Fluid mosaic model

The fluid mosaic model for membranes:

Membrane components and their functions: 

• Phospholipid: Hydrophilic head (phosphate), Glycerol, Hydrophobic fatty acid tail, forms a semipermeable phospholipid bilayer. 
• Carrier protein: protein with a specific shaw that complements the shape of the substance to be transported across the membrane, it is used in active transport and facilitated diffusion. 
• Cholesterol: Regulates membrane fluidity.
• Glycoprotein: Cell signalling and recognition and binding cells together.
• Glycolipid: Carbohydrate attached to lipid/phospholipid, cell signalling and cell recognition 
• Channel protein: Protein with a specific shape that complements the shape of the substance to be transported across the membrane, used in facilitated diffusion. 
• Cell signalling: Receptors on cells bind to hormones, drugs and other cells leading to a series of reactions within the cell

Temperature and permeability:

• A high temperature boosts the kinetic energy of the component molecules of the membrane and the transported substance, therefore the membrane becomes more permeable. 
• Very high temperatures will denature the protein molecules, changing their shape and making the membrane more permeable, eventually the membrane will be destroyed. 

Specialised membranes:

• Different cells’ membranes have varying properties, functions and capabilities. These depend on the glycoproteins, glycolipids, channel proteins and carrier proteins that are present. 
• Some membranes are folded to increase surface area for transport or absorption, e.g. microvilli.
• Membranes are fluid, so they can be folded by an organisms cytoskeleton to for vesicles. The active process (meaning it requires ATP) is part of endocytosis.
• Vesicles can fuse with the membrane as part of exocytosis. 

The structure of a cell membrane and phospholipids: 

Fluid mosaic model: Fluid - molecules within the membrane are free to move in relation to each other.

Mosaic - mixture of phospholipids and proteins.

Double layer of phospholipid molecules, phospholipid consists glycerol, to which are joined two fatty acids, and a phosphate, formed by a condensation reaction. The phosphate head is hydrophilic and the fatty acid tail is hydrophobic, meaning in the membrane the phospholipids are arranged as a bilayer, heads on the outside and tails on the inside. 

• Intrinsic proteins pass through the entire bilayer, some of the proteins have channels/pores, and some have binding sites and are carrier proteins. These proteins allow the transport of water soluble molecules.
• Extrinsic proteins are only in one layer, those on the outer side ofter act as receptors for hormones. 

Many of the proteins and phospholipids have carbohydrates attached forming glycolipids and glycoproteins that make up the glycocalyx. 

Movement: 

• Most molecules move across the membrane by diffusion down a concentration gradient, 
• Small molecules (water/gases) and lipid soluble molecules diffuse between the phospholipid, 
• Polar molecules require channel or carrier proteins to move them.
• Channels are water filled pores that can be open at all times or they can be gated. 
• Carrier proteins have a specific binding site for the molecules/ions, This cal me facilitated diffusion, a passive (no ATP required) process,
• Some molecules are actively transported across the membrane (against the concentrate gradient). This requires ATP (released in respiration). The ATP changes the shape of the protein to move the molecule across the membrane.