Membrane transport can occur passively through diffusion, osmosis, and facilitated diffusion which do not require energy, or actively through processes like ion pumps, cotransporters, and endocytosis/exocytosis which do require energy. Passive transport moves molecules along concentration gradients while active transport moves molecules against gradients using cellular energy from ATP. Cell membranes regulate the passage of substances in and out of cells through selective permeability and various transport mechanisms.
2. Transport across the cell membrane
• Passive transport does NOT require energy
• Diffusion – small uncharged molecules
• Osmosis - water
• Facilitated diffusion - glucose
• Active transport REQUIRES ENERGY
• Ion pumps
• Cotransport
• Endocytosis/Exocytosis
4. What does the membrane do?
• allows for different conditions between inside and
outside of cell
• subdivides cell into compartments with different
internal conditions
• allows release of substances from cell via vesicle
fusion with outer membrane:
5. Membrane Permeability
• Biological membranes are physical barriers, but which allow small
uncharged molecules to pass…
• They are described as semi-permeable
Because;
• Lipid soluble molecules and small molecules pass through
• Big molecules and charged ones do NOT pass through
6. Membrane Semi-permeability
1) lipid soluble solutes go through faster
1) smaller molecules go faster
1) uncharged & weakly charged go faster
2) Channels or pores may also exist in
membrane to allow transport of
larger molecules
1
2
7. Its about concentration
The concentration of the solution, with respect to
other solutions is important
Isotonic --- when both solutions have the same
concentration of dissolved substances
Hypertonic --- a solution with a higher
concentration of dissolved substances
Hypotonic --- a solution with a lower
concentration of dissolved substances
10. Passive Transport
• Involves concentration gradients ONLY.
• NO CELL ENERGY is used—this is why it is called “passive”
11. Passive Transport
3 types
• Diffusion- simple movement from regions of high
concentration to low concentration.
• Osmosis- specifically the diffusion of water across a
semi-permeable membrane.
• Facilitated diffusion - protein transporters which assist
in diffusion.
12. Diffusion
Small Particles & Air
Particles
Solution
Small Particles diffused
evenly into the Air Particles
Diffusion is the passive movement of particles from a
high concentration of particles to a lower
concentration until they are spread out evenly
14. Diffusion
• Movement generated by random motion of particles.
• Movement always from region of high concentration to
regions of low concentration.
• As a result of diffusion molecules reach an equilibrium
where they are evenly spread out.
This is when there is no net movement of molecules
from either side.
15.
16. Solutions
• A solution is made up of two parts: the solute and the solvent.
• The solute dissolves in the solvent
• Before a gas can diffuse across a membrane it must dissolve into a
liquid.
17. Biology, Cell membranes and Transport 17
Molecules that diffuse through cell membranes
1. Oxygen – Non-polar
so diffuses very
quickly.
1. Carbon dioxide –
Polar but very small
so diffuses quickly.
2. Water – Polar but
also very small so
diffuses quickly.
18. Diffusion & Gas Exchange
• Animal cells use oxygen, so oxygen is less concentrated
inside the cell than outside. This causes oxygen to diffuse
into the cell
• Carbon dioxide is produced in an animal cell, so it is more
concentrated inside than outside – so it diffuses out of the
cell
19.
20. Diffusion for Photosynthesis
• Diffusion of gases also happens in leaves:
• For photosynthesis to happen Carbon Dioxide has to get inside the
leaves. It diffuses in through stomata
• Water vapour and oxygen diffuse out of the leaf at the same time
Water + Carbon Dioxide → Oxygen and Glucose (& a little water)
22. Rate of Diffusion
• The rate of diffusion depends on:
• The conc. gradient: the higher it is = faster the rate.
• Electrical potential: moves to opposite charge
• Permeability Coefficient: (measure of ability of a
molecule to diffuse) more coeff.-move rapidly as water
(Size,charge & State of the particles-gas/liquid/solid)
• Hydrostatic pressure gradient: more P-more rate
• Thickness: of the exchange surface: thinner = faster
• Surface area available: larger = faster
• Temperature (eg. kinetic energy): hotter = faster
23. Osmosis
• Osmosis is a special type of diffusion.
• Osmosis is the diffusion of water.
Osmosis is the movement of water
molecules from a high concentration
of water to a low concentration of
water through a partially-permeable
membrane
25. Class Activity: Correctly Label
Weak Solution
Strong Solution
Solute
Molecule
Water Molecule
Partially Permeable
Membrane
Which way will
the water
flow???
27. Osmosis Experiment
1. Fill the partially
permeable membrane
with strong sugar
solution
2. Tie it to a capillary tube
and stand it in a weak
sugar solution
3. Use your ideas about
osmosis to explain why
the liquid rises in the
tube
Glass tube
Level of
sugar
solution
Weak sugar
solution
Visking Tubing
with Strong
Sugar solution
28. Osmosis and Animal Cells
haemolysis
Hypotonic
solution
Hypertonic
solution
30. Osmosis and Cells - Notes
• Cells are surrounded by a cell membrane that
separates the contents of the cell from the outside
environment. The cell membrane has tiny holes in it.
This allows small molecules to pass through, but not
large ones. The cell membrane is partially-permeable.
• Osmosis occurs when two solutions are separated by
a partially permeable membrane
31. Facilitated Diffusion
• Transport proteins carry specific
molecules across the cell membrane
• Movement is along a concentration gradient (i.e.
From higher to lower)
• Each type of transport protein will carry only one type
of molecule.
• This is how fructose/glucose is moved.
32. Fructose
Cell membrane
Transport protein
Fructose binds to the
transport protein
The transport protein turns over and releases Fructose
onto the inside of the cell, along the concentration gradient
Concentration
gradient
35. FASCILITATED DIFFUSION-Mechanism
• Carrier protein exist in 2 conformational states:
• PONG: exposed to high conc.of solute,solute binds to sp. Site,binding
induce change,that expose the carrier to low conc.of solute-PING
state,again go to previous state,PONG,reversible process.
• Depends upon:conc. Gradient,amount of carrier protein
available,affinity of solute,rapidity of conformational change and
hormone e.g.insulineby recruiting GLUT Transporters
36. Transport Proteins
• Move solutes faster across membrane
• Highly specific to specific solutes
• Can be inhibited by drugs
• Also involved in ACTIVE transport
37. FASCILITATED DIFFUSION-FACTORS
• Depedns upon:
• Concentration gradient,
• Amount of carrier protein available
• Affinity of the solute
• Rapidity of conformational change
• Hormone can regulate e.g.Insuline by recruiting Glut transporters.
39. TRANSPORT SYSTEMS-FUNCTIONAL TYPES
• UNIPORT:
Transport one type of molecule bidirectionally in or out depending upon
conc. of solute. e.g.Ca release from ER
• Cotransport/coupled transport: transport of one molecule depends upon
transfer of another molecule simultaneously or sequentially. Two types:
Symport:
Transport two different molecules in same direction simultaneously
e.g.glucose and sodium (also called coupled transport)
ANTIPORT:
Transport two different molecule,in opposite direction simultaneously
(e.g.Cl and HCO3 in proton pump).
41. Carrier molecule
Cell membrane
Transport protein
The carrier molecule binds to the transport protein, which
opens the pore allowing it to move through the cell membrane.
The pore closes once the carrier is inside the cell.
Concentration
gradient
It is possible to stop the action of transport protein with drugs
which will block the pore.
42. Active Transport
• Cells must maintain high and low levels of some
molecules.It is not possible with passive diffusion.
• Cell Energy is used to move substances across the cell
membrane against the concentration gradient i.e. from
where there is less to where there is more.
43. Ion Channels
• Work fast: No conformational changes needed
• Not simple pores in membrane:
• Are v selective,permit passage of specific ion (Na,K,
Ca,Cl)
• gates control opening, that open transiently
• Toxins, drugs may affect channels
• saxitoxin, tetrodotoxin
• cystic fibrosis
44. Ion Channels-types
• LIGANED GATED CHANNELS:
A specific molecule bind receptor n open it, e.g.Na,K,Ca,Cl (most cells)
• VOLTAGE GATED CHANNELS:
Open n close in response to potential changes in nerve cell
membrane,generate action potential,controlled by neurotransmitter.
• MECHANICALLY GATED CHANNELS:
Respond to mech.stimuli e.g.pressure,touch e.g.baroreceptors
45. Transport proteins
• Substances are moved molecule by molecule.
• It is similar to facilitated diffusion except that cell energy (ATP) is
used in the process.
ATP = Adenosine Triphosphate
46. Salt ion
Cell membrane
Transport protein
Ion binds to the
transport protein
The transport protein turns over and releases the ion
onto the inside of the cell, against the concentration gradient
Concentration
gradient
Energy is used
48. Active Transport and Villi
• When there’s lots of nutrients in the gut they diffuse
naturally into the blood. But...
• Sometimes there are less nutrients in the gut than in
the blood, so the villi cells of our gut use active
transport to absorb these nutrients against their
concentration gradient.
50. Na-K Pump Model: Part I
• 3 Na+ bind to inner region of protein
• Na+ binding triggers phosphorylation of protein. ATP
ADP + Pi
• Phosphorylation causes conformation change and Na+
binding site faces outside
• 3 Na+ released to outside
51. Na-K Active Pump: Part II
• 2 K+ ions on outside are able to bind
• K+ binding causes de-phosphorylation and new
conformation change
• 2K+ ions exposed to inside and released
Cyclic process uses ATP energy to drive Na & K ion
transport against conc. Gradient
57. Root Hair Cells and Active Transport
• Root hair cells take in minerals using active transport.
• If the concentration of minerals is higher in the root hair than in the
soil, then the cell needs to use energy to actively transport the
minerals into the cell against the concentration gradient.
58. Moving many large molecules at once—
Endocytosis
• Endocytosis
• Transports macromolecules and large particles into the
cell.
• Part of the membrane engulfs the particle and folds
inward to “bud off.”
• The cell membrane envelopes the material
• If material is liquid the process is called pinocytosis
• If material is solid the process is called phagocytosis
59. Pseudopodia extend to engulf food
A food vacuole is formed
Pinocytosis works the same, but with no food, only liquid
How Endocytosis works
60.
61. Moving many large molecules at
once—Exocytosis
• Material is packaged inside the cell and the package
fuses with the cell membrane while the material goes
out of the cell.
62. How exocytosis works
Vacuole containing particles
is moved close
to the cell membrane
Fuses with the cell
membrane to expel
the particles
•Animation;
•YouTube - Endocytosis & Exocytosis
66. ASSIGNMENT OF TODAY
• Define passive and active transport across the membrane,with types
and example for each.
• PASSIVE: Diffusion,Osmosis,Fascilitated diffusion
• ACTIVE:Ion channels,Cotransport,Endocytosis n exocytosis
Editor's Notes
We will cover gas exchange in more detail later in the course in Unit 4.
Weak solution = Low solute & lots of water
High water concentration = weaker solution
