Cell Membranes: Structure and Function

• Chapter 5 covers membrane structure, transport, and cell junctions.

Membrane Structure

• The cell or plasma membrane is a phospholipid bilayer that creates a selectively permeable barrier around the cell.
• Phospholipids are amphipathic molecules.
• Membranes also contain proteins, carbohydrates, and sterols.
• Integral membrane proteins are transmembrane or lipid-anchored proteins.
• Peripheral membrane proteins bind to integral membrane proteins or the polar heads of phospholipids.

Fluidity of Membranes

• Membranes are semifluid.
• Lipids move laterally within the membrane leaflet, or halves of the bilayer.
• "Flip-flop" of lipids from one leaflet to the opposite leaflet rarely occurs.
• Factors affecting membrane fluidity include the length of fatty acid tails, the presence of double bonds (unsaturated), and the presence of sterols like cholesterol.
• Lipid rafts form when certain lipids in the membrane strongly associate with each other and float together as a unit.
• The composition of the lipid raft differs from the rest of the membrane.
• Integral membrane proteins can be bound to components of the cytoskeleton.
• Membrane proteins may attach to proteins outside the cell, forming the extracellular matrix.

Membrane Transport

• The plasma membrane is selectively permeable, allowing some ions and molecules to pass through but not others, ensuring essential molecules enter, metabolic intermediates remain, and waste products exit.
• Transmembrane proteins provide a passageway for ions and hydrophilic molecules to move across membranes.
• The two classes of transmembrane proteins that provide a passageway are channels and carrier proteins.

Transport Proteins

• Channels provide an open passageway for the diffusion of ions or molecules across the membrane.
• Most channels are gated.
• Carrier proteins transport a solute across the membrane through a conformational change.
• A uniporter transports a single molecule or ion.
• A symporter transports two or more ions/molecules in the same direction.
• An antiporter transports two or more ions/molecules in opposite directions.

Movement Across Membranes

• Passive transport requires no energy input.
• In diffusion, solutes move down a gradient directly across the lipid bilayer.
• Osmosis is the diffusion of water down its gradient directly across a lipid bilayer and is influenced by solute concentration.
• In facilitated diffusion, solutes diffuse down a gradient with the help of a transport protein.
• Active transport requires energy, typically ATP.
• ATP, or adenosine triphosphate, fuels active transport.
• Active transport involves a transport protein moving a solute against its gradient.
• Vesicular transport utilizes membrane sacs or vesicles through endocytosis and exocytosis.
• Diffusion involves movement from a high concentration to a low concentration area.
• The diffusion rate depends on size, polarity, charge, and concentration gradient.

Osmosis

• Osmosis is the diffusion of solvent, or water, across a semi-permeable membrane.
• In osmosis, water 'chases' solutes.
• Osmotic pressure is related to osmosis, with tonicity describing the relative solute concentrations of two solutions separated by a membrane.
• Isotonic solutions have equal water and solute concentrations on either side of the membrane.
• A hypertonic solution has a higher solute concentration and lower water concentration on one side of the membrane.
• A hypotonic solution has a lower solute concentration and higher water concentration on one side of the membrane.
• Freshwater protists, like Paramecium, survive in a hypotonic environment by using contractile vacuoles to prevent osmotic lysis.
• Contractile vacuoles take up water and discharge it outside the cell, maintaining constant cell volume.

Active Transport

• Active transport moves a solute against its concentration gradient, from low to high concentration.
• Active transport requires energy input, such as ATP.
• Primary transport directly uses energy to transport a solute.
• Secondary transport uses a pre-existing gradient to drive transport

Sodium-Potassium Pump

• The sodium-potassium pump actively transports sodium and potassium against their gradients using the energy derived from ATP hydrolysis.
• The pump exports 3 Na+ for every 2 K+ imported into the cell
• The sodium-potassium pump is an antiporter because ions move in opposite directions.
• The pump is an electrogenic pump because it exports/imports ions or charged atoms.

Vesicular Transport

• Exocytosis occurs when material inside the cell is packaged into vesicles and excreted.
• Endocytosis occurs when the plasma membrane invaginates to form a vesicle that brings substances into the cell.
• Pinocytosis is also known as cell drinking
• Phagocytosis is also known as cell eating.
• Receptor-mediated endocytosis is when specific molecules bind receptor proteins, and a coated pit forms.

Intercellular Channels

• Intercellular channels allow the direct movement of substances between adjacent cells.
• Gap junctions connect animal cells.
• Plasmodesmata connect plant cells.
• Gap junctions are abundant where cells need to communicate.
• Gap junctions allow ions and small molecules to pass.
• Plasmodesmata are similar in function to gap junctions but differ in structure.
• Plasmodesmata form channels in the cell walls of adjacent cells.

Cell Junctions

• Cell junctions attach cells.
• Anchoring junctions link cells to each other and to the extracellular matrix.
• Anchoring junctions are formed by cell adhesion molecules or CAMs.
• Cadherins and integrins are cell adhesion molecules that form anchoring junctions.
• Tight junctions prevent leakage of materials across animal cell layers.
• Cells still need anchoring junctions for strength when joined by tight junctions.
• Tight junctions form a tight seal between cells.