Mechanisms of cellular transport: diffusion, osmosis, active transport
Cell biology • Transport in cells
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Key concepts
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Diffusion - definition and effect
Diffusion is the net movement of particles of any substance in solution or particles of a gas from an area of higher concentration to an area of lower concentration. Net movement occurs because more particles move in the direction of lower concentration than in the reverse direction, producing an overall flux until equilibrium is approached. The process is passive and does not use cellular energy, so diffusion occurs down a concentration gradient. Practical examples include oxygen moving from alveoli into blood and glucose moving from the small intestine into blood after a meal .
Diffusion - limiting factors and rate
The rate of diffusion increases when the concentration gradient is steeper, temperature is higher and the surface area for exchange is larger. Membrane thickness and particle size reduce the rate by increasing distance or lowering mobility. Multicellular organisms use adaptations such as thin exchange surfaces, ventilation and increased surface area to maintain adequate rates of diffusion over larger distances .
Osmosis - definition and tonicity
Osmosis is the net diffusion of water from a region of higher water concentration (a more dilute solution) to a region of lower water concentration (a more concentrated solution) across a partially permeable membrane. Water movement depends on relative solute concentrations: isotonic solutions show no net water movement, hypertonic solutions draw water out of cells and hypotonic solutions cause water to move into cells. Plant and animal cells show different responses because of cell walls and rigid structures that influence shape and turgor .
Osmosis - limiting factors and experimental evidence
Osmosis proceeds faster when the water concentration difference is larger and when membranes provide a larger effective surface area and shorter diffusion distance. Experimental investigations use plant tissue mass change in solutions of different solute concentrations to demonstrate osmotic water movement and to estimate the point of no net movement. Drying tissue before and after experiments prevents errors from surface water and allows accurate mass comparisons .
Active transport - definition and energy requirement
Active transport is the net movement of particles from a region of low concentration to a region of high concentration against a concentration gradient. Active transport requires metabolic energy supplied by respiration and often uses carrier proteins or pumps in cell membranes to move specific ions or molecules. The process allows cells to accumulate needed substances when external concentrations are low and to move substances where they are required, rather than relying solely on passive movement .
Active transport - biological examples
Root hair cells absorb mineral ions from soil by active transport when soil concentrations are lower than cytoplasm concentrations; cells in the small intestine use active transport to absorb sugars when gut concentrations are lower than blood concentrations. Active transport therefore supports nutrient uptake and ion balance even when diffusion is insufficient. Active transport declines when respiration is inhibited or ATP availability falls, reducing uptake efficiency .
Comparing diffusion, osmosis and active transport
Diffusion and osmosis are passive processes driven by concentration differences and do not use cellular energy; diffusion applies to solutes and gases while osmosis applies specifically to water across a partially permeable membrane. Active transport is an active, energy-dependent process that moves substances up a concentration gradient. The presence or absence of energy use, the direction relative to concentration gradients and the types of substance moved provide clear distinguishing features between the mechanisms .
Transport into and out of cells - how it occurs
Substances cross membranes by diffusion, osmosis or active transport depending on concentration differences and cellular needs. Oxygen and carbon dioxide diffuse across alveolar and capillary membranes because of steep gas concentration gradients. Water moves by osmosis into root cells from soil when soil water concentration exceeds root sap concentration. Nutrients and mineral ions move by diffusion when gradients favour inward movement and by active transport when inward movement occurs against a gradient. Membrane proteins and cellular respiration control selectivity and energy supply for these processes .
Key notes
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