B2.1 Membranes and membrane transport

A lipid bilayer forms because amphipathic phospholipids place water-compatible heads toward water and hydrocarbon tails away from it.

In water, this arrangement lowers unfavorable tail–water contact. The two leaflets therefore create a flexible sheet with a non-polar interior, allowing a cell to separate its inside from its surroundings.

A membrane has water on both sides: heads face each aqueous side while tails meet in the middle, forming one continuous barrier.

When phospholipids are placed in water, their hydrophilic heads remain in contact with water while their hydrophobic tails cluster away from it; a closed bilayer can therefore form without leaving exposed tails at its edge.

The bilayer is a consequence of amphipathic structure in an aqueous environment; it is not a rigid wall, and its hydrophobic core still permits some small non-polar molecules to cross.

The bilayer’s hydrophobic interior slows ions and most polar molecules, while small non-polar molecules cross more readily.

Crossing depends on how well a solute interacts with the tail region and on its size. The membrane can therefore separate concentration gradients without blocking every substance equally.

Use three checks: charge/polarity, size, and whether a transport protein is available. A matching pathway can change the prediction.

Oxygen can diffuse through the tail core, but a sodium ion needs a channel or carrier because its charge is incompatible with the hydrophobic interior.

‘Barrier’ does not mean ‘impermeable’. Selectivity comes from different crossing rates, not from stopping all movement.

Simple diffusion is net movement from higher concentration to lower concentration directly through the membrane, without ATP or a transport protein.

Random molecular motion is present in both directions, but more particles leave the high-concentration side, producing net movement down the gradient until equilibrium is approached.

Check: concentration gradient exists; molecule can cross the lipid core; no energy input or protein is required.

If oxygen is higher outside a cell than inside, oxygen molecules cross in both directions but net oxygen entry occurs until the gradient shrinks.

Diffusion is not movement toward ‘where the cell needs it’. The gradient and membrane permeability determine net direction.

Integral membrane proteins are embedded in or span the bilayer; peripheral proteins attach to a membrane surface or to another protein.

Hydrophobic amino-acid regions can sit among lipid tails, while polar regions face water or form pathways. Surface attachment does not require a protein to cross the hydrophobic core.

Classify by location: embedded/spanning means integral; surface-associated without entering the core means peripheral. Then connect location to likely function.

A channel crossing the membrane is integral; a cytoskeletal protein attached to its cytoplasmic face is peripheral.

A protein being near a membrane does not prove it spans the bilayer. Identify whether it enters the hydrophobic core.

Osmosis is net movement of water across a partially permeable membrane from higher water potential to lower water potential.

Solute lowers the water potential of a solution. Water molecules move randomly both ways, but the imbalance in water potential produces net movement toward the more concentrated solution; aquaporins can increase the rate.

State the membrane and compare water potential: higher ψ → lower ψ; then predict volume or cell-size change.

A cell placed in a solution with lower water potential loses water through its membrane and may shrink; aquaporins change speed, not the direction set by ψ.

Osmosis is not simply ‘water moves to higher solute’ without conditions: the membrane must allow water through and the water-potential comparison must be valid.

Channel proteins form selective hydrophilic pores through the bilayer, allowing particular ions or polar molecules to cross down an electrochemical gradient.

The pore exposes a compatible interior while the surrounding lipid tails remain excluded. Size, charge and binding sites determine which solutes pass; channels do not normally supply the energy for uphill movement.

For a channel claim, name the solute, pore selectivity and gradient direction.

An ion channel can let potassium move rapidly down its gradient while excluding a larger or differently charged ion.

A channel is not automatically an active pump. If movement is against a gradient, another energy-coupled mechanism is required.

Pump proteins use energy, commonly from ATP hydrolysis, to move selected substances against their concentration or electrochemical gradient.

A pump changes conformation when energy is supplied, alternately exposing a binding site to each side of the membrane. This couples an unfavorable transport step to a favorable energy-releasing reaction.

Trace: bind solute → energy changes protein shape → release solute on the opposite side → reset.

An ATP-driven pump can keep sodium higher outside a cell even though diffusion would tend to move sodium inward.

A protein moving a solute is not enough to identify active transport; look for uphill movement and an energy source.

A membrane is selectively permeable because each solute’s size, charge, polarity and available transport pathway determine whether and how fast it crosses.

The lipid core favors small non-polar solutes, while channels and carriers add controlled routes for particular polar or charged substances. Selectivity is therefore a property of the whole membrane, not lipids alone.

Judge a crossing claim using: solute chemistry, membrane core, matching protein, and gradient/energy condition.

A glucose molecule is polar and large, so it usually needs a specific carrier even when a small non-polar gas can cross directly.

Selective permeability is not the same as biological ‘choice’ or intention; it follows physical interactions and protein specificity.

Glycoproteins and glycolipids carry carbohydrate chains on the extracellular surface, where their shapes can act as recognition markers.

The carbohydrate pattern is exposed by the membrane orientation, and complementary receptors or immune molecules can bind it. The lipid or protein anchor keeps the label at the cell surface.

Identify: membrane anchor, outward-facing carbohydrate, and matching recognition partner. The label works only when the partner can bind its shape.

A receptor can distinguish two cells because their surface carbohydrate patterns differ even if the surrounding bilayer is similar.

A surface carbohydrate is not a universal identity tag; recognition depends on the exact molecular pattern and binding partner.

The fluid mosaic model describes a dynamic phospholipid bilayer containing proteins and carbohydrates that can move laterally and perform different jobs.

Hydrophobic interactions hold the bilayer together without making it rigid. Proteins are distributed like a mosaic: channels, carriers, receptors and anchors contribute different functions while the sheet remains flexible.

Explain a membrane feature by linking its component to a job: lipid core for barrier, protein for transport/signalling, carbohydrate for recognition.

A receptor can drift sideways within one leaflet while still presenting its binding site to an extracellular signal.

‘Fluid’ does not mean the membrane freely flips every molecule across the bilayer; lateral movement and transbilayer movement are different.