Principle of cellular physiology 1

Question 1

State the functions of the plasma membrane.

Answer 1

• 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

Question 2

Describe the structure of the plasma membrane.

Answer 2

• Composed mainly of phospholipids and proteins but also contains cholesterol
• Hydrophilic region face outward and hydrophobic inwards
• Forms lipid bilayer: fluid structure with proteins

Question 3

What is the function of structural proteins?

Answer 3

Help to give cell support and shape

Question 4

What is the function of receptor proteins?

Answer 4

Helps cells communicate with their external environment via hormones, neurotransmitters etc

Question 5

What is the function of channel proteins?

Answer 5

Serve to allow water, ions and proteins to flow passively through the bilayer

Question 6

What is the function of transport proteins?

Answer 6

Transport molecules across cell membranes

Question 7

What is the function of glycoproteins?

Answer 7

Have a carbohydrate chain attached to them
Embedded in cell membrane and help in cell to cell communications and adhesion.

Question 8

Describe how hydrophobic and hydrophilic substances move across the plasma membrane.

Answer 8

Hydrophobic substances - passively diffuse across the lipid bilayer
Hydrophilic - need to be transported from protein carriers

Question 9

How does water diffuse across the plasma membrane?

Answer 9

• Osmosis
• Specialised proteins: aquaporin which provide transmembrane ‘pores’ for water and are always open

Question 10

Where are aquaporin found?

Answer 10

In regions where water transport is physiologically important e.g. kidney, intestine

Question 11

How is the diffusion of water regulated?

Answer 11

• Osmosis: the diffusion of water from a higher water potential to a lower water potential through a selectively permeable membrane

Question 12

What happens to the cells when exposed to hypertonic solutions?

Answer 12

Cells will shrink due to osmotic water loss -cytoplasmic components become more concentrated

Question 13

What is meant by isotonic?

Answer 13

Concentration of the salt outside the cell is equal to the concentration of salt inside the cell - no water movement

Question 14

How do ions diffuse across the membrane?

Answer 14

Via specialised proteins that form ion channels

Question 15

What is the permeability of different ions determined by?

Answer 15

The number of ion channels that are open.

Question 16

What does the presence of ion channels mean for membrane permeability?

Answer 16

• 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+

Question 17

Describe the function of the Na-K pump

Answer 17

• 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

Question 18

What is the Na-K pump important for?

Answer 18

• In nerve/muscle
• In organs which transport ions - kidney
• Powers ionic movements

Question 19

What is meant by the chemical driving force on Na+?

Answer 19

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

Question 20

What is meant by the electrical driving force on Na+?

Answer 20

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

Question 21

What do the two driving forces on Na+ combine to give? - what does this mean for the cell?

Answer 21

An overall electrochemical driving force
- Means the cell has access to a source of potential energy

Question 22

How does the cell utilise the potential energy from the large driving force on Na+?

Answer 22

Allows cells to intake sugars/amino acids - secondary active transport

Question 23

How is the electrochemical driving force on Na+ maintained?

Answer 23

By a Na+ pump - active transport

Question 24

Describe the CHEMICAL driving force on K+

Answer 24

• As there is a high internal [K+] the chemical driving force is outwardly directed

Question 25

Describe the ELECTRICAL driving force on K+

Answer 25

• Inwardly directed as K+ is positive

Question 26

Describe how Ca2+ is extruded from the cytoplasm

Answer 26

• By a Ca2+ pump using ATP i.e. active transport

Question 27

Explain the inwardly directed Ca2+ gradient.

Answer 27

• 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+

Question 28

What happens when a drug activates an ion channel?

Answer 28

Allows ions (Na+/Ca2+) to move down concentration gradient and evoke cellular response e.g. skeletal/smooth muscle contraction, insulin secretion and nerve impulse

Question 29

State where the ions K+, Na+ and Ca2+ are high in concentration and low in concentration in the cell.

Answer 29

[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