Lecture: Plasma Membrane and Transport
I.Structure of the Plasma Membrane
- plasma membrane - the surface encapsulating a cell
- Fluid Mosaic Model
1.bilayer of phospholipids
a. hydrophilic heads - P04 end "water"
"loving" attracted to water on inner/outer
parts of cell
b. hydrophobic tails - fatty acids "water"
"fearing" attracted to each other on inside
of bilayer
c. glycolipids - some carbohydrates attached
to outer lipids (involved in cell to cell
recognition)
d. cholesterol - regulates fluidity of
membrane
2. proteins interspersed throughout the membrane
a.functions of membrane proteins
i.receptors - hormones, neurotransmitters
ii.enzymes - reactions in & out of cell
iii.transport - ions and molecules
- integral proteins - inserted into the bilayer
i. transmembrane - across entire bilayer
c.peripheral proteins - on inner & outer surface
d. glycoproteins - carbohydrates on outer surface
i. glycocalyx - outer carbohydrate coat (cell recognition and identification)
3.plasma membrane is fluid: it can easily shift
& flow
a. two layers can slide over one another
b. some proteins float freely throughout
membrane
c. many proteins attached to cytoskeleton
i. allows for regional specialization
4.Features of Plasma Membrane
a. microvilli - fingerlike extensions of cell
i.found in kidney and intestine
ii.increases surface area for absorption
iii. actin filaments for support
b. tight junctions - cell-cell adhesion proteins
i.generally at surface of epithelium
ii.prevent passage between cells
iii. "seal" layer of cells into a sheet
c. desmosomes - anchor cells to cells &
basement
i. carbohydrates of glycoprotein
intermingle
ii. keratin filaments anchor to cytoplasm
iii. hemidesmosome - anchor to basement
membrane
II.Plasma Membrane Transport
A.General Features
1.interstitial fluid - bathes all cells and tissues
a. released by capillaries into organs/tissues
b. recaptured by lymph vessels back to heart
c. contains salts, nutrients, hormones, etc.
2.selectively permeable - only certain things pass
a. passive transport - nature does the work
b. active transport - cell must use energy
(ATP)
B.Passive Transport Processes (no cellular energy required)
1. diffusion - movement of particles from area
of HIGH concentration to area of LOW
concentration until equal
a. concentration gradient - difference in
concentration between HIGH and LOW
areas
i. larger gradient - larger driving force
ii. faster = higher temperature or smaller
particle
2. simple diffusion across the cell membrane
a. nonpolar molecules (oxygen, carbon
dioxide, urea)
i. oxygen blood (high) cells (low)
ii. CO2 cells (high) blood (low)
iii. ureacells (high) blood (low)
b. fat soluble molecules (small fats and
steroids)
3.osmosis - the movement of a solvent (such as WATER) from an area of LOW solute concentration (such as NaCl) to an area of HIGH solute concentration
solution = solvent + solute
(dissolving liquid) (dissolved particles)
- molarity - moles of solute / liters of solvent (moles/liter = Molar)
- mole - grams of substance = mol. wt. substance
l mole H = 1 gram H
1 mole C = 12 grams C
1 mole NaCl = 58 grams NaCl
1 mole C6H12O6 = 180 grams C6H12O6
58 grams NaCl/l liter water = 1 mole NaCl/liter = 1 Molar NaCl (lM NaCl)
180 g Glucose/1 liter water = 1 mole glucose/liter = 1 Molar glucose (1M Glucose)
b. osmolarity - measure of concentration of
particles in a solution
i. 1 molar Glucose = 1 osmol Glucose
ii.1 molar NaCl = 2 osmol NaCl
WHY? in water NaCl dissociates Na+ + Cl-
(for each salt molecule their are 2 parts)
Movement Across Membrane Permeable to Water Only (not solutes)
Conditions Water Movement Terminology
osmo(in) = osmo(out) no net movementisotonic
osmo(in) > osmo(out) water moves IN inside is hypertonic
osmo(in) < osmo(out) water moves OUT inside is hypotonic
c. osmotic pressure - driving force generated by
the concentration gradient
*the larger the difference in concentrations between the INSIDE and OUTSIDE, the larger the osmotic pressure (driving force is greater)
- hydrostatic pressure - pressure of cell wall in plant cells that balances the osmotic pressure, preventing more water from entering the cell
e. observable implications of osmosis
- crenate - water moves out and cell shrinks
- lyse - water moves in and cell bursts
f. clinical implications of osmosis
i. isotonic I.V. - Ringers (0.9% NaCl; 5% glucose)
ii. hypertonic I.V. - to treat edema (water excess)
iii. hypotonic I.V. - to treat dehydration
4. filtration - hydrostatic pressure > osmotic
pressure (Squeezing a leaky water balloon)
- WATER moves from HIGHER osmo LOWER osmo
5. facilitated diffusion - see-saw protein carries across or channels allow through (goes with the concentration gradient so it is still a form of passive transport)
a.carrier protein - "open outside" <-> "open inside"
i. very specific for the molecule transported
ii. uses energy of natural diffusion (water-
wheel)
iii. glucose carrier is typical
b. protein channels - passage of charged & polar
i. Na+, K+, Cl- channels are very specific
can be opened or closed on command
C. Active Transport Processes (energy of the cell required)
1. active transport - transport solutes against a concentration gradient (goes against diffusion)
a. solute pumps - Na+, K+, Ca++, amino acids
(relies on ATP energy source)
i.rely on energy of ATP to overcome forces of nature
ii.uniport - one specific particle only
iii. coupled system - two particles together
symport - same direction
antiport - opposite directions
b.Na+-K+ ATPase Pump - creates ion concentration gradient for cell [Na+]OUT HIGH; [K+]IN HIGH
i.ATP is used by this pump to move 3 Na+ out of the cell and bring 2 K+ into the cell
ii. Na+ will want to move INTO cell; K+ will want to move OUT of cell
2.bulk transport - cell membrane pouching process
a. exocytosis - cell vesicle moves to
membrane with contents, merges, then
releases material
i.hormone/neurotransmitter release; mucus secretion; expulsion of extracellular proteins (collagen, elastin, matrix)
b. endocytosis - engulfment by cell membrane pouch which then buds off into the cytoplasm
i.phagocytosis ("eat" "cell"" process") - plasma membrane raps around large mass
(bacteria, dead cell, cell debris)
phagosome lysosome (digestive enzymes)
macrophages - immune cells that engulf
ii. pinocytosis - "drink" "cell"" process"
iii. receptor-mediated endocytosis -
receptors on the cell surface bind to
desired molecule before the
engulfment
insulin, low density lipoproteins (LDL), and Fe++ can be ligands for such receptors
III.The Resting Membrane Potential (voltage acrossthe membrane)
A. voltage - energy that results from separation of charges (also called potential difference - potential)
1. The Na+-K+ ATPase Pump creates concentration gradients for both Na+ and K+
a.[Na+]OUT > [Na+]IN
b.[K+]IN > [K+]OUT
- Results in NET flow of positive charge out of the cell
1cycle = 3Na+ out & 2K+ in
3.Na+ Channels normally closed so that Na+ cannot easily move back into the cell.
4.K+ Channels normally slightly open so that K+ can slowly leak out
5.The net movement of Na+ and leaking of K+ to the outside of the cell causes a POTENTIAL DIFFERENCE (voltage) across the membrane.
6.resting membrane potentials for cells generally range: -20 mV to -200mV
7.electrochemical gradient - charge & concentration
i. Na+: {electro-IN; chemical-IN}
ii. K+: {chemical-OUT = electro-IN}
IV.Functions of Glycoproteins on Cell Membrane (Glycocalyx)
A.Determination of ABO Blood Types
1.Sugar moiety on glycoprotein of red blood
cell (RBC)
a.signature for immune response of foreign blood
B. Binding of Dangerous Toxins
1.proteins of cholera and tetanus bind to cell by identifying specific carbohydrates
on proteins
C. Identification of Specific Cell Types
1.Sperm knows egg by specific glycoproteins
2.Cell-cell interaction during embryogenesis and tissue differentiation
3.Immune cells identifying foreign cells and material such as bacteria, viruses, and cancer cells