Chapter 7: Membrane Structure and Function Essential Knowledge 2.b.1 Cell membranes are selectively permeable due to their structure (7.1 & 7.2). 2.b.2 Growth and dynamic homeostasis are
maintained by the constant movement of molecules across membranes (7.3-7.5). Plasma Membrane The membrane at the boundary of every cell Functions as a selective barrier for the passage of materials in and out of cells. Called a semi-permeable membrane (regulates
crossing of materials) Membrane Composition Lipids Most numerous Phospholipids and cholesterol
Proteins Very large (in size) Peripheral and integral Carbohydrates Historical Cell Membrane Models
Davson-Danielli Model 1935 Lipid bilayer Proteins coat the surfaces Sometimes called the sandwich model Evidence: Biochemical work TEM pictures showed
double line Accepted until 1960s Problems w/ Davson model Not all membranes in a cell were the same How could the proteins stay in place? Protein placement was confusing
Result - the model was questioned and tested by scientific process Fluid Mosaic Model 1972 Current/New model to fit the new evidence Example of Science as a Process Refers to the way the lipids and
proteins behave in a membrane Fluid Refers to the lipid bilayer Molecules are not bonded together, so are free to shift. Must remain "fluid" for membranes to function. Fluid = dynamic, changing
Cell membrane will remain fluid until temperature drops to extreme levels Ways to keep the membrane fluid Lipid changes or shifts: Plants: Cold hardening (shift to unsaturated fatty acids remember unsaturated fats are kinked in shape)
Animals: Hibernating - Cholesterol amount increases eat more fatty foods right BEFORE they hibernate Sat = NO double bonds Unsat = double bonds Mosaic
Proteins: float in a sea of lipids Proteins form a collage or mosaic pattern that shifts over time Evidence for fluid-mosaic TEM pictures of fractured membranes
Cell fusion studies Tagging of membrane proteins by antibodies ME MO R T HI I Z E
S!!! Protein Function in Membranes Main function: Determine cells specific function Other functions: Transport Enzymatic activity Receptor sites for signals (hormones)
Cell adhesion Cell-cell recognition (immunity) Attachment to the cytoskeleton (cilia and flagella) 2 types of transport: a) Channel b) Carrier
Types of Membrane Proteins 1) Integral Inserted into the lipid bilayer Go through ENTIRE bilayer 2) Peripheral Not embedded in the lipid bilayer Attached to the membrane surface Question?
How do the integral proteins stick to the membrane? By the solubility of their amino acids Protein folding/structure type Hydrophilic Amino Acids Hydrophobic
Amino Acids Hydrophilic Amino Acids Membranes are Bifacial The lipid composition of the two layers is different The proteins have specific orientations.
Carbohydrates are found only on the outer surface Membrane Carbohydrates Branched oligosaccharides form glycolipids and glycoproteins on external surface Glyco = carbohydrate Made from modification of existing cellular
molecules Glycolipids = Lipids+carbohydrates Glycoproteins = Proteins + carbohydrates Function: recognition of "self" vs "other 1 2 3
4 Carbohydrates Questions How do materials get across a cell's membrane? Do they use energy/no energy?
Do the molecules move against/with concentration gradient? REMEMBER: Cell membrane is regulatory membrane (semi-permeable) Problems with using cell membrane to move materials
1) Lipid bilayer is hydrophobic Hydrophilic materials don't cross easily Ex: ions, H2O, polar molecules Hydrophobic materials will cross easily Ex: CO2, O2, hydrocarbons 2) Large molecules don't cross easily Too big to get through the membrane (without assistance or through the use of energy)
Proteins play HUGE role in getting certain molecules across (hydrophilic) Two Mechanisms for Movement 1. Passive Transport 2. Active Transport * Both involve concentration gradients (movement of molecules from areas of high/low concentrations)
Passive Transport Movement across membranes that does NOT require cellular energy Types: Diffusion Osmosis Facilitated Diffusion Diffusion
The net movement of atoms, ions or molecules down a concentration gradient Ex: smells crossing room Movement is from: High Low io s
u f Dif n ie v o m
Equilibrium When the concentration is equal on both sides There is no net movement of materials Molecules are constantly in motion (dont stop moving!) However, CONCENTRATION STAYS THE SAME!!!
Factors that Affect Diffusion 1. Concentration a) Of solute or solvent/s 2. Temperature 3. Pressure 4. Particle size a) Smaller size = quicker movement
5. Mixing a) More = faster diffusion Osmosis Diffusion of water Water moving from an area of its high concentration to an area of its low concentration. No cell energy is used
Passive transport Relies upon tonicity of solutions (both internally and externally) Tonicity The concentration of water relative to a cell. 1. Isotonic (same) 2. Hypotonic (below)
a) The hypotonic solution has a solute concentration BELOW that of the cell 3. Hypertonic (above) Tonicity Tutorial Isotonic Isosmotic solution Cell and water/solution are equal in solute concentration
No net movement of water in or out of the cell Water still MOVES, but concentration of solutes stays the same!!! RESULT: No change in cell size Ex: Marine mammals Hypotonic
Hypoosmotic solution Cell's water is lower than the outside water (more solutes) Water moves into the cell RESULT: Cell swells, may burst or the cell is turgid Ex: place raisin in water (raisin will swell, because water rushes in to attempt to EQUAL out
concentrations of solutes and water) Ex: place egg in vinegar/water solution Hypertonic Hyperosmotic solution Cell's water is higher inside than the outside water (less solutes) Water moves out of the cell RESULT: Cell shrinks or plasmolysis occurs
Ex: Placing RBC in salt solution (RBC shrivels to attempt to push water out of cell to = solute ie v concentrations) o sm i s
o Ex: Placing egg in corn syrup Osm Ex: onion cell w/ salt solution added Facilitated Diffusion Transport protein that helps materials through the cell membrane Polar molecules and ions USE this!!!
Doesn't require energy (ATP) Still passive transport Works on a downhill concentration gradient F D d e
t a acilit e i v o iff m
Channel proteins Carrier proteins Active Transport Movement across membranes that DOES require cellular energy Uses ALL carrier proteins Allows cells to differ in solute concentration
Why? Important in Ps, Rs and hormones Types: Carrier-mediated Endocytosis Exocytosis Key terms for Active Transport Membrane potential: Voltage across membrane (opposite charges and
concentration of them) likelihood charges will pass across the membrane Electrochemical gradient: Electro change in charge Chemical change in concentration Carrier-Mediated Transport General term for the active transport of
materials into cells AGAINST the concentration gradient Movement is: low high Examples: Na+ - K+ pump Electrogenic/H+ pump Cotransport
Na+- K+ pump Moves Na+ ions out of cells while moving K+ ions in Occurs in animals Sodium ions increase outside cell while potassium ions increase inside cell Electrogenic or H+ pumps Also called Proton pumps.
Create voltages (change of charge) across membranes for other cell processes Ps and Rs Used by plants, fungi and bacteria. Transports H+ OUT of cell This change in charge (or change in voltage) allows for storage of ATP
Used in later rxns Cotransport Movement of H+ that allows other materials to be transported into the cell as the H+ diffuses back across the cell membrane Example - Sucrose transport This is how plants transport food/sugar to non-
photosynthetic organs (like the roots) Outside cell Inside cell Exocytosis Moves bulk material out of cells
Uses Golgi vesicles to do this Example: Secretion of enzymes Hormone movement Secretion of insulin by pancreas Endocytosis Moves bulk materials into cells Cell forms new transport vesicles (from parts of
cell membrane) Types of Endocytosis: 1. Pinocytosis move liquids 2. Phagocytosis move solids 3. Receptor Mediated - uses receptors to "catch" specific kinds of molecules. Summary
Recognize the Davson-Danielli model of cell membranes. Identify the components and structure of the fluid mosaic model of cell membranes. Identify methods that keep cell membranes fluid.
Identify methods that cells use for transporting small molecules across membranes. Recognize the conditions that regulate osmosis and tonicity in cells. Identify methods that cells use for transporting large molecules across membranes. Be able to solve problems in osmosis pgs. 140 and 141 #1-6
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