Membrane and transport

• The cell membrane is composed of phospholipids, cholesterol, proteins, carbohydrates, and glycolipids.
• Cholesterol helps maintain fluidity and stability of the membrane by interacting with phospholipid tails.
• Phospholipids are amphiphilic molecules with hydrophobic tails and polar heads that form the bilayer structure of the cell membrane.
• Active transport requires input of energy and uses transporter proteins in the membrane to move molecules against their concentration gradient.
• Osmosis is an example of simple diffusion across membranes and involves the net movement of water molecules through a semi-permeable membrane from an area of high water potential to low water potential until equilibrium is reached.
• Facilitated diffusion is the transport of molecules across membranes with the aid of transport proteins that provide pathways without requiring an input of energy.
• Carbohydrates (glycoproteins) can be found on the surface of cells and act as recognition markers.
• Passive transport does not require energy and moves substances down their concentration gradients through diffusion or osmosis.
• Facilitated diffusion involves carrier proteins that bind specific solutes and undergo conformational changes to allow passage across the membrane.
• Simple diffusion occurs when small nonpolar molecules can pass directly through the lipid bilayer without the aid of carrier proteins.
• Facilitated diffusion occurs when substances passively cross the plasma membrane using carrier or channel proteins.