General Biology New updated Chapter 7

Fluid mosaic model

The membrane is a fluid lipid bilayer with different proteins embedded in or attached to it.

Amphipathic molecule

A molecule with both a water-loving (hydrophilic) region and a water-fearing (hydrophobic) region.

Hydrophobic interactions

Weak interactions that help hold the phospholipid bilayer together as hydrophobic parts avoid water.

Frye–Edidin experiment

Fusing mouse and human cells showed that membrane proteins move sideways within the membrane.

Membrane fluidity: unsaturated tails

Double-bond kinks stop phospholipids packing tightly, keeping the membrane more fluid in cold conditions.

Membrane fluidity: saturated tails

Straight tails pack tightly together, making the membrane less fluid.

Cholesterol as a fluidity buffer

At warm temperatures cholesterol restrains phospholipid movement; at cold temperatures it prevents tight packing.

Cold-water fish membrane adaptation

They have many unsaturated fatty acid tails to help keep membranes fluid in cold water.

Extreme-environment lipid adaptations

Cold-water fish use unsaturated fatty acids; hot-spring microbes have lipids that resist excessive melting.

Integral membrane protein

A protein embedded in the membrane’s hydrophobic core; many span the whole membrane.

Peripheral membrane protein

A protein loosely attached to the membrane surface and not embedded in its hydrophobic core.

Transmembrane protein: hydrophobic region

Usually contains nonpolar amino acids arranged as alpha helices.

Six functions of membrane proteins

Transport, enzymatic activity, signal transduction, cell-cell recognition, intercellular joining, and attachment to the cytoskeleton or ECM.

CD4 and CCR5

Cell-surface proteins HIV binds to: CD4 is the main receptor and CCR5 is the co-receptor.

Membrane carbohydrates

Carbohydrates on the outer membrane surface act as cell-identification tags, such as blood-type markers.

Glycolipid

A carbohydrate covalently bonded to a lipid.

Glycoprotein

A carbohydrate covalently bonded to a protein.

Membrane component synthesis and processing

Membrane proteins and lipids are initially made in the ER and modified in the Golgi.

Membrane sidedness

When transport vesicles fuse with the plasma membrane, the vesicle’s inner face becomes the cell’s outer face.

Selective permeability

The membrane lets some substances cross more easily than others, depending largely on their properties.

Molecules that cross the lipid bilayer easily

Small nonpolar, hydrophobic molecules such as O2, CO2, and hydrocarbons.

Why ions and polar molecules cross poorly

They are hydrophilic and cannot easily pass through the membrane’s hydrophobic interior.

Channel protein

A membrane protein that forms a hydrophilic passage for particular substances.

Carrier protein

A membrane protein that binds a substance and changes shape to move it across.

Aquaporin

A channel protein specialized for rapid water transport across membranes.

Passive transport

Movement down a concentration gradient without cellular energy input.

Osmosis

Passive movement of water across a selectively permeable membrane toward the higher solute concentration.

Tonicity

A solution’s ability to make a cell gain or lose water, based on nonpenetrating solutes.

Isotonic solution

A solution causing no net water movement; an animal cell stays at a stable volume.

Hypertonic solution (animal cell)

A solution that draws water out of a cell; an animal cell shrivels.

Hypotonic solution

A solution that causes water to enter a cell; an animal cell may swell and burst.

Plant cell in a hypotonic solution

Water enters and the cell becomes turgid as the cell wall resists expansion.

Plant cell in an isotonic solution

There is no net water movement, so the cell becomes flaccid or limp.

Plant cell in a hypertonic solution

Water leaves and the plasma membrane pulls away from the cell wall; this is plasmolysis.

Osmoregulation

Control of water balance and solute concentrations in a cell or organism.

Contractile vacuole

An organelle in freshwater protists that pumps excess water out of the cell.

Facilitated diffusion

Passive movement of hydrophilic substances down their gradient through channel or carrier proteins.

Ion channel

A channel protein that transports ions; gated channels open or close in response to stimuli.

Active transport

Movement of a substance against its concentration gradient using energy, usually ATP, via carrier proteins.

Sodium-potassium pump

Using ATP, it moves 3 Na+ out of the cell and 2 K+ into the cell per cycle.

Membrane potential

The voltage across a membrane; a resting cell’s inside is usually negative relative to the outside.

Electrochemical gradient

The combined effect of an ion’s concentration gradient and the membrane’s electrical force.

Electrogenic pump

A transport protein that creates voltage by moving a net charge across a membrane.

Major electrogenic pumps

Animal cells mainly use the sodium-potassium pump; plants, fungi, and bacteria mainly use proton pumps.

Cotransport

A transporter uses one substance moving downhill to drive another substance uphill against its gradient.

H+/sucrose cotransport

In plants, H+ moving back into cells drives sucrose uptake into leaf veins.

Na+/glucose cotransport

Intestinal cells use Na+ movement to help absorb glucose; oral rehydration solutions use this to treat severe diarrhea.

Bulk transport

Movement of large materials in membrane-bound vesicles by endocytosis or exocytosis.

Exocytosis

A vesicle fuses with the plasma membrane and releases its contents outside the cell.

Endocytosis

The membrane folds inward, surrounds material, and pinches off to form an internal vesicle.

Phagocytosis

A type of endocytosis in which a cell engulfs large particles using pseudopodia; the vesicle can fuse with a lysosome.

Pinocytosis

A type of endocytosis that takes in droplets of extracellular fluid into tiny vesicles

Receptor-mediated endocytosis

Specific substances bind to membrane receptors and are taken in concentrated amounts, often through coated pits.