Movement in and out of the cell
membrane
Fluid compartments in our bodies are
separated by membranes
Composition of body fluids
Distribution of Total body fluids
Greater number of osmotically active particles
Some organisms have a CELL WALL
•
•
•
•
Plants (cellulose)
Algae (polysaccharide)
Fungi (chitin)
Prokaryotes(peptidoglycan)
Why have a cell wall?
1.
2.
3.
4.
5.
Mechanical support
Protection
Cell-cell communication
Maintenance of structure (turgor)
Prevent water loss
Membranes
2007-2008
Where are membranes located?
• Plasma membrane
(double membrane)
• Membrane-bound
organelles (single or
double)
• Secret Universe
• Introduction to cell
membrane
Why do we need plasma membranes made
Our cells inhabit an aqueous environment
• …but they must let ‘stuff’ (nutrients, ions,
molecules,waste products) in and out
• …..and they need to communicate with each
other…
How is this achieved?....
Functions of the Cell membrane
1. Protective barrier
2. Cell-Cell signalling
3. Transport of nutrients, products and waste
products
4. Localisation of function within organelles
5. Semi-permeable: controls entry and exit of
substances
6. Self-sealing!
7. Flexible, mobile fluid mosaic
Cell membrane function
Let’s meet the components of the cell
membrane
• Phospholipid bilayer
• Cholesterol
• Carbohydrates
(glycoproteins)
• Proteins
Phospholipid bilayer
polar
hydrophilic
heads
nonpolar
hydrophobic
tails
polar
hydrophilic
heads
Membrane Proteins
• Proteins determine a
membrane’s specific
functions
• Cell membrane &
organelle membranes
each have unique
collections of proteins
• Channels, pumps,
receptors
Examples
H+
Retinal
chromophore
NH2
water channel
in bacteria
Porin monomer
b-pleated sheets
Bacterial
outer
membrane
Nonpolar
(hydrophobic)
a-helices in the
cell membrane
COOH
H+
Cytoplasm
proton pump channel
in photosynthetic bacteria
function through conformational
change (shape change)
Membrane glycoproteins
(carbohydrates)
‘Chemical identification cards’
Play a key role in cell-cell
recognition
• ability of a cell to distinguish
one cell from another
– Antigens
• basis for rejection of
foreign cells by
immune system
Let’s Review…
Let's build a membrane from scratch…
And now…let’s make a membrane!
Movement across the
Cell Membrane
How do things get into and out of our
cells?
PASSIVE MECHANISMS
These don’t require energy
Simple diffusion
Facilitated diffusion
Osmosis
ACTIVE MECHANISMS
These require energy
(usually ATP) to transport
substances (often against
their concentration
gradient)
Protein pumps
Endocytosis
(cotransport)
Simple diffusion…in the real world
• Gas exchange in the lungs
• Gas exchange in plants
Simple Diffusion
2nd Law of Thermodynamics governs biological
systems: the universe tends towards disorder (entropy)
Diffusion: movement of small, soluble particles from high
 low concentration
Diffusion
Movement is from HIGH to LOW concentration
• “passive transport”
• no energy needed
diffusion
movement of water
osmosis
Factors affecting diffusion
1. Temperature
2. Surface area for diffusion
3. Distance for diffusion
4. (size of particle)
5. (charged or uncharged)
Diffusion in the real world
Diffusion animation 2
• Gas exchange at the alveoli — oxygen from air to
blood, carbon dioxide from blood to air.
• Gas exchange for photosynthesis — carbon dioxide
from air to leaf, oxygen from leaf to air.
• Gas exchange for respiration — oxygen from blood to
tissue cells, carbon dioxide in opposite direction.
• Transfer of neurotransmitter substance at a nerve
synapse.
• Osmosis — diffusion of water through a
semipermeable membrane.
Facilitated Diffusion
Facilitated diffusion is diffusion of specific molecules
through protein channels in the cell membrane
– no energy is required
facilitated = with help
open channel = fast transport
high
low
Channels for facilitated diffusion
Membrane becomes semi-permeable with protein
channels : specific channels allow specific material
across cell membrane
inside cell
NH3
H2O
salt
aa
sugar
outside cell
Which substances are transported by
facilitated diffusion?
•
•
•
•
Glucose
Urea
Amino acids
Animation
Factors affecting the rate of facilitated
diffusion are the same as for simple
diffusion
1. Temperature
2. Surface area for diffusion
3. Distance for diffusion
Osmosis
A special case:
facilitated diffusion
of water molecules
across a semipermeable
membrane
Osmosis is facilitated diffusion of
water
Facilitated diffusion of
water from
high concentration of
water to low
concentration of water
across a semi-permeable
membrane
Examples of Osmosis in Biology
• Absorption of water by plant roots.
• Re-absorption of water by the proximal and
distal convoluted tubules of the nephron.
• Re-absorption of tissue fluid into the venule
ends of the blood capillaries.
• Absorption of water by the alimentary canal
— stomach, small intestine and the colon.
Water passes through special water
pores - Aquaporins
Aquaporins
• Structure, function and dynamics of
aquaporins
Peter Agre
Roderick MacKinnon
John Hopkins
Rockefeller
Water moves across
a membrane from
the hypotonic
solution to the
hypertonic solution
• Animation of osmosis 1
• Animation of osmosis 2:
why water balance
matters
Comparing ‘water concentration’ of
different solutions
Direction of osmosis is determined by comparing
total solute concentrations on either side of the
membrane:
• Hypertonic - more solute, less water
• Hypotonic - less solute, more water
• Isotonic - equal solute, equal water
water
hypotonic
hypertonic
net movement of water
freshwater
balanced
saltwater
Water potential?
• It can be confusing to talk about the concentration of
water molecules, since ‘concentration’ is usually
reserved for a solute (e.g. glucose,
• Instead we use the term water potential
• High water potential = lots of water (high water
concentration/ low solute concentration
• LOW water potential = little water (low water
concentration/high solute concentration)
Water moves from a region of high water
potential to a region of low water potential
High water potential
(relatively few water molecules
compared to solute molecules)
Low water potential
(relatively many water molecules
compared to solute molecules)
How do things get into and out of our
cells?
PASSIVE MECHANISMS
These don’t require energy
Simple diffusion
Facilitated diffusion
Osmosis
ACTIVE MECHANISMS
These require energy
(usually ATP) to transport
substances (often against
their concentration
gradient)
Protein pumps
Endocytosis
(cotransport)
Active transport 1: Protein pumps
• Active transport uses
• Animation
energy (ATP) to
• protein pumps in plants
transport substances
AGAINST a
concentration gradient low
into/out of the cell
• The energy is used to
change the shape of the
ATP
‘protein pump’ and thus
import/export specific
molecule
high
Active transport in the real world
• Re-absorption of glucose,
amino acids and salts in the
kidney.
• Sodium/potassium pump in
cell membranes (especially
nerve cells)
• Uptake of nutrients (e.g.
nitrate ions)in plant root
hair cells
Active transport 1:
Endocytosis/Exocytosis
• A simple one...
• McGRaw Hill
phagocytosis
Endocytosis/
exocytosis are
import/export of
materials by
infolding/outfolding of
the cell membrane
Transport summary
simple
diffusion
facilitated
diffusion
active
transport
ATP