Unit 1.1.2: Cell Membranes

The Role of Membranes

• Seperating cell contents from the outside environment.

• Separating cell components from the cytoplasm.

• Holding the components of some metabolic pathways in place. 

Plasma membranes are partially permeable membranes - they are permeable to water and other solutes.

The Fluid Mosaic Model

The fluid mosaic model describes the molecular arrangement in cell membranes. Main features: 

• A bilayer of phospholipid molecules forming the basic structure.
• Various protein molecules floating in the phospholipid bilayer, some freely, some bound to other components within the cell.
• Extrinsic and intrinsic proteins. Extrinsic = partially embedded in the bilayer. Intrinsic proteins = spanning the bilayer. 

-The phospholipid bilayer is the basic structural component of plasma membranes. 

-Phospholipid molecules consist of a phosphate head, that is hydrophilic (water-loving) & 2 fatty acid tails which are hydrophobic (water hating). 

The Fluid Mosaic Model

• If you surround them with water, a bilayer will form. 

• The hydrophobic tails are held away from the water molecules 

• In this state, the phospholipid molecules can move freely within the membrane.

• The hydrophobic region, giving the bilayer stability.

• This creates a barrier that separates cell contents from the outside world - as many metabolic reactions take place in a water-based environment.

Communication and Cell Signaling

Cell Signaling. Is the communication between cells to trigger a response.

• Many molecules can act as signals. E.g. Cytokines
• Signal Molecules fit into receptors on cell surface membranes, due to their shapes being complementary to each other. 
• Any cell with a receptor for the hormone molecules is called a target cell.

Medicinal Drugs. Have been developed that are complementary to the shape of a type of receptor molecule. 

• Such drugs are intended to block receptors. 
• Viruses enter cells by binding to the receptors on the cell's plasma membrane. 
• Some poisons also bind with receptors. 

Passive Transport. Is the processes that depend on the kinetic energy in water/gas molecules.

Diffusion. Is the movement of molecules from a region of high concentration of that molecule to a region of low concentration of that molecule down a concentration gradient.

Diffusion is affected by: 

Passive Transport

• Lipid-based molecules can simply pass through the bilayer.
• Very small molecules/ions such as Oxygen & Carbon Dioxide molecules are small enough to pass between the phospholipid molecules. 
  
• Small, charged particles such as Sodium Ions or larger molecules - Glucose, cannot pass.
• They diffuse by facilitated diffusion, in which the membrane proteins allow the substances to diffuse through the membrane. 

• Channel Proteins. From pores in the membrane to allow only one type of ion through. 
• Carrier Proteins. Shaped so a specific molecule (E.g. Glucose) can fit into them at the membrane surface. They can change shape to allow the molecule through to the other side of the membrane.

Active Transport. Is the movement of molecules or ions against a concentration gradient and across membranes, using energy in ATP from respiration.

• Carrier proteins, act as pumps to allow this to happen. They are shaped in a way that is complementary to the molecule they carry.
• They carry larger molecules & ions which cannot pass through the lipid bilayer by diffusion. 

• They differ from the proteins in facilitated diffusion by:
  • They carry specific molecules one way across the membrane.
  • They use metabolic energy in the form of ATP.
  • They can carry molecules against the concentration gradient. 
  • They can carry molecules at a much faster rate than diffusion. 
  • Molecules can be accumulated either inside cells or organelles. 

Osmosis

Osmosis. Is the movement of water molecules from high to low water potential across a partially permeable membrane.