Cell Physiology L2/3: Osmosis and Diffusion Flashcards
What is homeostasis?
maintain a stable internal environment for all those critical physiological processes to occur
How is homeostasis maintained?
- Each cell contributes by the exchange of nutrients to and from the intra- and extra-cellular fluid in specialised ways.
- Done by cell membrane
What does the phospholipid bilayer consist of?
- Hydrophilic (polar heads)
- Hydrophobic (non-polar tails)
What is the structural function of the phospholipid bilayer?
Gives cells shape
Is the phospholipid layer flexible? Explain.
Yes, is flexible
- Allows cells to change shape
Does the phospholipid layer acts as a barrier? Explain.
Yes, is a barrier
- Prevents water soluble substances passing through
What is the phospholipid layer permeable to? Explain.
Permeable to:
- Lipid-soluble molecules (eg. steroid)
- Small uncharged molecules (eg. O2, CO2, urea can move freely between phospholipids)
What is the phospholipid layer impermeable to? Explain.
Impermeable to:
- ions & large molecules (e.g. sodium, proteins, glucose)
Is the phospholipid layer permeable to H20?
Yes, but is a bit more tricky- thorough osmosis
What simple diffusion?
Anything which passes directly through the bilayer
How does simple diffusion occur?
- Driven by concentration gradient
- The bigger the gradient (difference) –> faster the molecules will pass through
What are the 2 things that the movement across a membrane depends on?
- Concentration gradient
- Permeability of membrane
What is Fick’s Law?
All factors affect simple diffusion through a membrane
What is osmosis?
Movement of water from a high water concentration to a low water concentration
- Water moves from high [water] low [water]
- Low [solute] –> high [solute]
Why would osmosis not occur?
- Water moves down its (concentration) activity gradient o No activity = no osmosis
- Water activity same on both sides because [solute] same on both sides.
- No net flux
For osmosis, in the presence of solutes, there will be _____ (increased/decreased activity)? Why?
Decreased; water is a solvent
If there is a non-penetrating solute occurring on one side of the membrane, will osmosis occur? How?
Yes, water activity gradient
- Net flux to the side of the non-penetrating solute
Do all solutes behave the same in terms of influencing water activity, and thus osmosis?
No- differ in 2 ways
What are the 2 different ways that solutes behave, which influences water activity and in turn osmosis?
- Whether they are (as already considered)
- membrane penetrating (e.g. urea), or
- non-penetrating (e.g. glucose and ions).
- Whether they dissociate in solution.
- e.g. compare solutions of glucose and sodium chloride (NaCl)
- Break apart = more particles in water = eg. NaCl = Na+2 + Cl- = higher solute concentration = need twice as much water = water activity is lowest and every molecule dissociates into 2 ions.
How do we describe the difference between solutions that contain solutes that dissociate in solution and those that do not?
Osmolarity of solution
What is osmolarity (equation)? Use glucose and NaCl as examples.
Osmolarity of a solution = ∑ {(solute concentration) x (#dissociated species)}
E.g.
- 1M glucose = (1M x 1) = 1 Osmolar (OsM)
- 1M NaCl = (1M x 2) = 2 Osmolar (OsM)
- 2M CaCl2 + 0.5M KCl = (2M x 3) CaCl2 + (0.5M x 2) KCl = 7 OsM
N.B. Water activity is inversely proportional to osmolarity (i.e. water activity is low when osmolarity is high)
Water activity is __________(proportional/inversely proportional) to osmolarity.
inversely proportional (i.e. water activity is low when osmolarity is high)
What is the importance of osmosis?
Cell volume (keep shape)
Intracellular fluid (ICF) is normally _____ mOsM
300mOsM
Explain if any processes occur with 0.15M of NaCl?
(i) as ions, Na+ and Cl- are non-penetrating solutes
(ii) the osmolarity of this solution is: (0.15M x 2) = 0.3 OsM = 300 mOsM* *same as normal ICF
- –> no osmolarity gradient
- –> no water activity gradient – no osmosis
- –> no change in cell volume
**Intracellular fluid (ICF) is normally 300 mOsM**
What is tonicity?
Effect of bathing solutions on cell voume
What are the 3 types of tonicity?
- Isotonic: no change on cell volume
- Hypotonic: cells swell
- Hypertonic: cells shrink
What is the cause of the 3 types of tonicity?
If ECF in body is too weak or too concentrated (i.e hydration) your cells swell or shrink
What is an isotonic cell?
no change on cell volume
What is a hypotonic cell?
cells swell
What is a hypertonic cell?
cells shrink
Specialised membrane proteins enable trans-membrane solute movement (carrier-mediated transport). What are the 4 types?
- Simple diffusion (of ions, through channels) 2
- Facilitated diffusion (larger molecules)
- Primary active transport
-
Secondary active transport
- (both requiring energy to move solutes against their concentration gradient)
- [low] = [high]
All display: specificity, saturation and competition
The 4 types of carrier-mediated transport all display ___, _____ and ______.
specificity, saturation and competition
What is specificity in terms of carrier-mediated transport?
Each carrier protein is specialised to transport one or, at most, a few closely related substances
What is saturation in terms of carrier-mediated transport?
Limited number of carriers in the membrane + limited number of binding sites for a particular substance, therefore can become ‘full’ - known as the transport maximum (Tmax)
What is competition in terms of carrier-mediated transport?
If closely related substances can use the same carrier they will compete for the use of that carrier
Carrier-mediated transport: What is simple diffusion?
Channels are an easy way to go ions
They come in many types, depending on:
- Ion selectivity (e.g. for Na+,K+, Cl-, etc.)
- Gating
- ungated: always open (leak in/out)
- gated: voltage, ligand (hormones) or mechanically-gated (move, change shape)
What are the typical ion concentrations in simple diffusion?
Carrier-mediated transport: What is facilitated diffusion? What are the 4 steps?
Step 1: Transported solute binds weakly to a carrier protein (trans-membrane protein, but no pore)
Step 2: Binding of the solute molecules induces change in conformation of carrier protein Once molecule is full = Flip and change shape
Step 3: Transported solute detaches from carrier protein in area of low concentration
Step 4: Carrier protein reverts to original shape
Carrier-mediated transport: What is step 1 of facilitated diffusion?
Step 1: Transported solute binds weakly to a carrier protein (trans-membrane protein, but no pore)
Carrier-mediated transport: What is step 2 of facilitated diffusion?
Step 2: Binding of the solute molecules induces change in conformation of carrier protein
Once molecule is full = Flip and change shape
Carrier-mediated transport: What is step 3 of facilitated diffusion?
Step 3: Transported solute detaches from carrier protein in area of low concentration
Carrier-mediated transport: What is step 4 of facilitated diffusion?
Step 4: Carrier protein reverts to original shape
Carrier-mediated transport: What is active transport in primary and secondary active transport?
- “uphill” movement, thus work to be done …needs energy
- energy direct from ATP hydrolysis: …primary active transport
- energy derived from existing concentration
- gradient of another solute: … secondary active transport
- both involve a protein carrier that binds one or more solutes, thus substrate specific & saturable
Uphill movement of active transport needs _____.
Energy
Primary active transport means that energy in active transport is direct from _________.
ATP hydrolysis
Energy in active transport is derived from _________ (new/existing) concentration.
Existing
Secondary active transport means that the _____ of another solute.
gradient
Active transport both involve a protein carrier that binds one or more solutes, thus______ and _______.
substrate specific; saturable
What is the most important thing about primary active transport? Why?
- Most important is Na2+/K+ pump
- Maintain low Na2+ and high K+ (inside cells)
- Pump out 3 Na2+
- Pump in 2 K+
What is secondary active transport?
- Similar steps as those in facilitated diffusion but ‘extra’ solute transported against concentration gradient
- Example: Na+/glucose symport
- Moves glucose from intestine into epithelial cells
- Na+ concentration gradient used to drive glucose ‘uphill’
What are 6 important active transporters?
- Na+/K+ – ubiquitous sodium/potassium pump
- K+/H+ – gastric acid pump (stomach)
- Ca2+ – ubiquitous calcium extrusion pump
- H+ – ubiquitous acid extrusion pump
- Na+/H+ antiport – renal tubules (removes acid from body, makes urine acidic)
- Na+-coupled amino acid transporters (allow cells to take in amino acids to make proteins)
In secondary active transport, there are similar steps as those in ____ diffusion but ‘extra’ solute transported ______ (towards/against) concentration gradient
facilitated; against
In secondary active transport, moves glucose from ______ into _______ cell.
intestine; epithelial cell
Water moves from an area of ___ (high/low)osmolarity to an area of ____ (high/low) osmolarity.
low; high
