Cell Membrane

IS3

Functions•Protects the cell•Semi-permeable barrier: selects incoming and outgoing substances (passive/active transport)•Maintain concentration of various substances•Allows cell recognition/communication•Allows receptivity (hormones, etc)•Allows the cell to maintain its shape•Helps compartmentalize subcellular domains

Phospholipids are a major component of cell membranes.

Fluid Mosaic Model (1972)

- Phospholipid bilayer with proteins forming a mosaic pattern as they float/penetrate through the membrane•Hydrophilic end: phosphate = polar•Hydrophobic end: fatty acid = non-polar

Credit: Stephen Taylor

Transport through membranes•Active: ATP needed / proteins are used as pumps to move substances against the concentration gradient.•Passive: no ATP needed / small particles cross the membrane through special proteins or lipid bilayer

Passive TransportDiffusion: •movement of particles through the lipid bilayer •from an area of high concentration to an area of low concentration •O2, H2O, urea, glycerol, CO2

Facilitated Diffusion: •diffusion of particles through special transport proteins •charged molecules (even small ones) and large uncharged molecules•Na+, K+, glucose

Passive Transport

Osmosis: diffusion of water across a semi-permeable membrane: plasma membrane permeable to water but not to solute – Solute = dissolved particle – Solvent = liquid medium in which

particles may be dissolved

Movement: from low solute concentration to

high solute concentration

Credit: Stephen Taylor

http://education.uoit.ca/lordec/ID_LORDEC/diffusion_osmosis/garib_diffusion_osmosis.swf

Solutions can be... Isotonic: The solutions being

compared have equal concentration of solutes.

Hypertonic: The solution with the higher concentration of solutes.

Hypotonic: The solution with the lower concentration of solutes.

http://cc.hccs.edu/Biology/AllStudyPages/Diffusion_Osmosis/Elodeagif.swf

Active Transport•Movement across membrane against concentration or electrochemical gradient (from low to high concentrations )•Requires:• specific integral membrane proteins • requires energy to overcome the concentration and electrochemical gradient

•It is used to pump specific compounds in or out of the cell • Example: sodium, potassium, calcium

•It allows cells to accumulate needed substances even when the concentration is lower outside

The K+/Na+ Pump: An example of Active Transport

•Essential for the nerve impulse to be produced

•The pump is an integral membrane protein

– Cellular [K+] is high and [Na+] is low– Protein pumps K+ in and Na+ out– This pump works independent of

concentration gradient

•K+/Na+ Pump

Types of transport

Endocytosis/Exocytosis

•Endocytosis: ingesting...– particles = phagocytosis – liquids = pinocytosis

•Exocytosis: releasing particles/wastehttp://www.maxanim.com/physiology/Endocytosis%20and%20Exocytosis/Endocytosis%20and%20Exocytosis.htm

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