Membranes Flashcards
Describe function and components of the plasma membrane
F- regulates materials entering and exiting the cell
S- 2 layers of phospholipids, proteins
The fluid mosaic model
Lipids :
Phospholipids
Phospholipids -Polar (charged phosphate head) Hydrophilic (attracted to water) Choline Phosphate Glycerol - long non polar lipid tails ( repelled by water) Fatty acids
Micelle (round with tails meeting in the middle)
Liposome (bilayer)
- phospholipid bilayer is one way the tail can be removed from the water
- phospholipid molecules can flow past each other laterally but not move veritically
Lipids : cholesterol
- polar head group
- steroid ring structure
- nonpolar hydrocarbon tail
- it makes the phospholipids pack more tightly and regulates the fluidity and flexibility of the membrane
Proteins
Integral proteins - permanently embedded
Peripheral proteins - penetrate just on surface
Diverse in S&F
TRACIE
Transport - protein channels(facilated) and protein pumps (active)
Receptors: peptide based hormones (insulin)
Anchorage: cytoskeleton attachments and extra cellular matrix
Cell recognition: antigens
Intracellular joinings: tight junctions and plasmodesmata
Enzymatic activity: metabolic pathways
Movement of large molecules
Endocytosis (movement in)
Exocytosis (movement out)
Movement of small molecules
A. Simple diffusion
B. Facilitated transport(passive transport)
C. Active transport
Movement of large molecules
1
Vesculation : vesicles can move large molecules in endoytosis
Endocytosis can either be
Pinocytosis: ‘cellular drinking’
Phagocytosis: cellular eating
E.g. white blood cells engulfing foreign bacteria
Movement of large molecules
Exocytosis (movement out)
Can be :
Constitutive (continuous)
Regulated (triggered by receptors)
E.g of regulated exocytosis is histamine release from mast cells following exposure to allergen
Movement of small molecules
Small diffusion
Protein - free bilayers are impermeable to ions but freely permable to water
Facilitated diffusion
- rate of movement is much faster than simple diffusion
- FD exhibits saturation kinetics
- FD is selective (i.e specific binding sites)
- also susceptible to competitive and non competitive inhibition
- membrane transport proteins can be carriers or channels
- channel proteins form water- filled pores and usually allow ions to pass thro (ie 10^6 - 10^7s-1)
- carrier proteins bind a specific solute and undergo confirmational change (100x slower 10^4 molecules S^-1)
Active transport
- mediated by carrier proteins coupled to an energy source
- different to FD in that it moves solutes against the concentration gradient
- mechanism can be symporter and antiporter
- intracellular pH is controlled by Na+/H+ antiporter
- this pumps H+ out of the cell to maintain cellular pH at 7.1
Understand how membrane transport can create and maintain solute gradients
Active transport Proteins - 2 main families of transport proteins use ATP hydrolysis to pump molecules across membranes
1. P class transporters (ion pumps) These couple phosphorylation and conformational change to pump ions across membranes
- ABC transporters
Have structure and mechanism
- P class transporters (ion pumps)
E.g
The Na+/K+ antiport pump in the plasma membrane higher eukaryotes
3x Na+ go out
2x K+ go in
Both being pumped against their concentration gradients
Active transport driven by ATP hydrolysis
(Blood is salty , ie high in Na+ and high in Cl- whereas cells have low Na+ and high K+)
ABC transporters
- these proteins were first found in bacteria
- called the ABC transporters because the contain 2 highly conserved ATP - binding cassettes (regions)
- Bacterial ABC transporters are used for both import and export
- Eukaryotic ABC transporters are mostly specialised for export. E.g the multi- drug resistance (MDR) proteins
ABC transporter example :
Bacterial permease in bacterial plasma membranes
Bacterial permease uses ATP hydrolysis to transport nutrients into the cell
Since bacteria often grow in nutrient- poor environments. E.g pond water, permease enables cell to take in nutrients against very substantial concentration gradients
Give example of a drug which exerts its affects by blocking membrane transport
Example : Digitalis
- an ancient herbal remedy for the treatment of heart disease
- derived from dried leaves of poisonous foxglove plant Digitalis purpurea
- contents a potent mixture if several compounds, some of which act in heart muscle cells by specifically blocking ion transport across the plasma membrane
- nowadays medicines derived from digitilis are used to treat congestive heart failure
Give an example of a disease where MDR (multi drug) transporters cause drug resistance to develop
- When MDR is over expressed in cancer cells, it can make them simultaneously resistant to a variety of chemically unrelated cytotoxic drugs
- When MDR is over expressed in plasmodium falciparum (the protozoan parasite which causes malaria), the parasites develop resistance to anti malarial drugs such as chloroquine
