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L8 Structure of cells and movement of molecules across cell membranes…
L8 Structure of cells and movement of molecules across cell membranes
Total body water (TBW)
(60% BW)
:star:Intracellular
2/3 of TBW
(40% BW)
Intracellular fluid
Cell membranes
Plasma membrane Structure
lipid bilayer membranes
3 main types membrane lipids
Phospholipids
amphipathic
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two major classes
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Cholesterol
determines fluidity and permeability
(Within membrane)
Glycolipids
Properties
fluid-mosaic model
Noncovalent assemblies
Each monolayer is a two dimensional liquid
Lipids & proteins can diffuse sideways freely within their own monolayer
Permits functional interactions
Rarely ‘flip flop’except for cholesterol ( Several hours)
Factors affecting membrane fluidity & permeability
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Lateral diffusion 2. Rotation 3. Swing
Flexion 5. Transverse diffusion (flip flop)
Asymmetric
Different phospholipids make up external and internal layers
Protein
2 Types
Integral proteins
{full or partial)
(Permanently attached to the cell membrane lipid bilayer)
-Types-
Span entire membrane
• once or multiple times
• many a helices
• Examples –voltage gated ion channels , hormone receptors eg insulin
Integral monotopic proteins
• Examples enzymes -monoamine oxidase
• Do not span the entire biological membrane
Peripheral proteins
(surface)
Non covalently bound often attached to integral proteins
Functions
Transport
Hydrophilic channel
ATP-pump
Enzyme functions
Active site exposed
Signal transduction
Receptors for external signals
Cell recognition
Molecules recognised by other cells proteins
Control adhesion
between cells and extracellular matrix
Cytoskeletal attachment
Microfilaments
Carbohydrates linked to proteins & lipids
Transport of molecules across cell membranes
Two basic processes
Diffusion
Simple diffusion
Through pores
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Through protein channels
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Through membrane
Facilitated Diffusion
(carrier mediated)
concentration or
electrical gradient
Active transport
Primary active transport (ATP dependent)
Secondary active transport (co-transport)
Large molecule transport
Endocytosis
Exocytosis
Functions
Transport
Contain specific molecular pumps and channels for transporting solutes
Control information flow
Specific receptors for external signalling
Generate chemical & electrical signals
Organization
Form compartments > surround organelles
Help maintain separate and distinct molecular environments
Cell-cell interactions
Cell recognition
Cell adhesion
Selective Barrier
Surround cells to hold enzymes and metabolites inside
Regulates flow of material into and out of the cell
Reactions
Enzyme activity
composed of a phospholipid bilayer that regulates what comes into the cell and what goes out.
Extracellular
1/3 of TBW
(20% BW)
interstitial fluid
Movement
Bulk flow and Starling’ Forces
Diffusion
Facilitated diffusion
Active transport
Co-transport
Flux = concentration difference x surface area x diffusion coefficient / thickness
