What is a membrane?

Question 1

what do membranes control?

Answer 1

entry & exit of waste and secretory products (like neurotransmitters, hormones, enzymes etc)

Question 2

what does membrane maintain?

Answer 2

difference in ion concentration inside & outside the cell

Question 3

what are the 2 principal constituents of a cell membrane?

Answer 3

lipid & protein

Question 4

what is composition of cell membrane?

Answer 4

= phospholipid bilayer, that have fatty acids tails pointing inwards that are hydrophobic
and hydrophilic heads that point out and are charged and polar

Question 5

what happens when phospholipids mixed in water?

Answer 5

they organise themselves into structures →lipid bilayer = 2 parallel sheets

Question 6

what determines how densely individual phospholipid molecules are packed?

Answer 6

nature of individual head group

Question 7

what determines width of phospholipid bilayer?

Answer 7

fatty acid tails

Question 8

what is rate of movement of phospholipids dependant on?

Answer 8

temperature

Question 9

how is cholesterol involved in plasma membrane?

Answer 9

cholesterol interacts with adjacent phospholipid molecules →aid in stiffening bilayer = amount of cholesterol found, varies for each cell

Question 10

what are some examples of different head groups a phospholipid can have?

Answer 10

• inositol
• serine
• choline

Question 11

can charged molecules cross phospholipid bilayer?

Answer 11

no - membrane impermeable to charged molecules (almost any water-soluble substance)
e.g. ions, proteins & sugars

Question 12

what are examples of molecules that can freely cross membrane?

Answer 12

O2, CO2, NH3 and H2O

Question 13

what does membrane look like on electron micrograph?

Answer 13

trilaminar appearance

Question 14

what are the 3 important functions of lipid bilayer?

Answer 14

1. basic structure that makes plasma membrane
2. hydrophobic interior serving as barrier - The cell can maintain differences in solute composition and concentrations inside/outside the cell
3. it is responsible for fluidity of membrane - Enables cells to change shape (e.g.RBC or a skeletal muscle cell)

Question 15

what are the 2 broad classes of membrane proteins?

Answer 15

peripheral and integral

Question 16

what are peripheral membrane proteins?

Answer 16

• not embedded within membrane
• adhere tightly to integral proteins in membrane or extracellular surface of lipids

Question 17

what are 3 types of integral membrane proteins?

Answer 17

• Some proteins span the lipid bilayer once or several times – transmembrane proteins
• Some are embedded but do not cross the bilayer
• Some are linked to a lipid component of the membrane or a fatty-acid derivative that intercalates into the membrane

Question 18

what are some functions of integral membrane proteins?

Answer 18

• ligand binding receptors
• adhesion molecules
• pores or channels
• carriers
• pumps
• enzymes
• participate in extra cellular signalling
  -docking marker acceptors

Question 19

what are docking marker acceptors?

Answer 19

• a function of integral membrane protein
• located on inner membrane surface
• interact with secretory vesicles leading to exocytosis of vesicle contents

Question 20

how can integral membrane proteins participate in intracellular signaling?

Answer 20

• Associated with the cytoplasmic surface of the PM
• E.g. GTP-binding proteins, kinases

Question 21

what are pumps?

Answer 21

integral membrane proteins that Use energy that is released through the hydrolysis of ATP to drive the transport of substances into or out of cells against energy gradients

Question 22

what are carriers?

Answer 22

integral membrane proteins that either facilitate the transport of a specific molecule or couple the transport of a molecule to that of other surfaces (usually transmembrane proteins that span membrane multiple times)

Question 23

what are adhesion molecules?

Answer 23

→integral membrane proteins that Form physical contacts with the surrounding extracellular matrix or with cellular neighbours

→Important in regulating cell shape, growth and differentiation, allowing the cell to adapt to its immediate surroundings
E.g. integrins: cell-matrix adhesion molecules
E.g. cell adhesion molecules: incl. cadherins (Ca2+-binding glycoproteins)

Question 24

where are membrane carbohydrates? and what are they?

Answer 24

• a small amount of membrane carbohydrate is located on outer surface of cells (sugar coating)
• they’re short carbohydrate chains that are often bound to membrane proteins and to a lesser extent lipids e.g. glycoproteins & glycolipids (together form layer called glycocalyx)

Question 25

how do membrane carbohydrates act as self identity markers?

Answer 25

they’re short carbohydrate chains on the outer membrane surface and serve as self-identity markers since they can enable cells to identify and interact with one another

different cells = different markers (important in cell-cell interactions especially embryonic development)

• has role in tissue growth so cells don’t overgrow their own territory

Question 26

what are 3 types of specialised cell junctions?

Answer 26

tight junctions, desmosomes, gap junctions

Question 27

what are tight junctions?

Answer 27

Join the lateral edges of epithelial cells near their lumenal (apical) membranes. (tight or leaky)

• help prevent movement of substances through layer, individual epithelial cells are polarised (apical and basolateral side), so important role is to maintain distinct faces (basolateral = blood facing side)

Question 28

what are desmosomes?

Answer 28

Adhering junctions that anchor cells together, especially in tissues subject to stretching. (e.g. skin, heart, uterus)

• rivets that hold neighbouring cells together, provide mechanical stability particularly in skin that have lots of stress

Question 29

what are gap junctions?

Answer 29

‘Communicating’ junctions that allow the movement of charge carrying ions and small molecules between two adjacent cells.

• channels that run between adjacent cells, found in cardiac muscle cells, gap junctions in heart allow rapid spread of electrical conduct to allow contraction