Absorption & secretion of materials
Plant & animal cells in solutions of different strength.
Absorption and secretion of materials
Cell Walls
•
The cell wall is a non-living layer composed mainly
of cellulose – a fibrous criss-crossing carbohydrate.
•
Cell walls allow free movement of most substances
into or out of the cell and provide a continuous
water-conducting path around the plant.
•
The cell membrane is selectively permeable !
 The molecules of liquids and gases move
about freely all the time.
 Molecules move from a region of high
concentration of that substance to a region of
low concentration of that substance.
 Diffusion continues until the concentration
becomes equal.
Diffusion
Factors affecting the rate of diffusion across membranes include:
The steepness of the
concentration gradient. The
greater the difference in
concentration, the faster the rate.
Temperature: Molecules have
more kinetic energy at high
temperatures and diffuse
faster
Diffusion is the net movement
of molecules down a
concentrated gradient.
The type of molecule or ion: Large
molecules diffuse more slowly than
small ones. Non-polar molecules
diffuse faster
Surface area: the
greater the surface
area, the more
molecules or ions
that can cross it.
Osmosis in animal cells – red blood cells
Movement of water into or out of red blood cells by osmosis in solutions
of different concentration
Red cell bursts
In (hypotonic) pure
water or dilute
solution
Low concentration
of solute molecules,
high concentration
of water molecules
Red cell remains
normal
Red cell shrinks
In a (isotonic) solution In a (hypertonic) more
concentrated solution
with the same
concentration as the
red cell
High concentration
of solute molecules,
low concentration of
water molecules
Hypotonic
=
contains more water
Isotonic
=
contains equal water
Hypertonic
=
contains less water
Lipids
Simple lipids (Triglycerides)
The most common forms of lipids are triglycerides (fats
and oils)
These are made by the combination of 3 fatty acid
molecules with 1 glycerol molecule.
G
L
Y
C
E
R
O
L
FATTY ACID
FATTY ACID
FATTY ACID
Lipids
Phospholipids
The third fatty acid has been replaced by a phosphate
group.
Phosphate groups are hydrophilic.
2 representations
of a phospholipid
molecule
Non-polar tail
Polar
head
Lipids
Phospholipids and membranes
Cell
membrane
cross
section
1 phospholipid
molecule
A cell membrane is made up
of a double phospholipid layer
Membrane structure
When mixed with water, phospholipid molecules
spontaneously assemble to form membrane-like
structures.
Their polar heads point outwards towards the
surrounding charged water molecules, and their
non-polar tails point inwards.
Under certain conditions they form bilayers, the
basis of cell membranes
DETAILED
VIEW OF
BILAYER
PHOSPHOLIPID MOLECULE
HYDROPHOBIC TAIL
HYDROPHILIC
HEAD
Membrane structure
Features of the fluid mosaic model
The double line seen at very
high power is thought to be
the 2 phospholipid layers. The
bilayer is about 7 nm wide.
Membranes also contain
proteins and the model of
membranes accepted at
present is called the ‘fluid
mosaic’ model.
The phospholipid bilayer is the
fluid part because phospholipid
molecules can move around
The protein molecules form a
mosaic pattern set in the
phospholipid bilayer
Membrane structure
Features of the fluid mosaic model
outside
PHOSPHOLIPID
LAYER
3 INTRINSIC PROTEINS
inside
EXTRINSIC
PROTEIN
Most protein molecules
are mobile, moving
around freely. Others
are fixed like islands to
structures in the
membrane and do not
move
Membrane structure: the fluid mosaic model
•
Phospholipids consist of a phosphate group ‘head’
and two fatty acid tails.
•
The phosphate head is hydrophilic and the tail is
hydrophobic; in water the molecules spontaneously
arrange themselves into membrane like structures.
•
Under certain conditions they form bilayers.
•
Membranes also contain various proteins, set in the
phospholipid layer in a mosaic pattern.
Torrance p.8 Questions 1 – 3
Transport across the plasma membrane
Active transport
Active transport is the pumping
of ions across membranes
against a diffusion gradient.
ATP is required to change the
shape of the protein and move the
ion or molecule across.
Transport across the plasma membrane
Bulk transport - Endocytosis
Diffusion, osmosis and active
transport refer to the
movement of individual
particles across membranes
Ingestion
Release of
microbial
debris
Destruction of
microbe
Stages in phagocytosis of a
bacterium by a white blood cell
Phagocytosis or ‘cell eating’. The bulk
uptake of solid materials. Cells which do
this are phagocytes, e.g. some white blood
cells
Mechanisms also exist for the
bulk transport of materials in
and out of cells (endo- and
exocytosis).
Transport across the plasma membrane
Bulk transport - Exocytosis
Exocytosis is the reverse
of endocytosis
It happens, for example, in
the secretion of digestive
enzymes from the pancreas
Secretory vesicles from the
Golgi body carry the
enzymes to the cell surface
and release them to the
outside of the cell
Diagram of Golgi
apparatus secretion
Exocytosis
EM of pancreatic
acinar cell secreting
protein
Golgi apparatus
Secretory vesicle
containing
secretory product,
e.g. enzyme
Torrance pp. 15/16 Questions 1 – 3
Do either 1. a) or 1. b) and either 3. a) or 3. b)
ATP and energy release
Aerobic respiration
ATP and energy release
•
In respiration, glucose is broken down to
release a large amount of energy.
•
This energy must be transferred to the point in
the cell where it is needed.
•
Adenosine triphosphate (ATP) is the molecule
which acts as an energy carrier in all living cells.
Structure of ATP
ATP
energy released
energy required
(high energy state)
ADP + Pi
(Low energy state)
Turnover of ATP
•
The total ATP in the body would be used up, at
rest, in about 90 seconds.
•
Therefore, ATP is constantly being reformed
from ADP using energy released during the
breakdown of glucose.
•
The net result is a constant quantity of ATP in
the body; about 50g.
Torrance pp. 22/23 All questions