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Cell Membrane/Plasma Membrane
• 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
•lipid bilayer - with embedded proteins and carbohydrates
•about 75% of these lipids are phospholipids
•also made up of cholesterol and glycolipids
Phospholipids
• similar to fat molecules - glycerol + 2 fatty acids
+ a phosphate group
• phosphate gp
hydrophilic “head”
• fatty acid gps
hydrophobic “tails”
-this gives phospholipids both polar and non-polar
characteristics = amphipathic
http://www.bio.davidson.edu/people/macampbell/111/memb-swf/membranes.swf
A. Composition:
polar heads out
non-polar tails in
-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 rings
face inward
• membrane proteins
1. peripheral or extrinsic
-bind to the outside only
e.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 attachment
of carbohydrates = ‘glyco’proteins & ‘glyco’lipids
•glycoproteins & glycolipids form a superficial coat around the
cell = ‘glycocalyx’
Functions of Integral Proteins
-in addition:
4. enzymes
5. linkers – anchor proteins of the PM
to the protein filaments inside or to
neighboring cells
6. cell-identity markers – used in
identifying “self” by the immune system
e.g MHC proteins
e.g. ABO blood typing
• ion channels = gates for specific ions only
-open in response to: 1. changes in voltage
2. binding of a ligand
e.g. calcium
sodium
chloride
potassium
-affected by drugs
e.g. anti-hypertensives - calcium, potassium
local anesthetics - sodium
diuretics - sodium
muscle relaxants - chloride
anti-diabetics - potassium
-disease states affect channel function
e.g. cystic fibrosis
B. Membrane function:
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
2. Integrity of cell - cell shape and size
-increase cell size, increase surface area/volume
-increase exchange surface
3. Sensitivity - first part of cell that is affected by changes in the
extracellular environment
4. Structural support - connections between cells provides tissues
with support and stability
5. Controls transport = “selectively permeable”
-two types: Passive - Diffusion, Osmosis, Facilitated
Active - Active transport, Exocytosis,
Endocytosis,
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
diffusion
osmosis
facilitated
active transport
endocytosis
(pinocytosis
phagocytosis
receptor-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 concentration
gradient – 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 through
channel 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 = osmotic
pressure
-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 how
much leaves to surround the cells in your tissues
-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
e.g. isotonic = [S]in = [S]out, hypotonic = [S]in > [S]out, hypertonic = [S]in < [S]out
water enters cell
water exits cell
no water movement
-medical uses of solutions requires careful consideration of osmolarity
e.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 kidneys
e.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 undergo
and it has to do with the # of carrier proteins on the PM
-molecules that are insoluble, too polar or
too large
e.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:
http://highered.mcgrawhill.com/sites/0072437316/student_view0/chapter6/a
nimations.html#
-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 K
are pumped into the cell (Na/K ATPase)
-maintains a specific concentration of Na within the cell and K outside the
cell
-Na binds to the pump, ATP then binds and hydrolyzes, a P group attaches to the
pump 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
2. secondary active transport:
-the energy stored in a concentration gradient is used to drive the transport
of other materials
e.g Na/Ca antiporter – opposite direction for Na and Ca movement
– primary transport establishes high [Na] outside the
cell – this concentration gradient creates potential energy which is stored
by the antiporter pump
- as Na leaks back in – this potential energy is converted into kinetic energy
which drive the movement of a Ca ion against its gradient
-some pumps can also pump two materials in the same direction = symporter
e.g. Na/glucose symporter
-most of our cells use the energy created by the Na gradient to power the
movement of other ions
http://highered.mcgrawhill.com/sites/007243731
6/student_view0/chapter6
/animations.html#
diffusion
diffusion
diffusion
Low Na, low Ca
High glucose, high amino acids
diffusion
-primary active transport
and 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 the
second 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 cell
e.g. nerve cells - neurotransmitter release
http://highered.mcgrawhill.com/sites/0072437316/
student_view0/chapter6/a
nimations.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 -> internalization
into the cell
-occurs at specific sites within the PM -> clathrin-coated
pits
-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