CHAPTER 3: MOVEMENT OF SUBSTANCES ACROSS THE PLASMA MEMBRANE
3.1 STRUCTURE OF PLASMA MEMBRANE
Cells need oxygen and nutrients to carry out cell metabolism and also to eliminate carbon dioxide
and nitrogenous waste products produced. The movement of these substances in and out of cells
is regulated by the plasma membrane. This regulation is necessary for it to function effectively.
 The structure of the plasma membrane
The plasma membrane is the boundary that separates substances in the cytoplasm from its
surroundings. According to the fluid mosaic model proposed by S.J. Singer and G.L. Nicolson
in 1972, the structure of the plasma membrane is as follows:1) The plasma membrane is made up of a ________________bilayer with various types of
______________ molecules and other components (cholesterol/ glycoprotein) embedded in
it to form a mosaic pattern. The phospholipid and protein molecules are not static but is
_________in structure and ___________ in nature, thus making the structure strong and
flexible.
2) Each phospholipid molecule has a polar head which is ____________ (attracted to water)
and non-polar tail which is _______________ (repelled by water).
 The plasma membrane is semi-permeable/ selectively permeable because:a) the phospholipid bilayer allows __________ molecules such as fatty acids, glycerol,
steroids, hormones and vitamins A, D, E, K, small uncharged/ non-polar molecules such as
oxygen and carbon dioxide and small molecules such as water to move across the plasma
membrane.
b) pore/channel proteins allow small ______________ molecules and_________ such as K+,
Na+, Ca+ and CI- to move across the plasma membrane.
c) carrier proteins allow large _____________ molecules such as glucose, amino acids and
vitamin C to move across the plasma membrane. They have binding sites to take up these
molecules, change shape and transport them across the plasma membrane.
d) ________________ like sucrose, starch and proteins cannot pass across the plasma
membrane.
3.2 CONCEPT OF MOVEMENT OF SUBSTANCES ACROSS A PLASMA MEMBRANE
Substances can move across the plasma membrane by
1) ____________ transport such as simple diffusion, osmosis and facilitated diffusion
2) _____________ transport
Passive transport is the movement of substances across the plasma membrane _________ the
concentration gradient (from a region of ________ concentration to a region of
_________concentration) and no _________ is required.
a) Simple diffusion is the movement of molecules or ions from a region of
________concentration to a region of _________ concentration. Molecules will move
down the concentration gradient until an equilibrium is achieved whereby the molecules are
distributed equally on both regions.
Eg: Lipid-soluble molecules and small uncharged molecules pass through the phospholipid
bilayer of the plasma membrane by simple diffusion.
b) Osmosis is the movement of _________ molecules from a region of _________ water
concentration (hypotonic solution) to a region of _________ water concentration
(hypertonic solution) across a semi-permeable membrane. As the plasma membrane is a
semi-permeable membrane, water molecules move in and out of the plasma membrane of
cells by osmosis.
(A region of high concentration refers to a hypertonic solution whereby the solution has a
lot of solutes dissolved in it whereas a region of low concentration refers to a hypotonic
solution whereby the solution has little solutes dissolved in it.
c) Facilitated diffusion is the movement of substances across the plasma membrane aided
by ______________ (pore proteins and carrier proteins).
Eg: - Small water-soluble molecules and ions pass through the ________________ in the
plasma membrane by facilitated diffusion. The pore proteins have specific interior
characteristics that only allow specific ions to pass through them.
– Large water-soluble molecules pass through the plasma membrane aided
by___________________. The molecules bind to the ______________ of the carrier
protein. The carrier protein then changes shape to allow the molecules to pass through
to the other side of the plasma membrane.
Active transport is the movement of molecules or ions across the plasma membrane
___________ the concentration gradient and _________ is required. It involves carrier
proteins and only takes place in living cells (active transport cannot take place in cells that
have been killed/ poisoned as cellular respiration stops). The carrier protein has an active site
which binds to the ______ molecule. The carrier protein changes _________ when the
phosphate group from the ATP molecule binds to it. The particular molecule is then moved
across the plasma membrane.
 Comparison between passive transport and active transport in living organisms
Similarities: Both involve moving a substance across a membrane and occur through a selectively
permeable membrane.
Differences:
Passive transport
Active transport
1)
1)
2)
2)
3)
3)
Compare the 4 different processes involved in the movement of substances:
Process
Aspects
1) Movement of
substances
Simple diffusion
Osmosis
Facilitated
Diffusion
Active
transport
Need plasma
membrane
Need plasma
membrane
Ions, nucleic
acids,
glucose,
amino acids,
water soluble
molecules
Sodium ions &
potassium ions
in animals,
iodide ions in
algae, mineral
ions in plants.
Yes. Need a
living cell
that has
carrier
proteins &
pore proteins
Yes. Need a
living cell to
provide energy
& carrier
proteins
Absorption of
glucose &
amino acids
through villi
of small
intestine
Uptake of
mineral salts by
root hairs. Na+
& K+ pump in
nerve cells.
2) Concentration
gradient
3) Cellular
energy
4) Membrane
5) Outcome
process
Need semipermeable
membrane
of
6) Substance
involved
7) Needs
cells?
Not necessary
Small molecules
Solvents e.g. water
e.g. O2,CO2, lipid
soluble molecules
in tissues, or other
substances
living No. Can occur in
non- living
environment, e.g.
dye in beaker
No. Can occur in
non-living
environment, e.g.
Visking tube
8) Movement
across plasma
membrane
9) Examples in In lungs, diffusion Water moves by
living
of O2 from alveoli osmosis from soil
organisms
into blood
into root hairs
3.3 MOVEMENT OF SUBSTANCES ACROSS A PLASMA MEMBRANE IN LIVING
ORGANISMS
(a) Gaseous exchange in the alveoli & blood capillaries
- Gaseous exchange in the alveoli of the lungs occurs by __________________.
- The concentration of _________ in the alveoli is higher than that of the blood capillaries.
- Hence, oxygen diffuses from the alveoli across the epithelial cells into the blood capillaries
_________ the concentration gradient.
- The concentration of _________________ in the blood capillaries is higher than that of the
alveoli.
- Hence, carbon dioxide diffuses from the blood capillaries into the alveoli down the
concentration gradient.
- No __________ is used for this process
(b) Absorption of water by the root hairs of a plant
- The absorption of water by the root hairs of a plant occurs by ___________.
- The concentration of water molecules in the soil solution is __________ than that of the
cell sap in the root hairs (or the concentration of solutes in the soil is lower than in the cell
sap)
- Therefore water diffuses from the soil across the plasma membrane of the root hairs into
the root hairs of the plant by osmosis.
- This process takes place __________ the concentration gradient.
- No ___________ is used for this process.
(c) Absorption of digested food in the villus
- The absorption of digested food such as glucose & amino acids in the villus of the small
intestine occurs by ______________________.
- The concentration of glucose & amino acids is ____________ in the lumen of the small
intestine (ileum) than in the blood capillaries inside the villus
- Hence, glucose & amino acids diffuse across the epithelial cells of the villus into the blood
capillaries _______ the concentration gradient.
- Carrier proteins in the plasma membrane of the epithelial cells are needed to transport
the glucose & amino acid molecules into the blood capillaries.
- However, no _________ is used for this process.
Ion intake by the root hairs of a plant
- Ion intake by the root hairs of a plant occurs by ___________________.
- The concentration of mineral ions in the cell sap of the root hairs (epidermal cells) is
______________ than its concentration in the soil solution.
- As the plant needs the mineral ions, it has to pump the ions by active transport across the
membrane of the root hairs into the cells against the concentration gradient.
- ________________ in the plasma membrane of the root hairs transport the mineral ions
across the membrane.
- Cellular energy from _____________ is used for this process.
3.4 MOVEMENT OF SUBSTANCES ACROSS A PLASMA MEMBRANE AND ITS
APPLICATION IN DAILY LIFE
1) Phenomenon of Plant Wilting. When excessive fertilisers are used, soil water becomes
________________ to the cell sap of the root hair cells. Root hair cells become
____________because water molecules diffuse out of the cell vacuole by osmosis. The plant
_________ and eventually dies.
2) The addition of excessive salt/ sugar to food during food preservation causes the salt/ sugar
solution to be____________ to the cell sap of the plant cell. Water molecules diffuse out of the
cells of the food substances by osmosis causing the food to be ______________. Dehydrated
food prevents growth of bacteria and fungi. Microorganisms also face plasmolysis and die.
3) Reverse osmosis process in water purification.
4) Application of movement of substances across a plasma membrane in daily life:
a. Rehydration drinks
b. Saline solutions
c. Liposomes
d. Isotonic drinks