Transport Mechanisms Flashcards
In Transport Mechanisms, in which part of the cell are we particularly interested in?
The Cell Membrane
What are the Permeability Characteristics of the Cell Membrane?
Highly permeable to:
- H20, Lipid-soluble substances, Dissolved Gases (O2, CO2) and small uncharged molecules
Less permeable to:
- Larger molecules and charged particles
Impermeable to:
-Very large molecules
Describe the Cell Membrane.
It has a Phospholipid Bilayer with hydrophilic heads (polar, on the outside) and hydrophobic tails (non-polar, on the inside)
What are hydrophilic heads composed of?
Carboxyl and hydrogen
What are hydrophobic tails composed of?
Tricglycerides and cholesterol
What is cholesterol’s function for in the Cell Membrane?
Acts as a buffer to prevent lower/increase TEMPERATURES for inhibiting/increasing FLUIDITY.
Involved in formation of vesicles that pinch off the plasma membrane
There are 2 types of protein pertaining to the Cell Membrane. What are they and where are they located?
Integral proteins: Closely associated with phospholipids (Trans-membrane)
Peripheral proteins: loosely associated on the cytoplasmic side
What is glycocalyx and what are it’s main functions?
It’s a fuzzy coating surrounding the cell membrane formed of glycans, glycoproteins and glycolipids
Functions: Contributes to cell-cell recognition, communication, adhesion and protection. Helps control vascular permeability
What are the functions of Plasma Membrane Proteins?
They act as selective or enzyme transporter, cell surface receptor or identity marker and cell-cell adhesion or attachment to the cytoskeleton
What are the 2 transmembrane pathways?
- Via the phospholipid bilayer
- Via interaction with a transmembrane protein
There are 2 Cell Membrane Transport Mechanisms. What are they and by which mechanisms?
- Passive (energy indepedent): Diffusion, Facilitated Diffusion and Osmosis
- Active (energy dependent): Active Transport (primary,secondary) and Pino/Phagocytosis
What is Diffusion?
It’s the movement of molecules from one location to another as a result of random thermal motion.
It happens even in the presence of a mechanical partition (membrane) as long as it is permeable
How does diffusion flux and net flux work?
Diffusion flux: It’s the amount of particles crossing a surface per unit time
Net flux: Movement of particles from high to low concentration
What happens to diffusion flux and net flux at equilibrium?
Diffusion flux: They become equal
Net Flux = 0
How does diffusion relates in terms of time and distance?
Diffusion time increases in proportion to the square distance travelled by the solute molecules
What affects Cell Membrane Diffusion?
- Mass of molecule
- Concentration gradient
- Lipid Solubility
- Electrical charge
- Ion Channels
- Membrane carriers
How can diffusion happen at the cellular level and what is it related to?
- Diffusion across the lipid bilayer
- Diffusion of ions through channels
It is related to the concentration gradient
What are Ion Channels?
Transmembrane proteins that show ion selectivity
How is the movement of an ion is affected?
By the presence of an electrical gradient and it’s own concentration gradient = electrochemical gradient
Ion channels can exist in open or closed state and may be gated but in what ways?
- Ligand gated (chemical messengers)
- Voltage gated
-Mechanically gated
Name the different Voltage Gated Ion Channels
Na+ channels
K+ channels
Ca+ channels
Cl- channels
Current flow through single ion channels depends upon what?
- Channel conductance
- Channel open time
- Frequency of channel opening
What is Mediated Transport?
The movement of ions and other molecules by integral membrane proteins called transporters or carriers.
What are the characteristics of Mediated Transport?
- Specificity (1 type of molecule transported)
- Saturation (Rate of transport when all binding sites are occupied - Transport max)
- Competition (When similar substances compete for binding sites)
What are the factors that determine Mediated Transport?
- Solute concentration
- Affinity of transporter for the solute
- Numbers of transporters
- Rate of transporter conformational change
What are the Mediated Transport Systems?
- Facilitated Diffusion (Passive)
- Active Transport via:
- Primary Active Transport
- Secondary Active Transport
Summarize the process of Facilitated Diffusion
- Transported Solute binds to Transporter Protein’s binding site
- Transporter binding site changes configuration as well as the protein
- Solute is delivered to other side of membrane
- Transporter resumes original configuration
What are the Characteristics of Facilitated Diffusion?
- Transporter Mediated
- Passive
- Net flux going from hight to low concentration
- Hormones may increase the number and/or affinity of transporters in some membranes
- Glut-4 transports glucose in muscle that increased by insulin
What are the Characteristics of Active Transport?
- Transporter-Mediated
- Requires supply of chemical energy (enzymatic hydrolysis of ATP)
- Susceptible to metabolic inhibitors
- Can transport solute AGAINST it’s concentration gradient
Explain Primary Active Transport
It involves the hydrolysis of ATP by a transporter.
Phosphorylation of the transporter changes the conformation of the transporter and it’s solute binding affinity
Describe Na+/K+-ATPase transport process
- Na+/K+-ATPase - ATP binds 3Na+
- Phosphorylation of ATP–>ADP
- Conformational change exposing Na to ECF and releasing them
- Change increase K+ affinity and binds it to site
- Dephosphorylation + return to original conformation
- Release of K+ in ICF
What are the other Active Transporters and their functions?
- Ca2+-ATP-ase: Maintain low intracellular Ca2+ levels
- H+-ATP-ase: Maintain low lysosomal pH
- H+/K+-ATPase: Acidification of the stomach
How does Secondary Active Transport work?
Movement of Na+ down its concentration gradient (High to low) is coupled to the transport of another solute molecule (Glucose or aa).
It uses the energy stored of the electrochemical gradient to move both molecules
Summarize Na+/K+ pump
- Na+ binds to a transporter outside the cell allowing glucose or aa to also bind
- Change in conformation = release of both molecules into the cell
- Back to normal conformity and Na+ is extruded from cell by Na+/K+ -ATPase
Describe the 2 Secondary Active Transport Mechanisms
Symport: When solute x is transported in the same direction as Na+
Antiport: When solute x is transported in the opposite direction as Na+
Name examples of Symports and Antiports
Symports: Na+/HCO3- cotransporter, Na+/aa cotransporter, Na+/glucose cotransporter
Antiports: Na+/H+ exchanger, Na+/Ca2+ exchanger
What is Endocytosis and Exocytosis?
They are an active transport mechanisms involving participation of the cell membrane itself
Endocytosis: Cell Membrane invaginates and pinches off to form a vesicle
Exocytosis: An intracellular vesicle fuses with the cell membrane and its contents are released into ECF
What are the 2 types of Exocytosis?
Constitutive Exocytosis: Non-Regulated. Replace plasma membrane, deliver membrane proteins to CM and get rid of substances from the cell
Regulated Exocytosis: Tends to be triggered by extracellular signals and the increase of cytosolic Ca2+. Responsible for secretion of: Hormones, Digestive Enzymes and Neurotransmitters
There are 3 types of Endocytosis. What are they?
- Pinocytosis (Fluid Encocytosis)
- Phagocytosis
- Receptor Mediated Endocytosis
Explain Pinocytosis
Endocytotic vesicle that engulfs ECF including present solutes. Fuse with other vesicles like endosomes or lysosomes. Non-specific.
Explain Phagocytosis
Pseudopodia fold around particle and form a phagosome that fuses with lysosomes. Specific and triggered
Explain Clathrin-dependent receptor -mediated endocytosis
- Ligands bind to receptor ie cholesterol
- Clathrin recruited to PM and linked to ligand-receptor
- Clathrin coated pit forms a vesicle
- Clathrin released from vesicle and recycled to PM
- Vesicles can: Fuse with organelle, endosomes, lysosomes or PM
Explain Potocytosis
Molecules are sequestred and transported by tiny vesicles calle caveolae. They can:
- Deliver contents in cell cytoplasm, endoplasmic reticulum, organelle or PM
Implicated in uptake of vitamins (low M.W molecule)
What is osmosis?
The net diffusion of H2O accross a semipermeable membrane
What is Osmotic Pressure?
The pressure required to prevent the movement of water across a semi-permeable membrane.
Describe the capillary wall.
A single layer of flattened endothelial cells and a supporting basement membrane
Describe Capillary Structure and Permeability.
Structure: Endothelial cell separated by narrow water filled pores called intercellular clefts.
Permeability: via intercellular clefts and endo-exocytosis
How does transport happened across capillary wall?
Diffusion
Transcytosis (endocytosis)
Bulk Flow: Plasma<–ECF–>ISF
