Membrane ultrastructure and function 14.10.22 Flashcards
What is a cell?
- Cell-specific functions
- Growth and division: cell cycle
What is in the nucleus?
Genome:
- Instructions
- Inherited disease
- Cancer
What is the smooth ER?
- NO ribosomes
- Site of lipid synthesis
- Some drug metabolism
What is the golgi body?
- Mediates protein sorting to specific sites
What is the Rough ER?
- Studded with ribosomes
- Site of protein synthesis
What is a ribosome?
Translate mRNA into protein
What are microtubules?
Give structure to cell
What is the plasma membrane?
- Keeps stuff in and out
- Selectively permeable
What are mitochondria?
- TCA cycle
- oxidative phosphorylation
- Maternal inheritance only
What are lysosomes?
Cell’s dustbin
What is endocytosis?
Energetic process to absorb/engulf molecules into a cell. Some extracellular fluid is usually engulfed too along with the molecule e.g. a portion of the membrane is invaginated to form a membrane bound vesicle called an endosome
* Occurs in neutrophils & macrophages - they implement phagocytosis (eating)
whereby they engulf entire cells/macromolecules to form a phagosome
* Pinocytosis (drinking) - bringing in dissolved solutes
* Receptor mediated - specific, found in depressed areas (coated pits) - allows the cell
to get the molecules it needs. Ligands bind to receptor, this complex is engulfed -
releasing the ligand into the cytosol (fluid portion of the cytoplasm outside the cell
organelles)
What is exocytosis?
Vesicle from the golgi apparatus, fuse with the plasma cell membrane, resulting in the expulsion of waste or the secretion of enzyme/hormones.
What type of things can be inside the phospholipid bilayer?
- Cholesterol (charged)
- Integral or intrinsic proteins e.g. ion channels
- Peripheral (extrinsic) protein
- Hydrophilic head
- Hydrophobic tail
contains; glycolipids: communication, joins cells to form
tissues + stability, glycoproteins: for cell to cell recognition + acts as receptors,
cholesterol: maintains fluidity in membrane
What is the membrane permeability?
- Selective permeability
- The fluidity is modified by cholesterol and temperature
What is the membrane freely permeable to?
- Water: aquaporins (small channels for water)
- Gases: CO2, N2, O2
- Small uncharged polar molecules: Urea, ethanol
What is the membrane impermeable to?
- Ions: Na+, K+, Cl-, Ca2+
- Charged Polar molecules: ATP, Glucose-6-phosphate
- Large uncharged polar molecules: Glucose
What does simple diffusion transport?
- Blood gases, water
- Urea, free fatty acids
- Ketone bodies
What is facilitated diffusion?
- The movement of solutes from a region of their high concentration to a region of their low concentration through protein channels
- This continues until dynamic equilibrium is reached. e.g. Glucose - protein assisted which is regulated by insulin. Voltage gate channels activated by action potentials
- no real energy expended
What is active transport?
- The movement of solutes from a region of low concentration to a
region of high concentration against the concentration gradient. - Both transmembrane carrier protein and ATP is required. e.g Na/K ATPase pump (sodium out and potassium into cell) - going against chemical and electrical gradients
- This is primary example
What is secondary active transport?
Secondary: sodium, glucose chain. Transports sodium and glucose across membrane. Energy not used directly it is derived from Na/K ATPase as lots of sodium out of cell so now high to low and means sodium can go down its gradient and take glucose with it
What is an osmotic drag?
When water also moves through the membrane with it.
What examples of ion channels?
- Put a channel in and ions can move in and out without having to interact with the membrane
- E.g. Voltage-gated and “Leak” channels
- Can also have pino-/phago-cytosis that are large molecules transported through vesicles.
Why are membranes and membrane proteins needed?
- Cell polarisation
- Compartmentalisation which allows cell to set up: ion gradients, diffusion, membrane potential
- Tightly regulated
- Disease disrupts this: heart disease, kidney failure
These processes are tightly regulated and diseases can set in
What is the membrane potential?
(Em = membrane potential)
- Potential difference across the cell membrane generated by different ion concentrations
- Potassium is the major determinant of Em
- Stable in most cells but sensitive to ionic imbalance
- When ions can’t diffuse anymore = equilibrium
What is the membrane potential?
- Extracellular fluid potential = 0mV
- Membrane potential is that on intra-cellular membrane
- Composed of various individual diffusion potentials
What does hyperpolarised mean?
When we take away positive from inside the cell, the intra-cellular becomes more negative e.g. K+
Or more rarely when negative ions move into the cell e.g. Cl-
What does depolarised mean?
When positive ions move into the cell e.g. Na+, Ca2+
How do we measure the diffusion potential?
The Nernst equation
Eion = (RT/zF)ln ionE/ionI
Log ration of the intracellular ion concentration and extracellular concentration
First part of equation = describes ion diffusion work done
Second part of equation (zFV) describes the electrical work done
How does permeability determine membrane potential?
- Ion conductance (permeability) is a key determinant of Em
- Permeability dependent on: channel numbers, and channel gating (open or close)
- If we change the ion permeability, it will change the membrane potential.
Describe the K+ diffusion potential
- Major role in K+ homeostasis
- Renal failure
- If we raise the extracellular potassium and what happens is the membrane potential becomes less negative and more positive so starts to depolarise.
Clinically called hyperkalaemia - If we decrease the extracellular potassium then the membrane potential becomes more negative
Clinically called hypokalaemia
What is Ischaemia?
- Can’t produce ATP anymore
- The K ATP channel opens too early so potassium can leak out when it shouldn’t
- So depolarisation occurs
- So the fast sodium channels can’t open anymore so sodium and calcium enter very slowly.
What does the epithelia require?
- polarisation of plasma membrane
- The epithelial permits cell-specific function
- It can strongly adhere to neighbours - tight junctions
What do tight junctions do?
Occluding: tight junctions help seal cells together in an epithelial sheet to prevent
leakage of molecules between them
What are gap junctions?
Communicating: gap junctions - allows the passage of small water-soluble ions and molecules
How do actin filaments provide anchoring?
Actin filaments: enable cell to cell adhesion through adherens
junctions (ADHERENS JUNCTION JOINS ACTIN BUNDLE IN ONE CELL TO A
SIMILAR BUNDLE IN ANOTHER CELL - HELPS KEEP CELLS TOGETHER) & cell to matrix (external to cell) adhesion through adherens junctions too.
How do intermediate filaments provide anchoring?
INTERMEDIATE FILAMENTS; enable cell to cell adhesion through desmosomes
(cell surface adhesion proteins + intracellular keratin cytoskeletal filaments - they
resist shearing forces & JOIN THE INTERMEDIATE FILAMENTS IN ONE CELL
TO THOSE IN A NEIGHBOUR) & cell to matrix adhesion through focal adheren
junctions. HEMIDESMOSOMES; anchor intermediate filaments in a cell to the basal lamina
What are cell receptors?
Gateway to intracellular signals: Examples; open a channel, activate a intracellular enzyme, induce second messenger (peptide hormone binds to receptor) & migrate nucleus to receptor-ligand complex
What is an example of an enzyme linked receptor?
e.g. tyrosine kinase - transfers a phosphate group from ATP to a protein in a cell thus acts like an on/off switch,
What is an example of an ion channel linked receptor?
participate in rapid signalling events found in electrically active cells
like neurons, also referred to as ligand gated ion channels,
What is a G-protein coupled receptor?
sense molecules outside the cell and activate inside signal transduction pathways to ultimately illicit a cellular response
What environmental changes can alter the membrane function?
- pH
- Calcium concentrations
- temperature (too cold: proteins slow down, too hot: proteins denature)
