Chapter 3: Movement of Substances across a Plasma Membrane
Form 4 Biology Bab 3: Movement of Substances across a Plasma Membrane
3.1 Structure of Plasma Membrane
Cells need substances from the environment and must remove metabolic waste; movement is regulated by the plasma membrane. (p.46)
Fluid mosaic model: protein molecules float within the phospholipid bilayer, forming a changing mosaic pattern. (p.46)
Each phospholipid has a polar hydrophilic head and a non-polar hydrophobic tail; heads face outward (extracellular fluid and cytoplasm), tails face each other. (p.47)
Channel proteins form channels; carrier proteins act as carriers. (p.47)
Glycoprotein and glycolipid act as hormone receptors (e.g. insulin), stabilise the membrane and act as antigens for cell identification. (p.47)
Cholesterol makes the bilayer stronger, more flexible and less permeable to water-soluble substances such as ions. (p.47)
The fluid mosaic model was proposed by S. J. Singer and G. L. Nicholson in 1972. (p.47)
The plasma membrane is selectively permeable: it allows certain molecules through and limits others. (p.48)
3.2 Concept of Movement of Substances across a Plasma Membrane
Movement depends on molecule size, polarity and ionic charge. (p.49)
Small molecules (water, O2, CO2) diffuse through; ions and large molecules (glucose, amino acids) need channel or carrier proteins. (p.49)
Visking tubing experiment: starch is too large to pass out; glucose is small and diffuses out (iodine test and Benedict's test on tubing and beaker). (p.50)
Simple osmometer: sucrose solution in Visking tubing draws water in, so the level in the capillary tube rises. (p.51)
Passive transport requires no energy: simple diffusion, osmosis and facilitated diffusion. (p.52)
Simple diffusion: movement from high to low concentration until dynamic equilibrium; lipid-soluble molecules, O2 and CO2 diffuse through the bilayer. (p.53)
Osmosis: net movement of water from high water potential (low solute concentration) to low water potential (high solute concentration) across a selectively permeable membrane. (p.53)
Facilitated diffusion: ions and large molecules move down the concentration gradient with transport proteins, without energy. (p.53)
Channel proteins let specific small solutes and ions through; carrier proteins have specific binding sites (e.g. glucose carrier). (p.54)
Active transport: against the concentration gradient, requires ATP energy, specific carrier proteins (pumps) that change shape when a phosphate group attaches. (p.54)
Sodium-potassium pump: 3 Na+ bind, ATP splits to ADP and phosphate, protein changes shape, Na+ exits; 2 K+ enter; phosphate leaves and shape is restored. (p.55)
Proton pump: uses ATP to move H+ out of stomach epithelial cells, causing stomach acidity. (p.55)
Passive vs active: passive needs no energy and follows the gradient until equilibrium; active needs energy and goes against the gradient, causing accumulation or disposal. (p.56)
3.3 Movement of Substances in Living Organisms
Passive transport in organisms: gas exchange at alveoli (simple diffusion), water reabsorption in renal tubule (osmosis), water absorption by root hair cells (osmosis), fructose absorption in villus (facilitated diffusion). (p.57)
Active transport in organisms: absorption of glucose and amino acids in villus, glucose reabsorption in renal tubule, sucrose transport from leaf to phloem, mineral ion absorption by root hair cells. (p.57)
Isotonic: same solute concentration, no net water movement. (p.58)
Hypotonic: low solute concentration, high water potential; water moves out of it by osmosis. (p.58)
Hypertonic: high solute concentration, low water potential; water moves into it by osmosis. (p.58)
Red blood cells in hypotonic solution swell and burst (haemolysis). (p.59)
Red blood cells in isotonic solution keep normal shape. (p.59)
Red blood cells in hypertonic solution shrink (crenation). (p.59)
Plant cells in hypotonic solution become turgid; the rigid cell wall prevents bursting; turgor gives support and keeps stomata open. (p.60)
Plant cells in hypertonic solution undergo plasmolysis (membrane pulled from cell wall); leaves and stems wilt; deplasmolysis occurs if returned to hypotonic solution. (p.60)
Plant cells in isotonic solution become flaccid. (p.60)
Chicken blood experiment: distilled water, 0.15 M NaCl and 0.50 M NaCl show haemolysis, normal cells and crenation. (p.61)
Onion epidermis experiment: 0.50 M and 1.0 M sucrose show plasmolysis; distilled water shows turgidity. (p.62)
3.4 Application in Daily Life
Potato strip experiment (distilled water and 0.1–0.6 M sucrose): the concentration at which mass does not change is isotonic to potato cell sap. (p.63)
Too much fertiliser makes soil water hypertonic to root cell sap; roots lose water and plants wilt. (p.65)
Food preservation using salt, sugar and vinegar applies osmosis (pickles, salted fish). (p.65)
Malaysian innovation: Sil-RH Membrane Distillation from paddy chaff for desalination. (p.65)
Saline solutions are isotonic to blood plasma (0.85–0.90 g NaCl per 100 ml). (p.66)
Oral rehydration salts and isotonic drinks replace water and electrolytes. (p.66)
Liposomes (phospholipid bilayer vesicles) protect drugs from gastric juice; water-soluble drugs in the hydrophilic area, water-insoluble drugs in the hydrophobic area. (p.66)
Reverse osmosis: pressure pushes seawater through a semi-permeable membrane, letting only water molecules through to give fresh water. (p.66)