Cell Physiology L2/3: Osmosis and Diffusion Flashcards by Vanessa Chuah

What is homeostasis?

maintain a stable internal environment for all those critical physiological processes to occur

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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

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What does the phospholipid bilayer consist of?

Hydrophilic (polar heads)
Hydrophobic (non-polar tails)

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What is the structural function of the phospholipid bilayer?

Gives cells shape

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Is the phospholipid layer flexible? Explain.

Yes, is flexible

Allows cells to change shape

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Does the phospholipid layer acts as a barrier? Explain.

Yes, is a barrier

Prevents water soluble substances passing through

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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)

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What is the phospholipid layer impermeable to? Explain.

Impermeable to:

ions & large molecules (e.g. sodium, proteins, glucose)

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Is the phospholipid layer permeable to H₂0?

Yes, but is a bit more tricky- thorough osmosis

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What simple diffusion?

Anything which passes directly through the bilayer

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How does simple diffusion occur?

Driven by concentration gradient
- The bigger the gradient (difference) –> faster the molecules will pass through

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What are the 2 things that the movement across a membrane depends on?

Concentration gradient
Permeability of membrane

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What is Fick’s Law?

All factors affect simple diffusion through a membrane

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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]

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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

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For osmosis, in the presence of solutes, there will be _____ (increased/decreased activity)? Why?

Decreased; water is a solvent

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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

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Do all solutes behave the same in terms of influencing water activity, and thus osmosis?

No- differ in 2 ways

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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.

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How do we describe the difference between solutions that contain solutes that dissociate in solution and those that do not?

Osmolarity of solution

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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)

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Water activity is __________(proportional/inversely proportional) to osmolarity.

inversely proportional (i.e. water activity is low when osmolarity is high)

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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?

1. Isotonic: no change on cell volume 2. Hypotonic: cells swell 3. 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?

1. **Simple diffusion** (of ions, through channels) 2 2. **Facilitated diffusion** (larger molecules) 3. **Primary active transport** 4. **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 (T_max)

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?

1. Na+/K+ – ubiquitous sodium/potassium pump 2. K+/H+ – gastric acid pump (stomach) 3. Ca2+ – ubiquitous calcium extrusion pump 4. H+ – ubiquitous acid extrusion pump 5. Na+/H+ antiport – renal tubules (removes acid from body, makes urine acidic) 6. 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