How do cellular membranes control diffusion?

Cellular membranes control diffusion by acting as a selective barrier, allowing only certain substances to pass through.

Cellular membranes are composed of a phospholipid bilayer with embedded proteins, which creates a semi-permeable barrier between the cell and its external environment. This structure is crucial in controlling the movement of substances in and out of the cell, a process known as diffusion.

Diffusion is the movement of particles from an area of high concentration to an area of low concentration. It's a passive process, meaning it doesn't require energy. However, not all substances can diffuse across the cell membrane. The phospholipid bilayer is hydrophobic (water-repelling) in its interior, so only small, non-polar molecules like oxygen and carbon dioxide can diffuse directly through the lipid bilayer.

Larger or polar molecules, such as glucose or ions, cannot pass through the lipid bilayer unaided. Instead, they require the help of transport proteins embedded in the membrane. These proteins provide a channel or carrier for these substances to pass through, controlling their diffusion into or out of the cell. This is known as facilitated diffusion, which is also a passive process.

In addition to this, the cell membrane can also control diffusion through active transport. This is the movement of substances against their concentration gradient, from an area of low concentration to an area of high concentration. This process requires energy in the form of ATP and is carried out by specific transport proteins.

In summary, the cellular membrane controls diffusion by acting as a selective barrier. It allows the passive diffusion of small, non-polar molecules directly through the lipid bilayer, facilitates the diffusion of larger or polar molecules via transport proteins, and controls active transport of substances against their concentration gradient.