M1B: Topic 6 - The Plasma Membrane & Cellular Transport

Plasma Membrane

AKA Cell Membrane

• Function

• Separate the contents of the

cell (cytoplasm) from the

environment

• Regulates movements into

and out of the cell – transport!

• Phospholipid bilayer

Fluid Mosaic Model

Fluid: components can all shift around

within the bilayer

• Many lipids and proteins move

laterally within the membrane

• The membrane is dynamic rather

than rigid

• Mosaic: Multiple components:

1. Phospholipids

2. Proteins

3. Carbohydrates

4. Sterols

• (Cholesterol in animals)

Phospholipids

• Hydrophilic heads face water

• Hydrophobic tails face

inward

• Amphipathic phospholipids

self-assemble into a bilayer

Proteins

Membrane-associated proteins

• Integral membrane proteins

• Span full width of

membrane

• Peripheral membrane

proteins

• Associated with only one

surface (leaflet)

Proteins (Part 2)

Channels or pumps transport

materials in/out of the cell

• Receptors detect molecules

outside of the cell

• Transmit signal to cytoplasm

• Anchor proteins

• Anchor cell components

• Enzymes catalyze chemical

reactions

Carbohydrates

Short carbohydrate chains

attached to membrane proteins

and lipids

• Project from extracellular

surface

• Contribute to cell recognition

and communication

Cholesterol

Helps stabilize membrane fluidity

• Restrains phospholipid

movement at higher

temperatures

• Prevents phospholipids from

packing too tightly at lower

temperatures

Selective Permeability

Small nonpolar molecules cross the lipid

bilayer easily

• Small uncharged polar molecules cross more

slowly

• Ions and most large polar molecules require

transport proteins

• The membrane controls exchange rather than

simply blocking substances

Cell Membranes

Passive transport: Moves substances down a

gradient without cellular energy

• Simple Diffusion

• Osmosis

• Facilitated diffusion

• Active Transport: Moves substances against a

gradient using cellular energy

• Vesicular transport: Moves large materials or

bulk quantities through endocytosis or

exocytosis.

Passive Transport - Diffusion

Net movement of particles from

higher to lower concentration

• Occurs because particles are in

constant random motion

• Moves down the concentration

gradient and does not require

cellular energy

• Example: O2 and CO2 crossing

the plasma membrane

• At equilibrium, particles

continue moving, but there is no

net change in concentration

Passive Transport - Osmosis

A specific case of simple diffusion

Net movement of water across a selectively permeable membrane

Net water movement depends on differences in solute concentration across the membrane

Osmosis (pt 2)

1. Isotonic solution

2. Hypotonic solution

3. Hypertonic solution

• Each term describes the extracellular solution relative to the cell.

Summary of Osmosis

Osmosis is the net movement of water across a selectively permeable

membrane toward the side with the greater concentration of

nonpenetrating solute.

• Osmosis changes cell volume and contributes to fluid balance

Why does Osmosis matter?

Osmosis affects cell volume and helps maintain fluid balance between

body compartments

Osmosis (pt 3)

Isotonic:

• solute concentration balanced

• no net water movement; no change in cell

volume

• Hypertonic:

• Solute concentrations higher in extracellular

fluid

• Water leaves the cell

• Hypotonic:

• Solute concentrations lower in extracellular fluid

• Water enters the cell

Passive Transport – Facilitated Diffusion

Net movement down a

concentration or electrochemical

gradient

• Requires a specific channel or

carrier protein.

• Does not use require ATP.

• Examples: ions moving through

channels and glucose moving

through GLUT carriers

Active Transport

Moves substances against their

concentration or electrochemical

gradients

• Requires a specific membrane

pump or carrier

• Requires cellular energy, often

supplied by ATP

• Example: Sodium-potassium

pump moves 3Na+ out and 2K+

into the cell for each ATP used

Vesicular Transport

Moves materials using membrane-

bound vesicles

•Transports substances too large for

channels or carrier proteins

•Requires cellular energy

•Endocytosis brings material into the

cell

•Exocytosis releases material outside

the cell

Endocytosis

The plasma membrane surrounds material

and forms an internal vesicle

•Phagocytosis engulfs large particles,

including microorganisms and cell

debris

•Receptor-mediated endocytosis brings

specific substances into the cell

•Membrane receptors recognize and

bind their targets

Exocytosis

An intracellular vesicle fuses with the

plasma membrane

•Releases the vesicle contents

outside the cell

•Adds vesicle membrane to the

plasma membrane

•Requires cellular energy and

regulated membrane fusion

Examples: Neurotransmitters, Peptide hormones, including insulin, Digestive enzymes