4.1- STRUCTURE OF THE CELL-SURFACE MEMBRANE Flashcards
what is the term cell-surface membrane specifically given to?
plasma membrane that surrounds cells + forms boundary between cell cytoplasm + environment
What does the cell-surface membrane allow?
different conditions to be established inside + outside of cell
What is the function of the cell-surface membrane?
controls movement of substances in and out of cell
What do phospholipids form?
a bilayer
Where does the hydrophilic heads of both phospholipid layers point to?
outside of cell-surface membrane
Why does the hydrophilic heads point to outside of the cell surface membrane?
attracted by water on both sides
Where does the hydrophobic tails of both phospholipid layers point?
into the centre of the cell membrane
Why does the hydrophobic heads point into the centre of the cell membrane?
repelled by water on both sides
How do lipid-soluble materials move through the membrane?
via phospholipid portion
Functions of phospholipids in membrane? (3)
allow lipid-soluble substances to enter + leave cell
prevent water-soluble substances entering + leaving cell
make membrane flexible + self-sealing
Are proteins interspersed throughout the cell surface membrane?
yes
One way in which proteins embedded in phospholipid bilayer?
proteins occur in surface of bilayer + never extend completely across it
What do proteins that occur on the surface of the bilayer and not extend completely across it do?
give mechanical support to membrane or in conjunction with glycolipids, as cell receptors for molecules i.e. hormones
Another way in which proteins embedded into phospholipid bilayer?
proteins completely span phospholipid bilayer from one side to other
examples of proteins that completely span phospholipid bilayer from one side to other?
protein channels
carrier proteins
What do protein channels do?
form water-filled tubes to allow water-soluble ions to diffuse across membrane
What do carrier proteins do?
bind to ions or molecules like glucose and amino acids, then change shape to move these molecules across membrane
Function of proteins in membrane? (6)
provide structural support
act as channels transporting water-soluble substances across membrane
allow active transport across membrane through carrier proteins
form cell-surface receptors for identifying cells
help cells adhere together
act as receptors, i.e. for hormones
Where do cholesterol molecules occur within?
phospholipid bilayer of cell-surface membrane
What do the cholesterol molecules do?
add strength to membranes
Are cholesterol molecules hydrophobic or hydrophilic?q
very hydrophobic
As cholesterol molecules are very hydrophobic what important role do they play?
preventing loss of water + dissolved ions from cell
What other important role does cholesterol molecules carry out regarding fatty acid tails?
pull together fatty acid tails of phospholipid molecules, limiting their movement and that of other molecules but without making membrane as a whole too rigid
Function of cholesterol in membrane? (3)
reduce lateral movement of other molecules including phospholipids
make membrane less fluid at high temperatures
prevent leakage of water + dissolved ions from cell
What are glycolipids made up of?
a carbohydrate covalently bonded with a lipid
Where does the carbohydrate portion of the glycolipid extend from into?
from phospholipid bilayer into the watery environment outside cell where it acts as a cell-surface receptor for specific chemicals
An example where glycolipids act as a receptor for specific chemicals?
human ABO blood system operates as a result of glycolipids on the cell-surface membrane
Functions of glycolipids in the membrane? (3)
act as recognition sites
help maintain stability of membrane
help cells to attach to one another and so form tissues
What are glycoproteins made up of?
carbohydrate chain attached to many extrinsic proteins on the outer surface of the cell membrane
What do the glycoproteins act as?
cell-surface receptors
What do glycoproteins act as receptor for more specifically?
hormones and neurotransmitters
Function of glycoproteins in the membrane? (3)
act as recognition site
help cells attach to one another and so form tissues
allow cells to recognise one another, e.g. lymphocytes can recognise an organism’s own cells
Why do most molecules not freely diffuse across the cell-surface membrane? (4)
not soluble in lipids
too large
of same charge as charge on protein channels
electrically charged
Why are molecules that are not soluble in lipids not able to diffuse across the cell-surface membrane?
they cannot pass through the phospholipid bilayer
Why are large molecules not able to diffuse across the cell-surface membrane?
cannot pass through the channels in the membrane
Why are molecules that have the same charge as the protein channels unable to diffuse across the cell-surface membrane?
they’re repelled
Why are molecules that are electrically charged unable to diffuse across the cell-surface membrane?
have difficulty passing through the non-polar hydrophobic tails in the phospholipid bilayer
What is the arrangement of all the various molecules combined into the structure of the cell-surface membrane known as?
fluid-mosaic model
Why is the model known as ‘fluid’ in fluid-mosaic?
individual phospholipid molecules can move relative to one another
What does the fluidity of the phospholipid molecules give?
give membrane a flexible structure that’s constantly changing in shape
Why is the model known as ‘mosaic’ in fluid-mosaic?
as proteins that are embedded in phospholipid bilayer vary in shape, size and pattern in same way as stones/tiles in mosaic
