Membrane transport lab
A close-up of the phospholipid bilayer. Small non-polar molecules slip straight through, glucose needs a carrier, ions need channels, and the Na⁺/K⁺-ATPase spends ATP to pump ions against their gradients.
Osmosis
Change the concentration of the surrounding solution. Water moves across the membrane towards the more concentrated side until water potentials balance; an animal cell may burst or shrink, while a plant cell is protected by its wall.
The secretory pathway
Follow one protein from gene to destination through the endomembrane system. The address label in its sequence decides where it ends up.
Build a cell for the job
Cells specialise by changing their mix of organelles. Pick a job, drag organelles from the tray into the empty cell (drag them out again to remove), then check whether your cell could do its work.
Key facts to remember
- Plasma osmolality is 275–295 mOsm/kg. Calculated osmolality ≈ 2[Na⁺] + glucose + urea (all in mmol/L).
- 0.9 % NaCl = 154 mmol/L of Na⁺ and of Cl⁻ ≈ 308 mOsm/L: isotonic. 5 % dextrose is iso-osmotic in the bag but hypotonic in the body, because the glucose is metabolised and leaves free water.
- Red cells swell and haemolyse below about 150 mOsm/L (0.45 % NaCl); they crenate in hypertonic fluid. Sterile water is never infused.
- This is the basis of the osmotic fragility test: normal red cells start to lyse at about 0.45 % NaCl and lyse completely by about 0.30–0.35 %. Spherocytes (hereditary spherocytosis) lyse at higher concentrations because they have less spare surface area; target cells (thalassaemia, iron deficiency) are more resistant.
- Na⁺/K⁺-ATPase: 3 Na⁺ out, 2 K⁺ in per ATP. Digoxin (like ouabain) blocks it; hypokalaemia (K⁺ < 3.5 mmol/L, e.g. after diarrhoea or diuretics) makes digoxin toxicity more likely.
- Low-osmolarity ORS (WHO/UNICEF): Na⁺ 75, K⁺ 20, Cl⁻ 65, citrate 10, glucose 75 mmol/L; 245 mOsm/L. Glucose drives Na⁺ and water uptake through SGLT1 in the gut.
- O₂ and CO₂ cross by simple diffusion (Fick): rate ∝ area × gradient ÷ thickness. Oxygen therapy steepens the gradient; pneumonia and pulmonary oedema reduce area and thicken the barrier.
- Selective antibiotics hit what human cells lack or build differently: 70S ribosomes (gentamicin, doxycycline, azithromycin) and the peptidoglycan wall (penicillins).
Sources: WHO/UNICEF reduced-osmolarity ORS statement; WFSA Update in Anaesthesia on paediatric IV fluids; standard physiology and pharmacology texts. Educational summary: follow current national and hospital protocols and your supervisors.
Cells: the units of life
Cell theory states that all organisms are made of cells, the cell is the basic unit of structure and function, and cells arise only from pre-existing cells. Prokaryotes (bacteria and archaea, typically 1–5 µm) have no nucleus or membrane-bound organelles; their DNA lies in a nucleoid and they have 70S ribosomes (S, the Svedberg unit, measures how fast a particle sinks in a centrifuge, so 70S ribosomes are smaller than the 80S ribosomes of eukaryotes). Eukaryotes (typically 10–100 µm) compartmentalise their chemistry in membrane-bound organelles. Mitochondria and chloroplasts are descendants of engulfed bacteria (endosymbiosis): they have two membranes, circular DNA and 70S ribosomes.
Compartments let incompatible reactions run side by side: acid hydrolases work at pH ≈ 4.5–5 inside lysosomes while the cytosol stays near pH 7.2; the ER lumen is oxidising so disulfide bonds can form; the mitochondrial inner membrane holds a proton gradient.
The plasma membrane: fluid mosaic
A 7–8 nm phospholipid bilayer with hydrophilic heads facing water and hydrophobic tails inside, studded with proteins (about 50 % of membrane mass), cholesterol (which buffers fluidity) and glycolipids/glycoproteins forming the glycocalyx. The hydrophobic core is what makes the membrane selectively permeable.
Transport across membranes
Simple diffusion of small, non-polar molecules (O₂, CO₂, N₂, steroid hormones) follows Fick's law. Intuition: each molecule wanders at random, so more of them happen to cross from the crowded side than from the sparse side, and the net flow stops when the two sides are equal. The flux is proportional to the concentration difference:
Here $J$ is the flux (amount crossing per unit area per second), $D$ the diffusion coefficient, $dC/dx$ the concentration gradient (the minus sign means flow runs from high to low) and $P$ the permeability coefficient (m/s), which lumps $D$ and membrane thickness together. Facilitated diffusion uses channels (fast, selective pores such as K⁺ channels and aquaporins) or carriers that change shape (such as GLUT1 for glucose). Because there are only so many carriers, adding more glucose eventually stops speeding things up: carriers saturate, following Michaelis–Menten kinetics, where $[S]$ is the solute concentration, $J_\text{max}$ the top speed with every carrier busy, and $K_m$ the concentration at which the carriers run at half speed:
For GLUT1, $K_m \approx 1$–3 mM. Both forms of diffusion are passive and run down the electrochemical gradient. Active transport uses energy to move substances against their gradient. The Na⁺/K⁺-ATPase (primary active transport) hydrolyses one ATP to export 3 Na⁺ and import 2 K⁺; because it moves one net positive charge out per cycle it is electrogenic. It uses about 20–25 % of the body's resting ATP (up to two-thirds in neurons). Secondary active transporters such as SGLT1 use the Na⁺ gradient it creates to import glucose.
Osmosis and water potential
Osmosis is the net diffusion of water across a selectively permeable membrane from a region of higher water potential (lower solute concentration) to lower water potential. Water potential $\Psi$ (Greek psi, measured in kPa or MPa) is simply the tendency of water to move: pure water at atmospheric pressure is defined as 0, and water always flows towards the more negative value. Solutes lower water potential by the osmotic (solute) potential $\Psi_s$, given by the van 't Hoff relation, and pressure $\Psi_p$ adds to it:
where $C$ is molar concentration, $i$ the number of particles per formula unit ($i \approx 2$ for NaCl, which splits into Na⁺ and Cl⁻), $R$ the gas constant (8.314 J mol⁻¹ K⁻¹) and $T$ the absolute temperature (K). Osmolarity, $iC$, is quoted in mOsm/L (milliosmoles of dissolved particles per litre). Body fluids are about 285–295 mOsm/L; 0.9 % NaCl (154 mM, ≈ 308 mOsm/L ideally) is isotonic. Red cells behave as osmometers following the Boyle–van 't Hoff law $V = b + (V_0 - b)\,C_0/C$, where $b$ (≈ 40 % of $V_0$) is the osmotically inactive volume. They lyse when swollen beyond about 1.6 times their normal volume (≈ 150 mOsm/L, 0.45 % NaCl).
Plant cells have a rigid cellulose wall. In a hypotonic solution water enters until wall pressure ($\Psi_p$, turgor) balances the solute potential: the cell is turgid, which keeps non-woody plants upright. In a hypertonic solution the protoplast loses water and pulls away from the wall: plasmolysis. Wilting is the loss of turgor.
The endomembrane system and protein targeting
Proteins made on free ribosomes stay in the cytosol or go to the nucleus, mitochondria, chloroplasts or peroxisomes. Proteins with an N-terminal signal peptide are recognised by the signal recognition particle (SRP) and translated into the rough ER. From there COPII-coated transport vesicles carry them to the Golgi, which modifies glycans and sorts cargo at the trans-Golgi network: secretory granules, plasma membrane, or (with a mannose-6-phosphate tag) lysosomes. Fusion of vesicles with target membranes is driven by SNARE proteins.
| Structure | Membrane | Main function | Animal | Plant | Bacteria |
|---|---|---|---|---|---|
| Plasma membrane | Single bilayer | Selective barrier, signalling | ✓ | ✓ | ✓ |
| Nucleus | Double (envelope with pores) | Holds DNA; transcription | ✓ | ✓ | ✗ (nucleoid) |
| Ribosomes | None | Protein synthesis | 80S | 80S | 70S |
| Rough ER | Single | Synthesis of secreted and membrane proteins | ✓ | ✓ | ✗ |
| Smooth ER | Single | Lipids, steroids, detoxification, Ca²⁺ store | ✓ | ✓ | ✗ |
| Golgi apparatus | Single (stacked cisternae) | Modify, sort and package proteins | ✓ | ✓ | ✗ |
| Mitochondria | Double (cristae) | Aerobic respiration, ATP | ✓ | ✓ | ✗ |
| Chloroplasts | Double + thylakoids | Photosynthesis | ✗ | ✓ | ✗ |
| Lysosomes | Single | Intracellular digestion | ✓ | Lytic vacuole instead | ✗ |
| Peroxisomes | Single | Fatty acid oxidation, H₂O₂ breakdown | ✓ | ✓ (photorespiration) | ✗ |
| Centrosome / centrioles | None | Organise microtubules, spindle | ✓ | ✗ (most land plants) | ✗ |
| Large central vacuole | Single (tonoplast) | Turgor, storage | Small vacuoles only | ✓ | ✗ |
| Cell wall | None | Support, prevents bursting | ✗ | ✓ cellulose | ✓ peptidoglycan |
| Plasmodesmata | Lined by membrane | Cytoplasmic channels between cells | ✗ (gap junctions) | ✓ | ✗ |
1. Sizes and magnification
A mitochondrion 1.5 µm long appears 30 mm long in an electron micrograph. Magnification $M = \text{image}/\text{actual} = 30\ \text{mm} / 1.5\ \mu\text{m} = 30\,000\ \mu\text{m}/1.5\ \mu\text{m} = 20\,000\times$.
The same micrograph shows a ribosome as a 0.5 mm dot: actual size $= 0.5\ \text{mm}/20\,000 = 25$ nm.
2. Surface area to volume
A spherical cell of radius 10 µm: $A = 4\pi r^2 = 1257\ \mu\text{m}^2$, $V = \tfrac43\pi r^3 = 4189\ \mu\text{m}^3$, $A/V = 3/r = 0.30\ \mu\text{m}^{-1}$.
Double the radius to 20 µm: $A/V = 0.15\ \mu\text{m}^{-1}$. Halving the surface available per unit volume limits how large a cell can grow and still exchange enough O₂ and nutrients by diffusion. This is one reason cells divide, and why absorptive cells carry microvilli.
3. Isotonic saline
0.9 % NaCl = 9 g/L. Molar mass 58.44 g/mol, so $C = 9/58.44 = 0.154$ mol/L. NaCl dissociates into 2 ions: ideal osmolarity $= 2 \times 154 = 308$ mOsm/L (about 286 measured, because the ions interact). Plasma is ≈ 290 mOsm/L, so 0.9 % saline is isotonic.
Solute potential at 25 °C: $\Psi_s = -CRT = -(300\ \text{mol m}^{-3})(8.314)(298) = -7.43\times10^{5}$ Pa $= -0.74$ MPa.
4. Red cell volume in a hypotonic solution
Boyle–van 't Hoff with $b = 0.4V_0$, $C_0 = 300$ mOsm/L. At 200 mOsm/L: $V/V_0 = 0.4 + 0.6 \times 300/200 = 1.30$: the cell swells by 30 %.
Lysis at $V/V_0 = 1.6$: $0.4 + 0.6 \times 300/C = 1.6 \Rightarrow C = 150$ mOsm/L (≈ 0.45 % NaCl). Try these values in the osmosis lab.
5. Plant cell turgor
Cell sap 400 mOsm/L: $\Psi_s = -0.4 \times 8.314 \times 298 / 1000 = -0.99$ MPa. In pure water ($\Psi = 0$) at equilibrium $\Psi_p = +0.99$ MPa, about ten times atmospheric pressure, held by the wall.
In 600 mOsm/L sucrose ($\Psi = -1.49$ MPa) the cell loses water until its sap reaches 600 mOsm/L: protoplast volume $\approx 400/600 = 67$ % and the cell is plasmolysed.
6. Fick's law
O₂ permeability of a lipid bilayer is roughly $P \approx 0.1$ m/s (very high). With 0.1 mol/m³ outside and 0.02 mol/m³ inside: $J = 0.1 \times 0.08 = 8\times10^{-3}$ mol m⁻² s⁻¹. Glucose's permeability is about $10^{-9}$ m/s, eight orders of magnitude less, which is why cells need GLUT carriers.
Guided challenges
Each challenge is checked automatically as you use the labs above.
Cell biology at the bedside
Tonicity is what matters for cells. Osmolarity counts all dissolved particles; tonicity counts only those that cannot cross the cell membrane and therefore pull water. Urea crosses membranes, so it adds to osmolality but not to tonicity; glucose in 5 % dextrose is taken into cells and metabolised. That is why a fluid's label osmolarity and its effect on cells can differ.
| Fluid | Main contents (mmol/L) | Osmolarity (mOsm/L) | Effect on cells once infused | Typical use (as taught) |
|---|---|---|---|---|
| 0.9 % sodium chloride | Na⁺ 154, Cl⁻ 154 | ≈ 308 | Isotonic | Resuscitation, replacement of losses |
| Ringer's lactate (Hartmann's) | Na⁺ ≈ 130, K⁺ 4–5, Ca²⁺ 2, Cl⁻ ≈ 110, lactate ≈ 28 | ≈ 255–275 | Close to isotonic | Resuscitation; Plan C for severe dehydration |
| 5 % dextrose | Glucose 278 (50 g/L) | ≈ 250–280 in the bag | Hypotonic (free water after the glucose is used) | Not a resuscitation fluid; a source of free water and some glucose |
| 0.45 % sodium chloride | Na⁺ 77, Cl⁻ 77 | ≈ 154 | Hypotonic | Specialist use only; risk of hyponatraemia in children |
| 3 % sodium chloride | Na⁺ 513, Cl⁻ 513 | ≈ 1027 | Hypertonic | Specialist use (severe symptomatic hyponatraemia, raised intracranial pressure) |
| Low-osmolarity ORS (oral) | Na⁺ 75, K⁺ 20, Cl⁻ 65, citrate 10, glucose 75 | 245 | Absorbed by Na⁺–glucose co-transport | Plans A and B for diarrhoea, with zinc |