Cell Membrane Structure and Function

Prof. Dr. Turgut Ulutin

Why do we need the plasma membrane?

“Keeping the goods concentrated”

Keeping harmful materials out

Transports substances in and out of the cell

Communication with other cells – chemical signals

Physical isolation

Cell membrane functions include:

•Regulation of exchange with the environment

Changes in ECF, pH, receptor recognition

•Structural support

The cell membrane is a phospholipid bilayer with proteins, lipids and carbohydrates.

The Cell Membrane

Membrane Structure

Primarily made up of lipids

With lipids

With carbohydrates

1. Lipids are the most abundant

Membrane called phospholipid bilayer

Outermost portions - hydrophilic

Innermost layers - hydrophobic

Four Components of the Plasma Membrane

Phospholipid Bilayer

Cholesterol

Proteins

Glycocalyx

Membranes are “Fluid Mosaics”

Phospholipids are the fluid part of the membrane

Changes shape without breaking apart

The proteins move through the phospholipids

Think of it as icebergs moving through the ocean

Phospholipid Bilayer

Phospholipids–two fatty-acid chains and a polar phosphate group attached to glycerol

Phospholipids in water -two layers, heads pointed out, and tails pointed in Outside of cell mostly water and inside of cell mostly

water so polar heads point towards the watery extracellular fluid and watery cytoplasm inside the cell

Passage through bilayer–lipid center is a barrier to passage of large hydrophilic molecules, but it allows nonpolar, hydrophobic molecules to pass. Small molecules like water and uncharged, lipid

soluble moleucles can pass freely

Cholesterol

Lipid material

Two Functions

Acts as patching substance on the bilayer that prevents passage of some molecules (such as ions or simple sugars)

Helps keep membrane stronger and flexible

Proteins

Two types with many functions Integral - span entire membrane, popping out on both

sides or extending partway into the membrane; bound to the hydrophobic interior

Peripheral - lie on either side of the membrane, not bound to hydrophobic interior

Function Structural support–attach to cytoskeleton Recognition– binding sites on some proteins identify the cell to

other cells help immune system determine self from foreign cells

Communication–signals transferred through receptor proteins Transport–molecules can pass

Glycocalyx – “sugar coat”

Sugar components protruding from lipids and proteins

Binding sites for proteins in communication and recognition

Lubricate cells

Stick cells down – sticky adhesion layer

Membranes serve a wide variety of functions

1. Barriers to separate cells from environment

(plasma membrane)

and one compartment from another (organelle membranes)

2. Transport – Passive, active, and vesicular transport

3. Sites for enzyme systems, e.g.

a) ATP synthase

b) Cytochrome p450s

4. Conduct electrical signals

5. Recognize and respond to molecules or surfaces

a) Signal mol. Receptors

b) Immune response to antigens

Moving Materials In and Out: Diffusion and Gradients

A. Random Movement and Diffusion

Concentration - # of molecules in a given unit of volume

Concentration gradient = difference between the highest and lowest concentration of a solute

Example – bike coasting down a hill

Diffusion = movement of molecules from region of higher to lower concentration

The greater the concentration gradient the faster the rate

Diffusion will continue until concentration gradient is eliminated (as long as no other processes interfere)

Structure and Function of Organelles

cell (plasma) membrane

semi-permeable membrane which regulates the passage of substances into and out of the cell

composition: protein molecules dispersed throughout a double layer of lipid (fluid mosaic)

The ease with which substances can cross the cell membrane

Permeability

•Nothing passes through an impermeable barrier

•Anything can pass through a freely permeable

barrier

•Cell membranes are selectively permeable

Selective permeability is based on size, electrical charges, molecular

shape, and lipid solubility.

Transport of substances across the membrane can be Passive or Active

Active transport requires energy to occur

Passive transport does not require energy

Diffusion, Osmosis and Active Transport are different types of movement

Factors affecting permeability of cell membrane to a particular

substance:

1. molecular size of the substance

2. electrical charge on the atom or molecule of the substance

3. solubility of the substance in water

4. solubility of the substance in lipid

What determines the direction of the movement of molecules (diffusion)

across the cell membrane?

concentration of substances on each side of the membrane

Diffusion is a form of passive transport.

(no energy required)

Osmosis

diffusion of water through a semi-permeable membrane from an area where the water molecules are more concentrated to an area where the water molecules are less concentrated

Diffusion through Membranes

Permeability verses selectively permeable Permeable

Membrane permeable to both water and solute solute diffuses down its concentration gradient diffuses through the membrane becoming evenly distributed on both sides

Selectively permeable water freely moves, but solute cannot more solute on one side of the membrane water moves both ways, but more flows to the greater concentration of

solutes result – more solution on one side of the membrane than the other

Osmosis - net movement of water across a selectively permeable membrane from an area of lower solute concentration to higher solute concentration

Importance of osmosis to membrane function in animals and plants Cells shrinking or bursting in animals Cells expanding and shrinking in plants (wilting and standing up)

Two Main Types of Transport across Cell Membranes

Passive Transport – no energy expended

Simple diffusion - membrane is permeable to water, gases, and fat-soluble molecules, so can enter without energy output by cell

Facilitated diffusion - membrane is impermeable to larger polar molecules, so to travel down the concentration gradient, need help from a transport protein

Channel protein Carrier protein

Osmosis

Active Transport– molecules passing across the membrane up their concentration gradient cannot use diffusion - must expend energy (ATP)

Transport protein binds with ATP and the molecule being transported. The protein changes shape to move the ion across the membrane

Ex. Sodium (Na) – Potassium (K) pump Maintain high K inside the cell and high Na outside the cell

Think of it as pumping out seawater from a leaking boat

Active transport requires energy

A transport protein actively pumps a specific solute across a membrane against the solute’s concentration gradient; i.e., away from where the solute is less concentrated. Membrane proteins use ATP as their energy source for active transport.