Structure, permeability and transport functions of cell membrane

Question 1

Cell membranes are composed of:

Answer 1

Phospholipids, proteins, cholesterol, glycolipids which means the cell membrane is permeable to lipid soluble substances like CO2, O2, fatty acids, steroid hormones but not to water soluble substances like ions, glucose, amino acids.

Question 2

Lipid component of cell membrane

Answer 2

primarily composed of phospholipids, glycolipids, cholesterol

Question 3

Phospholipid structure and types:

Answer 3

Structure: alcohol(glycerol), phosphate, fatty acid

types: phosphatydlcholine, sphingomyelin, phosphatydlethanolamine, phosphotydlserine, phosphotydlinositol

Question 4

glycolipids structure, types and function:

Answer 4

structure: sugar and fatty acid
types: glycosylphosphotydlinositol
function: receptors and antigens

Question 5

cholesterol structure and function

Answer 5

structure: hydroxyl group, fatty acid, steroid appearance with 4 hydrocarbon rings.

function: increases stability of membrane
by limiting movement of phospholipids and reduces fluidity of membrane and makes membrane less permeable to ions

Question 6

Phospholipids are:

Answer 6

amphipathic: have hydrophobic fatty acid tail pointing inwards and hydrophilic glycerol head

Question 7

Role of phospholipids in signal transduction:

Answer 7

With Gq GPCR activation, phosphatydlinositol bisphosphate in the cell membrane is cleaved by Phospholipase C into IP3(increases intracellular calcium) and DAG( activates PKC)

Question 8

Protein component of cell membrane:

Answer 8

• Integral membrane proteins(anchored by hydrophobic interaction for example transmembrane proteins like G proteins, sodium potassium pump, hormone binding receptors)
• Peripheral proteins: loosely attached by electrostatic interaction
• Lipid anchored protein for example GPI bound proteins
• Glycoproteins where carbohydrate faces extracellularly

Question 9

Functions of proteins

Answer 9

selective transport of molecules, cell recognition, cell to cell communication, enzyme activity, determination of cell shape

Question 10

Classification of transport processes throughout the Plasma membrane:

Answer 10

Simple diffusion, protein mediated membrane transport, vesicular transport

Question 11

Simple diffusion definition:

Answer 11

passive transport by concentration gradient from high to low concentration, net rate of movement due to random or Brownian motion. Can be characterised by ficks 1st law that the flux is equal to diffusion coefficient times concentration gradient/area covered

Question 12

what molecules use simple diffusion:

Answer 12

CO2, O2, NO, CO and uncharged polar molecules

Question 13

If simple diffusion is across a membrane, we have to take into account:

Answer 13

thickness of membrane and concentration difference between both sides

Question 14

Proteins of protein mediated transport:

Answer 14

Transport can be either:

• Carriers: which are enzymes(eg. pumps) and can be pumps, and have passive or active cycle. Often have slow rate of transport, can be saturated(enzyme kinetics, max, etc)
• Channels: which are gates(eg.ion channel) that can alter between blocking or allowing passive transport. Often have much faster rate of transport and can be saturated but only with rare extremely high ion concentration

Question 15

Types of protein mediated transport

Answer 15

facilitated transport: no metabolic energy needed

Active transport: metabolic energy or ATP is required

Superfamily(ABC transporters or atp binding cassette)

Water channels

Question 16

Facilitated transport

Answer 16

Glucose tranporters:

• There are 7 types: GLUT 1-7
• Helps in glucose uptake of cells
• GLUT 1: RBC’s (extracellular to intracellular transport). Isoform specific kinase, is a uniport
• GLUT 2: pancreatic beta cells, liver cells. they monitor concentration of glucose
• GLUT 4: in skm and adipose tissue and localisation depends on insulin. if insulin increases, glucose taken into cells
• can look at rate of transport graph in notes. physiological range of glucose is between 2 and 5 mM

Ion exchanger:

• Chloride/bicarbonate exchanger(anti-port which works in both directions, facilitated transport)
• Found in RBC’s for CO2 transport
• passive, electronuetral

Question 17

Active transport

Answer 17

Sodium potassium atpases:

• all cells have this pump
• antiporter
• 3 Na to intracellular, 2 K+ extracellular
• uses app
• Oubain is inhibitor
• electrogenic
• works in cycle

Calcium atpases:
-works in plasma membrane or transports calcium to ER like SERCA(lowering intracellular calcium levels) or H-K+ atlases in gastric parietal cells and in alpha intercalated cells of renal collecting duct

Question 18

Superfamily of ABC transporters

Answer 18

Primary active transporter:

• transport hydrophobic molecules like bile and cholesterol
• MDR proteins (multi drug resistant) remove drugs like cytostatids that are used to treat cancer

CFTR channel:

• Chloride channel
• lost atpase activity, just a regulator
• is a cystic fibrosis(chloride transport impaired) transmembrane regulator

Question 19

Water channel

Answer 19

• aquaporins(11 isoforms)
• passive transport
• water flows in direction where osmotic concentration is higher
• semipermeable membrane: permeable for water but impermeable for solute

Question 20

Vesicular transport mechanisms

Answer 20

Endocytosis

• pinocytosis: ECF, small molecules and water
• phagocytosis: proteins, larger molecules like bacteria and debris
• receptor mediated: lipids and important molecules, Cathrin coated pits, etc

Exocytosis

• constitutive: continuously occurs
• regulated:signal->intracellular->regulated membrane fusion eg)histamine release from cell

Question 21

Secondary active transport:

Answer 21

• Uses the potential energy stored by the concentration gradient created by primary active transport
• can be characterised by symport or antiport
• Examples include SGLT(in renal proximal tubule and intestinal epithelium it is sodium glucose cotransport), sodium amino acid con transport, Sodium/ potassium /2 chloride cotransporter in TAL of renal tubule

Question 22

Transcellular vs paracellular

Answer 22

Transcellular: uses pumps, protein mediated, facilitated diffusion, water and ion channels

Paracellular: no intracellular compartment, passive transport thru tight junctions. Examples include between epithelial cells of small intestine which is permeable to many kinds of molecules and collecting duct of nephron

-there are examples of transepithelial transport in notebook (nacl absorption in frogs, glucose reabsorption in kidney and gi, salivary gland chloride secretion, stomach protein secretion)

Question 23

Regulation of transporters

Answer 23

1) Decrease or increase activity of transporter eg)camp->CFTR activity increases
2) Trafficking of transporters(H+ pump, insulin->GLUT 4 translocation to cell membrane, Aldosterone->EnAC synthesis)

Question 24

Osmosis

Answer 24

flow of water through semipermeable membrane from low to high solute concentration

Question 25

osmolarity vs osmolality

Answer 25

Osmolarity: concentration of osmotically active particles in solution( Osmol/Liter). This is temperature dependent

Osmolality:concentration of osmotically active particles in solution (Osmol/Kg)

Question 26

Vant Hoff law

Answer 26

diagram in notebook

Question 27

osmotic pressure

Answer 27

hydrostatic pressure needed so that there is no net water flow across semipermeable membrane

Question 28

oncotic pressure

Answer 28

osmotic pressure for larger molecules like proteins

Question 29

tonicity

Answer 29

ability of cell to move ECF in and out of cell

=coefficent times osmotic concentration

Question 30

structure of transepithelial cells

Answer 30

has apical/luminal side and basolateral side(side facing interstitium)