Cell
membranes
June 2002
Question 3
(a) –i-Phospholipid molecule.
-ii- It is consists of one phosphate group forming the hydrophilic head , one glycerol and
two fatty acids forming the hydrophobic tail.
-iii-It is fluid because phospholipid molecules are in continuous vibrating movement,
exchanging positions with each other but within their monolayer, also it is mosaic due to
the pattern of distribution of proteins among phospholipids heads.
(b) Receptors are proteins formed of different amino acids with different R-groups, therefore
the primary structure which is the specific sequence of amino acids leads to the formation
of specific globular shape as a result of formation of different bonds between its Rgroups, and this specific shape is complementary to specific antigen.
November 2003
Question 5
(a) Glycoproteins
Can be used as cell surface antigens for cell recognition also can be used as receptors
or structures for hormonal attachment, and can form hydrogenbonds with the
surrounding water to stabilize membrane structure.
Carrier proteins
Can be used to carry certain molecules or ions such as sodium ions by activetransport
( against their concentration gradient using energy from ATP) or used in the process
of facilitated diffusion in which polar molecules or ions which cannot pass across
phospholipids bilayer can cross the membrane down their concentration gradient.
Cholesterol
Regulates fluidity of cell membranes as it prevents membranes from being too rigid
or too fluid also it has a role in controlling leakage of water and watersoluble
molecules through the cell membranes.
(b) A can cross the cell surface membrane from inside the cell where its concentration is
lower to the surrounding medium where its concentration is higher only by active
transport using energy and carrier protein.
B can cross the cell surface membrane from the surrounding medium where its
concentration is higher to the inside of the cell where its concentration is lower by simple
diffusion through the phospholipids bilayer as it is a non-polar molecule.
(c) Bacteria can be marked by antibodies or plasma proteins known as opsonin which can
bind to specific receptors on the cell surface membrane of the phagocyte and this
induces phagocyte to surround and envelope bacteria to be taken in a vesicle or a
phagosome.
(d) Lysosomes contain hydrolytic or digestive enzymes such as protease to act on the
engulfed bacteria.
November 2005
Question 2
(a)Notice :Annotated diagram means diagram with labels and brief description or
explanation.
Phospholipid heads being hydrophilic are directed outwards to form bonds with the
surrounding water while its tails being hydrophobic are directed inwards to be away from
water attracting each other with hydrophobic interaction, so they can form a
phospholipids bilayer.
Question 5
(a) To have the same volume as that of the large cube.
(b) In both there is rapid increase in mass for the first three hours but after that the %
increase starts to be slower between 3 to about 25 hours, then the percentage increase
remains constant . The difference between the two results is that the % increase for the 8
cubes is higher as it reaches about 17% while that of the large cube is only about 7.5%.
(c) The cells of the cubes of yam have lower water potential than that of the surrounding
medium as they contain different solutes especially in their sap vacuole such as sugars
and amino acids therefore water enters cells by osmosis down its concentration gradient
through the partially permeable cell membranes of the cells.
(d) Because the surface area to volume ratio in the 8 cubes is (6:1) larger than that of the
cube of sides 2cm which is (3:1) therefore more water per unit time enter the cells, also
the outer cells of the large cube may become fully turgid restricting inner cells from
obtaining water and become enlarged.
June 2007
Question 6
(a) 7.0 nm
(b)K : Allow the movement of ions or polar molecules and water soluble molecules through
the plasma membrane by facilitated diffusion or active transport.
L : Acts as surface antigen for cell recognition or to act as receptors or binding sites for
certain molecules such as hormones and to form hydrogen bonds with the surrounding
water molecules to stabilize membrane structure.
M : Necessary for membrane fluidity, and act as barrier for water and
water soluble compounds and ions but allow the passage of lipid soluble substances.
N : Regulates fluidity of the plasma membrane and has a role in controlling leakage of
water.
(c) Because glucose is a large water soluble ( hydrophilic) molecule therefore it cannot
diffuse through the phospholipids bilayer .
(d) Increase concentration of glucose leads to an increase in rate of uptake
of glucose indicating that diffusion takes place down its concentration gradient and
because the rate of diffusion remains constant above concentration 30 arbitrary units, this
indicates that facilitated diffusion takes place as no enough protein channels are available
to increase rate of diffusion by increasing concentration of glucose in the surrounding
medium.
(e) Active transport requires energy while facilitated diffusion is a passive process does not
need energy also in facilitated diffusion molecules move down their concentration
gradient while in active transport molecules move against their concentration gradient.
June 2008
Question 2
(d)–i- As a general trend, increasing temperature leads to a decrease in % transmission of
light. Between 20 and 60 oC % transmission decreases from 95% to 70%, but
between 60 and 70 oC % transmission decreases sharply from 70 to 19% ,between
70 and 80 oC % transmission decreases from 19 to 6%.
-ii- By increasing temperature diffusion of betalain increases as temperature causes
membrane proteins to be denatured and this alters their tertiary structure making
betalain released easily out the membranes also temperature increases fluidity of
membranes as it increases kinetic energy of phospholipid molecules to vibrate faster.
November 2008
Question 1
(a) -i- Draw a bracket across the whole bilayer .
-ii-Fluid means that phospholipid molecules and proteins are in continuous motion ,
vibrating , and exchanging position with each other within their monolayer. Mosaic
refers to the pattern of distribution of protein between the phospholipid molecules of
the cell membrane.
November 2009/21
Question 1
(c) 1- Can act as a carrier protein for active transport where molecules canbe carried against
their concentration gradient.
2- Can act as channel protein for facilitated diffusion where ions that cannot pass
through the phospholipid bilayer can pass down their concentration gradient.
3- Can be used as cell recognition structures.
4- Can act as an enzyme attached to the membranes such as enzyme in mitochondrial
membranes used for ATP synthesis.
5- Can be used as receptors for hormonal attachment.
6- Can be used as receptors on the surface of lymphocytes and phagocytes.
November 2009/22
Question 1
(a) –i- Because calcium ions are charged and water soluble therefore they cannot pass
through the hydrophobic tails of the phospholipid molecules.
-ii-Name active transport or active uptake
(Notice that facilitated diffusion cannot be involved in this question as it states that
molecules are forced against their concentration gradient, also this is shown in the
diagrams)
(a) (ii)Description calcium ions bind to the specific binding site of the protein
carrier,and then using energy supplied by ATP, the carrier protein changes its shape
making ions to be forced outside against their concentration gradient.
(b) Bacterial cells are taken in vesicles, lysosomes fuse with the vesicles where lysosomal
enzymes such as protease act on proteins to be digested into amino acids and lipase to
act on its lipids, also contains enzymes to act on bacterial cell walls and nucleic acids to
be digested.
November 2010/22
Question 1
(b) The materials needed to be exported outside the cell (such as enzymes , hormones and
glycoprotein ) are carried in a vesicle which is directed towards the cell surface
membrane using energy from ATP and guided by microtubules, cell surface membrane
and membrane of vesicle fuse together so that the contents of the vesicle is released
outside the cell.
November 2010/23
Question 1
(a) –i- A- phospholipid
B- protein
-ii- It has a hydrophilic head directed outwards to form bond with the surrounding water
while its tail being hydrophobic directed inwards as it is expelled by water ,at the
same time it forms hydrophobic interaction with the other hydrophobic tails of the
other phospholipid molecules.
(b) C- Channel protein used in facilitated diffusion of polar molecules that cannot diffuse
through the phospholipid bilayer.
D- they are glycoproteins and glycolipids used in cell recognition, also act as receptors
and stabilise cell membranes by formation of bonds with the surrounding water.
November 2011/21