Cellular Membranes and Transport

Plasma Membrane

• Some small molecules cross the cell membrane through passive transport, which requires no energy input, but may need transport proteins.
• Small molecules use active transport, which requires both energy and a transport protein.
• Large molecules move in and out via bulk transport, using exocytosis or endocytosis.

Cellular Membranes

• Lipids and proteins are the main membrane components, but carbohydrates also contribute.
• Membranes consist mainly of phospholipids.
• Phospholipids are amphipathic molecules, containing hydrophobic ("water-fearing") and hydrophilic ("water-loving") regions.
• Phospholipids form a bilayer with hydrophobic tails inside and hydrophilic heads exposed to water on either side.
• Most membrane proteins are also amphipathic.
• Protein hydrophilic regions orient toward the cytosol and extracellular fluid.
• Protein hydrophobic regions are embedded in the bilayer.

Fluid Mosaic Model

• The membrane structure follows the fluid mosaic model with a mosaic of protein molecules bobbing in a fluid bilayer of phospholipids.
• Proteins are not randomly distributed, but form groups that carry out common functions.
• Membranes are held together by weak hydrophobic interactions.
• Lipids and some proteins can move sideways within the membrane.
• Lipids rarely flip-flop across the membrane from one phospholipid layer to the other.

Membrane Fluidity

• As temperatures cool, membranes transition from a fluid to a solid state.
• The temperature at which solidifying occurs depends on the types of lipids.
• Membranes rich in unsaturated fatty acids are more fluid than those rich in saturated fatty acids.
• Membranes must be fluid to function properly, affecting both permeability and transport protein movement.
• Membranes that are too fluid cannot support protein function.
• Organisms in extreme temperatures have adaptive differences in membrane lipid composition.

Cholesterol's Role

• Cholesterol in animal cell membranes has variable effects on membrane fluidity at different temperatures.
• At relatively high temperatures (like 37°C), cholesterol restrains the movement of phospholipids.
• At low temperatures, cholesterol maintains fluidity by preventing tight packing.
• Plants use different but related steroid lipids to buffer membrane fluidity.

Membrane Proteins

• Phospholipids form the main membrane fabric, but proteins determine most functions.
• The protein composition of membranes varies among organisms and intracellular membranes within a cell.
• Two major types of membrane proteins exists; peripheral and integral proteins
• Peripheral proteins are bound to the membrane surface.
• Integral proteins penetrate the hydrophobic core.
• Transmembrane proteins are integral proteins that span the membrane, with hydrophobic regions of nonpolar amino acids often coiled into a helices.

Membrane Protein Functions

• Cell-surface membrane proteins carry can perform, transport, enzymatic activity, signal transduction, cell-cell recognition, intercellular joining, and attachment to the cytoskeleton and extracellular matrix (ECM)..

Membrane Permeability

• The plasma membrane controls the exchange of materials between a cell and its surroundings.
• Membranes exhibit selective permeability; some substances cross more easily than others.
• The fluid mosaic model explains how membranes regulate molecular traffic.
• Hydrophobic (nonpolar) molecules dissolve in the lipid bilayer and rapidly pass through the membrane.
• The hydrophobic interior of the membrane impedes the passage of hydrophilic (polar) molecules.

Transport Proteins

• Hydrophilic substances cross membranes more quickly through transport proteins.
• Channel proteins have a hydrophilic channel used as a tunnel for certain molecules or ions..
• Carrier proteins bind to molecules, changing shape to shuttle them across the membrane.
• Aquaporins are channel proteins that greatly increase the rate of water molecule passage.

Passive Transport

• Diffusion is the movement of particles to spread out evenly into the available space.
• Though each molecule moves randomly, the diffusion of a population of molecules is directional.
• At dynamic equilibrium, the same amount of molecules cross the membrane in both directions.
• Substances diffuse down their concentration gradient, where the density of a chemical substance decreases.
• Each substance moves unaffected by the concentrations of other substances.
• The diffusion across a biological membrane is passive transport because no energy is used.
• The concentration gradient contains potential energy that fuels diffusion.

Osmosis

• Osmosis is the diffusion of free water across a selectively permeable membrane.
• Water molecules diffuse from regions of lower solute concentration to regions of higher solute concentration.
• Water continues to move until the solute concentration is equal on both sides.
• Tonicity is the ability of a surrounding solution to cause a cell to gain or lose water, dependent on the concentration of solutes that cannot cross the membrane, compared to that inside the cell.
• If the surrounding solution has a higher concentration of these solutes, water will tend to leave the cell and vice versa

Facilitated Diffusion

• In facilitated diffusion, transport proteins accelerate passive movement of molecules across the plasma membrane.
• Transport proteins include channel and carrier proteins.
• Channel proteins provide corridors for specific molecules or ions to cross the membrane.
• Aquaporins facilitate water diffusion, while ion channels transport ions.
• Some ion channels, called gated channels, open or close in response to a stimulus.
• Carrier proteins undergo a shape change that moves the solute-binding site across the membrane.
• This shape change can be triggered by the transported molecule binding and releasing.
• Carrier proteins in facilitated diffusion move substances down their concentration gradients: no energy input required.

Active Transport

• Facilitated diffusion is passive, moving solutes down their concentration gradient and requiring no energy
• Some transport proteins use energy to move solutes against their concentration gradients
• Active transport needs energy (often from ATP hydrolysis) to move substances
• All proteins enabling active transport are carrier proteinsts.
• Active transport allows cells to maintain solute concentrations that differ from the environment.

Membrane Potential

• Membrane potential is the voltage across a membrane.
• Voltage comes from differences in distribution of +/- ions across a membrane
• The inside of the cell is relatively negative in charge, favoring passive transport of cations into the cell
• Two combined forces, collectively called the electrochemical gradient, drive diffusion of ions across a membrane, including: a chemical and electrical force
• An ion diffuses down its electrochemical gradient.
• An electrogenic pump generates voltage across a membrane, storing energy for cellular work.

Cotransport

• Cotransport is when active transport of solute drives transport of other substances.
• The “downhill” diffusion is coupled with the “uphill” against is gradient.

Endocytosis

• In endocytosis, macromolecules are transported into the cell in vesicles.
• The membrane forms a pocket that deepens, pinching off to form a vesicle around the material
• Types of endocytosis: phagocytosis ("cellular eating"), pinocytosis ("cellular drinking"), and receptor-mediated endocytosis.

Exocytosis

• In exocytosis, transport vesicles move, fuse with the membrane, releasing the contents.
• Many secretory cells export products through exocytosis.
• Pancreatic cells secrete insulin via exocytosis.