Principle of cellular physiology 1 Flashcards
State the functions of the plasma membrane.
- Physical barrier: protects cellular contents + provides support
- Selective permeability: regulates entry and exit of ions, nutrients and waste molecules
- Electrochemical gradients: establishes and maintains an electrical charge difference
- Communication: has receptors that recognise and respond to molecular signals
Describe the structure of the plasma membrane.
- Composed mainly of phospholipids and proteins but also contains cholesterol
- Hydrophilic region face outward and hydrophobic inwards
- Forms lipid bilayer: fluid structure with proteins
What is the function of structural proteins?
Help to give cell support and shape
What is the function of receptor proteins?
Helps cells communicate with their external environment via hormones, neurotransmitters etc
What is the function of channel proteins?
Serve to allow water, ions and proteins to flow passively through the bilayer
What is the function of transport proteins?
Transport molecules across cell membranes
What is the function of glycoproteins?
Have a carbohydrate chain attached to them
Embedded in cell membrane and help in cell to cell communications and adhesion.
Describe how hydrophobic and hydrophilic substances move across the plasma membrane.
Hydrophobic substances - passively diffuse across the lipid bilayer
Hydrophilic - need to be transported from protein carriers
How does water diffuse across the plasma membrane?
- Osmosis
- Specialised proteins: aquaporin which provide transmembrane ‘pores’ for water and are always open
Where are aquaporin found?
In regions where water transport is physiologically important e.g. kidney, intestine
How is the diffusion of water regulated?
- Osmosis: the diffusion of water from a higher water potential to a lower water potential through a selectively permeable membrane
What happens to the cells when exposed to hypertonic solutions?
Cells will shrink due to osmotic water loss -cytoplasmic components become more concentrated
What is meant by isotonic?
Concentration of the salt outside the cell is equal to the concentration of salt inside the cell - no water movement
How do ions diffuse across the membrane?
Via specialised proteins that form ion channels
What is the permeability of different ions determined by?
The number of ion channels that are open.
What does the presence of ion channels mean for membrane permeability?
- Permeability to K+ is high whilst permeability to Na+ and Ca2+ is very low
- Intracellular fluid is high in K+ and low in na+ and Ca2+ while extracellular fluids are high in Na+ and Ca2+ and low in K+
Describe the function of the Na-K pump
- An enzyme that hydrolyses ATP
- Transports Na+ out of cell in exchange for K+
- Energy from ATP used to transport molecules against a concentration gradient
What is the Na-K pump important for?
- In nerve/muscle
- In organs which transport ions - kidney
- Powers ionic movements
What is meant by the chemical driving force on Na+?
Na-K pump activity establishes and maintains an inwardly-directed concentration gradient for Na+ which results in Na+ wanting to diffuse down the concentration gradient
What is meant by the electrical driving force on Na+?
Resting membrane potential means inside of cell is slightly more negative than outside - Na+ has positive charge so is attracted to the inside of the cell
What do the two driving forces on Na+ combine to give? - what does this mean for the cell?
An overall electrochemical driving force
- Means the cell has access to a source of potential energy
How does the cell utilise the potential energy from the large driving force on Na+?
Allows cells to intake sugars/amino acids - secondary active transport
How is the electrochemical driving force on Na+ maintained?
By a Na+ pump - active transport
Describe the CHEMICAL driving force on K+
- As there is a high internal [K+] the chemical driving force is outwardly directed
Describe the ELECTRICAL driving force on K+
- Inwardly directed as K+ is positive
Describe how Ca2+ is extruded from the cytoplasm
- By a Ca2+ pump using ATP i.e. active transport
Explain the inwardly directed Ca2+ gradient.
- Huge chemical gradient and electrical gradient
- Driving force on Ca2+ is larger than the one on na+
- Ca2+ wants to enter cell but plasma membrane is very impermeable to Ca2+
What happens when a drug activates an ion channel?
Allows ions (Na+/Ca2+) to move down concentration gradient and evoke cellular response e.g. skeletal/smooth muscle contraction, insulin secretion and nerve impulse
State where the ions K+, Na+ and Ca2+ are high in concentration and low in concentration in the cell.
[K+] = high in cytoplasm and low in extracellular fluid
[Na+] = low in cytoplasm and high in extracellular fluid
[Ca2+] = low in cytoplasm and higher in extracellular fluid
