Chapter Summary

• Biological membranes consist of lipids, proteins, and carbohydrates. The fluid mosaic model of membrane structure describes a phospholipid bilayer in which proteins can move about within the plane of the membrane. Review Activity 6.1

• The two layers of a membrane may have different properties because of their different lipid compositions. Animal cell membranes may contain high concentrations of cholesterol (up to 25%). Review Activity 6.2

• The properties of membranes also depend on the integral membrane proteins and peripheral membrane proteins associated with them. Some proteins, called transmembrane proteins, span the membrane. Review Focus: Key Figure 6.1

• Carbohydrates, attached to proteins in glycoproteins or to phospholipids in glycolipids, project from the external surface of the cell membrane and function as recognition signals.

• Membranes are not static structures, but are constantly forming, exchanging components, and breaking down.

• In order for cells to assemble into tissues, they must recognize and adhere to one another. Cell recognition and cell adhesion depend on membrane-associated proteins and carbohydrates. Review Figure 6.6

• Adhesion can involve binding between identical (homotypic) or different (heterotypic) molecules on adjacent cells.

• Cell junctions connect adjacent cells. In some animal cells, tight junctions prevent the passage of molecules through the intercellular spaces between cells, and they restrict the migration of membrane proteins over the cell surface. Desmosomes cause cells to adhere firmly to one another. Gap junctions provide channels for communication between adjacent cells. Review Figure 6.7, Activity 6.3

• Integrins mediate the attachment of animal cells to the extracellular matrix and to each other. Detachment and recycling of integrins allow cells to move. Review Figure 6.8

• Membranes exhibit selective permeability, regulating which substances pass through them. Substances can cross the membrane by either passive transport, which requires no input of chemical energy, or by active transport, which uses chemical energy. Review Figure 6.9

• Diffusion is the movement of a solute from a region of higher concentration to a region of lower concentration. Equilibrium is reached when there is no further net change in concentration.

• In osmosis, water diffuses across a membrane from a region of higher water concentration to a region of lower water concentration.

• In an isotonic environment, total solute concentrations on both sides of the cell membrane are equal. If the solution surrounding a cell is hypotonic to the cell interior, more water enters the cell than leaves it, causing it to swell. In plant cells, this contributes to turgor pressure. In a hypertonic solution, more water leaves the cell than enters it, causing it to shrivel. Review Figure 6.10

• A substance can diffuse passively across a membrane by either simple diffusion or facilitated diffusion, via a channel protein or a carrier protein.

• Ion channels are membrane proteins that allow the rapid facilitated diffusion of ions through membranes. Gated channels can be opened or closed by chemical ligands, or changes in membrane voltage, or mechanical stimuli. Review Figure 6.11

• Aquaporins are water channels. Review Investigating Life: Aquaporins Increase Membrane Permeability to Water

• Carrier proteins bind to polar molecules such as sugars and amino acids and transport them across the membrane. The maximum rate of this type of facilitated diffusion is limited by the number of carrier (transporter) proteins in the membrane. Review Figure 6.12, Activity 6.4

• Active transport requires the use of chemical energy to move substances across membranes against their concentration or electrical gradients. Active transport proteins may be uniporters, symporters, or antiporters. Review Figure 6.13

• In primary active transport, energy from the hydrolysis of ATP is used to move ions into or out of cells. The sodium–potassium pump is an important example. Review Figure 6.14

• Secondary active transport couples the passive movement of one substance down its concentration gradient to the movement of another substance against its concentration gradient. Energy from ATP is used indirectly to establish the concentration gradient that results in the movement of the first substance. Review Figure 6.15

• Endocytosis is the transport of small molecules, macromolecules, large particles, and small cells into eukaryotic cells via invagination of the cell membrane and the formation of vesicles. Phagocytosis and pinocytosis are types of endocytosis. Review Figure 6.16A

• In exocytosis, materials in vesicles are secreted from the cell when the vesicles fuse with the cell membrane. Review Figure 6.16B

• In receptor-mediated endocytosis, a specific receptor protein on the cell membrane binds to a particular macromolecule. Review Figure 6.17