeptidases and dipeptidases: Endopeptidases hydrolyse peptide bonds within a polypeptide. Exopeptidases hydrolyse bonds at the ends, removing single amino acids. Membrane-bound dipeptidases hydrolyse dipeptides into two amino acids.
How are monosaccharides and amino acids absorbed in the ileum?: By co-transport with sodium ions: Na⁺ is actively pumped out of the epithelial cell into the blood, keeping its concentration low in the cell, so Na⁺ diffuses in from the lumen through a co-transporter protein, carrying glucose or amino acids with it.

Bohr effect: A higher carbon dioxide concentration shifts the oxygen dissociation curve to the right, so haemoglobin unloads oxygen more readily at a given partial pressure (as in respiring tissue).
Cardiac output: Cardiac output = stroke volume × heart rate.
How is tissue fluid formed and returned?: At the arteriole end, high hydrostatic pressure forces fluid out of the capillary; plasma proteins stay behind, lowering blood water potential. At the venule end, water returns by osmosis. Excess drains into the lymph system.
Cohesion–tension theory of water movement in xylem: Water evaporates from leaves (transpiration), creating tension. Hydrogen bonds make water molecules cohesive, so the column is pulled up the xylem as a continuous chain; adhesion to the walls helps.
Mass flow hypothesis of translocation in phloem: Sucrose is loaded into phloem at the source; water follows by osmosis, raising hydrostatic pressure. Sap flows toward the sink, where sucrose is removed or used.