1.5: Cell Membrane

• The plasma membrane separates living cells from their environment and exhibits selective permeability.

Plasma Membrane Structure

• Phospholipids are a key part of the plasma membrane structure.
• Phospholipids are amphipathic, containing both hydrophilic and hydrophobic regions.
• Hydrophilic heads interact with water.
• Hydrophobic tails avoid water.

Fluid Mosaic Model

• The phospholipid bilayer has hydrophobic regions of protein within.
• The phospholipid bilayer also contains hydrophilic regions of protein.

Fluidity of Membranes

• Lipids and proteins drift laterally.
• Molecules rarely flip-flop transversely.
• Lateral movement occurs frequently (~107 times per second).
• Flip-flop movement occurs infrequently (~ once per month).

Membrane Fluidity

• Unsaturated hydrocarbon tails with kinks increase membrane fluidity.
• Saturated hydrocarbon tails decrease fluidity.

Cholesterol's Role

• At warm temperatures (37°C), cholesterol restricts phospholipid movement, reducing membrane fluidity.
• At cool temperatures, cholesterol maintains fluidity and prevents solidification by hindering tight packing of phospholipids.

Membrane Protein Types

• Peripheral proteins are bound to the surface of the membrane
• Integral proteins penetrate the hydrophobic core, called transmembrane proteins.
• Hydrophobic regions of integral proteins consist of nonpolar amino acids often coiled into alpha helices.

Functions of Membrane Proteins

• Proteins determine membrane's specific functions.
• Transport: proteins facilitate the movement of substances across the membrane.
• Enzymatic activity: proteins act as enzymes to catalyze reactions.*
• Signal transduction: proteins transmit signals from the outside to the inside of the cell.
• Cell-cell recognition: proteins enable cells to recognize each other.
• Intercellular joining: proteins mediate cell adhesion.
• Attachment to the cytoskeleton & extra cellular matrix (ECM): proteins anchor the membrane to the cytoskeleton and ECM.

Selective Permeability

• Membrane structure results in selective permeability.
• Cells exchange materials with their surroundings.
• Membranes are selectively permeable, regulating molecular traffic in and out of cells.
• Nonpolar/hydrophobic molecules (hydrocarbons, CO2, O2) can dissolve in the lipid bilayer and pass through the membrane rapidly.
• Polar/hydrophilic molecules (sugars, water) do not cross the membrane easily and require transport proteins.

Transport Proteins

• Transport proteins facilitate the passage of hydrophilic substances across the membrane.
• Channel proteins have a hydrophilic channel for certain molecules or ions to use as a tunnel.
• Carrier proteins bind to molecules and change shape to shuttle them across the membrane.
• Transport proteins are specific for the substance they move.
• Aquaporins are for the passage of water.
• A glucose transporter is specific for glucose.

Passive Transport

• Diffusion of a substance across a membrane requires no energy investment.
• Substances move from an area of high concentration to low concentration.
• Substances diffuse down their concentration gradient.

Osmosis

• Water diffuses across a membrane from an area of low solute concentration to high solute concentration.

Tonicity and Water Balance Of Cells

• Tonicity determines a cell's ability to gain or lose water.
• In an isotonic solution the solute concentration is the same as that inside the cell.
• In a hypertonic solution, the concentration is greater than that inside the cell.
• In a hypotonic solution, the concentration is less than that inside the cell.

Cell Behavior in Different Solutions

• In a hypotonic solution, an animal cell will lyse.
• In an isotonic solution, an animal cell will be normal.
• In a hypertonic solution, an animal cell will shrivel.
• In a hypotonic solution, a plant cell will be turgid (normal).
• In an isotonic solution, a plant cell will be flaccid.
• In a hypertonic solution, a plant cell will become plasmolyzed.

Osmoregulation

• Osmoregulation controls water balance.
• A contractile vacuole system controls water balance, for example in Paramecium.
• Contractile vacuoles fill with fluid from radiating canals throughout the cytoplasm.
• When full, the vacuole and canals contract, expelling fluid from the cell.

Facilitated Diffusion

• Transport proteins speed up the passive movement of molecules.

Channel Proteins

• Channel proteins provide corridors that allow specific molecules or ions to cross, such as aquaporins for water and ion channels that open or close in response to stimuli (gated channels).

Carrier Proteins

• Carrier proteins undergo a subtle change in shape that translocates the solute-binding site across the membrane.

Active Transport

• Active transport moves substances against their concentration gradient.
• It requires energy, typically in the form of ATP.
• It is performed by proteins embedded in the membranes.
• One type of active transport system is the sodium-potassium pump.

How Ion Pumps Maintain Membrane Potential

• Voltage is created by differences in the distribution of positive and negative ions.
• Membrane potential is the voltage difference across a membrane.
• The electrochemical gradient drives the diffusion of ions across a membrane.
• Chemical force (the ion's concentration gradient).
• Electrical force (the effect of the membrane potential or voltage difference) influences the ion's movement.

Electrogenic Pumps

• Electrogenic pumps are transport proteins that generates voltage across a membrane, for example:
• sodium-potassium pump (animal cells).
• proton pump (plants, fungi, bacteria).

Cotransport

• Cotransport occurs when active transport of a solute indirectly drives transport of another solute.
• Plant use the gradient of H+ ions generated by proton pumps to drive active transport of nutrients (sugar) into the cell.

Bulk Transport

• Bulk transport involves exocytosis and endocytosis processes.
• Small molecules and water enter or leave the cell across the lipid bilayer or by transport proteins.
• Large molecules (e.g., polysaccharides and proteins) cross the membrane in bulk via vesicles, requiring energy.

Exocytosis

• Transport vesicles migrate to the membrane, fuse with it, and release their contents.
• Many secretory cells, use exocytosis such as pancreatic cells exporting their products.

Endocytosis

• Cells take in macromolecules by forming vesicles at the plasma membrane.
• Three types of Endocytosis:
• Phagocytosis is cellular eating.
• Pinocytosis is cellular drinking.
• Receptor-mediated endocytosis.

Phagocytosis

• A cell engulfs a particle in a vacuole.
• The vacuole fuses with a lysosome to digest the particle.

Pinocytosis

• Molecules are taken up when extracellular fluid is "gulped" into tiny vesicles.

Receptor-Mediated Endocytosis

• Binding of ligands to receptors triggers vesicle formation.
• A ligand is a molecule that binds specifically to a receptor site of another molecule.