transport across cell membranes

Arrangement of phospholipids

They align as a bilayer with the hydrophilic heads outwards and hydrophobic tails inwards

Purpose of cholesterol in membrane

• Restricts lateral movement of other molecules in the membrane
• so it makes the membrane less fluid at high temperatures and prevents water and dissolves ions from leaking out of the cell

Proteins in the membrane

Peripheral proteins - provide mechanical support or connected to proteins or lipids to make glycoproteins or glycolipids (function - cell recognition as receptors)
integral proteins - they are carrier / channel proteins involved in transport of molecules across membrane

• protein channels fill with water to enable water soluble ions to diffuse whereas protein carriers wind with larger molecules (glucose, amino acids) and change shape to transport them to the other side

Simple diffusion

• net movement of molecules from an area of high concentration to an area of low concentration
• does not require ATP

Facilitated diffusion

• Movement of ions and polar molecules across cell membranes through protein channels and carrier proteins
• doesn't require ATP

Osmosis

Movement of water from high water potential to lower water potential through a partially permeable membrane

Active transport

• Movement of molecules from low concentration to higher concentration
• requires ATP and carrier proteins
• carrier proteins act as a pump very selective as only certain molecules can bind to carrier proteins
• ATP binds to the inside of the protein and is hydrolysed into ADP + Pi causing protein to change shape and allows it to be released on the other side, the Pi is released and protein returns to original shape