Plasma Membrane: Phospholipids and Proteins

• Plasma membrane functions include defining the outer borders of cells and organelles, managing what enters and exits the cell, receiving external signals, initiating cellular responses, and adhering to neighboring cells.

Phospholipids

• The membrane consists of amphiphilic lipid molecules known as phospholipids.
• Phospholipids contain two nonpolar fatty acid chains, a glycerol molecule, and a polar phosphate group.
• Fatty acids can be saturated, containing carbons saturated with hydrogen and all single C-C bonds, or unsaturated, containing at least one double C=C bond.

Proteins

• Proteins constitute the second major component of membranes.
• Proteins function as transporters, receptors, enzymes, or in binding and adhesion.
• Integral proteins are integrated completely into the bilayer.
• Peripheral proteins appear only on the surfaces of the membrane.

Facilitated Passive Transport

• Facilitated transport, also known as facilitated diffusion, moves substances down their concentration gradients through transmembrane, integral membrane proteins.
• Ions and small polar molecules diffuse the way of facilitated passive transport.
• Channel proteins and carrier proteins are two types of facilitated transport proteins.

Passive Transport

• Diffusion is the simplest type of passive transport
• A substance moves from an area of high concentration to an area of low concentration.
• Across membranes, diffusion occurs through the lipid bilayer.
• Net movement stops once equilibrium is achieved.
• Small nonpolar molecules like O2 and CO2, as well as biological molecules are able to move using passive transport.

Channel Proteins

• Channel proteins have a core composed of hydrophilic amino acids that attract ions/polar molecules.
• Some channel proteins are open all the time.
• Others are gated, opening only when a signal is received.
• Aquaporins are channel proteins specific to H₂O.
• Muscle cells have gated ion channels that allow for muscle contraction when opened.

Osmosis

• Osmosis is the diffusion of water across a membrane.
• Water moves from higher to lower water concentration areas.
• Differences in water concentration are present when a solute cannot pass through the selectively permeable membrane.

Tonicity

• Tonicity is how an extracellular solution changes the volume of a cell via osmosis.
• Tonicity is often associated to osmolarity of a solution.
• Osmolarity is the solution's total solute concentration, which includes permeable and non-permeable solutes.
• When solutions with different osmolarities are separated by a membrane permeable to water but not the solute, water moves from the solution with the lower osmolarity through the membrane.
• Hypertonic, isotonic, and hypotonic describe the osmolarity of the cell relative to that of its extracellular fluid.
• A hypertonic extracellular fluid has lower osmolarity than the cytosol - water leaves the cell.
• An isotonic extracellular fluid has the same osmolarity than the cytosol – water does not move.
• A hypertonic extracellular fluid has a higher osmolarity than the cytosol - water enters the cell.
• Animal cells work best when extracellular fluids are isotonic.

Active Transport

• Active transport is used when an ion or molecule e.g. glucose) is transported through a membrane protein:
  • Against its concentration gradient, from low to high concentration.
  • Against electromagnetic gradient, for example, H+ ions to a more positive solution.
• Energy is always required for active transport.
• There are two kinds of active transport:
  • Primary - where ATP provides the energy.
  • Secondary - where an electrochemical gradient provides the energy.

Endocytosis

• Phagocytosis, or cellular eating, involves the cell membrane engulfing a particle.
• Pinocytosis, or cellular drinking, involves the cell membrane invaginating, surrounding a small volume of fluid, and pinching off.
• Receptor-mediated endocytosis targets the uptake of a specific substance via binding to receptors on the external membrane surface.

Explore the structure and function of the plasma membrane. The plasma membrane is composed of phospholipids and proteins. Integral proteins are integrated completely into the bilayer, whereas peripheral proteins appear only on the surfaces of the membrane.