Cell membranes: active recall

Their hydrophilic heads interact with water, while their hydrophobic tails are directed away from it. Two layers place the tails together inside the membrane and the heads towards the aqueous environments on either side.

Phospholipids and many membrane proteins can move laterally, so the structure is fluid. Different embedded or associated proteins form the mosaic. The membrane is not a rigid sheet with every protein fixed in one position.

Cholesterol affects phospholipid movement, stability and permeability. It can restrain movement at higher temperatures and limit tight packing at lower temperatures. Its effect is not adequately described as always making a membrane more fluid.

Particles move randomly in both directions, but there is net movement down a concentration gradient. Simple diffusion does not require ATP or a membrane transport protein. Small non-polar molecules such as oxygen can cross the phospholipid bilayer.

It uses membrane channel or carrier proteins. Net transport is still down the relevant gradient and does not directly require ATP. Ions and many polar molecules cannot freely pass through the hydrophobic membrane core.

Channels provide a pathway through which suitable particles can pass. Carriers bind a transported substance and change conformation during transport. Specificity depends on the protein’s structure and interactions with the substance.

Osmosis is the net movement of water through a partially permeable membrane from higher to lower water potential. A higher water potential is less negative when both values are below zero. Water molecules continue to move in both directions.

There is no net water movement under those conditions, although water molecules continue to cross in both directions. Equal water potential is not the same as saying that molecular movement has stopped.

It requires an energy supply and can move a substance against its gradient using transport proteins. In primary active transport, ATP hydrolysis is coupled to the transport process. A concentration difference alone is not evidence that ATP was used.

Sodium enters with glucose through a co-transporter, using the sodium electrochemical gradient. A sodium–potassium pump helps maintain that gradient using ATP. The glucose co-transporter therefore depends indirectly on an energy-consuming process.

If pigment normally contained inside cells escapes into the surrounding solution, the barriers retaining it have become more permeable or damaged. Temperature can affect lipid organisation and membrane proteins. The colour measurement alone does not identify every molecular change.

There is less surface area available per unit volume of material needing exchange. Larger organisms may also have longer internal diffusion distances. Specialised exchange surfaces and transport systems help meet those demands.