The Structure of Biological Membranes

• The plasma membrane physically separates the interior of the cell from the extracellular environment, receives information about changes in the environment, regulates the passage of materials into and out of the cell, and communicates with other cells.

• Biological membranes form compartments within eukaryotic cells that allow a variety of separate functions.

  • Membranes participate in and serve as surfaces for biochemical reactions.

• According to the fluid mosaic model, membranes consist of a fluid phospholipid bilayer in which a variety of proteins are embedded.

  • The phospholipid molecules are amphipathic: they have hydrophobic and hydrophilic regions.

  • The hydrophilic heads of the phospholipids are at the two surfaces of the bilayer, and their hydrophobic fatty acid chains are in the interior.

• In almost all biological membranes, the lipids of the bilayer are in a fluid or liquid-crystalline state, which allows the lipid molecules to move rapidly in the plane of the membrane.

  • Proteins also move within the membrane.

• Lipid bilayers are flexible and self-sealing and can fuse with other membranes.

  • These properties allow the cell to transport materials from one region of the cell to another; materials are transported in vesicles that bud from one cell membrane and then fuse with some other membrane.

• Integral membrane proteins are embedded in the bilayer with their hydrophilic surfaces exposed to the aqueous environment and their hydrophobic surfaces in contact with the hydrophobic interior of the bilayer.

  • Transmembrane proteins are integral proteins that extend completely through the membrane.

• Peripheral membrane proteins are associated with the surface of the bilayer, usually bound to exposed regions of integral proteins, and are easily removed without disrupting the structure of the membrane.

Overview of Membrane Protein Functions

• Membrane proteins:

  • Anchor cells

  • Transport materials

  • Act as enzymes or receptors

  • Recognize cells and communicate with them

  • Structurally link cells

Cell Membrane Structure and Permeability

• Biological membranes are selectively permeable membranes: they allow the passage of some substances but not others.

  • By regulating passage of molecules that enter and leave the cell and its compartments, the cell controls its volume and the internal composition of ions and molecules.

• Membrane transport proteins facilitate the passage of certain ions and molecules through biological membranes.

  • Carrier proteins are transport proteins that undergo a series of conformational changes as they bind and transport a specific solute.

  • ABC transporters are carrier proteins that use energy from aTP to transport solutes.

• Channel proteins are transport proteins that form passageways through which water and certain ions travel through the membrane.

  • Porins are channel proteins that form relatively large pores through the membrane for passage of water and certain solutes.

Passive Transport

• Diffusion is the net movement of a substance down its concentration gradient from a region of greater concentration to one of lower concentration.

  • Diffusion and osmosis are physical processes that do not require the cell to directly expend metabolic energy.

• In simple diffusion through a biological membrane, solute molecules or ions move directly through the membrane down their concentration gradient.

  • Facilitated diffusion uses specific transport proteins to move solutes across a membrane.

  • As in simple diffusion, net movement is always from a region of higher to a region of lower solute concentration.

  • Facilitated diffusion cannot work against a concentration gradient.

• Osmosis is a kind of diffusion in which molecules of water pass through a selectively permeable membrane from a region where water has a higher effective concentration to a region where its effective concentration is lower.

• The concentration of dissolved substances (solutes) in a solution determines its osmotic pressure.

  • Cells regulate their internal osmotic pressures to prevent shrinking or bursting.

• An isotonic solution has an equal solute concentration compared with that of another fluid, for example, the fluid within the cell.

• When placed in a hypertonic solution, one that has a greater solute concentration than that of the cell, a cell loses water to its surroundings; plant cells undergo plasmolysis, a process in which the plasma membrane separates from the cell wall.

• When cells are placed in a hypotonic solution, one that has a lower solute concentration than the solute concentration of the cell, water enters the cells and causes them to swell.

• Plant cells withstand high internal hydrostatic pressure because their cell walls prevent them from expanding and bursting.

  • Water moves into plant cells by osmosis and fills the central vacuoles.

  • The cells swell, building up turgor pressure against the supportive cell walls.

Active Transport

• In active transport, the cell expends metabolic energy to move ions or molecules across a membrane against a concentration gradient.

  • For example, the sodium–potassium pump uses aTP to pump sodium ions out of the cell and potassium ions into the cell.

• In cotransport, also called indirect active transport, two solutes are transported at the same time.

  • An ATP-powered pump maintains a concentration gradient.

  • Then a carrier protein cotransports two solutes.

  • It transports one solute down its concentration gradient and uses the energy released to move another solute against its concentration gradient.

Exocytosis and Endocytosis

• The cell expends metabolic energy to carry on exocytosis and endocytosis.

  • In exocytosis, the cell ejects waste products or secretes substances such as mucus by fusion of vesicles with the plasma membrane.

  • This process increases the surface area of the plasma membrane.

• In endocytosis, materials such as food particles are moved into the cell.

  • A portion of the plasma membrane envelops the material, enclosing it in a vesicle or vacuole that is then released inside the cell.

  • This process decreases the surface area of the plasma membrane.

• Three types of endocytosis are phagocytosis, pinocytosis, and receptor-mediated endocytosis.

  • In phagocytosis, the plasma membrane encloses a large particle such as a bacterium, forms a vacuole around it, and moves it into the cell.

  • In pinocytosis, the cell takes in dissolved materials by forming tiny vesicles around droplets of fluid trapped by folds of the plasma membrane.

  • In receptor-mediated endocytosis, specific receptors in coated pits along the plasma membrane bind ligand molecules.

    • These pits, coated by the protein clathrin, form coated vesicle by endocytosis.

    • The vesicles fuse with lysosomes, and their contents are digested and released into the cytosol.

Cell Junctions

• Cells in close contact with one another may form intercellular junctions.

  • Anchoring junctions include desmosomes and adhering junctions; they are found between cells that form a sheet of tissue.

  • Desmosomes spot-weld adjacent animal cells together.

  • Adhering junctions are formed by cadherins, transmembrane proteins that cement cells together.

• Tight junctions seal membranes of adjacent animal cells together, preventing substances from moving through the spaces between the cells.

• Gap junctions, composed of the protein connexin, form channels that allow communication between the cytoplasm of adjacent animal cells.

• Plasmodesmata are channels connecting adjacent plant cells.

  • Openings in the cell walls allow the plasma membranes and cytosol to be continuous; certain molecules and ions can pass from cell to cell.