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
MEMORIZE THIS!!!
Protein Function in Membranes
Main function: Determine cell’s 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) Channelb) 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”
Carbohydrates
1
3
4
2
3
2
3
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 Transport2. 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
Diffusion movie
Equilibrium
When the concentration is equal on both sides
There is no net movement of materials Molecules are constantly in motion (don’t stop
moving!) However, CONCENTRATION STAYS THE SAME!!!
Factors that Affect Diffusion
1. Concentrationa) Of solute or solvent/s
2. Temperature3. Pressure4. 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 concentrations)
Ex: Placing egg in corn syrup Ex: onion cell w/ salt solution added
Osmosis movie
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
Facilitated Diff movie
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
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