L2. Diffusion Flashcards
Diffusion?
Substances move by random thermal motion from high to low concentration until they become evenly uniformly distributed in the volume
- Diffusion is very rapid over short distances, but very slow over long distances
What are the two things required for diffusion?
- A driving force
- The membrane must be permeable to the substance
What are the determinants of diffusion speed?
The time it takes for substances to reach equilibrium via diffusion is an important determinant of:
1. The upper limits of cell dimensions
2. Large multicellular organisms requiring a circulatory system
What is Flux?
The amount of substance crossing a given surface area per unit time (moles/(cm2/sec))
Osmosis?
Diffusion of water across a selectively permeable membrane
- Water diffuses through aquaporins (also from high to low concentraion)
How can you change the “concentration” of water?
- Adding solute lowers the concentration of water
- Decreasing solutes increases the concentration of water
Concentration is measured in mosmol/L
Define osmolarity?
The total number of solute particles dissolved in the solution
–> It’s a number and is a measured concentration of solutes in comparison to normal ICF
–> It makes no reference to the biological activity of the solutes
Dissociation in water?
- Molecules bound by covalent bonds don’t dissociate in water e.g. glucose, urea
(e.g. concentration: 100 mmol/L solution of glucose,
osmolarity: 100 mosmol/L solution of glucose) - Molecules joined by ionic bonds dissociate in water e.g. NaCl (e.g. concentration: 100 mmol/L of NaCl, osmolarity: 100 mosmol/L of Na+ and 100 mosmol/L of Cl- = 200 mosmol/L of NaCl
What is the normal osmolarity of the ICF and ECF?
275 - 300 mosmol/L
Hyposmotic solution?
- Less solute molecules per L
- Lower osmolarity
- Higher water concentration
- < 275
Isosmotic solution?
- Same number of solute molecules per L
- Same osmolarity
- Same water concentration
Hyperosmotic solution?
- More solute molecules per L
- Higher osmolarity
- Lower water concentration
- > 275
Driving force for osmosis?
- Water diffuses from an area of high water concentration (lower osmolarity) to an area of low water concentration (higher osmolarity)
- The driving force for osmosis (water movement) is a difference in osmolarity
Define tonicity?
The effect a solution has on the cell volume
–> A qualitative description of what happens to the size of the cell when it’s placed in the solution
–> Tonicity (whether the cell changes volume) depends on whether there’s an electrochemical gradient for a solute to cross the cell membrane and whether the cell membrane is permeable to that solute (penetrating vs non-penetrating solutes)
Hypotonic solution?
- Osmolarity outside is lower than inside the cell
- Water concentration outside is higher that inside the cells
- Net movement of water into the cells
- Cells swell
Isotonic solution?
- Same osmolarity inside and outside the cell
- No net movement of water
- Cells stay the same size
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
Simple diffusion?
Solutes move down their concentration gradient directly across the phospholipid bilayer e.g. O2
- Flux is limited only by the concentration gradient
Facilitated mediated diffusion?
Movement of substances across the plasma membrane through open channels or other carrier protein transporters
- Diffusion provides the driving force and a channel or transporter provides the pathway
Channels?
- Channels transition between open and closed state (gating)
- Can be ligand or voltage gated
Carrier/transport protein?
The solute acts as a ligand that binds to the transporter protein
- The binding triggers a conformational change in the protein that causes the release of the solute on the other side of the membrane
Active transport mediated transport?
Transport systems for moving substance across the plasma membrane through membrane proteins (channels and transporters)
Active transport?
Energy-requiring process that moves material across a cell membrane against their electrochemical gradient
Primary active transport?
Directly uses energy to maintain an electrochemical gradient e.g. Na+/K+ATPase
Secondary active transport?
Indirectly uses energy by moving one solute (e.g. Na+) down its electrochemical gradient to move another solute (e.g. glucose) up its electrochemical gradient
- Cotransport (symport) e.g. SGLT1 (sodium-glucose transporter)
- Countertransport (antiport) e.g. NHE (sodium-hydrogen exchanger)
Endocytosis?
A way in which some substances may enter the cell without passing through the plasma membrane
Three forms;
1. Pinocytosis
2. Phagocytosis
3. Receptor-mediated endocytosis
Pinocytosis?
Solutes and water are non specifically brought into the cell from the ECF via vesicles
Phagocytosis?
- Specialised cells form extensions of the membrane which engulf bacteria or debris
- The vesicles then fuse with lysosomes that destroy the vesicle contents
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
Exocytosis?
- Membrane bound vesicles fuse with the plasma membrane discharging their contents outside the cell
- This serves to replace membrane removed from the cell by endocytosis
- Provides a route for controlled discharge of large molecules into the extracellular space e.g. exporting/secreting substances - such as peptide hormones
Exocytosis is generally triggered by events that lead to a transient increase in cytosolic calcium concentration e.g. nerve terminal releasing neurotransmitters from nerve terminals by exocytosis is regulated by calcium flux in the nerve terminal
