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Higher Human Biology
Chapter 6: Cell Membrane
and Transport of Materials
Unit 1: Cell function and inheritance
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Lesson Aims
• To study the structure and function of the
plasma membrane
• To examine the transport of substances
across the membrane by diffusion, active
transport, endocytosis, exocytosis,
pinocytosis, phagocytosis,
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All living things are surrounded by a thin
boundary called a cell membrane (plasma
membrane).
•Cell organelles (e.g. Nucleus mitochondria, Golgi
apparatus and lysosomes) are also bounded by a cell
membrane.
• The only
exceptions to this
rule are the nucleoli
and ribosomes.
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Functions of Cell Membrane
• All living things are surrounded by a thin
boundary called a cell membrane (plasma
membrane).
• Controls entry/exit of materials including
water and soluble molecules.
• Communicates with other cells e.g.
hormones binding with receptors.
• Important for the connections between
cells.
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Investigating the chemical
nature of the cell membrane
• The cell sap present in
the central vacuole of a
beetroot cell contains a (Red cell sap)
red pigment. Bleeding
(the escape of this
pigment) indicates that
the cell’s plasma and
vacuolar membranes
have been damaged.
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Plasma
Membrane
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Beetroot experiment con’t
• 4 identical cylinders of beetroot are
prepared using a cork borer.
• These are washed to remove red cell sap
from the outer damaged cells.
Water
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Acid
Water bath at
25oC Mrs Smith
Alcohol
Water
Water bath at
6
70oC
Beetroot experiment CONCLUSION
• Bleeding is found in tubes B,C and D showing that the
membranes have been destroyed.
A:Water
Water bath at
25oC
B:Acid C:Alcohol
D:Water
Water bath at
70oC
• Molecules of protein are known to be denatured when exposed
to acid or high temperatures. Molecules of protein are known to
be soluble in alcohol.
• It is therefore concluded the cell membrane contains protein
(results b and d – denatured protein allowed pigment to leak
out) and lipid (results c – lipid dissolved in alcohol causing the
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same.)
Danielli and Davson 1930's-40's
• Studied triglyceride lipid
bilayers over a water surface.
• Lipids arranged themselves
with the polar heads facing
outward.
• Formed droplets (oil in water)
and the surface tension was
Danielli and Davson’s
much higher than that of cells
early model of the cell • Added proteins, the surface
membrane.
tension was reduced and the
membranes flattened out
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Structure of the plasma membrane
• The PM consists of protein and
phospholipid molecules.
• The best model we have for the
arrangement of these molecules is called
the FLUID MOSAIC MODEL of cell
membrane structure.
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Fluid mosaic model
• Here the plasma membrane consists of a fluid
bilayer of constantly moving phospholipid
molecules
containing a patchy mosaic of
protein molecules
• Proteins act as:
channel
receptors
support
carriers
enzymes
antigens
• Phospholipids form a bilayer that acts like a
barrier between the cell and the environment.
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Fluid Mosaic Model
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Remember Phospholipids- have
different properties at each end!
• The phosphate (‘head’) end
of the molecule is
hydrophilic (water loving)
and is therefore soluble in
water.
• The fatty acid (‘tail’) end of
the molecule is hydrophobic
(water-hating) and is
therefore insoluble in water.
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Phospholipid Bilayer Structure
In the company of other similar molecules,
phospholipid molecules arrange themselves into a
bilayer.
Where the Phospholipid:
• Hydrophilic head points towards the outside
environment and the cytoplasm, this makes up the
two outside surfaces of the bilayer.
• Hydrophobic tail repels water and points in.
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Phospholipid bi-layer
Hydrophilic
phosphate
head
Hyrdophobic
lipid tail
• The arrangement of phospholipid molecules is fluid yet at the
same time it forms a stable and effective boundary round the
cell.
• It allows tiny molecules such as water to pass through it rapidly.
• Larger molecules such as glucose depend on the membranes
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protein molecules for entry Mrs
and
exit from the cell.
Freeze-fracture of Plasma
Membrane
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Freeze-fracture of Plasma
Membrane
• This electron
micrograph shows the
inside of a membrane
and bumps, grooves,
ridges. These were
later found to be
proteins.
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The membranes possessed by the cell’s various
organelles tend to vary from one
another in function and arrangement.
• The nucleus, has a
double membrane
perforated by pores
to allow exit of
mRNA for protein
synthesis.
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• The inner membrane
of mitochondrian is
convoluted giving a
large surface area
for aerobic
respiration to occur
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The membranes possessed by the cell’s various
organelles tend to vary from one
another in function and arrangement.
• The membranes of • The enzymes
around a lysosome
the Golgi apparatus
isolates digestive
and endoplasmic
enzymes until they
reticulum become
are required.
pinched of releasing
vesicles containing
protein.
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Membrane composition
Membrane Type
% Lipid
% Protein
Myelin
Mitochondrion
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Red blood cell
50
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Despite this diversity of function, all of these
membranes have the same basic structure as the
plasma membrane and operate as selective
barriers, each contributing in its own way to the
integrated
working of the
cell.
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Constant cell environment.
• For a cell to function efficiently, its internal
environment must remain fairly constant with
respect to water and soluble substances present
in the cytoplasm.
• E.g. If O2 was to be used up without being
replaced, the cell would eventually be unable to
respire aerobically.
• It is the cell membrane which regulates the entry
and exit of substances to maintain a constant
environment.
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TASK – Testing you knowledge!
• Complete Torrance Pg 46 Questions 1-3
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Essay Question
• Describe the function of lipids and proteins
within the plasma membrane.
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Modes of transport – 3 main types.
• The movement of small molecules or ions
into or out of a cell normally occurs as a
result of:
Which depends on
1. Diffusion
the nature of the
2. Osmosis
substance involved.
3. Active transport.
• In addition:
4. Endocytosis: cells engulf and take in large
molecules
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5. Exocytosis: discharge
secretory products. 23
Passive Transport: Diffusion & Osmosis
• Molecules move down the
concentration gradient.
• No energy is required .
• Diffusion - molecules moves
from high concentration to low
concentration e.g. minerals,
oxygen and carbon dioxide
Channel-forming proteins
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• Osmosis - water molecules
through a selectivelypermeable cell membrane from
HWC to LWC
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Role of the plasma membrane in diffusion:
IT IS SELECTIVLY PERMEABLE.
• The PM possessed tiny pores and is
freely permeable to small molecules
e.g.O2.
• It is not equally permeable to all
substances.
• Larger molecules pass by diffusion slowly
• Even larger molecules are unable to pass
by diffusion.
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Role of the plasma membrane in OSMOSIS:
IT IS SELECTIVLY PERMEABLE.
• Whenever a cell is in contact with a solution of
different water concentration, osmosis occurs.
• This because the PM is selectively permeable.
• Allowing rapid movement of H2O through it (but
allows larger molecules to move slowly or not at
all).
• The direction of water molecules depends upon
the water concentration of the liquid the cell is in
compared with the cell contents.
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Osmosis in Red Blood Cells
When RBC’s are placed
in salt solution, there is
a water there is a HWC
inside and LWC
outside. Water moves
out of cells and cells
shrink
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Osmosis in Red Blood Cells
When RBC’s are placed
in water there is a LWC
inside and HWC
outside. Water moves
into cells and cells
burst.
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Osmosis in Red Blood Cells
When the water
concentration of the RBC
inside = water
concentration outside. No
net gain or loss of water
It is essential that the water and solute
concentration of blood plasma and body cells is
maintained at a steady state so that the body is
not damaged by the cells
bursting or shrinking 29
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Ecstasy tablets: interfere with
the water balance in cells
• Immediate effects include an
increased heart rate and raised
blood pressure. Dry mouth and
raised body temperature.
Ecstasy interferes with the
body's fluid control mechanisms
and salt balance, and it's easy
to drink too much fluid and
overhydrate (water poisoning,
causing the brain to swell
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dangerously.
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Active Transport
• Molecules move across the PM
against the concentration
gradient.
• Energy (ATP from respiration)
required.
• Proteins act as carrier
molecules
• Proteins are specific
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• Rate affected by temperature,
respiratory substrate, oxygen
concentration
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Sodium/potassium pump
• Active transport carriers = pumps. Some carriers have a duel
role, they exchange one type of ion for another.
• Above shows Na ions being pumped out of the cell and K ions
being pumped into the cell against the concentration gradient.
The same carrier molecule is able to convert ATP to ADP + Pi
to
produce energy for this process.
This balance is very 32
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important for nerve cells.
Conditions required for active
transport
Factors such as
– Temperature
– Availability of oxygen.
– Availability of respiratory substrate (e.g. Glucose)
• All affect the cell’s respiration rate also affect the
rate of active transport.
• For example increase in temp causes an increase
in ion uptake until at high temps the enzyme
becomes denatured and the cell dies.
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Endocytosis
• Cells absorb material e.g. Proteins, hormones or lipids
from the outside by engulfing it with cell membrane.
• This involves the PM folding inwards to form a pouch.
When this becomes closed off and detached from the cell
membrane, it is called an INTRACELLULAR VESICLE.
• It is used by all cells of the body because most substances
important to them are large polar molecules so cannot
pass through the hydrophobic plasma membrane.
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Phagocytosis
• The engulphing of large solid particles by the cell is
called PHAGOCYTOSIS “Cell-eating”.
• The contents of the vesicle are then digested.
• e.g. white blood cell eating fungus Candida albicans
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Pinocytosis
• The formation of
small liquid-filled
vesicles by the cell
membrane is called
PINOCYTOSIS “Celldrinking”
• e.g. Amoeba showing
pinocytosis
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Exocytosis
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• This is the reverse of
endocytosis
• Vesicles formed inside the
cell fuse with the PM
allowing their contents to
be expelled from the cell
membrane.
• Vesicles contain:
soluble proteins to be
secreted to the
extracellular environment
membrane proteins and
lipids that are sent to
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cell membrane
TASK – Testing you knowledge!
• Complete Torrance Pg 49/50 Questions 1-3
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