Structure and Function of cell membrane 1.1 and 1.2

Question 1

Why is chemical composition of ECF and ICF different?

Answer 1

Difference comes about due to the presence of the cell membrane (aka plasma membrane) which separates ECF and ICF.

Question 2

What are the chemical differences between ECF and ICF?

Answer 2

ECF-
has higher composition of sodium and chloride ions.
More bicarbonate ions.
Protein only in plasma compartment.

ICF-
has higher concentration of potassium.
More phosphates and sulphates.
Protein more abundent in ICF.

Question 2

What does cell membrane separate?

Answer 2

ECF and ICF

Question 2

What is part of ECF ?

Answer 2

Plasma - interstitial

Question 3

What is the distribution of fluids?

Answer 3

2 x ICF (28L) than ECF (14L)

Question 4

What is distribution of fluid in ECF?

Answer 4

80% in interstitial (11L)
20% in plasma (3L)

Question 5

What does capillary wall separate?

Answer 5

Separates plasma from interstitial

Question 6

Where can water pass between compartments?

Answer 6

Water passes freely between all compartments; from plasma to ISF between the capillary cells of the blood vessel wall and from ISF to ICF via aquaporin channels.

Question 7

How does cell membrane let water through?

Answer 7

The cell membrane is studded with aquaporin channels – channels that are permanently open to water. This allows water to move, unhindered, between the ECF and ICF.
There are no barriers to H2O movement

Question 8

What about gases ability to move between compartments?

Answer 8

Gases also pass freely between all compartments

Question 9

What about nutrients and waste’s ability to move between compartments?

Answer 9

Nutrients and waste also pass easily, but sometimes need help crossing the cell membrane (see later)

Question 10

What about ion’s ability to move between compartments?

Answer 10

Ions pass freely across the capillary wall so exchange readily between plasma and ISF. They do not penetrate the cell membrane.

Question 11

Ions distribution across capillary wall?

Answer 11

Composition of ions across plasma and interstitial fluid is the same

Question 12

Difference in proteins between plasma and ISF?

Answer 12

Plasma has plasma proteins- different to ISF

Question 12

How is ECF different to ICF in terms of ions?

Answer 12

Where sodium is high in ECF, low in ICF.
Where potassium low in ECF, high in ICF.

Question 13

Why do we find lots of proteins in ICF?

Answer 13

Making up all the organelles

Question 14

Why are these differences in the composition of fluid in the extracellular and intracellular compartments fundamental for life?

Answer 14

many physiological and biochemical processes rely on the concentration gradients between different compartments

Question 15

Describe the way in which cell membrane is selective barrier?

Answer 15

It is freely permeable to some substances e.g. O2 and CO2, but the difference in composition between ECF and ICF shows that permeability is selective and not universal.

Question 16

Can permeability of cell membrane vary?

Answer 16

Permeability can vary, may increase or decrease at different times, fundamentally important for various cell functions e.g. transmission of the nervous impulse.

Question 17

Describe what a membrane being dynamic means?

Answer 17

Membranes are dynamic, continually being formed and maintained or dismantled and metabolized according to the needs of the cell.

Question 18

Describe the physical characteristics of membranes?

Answer 18

Membranes are very thin double layers of sheets of lipids (the lipid bi-layer)

Question 19

Describe physical characteristics of major membrane lipids?

Answer 19

Major membrane lipids are phospholipids which have a hydrophilic (water loving) phosphate head and a hydrophobic (water repellent) fatty acid tail

Question 20

Major membrane lipid arrangement in aqueous environment?

Answer 20

In an aqueous environment (i.e. ECF ad ICF!) arrange themselves so that the polar (hydrophilic) part is on the outside of the membrane and the fatty acid chains are on the inside, away from H2O.
Hydrophilic = lipophobic (lipid repellent);
Hydrophobic = lipophilic (lipid loving)

Question 21

Why are membranes very flexible?

Answer 21

because of fatty acids in vivo, behave like an oil, therefore can stretch, although may rupture if over-stretched.

Question 22

Why are cell membranes good insulators?

Answer 22

Membranes are excellent insulators against movement of electrical charge. This prevents the passage of electrons – important in maintaining electrical stability of cell

Question 23

What are the types of proteins involved in cell membrane?

Answer 23

Integral proteins - both sides of membrane
Peripheral proteins-one side of membrane

Question 24

Where are carbohydrate groups in membrane?

Answer 24

Carbohydrate replacing phospholipid head, directly attached to lipid component (glycolipid)
Carbohydrate attaching to protein (glycoprotein)

Question 25

Where are carbohydrate groups found?

Answer 25

always orientated on extracellular. Important in cell to cell communication. Flag to tell immune system the identity of the cell

Question 26

What are the common functions of the membrane proteins?

Answer 26

Receptors
Transport (transporter and channel proteins)
Enzymes
Maintenance of cell structure (anchorage)
Communication

Question 27

Characteristics of integral proteins?

Answer 27

integral to the membrane structure. Penetrate the membrane from ECF to ICF. Allow communication of an extracellular signal e.g. neurotransmitter or hormone, to the intracellular space to create a cellular response.

Question 28

How is extracellular signal communicated to intracellular side of cell?

Answer 28

Ligand (extracellular chemical signal) binds to receptor protein.
Causes a conformational change in protein and this is transmitted to intracellular side.
Changes events in cell and cell behaviour.

Question 29

Would all tissues respond to extracellular signal?

Answer 29

No. Only tissues that express receptors for a particular hormone or neurotransmitter will respond to chemical.

Question 30

Characteristics of transport proteins?

Answer 30

Transporters – integral proteins which allow movement of ions or molecules across the membrane

Question 31

What are the two forms of transport proteins?

Answer 31

Channels and Carrier mediated transport proteins (transporters)

Question 32

What do channel proteins do?

Answer 32

– create a pore through the membrane through which molecules, usually water and ions, flow.

Question 33

Types of channel proteins?

Answer 33

Can be open (water) or gated (ions).

Question 34

Characteristics of open channels?

Answer 34

Create a water filled pore (Aquaporin)

Question 35

Characteristics of gated channels?

Answer 35

Pores in membrane but most of time are closed and only open to respond to certain stimuli.

Question 36

Characteristics of carrier proteins?

Answer 36

do not create a continuous pore from ECF to ICF. Open to ECF, then ICF, but never at the same time. Typically move larger molecules than channels e.g. glucose.

Question 37

Where are glucose transporters present?

Answer 37

Glucose transporters permanently present in cell membranes are insulin independent e.g. nerve cells. Insulin only required for glucose uptake in muscle and adipose tissue.

Question 38

What are important stores of glucose in body?

Answer 38

Muscle and adipose tissue are main storage sites.

Question 39

Membrane proteins as enzymes, what is their role?

Answer 39

membrane enzymes catalyse chemical reactions on the cell membrane. Enzymatic part may be on external side of membrane

Question 40

Give an example of membrane proteins found on external side in liver?

Answer 40

those found in small intestine which break down nutrients into smaller units, or internal side such as those associated with converting signals carried from receptors into an intracellular response e.g.:

Question 41

Describe the process of membrane protein enzymes?

Answer 41

-when ligand binds to receptor, signal is transmitted to intracellular side. Change in activity of intracellular enzyme region

Question 42

What do structural proteins of cell membranes do?

Answer 42

anchor the cell membrane to the intracellular skeleton, to the extracellular matrix (collagen) and/or to other cells

Question 43

What happens if there is dysfunction or loss of structural proteins?

Answer 43

Dysfunction or loss can cause serious debility e.g. lack of dystrophin protein in Duchenne’s Muscular Dystrophy.

Question 44

Membrane proteins role in communication?

Answer 44

this can be between cells or within cells.

Question 45

What do receptor/enzyme complexes and G proteins do?

Answer 45

involved with communicating extracellular messages carried by hormones (or drugs) to the intracellular environment i.e. they convert extracellular chemical signals into intracellular responses

Question 46

What do G proteins specifically do?

Answer 46

The latter are peripheral proteins that relay a signal along the membrane from an integral receptor protein to an integral enzyme protein.

Question 47

What is the protein content of the myelin membrane?

Answer 47

a specialised type of membrane that serves as an insulator around myelinated nerve fibres has a low content of protein (18%), major component is lipid, very good insulator so ideal for function

Question 48

Protein content of plasma membrane?

Answer 48

Plasma membranes of most other cells have much greater activity and protein content is typically 50%.

Question 49

Energy transduction membrane protein content?

Answer 49

Membranes involved in energy transduction such as the inner membranes of mitochondria, have highest protein content, 75%.