Download Diffusion, Osmosis, Active Transport

Diffusion, Osmosis, Active
Transport
Biology Notes
GCE Study Buddy
Diffusion
 Diffusion is the net movement of ions or molecules from a
region of higher concentration to a region of lower
concentration, down a concentration gradient
 In liquids and gases, molecules possess kinetic energy and
constantly move about
 As this movement is random, an equilibrium is reached when
the molecules are evenly spread out, where there is no more
net change – dynamic equilibrium reached
 The steeper the gradient for a substance, the faster the rate of
diffusion
 No energy input is required for diffusion because the particles
already are in motion.
 Hence, it is know as passive transport
Diffusion
permeable
membrane
water
20
potassium
iodide ions
20
copper
sulphate ions
Key
potassium iodide ions
copper sulphate ions
10
potassium
iodide ions,
10
copper
sulphate ions
10
copper
sulphate ions,
10
potassium
iodide ions
Factors affecting diffusion
 What are some factors affecting the rate of
diffusion?





Concentration gradient
Temperature
Pressure
size of particles
surface area to volume ratio
Osmosis
 Osmosis is the movement of water molecules from a solution
of higher water potential to a solution of lower water
potential through a partially permeable membrane
 Water potential is a measure of the tendency of water to move
from one place to another
 A water potential gradient is established when a
partially-permeable membrane separates two solutions
of different water potentials.
 A dilute solution contains more water molecules per unit
volume than a concentrated solution so it has a higher water
potential than a concentrated solution
Osmosis
 Osmotic pressure is the pressure that must be applied
to a solution to prevent the inward flow of water
across a partially permeable membrane .
 Osmotic pressure is the opposite of water potential.
 A partially permeable membrane is a membrane that
will allow certain molecules or ions to pass through it
by diffusion.
 The cell membrane is partially permeable.
 What other structures or organs are partially
permeable?
Osmosis
rise in level
A
B
A
5%
sucrose
solution
(more
water,
less
sucrose)
B
A
B
drop in
level
water
molecules
diffuse
from B to
A.
partially
permeable
membrane
8 sucrose
molecules,
9 water
molecules
4 sucrose
molecules,
18 water
molecules
Key
sucrose molecule
water molecule
Net movement of water
molecules
8 sucrose
molecules,
18 water
molecules
4 sucrose
molecules,
9 water
molecules
Hypotonic, Isotonic, Hypertonic
(animal cells only)
Lower water potential Equal water potential Higher water potential
Plant cell in high water
potential solution
 As water enters the cell, the vacuole increases in size and
pushes the cell contents against the cellulose cell wall.
The cell wall prevents over expansion of the cell by
exerting an opposing pressure preventing the entry of
more water
 A plant in this state becomes turgid
 The turgidity of the cell with water is called turgor
pressure
 The plant cell does not burst because the cell wall is
strong and relatively inelastic
Animal cell in high water
potential solution
 Animal cell will swell and may bust in a hypotonic
solution because it does not have a cell wall
in a solution with high
water potential
Plant cell in low water
potential solution
 The water potential of the cell sap is higher than that of the
solution outside the cell. Osmosis takes place and water
moves out of the cell
 As water is lost from the cell, the vacuole decreases in size
and the cytoplasm shrinks away from the cellulose cell wall.
 This shrinkage of the cytoplasm away from the cell wall when
plant cells are immersed in a solution of low water potential is
known as plasmolysis
 The cells are said to be plasmolysed. The cells can be restored
by placing them in water or solution of high water potential
Plant cell in low water
potential solution
water leaves by osmosis
Cytoplasm shrinks away from cell
wall
in a solution with low
water potential
As the cell loses water, the vacuole
decreases in size.
The cell becomes plasmolysed
Animal cell in low water
potential solution
 Animal cells in hypertonic solution: it will lose water
and the cell membrane will form little spikes as water is
lost and the cell shrinks, a process called crenation. The
cell dies eventually
Spikes form
in a solution with low
water potential
Turgor in plants
 Turgor is very important in maintaining the shape of soft
tissues in plants
 Young stems and most leaves, esp. those of herbaceous or
non-woody plants, can remain firm and erect because of the
turgor pressure within their cells.
 When there is a high rate of evaporation of water from the
cells, they lose their turgidity and the plant wilts
 The movement of certain plants are due to changes in turgor
 Eg opening and closing of the stomata, folding of leaflets of the
Mimosa plant
Active Transport
 Active transport is the process in which energy is used to
move the particles of a substance against a concentration
gradient from a region of lower concentration to a region
of higher concentration
 Occurs in living cells because only living cells respire
Active transport
 The energy used in cells is called “ATP”.
 Active transport requires:
 Protein transporters
 ATP (or energy)
 Active transport occurs in:
 Absorption of mineral salts by root hair cells.
 Absorption of glucose and amino acids by cells in the
small intestine.
 Na-K pump
Example of Active
Transport
 Absorption of mineral salts by root hair cells
 The soil solution has to have a higher water potential than the root
hair cell in order for water to move into the root hair cell by osmosis.
 This means that the soil solution has a lower concentration of ions
(ie. mineral salts) than the root hair cell.
 Therefore, active transport is used to transport ions into the root hair
cell.
 Specific transporter proteins
 Energy expended.
Transport of large
molecules
 Some substances are too large to cross the
plasma membrane by diffusion, facilitated
diffusion and osmosis
 Other means of transport are necessary:
 Endocytosis
 Exocytosis
Surface area: Volume ratio
The greater the area
of cell surface
membrane, the
faster will be the
rate of diffusion of
a substance for a
given concentration
gradient
Surface area: Volume ratio
 The greater the surface area to volume ratio, the
faster the rate of diffusion.
 This explains why cells are so small: nutrients, oxygen
and waste substances have to diffuse into and out of
cells quickly.
 Eg.
Question
 Four clear agar blocks were placed into solutions of
methylene blue as shown. Which agar block would be
first to become completely stained?
A
D
C
B
6mm
4mm
4mm
2mm
2mm
2mm
2mm
2mmRanee Mohan
2mm
2mm
4mm
4mm
Question
 An experiment to investigate
osmosis is set up as shown.
What happens?
A
B
C
D
Volume of
the liquid
in tube X
Decreases
Decreases
Increases
Increases
Concentration
of the sugar
solution
Decreases
Increases
Decreases
Increases
Ranee Mohan
Tube X
Partially
permeable
membrane
Question
 A plasmolysed cell is shown. Which numbered
structures are partially-permeable?
A. 1 only
B. 2 only
C. 1 and 2 only
1
2
D. 1 and 3 only
3
Question
Four identical pieces of potato are treated in two
stages, as shown. Which piece of potato will be
the largest after two hours?
Stage 1
A
Stage 2
Placed in 10% salt solution
C
Boiled in water
for 10 min
Boiled in water
for 10 min
Unboiled
D
Unboiled
Placed in distilled water
B
Ranee Mohan
Placed in distilled water
Placed in 10% salt solution
Question
 Which one of the following enters plant cells by active
transport?
A. Carbon dioxide
B. Nitrate ions
C. Water
D. Oxygen