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 passive net movement down a concentration gradient directly between phospholipids, without ATP or a transport protein.

Particles move randomly in both directions, but more leave the higher-concentration side, producing net movement until dynamic equilibrium is approached. Small non-polar oxygen and carbon dioxide dissolve in the lipid core and cross readily.

Diffusion is faster with a steeper gradient, larger surface area, shorter diffusion distance and greater particle motion at higher temperature, provided the molecule can enter the bilayer.

Oxygen can diffuse into a respiring cell while carbon dioxide diffuses out when their respective concentration gradients point in those directions.

The cell does not direct diffusion toward need. Each gas moves according to its own gradient, and movement continues both ways even when net movement is zero.

Integral proteins are embedded in one or both lipid layers and may span the membrane; peripheral proteins attach to one surface or to another membrane protein.

Hydrophobic amino-acid regions of integral proteins interact with lipid tails, while hydrophilic regions face water or form pores. Peripheral proteins remain surface-associated and can detach without crossing the hydrophobic core.

A channel spanning the bilayer is integral, while a cytoskeletal protein attached to its cytoplasmic face is peripheral.

Function alone does not determine the category. Classify a protein by whether it enters the hydrophobic bilayer core or remains surface-associated.

Osmosis is passive net movement of water across a membrane that is permeable to water but not freely permeable to the solutes involved.

Water molecules move randomly both ways. A difference in solute concentration creates a difference in water potential, so net water movement is from the lower-solute, higher-water-potential side toward the higher-solute, lower-water-potential side.

Aquaporins are selective protein pores that greatly increase water permeability. They change the rate of osmosis, not the direction set by the water-potential difference.

A cell in a solution with higher solute concentration than its cytoplasm loses water and shrinks; aquaporins allow that volume change to occur faster.

Do not state only that 'water follows solute'. Identify the selectively permeable membrane, compare the two solutions and distinguish net movement from random movement in both directions.

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.

Membrane permeability depends on both the lipid bilayer and transport proteins. Simple diffusion is not protein-selective; facilitated diffusion and active transport are selective because specific proteins recognize particular particles.

For simple diffusion, crossing rate follows physical properties such as particle size and hydrophobicity. Channels, carriers and pumps add binding sites or pores that admit selected ions or molecules under defined gradient and energy conditions.

A small non-polar gas crosses the lipid core, whereas glucose usually requires a specific carrier and sodium requires a selective channel or pump.

Selective permeability is not biological intention. It emerges from molecular interactions and the transport proteins present in that membrane.

Glycoproteins and glycolipids are membrane proteins or lipids with short carbohydrate chains exposed only on the extracellular surface.

Together the outward carbohydrate chains form much of the glycocalyx. Their three-dimensional patterns bind complementary molecules, supporting cell recognition, cell-cell adhesion and signalling at the aqueous surface.

Read the structure as three linked parts: protein or lipid anchors the molecule; carbohydrate projects outside; a specific receptor, antibody or neighbouring-cell molecule binds the pattern.

Different red-cell carbohydrate antigens can be distinguished by antibodies, while adhesion glycoproteins can bind matching partners on an adjacent cell.

Membrane carbohydrates are asymmetric: they face the extracellular side, not both surfaces. Recognition and adhesion require a compatible binding partner.

The fluid mosaic model describes a dynamic phospholipid bilayer containing laterally mobile lipids and a mosaic of integral and peripheral proteins, glycoproteins and cholesterol.

Hydrophilic heads face aqueous solutions and hydrophobic tails form the core. Integral proteins enter or cross that core, peripheral proteins attach at surfaces, carbohydrates project extracellularly, and cholesterol fits among animal-cell phospholipids.

A correct two-dimensional drawing labels: two phospholipid layers; hydrophilic heads; hydrophobic tails; an integral protein; a peripheral protein; a glycoprotein with outward carbohydrate; and cholesterol between tails.

A receptor can diffuse laterally while maintaining its extracellular binding domain and hydrophobic membrane-spanning region, illustrating both fluidity and mosaic organization.

Fluidity mainly describes lateral movement and flexibility; it does not imply that components freely flip between leaflets or that every membrane contains identical proportions.