2.1.5 Biological Membranes

What are the roles of membranes?

• Partially permeable barriers to control the exchange of materials in & out of cells

• Site of cell communication

• Site of chemical reactions

• Separates different areas of the cell

What is the Fluid Mosaic Model?

It explains how biological molecules are arranged to form cell membranes

• Phospholipids & proteins move around via diffusion

What is the role of phospholipids?

Forms the phospholipid bilayer, which acts as a barrier that controls which substances leave & enter the cell

What is the role of cholesterol?

• Regulates the fluidity of the cell

→Fitted between phospholipids → prevents them from packing too closely together

• Allows membrane to be impermeable to ions

→ Increasing strength & stability

What is the role of glycolipids?

• Maintain cell membrane stability

• Act as receptors for cell signalling

• Involved in cell recognition

• Involved in cell adhesion

What is the role of glycoproteins?

• Act as receptors for cell signalling

• Act as cell-markers/antigens

• Involved in cell recognition

• Involved in cell adhesion

What is the role of transport proteins?

They create hydrophilic channels → allows ions & polar molecules to pass through

e.g. channel (pore) proteins & carrier proteins

How do glycolipids and glycoproteins act as receptor molecules?

They can bind with certain substances at the cell’s surface 

What factors affect membrane structure and permeability?

• Temperature

• Solvent concentration

How does temperature affect the permeability of membranes?

• Temperature increases → lipids become more fluid

→ Increased fluidity reduces effectiveness of cell membrane as a barrier to polar molecules (can then pass through) → Increased permeability

• Diffusion occurs at higher speed (increased kinetic energy)

How does solvent concentration affect permeability of membranes?

• Organic solvents increase cell membrane permeability

→ dissolves lipids in membrane, causing the membrane to lose structure

What is diffusion?

(Passive) The net movement of substances (e.g. molecules, ions) from a region of its higher concentration to a region of its lower concentration 

What is facilitated diffusion?

Requires a channel protein to transport polar molecules, charged & water soluble molecules across a cell membrane

What is active transport?

The net movement of molecules and ions through a cell membrane from a region of lower concentration to a region of higher concentration using energy from respiration