Plasma Membrane Structure and Function Notes

Functions of Plasma Membrane

• Plasma membrane defines the boundary of all cells.
• Membranes separate organelles from the cytoplasm and the cell membrane.
• Cell and organellar membranes manage what enters and exits the area bounded by the membrane.
• Membranes can receive external signals and respond.
• Plasma membranes separate extracellular from intracellular environments, and signals must be transduced through the membrane.
• Proteins perform specific jobs when associated with or passing through a membrane.

Representation of Plasma Membrane

• The plasma membrane includes lipid-anchored proteins, integral membrane proteins, and peripheral membrane proteins.
• It consists of a phospholipid bilayer with hydrophilic head groups and hydrophobic fatty acyl side chains.
• The structure includes polar head groups and a hydrophobic interior.
• Key components: Hydrogen, Carbon, Oxygen, Nitrogen, and Phosphorus.

Fluid Mosaic Model of the Plasma Membrane

• The plasma membrane is a combination of lipids (phospholipids, cholesterol), proteins, and carbohydrates.
• It has a fluid character.
• Proposed in 1972 by S.J. Singer and G.L. Nicolson.
• Gly = sugar

Cell Membrane Lipids

• Cell membrane lipids have an amphipathic nature.

Phospholipids

• Phospholipids are the main lipid components and are amphiphilic.
  • Composed of:
    • 2 fatty acid chains (nonpolar)
    • a glycerol molecule
    • a phosphate group (polar)
• Fatty acids can be saturated or unsaturated.
  • Saturated: Carbons are saturated with H; all single C-C bonds.
  • Unsaturated: At least one double C=C bond occurs.

Phospholipid Bilayer

• Phospholipids arrange themselves in a bilayer.
  • Polar heads face outward (water interactions).
  • Hydrophobic tails face inward, interacting with each other (non-polar to non-polar).

Hydrophilic Interactions

• Example: Acetone in Water

Hydrophobic Interactions

• Example: 2-methylpropane in Water

Phospholipid Bilayers

• Phospholipid bilayers are strong and semi-permeable.
• Permeability:
  • Impermeable to ions (e.g., Na^+
  • Impermeable to large molecules (e.g., Sucrose)
  • Permeable to small nonpolar molecules (e.g., N_2
  • Permeable to small polar molecules (e.g., H_2O, Ethanol)

Membrane Phospholipid Behavior

• Membrane phospholipids exhibit various behaviors within the lipid bilayer.

Membrane Assembly in the ER

• Newly synthesized phospholipids are added to the cytosolic side of the ER membrane.
• They are then redistributed by transporters from one half of the lipid bilayer to the other.

Phospholipid Distribution

• Some phospholipids are confined to one side of the membrane.
• Flippases help establish and maintain the asymmetric distribution of phospholipids in animal cell membranes.

Membrane Orientation

• Membranes retain their orientation during transfer between cell compartments.
• Pay attention to which side of the membrane the sugar is on.

Asymmetric Distribution

• Phospholipids and glycolipids are distributed asymmetrically in the lipid bilayer of an animal cell plasma membrane.

Proteins in Plasma Membrane

• Proteins are a major component of membranes.
• Functions:
  • Transporters
  • Receptors
  • Enzymes
  • Binding and adhesion
• Types:
  • Integral proteins: Span across the bilayer.
  • Peripheral proteins: Occur only on the surface or partially embedded on one side (extracellular or cytosolic).

Integral Proteins

• Integral membrane proteins have hydrophobic and hydrophilic regions.
• The locations and number of regions determine their arrangement within the bilayer.
• Polypeptide chains can cross the lipid bilayer as an α helix, β sheet, or both.

Amino Acids

• Amino acids are categorized by their R groups: polar/uncharged, nonpolar/aliphatic, negatively charged, positively charged, and nonpolar/aromatic.
• Examples of amino acids and their structures are provided.

Plasma Membrane Proteins Functions

• Plasma membrane proteins perform a variety of functions.

Plasma Membrane Proteins Examples

• Examples of plasma membrane proteins and their specific functions:
  • Transporters: Na^+ pump (actively pumps Na^+ out of cells and K^+ in).
  • Ion channels: K^+ leak channel (allows K^+ ions to leave cells, influencing cell excitability).
  • Anchors: Integrins (link intracellular actin filaments to extracellular matrix proteins).
  • Receptors: Platelet-derived growth factor (PDGF) receptor (binds extracellular PDGF, generating intracellular signals).
  • Enzymes: Adenylyl cyclase (catalyzes the production of cyclic AMP in response to extracellular signals).

Membrane Proteins Association

• Membrane proteins associate with the lipid bilayer in different ways.

Polypeptide Chain

• A polypeptide chain usually crosses the lipid bilayer as an α helix.
• Review alpha helix Hydrogen bonding

Transmembrane Hydrophilic Pore

• A transmembrane hydrophilic pore can be formed by multiple amphipathic α helices.

Beta Sheet

• A 16-stranded β sheet can curve around itself to form a transmembrane water-filled channel (Porin protein).
• Porin proteins form water-filled channels in the outer membrane of a bacterium.

Plasma Membrane Reinforcement

• The plasma membrane is reinforced by the underlying cell cortex.
• In animal cells, the cortex is a meshwork of filamentous proteins, largely made of spectrin in red blood cells.
• A cell can restrict the movement of its membrane proteins.

Carbohydrates

• Oligosaccharide carbohydrates are the third major component.
• Located on the exterior surface of the plasma membrane, bound to proteins (glycoproteins) or lipids (glycolipids).
• Function in cell-cell recognition and attachment.

Membrane Fluidity

• The membrane needs to be flexible but not too fluid to maintain its structure.
• Fluidity is affected by:
  • Phospholipid type: Saturated fatty acids pack more closely, making the membrane more rigid.
  • Temperature: Cold temperatures compress molecules, making membranes more rigid.
  • Cholesterol: Acts as a fluidity buffer, keeping membranes fluid when cold and preventing it from becoming too fluid when hot.

Plasma Membranes Asymmetry

• Plasma membranes are asymmetric; the inner surface differs from the outer surface.
• Examples:
  • Interior proteins anchor fibers of the cytoskeleton to the membrane.
  • Some exterior proteins bind to the extracellular matrix, interact with signaling proteins, or assist with importing materials into the cell.

Cell-Surface Carbohydrates

• Recognition of cell-surface carbohydrates on neutrophils allows the immune cells to migrate out of the blood into infected tissues.

Transport

• The plasma membrane is selectively permeable, allowing some molecules to pass through but not others.
• Cytosol solutions differ from extracellular fluids, maintaining an imbalance of sodium and potassium ions.
• Transport can be passive (no energy required) or active (energy/ATP required).

Passive Transport

• The simplest type of passive transport is diffusion.
• Diffusion occurs when a substance moves from an area of high concentration to an area of lower concentration down its concentration gradient.
• In membranes, this occurs across the lipid bilayer.
• Net movement ceases once equilibrium is achieved.
• Only small nonpolar molecules (e.g., O2, CO2, lipid hormones) can diffuse through biological membranes.

Factors Affecting Diffusion Rates

• Concentration gradients: Greater difference, faster diffusion.
• Mass of the molecules: Smaller molecules diffuse more quickly.
• Temperature: Molecules move faster at higher temperatures.
• Solvent density: Dehydration increases density of cytoplasm, reducing diffusion rates.
• Solubility: More nonpolar (lipid-soluble) materials diffuse faster.
• Surface area: Increased surface area speeds up diffusion rates.
• Distance traveled: Greater distance, slower rates; affects the upper limit of cell size.
• Pressure: In some cells, blood pressure forces solutions through membranes, speeding up diffusion rates.

Osmosis

• Osmosis is the diffusion of water across a membrane.
• Water moves from higher to lower water concentration.
• Differences in water concentration occur when a solute cannot pass through the selectively permeable membrane.
• Many chemical structures interact with water, reducing free water available.

Tonicity

• Tonicity describes how an extracellular solution can change the volume of a cell by affecting osmosis.
• Tonicity is often correlated to the osmolarity of a solution.
• Osmolarity describes the total solute concentration of a solution (permeable and non-permeable solutes).
• Water moves from the solution with lower osmolarity through the membrane when solutions with different osmolarities are separated by a membrane permeable to water but not the solute.
• Hypertonic, isotonic, and hypotonic describe the osmolarity of the cell relative to its extracellular fluid.

Tonicity Details

• Hypertonic extracellular fluid: Lower osmolarity than the cytosol; water leaves the cell.
• Isotonic extracellular fluid: Same osmolarity as the cytosol; net water movement is zero.
• Hypotonic extracellular fluid: Higher osmolarity than the cytosol; water enters the cell.
• Animal cells function best when extracellular fluids are isotonic.

Osmoregulation

• Organisms with cell walls (plants, fungi, bacteria, protists) prefer hypotonic extracellular solutions.
• Turgor pressure (pressure exerted by the plasma membrane against the cell wall) is critical to organismal growth and functions.
• Hypertonic solutions cause plasmolysis – plasma membrane (PM) detaches from the cell wall (CW).

Turgor Pressure

• Turgor: water pressure inside of plant cells.
• Without adequate water, plants lose turgor pressure and wilt; turgor pressure is restored by forcing water into the plant cells.

Osmoregulation by Other Organisms

• Freshwater protists (e.g., paramecia, amoebas) use contractile vacuoles to pump water out of their cells to prevent bursting.
• Marine invertebrates have internal salt concentrations matching their environment.
• Fishes excrete diluted urine to get rid of excess H_2O or salts.
• Osmoreceptors of brain cells monitor solute concentrations in our blood, releasing hormones that affect kidney function.