Cell Membrane/Plasma Membrane

•lipid bilayer - with embedded proteins and carbohydrates•about 75% of these lipids are phospholipids•also made up of cholesterol and glycolipids

• functions: 1. integrity of the cell 2. controls transport = “selectively permeable” 3. excludes unwanted materials from entering the cell 4. maintains the ionic concentration of the cell & osmotic pressure

of the cytosol 5. forms contacts with neighbouring cells = tissue

Phospholipids

• similar to fat molecules - glycerol + 2 fatty acids+ a phosphate group

• phosphate gp hydrophilic “head”• fatty acid gps hydrophobic “tails”

http://www.bio.davidson.edu/people/macampbell/111/memb-swf/membranes.swf

-this gives phospholipids both polar and non-polar characteristics = amphipathic

A. Composition:

-the polar and non-polar attributes of the lipids results in a bilayer arrangement-cholesterol is also polar (OH group) and non-polar (steroid rings) and contributes to this arrangement – OH group faces out and the steroid ringsface inward

polar heads out

non-polar tails in

• membrane proteins

1. peripheral or extrinsic -bind to the outside onlye.g. enzymes

2. integral or intrinsic -globular and amphipathic-can span 1 or both layers-most are transmembrane (long, rodlike)

•many lipids are proteins are modified by the attachmentof carbohydrates = ‘glyco’proteins & ‘glyco’lipids•glycoproteins & glycolipids form a superficial coat around thecell = ‘glycocalyx’

Functions of Integral Proteins

-in addition:4. enzymes5. linkers – anchor proteins of the PMto the protein filaments inside or to neighboring cells6. cell-identity markers – used in identifying “self” by the immune system

e.g MHC proteinse.g. ABO blood typing

• ion channels = gates for specific ions only-open in response to: 1. changes in voltage

2. binding of a ligande.g. calcium

sodium chloride

potassium

-affected by drugse.g. anti-hypertensives - calcium, potassium local anesthetics - sodium diuretics - sodium

muscle relaxants - chloride anti-diabetics - potassium

-disease states affect channel functione.g. cystic fibrosis

B. Membrane function:

5. Controls transport = “selectively permeable”-two types: Passive - Diffusion, Osmosis, Facilitated

Active - Active transport, Exocytosis,Endocytosis,

2. Integrity of cell - cell shape and size-increase cell size, increase surface area/volume-increase exchange surface

1. Physical isolation - from the surrounding ECF-allows the cell to create different environments outside

and inside-allows for the creation of gradients – electrical and

chemical

3. Sensitivity - first part of cell that is affected by changes in theextracellular environment

4. Structural support - connections between cells provides tissueswith support and stability

Membrane Gradients

• selective permeability of the PM allows the cells to control the concentration of ions within the cell and outside the cell (in the ECF)

• this results in a distinct distribution of positive and negative ions inside and outside the cell– typically the inside of the cell is more negatively charged

• this difference in electrical charge between inside and outside = electrical gradient

• because it occurs across the PM – we call this difference in charge = membrane potential

• can be measured with tiny glass electrodes• varies from cell to cell• very important in the functioning of neurons and muscle

cells

Membrane Permeability and Transport

•permeability = property that determines the effectiveness of the PM as a barrier

•permeability varies depending on the organization and characterization of the membrane lipids and proteins

•transport across the membrane may be passive or active

passive transport

diffusionosmosisfacilitated

active transport

endocytosis (pinocytosisphagocytosisreceptor-mediated)exocytosis

http://programs.northlandcollege.edu/biology/Biology1111/animations/transport1.html

-materials may cross into a cell based on concentration and size-if they cross from [high] to [low] – they are traveling with their concentrationgradient – requires no energy (Passive)-if they cross against the concentration gradient – requires energy (Active)-small particles may cross through the lipid bilayer-others may require integral proteins that help (e.g. channels or pores)-others may enter through the fusion of tiny vesicles with the PM

A. Diffusion = movement of materials from [high] to [low]-random movement, no energy needs to be

synthesized-the movement is driven by the inherent kinetic energy of the particles moving down their concentration gradient-movement could be through the bilayer itself or throughchannel proteins-three ways to diffuse:

1. through the lipid bilayer: lipid soluble (non-polar), alcohol, gases, ammonia, fat-soluble vitamins

2. through a channel: charged, small ions (polar)-some channels are “gated” – open and close

3. facilitated diffusion: larger molecules too big for channels

B. Osmosis = diffusion of water from [high] to [low]OR movement of water from [low solute] to [high solute]

-in osmosis – the membrane is permeable to water and NOT to the solutes-but it is the concentration of solutes that causes the water to move

-experiment – U shaped tube divided by a membrane permeable to water only-increase the solute concentration in the right half of the tube-this increases the pressure caused by the increase solutes = osmoticpressure-therefore increasing solute concentration increases osmotic pressure-water will move in to decrease this OP

-OP is important in determining how much fluid remains in your blood and howmuch leaves to surround the cells in your tissues

hypotonic = [S]in > [S]out, water enters cell

hypertonic

-Osmosis is controlled by tonicity = degree to which a the concentration of a specific solute surrounding a cell causes water to enter or leave the cell

hypertonic = [S]in < [S]out, water exits cell

e.g. isotonic = [S]in = [S]out, no water movement

-medical uses of solutions requires careful consideration of osmolaritye.g. can cause destruction of red blood cells if these cells are placed in hypotonic or hypertonic solutions

-typical saline solutions are 0.9% NaCl = isotonic saline-other IV solutions are also isotonic

e.g. D5W – 5% dextrose in water-but hypertonic and hypotonic solutions can be used in specific situations

e.g. cerebral edema = water is forced out of the blood and into the brain tissue -treatment with hypertonic saline causes water to leave the brain tissue back into the

where it is removed by the kidneyse.g. dehydration – treatment with hypotonic solutions to increase water content of ECF

C. Facilitated transport = molecules move by a carrier protein from

[high] to [low]-binds to a receptor site on the plasma membrane-transported by the carrier protein-no energy required-but there is a limit to the amount of FD cells can undergoand it has to do with the # of carrier proteins on the PM-molecules that are insoluble, too polar or

too largee.g. glucose

amino acids

Medical application

-the number of transporters during homeostasis remains constant-but cells can increase or decrease the expression of these carriers in response to the environment-increased blood sugar – production of insulin by the pancreas- insulin causes cells (e.g. adipose cells, liver cells, muscle cells) to increase their expression of a glucose transporter (GLUT proteins)on the surface-this increases the uptake of sugar from the blood-failure to produce enough insulin or failure of cells to express GLUT transporters in response to insulin = diabetes mellitus

A. Active transport = molecules are moved against the the concentration gradient

i.e. from [low] to [high]

-two kinds: primary and secondary-primary active transport:

-requires a protein carrier and ATP-carrier is often called a pump-ATP binds to the pump and changes its shape (ATPase)e.g. sodium/potassium pump – three Na are pumped out of a cell and 2 Kare pumped into the cell (Na/K ATPase)-maintains a specific concentration of Na within the cell and K outside thecell-Na binds to the pump, ATP then binds and hydrolyzes, a P group attaches

to thepump and changes its shape – expels the Na out of the cell-K then binds the pump and causes the release of the P, the pump returns

to its original shape, bringing K into the cell

http://highered.mcgraw-hill.com/sites/0072437316/student_view0/chapter6/animations.html#

2. secondary active transport:-the energy stored in a concentration gradient is used to drive the transportof other materialse.g Na/Ca antiporter – opposite direction for Na and Ca movement– primary transport establishes high [Na] outside thecell – this concentration gradient creates potential energy which is storedby the antiporter pump- as Na leaks back in – this potential energy is converted into kinetic energywhich drive the movement of a Ca ion against its gradient-some pumps can also pump two materials in the same direction = symportere.g. Na/glucose symporter-most of our cells use the energy created by the Na gradient to power themovement of other ions

http://highered.mcgraw-hill.com/sites/0072437316/student_view0/chapter6/animations.html#

diffusiondiffusion

diffusion

diffusionLow Na, low CaHigh glucose, high amino acids

-primary active transportand ATP hydrolysis pump Na out of the cell and creates a sodium gradient-increased sodium gradient = increased membrane potential energy

-when sodium diffuses back into the cell through the symporter or antiporter, potential energy is converted into kinetic energy and thesecond ion can be pumped against its gradient-same direction as Na = symporter-opposite direction as Na = antiporter

B. Exocytosis = secretion of a substance outside the cell-made within the cell, packaged into transport vesicles-> fusion with the plasma membrane and release outside the celle.g. nerve cells - neurotransmitter release

http://highered.mcgraw-hill.com/sites/0072437316/student_view0/chapter6/animations.html#

C. Endocytosis = reverse of exocytosis, internalization of substances

-3 forms: 1. pinocytosis = “cell drinking”

2. phagocytosis = “cell eating”

3. receptor-mediated = internalization of specific substances-binding of a ligand with its receptor -> internalizationinto the cell-occurs at specific sites within the PM -> clathrin-coatedpits-internalization at pits -> clathrin-coated vesicle-vesicle fuses with endosomes - processing

Medical application

• HIV and receptor-mediated endocytosis• binding of HIV virus to the CD4 protein on the

surface of T helper cells and macrophages results in the RME of the HIV virus

• the HIV viral particles are made by the host cell protein synthesis machinery and assembled at the host’s PM – released from the cell = exocytosis

• the infected T cells are killed leading to low T cell counts in infected people