Organelles, the Fluid Mosaic Model and Membrane Transport

The nuclear envelope, ER, Golgi, vesicles, lysosomes and plasma membrane together form the endomembrane system, and they work like a factory production line. Follow a molecule of the hormone insulin in a pancreatic cell. Its mRNA leaves the nucleus and is translated by a ribosome bound to the rough ER. The growing chain is threaded into the ER lumen, where it folds. A transport vesicle buds off and carries it to the Golgi apparatus, which modifies it, sorts it and packages it into a secretory vesicle. When blood sugar rises, the vesicle fuses with the plasma membrane and releases insulin outside the cell by exocytosis. Lysosomal enzymes travel the same route but are tagged at the Golgi for delivery to lysosomes instead. Mitochondria and chloroplasts are not part of this system: their double membranes, own circular DNA and bacteria-like ribosomes are evidence that they descend from bacteria engulfed by an ancestral cell (endosymbiosis).

• Phospholipid bilayer: hydrophilic phosphate heads face the watery cytoplasm and extracellular fluid; hydrophobic fatty acid tails face each other in the middle. The nonpolar core is the barrier.
• Fluid: phospholipids and many proteins drift sideways within their layer, like people moving through a crowd. Unsaturated (kinked) tails keep the membrane more fluid.
• Mosaic: the membrane is studded with many different proteins, like tiles in a mosaic. Integral proteins span or sit within the bilayer (channels, carriers, pumps, receptors); peripheral proteins attach loosely to the surface (enzymes, links to the cytoskeleton).
• Cholesterol (animal cells) wedges between phospholipids. It is a fluidity buffer: it keeps the membrane from becoming too runny when warm and too stiff when cold.
• Glycoproteins and glycolipids carry short carbohydrate chains on the outer surface. They act as identity tags for cell recognition (for example ABO blood types and immune self-recognition) and help cells stick together.
• Selective permeability: small nonpolar molecules (${\text{O}}_2$, ${\text{CO}}_2$) and lipid-soluble molecules dissolve through the core easily. Water crosses slowly (fast through aquaporin channels). Ions and large polar molecules such as glucose cannot cross without protein help.

Molecules are always moving randomly, so wherever there is a difference in concentration (a concentration gradient), more molecules move from the crowded side to the less crowded side than the other way. The net movement from high to low concentration is diffusion, and it needs no energy from the cell: it is passive transport. It stops (reaches dynamic equilibrium) when concentrations are equal. Steeper gradients, higher temperatures, smaller molecules and larger surface areas make diffusion faster. Osmosis is the diffusion of water across a selectively permeable membrane, from a region of lower solute concentration (more free water) to higher solute concentration (less free water). Facilitated diffusion is still passive and still downhill, but the substance needs a membrane protein to cross. Channel proteins form water-filled pores for specific ions or water (potassium channels, aquaporins), and many are gated to open only on a signal. Carrier proteins bind the molecule, change shape and release it on the other side (the GLUT transporters for glucose). Because a cell has a limited number of carriers, facilitated diffusion saturates at high concentrations, exactly like an enzyme.

Active transport moves a substance against its concentration gradient, from low to high concentration, so it costs energy, usually ATP, and uses carrier proteins called pumps. The most important is the sodium-potassium pump. In each cycle it binds 3 ${\text{Na}}^{+}$ from inside the cell, ATP transfers a phosphate to the pump, the pump changes shape and releases the 3 ${\text{Na}}^{+}$ outside, then binds 2 ${\text{K}}^{+}$ from outside; losing the phosphate returns it to its original shape and it releases the 2 ${\text{K}}^{+}$ inside. Net result: 3 Na⁺ out and 2 K⁺ in per ATP. Because it moves more positive charge out than in, the inside of the cell becomes negative relative to the outside, which helps create the membrane potential that neurons use for nerve impulses. The pump uses a large share of a resting body's ATP. The ${\text{Na}}^{+}$ gradient it builds also powers secondary active transport: in the small intestine, ${\text{Na}}^{+}$ flowing back into cells drags glucose in with it through a shared carrier. Large particles and bulk fluid cannot pass through proteins, so cells engulf or expel them in vesicles, which also costs energy. In endocytosis, the membrane folds in to take material in. Phagocytosis (cell eating) engulfs large particles such as bacteria, as white blood cells do; the vesicle then fuses with a lysosome to digest them. Pinocytosis (cell drinking) takes in droplets of extracellular fluid non-selectively. Receptor-mediated endocytosis is selective: receptors in coated pits bind a specific molecule (such as LDL cholesterol particles) before the membrane folds in. People with familial hypercholesterolemia have faulty LDL receptors, so cholesterol stays in their blood. In exocytosis, vesicles fuse with the plasma membrane and release their contents outside, which is how cells secrete hormones, digestive enzymes, mucus and neurotransmitters.

1. Use a cork borer to cut potato cores of the same diameter from one potato, trim them to the same length, blot them dry and record each starting mass.
2. Place one core in each of a series of solutions, e.g. distilled water and 0.2, 0.4, 0.6 and 0.8 mol/L salt (or sucrose) solution, with the same volume in each, and leave them for the same time (for example 24 h). Run at least three cores per concentration.
3. Remove, blot dry the same way and reweigh. Record observations too: cores in distilled water become firm (turgid); cores in strong salt water become soft and bendy (flaccid and plasmolyzed).
4. Calculate percent change in mass for each core, because the cores do not start at exactly the same mass: $\%\text{change}=\frac{\text{final}-\text{initial}}{\text{initial}}\times 100\%\text{.}$
5. Graph average percent change against solution concentration. Positive values mean water entered (the solution was hypotonic to the potato cells); negative values mean water left (hypertonic). Where the line crosses zero, the solution is isotonic with the cells, which estimates the potato's own solute concentration.