Osmosis and membrane transport

Osmosis and Water Potential

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

The phrase “free water molecules” refers to water molecules not bound to solute particles. When solutes dissolve in water, they attract water molecules, reducing the number free to move. A solution containing more solute therefore has fewer free water molecules available to cross the membrane.

Water potential [the tendency of water to move from one place to another] is given the symbol ψ (psi) and measured in kilopascals (kPa). Pure water has the highest water potential, set at 0 kPa by convention. Adding any solute lowers water potential, making it more negative. Water always moves from a less negative water potential to a more negative one.

The membrane involved is partially permeable, which means it allows water through freely but restricts the movement of larger solute particles. Water continues to move until the water potential is equal on both sides, reaching equilibrium. Osmosis is a passive process. No ATP is needed, because water moves down its own water potential gradient.

In animal cells, water entering by osmosis can cause the cell to swell and burst by lysis. Plant cells avoid this because the rigid cell wall provides support, allowing the cell to become turgid instead.

MisconceptionStudents often say water moves “to where there is more water” or “to the higher concentration of solute”. Osmosis must always be described in terms of water potential, not solute concentration alone.

Exam TipAlways use the phrase “from a region of higher water potential to a region of lower water potential” for full marks.

Passive Transport: Diffusion and Facilitated Diffusion

Passive transport is the movement of substances across a membrane down a concentration gradient, requiring no metabolic energy from the cell.

Two types of passive transport occur across the cell surface membrane: simple diffusion and facilitated diffusion. Both move substances from a region of higher concentration to a region of lower concentration. The energy comes from the random kinetic motion of the particles themselves.

Simple diffusion is the net movement of particles directly through the phospholipid bilayer. Only small, non-polar molecules can pass this way, because the hydrophobic interior of the membrane repels charged particles. Examples include oxygen, carbon dioxide, and small lipids such as steroid hormones.

Facilitated diffusion moves larger or charged molecules across the membrane with the help of transport proteins. These molecules cannot cross the bilayer alone because they are either too big or repelled by the hydrophobic core. Examples include glucose, amino acids, and ions such as Na⁺ and Cl⁻.

The rate of diffusion increases with a steeper concentration gradient, higher temperature, larger surface area, and shorter diffusion distance. For facilitated diffusion specifically, the rate also depends on the number of available transport proteins. Once all proteins are saturated, the rate plateaus.

Active Transport and the Role of ATP

Active transport is the movement of substances across a membrane against their concentration gradient, using energy released from ATP hydrolysis.

Unlike passive transport, active transport allows cells to accumulate substances even when their concentration outside the cell is already low. This is essential for processes such as nerve impulse transmission, mineral uptake in root hair cells, and absorption in the small intestine.

The process always involves a carrier protein, sometimes called a pump. The substance binds to a specific site on the carrier protein. ATP then binds and is hydrolysed to ADP and an inorganic phosphate (Pi). The energy released causes the carrier protein to change shape, transporting the substance across the membrane.

ATP [adenosine triphosphate] acts as the immediate source of energy. This means ATP itself donates the energy directly, rather than glucose or another fuel being used as the energy source. After hydrolysis, the carrier protein returns to its original shape and the cycle repeats.

A classic example is the sodium-potassium pump found in nerve cells. For each ATP hydrolysed, three Na⁺ ions are pumped out of the cell and two K⁺ ions are pumped in. Both ions move against their concentration gradients, helping to maintain the resting potential.

Examiner InsightMany students lose marks by saying active transport “uses energy” without specifying ATP. The syllabus explicitly requires you to identify ATP as the immediate energy source and link it to its hydrolysis.

Exam TipAlways write “ATP is hydrolysed to release energy”. Naming the molecule and the process earns the mark.

Endocytosis and Exocytosis

Cells move large molecules and bulk amounts of material into and out of the cell using vesicle-based transport, called endocytosis and exocytosis.

These processes are necessary because some substances are too large to cross the membrane via diffusion or transport proteins. Examples include proteins, large polysaccharides, and entire microorganisms. Both processes require ATP, making them active processes.

Endocytosis moves substances into the cell. The cell surface membrane folds inwards (invaginates) around the material, eventually pinching off to form a vesicle inside the cell. Two main forms exist:

• Phagocytosis (“cell eating”): engulfment of solid particles such as bacteria by white blood cells.
• Pinocytosis (“cell drinking”): uptake of dissolved substances and fluids in small vesicles.

Exocytosis moves substances out of the cell. A vesicle inside the cell, often produced by the Golgi apparatus, moves towards the cell surface membrane. The vesicle membrane fuses with the cell membrane, releasing the contents to the outside.

Exocytosis is essential for secretion. Examples include the release of digestive enzymes from pancreatic cells, hormones such as insulin, and neurotransmitters from synaptic terminals.

Carrier and Channel Proteins in Membrane Transport

Carrier and channel proteins are intrinsic [embedded across the bilayer] proteins that allow specific substances to cross the cell surface membrane.

These proteins are essential because the phospholipid bilayer alone is impermeable to large, polar, or charged molecules. Each protein is highly specific to a particular substance, similar to the way enzymes are specific to substrates. This specificity allows the cell to control exactly what enters and leaves.

Channel proteins form a hydrophilic pore through the membrane. Polar molecules and ions pass through this pore down their concentration gradient. Many channel proteins are gated, meaning they open or close in response to a signal, such as a voltage change or chemical messenger. Channel proteins are only used in facilitated diffusion. They are passive and never use ATP.

Carrier proteins work differently. The substance binds to a specific site, which causes the protein to change shape and release the substance on the other side. Carrier proteins can be involved in either facilitated diffusion (passive) or active transport. The difference is that active transport requires ATP to drive the shape change against the concentration gradient.