4 Membranes and Lipids Flashcards
Describe cell membranes
Membranes are lipid structures:
- Outer membrane separates the cell from the external environment
- Organelles are surrounded by membranes isolating them from the cytosol
Describe what else cell membranes can allow (apart from protection/separation)
Membranes allow concentration and separation of enzymes and biomolecules:
- Transporters, channels, and receptors control the local environment
- Important for reaction sequencing, energy conservation, and communication
Describe the physical properties of membranes
- Flexible
- Self-healing
- Selectively permeable
This allows for:
- Exocytosis, endocytosis
- Cell division
Describe how a membrane can be selectively permeable
With its hydrophobic interior, the lipid bilayer restricts the movement of ions and biomolecules
- this allows concentration gradients to develop (with the help of active transport systems)
- Molecules that don’t cross the lipid bilayer need to be transported, either passively through channels, or actively (using energy) by transporters
- ion concentrations vary between ECF and the cell, and within different cellular compartments:
> [K+] high inside, low outside
> [Na+] low inside, high outside
> H+ gradient in mitochondria
Describe the plasma membrane
All mammalian cells are enclosed by a plasma membrane composed of a lipid bilayer (hydrophobic, 5-8nm thick)
- sometimes referred to as ‘fluid mosaic’, as lipids move laterally
The plasma membrane also contains:
- Proteins: can be integral (membrane-spanning), peripheral, or anchored
- Carbohydrates: The glycocalyx is a layer of associated carbohydrates, attached via glycoproteins and glycolipids
Describe membrane lipids
Lipids are formed from fatty acids, which consist of:
- A hydrophobic hydrocarbon chain
- A terminal carboxyl group
Fatty acids may be:
- Saturated (no double bonds)
- Unsaturated (1 or more double bonds)
Phospholipids are a common and diverse class of lipids - they contain a phosphate group linked to the fatty acid via a phosphodiester linkage
Describe how lipids form micelles
Lipids naturally form micelles in an aqueous solution
- Bilayer formation is favoured if the cross-sectional areas of the head group and acyl side chains are similar
- If ‘edges’ are exposed, then vesicles form
Describe the 3 principle lipid components bilayers have:
- Phosphoglycerides; these are phospholipids
- Spingholipids; mostly glycolipids (contain carbohydrate)
- Cholesterol; a sterol
Describe phosphoglycerides
- They have a polar head group (phosphate and -OH)
- Hydrophobic tails (2 Fatty acids esterified to the glycerol backbone)
By convention, phosphate attached to C3
e.g. phosphatidylcholine
Describe sphingolipids
Sphingolipids contain the sphingosine moiety, which includes a fatty amine chain
- they have 2 hydrophobic tails:
> one is the tail of sphingosine
> the other a fatty acid residue
They are common in neuronal membranes
e.g. Sphingomyelin in the myelin sheath
Describe cholesterol
Sterols are lipophilic compounds characterised by 4 hydrocarbon rings
The sterol Cholesterol:
- is structurally more rigid than other membrane lipids
- comprises ≥ 10% of total lipid in the plasma membrane and Golgi
- cholesterol helps to maintain the membrane fluidity (by sitting in pockets formed by cis-conformation fatty acids)
- plants contain phytosterols, so a vegan diet is cholesterol-free
Describe the structure of the plasma membrane
The lipid bilayer is asymmetric
- the lipid composition varies in the outer and inner membrane
e. g. the outer leaflet contains more phosphatidylcholine and the inner leaflet phosphatidylserine - Lipid rafts important in signalling are formed by sphingolipids and cholesterol
The lipid bilayer is also dynamic
- mixing of compartments (e.g. vesicle formation, export from golgi_
- movement within leaflets is rapid
- movement between leaflets is very slow
Describe membrane proteins
roles, different types, etc.
Orientation of proteins in membranes is asymmetric (essential for function)
- Integral Membrane Proteins contain transmembrane domains and can have hydrophilic domains protruding into the extracellular and/or cytoplasmic spaces
- Peripheral Membrane Proteins are associated with the lipid bilayer, but generally separate with the aqueous component in cellular fractionation
- Lipid Anchored Proteins can be bound to either the inner or outer membrane surface (e.g. proteins with a GPI anchor)
Describe membrane protein functions
- Transporters (require energy) and channels (do not)
- Enzymes
- Receptors (not all are membrane proteins)
- Cell-cell recognition
- Intracellular joining
- Attachment to the cytoskeleton and extracellular matrix
These classes can overlap
- e.g. receptors can be ion channels and enzymes
- joining proteins can be pores
