Transport across
membranes
By the end of this lesson you should be
able to describe how the components of
a membrane allow both passive and
active transport of molecules into and
out of cells
Metabolism and cell survival
• Reactions in living cells require molecules to be
transported into the cells.
• Some reactions make useful products that need to be
transported out of the cells
• Many reactions produce waste products that need to be
removed from the cells.
• All molecules that need to pass into or out of a cell will
have to pass across a membrane.
• How do you think this happens?
Types of transport
• Simple Diffusion
• Facilitated diffusion through “channel”
proteins
• Facilitated diffusion through “carrier” proteins
• Facilitated diffusion through “gated” proteins
• Active transport which requires energy in the
form of ATP
Diffusion
• The NET movement of molecules from an
area of high concentration to an area of
low concentration
• This means that molecules are moving
randomly in all directions but the final
outcome appears to be in one direction.
Factors that affect Rate of Diffusion
across a membrane
•
•
•
•
Temperature
Concentration gradient
Surface area of the membrane
Distance/thickness of the
membrane
• Size of molecules
Diffusion in cells
Passive (simple) Diffusion
http://www.youtube.com/watch?v=s0p1ztrbX
PY&feature=related
Simple diffusion through the
phospholipid bilayer
• Lipid based molecules can slip through the hydrophobic core and
passively move down the concentration gradient
• Very small or non-polar molecules E.g. Oxygen, carbon dioxide and
water can easily pass through the bilayer
• Larger or charged molecules cannot pass between the
phospholipids, so they require help through facilitated diffusion
• Channel proteins: gated or ungated ‘pores,
Ungated pores are always open, Gated pores need another
substance to bind to them to open the gate.
• Carrier proteins: These are “molecule-specific” carriers that change
shape when activated to allow passage through the membrane
Which
is
which?
Gated and non gated
http://www.stolaf.edu/people/giannini
/flashanimat/transport/channel.swf
Copy blue box bottom of page 23
Active Transport
http://www.stolaf.edu/people/giannini/f
lashanimat/transport/channel.swf
When concentration
gradients can’t help!
Find 2 examples
•‘Pump’ like carrier
protein
•Complementary shape
to molecule
•Require ATP
•Go against
concentration gradient
•Can transport at a
faster rate than
diffusion
•Molecules can be
accumulated on either
side of a membrane
See how Active Transport
happens!
• YouTube - Active Transport
Endo- and
Exocytosis
Used for bulk
transport
Complete notes by
reading through
p25
See how Endocytosis and
Exocytosis happen
YouTube - Cell Membrane, Exocitosis &
Endocitosis
Cell signalling
• Signalling and receiving signals is vital for cell survival and
communication
• When looking at the bilayer, chains ‘sticking out’ from the
surface are visible (called?)
• These allow for ‘self’ recognition of cells by the immune
system, to prevent auto immune responses
• The surface also has membrane bound receptors
associated with these projections that are modified protein
molecules.
• They receive ‘messages’ about cell activities via chemical
messengers (called?)
• Like jigsaw pieces a hormone and receptor will only bind
with their complementary partner causing the target cell to
respond. (similar idea to enzyme active site and substrate)
For cover work:
•
•
•
•
Research in as much detail as you can the
communication and cell signalling involved with:
Insulin
AIDS
BOTOX and
Beta blockers
Please use p20-21 to help guide you and help you
begin your research