Biological Membranes and Transport

Flashcards about Biological Membranes and Transport

Lipid Bilayer

The basic structural element of membranes.

Micelles, Bilayers, Liposomes

Structures formed when lipids aggregate in water, depending on the type of lipid and concentration.

Micelle

Forms in the solution of amphipathic molecules with a larger, more polar head than tail.

Membrane Bilayer

Composed of two leaflets of lipid monolayers and forms when lipids with polar head groups and more than one lipid tail are in aqueous solution.

Vesicle (Liposome)

Small bilayers that spontaneously seal into spherical vesicles in a concentration-dependent manner.

Membranes

Complex lipid-based structures that form pliable sheets and are composed of a variety of lipids and proteins.

Functions of Membranes

Defines the boundaries of the cell, allows import and export, retains metabolites and ions, senses external signals, provides compartmentalization, produces nerve signals, stores energy, and supports ATP synthesis.

Common Features of Membranes

Sheet-like flexible structure, composed of two leaflets of lipids, formed spontaneously in aqueous solution, asymmetric and fluid.

Fluid Mosaic Model of Membranes

Proposed in 1972, lipids form a viscous, two-dimensional solvent into which proteins are inserted and integrated.

Integral Proteins

Firmly associated with the membrane, often spanning the bilayer.

Peripheral Proteins

Weakly associated with the membrane and can be removed easily.

Lipid Composition of Membranes

Varies by organisms, tissues, and organelles.

Membrane Bilayers Are Asymmetric

Two leaflets have different lipid compositions, and the outer leaflet is often more positively charged.

Archaea Unique Membrane Constituents

Unique glycerol chirality, unique fatty acids, unique linkages, and membrane topology.

Functions of Proteins in Membranes

Detecting signals from outside, channels, gates, pumps, and enzymes.

Peripheral Membrane Proteins

Can be dissociated from the membrane fairly easily during changes in ionic strength or pH.

Amphitrophic Membrane Proteins

Can be conditionally attached to the membrane by covalent interaction with lipids or carbohydrates attached to lipids.

Lipid-linked Membrane Proteins

Contain a covalently linked lipid molecule.

Farnesylation of Proteins

Proteins can be targeted to the inner leaflet of the plasma membrane.

Integral Membrane Proteins

Span the entire membrane and are tightly associated with the membrane.

Amino Acids in Membrane Proteins

Transmembrane segments are predominantly hydrophobic.

Physical Properties of Membranes

Dynamic and flexible structures, exist in various phases, not permeable to large polar solutes and ions, and permeable to small polar solutes and nonpolar compounds.

Membrane Phases

Depending on their composition and temperature, the lipid bilayer can be in gel or fluid phase.

More Fluid Membranes

Requires shorter and more unsaturated fatty acids.

Sterols and Hopanols

Increase membrane rigidity and permeability.

Membrane Dynamics: Lateral Diffusion

Individual lipids undergo fast lateral diffusion within the leaflet.

Membrane Dynamics: Transverse Diffusion

Spontaneous flips from one leaflet to another are rare.

Membrane Diffusion: Flippases

Catalyze transverse diffusion.

Study of Membrane Dynamics: FRAP

Allows us to monitor lateral lipid diffusion by monitoring the rate of fluorescence return.

Membrane Rafts

Contain clusters of glycosphingolipids with longer-than-usual tails and allow segregation of proteins in the membrane.

Caveolin

Forces membrane curvature.

T-SNARE Proteins

Assembly on the target membrane.

V-SNARE Proteins

Assembly on the Vesicle membrane

Q-SNARE proteins

Regulatory proteins that are Ca2+ induced

Transport Across Membranes

Cell membranes are permeable to small nonpolar molecules that passively diffuse through the membrane.

Concentration Dependence

Solute moves toward equilibrium across the membrane

Electrochemical Dependence

Solute moves toward charge equilibrium across the membrane

Polar Solutes

Need alternative paths to Cross Cell Membranes.

Simple Diffusion

Nonpolar compounds, concentration only, down gradient.

Facilitated Diffusion

Down electrochemical gradient.

Primary Active Transport

Against electrochemical gradient, driven by ATP.

Ionophore-mediated ion transport

Down electrochemical gradient.

Ion channel

Down electrochemical gradient; may be gated by a ligand or ion

Secondary Active Transport

Against electrochemical gradient, driven by ion moving down its gradient

Uniport

Moves a single Molecule

Symport

Moves 2 molecules in the same direction

Antiport

Moves 2 molecules in the opposite direction

Proton Transport and Chemical Energy of ATP

Energy of ATP hydrolysis can be used to drive protons through the membrane.