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MEMBRANES AND CELL
ORGANELLES 1
CELL STRUCTURE:
MEETING THE NEEDS OF MOLECULES

Molecules need to:
– move in and around cell at a certain rate to
reach sites of specific activity (ie where they
will react with other molecules)
– be in adequate concentrations (ie there needs
to be enough of them) for chemical reactions to
occur at the right rate.

Cell structure therefore needs to facilitate the
movement of molecules and maintain them in
adequate concentrations to maintain cell function
(ie so the cell doesn’t die)
The Surface area conundrum

Cells need to maximise their surface area
to ensure the rapid movement of
molecules

Problem:
– As volume increases, surface area
decreases!
– How do cells deal with this?
Membranes!

Prokaryotic
Types of cells
– Very small: less
than 2mm in
diameter
– Lack internal
compartments
– Bacteria and
archaeans

Eukaryotic
– Much larger: 10100mm in diameter
– More complex
structure –
compartments
called organelles
– Animals, plants,
fungi and protists
Organelles

Large eukaryotic cells increase
their surface area by having
folded membranes and
internal compartments called
organelles

Organelles also allow
different chemical reactions
to occur at the same time in
different places without
interfering with each other

Organelles maintain the
concentration of molecules
at levels that ensure they will
react with each other at
optimum rates
CELL STRUCTURE
We are now going to learn about the structure of
eukaryotic cells and their various organelles in the
context of cellular processes.
What is a cell?
A fluid filled compartment containing atoms and
molecules
INTRACELLULAR
AQUEOUS
ENVIRONMENT – CYTOSOL
or CYTOPLASM
EXTRACELLULAR
AQUEOUS
ENVIRONMENT
CELL BOUNDARY
(PLASMA MEMBRANE)
Cell membrane - structure
A plasma membrane is an ultra thin and pliable layer
with an average thickness of less than 0.01 μm
(0.00001 mm).
Cell membrane - structure
Called fluid mosaic model
 Lipids are the fluid part of the membrane
 Proteins are the mosaic part of the
membrane

Cell membrane - functions

Define cell boundary

Provide permeability barrier (acts like a sieve)

Provide sites for specific functions

Regulate transport of solutes

Detect electrical and chemical signals

Assists in cell to cell communication
Summary: crossing the cell membrane
Type
Diffusion
Osmosis
Facilitated
diffusion –
carrier proteins
Facilitated
diffusion –
channel proteins
Active transport
Endo/Exocytosis
Description
Molecules
1. Diffusion
The movement of molecules from areas of
high solute concentration to area of low
solute concentration.
i.e.. Down the concentration gradient.
No energy is involved!
Diffusion depends
on…
Permeability
Surface Area
Concentration Gradient
Distance of Diffusion
Which molecule
will diffuse?
Fick’s
Diffusion Law
Surface area
of membrane
Difference in concentration
X
across the membrane
Length of the diffusion path
(thickness of the membrane)
Ways to increase
diffusion
Increasing
concentration
Increasing temperature
Increasing surface area
Permeable membrane
If the membrane is permeable to both the
solute and the solvent, the pattern of
diffusion is unchanged.
Concentration Gradients
Diffusion
High concentration
Low concentration
No net movement!
Once diffusion is complete the
molecules keep moving but the
overall distribution remains
constant = equilibrium.
Partially Permeable
Membrane
If the membrane is partially permeable, the
solvent can move through but the solute
cannot.
Concentration Gradients
Partially permeable membrane
High concentration
Low concentration
2. Osmosis
A special type of diffusion!
Solute
Water
molecules
The Add
solute
cannot cross the membrane. To try and
Solute
balance the concentrations, the water molecules
move to dilute the solution.
High concentration
solute
Low concentration
solute
The solute cannot cross the membrane. To try and
balance the concentrations, the water molecules
move to dilute the most concentrated solution.
Osmotic Gradient
Concentrated solute
Dilute solute
The pressure that makes the water move is
called the osmotic pressure.
Hypotonic =
extracellular fluid
lower
concentration
than intracellular
fluid and water will
diffuse into cell
Isotonic = extra
and
intracellular
fluid are same
concentration
and there will
be no net
movement of
water
Hypertonic =
extracellular
fluid higher
concentration
than
intracellular
fluid and water
will diffuse out
of cells
The net movement of water
from a region of low solute
concentration to a region of
high solute concentration is
called:
A.
B.
C.
D.
Osmosis
Diffusion
Facilitated diffusion
Active transport
3. Facilitated Diffusion

Most molecules are too large or too polar to
cross membrane by simple diffusion

Protein assisted movement down a
concentration gradient – facilitated diffusion
can occur in a few different ways
HIGH
CONCENTRATION
GRADIENT
LOW
Facilitated Diffusion
Special channels in the membrane help the diffusion.
This channel or carrier mediated movement is selective
and can become saturated. This may inhibit the
movement of another molecule. No energy is used.
Facilitated diffusion: carrier protein
The molecule binds to its carrier protein,
potentially changing its shape, and is
carried to the other side
Facilitated diffusion: channel protein
Channel proteins form pores in the membrane
that fill with water and dissolve hydrophillic
molecules.
Both simple diffusion and
facilitated diffusion involve:
A.
B.
C.
D.
Energy expenditure by the cell
Movement of a substance down its
concentration gradient
A protein in the plasma membrane
acting as a carrier molecule
A substance moving from outside to
inside a cell across the membrane.
4. Active transport
When the cell spends energy to
move molecules against the
concentration gradient.
Concentration Gradients
Active transport
High concentration
Low concentration
Against the concentration gradient!
Transport/Carrier proteins
Form a channel for molecules to pass
through.
They are selective, may become saturated
and inhibit the movement of other
molecules.
Space filling model of rabbit
calcium ATPase. Calcium
ATPase is a membrane
transport protein which
transfers calcium after a
muscle has contracted.
Extracellular fluid
Sodium-Potassium
Pumps
Na+
Na+
The sodiumpotassium pump
is a protein in the
membrane that
exchanges sodium
ions (Na+) for
potassium ions (K+)
across the
membrane.
K+
Na+
Plasma
membran
e
Carrier
protein
K+
ATP
Na+
Na+ moves to
its binding site
Cell cytoplasm
K+
Proton
Pumps
Proton pumps use
the energy from
ATP to move
hydrogen ions
(H+) from inside
the cell to the
outside.
Extracellular fluid
H+
H+
H+
H+
H+
H+
Carrier
protein
Plasma membrane
ATP
H+
Cell cytoplasm
Coupled
Transport
Coupled transport
is also called cotransport.
Plant cells use the
hydrogen gradient
created by proton
pumps to actively
transport nutrients
into the cell.
Extracellular fluid
Diffusion of hydrogen
ions down their
concentration
gradient
H+
H+
Sucrose
H+
Carrier
protein
H+
Plasma
membrane
H+
Cell cytoplasm
Summary-Membrane Pumps
Membrane pumps are proteins,which
require energy (often as ATP) to transport
molecules across the cell membrane.
.
Extracellular fluid
Na+
Na+
K+
Na+
H+
H+
H+
H+
H+
Plasma
membrane
H+
ATP
K+
Na+
Cell cytoplasm
K+
ATP
H+
H+
Summary-Membrane Pumps
The activity of pumps may be coupled, e.g.
the accumulation of H+ from the proton
pump is used to drive the transport of
sucrose against its concentration gradient.
Extracellular fluid
Na+
Na+
K+
Na+
H+
H+
H+
H+
H+
Plasma
membrane
H+
ATP
K+
Na+
Cell cytoplasm
K+
ATP
H+
H+
The role of proteins and
protein complexes in the
plasma membrane of a cell
includes their role as:
A.
B.
C.
D.
A receptor protein
A channel or pore
An antigen
All of the above
5. Cytosis
When the cell spends energy to
move LARGE molecules.
Moving large molecules

Sometimes, large molecules need to be
moved around in the cell, stored within, or
moved outside the cell

To do this, cells make very small containers
or sacs called vesicles from the plasma
membrane

Transporting out of the cell: exocytosis

Transporting into of the cell: endocytosis
Active Transport: Cytosis
Membrane-bound
vesicles or vacuoles
are formed by
infolding
(invagination) or
outfolding
(evaginated) to
transport substances
across the
membrane.
Membranebound vesicle
Plasma
membrane folding
inwards
This cell is carrying out a form of
endocytosis called pinocytosis in which the
plasma membrane forms invaginations to
enclose liquids and bring them into the cell.
Phagocytosis
During endocytosis the
plasma membrane
invaginates (folds
in) around the
molecules to be
transported into the
cell.
Solid particle
CDC
Endocytosis
Pinocytosis
Membranebound vesicle
Endocytosis
1
Materials that are to be
collected and brought into the
cell are engulfed by an
invagination of the plasma
membrane.
2
Plasma
membrane
Vesicle buds off from
the plasma
membrane.
3
Cell cytoplasm
The vesicle carries molecules
into the cell. The contents may
then be digested by enzymes
delivered to the vacuole by
lysosomes.
Types of endocytosis:



phagocytosis: the engulfment of solid
particles.
pinocytosis: the engulfment of liquid
particles.
receptor mediated: engulfment of specific
particles according to membrane receptors.
Phagocytosis
Food particle
(cell eating)
The particles are contained
within a membrane
enclosed sac (a vacuole).
Digestion of the particles
occur when the vacuole
fuses with a lysosome
containing digestive
enzymes.
Amoeba
pseudopod
Engulfed
bacterium
Pinocytosis
Invaginations of the plasma
membrane enclose the
liquid droplets within
small vesicles.
Plasma membrane
engulfing liquid
substance.
Membranebound vesicle
The fluid within the vesicle
is transferred to the
cytosol.
Pinocytosis by a capillary endothelial
cell. TEM (X12,880)
Receptor-Mediated Endocytosis
The cell membrane has
regions of specific
receptor proteins
exposed to the
extracellular
environment.
The receptor proteins
occur in clusters (called
coated pits) and have
binding sites that will
only bind specific
molecules.
Extracellular
fluid
Receptor
protein
Cytoplasm
Plasma
membrane
Receptor-Mediated Endocytosis
The cytoplasmic side of the
coated pit is lined with a special
protein called clathrin protein,
which provides membrane
stability (right).
Target
molecule
Clathrin
protein
Coated
vesicle
When the target molecule (ligand)
binds to the receptor protein (left),
a coated vesicle forms around it,
allowing the molecule to be
imported into the cell.
A cell that is
phagocytosing a bacteria
cell could be expected
to:
A.
B.
C.
D.
Have a cell wall
Be expending energy
Be producing oxygen
Contain a chloroplast
Exocytosis
Exocytosis releases
molecules from the inside of
the cell to outside of the
cell.
Exocytosis occurs by fusion of
a vesicle membrane with the
plasma membrane. The
vesicle contents are then
released to the outside of
the cell.
Transport
vesicle
Cross section through the plasma
membrane of cardiac muscle showing
the presence of transport vesicles.
TEM X 162,000
Exocytosis
3
2
1
Vesicle carrying molecules
for export moves to the
perimeter of the cell.
The contents of the vesicle are
expelled into the intercellular space
(which may be into the bloodstream).
Vesicle fuses with
the plasma
membrane.
Plasma membranes that are
able to bend and fold are
necessary for the movement
of which substances into or
out of a cell?
A.
B.
C.
D.
Glucose molecules
Sodium ions
Fatty acid molecules
Protein molecules
Summary
There are two types of
transport in a cell.
1. Passive (not requiring energy)
Plasma membrane
Cell cytoplasm
diffusion and facilitated diffusion
osmosis
2. Active or energy requiring
Active transport
Cytosis (exocytosis, endocytosis etc)
The plasma membrane is partially
permeable, allowing some molecules to
pass through, and preventing the
passage of others.
E
The three types of movement across a
membrane are correctly described as
X
Y
Z
A
active
transport
diffusion
facilitated
diffusion
B
active
transport
facilitated
diffusion
diffusion
C
facilitated
diffusion
active
transport
diffusion
D
diffusion
active
transport
facilitated
diffusion
SUMMARY