BIO 1330 week 3- Phospholipid bilayer, membranes, & transport across them

Phospholipid

Amphipathic molecule with a hydrophilic head and two hydrophobic tails; forms bilayers in water

Amphipathic

Having both hydrophilic and hydrophobic regions

Four components of membranes

Phospholipids

proteins

cholesterol

sugar chains

Cholesterol

Amphipathic molecule that stabilizes membrane by reducing fluidity and permeability

integral proteins

Embedded within the membrane, function as transporters, receptors, enzymes

Peripheral proteins

Loosely attached to membrane surface; assist in signaling and support

Factors affecting fluidity

More cholesterol = less fluid

more unsaturated tails = more fluid

higher temperature = more fluid.

Selective permeability

Membranes allow some substances to pass while blocking others.

Passive transport

Movement of substances down their concentration gradient without energy (includes diffusion and osmosis).

Active transport

Movement of substances against their concentration gradient using energy (ATP) and pumps

Simple diffusion

Direct movement across the bilayer; works for small nonpolar molecules (O₂, CO₂).

Facilitated diffusion

Movement via channel or carrier proteins; for ions and large polar molecules.

Channel proteins

Form pores for ions and small polar molecules (e.g., aquaporins for water).

Carrier proteins

Bind and transport large polar molecules like glucose

Osmosis

Diffusion of water across a membrane; occurs via aquaporins or slowly by direct diffusion

Hypertonic solution

Higher solute concentration outside cell; water moves out; cell shrinks.

Hypotonic solution

Lower solute concentration outside cell; water moves in; cell swells.

Isotonic solution

Equal solute concentration inside and outside; no net water movement.

Relative permeability

Small nonpolar molecules cross easily

small polar molecules cross slowly

large polar molecules and ions require transport proteins.