Movement into and out of cells

Simple diffusion is the net movement of molecules or ions from a region of higher concentration to a region of lower concentration, down a concentration gradient, as a result of their random kinetic energy. It is passive — no ATP is required — and continues until the concentrations are equal (equilibrium), at which point net movement ceases.

Across the cell-surface membrane, simple diffusion occurs directly through the phospholipid bilayer for molecules that meet two criteria:

• They are small enough to slip between phospholipids.
• They are lipid-soluble (non-polar / uncharged), so they can dissolve in the hydrophobic core of the bilayer.

Examples include $\text{O}_2$, $\text{CO}_2$, $\text{N}_2$, steroid hormones, and other small non-polar molecules. Water also crosses the bilayer slowly by simple diffusion, although most water transport is via specific channel proteins called aquaporins.

Factors affecting the rate (Fick's law in qualitative form):

• Steepness of the concentration gradient — steeper gradient = faster rate.
• Surface area — larger area available for diffusion = faster rate (e.g. microvilli, alveoli, root hairs).
• Thickness of the membrane — thinner = faster (e.g. alveolar epithelium is one cell thick).
• Temperature — higher = more kinetic energy = faster (within the limit at which membranes are damaged).
• Size and lipid-solubility of the molecule — smaller and more lipid-soluble = faster.

Facilitated diffusion is the passive movement of polar molecules and ions down a concentration gradient through specific transport proteins in the membrane. Like simple diffusion, it requires no ATP and is driven by the random kinetic energy of the diffusing particles. The role of the protein is simply to provide a polar pathway through the hydrophobic bilayer.

Two kinds of transport proteins facilitate diffusion:

• Channel proteins — form hydrophilic pores through the bilayer through which specific ions (e.g. $\text{Na}^+$, $\text{K}^+$, $\text{Cl}^-$, $\text{Ca}^{2+}$) diffuse. Many channels are gated: voltage-gated (open in response to membrane potential changes), ligand-gated (open when a signalling molecule binds), or stretch-gated (open in response to mechanical force). Aquaporins are a specialised channel for water.
• Carrier proteins — bind a specific molecule (e.g. glucose, amino acid) on one side of the membrane, undergo a conformational change, and release the molecule on the other side. In facilitated diffusion this conformational change is reversible and requires no ATP — it is driven by the concentration gradient itself. (When ATP IS used to power the shape change, the same kind of protein performs ACTIVE transport.)

Specificity. Each transport protein moves only one substance (or a small family). This is determined by the shape and chemistry of its binding site, in the same way an enzyme's active site is specific to its substrate.

Saturation. Because facilitated diffusion depends on a finite number of proteins, the rate can be saturated: when all proteins are working at maximum, the rate reaches a maximum ($V_{\max}$) and cannot rise further even if the gradient steepens. Simple diffusion has no such limit.

Osmosis is the special case of diffusion that applies to water across a partially permeable membrane. At Cambridge A Level it is defined precisely in terms of water potential:

Osmosis is the net movement of water molecules from a region of higher water potential to a region of lower water potential through a partially permeable membrane.

Water potential ($\psi$) (Greek letter 'psi') is a measure of the tendency of water to move out of a region. It is expressed in kilopascals (kPa). By convention:

• Pure water has a water potential of zero ($\psi = 0$ kPa) at standard atmospheric pressure.
• The presence of any solute lowers water potential, so all solutions have negative water potentials.
• The more concentrated the solution, the more negative its water potential.

When two solutions are separated by a partially permeable membrane (e.g. a cell membrane, which is permeable to water but not to most solutes), water moves down the water potential gradient — from the less negative side to the more negative side — until the two values are equal.

Why use water potential? Because it correctly predicts the direction of water movement when both solute concentration and hydrostatic pressure are present (as inside a plant cell), and because Cambridge mark schemes have moved away from older 'high to low concentration of water' wording.

Whether a cell takes up or loses water by osmosis depends on the water potential of the surrounding solution relative to the cell's contents.

Animal cells (e.g. red blood cells) have only the cell-surface membrane.

• In distilled water (higher $\psi$ than the cell): water moves in by osmosis; the cell swells and the membrane stretches until it ruptures — haemolysis (also called cytolysis).
• In an isotonic solution ($\psi$ equal to the cytoplasm): no net movement; the cell keeps its normal shape.
• In a hypertonic solution (more negative $\psi$ than the cell): water moves out; the cell shrinks and its surface wrinkles / develops spikes — crenation.

Red blood cells are normally kept in a slightly hypotonic environment (plasma is close to 0.9% NaCl, which is isotonic to the cell), so any large change in blood osmolarity disrupts cell function quickly.

Plant cells have an additional structure: the strong, inelastic cellulose cell wall.

• In a dilute solution / pure water (higher $\psi$ than the cell sap): water moves in by osmosis; the vacuole expands and pushes the cytoplasm against the cell wall. The wall resists further expansion, exerting an inward pressure (turgor pressure) that prevents further water uptake. The cell becomes turgid (firm). The wall stops the cell from bursting. Turgor is essential for the mechanical support of non-woody plant tissues — wilting occurs when cells lose turgor.
• In a hypertonic solution (more negative $\psi$ than the cell sap): water moves out by osmosis from the vacuole and cytoplasm. The cytoplasm shrinks and pulls away from the cell wall — this is plasmolysis. The point at which the cytoplasm just begins to detach is incipient plasmolysis. Severe plasmolysis is often fatal.
• Iso-osmotic solution: no net movement; the cell wall provides no inward pressure (cell is flaccid, neither turgid nor plasmolysed).

Active transport is the movement of a substance against its concentration gradient (from lower to higher concentration). Because this is the energetically unfavourable direction, it requires an input of energy — supplied by the hydrolysis of ATP to ADP and inorganic phosphate. Active transport always uses carrier proteins; channel proteins (which simply provide a pore) cannot perform active transport.

Mechanism (single-pump example).

1. A substance binds to a specific site on the carrier on one side of the membrane.
2. ATP is hydrolysed (often by the carrier itself, which is an ATPase) and the released energy drives a conformational change in the carrier.
3. The substance is released on the other side, against its gradient.
4. The carrier returns to its original shape, ready for another cycle.

Key example: the sodium-potassium pump. Present in the membrane of all animal cells, it pumps 3 Na$^+$ out and 2 K$^+$ in for every ATP hydrolysed. This maintains the resting membrane potential, the low cytoplasmic Na$^+$, and the high cytoplasmic K$^+$ that are essential for nerve impulse transmission, muscle contraction, and other processes. It is one of the largest energy expenditures of the cell — consuming up to 25% of an animal's resting metabolic energy.

Other examples. Proton pumps (e.g. on the inner mitochondrial membrane, in stomach acid secretion), calcium pumps (sarcoplasmic reticulum), and the uptake of mineral ions by root-hair cells from low-concentration soil solutions.

Effects of factors. Because active transport depends on ATP, anything that reduces ATP supply slows or stops it:

• Low oxygen (limits aerobic respiration → less ATP).
• Low temperature (slows respiration enzymes).
• Respiratory inhibitors such as cyanide (block the electron transport chain → no ATP).

Facilitated diffusion is unaffected by these factors.

Molecules too large to cross the bilayer or to fit through any transport protein (e.g. proteins, polysaccharides, whole bacteria) are moved across the membrane in vesicles. These bulk processes always require ATP because they involve deforming the membrane and moving vesicles along the cytoskeleton.

Endocytosis — uptake INTO the cell.

The cell-surface membrane invaginates around the material, then pinches off inside the cell to form a vesicle containing the material. There are three main types:

• Phagocytosis ('cell eating'): uptake of large solid particles. The classic example is a macrophage engulfing a bacterium during the immune response; the resulting phagosome fuses with lysosomes whose hydrolytic enzymes digest the bacterium.
• Pinocytosis ('cell drinking'): uptake of fluid and dissolved solutes in small vesicles.
• Receptor-mediated endocytosis: specific receptors in the cell-surface membrane bind to particular extracellular molecules (e.g. LDL particles carrying cholesterol, transferrin carrying iron). The receptor-bound molecules concentrate in 'clathrin-coated pits' that pinch off to form vesicles.

Exocytosis — release OUT of the cell.

A vesicle (often formed at the Golgi apparatus) is moved along microtubules to the cell-surface membrane. The vesicle membrane fuses with the cell-surface membrane, releasing its contents to the outside. Examples include:

• Secretion of digestive enzymes by pancreatic acinar cells (e.g. trypsinogen, amylase, lipase).
• Hormone release — insulin from $\beta$-cells of the islets of Langerhans, adrenaline from chromaffin cells.
• Neurotransmitter release at synaptic terminals.
• Mucus release by goblet cells in the gut and airways.

Both endocytosis and exocytosis maintain a constant flow of membrane between the cell-surface membrane and internal compartments — this is the basis of membrane recycling.

Cambridge 9700 examines a classic practical to determine the water potential of plant tissue, typically using potato cylinders or carrot cylinders.

Method.

1. Prepare sucrose solutions of known water potential (e.g. 0.0 [distilled water, $\psi = 0$], 0.1, 0.2, 0.3, 0.4, 0.5, 0.6, 0.8, 1.0 mol dm$^{-3}$). A published reference table converts molarity to water potential in kPa.
2. Prepare standardised potato cylinders using a cork borer of fixed diameter; trim to the same length. Blot gently on paper towel and measure the initial mass of each cylinder on a balance.
3. Incubate equal numbers of cylinders in equal volumes of each sucrose solution at the same temperature for the same time (e.g. 30-60 min for length, 60-90 min for mass).
4. Remove, blot gently, and measure the final mass of each cylinder.
5. Calculate the percentage change in mass: $\%\,\text{change} = \dfrac{m_\text{final} - m_\text{initial}}{m_\text{initial}} \times 100$
6. Plot percentage change in mass (y-axis) against water potential of the sucrose solution (x-axis). Each point is the mean of repeats with error bars.
7. Read the x-intercept — the water potential at which percentage change in mass is zero. At this concentration, there is no net movement of water → $\psi_\text{tissue} = \psi_\text{solution}$.

Why % change? Different cylinders may have slightly different starting masses. Percentage change standardises the data so all points can be compared on the same axis.

Typical results. Fresh potato has $\psi \approx -700$ to $-1000$ kPa (about 0.25-0.35 mol dm$^{-3}$ sucrose); carrot is similar.

Sources of error. Variation between potato samples (age, region of the tuber), surface drying when blotting, evaporation during incubation, parallax error when reading the balance.