what is a membrane Flashcards
what do membranes control?
entry & exit of waste and secretory products (like neurotransmitters, hormones, enzymes etc)
what does membrane maintain?
difference in ion concentration inside & outside the cell
what are the 2 principal constituents of a cell membrane?
lipid & protein
what is composition of cell membrane?
= phospholipid bilayer, that have fatty acids tails pointing inwards that are hydrophobic
and hydrophilic heads that point out and are charged and polar
what happens when phospholipids mixed in water?
they organise themselves into structures →lipid bilayer = 2 parallel sheets
what determines how densely individual phospholipid molecules are packed?
nature of individual head group
what determines width of phospholipid bilayer?
fatty acid tails
what is rate of movement of phospholipids dependant on?
temperature
how is cholesterol involved in plasma membrane?
cholesterol interacts with adjacent phospholipid molecules →aid in stiffening bilayer = amount of cholesterol found, varies for each cell
what are some examples of different head groups a phospholipid can have?
- inositol
- serine
- choline
can charged molecules cross phospholipid bilayer?
no - membrane impermeable to charged molecules (almost any water-soluble substance)
e.g. ions, proteins & sugars
what are examples of molecules that can freely cross membrane?
O2, CO2, NH3 and H2O
what does membrane look like on electron micrograph?
trilaminar appearance
what are the 3 important functions of lipid bilayer?
- basic structure that makes plasma membrane
- hydrophobic interior serving as barrier - The cell can maintain differences in solute composition and concentrations inside/outside the cell
- it is responsible for fluidity of membrane - Enables cells to change shape (e.g.RBC or a skeletal muscle cell)
what are the 2 broad classes of membrane proteins?
peripheral and integral
what are peripheral membrane proteins?
- not embedded within membrane
- adhere tightly to integral proteins in membrane or extracellular surface of lipids
what are 3 types of integral membrane proteins?
- 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
what are some functions of integral membrane proteins?
- ligand binding receptors
- adhesion molecules
- pores or channels
- carriers
- pumps
- enzymes
- participate in extra cellular signalling
-docking marker acceptors
what are docking marker acceptors?
- a function of integral membrane protein
- located on inner membrane surface
- interact with secretory vesicles leading to exocytosis of vesicle contents
how can integral membrane proteins participate in intracellular signaling?
- Associated with the cytoplasmic surface of the PM
- E.g. GTP-binding proteins, kinases
what are pumps?
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
what are carriers?
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)
what are adhesion molecules?
→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)
where are membrane carbohydrates? and what are they?
- 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)
how do membrane carbohydrates act as self identity markers?
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
what are 3 types of specialised cell junctions?
tight junctions, desmosomes, gap junctions
what are tight junctions?
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)
what are desmosomes?
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
what are gap junctions?
‘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
