Course review
Review each learning objective in this topic, then open its concept explanation or question set.
Learning objective
• Amphipathic phospholipids have hydrophilic heads and hydrophobic tails • In water they spontaneously form continuous closed bilayers • Lipid bilayers are the structural basis of plasma membranes and vesicles
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Learning objective
• Hydrophobic fatty acid tails form a stable membrane core • The core has low permeability to ions and large or hydrophilic molecules • Membranes separate aqueous compartments and control entry and exit
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Learning objective
• Diffusion is passive movement down a concentration gradient using kinetic energy • Small non-polar molecules such as oxygen and carbon dioxide diffuse through the bilayer • Diffusion rate depends on factors such as gradient, distance, surface area, and temperature
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Learning objective
• Integral proteins are embedded in one or both lipid layers and may span the membrane • Integral proteins can act as channels, carriers, pumps, receptors, enzymes, or antigens • Peripheral proteins attach to membrane surfaces and can act as receptors or scaffolds
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Learning objective
• Osmosis is passive net movement of water across a partially permeable membrane • Water moves from lower solute concentration to higher solute concentration • Aquaporins are protein pores that increase water diffusion across membranes
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Learning objective
• Channel proteins provide hydrophilic pores for facilitated diffusion • Polar molecules, ions, ADP, ATP, and water use specific channels or transporters • Channels are selective and move substances down gradients without ATP
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Learning objective
• Pump proteins use ATP from respiration for active transport • They move specific molecules or ions against concentration gradients • Active transport is selective and uses carrier or pump proteins, not channel proteins
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Learning objective
• Simple diffusion depends mainly on particle size and hydrophobic or hydrophilic properties • Facilitated diffusion and active transport use proteins to create selective permeability • Selective transport is essential in roots, intestines, kidneys, and neurons
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Learning objective
• Glycoproteins and glycolipids have short carbohydrate chains on the extracellular surface • Together they form the glycocalyx • The glycocalyx supports recognition, adhesion, signalling, and interaction with water
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Learning objective
• The fluid mosaic model describes mobile lipids and proteins in a phospholipid bilayer • Membranes include integral proteins, peripheral proteins, cholesterol, glycoproteins, and glycolipids • Freeze-etching, protein extraction, and fluorescent tagging supported the model
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Learning objective
• Unsaturated fatty acid tails have kinks and increase membrane fluidity • Saturated fatty acid tails pack closely and strengthen membranes at higher temperatures • Homeoviscous adaptation changes lipid composition with temperature, as in lake sturgeon
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Learning objective
• Cholesterol has a polar hydroxyl group and mostly hydrophobic steroid structure • It sits between phospholipids and modulates animal membrane fluidity • It prevents stiffening at low temperature and over-fluidity at high temperature
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Learning objective
• Membrane fluidity allows vesicles to form, move, and fuse • Endocytosis takes material into cells by vesicle formation • Exocytosis exports material when vesicles fuse with the membrane
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Learning objective
• Gated ion channels are selective pores that open and close • Neurotransmitter-gated channels open when chemicals such as acetylcholine bind • Voltage-gated sodium and potassium channels respond to membrane potential during impulses
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Learning objective
• Sodium-potassium pumps are ATP-powered exchange transporters • Each cycle moves three sodium ions out and two potassium ions in • These gradients help maintain membrane potentials in nerve cells
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Learning objective
• Sodium-dependent glucose cotransporters move sodium and glucose into epithelial cells together • The sodium gradient is maintained by basolateral sodium-potassium pumps • Glucose is moved against its gradient by indirect active transport in intestine and nephron
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Learning objective
• Cell adhesion molecules are membrane proteins that bind cells to cells or extracellular matrix • Cadherins usually form cell-cell junctions; integrins usually form cell-matrix junctions • Tight, anchoring, gap, and signal-relaying junctions help organize animal tissues
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