Cell membranes

Plasma Membrane Structure and Function

The plasma membrane separates the internal environment of the cell from its surroundings.

The plasma membrane is a phospholipid bilayer with embedded proteins.

The plasma membrane has a fluidconsistency and a mosaic pattern of embedded proteins.

The cell-surface membrane controls the movement of substances into and out of the cell.

Cell membranes are made up of….

Phospholipids

Phospholipids allow….

• Lipid- soluble substances to enter and leave the cell

• Prevent water-soluble substances entering and leaving the cell

• Make the membrane flexible and self-sealing.

Make a note of this information

Proteins are also found in the bilayer….

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The fluid-mosaic model of the cell-surface

membrane

Movement across membranes

Molecules can pass across membranes by:

• Simple diffusion

• Facilitated diffusion

• Active transport

The Permeability of the Plasma Membrane

The plasma membrane is partially permeable.

Macromolecules cannot pass through because of their size, and tiny charged molecules do not pass through the nonpolar interior of the membrane.

Small, uncharged molecules such as oxygen and carbon dioxide pass through the membrane, down their concentration gradient.

How molecules cross the plasma membrane

Diffusion

Diffusion is the passive movement of molecules from a higher to a lower concentration until equilibrium is reached.

Gases move through plasma membranes by diffusion.

Process of diffusion

Gas exchange in the lungs occurs by diffusion

Facilitated diffusion

During facilitated diffusion, substances pass through a carrier protein following their concentration gradients.

Facilitated diffusion does not require energy.

For example, the carrier protein for glucose has two conformations and switches back and forth between the two, carrying glucose across the membrane.

Facilitated diffusion of glucose

Channel protein

Movement of materials across a membrane may be passive or active.

Passive transport does not use chemical energy; diffusion and facilitated diffusion are both passive.

Active transport requires chemical energy and usually a carrier protein.

Active transport

During active transport, ions or molecules are moved across the membrane against the concentration gradient – from an area of lower to higher concentration.

Energy in the form of ATP is required for the carrier protein to combine with the transported molecule and move it across the membrane.

Active transport

Transport by Carrier Proteins

Some biologically useful molecules pass through the plasma membrane through channel proteins and carrier proteins that span the membrane.

Carrier proteins are specific and combine with only a certain type of molecule.

Facilitated diffusion and active transportboth require carrier proteins.

Carrier protein

Carrier proteins involved in active transport are called pumps.

The sodium-potassium pump is active in all animal cells, and moves sodium ions to the outside of the cell and potassium ions to the inside.

The sodium-potassium pump carrier protein exists in two conformations; one that moves sodium to the inside, and the other that moves potassium out of the cell.

The sodium-potassium pump

FACILITATED DIFFUSIONMovement from high to lower concentrationUse of carrier/channel proteinProteins specificChanges shape of protein and passes through channel/membraneNo energy/ATP needed

ACTIVE TRANSPORTMovement against concentration gradientUse of carrier/intrinsic/pump proteinsProtein specific (to ion)Energy/ATP requiredEnergy used to change shape of proteins/attach ion to proteinIons moved through membrane as proteins change shape/position

Active transport Diffusion

May move substances

against concentration

gradient;

Requires ATP/energy;

Requires membrane

proteins/carriers

Substances moved

down concentration

gradient;

Does not require

ATP/energy;

Does not (necessarily)

require membrane

proteins/carriers

Homework – find out about the role of extrinsic proteins in a cell membrane. Be ready to explain this to a small group next lesson.

Cell recognition protein

In animal cells, the carbohydrate chains of cell recognition proteins are collectively called the glycocalyx.

The glycocalyx can function in cell-to-cell recognition, adhesion between cells, and reception of signal molecules.

The diversity of carbohydrate chains is enormous, providing each individual with a unique cellular “fingerprint”.

Receptor protein

Enzymatic protein

Summary

The structure of the plasma membrane allows it to be

differentially permeable.

The fluid phospholipid bilayer, its mosaic of proteins,

and its glycocalyx make possible many unique

functions of the plasma membrane.

Passive and active methods of transport regulate

materials entering and exiting cells.