Cell Membrane/Plasma Membrane d bilayer - with embedded proteins and carbohydrates about 75% of these lipids are phospholipids also made up of cholesterol and glycolipids ons: 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 press 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 s gives phospholipids both polar and non-polar acteristics = amphipathic
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Cell Membrane/Plasma Membrane lipid bilayer - with embedded proteins and carbohydrates about 75% of these lipids are phospholipids also made up of cholesterol.
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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”
-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
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
-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
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
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
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