Biological Membranes: Lipid-Protein Bilayers

Biological Membranes: Lipid-Protein Bilayers

• Biological membranes' general structure corresponds to the fluid mosaic model
• A phospholipid bilayer acts like a lake where proteins float

Key Component: Phospholipids

• Phospholipids feature polar, hydrophilic heads facing outward and hydrophobic fatty acid tails facing inward
• Phospholipids are diverse with varying fatty acid chain lengths, degrees of unsaturation, and phosphate groups

Membrane Interior: Fluidity

• The membrane interior has some fluidity allowing for lateral movement of molecules within the membrane
• Molecules rarely flip from one side of the membrane to the other
• Inner and outer sides of the bilayer may differ significantly

Fluidity Factors: Lipids and Temperature

• Lipid composition and temperature determine membrane fluidity
• Cholesterol and saturated fatty acids tightly pack together, making a less-fluid membrane
• Unsaturated long-chain fatty acids feature kinks and looser packing increasing membrane fluidity
• Membrane fluidity decreases with temperature drops

Adapting to Cold: Lipid Content Changes

• Some organisms adapt to cold by changing the lipid content of cell membranes
• They replace saturated fatty acids with unsaturated ones and use fatty acids with shorter tails

Protein Content

• The number of proteins in a membrane varies depending on membrane function
• Integral, peripheral and anchored represent the diverse types of membrane proteins

Integral Membrane Proteins

• Integral membrane proteins are at least partially embedded in the bilayer
• Hydrophilic domains of integral proteins extend into the cell's interior or exterior
• Hydrophobic domains interact with fatty acids in the membrane's interior
• Some integral proteins span the lipid bilayer completely, while others are only partially embedded

Peripheral Membrane Proteins

• Peripheral membrane proteins do not have hydrophobic regions and do not penetrate the bilayer

Anchored Membrane Proteins

• Anchored membrane proteins covalently attach to fatty acids or other lipids

Transmembrane Proteins

• Transmembrane proteins extend through the phospholipid bilayer, possessing one or more transmembrane domains
• Inner and outer domains can perform assorted functions

Peripheral Membrane Proteins Location

• Peripheral membrane proteins exist on one side of the membrane

Protein mobility

• Some membrane proteins freely move within the bilayer
• Experimentally fused cells show uniform distribution of proteins from each cell
• Other membrane proteins anchor to specific regions

Membrane Dynamics

• Membranes are dynamic, constantly forming, transforming, fusing, and breaking down
• The endomembrane system exemplifies this dynamism
• Subcellular membranes vary chemically and change when forming parts of organelles

Membrane Carbohydrates

• Membranes feature carbohydrates on the outer surface serving as recognition sites
• Glycolipids: carbohydrate + lipid
• Glycoproteins: carbohydrate (oligosaccharide) + protein
• Proteo-glycans: higher percentage of carbohydrates

Cellular Arrangement by Recognition and Adhesion

• Cells arrange into tissues through cell recognition and adhesion
• Surface proteins and carbohydrates facilitate this process
• Sponge cells easily separate and reassemble offering a model to study these processes

Cell Adhesion

• Cell adhesion results from carbohydrate, protein, or carbohydrate and protein interactions
• Homotypic adhesion: the same molecule extends from both cells and binds
• Heterotypic binding: the binding of different proteins between cells

Cell Junctions

• Cell junctions are specialized structures binding cells together
• Tight junctions ensure directional movement of materials
• Desmosomes act like spot welds
• Gap junctions enable communication between cells

Cell Adhesion to Extracellular matrix

• Cell membranes adhere to the extracellular matrix
• Integrin, a transmembrane protein facilitates this adhesion
• Integrin binds the matrix outside epithelial cells and actin filaments inside the cells
• Integrin binding is noncovalent and reversible

Integrin Role in Cell Movement

• Cells move within tissues via integrin binding and reattaching to the extracellular matrix
• Cell movement is important in developing embryos and cancer spread

Selective Permeability

• Cell membranes exhibit selective permeability meaning some substances pass through while others do not

Transport Types

• Passive transport requires no energy input as it is diffusion
• Active transport requires energy

Energy for Passive Transport

• The concentration gradient powers passive transport 

Diffusion

• Diffusion: Random movement towards equilibrium

Diffusion - Net Movement

• Net movement occurs until equilibrium is reached

Diffusion Movement

• Diffusion is the net movement from high to low concentration regions

Diffusion Rate

• Diffusion rate depends on:
  • Size and mass of the molecules or ions
  • Temperature of the solution
  • Density of the solution
  • Concentration gradient
  • Area and distance

Surface area

• A larger surface area allows more rapid diffusion.

Distance

• Diffusion occurs effectively over short distances like within a cell

Membrane: Permeability

• Permeable membranes allow solutes to cross easily
• Impermeable membranes restrict solute passage

Permeable Membrane

• Molecules move across a permeable membrane until the concentration is equal on each side

No Net Concentration

• Diffusion continues, but no net change in concentrations occur when equally distributed

Simple Diffusion

• Small molecules pass through the lipid bilayer
• Lipid-soluble molecules freely diffuse across the membrane
• Electrically charged and polar molecules cannot easily pass through

Permeability of Phospholipid Bilayers

• Small uncharged molecules can pass through the hydrophobic lipid bilayer

Osmosis

• Osmosis: Diffusion of water depending on water molecule concentrations
  • Isotonic: equal solute concentrations
  • Hypertonic: higher solute concentration
  • Hypotonic: lower solute concentration

Water Molecule Movement

• Water moves from high (hypotonic) to low (hypertonic) water concentration regions when separated by a water-permeable, solute-impermeable membrane

Animal and Plant Cells in Solutions

• Animal cells may burst in hypotonic solutions or shrink in hypertonic solutions
• Plant cells with rigid walls build turgor pressure

Facilitated Diffusion

• Facilitated diffusion: Polar and charged molecules diffuse along concentration gradients with protein assistance (channels or carriers)

Channel Proteins

• Channel proteins: Integral membrane proteins forming tunnels

Carrier proteins

• Carrier proteins: Membrane proteins binding substances and speeding diffusion

Ion channels

• Ion channels: Channel proteins with hydrophilic pores
• Most are gated and either close or open allowing ion passage
• Gates open upon protein shape change due to chemical signals (ligand) or voltage differences (voltage-gated)

Specialized Water Channels

• Water crosses membranes through aquaporins excluding ions

Carrier Proteins: Polar Molecule Transport

• Carrier proteins transport polar molecules like glucose across membranes in both directions
• Glucose binds the transporter and changes its shape releasing the glucose

Diffusion Rate

• Diffusion depends on the concentration gradient and carrier proteins
• The diffusion saturates if all carriers are loaded

Energy Needs

• Higher energy needs corresponds to an increased number of glucose transporters

Active Transport

• Active transport: Moves substances against concentration and/or electrical gradients; requires energy
• Typically, adenosine triphosphate (ATP) provides energy

Active Transport Types

• Three protein types drive active transport:
• Uniporter: moves one substance
• Symporter: moves two substances in one direction
• Antiporter: moves two substances in opposite directions

Active Transport Mechanism

• Primary active transport: Requires ATP's direct hydrolysis
• Secondary active transport: Uses energy from an ion gradient formed by primary active transport

Sodium-Potassium Pump Overview

• Sodium-potassium (Na+-K+) pump: An integral antiporter glycoprotein
• Transports two K+ ions into the cell and three Na+ ions out

Secondary Active Transport

• Na+-K+ pump establishes a Na+ concentration gradient where passive Na+ diffusion back into the cell powers glucose transport into the cell against its gradient.

Macromolecule Transport

• Macromolecules cannot cross the membrane needing membrane vesicles to enter or exit cells

Endocytosis

• Endocytosis: Brings molecules/cells into a eukaryotic cell
• The cell membrane invaginates around the material, forming a vesicle

Forms of Endocytosis

• Phagocytosis: engulfs molecules/cells
  • Some protists feed and white blood cells engulf foreign substances using this process
  • A food vacuole/phagosome fuses with a lysosome for digestion

Pinocytosis

• Pinocytosis: A vesicle brings small dissolved substances into a cell

Pinocytosis Vesicles

• Pinocytosis involves smaller vesicles than phagocytosis
• Pinocytosis constant in endothelial (capillary) cells

Receptor-Mediated Endocytosis

• Receptor mediated endocytosis: highly specific
• Macromolecules bind to receptor proteins, integral membrane proteins, at specific sites on the cell membrane.
• Sites are coated with proteins, like clathrin, on the inside

Mammalian Cholesterol

• Mammalian cells uptake cholesterol via receptor-mediated endocytosis
• The liver packages cholesterol into low-density lipoprotein, or LDL, and secretes it to the bloodstream
• LDL receptors sit in clathrin-coated pits

Exocytosis

• Exocytosis: Secretes materials (e.g., digestive enzymes, neurotransmitters) from a cell
• The vesicle membrane fuses with the cell membrane
• A pore forms without membrane fusion, releasing the vesicle’s contents