Membrane Structure and Function

Flashcards covering membrane structure and function, including lipids, proteins, fluidity, permeability, transport, and endocytosis/exocytosis.

Plasma membrane

The boundary that separates the living cell from its nonliving surroundings.

Plasma membrane

Exhibits selective permeability, allowing some substances to cross it more easily than others.

Lipids

Make up the bilayer of membranes.

Proteins

Traverse through the membrane.

Fluid mosaic model

A membrane is a fluid structure with a mosaic of various proteins embedded in it.

Singer and Nicolson's Fluid Mosaic Model (1972)

Proposed that the membrane is a mosaic of proteins dispersed and individually inserted into the phospholipid bilayer, with only the hydrophilic regions exposed to water.

Freeze-fracture

Splits a membrane along the middle of the phospholipid bilayer.

Movement of phospholipids

Phospholipids in the plasma membrane can move within the bilayer, mostly laterally.

Cholesterol's effect on membrane fluidity

At warm temperatures (such as 37°C), it restrains movement of phospholipids; at cool temperatures, it maintains fluidity by preventing tight packing.

Proteins

Determine most of the membrane’s specific functions.

Integral proteins

Penetrate the hydrophobic core and often span the membrane.

Transmembrane proteins

Integral proteins that span the membrane.

Functions of membrane proteins

Six major functions include: transport, enzymatic activity, signal transduction, cell-cell recognition, intercellular joining, and attachment to the cytoskeleton and extracellular matrix (ECM).

Membrane carbohydrates

May be covalently bonded to lipids (forming glycolipids) or more commonly to proteins (forming glycoproteins).

Selective permeability

Hydrophobic (nonpolar) molecules can dissolve in the lipid bilayer and pass through the membrane rapidly, while polar molecules do not cross the membrane easily.

Transport proteins

Allow passage of hydrophilic, polar substances across the membrane.

Channel proteins

Have a hydrophilic channel that certain molecules or ions can use as a tunnel

Aquaporins

Facilitate the passage of water.

Carrier proteins

Bind to molecules and change shape to shuttle them across the membrane.

Passive transport

Diffusion of a substance across a membrane with no energy investment.

Diffusion

The tendency for molecules to spread out evenly into the available space

Concentration gradient

The difference in concentration of a substance from one area to another.

Osmosis

The diffusion of water across a selectively permeable membrane.

Isotonic solution

Solute concentration is the same as that inside the cell; no net water movement across the plasma membrane.

Hypertonic solution

Solute concentration is greater than that inside the cell; cell loses water.

Hypotonic solution

Solute concentration is less than that inside the cell; cell gains water.

Tonicity

The ability of a solution to cause a cell to gain or lose water.

Osmoregulation

The control of water balance.

Plant cell

In a hypotonic solution, it swells until the wall opposes uptake; the cell is now turgid (firm).

Facilitated diffusion

Transport proteins speed movement of molecules across the plasma membrane.

Channel proteins

Provide corridors that allow a specific molecule or ion to cross the membrane.

Carrier proteins

Undergo a subtle change in shape that translocates the solute-binding site across the membrane.

Active transport

Uses energy to move solutes against their gradients.

Membrane potential

The voltage (charge) difference across a membrane.

Electrogenic pump

A transport protein that generates the voltage across a membrane.

Cotransport

Occurs when active transport of a solute indirectly drives transport of another solute.

Exocytosis

Transport vesicles migrate to the membrane, fuse with it, and release their contents.

Endocytosis

The cell takes in macromolecules by forming vesicles from the plasma membrane.

Phagocytosis

Cell engulfs particle in a vacuole (cellular eating).

Pinocytosis

Cell creates vesicle around fluid (cellular drinking).

Receptor-mediated endocytosis

Binding of ligands to receptors triggers vesicle formation.