bio - module 3

Cell Membranes and Functions  

• The plasma membrane is the boundary that separates the living cell from its nonliving surroundings 

• The PM exhibits selective permeability, allowing some substances to cross it more easily than others 

• Fluid mosaic model – cellular membranes are fluid mosaics of lipids and proteins 

  • Phospholipids are the most abundant lipid in the plasma membrane, forming a phospholipid layer 

  • Phospholipids are amphipathic (polar on one end and non-polar on another end and they interact in one specific ways when placed in an aqueous environment) molecules, containing hydrophobic and hydrophilic regions 

  • The fluid mosaic model states that a membrane is a fluid structure with a 'mosaic' of various proteins embedded in it 

The Fluidity of Membranes 

• Phospholipids in the plasma membrane can move within the bilayer  

• Most of the lipids, and some proteins, drift laterally 

• Rarely does a molecule flip-flop transversely across the membrane  

• Flippase moves an outer phospholipid leaflet to the inner phospholipid leaflet, floppase does the opposite 

• As temp. cool, membranes switch from a fluid state to a solid state 

• This temp at which a membrane solidifies depends on the types of lipids  

• Membranes rich in unsaturated fatty acids are less fluid that those rich in saturated fatty acids 

• Membranes must be fluid to work properly; they are usually about as fluid as salad oil  

• The steroid cholesterol, acting as essential building blocks of the plasma membranes, plays pivotal roles in maintaining the structural integrity and regulating the fluidity at different temp 

• At warm temp (such as 37 degrees), cholesterol restrains movement of phospholipids  

• At cool temp, it maintains fluidity by preventing tight packing 

Membrane Proteins and Their Functions  

• A membrane is a collage of different proteins embedded in the fluid matrix of the lipid bilayer  

• Proteins determine most of the membrane's specific functions 

  • Peripheral proteins are not embedded  

  • Integral proteins penetrate the hydrophobic core and often span the membrane 

  • Integral proteins that span the membrane are called transmembrane proteins 

Membrane Proteins and Their Functions 

• Transport 

• Enzymatic activity 

• Signal transduction 

• Cell-cell recognition 

• Intercellular joining  

• Attachment to the cytoskeleton and extracellular matrix (ECM) 

Membrane Transport 

• Movement across membranes: 

  • Passive transport 

    • Diffusion 

    • Facilitated diffusion 

  • Active transport 

    • Primary  

    • Secondary 

  • Exocytosis and Endocytosis 

Passive Transport  

• Diffusion of a substance across a membrane with no energy investment  

  • Diffusion is the tendency for molecules to spread out evenly into the available space 

  • Although each molecule moves randomly, diffusion of a population of molecules may exhibit a net movement in one direction and finally reaches a dynamic equilibrium  

  • Substances diffuse down their concentration gradient, the difference in concentration of a substance from one area to another 

  • No work must be done to move substances down the concentration gradient 

  • The diffusion of a substance across a biological membrane is passive transport because it requires no energy from the cell to make it happen 

Osmosis – effect on water balance 

• The diffusion of water across a selectively permeable membrane  

• The direction of osmosis is determined only by a difference in total solute concentration 

• Water diffuses across a membrane from the region of lower solute concentration to the region of higher solute concentration 

Water Balance of Cells Without Walls 

• Tonicity is the ability of a solution to cause a cell to gain or lose water 

  • Isotonic solution: solute concentration is the same as the inside the cell; no net water movement across the PM 

  • Hypertonic solution: solute concentration is greater than that inside the cell; cell loses water 

  • Hypotonic solution: solute concentration is less than that inside the cell; cell gains water 

• Animals and other organisms without rigid cell walls have osmotic problems in either a hypertonic or hypotonic environment. To maintain their internal environment, such organisms must have adaptations for osmoregulation, the control of water balance 

• In plant cells, due to osmosis, water will diffuse into the vacuole, placing pressure on the cell wall 

• The protist paramecium, which is hypertonic to its pond water environment, has a contractile vacuole that acts as a pump  

Facilitated Diffusion: Passive Transport Aided by proteins 

• In facilitated diffusion, transport proteins speed movement of molecules across the plasma membrane 

• Channel proteins provide corridors that allow a specific molecule or ion to cross the membrane 

• Carrier proteins undergo a subtle change in shape that translocate the solute-binding across the membrane  

Active Transport – using energy to move solutes against their gradients 

• Moves substances against their concentration gradient 

• Requires energy, usually in the form of ATP 

• Performed by specifics proteins embedded in the membranes 

• The sodium-potassium pump is one of the most important active transport system in animal cells  

Three classes of transporters for active transport: uniporters, symporters and antiporters 

• Uniport: movement of one molecule independent of the other molecules is known as a uniport 

• Symport: movement of two molecules in the same direction through a protein channel is known as symport 

• Antiport: movement of two molecules in the opposite direction through a protein channel is known as antiport 

Cotransport – coupled transport by a membrane protein 

• Occurs when active transport of a solute indirectly drives transport of another solute  

• Plants commonly use the gradient of hydrogen ions generated by proton pumps to drive active transport of nutrients into the cell 

Two Types of Active Transport 

• Primary (direct) active transport : the energy is derived directly from the breakdown of adenosine triphosphate (ATP) 

• Secondary (indirect) active transport (co-transport): uses an electrochemical gradient created by the primary active transport 

Bulk Transport across the Plamsa Membrane by: Exocytosis and Endocytosis 

• Small molecules and water enter or leave the cell through the lipid bilayer or by transport proteins 

• Large molecules, such as polysaccharides and proteins, cross the membrane via vesicles 

• Exocytosis  

  • An active process of moving materials from within a cell to the exterior of the cell 

  • In exocytosis, transport vesicles migrate to the membrane, fuse with it, and release their contents 

  • Many secretory cells use exocytosis to export their products 

• Endocytosis  

  • An active cellular process in which substances are brought into the cell 

  • The cell takes in macromolecules by forming vesicles from the plasma membrane  

  • A reversal of exocytosis, involving different proteins 

• Three types of endocytosis: 

  • Phagocytosis (cellular eating): cell engulfs particle in a vacuole 

  • Pinocytosis (cellular drinking) : cell creates vesicle around fluid 

  • Receptor-mediated endocytosis: binding of ligands to receptors triggers vesicle formation