MEMBRANE STRUCTURE AND FUNCTION BOOKLET ANSWERS
Page 4: Model Membranes
1
2 Plasma membranes are described as having a ‘fluid’ nature because the phospholipid
molecules they contain are able to move about within the phospholipid layer. ‘Mosaic’ refers to
the presence of proteins scattered throughout the phospholipid bilayer.
3 a Cholesterol molecules provide stability for the membrane structure.
b Carbohydrate molecules attached to the membrane surface are involved in cell recognition.
Page 5: Plasma Membrane
1. Some substances are allowed to pass through a differentially permeable
membrane but others are not able to pass through.
2. Your answer needs to include a description of the flexibility of the membrane
caused by the nature of the phospholipid bilayer, the range of different
proteins embedded within the membrane and the way they are resting on
the surface or penetrating through it.
3. a) ‘Fluid’ refers to the properties of the phospholipid bilayer. This allows the
cell to be flexible and change shape easily.
b) ‘Mosaic’ refers to the pattern of the proteins embedded within the
membrane.
4. Prokaryotic cell walls are made from a protein-carbohydrate compound
whereas plant cell walls are made from cellulose.
5.
Cell wall
Similarities
Plasma membrane
Surrounds the cell
Allows substances to pass through it
Differences Not found in all cell types
Found in all cell types
Fully permeable
Differentially permeable
Very strong
Fluid
Simple structure of proteincarbohydrate or cellulose
compound
Complex structure of
phospholipid molecules and
proteins
6. Flagella aid in movement of the cell, they rotate like a propeller. Pili help cells
stick to surfaces. They do not rotate and they are much smaller than flagella.
Refer to figure A.2.
7.
Prokaryotic
cell organelle
Plasma
membrane
Cell wall
Flagella and
Pili
Structure
(appearance)
Function
Phospholipid bilayer
with various
proteins embedded
within it
(see figure A.7)
Tough layer
surrounding the
plasma membrane
(see figure A.2,
A.17)
Flagella: whip like
tail
(see figure A.2,
A.14)
Pili: short, hair like
structures
(see figure A.2)
Maintains the
contents of a cell and
regulates the
movement of
substances in and out
Provides strength and
protection and gives
shape to a cell
Flagella: moves cell
Pili: help cells stick to
surfaces
Visible in a
light
microscope?
 or 



Page 6: The Plasma Membrane Revisited
1.
Membrane protein
Function
Transport proteins
Span the plasma membrane from one side to the other and thus allow certain
substances but not others to pass through
Receptor proteins
Bind hormones and other substances to the plasma membrane, and therefore cause
changes to the cell’s activities
Recognition
proteins
Attach to carbohydrate molecules on the surface of the plasma membrane and act
as markers, called antigens which allow the immune system to distinguish between
the body’s self cells and non-self cells
Adhesion proteins
Link cells together in multicellular organisms
2. a Water, oxygen, carbon dioxide and other small molecules as well as lipidsoluble molecules pass through the plasma membrane easily.
b Water-soluble molecules, charged molecules and atoms and many large
molecules do not pass easily through the plasma membrane.
3. Lipid-soluble molecules pass easily through the plasma membrane because it is
composed of a phospholipid bilayer. The lipids in the membrane form a barrier
to water-soluble molecules.
Page 7 & 8: The Structure of Membranes
1. The plasma membrane is located at the outer limit of the cell.
2. Membranes are composed of a phospholipid bilayer in which are embedded
proteins, glycoproteins, and glycolipids. The structure is relatively fluid and the
proteins are able to move within this fluid matrix.
3. a) controls the entry/exit of substances in and out of the cell
b) forms the outer limit of the cell
c) contains proteins that play an important role in the immune response and can
identify the cell as belonging to the body.
4. Many of the proteins play a role in the selective transport of certain substances across
the phospholipid bilayer, either acting as channels or active transport molecules. Others
function as receptors, which bind information-providing molecules, such as hormones,
and transmit corresponding signals based on the obtained information to the interior of
the cell. Membrane proteins may also exhibit enzymatic activity, catalyzing various
reactions related to the plasma membrane.
5.
Any of: Golgi, mitochondria, chloroplasts, nucleus, ER, vacuoles, lysosomes
6.
7.
8.
9.
a) Cholesterol lies between the phospholipids and prevents close packing. It thus
functions to keep membranes more fluid. The greater the amount of cholesterol
in the membrane, the greater its fluidity.
b) At temperatures close to freezing, high proportions of membrane cholesterol
is important in keeping membranes fluid and functioning.
This model accounts for the properties we observe in cellular membranes: its
fluidity (how its shape is not static and how its components move within the
membrane, relative to one another) and its mosaic nature (the way in which the
relative proportions of the membrane components i.e. proteins, glycoproteins,
glycolipids etc can vary from membrane to membrane). The fluid mosaic model
also accounts for how membranes can allow for the selective passage of
materials (through protein channels for example) and how they enable cell-cell
recognition (again, as a result of membrane components such as glycoproteins).
a) Non-polar substances diffuse more rapidly through the plasma membrane
polar substances because the non-polar fatty acid tails in the centre of the lipid
bilayer do not readily interact with polar molecules.
b) This means that large polar substances cannot diffuse across the plasma
membrane and that facilitated diffusion is required to get these substances into
the cell.
Page 9: The Role Of Membranes in Cells
1. a) Compartments within the cells allow enzymatic reactions in the cell to be
localised. This achieves greater efficiency in cell function and keeps potentially
harmful reactions and substances (e.g. hydrogen peroxide) contained.
b) Greater membrane surface area provides a greater area over which
membrane-bound reactions can occur. This increases the speed and efficiency
with which metabolic reactions can take place.
2.
a) Glycoproteins and glycolipids act as cell identity markers so that self and nonself cells can be recognised. Glycolipids also help cells to aggregate in the
formation of tissues.
b) Channel and carrier proteins facilitate selective transport of substances
through membranes. They can help to speed up the transport of substances into
and out of the cell, especially for enzymatic reactions requiring a steady supply
of substrate and constant removal of end-product e.g. ADP supply to the
mitochondrion during cellular respiration.
3.
Cholesterol can regulate the entry or exit of substances by acting as a selective
plug, allowing some substances but not others to enter and leave the cell.