Membrane structure and function

Key questions

• What is the structure and function of cellular membranes?

• How do different entities cross the plasma membrane?

The big picture

Cell membrane function:•Physically separate a cell from its environment•Control what goes in and out of the cell

Revisiting the lipid bilayerCell membrane structure:• When placed in water,

phospholipids spontaneously form into two layers because of the amphipathic nature of the molecule– Hydrophilic heads face the

outside of the membrane– Hydrophobic tails are in

the interior

• This is the framework of all membranes

Fluid mosaic model

Fluid because most of the components of the cell membrane move laterally within the membrane

Fluid mosaic model

Mosaic because the membrane contains a mixture of substances including:• proteins •cholesterol

Fluid because most of the components of the cell membrane move laterally within the membrane

Membrane proteins and their functions

There are two types of membrane proteins•Integral-penetrate the hydrophobic core of the lipid bi-layer•Some span the entire membrane

Peripheral•Not embedded in the bilayer•Peripheral appendages

Hydrophobic region

Hydrophilic region

Functions:

•Transporters •Can act as enzymes (peripheral)•Signal transduction •Recognition proteins •Fasten cells to adjacent cells •Attachment

Membrane proteins and their functions

(a) Transport

ATP

(b) Enzymatic activity

Enzymes

(c) Signal transduction

Signaling molecule

Receptor

(d) Cell-cell recognition

(e) Intercellular joining

(f) Attachment to the cytoskeleton and extracellular matrix (ECM)

Membrane permeabilityThe plasma membrane is selectively permeable

Membrane permeabilityThe plasma membrane is selectively permeable

What controls the flow of substances that can pass freely across the membrane?

Concentration gradient occurs when there is a different in concentration between two adjacent regions

Region 1 Region 2

What controls the flow of substances that can pass freely across the membrane?

Concentration gradient occurs when there is a different in concentration between two adjacent regions

Region 1 Region 2

What controls the flow of substances that can pass freely across the membrane?

Substances will move down their concentration gradients (from an area of high to low concentration)

This movement is called diffusion

What controls the flow of substances that can pass freely across the membrane?Diffusion-the movement of molecules or ions down a concentration gradient

What controls the flow of substances that can pass freely across the membrane?Diffusion-the movement of molecules or ions down a concentration gradient

Why does diffusion occur?

What controls the flow of substances that can pass freely across the membrane?

Diffusion occurs because:•Molecules are always in motion•Therefore they collide•They will collide more if they are crowded•These collisions tend to spread them out

Diffusion of a substance across a membrane does not require energy….. It is considered passive transport.

What controls the flow of substances that can pass freely across the membrane?

How quickly a substances diffuses depends on several factors:•Size•Temperature•Steepness of the concentration gradient•Charge•Pressure

What controls the flow of substances that can pass freely across the membrane?

Some things can pass freely across the membrane while others cannot

Some things can pass freely across the membrane while others cannot

How do molecules and ions that cannot freely cross the bilayer on their own enter or exit a cell?

Some things can pass freely across the membrane while others cannot

How do molecules and ions that cannot freely cross the bilayer on their own enter or exit a cell?

•With the help of transport proteins•By exocytosis and endocytosis

Transport protein•Selective permeability depends on membrane structure AND presence of transport proteins •Each type of transport protein moves a certain molecule across a membrane•There are several types of transport proteins

•Channel proteins (hydrophilic channel)•Ion channels allow ions to cross the membrane (many are gated)•Carrier proteins bind with passenger molecules, change shape and then let the passengers out on the other side

How do molecules and ions that cannot freely cross the bilayer on their own enter

and exit a cell?

There are two kinds of transport1.) Facilitated diffusion

•A concentration gradient drives the diffusion of a substance across a membrane with the assistance of a membrane protein. •No energy is required.

•Ie. glucose

http://highered.mcgraw-hill.com/sites/0072495855/student_view0/chapter2/animation__how_facilitated_diffusion_works.html

How do molecules and ions that cannot freely cross the bilayer on their own enter

and exit a cell?

There are two kinds of transport1.) Facilitated diffusion

•A concentration gradient drives the diffusion of a substance across a membrane with the assistance of a membrane protein. •No energy is required.

•Ie. glucose2.) Active transport

•A membrane protein assists in the transportation of a solute against its concentration gradient•Requires energy•Allows cells to maintain different internal conditions

http://highered.mcgraw-hill.com/sites/0072495855/student_view0/chapter2/animation__how_the_sodium_potassium_pump_works.html

How do molecules and ions that cannot freely cross the bilayer on their own enter

and exit a cell?

More on active transport:•Electrogenic pumps create a membrane potential (electrical potential energy due to a separation of opposite charges).•Two forces drive the passage of ions across a membrane: concentration gradient and charge (electrochemical gradient)

How do molecules and ions that cannot freely cross the bilayer on their own enter

and exit a cell?

EXTRACELLULARFLUID

H+

H+

H+

Proton pump

+

+ H+

H+

+

+

H+

– –

– –

ATP

CYTOPLASM –

How do molecules and ions that cannot freely cross the bilayer on their own enter

and exit a cell?More on active transport:CotransportAn ATP-powered pump that transports a specific solute can indirectly drive the active transport of other solute

Proton pump

–

–

–

–

–

+

+

+

+

+

ATP

H+

H+

H+

H+

H+

H+

H+

H+

Diffusionof H+

Sucrose-H+

cotransporter

Sucrose

Review of passive and active transport

Passive transport

Diffusion Facilitated diffusion

Active transport

ATP

Exocytosis•A vesicle moves to the cell’s surface•Its membrane fuses with the plasma membrane•Contents of the vesicle are released outside of the cell

Bulk transport

Endocytosis•Substances near the cell’s surface are taken into the cell•A small section of plasma membrane balloons inward and then pinches off•Contents are now vesicles which are delivered to their destination within the cell

Bulk transport

Endocytosis•Substances near the cell’s surface are taken into the cell•A small section of plasma membrane balloons inward and then pinches off•Contents are now vesicles which are delivered to their destination within the cell

Both endo and exocytosis involve motor proteins and require energy

Bulk transport

Take-home messages

You should understand:• Membrane structure and function• The types and roles of membrane proteins• Membrane permeability and how substances

that can and cannot flow freely across the membrane enter and exit cells