Download Crossing Membranes – Passive Processes

Crossing Membranes –
Passive Processes
Objectives:
• Describe and explain what is meant by
passive transport (diffusion and
facilitated diffusion)
• Describe the roles of membrane proteins
in passive transport.
What can you remember from GCSE?
What is diffusion?
• Define diffusion
• Describe the distribution of molecules
BEFORE & AFTER
• What factors affect the rate of
diffusion?
• What is meant by ‘net’ movement?
• What is meant by equilibrium?
• Why do organisms need diffusion?
• Is it passive/active?-What is the
difference?
• Give a biological example of diffusion
• What states does it occur in?
Diffusion
• Molecules move randomly and bump into
other molecules.
• So particles have a tendency to spread
out evenly to fill the space available.
• The particles are moving from an initial
area of high concentration to areas of
lower concentration, so they are moving
down a concentration gradient.
HIGH CONCENTRATION
CONCENTRATION
GRADIENT
LOW
CONCENTRATION
•Diffusion is PASSIVE i.e. does not
require energy
Diffusion Across Membranes
• Some small non-polar molecules are able
to diffuse straight through the lipid
bilayer. E.g. O2, CO2 and steroid
hormones
• Other very small charged particles
like water and small ions can also
diffuse directly through the lipid
bilayer.
• Larger or charged particles like glucose
or amino acids are unable to pass
through the lipid bilayer, and so must
pass through membrane proteins.
• integral membrane proteins allow the
cell to be selective about what passes
through the membrane.
• This is FACILITATED DIFFUSION.
• It is still passive as the molecules are
down a concentration gradient and it
does not require energy.
Facilitated Diffusion
Channel Proteins
• Pores in the membrane with a specific shape
that only allow a certain shaped ion through.
They can be gated, so they can open or close.
• Pores have a polar interior allowing polar
molecules to pass through.
Channel Proteins
Channel proteins include:
-ion channels which allow the passage of
ions (charged atoms or molecules) which
are associated with water
-gated channels which are opened or
closed in response to a stimulus e.g.
chemical or electrical
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Facilitated Diffusion
Carrier Proteins
Have a Specific 3D shape so that only a certain
molecule can bind causing a change in shape of
the protein that delivers the molecule to the
other side of the membrane.
It may saturate when all the carrier proteins are in
use
Factors affecting the rate of
diffusion
Temperature - ……
Conc. Gradient - ……
Stirring/moving - ………
Surface area - .......
Distance/thickness - ………
Size of molecule - …….
Control
• Different membrane have different carrier and
channel proteins in them, allowing a measure of
control over what enters and leaves the cell.
• NET diffusion will stop when there are an equal
number of one type of particle on one side of
the membrane as on the other. NB. This does
NOT mean diffusion STOPS, but that molecules
are entering at the same rate as they are
leaving.
• You could:
• Try the questions on Page 23
• Try the diffusion quiz at
http://www.austincc.edu/emeyerth/diffquiz1.
htm
Task
• Divide your page into 4.
• Draw diagrams of:
o Diffusion-lipid soluble substances
o Diffusion-small molecules & ions
o Facilitated diffusion by channel proteins
o Facilitated diffusion by carrier proteins
• Annotate each diagram
Active vs. Passive Transport
Active
Passive
• Requires energy in the
form of ATP
• Active transport
• Bulk Transport: endocytosis and
exocytosis
• No energy required
from ATP
• Diffusion
• Facilitated diffusion
• Osmosis