Give 3 cellular functions underpinned by differences in ECF and ICF compositions.
- Electrical activity in the nervous system
- Muscle contraction
- Formation of urine in the kidney
Give Fick’s Law of diffusion.
Jx = Px ([X]o – [X]i )
- Jx = flux
- Px = permeability coefficient, derived from membrane thickness, solubility of X in the membrane and diffusion coefficient.
- ([X]o – [X]i) = difference in concentration
How do pores allow for movement of substances?
Always open
Simple diffusion down a concentration gradient
Porins in mitochondria, perforins in lymphocytes, aquaporins
How do channels allow for movement of substances?
- Non-gated channels: potassium leak channels
- Gated channels: Na+ channels, K+ channels, Ca2+ channels, anion channels like Cl- channels
- Gated channels allow opening and closing by voltage, ligand or secondary messenger to allow movement of substances
How do carriers allow for movement of substances?
- Transporter that does facilitated diffusion
- Specific binding of solute, change of conformation, release of solute
- Transporters can have 1 solute, more than 1 solute in the same direction or more than 1 solute in opposite directions.
How do pumps allow for movement of substances?
Active transport of ions that utilises hydrolysis of ATP to move them against a concentration gradient.
- Sodium-potassium pump is ubiquitous and is present in all cells. Does secondary active transport – relies on a gradient set up by the active transport of another solute.
- Hydrogen-potassium pumps found in the stomach
Describe the sodium ion concentration of ECF, ICF and blood plasma.
ECF = higher
ICF = lower
blood plasma = higher
Describe the potassium ion concentration of ECF, ICF and blood plasma.
ECF = lower
ICF = higher
Blood plasma = lower
Describe the chloride ion concentration of ECF, ICF and blood plasma.
ECF = higher
ICF = lower
blood plasma = lower
Describe the protein concentration of ECF, ICF and blood plasma.
ECF = lower
ICF = higher
Blood plasma = higher
When is electroneutrality achieved?
For electroneutrality in each fluid compartment, the sum of the anion concentration must equal the sum of the cation concentration.
What is an electrochemical gradient?
When charged molecules diffuse down a chemical gradient, there is build up of charge. And so charged molecules then move down the electrical gradient in the opposite direction.
So, the driving force for passive transport is this electrochemical gradient.
Define equilibrium potential.
Equilibrium potential – when the electrical gradient and the chemical gradient are equal for a particular ion.
What is hypokalaemia and hyperkalaemia?
Hypokalaemia – too little K+. Causes muscle weakness and cardiac arrhythmias
Hyperkalaemia – too much K+. Causes cardiac arrhythmias
Where is calcium in the body?
99% of calcium in the body is inorganic, mineralised matrix of bone as hydroxyapatite.
Intracellular in endoplasmic reticulum. 0.00002% free in the cytosol. Extracellular fluid contains 0.1% calcium at about 2.5mmol/l.
- 50% of this is ionised Ca2+ (the biologically active calcium)
- 5% complexed Ca2+ salts and bicarbonates
- 45% is protein bound
What are some responses to calcium entering a cell as a secondary messenger?
When it enters the cell via calcium ion channels, calcium can activate intracellular processes and changes cellular function.
Cellular function is regulated by interaction with intracellular calcium binding proteins and calcium-sensitive protein kinases.
Biological responses include neurotransmitter release, contraction and secretion.
How is calcium gained, exchanged and lost in the body?
- Calcium gained form diet
- Calcium exchanged in bone remodelling. There is more remodelling during growth or pregnancy.
- Calcium lost in urine, faeces and lactation
Name the 3 hormones involved in calcium homeostasis.
- Parathyroid Hormone (PTH)
- Calcitonin
- Active vitamin D (1,25-dihydroxycholecaliferol or 1,25-DHCC, calcitriol)
What is the parathyroid gland and what does it secrete?
PTG is a discrete area of tissue embedded in thyroid gland or near. Made up of chief cells which synthesise, store and secrete PTH.
What is the effect of PTH in response to low calcium?
Low calcium stimulates PTH that acts on bone to release more calcium ions from the ECF in the bone.
Can also act in the longer term to break down hydroxyapatite crystals, the mineralised from of calcium in the bone, but this is more of a long term effect, and is done by osteoclasts.
PTH increases absorption of calcium in the kidney, reducing the amount that is lost in the urine.
PTH promotes the formation of active vitamin D, which enhances the absorption of calcium in the intestine.
What do C-cells in the thyroid gland secrete?
Calcitonin is secreted. It is a peptide hormone that acts primarily on kidney and bone and reduces calcium ion levels.
How does calcitonin decrease blood calcium ion concentration?
- Reduces the calcium released from the skeleton into the plasma by acting on osteoclasts and inhibiting their activity.
- Binds directly to osteoclast and causes them to shrink and detach from the bone, and so they cannot break down the bone anymore.
- Calcitonin acts on the kidney to reduce absorption of calcium and increase the amount lost in urine.
How is active vitamin D produced in the body?
- When cholesterol is exposed to UV light in the skin, it is converted from cholesterol to cholecalciferol, vitamin D3.
- Vitamin D3 can also be provided through the diet.
- In the liver, cholecalciferol is converted to 25-hydroxycholecalciferol.
- In the kidney, the enzyme, renal 1 alpha hydroxylase, produces active vitamin D. this stage is regulated by PTH.
What is the effect of active vitamin D on blood calcium ion concentration?
Increases absorption of calcium ions from the intestine. It moves into gut epithelial cell and then moves into the nucleus, to cause the production of new proteins within the cell. This helps the cell deal with calcium, as the proteins could be calcium channels, calbindin, calcium pumps and exchangers. So allows calcium to move out of bloodstream and into the cells, to protect the bone.
