4 Semi-permeable Membranes Cell membranes are semi-permeable, also called selectively permeableWater moves across the membrane freelySmall molecules like oxygen and carbon dioxide diffuse through the phospholipidsSugars and amino acids diffuse through channel/transport/carrier proteinsIons (charged particles) do not move across the membrane freelyMacromolecules do not move across the membrane freely
5 Passive TransportDiffusion – the movement of substances from an area of higher concentration to an area of lower concentrationOsmosis – the movement of water from an area of higher water content to an area of lower water contentTonicity : Hypertonic, Hypotonic, & Isotonic Solutions
6 Active TransportThe movement of a substance against the concentration gradient (from lower concentration to higher concentrationActive transport requires the use of energy
7 Membrane proteins play an important role in passive and active transport
8 Very large molecules may be taken in by endocytosis
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