Chapter 7 Membrane Structure and Function

–The plasma membrane is located at the boundary of every cell –It functions as a selective barrier –It allows the passage of nutrients, oxygen and waste  The membrane is made up of phospholipids  Phospholipids are amphipathic –i.e.. It has both the hydrophobic and hydrophilic regions  Fluid mosaic: a term used to describe the structure of plasma membrane with freedom of movement of molecules within the membrane  Cholesterol molecules: strengthens the fluid mosaic and makes it more stable  Proteins: determine which particles can pass across the membrane –Some protein serves as enzyme –Some serve as markers involved in disease fighting

Traffic across the membrane  Plasma membrane serves as a communication media between a cell and it’s environment  In addition to regulation of gas exchange, it also regulates the concentration of inorganic ions – Na +, K +, Ca + and Cl  In addition to regulation of gas exchange, it also regulates the concentration of inorganic ions – Na +, K +, Ca + and Cl -   It allows the passage of certain molecules and prohibits others – selective permeability   Hydrophobic core impedes transportation of ions and polar molecules   Hydrophobic as hydrocarbons – CO2 and O2 can dissolve and pass with ease   Hydrophilic substances avoid contact with the lipid bi-layer by passing through the transport protein (selective)   Passive transport   Diffusion: the net movement of particles from area of high concentration to area of low concentration – –As a result of random movement of particles

Passive transport   Diffusion: the net movement of particles from area of high concentration to area of low concentration – –As a result of random movement of particle  Diffusion depends on concentration gradients –Concentration gradient is the difference in the concentration of substances across a space  Dynamic Equilibrium: a continuous movement but no overall change

Plasma Membrane (7)  Intracellular versus extracellular  Membrane potential  Glycolipids and glycoproteins  Fluid-mosaic model

Membrane Lipids (7)  Phospholipids form a lipid bilayer –Hydrophilic (water-loving) polar heads –Hydrophobic (water-fearing) nonpolar heads  Cholesterol: Determines fluid nature of membrane

Membrane Proteins (7) in  Integral or intrinsic –Extend from one surface to the other  Peripheral or extrinsic –Attached to either the inner or outer surfaces of the lipid bilayer

Movement through the Plasma Membrane  Passive Transport –Diffusion –Osmosis –Filtration  Mediated transport mechanisms –Facilitated diffusion –Active transport –Secondary active transport

Diffusion  Movement of solutes from an area of higher concentration to lower concentration in solution –Concentration or density gradient  Difference between two points –Viscosity:- the state of being thick, sticky, and semifluid in consistency, due to internal friction.  Oil is more viscous than water

Diffusion

Osmosis  Diffusion of water (solvent) across a selectively permeable membrane  Important because large volume changes caused by water movement disrupt normal cell function  Cell shrinkage or swelling –Isotonic: cell neither shrinks nor swells –Hypertonic: cell shrinks (crenation) –Hypotonic: cell swells (lysis)

Osmosis

Osmosis

Filtration  Works like a sieve  Depends on pressure difference on either side of partition  Moves from side of greater pressure to lower –Example: In kidneys in urine formation

Mediated Transport Mechanisms  Facilitated diffusion –Higher to lower concentration without metabolic energy  Active transport –Requires ATP –Moves particles against concentration gradient  Secondary active transport –Ions or molecules move in same (symport) or different direction (antiport)

Secondary Active Transport

Endocytosis  Internalization of substances by formation of a vesicle  Types –Phagocytosis –Pinocytosis –Receptor- mediated endocytosis

Pinocytosis and Receptor-Mediated Endocytosis

Exocytosis  Accumulated vesicle secretions expelled from cell  Examples –Secretion of digestive enzymes by pancreas –Secretion of mucus by salivary glands –Secretion of milk by mammary glands

The End