Lecture 4 - Cell Membrane Composition and Transport

Plasma membrane composition

Lipid bilayer and proteins in fluid mosaic

- cellular activity

- separates intracellular and extracellular fluid

What are the 3 basic parts of human cells?

plasma membrane, cytoplasm, nucleus

Membrane lipids

75% phospholipids

5% glycolipids

20% cholesterol (increases membrane stability)

Membrane protein function

Integral and peripheral proteins with various roles

-1/2 mass of plasma membrane

Integral proteins

penetrate the hydrophobic interior of the lipid bilayer

- function as transport proteins, enzymes or receptor

6 functions of membrane proteins

1. Transport

2. Receptors for signal transduction

3. Attachment to cytoskeleton and extracellular matrix

4. Enzymatic activity

5. Intercellular joining

6. Cell-cell recognition

3 Types of membrane transport

1. simple diffusion

2. osmosis

3. active transport

passive transport

Requires NO energy

- movement of molecules from high to low concentration (down gradient)

Fick's first law of diffusion

States that the rate of diffusion, or flux, J of a species is proportional to the concentration gradient.

Fick's second law of diffusion

describes the change in concentration at a particular point in the system with time

Stokes-Einstein Equation

D=KT/6πrη

- liquid diffusion, drag force is related to size and solvent viscosity of spherical particle

Permeability vs diffusion

permeability: rate of flow of liquid/gas through porous material

diffusion: passive movement of molec/particles down conc grad

4 types of diffusion across lipid bilayer:

1. small nonpolar (O2, CO2, steroid, hormones) = rapidly

2. small polar molec = slowly

3. large polar molec = very slowly, need help

4. ions = highly impermeable

2 types of membrane transport proteins

1. channels: aqueous pores

ex - aquaporin

2. transporters: bind specific solute and undergo conformational change

Speed of diffusion is influenced by

size of molecule and temperature

simple diffusion

Diffusion that doesn't involve a direct input of energy or assistance by carrier proteins.

facilitated diffusion

going high to low conc

- certain lipophobic molecules (like glucose, aa, ions) binding protein carriers, or water-filled channels

Carrier-mediated facilitated diffusion

Molecule binds to specific protein that changes shape to carry molecule across the plasma membrane

- limited number of carriers present

Channel-mediated facilitated diffusion

through a channel protein; mostly ions selected on basis of size and charge

2 types of channel-mediated

1. Leakage channel

2. Gated channel

Osmosis

Diffusion of water through a selectively permeable membrane

- through aquaporin channels

Osmolarity

measure of total concentration of solute particles

- h2o moves by osmosis until hydrostatic pressure and osmotic pressure equalize

Tonicity def

the ability of a surrounding solution to cause a cell to gain or lose water

Isotonic

Having the same solute concentration as another solution.

Hypertonic

Having a higher concentration of solute than cytosol

Hypotonic

Having a lower concentration of solute than cytosol

Cholesterol

Lipid that increases membrane stability

Sources of intracellular osmolarity

- macromolecules (little)

- mostly small organic molec (sugars, aa, nucleotides)

RBC osmolarity

hypotonic - water rushes in, cells burst

hypertonic - water leaves cell, shrink

Typical concentrations of ions in cells

K high inside

Na high outside

Cl high outside

2 types of active processes

active transport and vesicular transport

- both require ATP because solute to large, lipid not soluble or solute can't move down conc grad

Uniporter

transporter that carries one specific ion or molecule

coupled transporters

couple the uphill transport of one solute across the membrane to the downhill transport of another

- symporter & antiporter

Active transporters have

one or more binding sites

- undergoes conformational change

How to cells link transporters to energy souce?

1. coupled transporters

2. ATP driven pumps

3. light driven pumps (archea/bacteria)

active transport

requires energy and carrier proteins (specific)

primary active transport

Active transport that relies directly on the hydrolysis of ATP

ex: sodium-potassium pump

secondary active transport

use pre-existing gradient to drive transport of solute

symporter

transporter that carries two different ions or small molecules, both in the same direction

antiporter

A carrier protein that transports two molecules acrss the plasma membrane in opposite directions.

sodium-potassium pump

a carrier protein that uses ATP to actively transport sodium ions out of a cell and potassium ions into the cell

- keeps electrochemical gradient

Resting Membrane Potential (RMP)

Electrical potential energy produced by separation of oppositely charged particles across plasma membrane in all cells

- -50 to -100 mV

ABC transporter def

ATP binding cassette, contains 2 ATP binding sites in order for transport

ABC transporter function clincally

- cancer (challenge for treatment)

- malaria

- cystic fibrosis

vesicular transport

Transport of large particles and macromolecules across plasma membranes

- requires ATP

Vesicular transport functions

exocytosis, endocytosis, transcytosis, vesicular trafficking

Endocytosis and Transcytosis

Involve formation of protein-coated vesicles

- Often receptor mediated, therefore very selective

endocytosis example

phagocytosis and pinocytosis

Phagocytosis

Cell eating, pseudopods engulf solids

- form phagosome

Pinocytosis

Cell drinking

- plasma membrane infolds

- fuses with endosome

ex: nutrient absorption in small int.

Glycolipid

Lipid with polar sugar groups on outer membrane surface

receptor-mediated endocytosis

The movement of specific molecules into a cell by the inward budding of membranous vesicles

- clathrin-coated pits

types of coat proteins for receptor-mediated endocytosis

1. caveolae: capture specific molecules for transcytosis

2. coatomer: funct in vesicular trafficking

Exocytosis

Process by which a cell releases large amounts of material

- activated by cell-surface signal

- secretory vesicle

Tags exocytosis

1. v-SNARE (v = vesicle) find vesicle

2. t-SNARE (t = target) on membrane and bind

Glycocalyx

sugar coating at surface

- every cell has different pattern of sugars (used for cell-cell recognition)

** immune system to recognize "self" and "non self"

Cell junctions (3 types)

contact points between the plasma membranes of tissue cells

1. tight junctions

2. desmosomes

3. gap junctions

tight junctions

Membranes of neighboring cells are pressed together, preventing leakage of extracellular fluid

ex: endothelial cells, skin

Desmosomes

Anchoring junctions at plaques (thickening of plasma mem)

- linker proteins

- prevents cells subjected to mechanical stress from being pulled apart

ex: near joints

Gap junction

transmembrane proteins form pores (connexons) that allow small molecules to pass from cell to cell

ex: muscle for ion/signal transduction

Glycoprotein

Protein with polar sugar groups on outer membrane surface

Peripheral proteins

Proteins loosely attached to integral proteins

- filaments on intracellular surface

- function: enzymes

Receptors

Proteins that bind to specific molecules and initiate a cellular response

Cell-cell recognition

Process by which cells identify and interact with each other

Cell adhesion molecules (CAMs)

Proteins that anchor cells to the extracellular matrix or each other

Role of CAMs

- anchor to ECM

- assist in movement of cells

- attract WBCs to injury

Contact signaling

touching and recognition of cells

Chemical signaling

interactions between receptors and ligands that cause changes in cellular activities

Plasma membrane receptors

Proteins that bind to specific ligands and initiate a cellular response

Voltage-gated channel proteins

Proteins that open and close ion channels in response to changes in membrane voltage