Lecture 3: Membrane Transport Flashcards
What is diffusion?
The movement of substances across the membrane from high to low concentration
What is required for the net diffusion of a substance across a membrane?
- A driving force
- The membrane must be permeable to the substance
When both are present the substance will reach equilibrium over time, at the same concentration on both sides of the membrane
What is the relative rate of diffusion over short/long distances?
Diffusion is very rapid over short distances, but very slow over long distances
What does the time taken to reach equilibrium via diffusion determine?
- The upper limit of cell dimensions
- Large multicellular organisms requiring a circulatory system
If the distance is 10 μm, how long would it take to reach equilibrium?
0.0456s
What is flux? Give the units.
Refer to slide 9.
The amount of substance crossing a given surface area per unit time (how fast a substance is diffusing)
* moles/(cm2/sec)
What is Fick’s First Law of Diffusion?
J = P A (Co- Ci)
▪ J = net flux
▪ P = permeability coefficient (at a given temperature)
▪ A = membrane surface area
▪ (Co-Ci) = the concentration difference across the membrane
What is osmosis?
The process by which water moves from an area of high conc. to low conc. of water
- Driving force = difference in osmolarity
- Water diffuses into and out of cells via aquaporins
What is osmolarity? Give units.
The total number of solute particles dissolved in the solution
* mosmol/L
What type of molecules DON’T dissolve in water?
Molecules bound by covalent bonds don’t dissociate in water e.g. Glucose, Urea
* Concentration: 100 mmol/L solution of glucose
* Osmolarity: 100 mosmol/L solution of glucose
What type of molecules dissolve in water?
Molecules joined by ionic bonds dissociate in water e.g. NaCl
* Concentration: 100 mmol/L of NaCl
* Osmolarity: 100 mosmol/L of Na+ and 100 mosmol/L of Cl- : 200 mosmol/L of NaCl
What osmolarity is maintained in the ECF and ICF of the human body?
275 – 300 mosmol/L
What is a hyposmotic solution?
▪ LESS solute molecules per L
▪ LOWER osmolarity
▪ HIGHER water concentration
What is an isosomtic solution?
▪ SAME number of solute molecules per L
▪ SAME osmolarity
▪ SAME water concentration
What is a hyperosmotic solution?
▪ MORE solute molecules per L
▪ HIGHER osmolarity
▪ LOWER water concentration
What is tonicity?
The effect a solution has on the cell volume
What 3 factors does tonicity depend on?
- Electrochemical gradient
- Permeability to that solute
- Solute enters or leaves the cell
What is a hypotonic solution?
Osmolarity outside is LOWER than inside the cell
▪ Water concentration outside is HIGHER than inside the cells
▪ NET movement of water INTO the cells
▪ Cells swell
What is an isotonic solution?
Same osmolarity inside and outside the cell
▪ NO NET movement of water
▪ Cells stay the SAME size
What is a hypertonic solution?
Osmolarity outside is HIGHER than inside the cell
▪ Water concentration outside is LOWER than inside the cells
▪ NET movement of water OUT of the cells
▪ Cells shrink
What occurs in simple diffusion?
- Solutes move down their concentration gradient directly across the phospholipid bilayer e.g. O2
- Flux is limited only by the concentration gradient
What is mediated transport? Give examples.
Movement of substances across the plasma membrane through membrane proteins (channels and transporters)
e.g. Facilitated diffusion and active transport; secondary and primary
What is facilitated diffusion?
- Solutes move down their concentration gradient and cross the membrane through a channel or transporter
- Flux is limited by the number of available transporters
What are channels?
Channels transition between open and closed state (gating)
- Ligand gated
- Voltage gated
What are carriers/transport proteins?
- Solute acts as a ligand that binds to the transporter protein
- Binding triggers a conformational change in the protein
- Causes release of the solute on the other side of the membrane
What is active transport?
Energy is used (directly or indirectly) to move substances against their electrochemical gradient
How does primary active transport use energy?
Directly uses energy to maintain an electrochemical gradient e.g. Na+/K+ ATPase
How does secondary active transport use energy?
Indirectly uses energy by moving one solute (e.g. Na+) down it’s electrochemical gradient to move another solute (e.g. glucose) up it’s electrochemical gradient
What occurs in cotransport (symport)? Give an example.
Solutes move in the same direction e.g. SGLT1 (sodium-glucose transporter) - Uses the Sodium electrochemical gradient as a driving force
Sodium concentration gradient
* High = outside
* Low = inside
Sodium electrical gradient
* Negative RMP
What occurs in countertransport (antiport)? Give an example.
Solutes move in the opposite direction e.g. NHE (sodium-hydrogen exchanger)
What does exo and endocytosis allow substances to do?
It allows substances to enter or leave the cell without passing through the plasma membrane
What are the 3 forms of endocytosis?
- Pinocytosis
- Phagocytosis
- Receptor-mediated Endocytosis
What is endocytosis?
The plasma membrane folds inwards to form small pockets which pinch off to form membrane-bound vesicles
What is pinocytosis?
Solutes and water are non-specifically brought into the cell from the ECF via vesicles
What is phagocytosis?
Specialised cells form extensions of the membrane which engulf bacteria or debris. The vesicles then fuse with lysosomes that destroy the vesicle contents.
What is receptor-mediated endocytosis?
- Relatively specific process
- Binding of an extracellular molecule to a receptor in the plasma membrane triggers the process e.g. cholesterol, transferrin-iron
What is exocytosis?
Membrane-bound vesicles fuse with the plasma membrane discharging their contents outside the cell
What are the 2 roles of exocytosis?
- Replaces membrane removed from the cell by endocytosis
- Controlled discharge of large molecules into the extracellular space e.g. Exporting/secreting substances - such as peptide hormones
How is exocytosis triggered? Give an example.
By an increase in cytosolic calcium concentration e.g. nerve terminal - where release of neurotransmitters from nerve terminals by exocytosis is regulated by calcium flux in the nerve terminal
