Membrane Structure, Transport and Cell - Cell Communication

The plasma membrane

separates the living cell from its surroundings

Phospholipids

• are the most abundant lipids in most membranes

• amphipathic molecules, containing hydrophobic and hydrophilic regions

• can exist as a stable boundary between two aqueous compartments

At cool temperature membrane

switch from a fluid state to a solid state, depending on the type of lipids

A membrane remains
fluid to a lower temperature
if

it is rich in phospholipids with unsaturated hydrocarbon tails

Membranes must be

fluid to work properly

At warm temperatures (such as 37oC), cholesterol

restrains movement of phospholipids

At cool temperatures, cholesterol

maintains fluidity by preventing tight packing

fluid mosaic model

states that the membrane is a mosaic of protein molecules bobbing in a fluid bilayer of phospholipids

Passive transport

does not require an input of energy

Active transport

requires energy to move substances across the membrane

Transport proteins

are transmembrane proteins that transport molecules

Integral proteins

(e.g. transmembrane proteins) penetrate the hydrophobic interior of the lipid bilayer

Peripheral proteins

are loosely bound to the surface of the membrane

Enzymatic activity

active site exposed to substrates;may be organized as a team by sequential metabolic steps

Attachment to the cytoskeleton and extracellularmatrix (ECM)

maintain cell shape and protein location; may influence intra- and extracellular changes

Cell-cell recognition

glycoproteins serve as ID tags recognized by

proteins on the surface of other cells; usually short-lived binding

Intercellular joining

membrane proteins of nearby cells may hook together as a junction (e.g. gap or tight junctions); long-lasting binding

Signal transduction

1.) reception -(sense a signal),

2.) transductions pass the message along, and response (act on the message): two types: G -protein coupled receptors (receptors uses a middle man - a G protein - to send a message to another signaling molecule, which activates the response)

3.) Signaling molecule “ligand” binds and open an ion channel, allowing ions to move in and stimulate a response.

Transport proteins

allow passage of hydrophilic substances across the membrane

channel proteins

Aquaporins

facilitate the passage of water

Carrier Proteins

bind to molecules and change shape to shuttle them across the membrane

Passive transport

• Involves diffusion and facilitated diffusion

• Moves materials down their concentration gradient

• Does not require an input of energy

Types of Passive Transport

• Diffusion

• Facilitated diffusion

Active transport

• Moves materials against their concentration gradient

• Requires energy provided by ATP or an electrochemical gradient

Diffusion

is the tendency for molecules to spread out evenly into the available space

Facilitated Diffusion

• transport proteins speed the passive movement of molecules across the plasma membrane

Channel proteins

• provide corridors that allow a specific molecule or ion to cross the membrane

• include:

  1. Aquaporins: for facilitated diffusion of water

  2. Ion channels that open
     or close in response to a stimulus (gated channels)

Two types of transport proteins

channel proteins and carrier proteins

Carrier proteins

undergo a subtle change in shape that translocates the solute-binding site across the membrane

sodium-potassium pump

is one type of active transport system: exchanges Na+ for K+

Membrane potential

is the voltage across a membrane

electrochemical gradient

drive the diffusion of ions across a membrane:

electrogenic pump

• is a transport protein that generates voltage across a membrane

• help store energy that can be used for cellular work

Cotransport

occurs when active transport of a solute indirectly drives transport of other solutes against their own concentration or electrochemical gradient

Secondary Active Transport is

another name for

exocytosis

transport vesicles migrate to the membrane, fuse with it, and release their contents

Large molecules, such as polysaccharides and proteins, cross the

membrane in bulk by means of vesicles

Many secretory cells use

exocytosis

Protein Kinases ______ phosphate groups

add ; a process called phosphorylation; are reusable; turns on protein

Protein Phosphatases ____ phosphate groups

remove; turns off signal transduction pathway

Tight Junctions

Seal cells together to prevent leakage of molecules.

Gap junctions

Allow direct communication between cells by permitting the passage of ions and small molecules

integrins

transmembrane receptors that link the ECM to the cytoskeleton.

Desmosomes

• are made of proteins that link the cytoskeletons of adjacent cells

• are common in epithelial and muscle tissue
  § These proteins bind to each othero

  the proteins that anchor cytoskeletal intermediate filaments

Plasmodesmata

They act as a channel in which molecules can be transported in and out of the plant cells.

Symporter

moves the solutes in the same direction

Antiporter

moves the solutes in opposite directions

Signal Transduction

• reception (sense a signal), - signaling molecule causes a conformational change in the receptor of the target cell

• transduction (pass the message along) and response (act on the message);

• two types: G protein-coupled receptors (receptors uses a middle man – a G protein – to send a message to another signaling molecule,

• which activates the response) and ligand-gated ion channels