Roles of membranes Flashcards
Types of membranes
Two main types – plasma/cell membrane (keeps everything in the cell in), intracellular membranes of organelles (keeps organelles separate)
Plasma membrane coordinates interactions with the surrounding environment
Function of membranes
Acts as a permeability barrier – can control which substances are inside and outside cell, compartmentalises specific functions
Plasma membrane helps organise organelles functions (within organelles)
Mediates the transport process – cells require metabolically useful solutes e.g oxygen, cells must be able to dispose of metabolic waste products, regulate transport of solutes, passive transport (simple and facilitated diffusion, osmosis, filtration), active transport, endo/exocytosis
Mediates interaction with the outside of the cell – signal transduction, cell signalling:
1. Receptor binds to a molecules
2. Receptor binding generates secondary messengers
3. Secondary messengers alter gene expression and cell function
Signal transduction – where an extracellular signal is transmitted to the interior of the cell
Involved in cell-to-cell communication – joins cells of similar function together, cells connected and components exchanged via gap junctions (animals), plasmodesmata (plants), septa (fungi)
Sites of specific biochemical functions – hosting of essential cellular functions which include ETC, photosynthesis, translation of specific proteins, functional proteins are embedded in membranes, localise activity, exploit gradients to drive processes
A cell that contains multiple small structures greatly increases the SA which allows much more efficient solute exchange, allows biochemical processes to occur more efficiently, many cells have modified SA (e.g microvilli in gut)
Membrane proteins
Proteins determine membrane functions
Two types – integral and peripheral
Can be partially inserted into the membrane, exposed on only one surface, span the entire membrane
Integral proteins
- Include transmembrane proteins that span the membrane
- Membrane spanning regions/domains consist of hydrophobic amino acids
- Usually arranged into alpha-helices
- Cytoplasmic or extracellular parts are hydrophilic
- Peripheral proteins
- Contact the membrane temporarily
- Easily removed which allows them to be involved in cell singalling
Membrane structure
Key structural features – very thin (8 nm), physical barrier that keeps contents in an external materials out, barrier must be insoluble to water otherwise aqueous interior of cell will dissolve it but cannot be completely impermeable as it must allow certain molecules and water to permeate through
Main components of membrane
Sphingolipids
Glycolipids
Membrane proteins (enzymes, transporters, receptors)
Sterols (cholesterol (animals), ergosterol (fungi), phytosterol (plants))
Phospholipids
Amphipathic
Self assemble in water into bilayers
Placed in water form into micelles
Micelles – lipid molecules that arrange themselves in spherical form in aqueous solutions
Composition – glycerol, two fatty acids (uncharged, non polar tails), phosphate group (negatively charged, polar, hydrophilic head that is modified by a headgroup (e.g alcohol))
Properties of membranes
Lateral movement frequent – phospholipids may move up to 2 micrometres per second
Increased phospholipid movement leads to increased membrane fluidity
Composition of bilayers influences the movement of phospholipids
High levels of unsaturated fatty acid tails prevents tight molecular packing
Cholesterol reduces membrane fluidity (membrane made solid)
Proteins, glycoproteins and glycolipids float in the phospholipid bilayer, cholesterol is inserted into the lipophilic interior
Fluid mosaic model
Cell membrane as a tapestry of several types of molecules that are constantly moving
Movement helps the cell membrane maintain its role as a barrier between the inside and outside of the cell
Membrane synthesis
Phospholipids are synthesised in the SER
1. Membrane proteins have N-terminal sequences that direct ribosomes to the ER membrane
2. Attachment of the signal recognition particle (SRP)
Signal recognition particle – an abundant, cytosolic, universally conserved ribonucleoprotein (protein RNA complex) that recognises and targets specific proteins to the ER in eukaryotes and the plasma membrane in prokaryotes
3. Ribosomes dock on protein complex (translocon)
4. Polypeptide synthesis starts
5. Signal peptide is cleaved
6. Full synthesis
7. Folding
Membrane transport
Plasma membrane regulates traffic of molecules into and out of the cell
Membranes selectively permeable with the exception of gases (o2 and co2) and small hydrophobic molecules – most molecules cannot diffuse across the phospholipid bilayer at rates sufficient to cell needs
ATP-powered pumps, protein ion channels and protein transporters mediate the transport of ions, sugars, amino acids and other metabolites across cell membranes
ATP powered pumps
Utilise the energy released by ATP hydrolysis to power movement of specific ions or small molecules against a chemical concentration gradient – active transport
e.g P-class pumps, V-class proton pumps, F-class proton pumps, ABC superfamily
ABC transporters
A large family of active transporters
e.g peroxisomal ABC transporter that transports fatty acids into the peroxisome for degradation
When this gene is mutated in humans it causes a buildup of very long chain fatty acids damaging the myelin sheath of neurons of the brain
In plants it causes buildup of fatty acids in the leaf and prevents germination of seedlings
Protein/ion channels
Channels permit movement of specific ions (water or hydrophilic molecules) down their electrochemical gradient –facilitated diffusion/assisted transport
Most ion channels open only in response to specific chemical or electrical signals (gated channels)
Some ion channels are open much of the time (non-gated channels)
e.g Ligand-gated, mechanically gated, always open, voltage gated
An electrochemical gradient consists of two parts – the chemical gradient/difference in solute concentration across a membrane and the electrical gradient/difference in charge across a membrane
How to study function of voltage gated ion channels
Electrophysiologists use a technique called patch clamping
e.g voltage-clamp – voltage across the cell membrane is controlled by the experimenter, voltage-clamp enables researcher to investigate the opening, closing, regulation and ion conductance of a single ion channel
1. Glass electrode, filled with a current-conducting saline solution is applied with slight suction to the plasma membrane
2. The tip covers a region that contains only one or a few ion channels
3. A second electrode is inserted through the membrane into the cytosol
4. A recording device measures current flow only through the channels in the patch of plasma membrane
Protein transporters
Fall into three groups – uniporters, symporters, antiporters
Facilitate movement of specific small molecules or ions
Uniporter
- Transports a single type of molecule down its concentration gradient (high to low) e.g GLUT1 in mammalian cells
1. Binding of glucose to the outward facing site
2. Triggers a conformation change in the transporter that results in the binding site facing inward toward the cytosol
3. Glucose then is released to the inside of the cell
4. The transporter undergoes the reverse conformational change, regenerating the outward-facing binding site
If the concentration of glucose is higher inside the cell than outside, the cycle will work in reverse (4-1 instead of 1-4), resulting in higher net movement of glucose from inside to out
Symporter
- Transports molecules and ions in the same direction e.g two Na+/one glucose symporter
1. Simultaneous binding of Na+ and glucose to the outward-facing binding sites
2. Generates a conformational change resulting in inward-facing sites
3. Dissociation of the bound Na+ and glucose into the cytosol
4. Allows the protein to revert to its original outward-facing conformation ready to transport additional substrates
Antiporter
- Transport molecule and ions in the opposite direction e.g two Na+/proton antiporter
1. Proteins bind to the protein carrier in the outward-facing state causing it to switch to the inward-facing state
2. One the inside, protons are exchanged for Na+ and the protein carrier moves back to the outward-facing site
Uniporters – transport a single type of molecule down its concentration gradient (high to low)
Symporters and antiporters – transport a single type of molecule (black circle) against its concentration gradient (low to high) with the movement of one or more different ions down its concentration gradient = co transport
Vesicle transport
Vesicles – self-contained structure consisting of fluid or gas surrounded and enclosed by an outer membrane, control the transport of large substances into and out of the cells
Exocytosis – transports things out of the cell
Endocytosis – transport things into the cell
Endocytosis
Pinching off the PM to form vesicles
Phagocytosis – cell eating, internalise large particles
Pinocytosis – cell drinking, takes in substances from extracellular fluid
Exocytosis
Fusion of secretory vesicles with the plasma membrane
Discharges the vesicle contents into the extracellular space
e.g the release of neurotransmitters at the synaptic bulb
