Cell Bio Chapter 4 The Structure and Function of the Plasma Membrane

Compartmentalization

Separates cellular activities for regulation.

Scaffold for biochemical activities

Framework for effective molecular interactions.

Selectively permeable barrier

Controls movement of substances in and out.

Transporting solutes

Mechanisms for moving substances across membranes (against concentration and ion gradient)

Signal transduction

Process of responding to external signals.

Receptor

Molecule that recognizes and responds to stimuli.

Ligand

Type of stimulus recognized by receptors.

Intercellular interaction

Cell membranes facilitate communication between cells.

Energy transduction

Conversion of energy to chemical forms (ATP).

Fluid-mosaic model

Describes membrane structure as dynamic and varied.

Amphipathic lipids

Molecules with hydrophilic and hydrophobic regions.

Structural backbone of Membranes

Lipid bilayer

Part of the membrane that specifies determination

Proteins

3 main types of membrane lipids

phosphoglycerides, sphingolipids, cholesterol

Phosphoglycerides

Diacylglycerides with phosphate-linked head groups.

Sphingolipids

Ceramides formed from sphingosine and fatty acids.

Cholesterol

A smaller and less amphipathic membrane lipid found only in animals.

Membrane lipids can differ in:

Types of lipids, nature of the head groups, and the particular species of fatty acyl chains

Hydrophilic region

Water-attracting part of an amphipathic lipid.

Hydrophobic region

Water-repelling part of an amphipathic lipid.

Cell Fusion

A technique whereby two different types of cells, or cells from two different species, can be fused to produce one cell with a common cytoplasm and a single, continuous plasma membrane.

Liposomes

An artificially prepared spherical vesicle composed of a lipid bilayer that is Invaluable in membrane research.

liposomes function

Act as vehicles for drugs to a specific location or DNA delivery and protect drugs from the immune system.

Membrane Asymmetry

Different lipid compositions in inner and outer leaflets.

Membrane Carbohydrates

Account for 2-10% of the eukaryotic plasma membrane, covalently linked to lipids and proteins on the extracellular surface of the bilayer.

Glycoproteins

Carbohydrates linked to proteins on the membrane surface. It makes up more than 90% of membrane carbohydrates and is used for interaction with other cells.

Glycolipids

Carbohydrates linked to lipids. They make up more than 10% of membrane carbohydrates and are used as sites of cell-to-cell recognition.

Membrane Sidedness

Asymmetry of the membrane bilayer resulted from different membrane proteins attached to the bilayer.

Types of membrane proteins

integral(transmembrane), peripheral, lipid-anchored

Integral Proteins (transmembrane proteins)

Proteins spanning the entire lipid bilayer.

Integral Proteins Functions

Transport proteins (channels and carriers), Receptors that respond to stimuli, and Agents that transfer electrons.

Peripheral proteins

Located entirely outside of the bilayer on either the extracellular or cytoplasmic side, associated with non-covalent bonds.

Functions of peripheral proteins

Provide mechanical support, specialized coats, anchor integral proteins, act as enzymes, and transmit signals.

Lipid-anchored proteins

Located outside the lipid bilayer, on either the extracellular or cytoplasmic surface, it is covalently linked to lipids outside the bilayer.

Fluidity (Viscosity)

Physical state of lipid molecules

Liquid-crystal phase

Lipid molecules can rotate and move laterally.

Solid-crystal (gel) phase

Lipid movement is greatly restricted.

What determines the fluidity of lipid molecules?

Temperature

Transition temperature (TT)

Temperature where lipid phase changes from liquid to gel.

Higher than TT

Liquid phase

Lower than TT

Gel phase

Determinants for TT

Saturation, length of fatty acids, and cholesterol presence.

Saturation of fatty acids

More unsaturated fats lower the transition temperature.

Length of fatty acids

Shorter chains lower the transition temperature.

Cholesterol's role in TT

More cholesterol lowers the transition temperature.

Importance of membrane fluidity

Balances rigidity and fluidity for membrane function.

Simple diffusion

Movement of substances(water and ions) with/without protein channels.

Facilitated diffusion

Requires specific binding to protein transporters.(Glucose transporter)

Active transport (primary and secondary)

Requires ATP, moves against concentration gradient.

Selective permeability

Allows separation and exchange of materials across membranes.

Net flux

Difference between influx and efflux of materials.

Diffusion

Spontaneous movement from high to low concentration.

Diffusion of nonelectrolytes

depends on the concentration gradient

Diffusion of electrolytes

depends on the electrochemical gradient

Determinants of simple diffusion rate

polarity and molecular size

Partition coefficient

The ratio of a substance's solubility in a nonpolar solvent compared to its solubility in water.

Partition coefficient and polarity

Less polar, larger partition coefficient (faster penetration)

Size Effect on Diffusion

Smaller molecules penetrate membranes faster.

Osmosis

the diffusion of water through a semipermeable membrane. (low to high concentration)

Hypotonic Solution

Lower solute concentration; cells swell; lose ions

Hypertonic Solution

Higher solute concentration; cells shrink; gain ions

Isotonic Solution

Equal solute concentration; cells unchanged.

Turgor Pressure

Pressure in plant cells preventing swelling.

Plasmolysis

Plant cell shrinkage in hypertonic solutions.

Aquaporins

Proteins facilitating rapid water movement.

Ion Channels

Proteins allowing specific ion passage across membranes.

Gated Channels

Ion channels that can open or close.

Voltage-Gated Channels

Open in response to membrane potential changes (ionic charge).

Ligand-Gated Channels

Open when a specific molecule binds.

Mechano-Gated Channels

Open in response to mechanical forces.

Facilitated Diffusion

Passive transport via specific transporter proteins.

Saturation Kinetics

Transport limited by available transporter proteins.

Conformational Change

An alteration of the structure of the protein that impacts that protein's function

Active Transport

Movement against a concentration gradient using energy. (low to high concentration)

Hydrolysis of ATP

Exergonic process coupled with active transport releases energy by breaking phosphate group off.

Primary Active Transport

Direct use of ATP to transport substances.

Secondary Active Transport

Uses existing gradients from primary active transport to transport other solutes.

Cotransport

Simultaneous transport of two substances across membranes.