Forces across membrane IA%% (+

Question 1

Membrane info 1: selective barrier

Answer 1

1. Cell membrane is a selective barrier. It is freely permeable to some substances e.g. O₂ and CO_2, but the difference in composition between ECF and ICF shows that permeability is selective and not universal.

Question 2

Membrane info 2: permeability

Answer 2

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

Question 3

Membrane info 3: chemical recognition

Answer 3

Membranes provide binding sites for chemical recognition; embedded in them are receptors for chemical signals such as hormones and neurotransmitters which regulate cellular activity.

Only tissues that express receptors for that particular hormone or neurotransmitter can respond to the chemical.

Question 4

Membrane info 4: dynamic

Answer 4

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

Question 5

Membrane info 5 : structure

Answer 5

• Very thin bi-layer of lipids, embedded with proteins
• Phospholipids which have a hydrophilic (water loving) phosphate head and a hydrophobic (water repellent) fatty acid tail
• In an aqueous environment they 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.
  *

Question 6

Membrane info 6: flexible

Answer 6

Membranes are very flexible, due to presence of fatty acids. Behave like an oil, so can stretch, although may rupture if over-stretched.

Question 7

Membrane info 7 : insulators

Answer 7

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

Question 8

Receptors

Answer 8

Receptors – integral to the membrane structure. Penetrate the membrane from ECF to ICF. Allow communication of an extracellular signal to the intracellular space to create a cellular response.

Question 9

Transporters

Answer 9

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

i) Carrier Proteins – 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.
ii) Channel proteins – create a pore through the membrane through which molecules, usually ions, flow. Can be open or gated.

Question 10

Membrane Enzymes

Answer 10

Membrane enzymes catalyse chemical reactions on the cell membrane. Can be external e.g. those found in small intestine which break down nutrients into smaller units, or internal such as those associated with converting signals carried from receptors into an intracellular response.

Question 11

Peripheral membrane proteins

Answer 11

Peripheral membrane proteins – associated with the cell membrane but not incorporated into it. They are proteins that adhere only temporarily to the membrane with which they are associated. These proteins attach to integral membrane proteins, or penetrate the peripheral regions of the lipid bilayer.

The have 3 main roles:

i.Maintain structure of cells by anchoring membrane to intracellular cytoskeleton (network of interlinking filaments and tubules that extend throughout the cytoplasm, from the nucleus to the plasma membrane). Dysfunction or loss can cause serious debility .

ii. Attach cells to the extracellular matrix e.g. collagen
iii. Perform signalling functions within cells e.g. G Proteins

Question 12

Protein content difference in structures

Answer 12

Membranes differ in their protein content.

1. Myelin: a 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.
2. Plasma membranes of most other cells have much greater activity and protein content is typically 50%.
3. Membranes involved in energy transduction such as the inner membranes of mitochondria, have highest protein content, »75%.

Question 13

Membrane carbohydrates

Answer 13

• Small amounts linked to proteins and lipids as glycoproteins and glycolipids.
• ALL extracellular.
• Important role in self vs non-self recognition by the immune system.

Question 14

Diffusion definition

Answer 14

• Process by which a gas or substance in solution expands to fill all the available volume.
• molecules spread from regions of high concentration to regions of low concentration, until the concentration is uniform throughout the volume.

Question 15

Magnitude of net flux

Answer 15

Question 16

Diffusion through lipid bilayer

Answer 16

To pass rapidly through the lipid bilayer, molecules need to be:

1. Small
2. Lipophilic(hydrophobic)
3. UNCHARGED

• a)Small, uncharged, lipophilic molecules such as O₂ and N₂ pass easily through the lipid bilayer.
• b)Small, uncharged, lipophobic molecules such as CO₂ and urea, also diffuse easily across lipid bilayers.
• c)Large, uncharged lipophobic molecules such as glucose diffuse much more slowly through bilayer (neglible)

Question 17

Diffusion through channels

Answer 17

• These are transmembrane Integral membrane proteins that act as an aqueous route through the membrane for the diffusion of ions.
• Only allow the passage of mineral ions such as Na⁺ and K⁺, Cl-, Ca²⁺, H⁺ and H₂O. Too small to allow molecules such as glucose to go through.
• H₂O passes through aquaporins, family of water channels, ubiquitously distributed. There is NO barrier to water

Question 18

Gated channels

Answer 18

Ions can cross the membrane using (mainly) gated channels. They remain closed until a stimulus (chemical or change in electrical charge across the membrane) causes them to open.

Question 19

Osmosis

Answer 19

Net movement of H₂O from regions of high H₂O concentration to regions of low H₂O concentration

Note:

• ALL H₂O movements in the body are passive, (via aquaporins, protein water channels).
• H₂O concentration is inversely related to the concentration of solute, ie the more solute particles there are in solution, the more they will displace H₂O molecules lowering the concentration of H₂O

Question 20

Plasma composition

Answer 20

• High [Na⁺] 3 Na⁺ out for every 2 K⁺ in
• Low [K⁺]
• High [plasma protein] too big to pass through capillary wall

Question 21

ISF composition

Answer 21

• High [Na+] 3 Na+ out for every 2 K+ in
• Low [K+]
• low [plasma protein] too big to pass through capillary wall

Question 22

ICF composition

Answer 22

• Low [Na+] 3 Na+ out for every 2 K+ in
• High [K+]
• High [protein]

Question 23

Osmolarity

Answer 23

“Osmolarity” measures the concentration of biological solutions in units of “OSMOLES” and describes the number of particles/L of solution, (in reality use milliosmoles, because biological solutions are so weak).

285 —> 300 mOsmol/l in blood plasma (for ease)

Question 24

Osmolarity vs Tonicity

Answer 24

• Osmolarity describes total number of particles in solution
• Tonicity describes the number of non-penetrating particles in solution

Note: the volume of a cell at any time is dependent on the concentration of non-penetrating solutes on the either side of the membrane

Question 25

Osmolarity terms

Answer 25

• An isosmotic solution has the same total number of solute particles as ECF (plasma). 300 mOsmol/L
• Solutions with fewer total solute particles are hypo-osmotic.
• Solutions with greater number of total solute particles are hyper-osmotic

Question 26

Tonicity terms

Answer 26

• An isotonic solution has the same number of non-penetrating solute particles as ECF (plasma).
• Solutions with fewer non-penetrating solute particles are hypotonic.
• Solutions with greater number of non-penetrating solute particles are hypertonic.

Question 27

Facts

Answer 27

• Urea is a penetrating particle (disregard)
• (charged) Ions are non-pentrating (important)
• Brain is most sensitive organ to changes in tonicity.

Question 28

Define electrochemical gradient

Answer 28

A gradient of electrochemical potential, usually for an ion that can move across a membrane. The gradient consists of two parts, the chemical gradient, or difference in solute concentration across a membrane, and the electrical gradient, or difference in charge across a membrane.