2. Transport Mechanisms Flashcards
Cell membrane less permeable to
- larger molecules
- charged molecules
Cell membrane highly permeable to
- water
- lipid soluble substances
- dissolved gases
- small uncharged molecules
Cell membrane impermeable to
very large molecules
Phospholipid bilayer components (4)
- cholesterol
- integral proteins
- peripheral proteins
- glycocalyx
cholesterol function
maintains membrane fluidity
glycocalyx function (2)
- contributes to cell-cell recognition, communication, adhesion and protection
- helps control vascular permeability
integral proteins
transmembrane: cross the the membrane
peripheral proteins
mostly on cytoplasmic side
functions of plasma membrane proteins (6)
- selective transport
- enzymatic activity
- cell surface receptor
- cell surface identity marker
- cell adhesion
- attachment to cytoskeleton
Passive transport mechanisms (3)
- diffusion
- facilitated diffusion
- osmosis
Active transport mechanisms (3)
- primary active transport
- secondary active transport
- pino/phagocytosis
Diffusion definition
movement of molecules down a concentration gradient
diffusion equilibrium, net flux =
net flux = 0
Fick’s Law of Diffusion (formula)
J = PA(C0 - Ci)
J = flux
P = permeability constant
A = surface area
C0 - Ci = concentration gradient
factors affecting diffusion (6)
- mass of molecule
- concentration gradient
- lipid solubility
- electrical charge
- ion channels
- membrane carriers
Diffusion is effective over long distances, True/False?
False
ion channel
transmembrane protein that show ion selectivity
electrochemical gradient
simultaneous existence of electrical and concentration gradient for a particular ion
gating
conformational changes that open/close iron channels
3 gating ways:
- ligand gated = compound binds to channel
- voltage gated
- mechanically gated = stress
current flow through ion channels depends on: (3)
- channel conductance
- channel open time
- frequency of channel opening
Facilitated diffusion (def)
use of carriers to transport molecules down concentration gradient
Is facilitated diffusion active or passive?
passive
carrier/transporter is an
integral membrane protein
facilitated diffusion affected by: (4)
- solute concentration
- affinity of transporter for solute
- number of transporters
- rate of transporter conformational change
3 characteristics of mediated transports
- specificity
- saturation
- competition
Transport maximum (Tm)
when all binding sites on all transporters are occupied
steps for facilitated diffusion (4)
- solute binds transporter
- transporter changes configuration
- solute delivered to other side of membrane
- transporter resumes its original configuration
active transport
use of carriers to transport molecules against a concentration gradient using ATP
primary active transport
- involves hydrolysis of ATP by a carrier
- phosphorylation of carrier changes conformation of carrier and its solute binding affinity
example of primary active transport
Na+/K+ pump (ATPase):
- ATP binds to pump: 3Na+ bind to carrier
- phosphorylation: Na+ released outside membrane
- 2K+ binds to carrier
- dephosphorylation: K+ released inside membrane
secondary active transport
uses energy stored of electrochemical gradient to move both Na+ and the transported solute against the solute molecule’s concentration gradient
secondary active transport: glucose example
- Na+ binds to transporter outside the cell, allowing glucose to bind to the same carrier
- change in configuration: transporter delivers Na+ and glucose into cell
- transporter reverts to its original configuration
- Na+ removed from cell by ATPase
secondary active transport mechanisms (2)
- symport/cotransport
- antiport/counterport
symport/cotransport
solute transported in same direction as Na+
-> HCO3, amino acid, glucose
antiport/counterport/exchange
solute transported in opposite direction to Na+
-> H+, Ca2+
endocytosis/exocytosis
active transport mechanisms involving participation of cell membrane itself
3 types of endocytosis
- pinocytosis
- phagocytosis
- receptor-mediated endocytosis
endocytosis
cell membrane invaginates and pinches off to form a vesicle
pinocytosis - cell drinking
- non-specific and constitutive
1. endocytotic vesicle engulfs extracellular fluid
2. vesicles travel into cytoplasm and fuse with other vesicles
phagocytosis - cell eating
- specific and triggered
1. extensions of cell membrane (pseudopodia) fold around a particle to engulf it
2. pseudopodia fuse to form large vesicles that pinch off the membrane
3. created phagosome migrates to and fuses with lysosomes
4. phagosome content destroyed
receptor-mediated endocytosis
molecules in extracellular fluid bind with high affinity to specific protein receptors on plasma membrane
2 types of receptor-mediated endocytosis
- clathrin-dependent
- potocytosis
clathrin-dependent receptor-mediated endocytosis (process)
- ligand binds receptors on cell which undergoes conformational change
- adaptor proteins link the ligand-receptor to clathrin, leading to aggregation go ligand-bound receptors
- clathrin-coated pit formed and invaginates to form clathrin-coded vesicle
- vesicle pinches off, shedding its clathrin coat
- receptors and clathrin protein recycled back to cell membrane
example of clathrin dependent
low density lipoproteins (LDL) –> recognised by PM LDL receptors
potocytosis receptor-mediated endocytosis
- process by which molecules are sequestered and transported by tiny vesicles (caveolae)
- caveolae are clathrin independent: can deliver contents directly into organelles
- implicated in uptake of smaller molecules (lower Mr)
exocytosis
process of moving material from inside the cell to outside
2 types of exocytosis
- constitutive
- regulated
constitutive exocytosis
- continual and non-regulated
functions:
-replace plasma membrane
- deliver membrane proteins to cell membrane
- getting rid of substances from cell
regulated exocytosis
- triggered by extracellular signals and the increase of systolic Ca2+
- secretes hormones, digestive enzymes and neurotransmitters
osmosis
net diffusion of water across a semipermeable membrane
aquaporins
facilitate osmosis by forming water permeable channels
osmotic pressure
pressure required to prevent the movement of water across a semi-permeable membrane
osmotic pressure =
difference in hydrostatic pressures of 2 solutions
osmotic pressure is proportional to…
the number of particles in solution per unit volume
osmolarity (osm)
total solute concentration of a solution
1 osm =
1 mol of solute particles = 1 osm/l
isosmotic
solutions which have the same osmolarity as normal extracellular solution = 300mOsm
hyposmotic
solutions which have an osmolarity < 300mOsm
hyperosmotic
solutions which have an osmolarity > 300mOsm
non-penetrating
particles unable to cross membrane
–> Na+
isotonic
solution has concentration of non penetrating solute particles = 300mOsm
-> no net shift
hypotonic
solution has concentration of non penetrating solute particles < 300mOsm
-> water will enter cell: cell swelling
hypertonic
solution has concentration of non penetrating solute particles > 300mOsm
-> water will leave cell: cell shrinking
capillary transport mechanisms (3)
- Diffusion (across membrane or through pores)
- Transcytosis
- Bulk flow
transcytosis
exchangeable proteins are moved across by vesicular transport through endocytosis (to lumen side), then exocytosis (to instertital side)
bulk flow
distributes extracellular fluid volume between plasma and ISF
-> magnitude is proportional to hydrostatic pressure difference between plasma and ISF
Capillary wall allows protein-free plasma to move from capillaries to ISF. True/False?
True
