Physiology Flashcards
Why are membranes important?
- cell plasma membrane forms an outer boundary of every cell
- selectively permeable
- controls the entry on nutrients and exit of waste and secretory products
- maintains differences in ion concentration inside and outside the cell
- participates in joining of cell to form tissues and organs
- enhances a cell to respond to changes (signals) in the cells environment
- it is a mechanical barrier
- crucial for cell survival
- different cells interact in different way with essentially the same ECF
What are the two principal constituents of membranes?
Lipids and proteins
Describe the head of the phospholipid
- negatively charged
- polar
- hydrophilic
Describe the tail of the phospholipid
- uncharged
- non polar
- hydrophobic
What is the thickness of the membrane determined by?
The length of the fatty acid chain
How can phospholipids move?
- they are free to diffuse within the lipid bilayer
- they can mover laterally, rotate or flex around their own half of the membrane millions of times per second
Where do membrane lipids differ from one another?
- in the fatty side chain
- in the head group
What can cross freely across the membrane and which molecules find it more difficult?
Small uncharged molecules can cross fairly freely eg. O2, CO2, NH3 and water
Charged molecules find it more difficult to cross eg. ions, proteins and sugars
What are the three functions of the lipid bilayer?
- It forms the basic structure of the membrane
- Its hydrophobic interior serves as a barrier, the cell can maintain differences in solute composition and concentrations inside and outside the cell
- It is responsible for the fluidity of the membrane eg. red blood cells and skeletal muscle cells
What are the two kinds of membrane proteins?
- peripheral
- integral
How can you predict how proteins will interact with the membrane?
Some amino acids are more hydrophobic or hydrophilic than others so knowing the amino acid sequence allows for prediction
Name some functions of integral membrane proteins
- ligand binding receptors eg. hormone receptors
- adhesion molecules eg. integrins, cell matrix adhesion molecules including cadherins
- pores and channels
- transmembrane movement of water- soluble substances
- carriers
- pumps
- enzymes eg. carbonic anhydrase
- participate in intracellular signalling eg. GTP- binding protein kinases
What are docking marker acceptors?
- located on the inner membrane surface, interact with secretory vesicles leading to exocytosis of the vesicle contents
- examples, neurotransmitters, digestive enzymes and peptide molecules
What is the glycocalyx?
A layer composed of glycoproteins and glycolipids
What is the sugar coating of the membrane?
A small amount of membrane carbohydrate located on the outer surface of cells
What is a leak channel protein?
A channel that is always open
What are the three types of specialised cell junction?
- tight junctions
- desmosomes
- gap junctions
Describe tight junctions
- join the lateral edges of epithelial cells near their luminal (apical) membranes
- polar junctions
- the basolateral membrane always faces the blood
- nutrients must cross the apical membrane, through the cytoplasm and then through the basolateral membrane to get to the blood
- the sodium potassium pump is only found at the basolateral membrane of epithelial cells
Describe desmosomes
- adhering junctions that anchor cells together
- act to hold neighbouring cells tightly together
- cells that can undergo physical stress ie. skin, uterus
- still permits material exchange
Describe gap junctions
- between adjacent cells, allows substances to pass from on cell to the neighbouring cell
- much less specific
- found in cardiac muscle cells, allows for rapid spread of electrical current, allows for contraction and relaxation in a coordinated, organised manner
What two properties influence whether a particle can permeate the membrane without assistance?
- solubility of the particle
- size of the particle
What is required for movement across a membrane?
A pathway and a driving force
By what two methods can molecules be passively driven down across the membrane?
- diffusion down a concentration gradient
- movement along an electrical gradient
Describe the five points of the Ficks Law of diffusion
- several factors in addition to concentration gradient influence the rate of net diffusion across the membrane
1. the magnitude of the concentration gradient
2. the surface area of the membrane across which diffusion is taking place
3. the lipid solubility of the substance
4. the molecular weight of the substance
5. the distance through which diffusion must take place
What is the equation of Ficks law of diffusion?
Q = delta C x A x P
where delta c = concentration gradient of substance
A = surface area of membrane
What are the extensions contained on the apical membrane?
Microvilli
Which ions can move along the gradient when an electrical gradient exists between ICF and ECF?
Only ions that can permeate the membrane
Where are voltage gated channels found?
In excitable cells
What is an electrochemical gradient?
When both an electrical and a chemical gradient is acting on a particular ion at the same time
What are aquaporins?
Water channels
What is osmolarity?
The concentration of osmotically active particles present in a solutions, units = osmoles of solute per litre ( osmol/l)
What is tonicity and what are the three types of tonicity?
- the effect a solution has on cell volume
- isotonic ; no net diffusion of water, no cell volume change
- hypotonic; increases cell volume, outside the cell to inside the cell movement
- hypertonic; decreases cell volume, inside the cell to outside the cell movement
What two mechanisms can cells use for selective transport?
- carrier mediated transport
- vesicular transport
What is the two forms of carrier mediated transport?
- facilitated transport (not requiring energy)
- active transport (requiring energy)
What are the two forms of active transport?
- primary active transport
- secondary active transport
Describe primary active transport
Energy is directly required to move a substance against its concentration gradient
Describe secondary active transport
Energy is required but not used directly. The carrier does not split ATP- instead it moves a molecule by using second hand energy stored in the form of an ion concentration gradient (usually Na+ gradient)
What are the three roles of the sodium potassium pump?
- helps establish the Na+ and K+ concentration of solutes inside the cell
- helps regulate cell volume by controlling concentration of solutes inside the cell
- the energy used to drive the pump indirectly serves as the energy source for secondary active transport
What are the two secondary active transport mechanisms?
- symport (co-transport) ; the solute and Na+ move in the same direction
- anti-port (exchange or counter-transport); the solute and Na+ move in opposite directions
What is endocytosis?
Pinching off of the membrane to engulf substance
What is exocytosis?
Vesicle fuses with the plasma membrane, releasing it contents into the ECF eg. secretion of enzymes, protein hormones and a way of adding carriers, channels or receptors to the plasma membrane (glut 4 and insulin)
What is the membrane potential (Em)?
Separation of opposite charges across the membrane. Refers to the difference in charge between the thin layers of ECF and ICF located next to the inside and outside of the membrane respectively
What are action potentials?
Rapid, transient changes in membrane potential
Describe resting membrane potential
- constant in non-excitable cells and in excitable cells are rest
- usually around -70 mv
What does the magnitude depend on?
The number of opposing charges separate by plasma membrane
What ions have the greatest concentration outside the cell and inside the cell?
- Na+ and Cl- outside the cell
- K+ inside the cell
Which ion is a skeletal muscle at rest most permeable to?
K+ rather than Na+
At resting potential, the membrane is most permeable to what
It is 100x more permeable to K+ than Na+
The concentration gradient for K+ tends to move where?
Out of the cell
What are the two opposing forces acting on K+?
- The concentration gradient (tending to move K+ out of the cell)
- The electrical gradient ( tending to move K+ into the cell)
When both balance, the ion is at equilibrium
Which leak channel is greater expressed?
K+ leak channel as opposed to Na+ leak channels
What is the equilibrium potential for K+ ?
-90mv
What does increasing permeability mean?
Moving closer to the ions equilibrium
What does the Nernst equation show?
The equilibrium potential for a single ion
What is the equilibrium potential for Na+ ?
+61mv
The greater the permeability of an ion, the greater the what?
The greater the tendency for that ion to drive membrane potential towards the ions own equilibrium potential
What does the goldman-hodgkin- katz (GHK) equation show?
The membrane potential, takes into account the relative permeabilities and relative equilibriums
What will the Na+/K+ ATPase generate?
A hyperpolarising current
What is the single most important factor in setting Em?
The K+ gradient
