Concept 5.1   Biological Membranes Have a Common Structure and Are Fluid

• 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.
• The two layers of a membrane may have different properties because of their different phospholipid compositions, exposed domains of integral membrane proteins, and peripheral membrane proteins. Transmembrane proteins span the membrane. Review Figure 5.1, ACTIVITY 5.1, and ANIMATED TUTORIAL 5.1

Concept 5.2   Passive Transport across Membranes Requires No Input of Energy

• Membranes exhibit selective permeability that regulates which substances can pass through them.
• A substance can diffuse passively across a membrane by one of two processes: simple diffusion through the phospholipid bilayer or facilitated diffusion, either through a channel created by a channel protein or by means of a carrier protein. In both cases, molecules diffuse down their concentration gradients. Review Figure 5.4 and ANIMATED TUTORIAL 5.2
• In osmosis, water diffuses from a region of higher water concentration to a region of lower water concentration, largely through membrane channels called aquaporins. Ions diffuse across membranes through ion channels. Review Figure 5.3 and Figure 5.5
• Carrier proteins bind to polar molecules such as sugars and amino acids and transport them across the membrane. Review Figure 5.6

Concept 5.3   Active Transport Moves Solutes against Their Concentration Gradients

• Active transport requires the use of chemical energy to move substances across membranes against their concentration gradients. The sodium–potassium (Na⁺–K⁺) pump uses energy released from the hydrolysis of ATP. Review Figure 5.7 and ANIMATED TUTORIAL 5.3

Concept 5.4   Large Molecules Cross Membranes via Vesicles

• Endocytosis is the transport of molecules, large particles, and small cells into eukaryotic cells via the invagination of the cell membrane and the formation of vesicles. Review Figure 5.8A
• In receptor endocytosis, a specific receptor on the cell membrane binds to a particular macromolecule that is to be transported into the cell. Review Figure 5.9 and ANIMATED TUTORIAL 5.4
• In exocytosis, materials in vesicles are secreted from the cell when the vesicles fuse with the cell membrane. Review Figure 5.8B

Concept 5.5   The Membrane Plays a Key Role in a Cell’s Response to Environmental Signals

• Cells receive many signals from the physical environment and from other cells. Chemical signals are often at very low concentrations. Review Figure 5.10
• A signal transduction pathway involves the interaction of a signal (often a chemical ligand) with a receptor; the transduction and amplification of the signal via a series of steps within the cell; and a cellular response. The response may be short-term or long-term. Review Figure 5.11
• Cells respond to signals only if they have specific receptor proteins that can be activated by those signals. Many receptors are located at the cell membrane. They include ion channels, protein kinases, and G protein–linked receptors. Review Figure 5.13, Figure 5.14 and ANIMATED TUTORIAL 5.5

Concept 5.6   Signal Transduction Allows the Cell to Respond to Its Environment

• A cascade of events, one following another, occurs after a receptor is activated by a signal.
• Often, a soluble second messenger conveys signaling information from the primary messenger (ligand) at the membrane to downstream signaling molecules in the cytoplasm. Cyclic AMP (cAMP) is an important second messenger. Review Figure 5.16
• Activated enzymes may in turn activate other enzymes in a signal transduction pathway, leading to impressive amplification of a signal. Review Figure 5.17 and ANIMATED TUTORIAL 5.6
• Protein kinases covalently add phosphate groups to target proteins; cAMP binds target proteins noncovalently. Both kinds of binding change the target protein’s conformation to expose or hide its active site.
• Signal transduction can be regulated in several ways. The balance between the activation and inactivation of the molecules involved determines the ultimate cellular response to a signal. Review Figure 5.18
• The cellular responses to signals may include the opening of ion channels, changes in gene expression, or the alteration of enzyme activities.

See ACTIVITY 5.2 for a concept review of this chapter.