Membranes - Structure and Function Flashcards
Regulating movement in and out of cell
- transports raw materials into the cell
- transports products and wastes out of the cell
- prevents entry of unwanted matter in
- prevents escape of matter needed for cell function
What is the permeability of cell membrane?
Selectively permeable
Membrane functions
MOVE TO 1ST POSITION!!!!!!!
- Regulates movement in and out of cell
- Separates cell from outside environment
- Regulates molecular and ionic composition inside cell
- Contains structures for transport
- Internal membrane system (separates organelles)
- Controls flow of information
Giving cell individuality
- separates cell from external environment
- forms closed boundary between ICF (intracellular fluid) and ECF (extracellular fluid)
Regulating molecular and ionic composition of ICF
- i.e. solutes like minerals, ions, glucose, gas molecules
- conditions inside cell must remain near constant for function = homeostasis
Structures for transport
- contains specific pumps and gates for transport
- passage of materials (polar or nonpolar) through different methos
- i.e. passive and active
Internal membrane system
- forms boundaries between organelles
- enables separate compartments to exist where specialized metabolic functions take place
- i.e. mitochondria, chloroplasts
Controlling flow of information
- information exchange between cells and environment
- biological communication (i.e. hormones)
- infor into cell via messenger
- specific receptors or markers for external stimuli
Features of plasma membrane
- sheetlike structures, two layrs thick (6-10nm) = BILAYER
- 50% lipids and 50% proteins
- main functional components: phopholipids (amphiphatic/amphiphilic in nature)
Classification of Phospholipids
- differ in size, shape, and chemical makeup
- classes of phospholipids are determined by their type of molecule that is bound to the phosphate group
Types of phospholipids involved in cell membrane
Phosphatidylcholine
Phosphatidylthanolamine
Phosphatidylserine
How do membranes arrange themselves in water?
- when phospholipids are added to water, spontaneously (no energy added) from bilayers
- this is driven by thermodynamic
- hydrophobic tails face inwards
- hydrophilic heads are in contact with water through H-bonding (intermolecular)
How do phospholipids act as selective permeable barriers?
- various substances will pass through at varying rates depending on type of material and transport across
- membrane lipid based: nonpolar substances can pass through
- also small uncharged substances (i.e. o2, co2)
- down the concentration gradient by simple diffusion
Fluid Mosaic Model
1972 - Singer and Nicholson
- membrane is a dynamic structure, asymmetrical in nature
- floating protein “icebergs” in a lipid “sea”
- 10 main types of lipids
- each type of cell/organelle has a different percentage of each lipid, protein, carbohydrate
Phospholipids
- foundation of bilayer
- composed of hydrophilic heads and hydrophobic tails
i.e. phospatidylcholine - regulate the passage of materials in and out of cel (selective permeability)
- lipid soluble substances pass through easily
Internal (intrinsic) proteins
- proteins embedded in the lipid bilayer (one region interacts with hydrophobic core)
- many are transmembrane: span entire bilayer and peak out on either side
- transport small or large substances across the bilayer
Peripheral (external or extrinsic) proteins
- proteins on the surface of membrane
- do not interact with hydrophobic core
- held to surface non-covalent bonds (H, ionic) usually interacting with exposed portions of integral proteins and directly with the lipid molecules
- most are on the cytosol side of membrane
- function as catalysts, cell identity markers
- anchor or reinforce membrane shape
- give the membrane different shapes
- i.e. red blood cell
Carbohydrates
- covalently bonded to lipids or proteins forming glycolipids or glycoproteins
- function in cell recognition and also ensure cell interactions
Cholesterol
- lipid molecule composed of 17 carbon atoms arranged in 4 rings found embedded between phospholipids
- controls the fluidity of the membrane
What is membrane fluidity?
The viscosity (measure of resistance to flowing) of the lipid bilayer of the cell
Factors that can influence membrane fluidity
- Temperature
- Degree of unsaturation
- Length of the fatty acid chain
- Presence of the steroid cholesterol
Temperature
- increased temp leads to increased fluidity
- as temp lowers membrane switch from fluid state to solid state as phospholipids are more closely packed
Degree of unsaturation
- presence of double bonds creates kinks in the chains which don’t allow fatty acid chains to allign = increased fluidity
- no double bonds = decreased fluidity (membrane very strong and stacked tightly)
Length of the fatty acid chain
- longer chain = decreased fluidity
- because intermolecular interactions between the phospholipid tails adds rigidity to the membrane
Presence of the steroid cholesterol
- reduce fluidity but help prevent solidification at low temp by preventing tight packaging of phospholipids
