Transport Mechanisms Flashcards
Permeability of Cell Membrane
Permeable to water, lipid soluble substances, gases, non-polar molecules
Impermeable to large particles
Cell Membrane composition
Phospholipid Bilyaer
Cholesterol
Proteins
Glycocalyx
Cholesterol Role
Increase membrane fluidity
Helps pinch of vesicles
High temps: stabilize cell membrane
Proteins
Integral: transmembrane, amphipathic
Peripheral: cytoplasmic side
Glycocalyx
Carbs on the extracellular surface
Helps protect from infection and identification between cells
Protein Functions
Transport
Enzymes
Receptors
Identification markers
Cell adhesion
Attachement to cytoskeleton
Passive Transport
Diffusion
Carrier Mediated Facilitated Diffusion
Osmosis
Active Transport
Carrier-mediated active transport
Pinocytosis
Phagocytosis
Diffusion
Movement of molecules from one location to another due to thermal motion
Follows concentration gradient (high –> low)
Diffusion Equilibrium
Net flux = 0
Must be permeable to diffused molecules
Fick’s Law (Rate of Diffusion)
J = PA(Co - Ci)
- Diffusion time increases in proportion to square of distance travelled by solute (so only efficient over small distances)
Factors influencing diffusion
- Mass of molecules
- Concentration gradient
- Lipid solubility
- Electrical charge
- Availability
Ion Channels
Selectivity based on diameter and distribution of charges lining channel
Mediated Transport Systems
Movement of ions by integral proteins
1. Specific to certain molecules
2. Saturation: rate decreases when all binding sites are full
3. Competition: available binding sites
Factors Determining Flux Magnitude
- Solute concentration
- Affinity of transporter for solute
- Number of transporters
- Rate of transporter conformation change
List transports that are mediated systems
- Facilitated Diffusion
- Active transport
a) Primary active transport
b) Secondary active transport
Facilitated Diffusion
Diffusion with transporter (high–> low concentration)
a) solute binds to transporter
b) transporter changes conformation
c) solute releases on other side
d) transporter returns to original conf
Primary Active Transport
Hydrolysis of ATP by carrier
a) phosphorylation of transporter changes its conformation
sodium-potassium pump: 2K+ go in, 3 Na+
Secondary Active Transport
movement of Na+ down its concentration gradient is coupled to transport other molecules against their concentration gradient
Steps of secondary active transport
- Na+ binds to transport protein outside cell, allowing glucose to bind to same carrier
- Change in configuration transporter delivers Na+ and glucose to other side
- Transporter returns to original conformation
Endocytosis
membrane invaginates + pinches off to form a vesicle (enters)
Exocytosis
intracellular vesicle fuses with membrane + releases product to ECF
Constitutive (exocytosis)
Non-regulated, replaces membrane, delivers membrane proteins and gets rid of substances inside
Regulated (exocytosis)
Triggered by extracellular signals + increase of cytosolic Ca2+
Responsible for secretion of hormones, digestive enzymes and neurotransmitters
Pinocytosis (endocytosis)
Fluid endocytosis = vesicle engulfs extracellular fluid (w/ solutes)
Non-specific
Phagocytosis (endocytosis)
Cells bind + internalize particle matter = forms a phagosome vesicle in cytoplasm –> lysosome binds and degrades content
Specific and triggered
Receptor Mediated Endocytosis
Ligands bind w/ high affinity to protein receptors on membrane
Clathrin-Dependent Receptor Mediated Endo.
Ligands bind to receptors at membrane surface
Clathrin coats the inside
Clathrin coated pit vesicle forms
Clathrin is released from vesicle
Endosome binds with pit, separating the receptors and ligands = different ends, and breaks appart
Receptors return to surface
Ligand transport vesicle binds with lysosome = breaks them down into smaller pieces and released in cytoplasm
Potocytosis
Molecules are sequestered and transported by caveolae
Transported to cytoplasm or endoplasmic reticulum
Only low molecular weight molecules i.e: vitamins
Osmosis
Net diffusion of water across semipermeable membrane
- only water moves
Aquaporins
Proteins that facilitate the diffusion of water across membranes
Osmotic pressure
Opposes water movement
- pressure required to prevent water from moving across the membrane
- difference in hydrostatic pressures of solutions encourages water movement
Osmolarity of Cell
300 mOsm
Hypo osmotic
Osmolarity lower than 300 mOsm
Hyper osmotic
Osmolarity higher than 300 mOsm
Non Penetrating solutes
Unable to cross the membrane (they maintain the osmotic pressure)
Na+ is non penetrating
Isotonic Solution
300 mOsm of non penetrating solute
Hypotonic Solution
Less than 300 mOsm of non penetrating solute
- Causes water to enter cell = cell swells
Hypertonic Solution
Greater than 300 mOsm of non penetrating solute
- Causes water to leave = cell shrinks
Capillary wall transport
- Diffusion: through channels
- Transcytosis: ???
- Bulk flow: distributes ECF volume between plasma and ISF
