B2.1 Membranes and membrane transport

Phospholipids

Formed by a hydrophilic phosphate head bonding with two hydrophobic hydrocarbon (fatty acid) tails. They are amphipathic (having both hydrophilic and hydrophobic parts)

The lipid bilayer as a barrier

Large molecules cannot pass through as the hydrophobic region is tightly packed and has low permeability to large molecules. Polar molecules and ions cannot pass as they will not interact with the hydrophobic tails

Integral proteins

Amphipathic. They are embedded in the phospholipid bilayer, either across both layers or just one layer

Peripheral proteins

Hydrophilic. They are attached to either the surface of integral proteins or to the membrane via a hydrocarbon chain

Channel proteins

Pores that allow the passage of specific substances across a membrane

Carrier proteins

They can switch between two shapes, causing them to open or close on opposite sides of the membrane

Functions of membrane proteins

Transport, receptors, immobilised enzymes, cell adhesion, cell recognition

Glycoproteins

Cell membrane proteins that have a carbohydrate chain attached on the extracellular side

Simple diffusion

The net movement, as a result of the random motion of molecules or ions, of a substance from a region of higher concentration to a region of lower concentration

Examples of simple diffusion

Oxygen: Diffuses into cells from surrounding capillaries

Carbon dioxide: Diffuses out of cells into the surrounding capillaries

Factors affecting the rate of simple diffusion

The difference in concentration, temperature, surface area, properties of molecules or ions

Osmosis

The diffusion of water molecules from a region of high concentration to a region of low concentration across a partially permeable membrane

Facilitated diffusion

Substances such as large molecules, polar molecules, and ions can only cross the phospholipid bilayer through transport proteins (channel or carrier)

Active transport

The movement of molecules and ions across a cell membrane, from low to high concentration, using energy. It requires carrier proteins

Glycolipids

Lipids with carbohydrate chains attached, located on the outer surface of cell membranes

The function of glycoproteins and glycolipids

The carbohydrate chain enables them to act as receptor molecules

Fluid mosaic model

Phospholipids and proteins can move around within their own layers, and are therefore flexible

Saturated fatty acids

Straight, can pack together tightly, allowing them to maintain stability at higher temperatures

Unsaturated fatty acids

Have bends in the chain, they cannot pack together as tightly, therefore the membrane is more fluid and flexible

Cholesterol

A type of lipid. Amphipathic. It maintains membrane fluidity by disrupting the close-packing of phospholipids, increasing flexibility. It also holds the fatty acid tails together

Vesicles

Small spherical sacs of plasma membrane containing substances for transport

Endocytosis

The plasma membrane engulfs material, forming a sac around it. Example is phagocytosis

Exocytosis

Substances to be released are packaged into vesicles. The vesicles fuse with the membrane and release their contents outside the cell. Example is digestive enzymes from pancreatic cells

Absorption of glucose into the blood

Sodium potassium pumps actively transport sodium ions into the blood. Sodium ions move down their concentration gradient back into the cell via a cotransporter protein. Glucose is drawn into the cell along with the sodium ions

Cell adhesion molecules (CAMs)

A type of cell surface protein. They bind cells to other cells or with the extracellular matrix. Different CAMs are present in different cell junctions