17.1: Introduction

Small molecules like \(\rm O_2\) or \(CO_2\) can cross cellular membranes unassisted, since neither the hydrophilic surfaces nor the hydrophobic interior of the phospholipid bilayer are barriers to their transit. On the other hand, most molecules (even water) need the help of membrane transport proteins to get into, or out of cells and organelles.

Transport proteins can act as gates, which may be open or closed. When open, these gates permit the diffusion of molecules into or out of cells along a concentration gradient, so that their concentrations equalize across the membrane. Like the passive diffusion of small gases, facilitated diffusion by membrane proteins does not require an input of energy.

Unlike passive or facilitated diffusion, active transport uses chemical energy to transport substances across a membrane against a concentration gradient. Membrane transport proteins that do this are in fact, pumps. We already saw one in the cristal membrane of the mitochondrion. This pump uses the energy of electron transport to actively push protons across the cristal membrane to create a concentration gradient; the free energy in this gradient is coupled to ATP synthesis by oxidative phosphorylation. Other membrane pumps use chemical energy (typically from ATP hydrolysis) to move ions or molecules across membranes and to create or to maintain chemical gradients. For example, pumps that create sodium- and potassium-ion gradients are responsible for the excitability of cells. Recall that this is a fundamental property of life: the ability of cells and organisms to respond to stimuli. As you read this chapter, look for how allosteric changes regulate membrane function. We’ll consider the following:

• How membrane gates and pumps work
• How membrane protein interactions allow cells to self-assemble into tissues and organs
• How cells direct protein traffic (e.g., to the cytoplasm, into membranes, into organelles, or out of the cell
• How membrane proteins participate in direct communication between adjacent cells
• How membrane proteins are receptors for long-distance communications, responding to neurotransmitters, hormones, and other external chemical signals

Learning Objectives

1. Explain how/why one cell’s plasma membrane differs from that of another cell type.
2. Explain how/why the plasma membrane differs from other membranes with in the same cell.
3. Determine if a solute crosses a plasma membrane by passive or facilitated diffusion.
4. Explain how salmon can spend part of their lives in the ocean and part swimming upstream in freshwater to spawn, without their cells shriveling or bursting.
5. Explain how active transport stores chemical energy (recall electron transport).
6. Explain the role of active transport in maintaining/restoring a cell’s resting potential.
7. Compare and contrast different kinds of gated channels.
8. Describe the order of ion movements that generate an action potential.
9. Define and compare exocytosis, pinocytosis, phagocytosis and receptor-mediated endocytosis
10. Distinguish between signal molecules that enter cells to deliver their chemical message and those deliver their message only as far as the plasma membrane.
11. Trace an intracellular response to a steroid hormone to a likely cellular effect.
12. Trace a liver cell response to adrenalin from plasma membrane to glycogenolysis (glycogen breakdown).
13. Compare the signal transduction activities of different G-protein receptors leading to the first active kinase enzyme.
14. Explain how a liver cell can respond the same way to two different hormones (e.g., adrenalin and glucagon)…, and why this should be possible
15. Describe/explain how a phosphorylation cascade amplifies the cellular response to a small amount of an effector (signal) molecule.
16. Discuss the differences and interactions between the glycocalyx, basement membrane and extracellular matrix (ECM).
17. Explain ECM functions and identify components involved in those functions
18. Describe how the molecular structure of fibronectin supports its different functions.
19. Describe some structural relationships between cell surfaces and the cytoskeleton.
20. Compare and contrast the structures and functions of the different cell junctions.
21. Distinguish between the structures and functions of cadherins, clathrin, COPs, adaptin, selectins, SNAREs and CAMs.
22. State an hypothesis to explain why some cancer cells divide without forming a tumor