Objectives
• To understand the role that cellular membranes,
concentration gradients and membrane proteins have on
cells exchanging molecules in their environment
Cell Membrane• Protects the cell.
• Allows certain molecules to enter the cell and not others.
• Contains the cytoplasm and organelles.
• Is constituted by a bilayer of phospholipids, with proteins trapped in the membrane.
Membranes are formed of phospholipids.
• Phospholipids are amphipathic molecules: they have in the same molecule hydrophilic and hydrophobic regions
• Phospholipids are the most abundant lipid in membranes.
• Membrane proteins– in animals, proteins
constitute about 50% of the mass of the plasma
membrane
Functions of Membrane Proteins
Fig 11-20 Essential Cell Biology
Principles of a selectively-permeable membrane
Small
Hydrophobic
Molecules
O2
CO2
N2
Small
uncharged
polar
molecules
H2O
ethanol
glycerol
Larger
uncharged
polar
molecules
Glucose
Nucleotides
Amino acids
Ions
H+, Na+
Mg2+, K+
Ca2+, Cl-Cell membrane
Membrane permeability depends on:
• Molecule’s lipid solubility
• Molecule’s size
• Lipid composition of the membrane
Principles of a selectively-permeable membrane
• Passive transport:
- No energy is required
- Movement down a concentration gradient
• Active transport:
- Energy is required
- Movement against a concentration gradient
15
One way to categorize how molecules move across the
membrane is by their energy requirements
Active and Passive Transport
Passive Transport- Simple Diffusion
• Diffusion- movement from
an area of high molecule
concentration to low
concentration
• Continues until the
concentration is the same in
all regions- What is this
called?
• Equilibrium
Passive Transport- Facilitated Diffusion
• Facilitated diffusion
– Molecules that cannot cross membrane easily use membrane transport proteins to move down their concentration gradient
– NO ENERGY REQUIRED
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Passive transport: Use a protein as a carrier, or channel, from high concentration to low concentration.
One Direction
Channel protein vs. Carrier protein
Channel protein-
act like doorways in
that are either open or
closed-
Transportation is fast
Carrier protein-
like revolving doors in
that allow movement in
and out of the cell without
creating an open hole
Transportation is slower
Chanel protein- Ion channels
• Ion channels allow the passage of ions such as Na+, K+, Ca2+, and Cl- to pass membrane depending upon chemical gradient
• Channel proteins are either Open channels or Gated channels
21
Channel proteins- Ion Gated Channels
• What controls the opening and closing of ion gated
channels?
1. Mechanically gated channels- controlled by a
physical force such as increased temperature or
pressure on the membrane (e.g. auditory hair cells)
2. Chemically gated channels- controlled by biding to
chemical messenger (neurotransmitter)
3. Voltage-gated channels- open and close when the
electrical state of the cell changes
Carrier proteins
• How it works
• Why do you think diabetics lose
glucose in their urine?
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• Molecules that otherwise can’t cross
the membrane use carrier proteins to
help transport both ions and other
solutes across the membrane
• Never form a direct connection
between the intracellular and
extracellular fluid
• Concentration gradient
required, but energy is not
Properties
-Selective for the molecule they
transport such as the GLUT transporter
can only transport glucose across the
membrane
-They can be saturated (i.e. rate of
transport limited by number of
transporters)
Competition occurs between transport
proteins binding sites and other related
substrate that the protein can anneal to
Osmosis: Waterdiffusion
• Osmosis- net diffusion of water across a membrane toward a higher solute concentration
• Definitions
– Water is a solvent
– Substances dissolved in a solvent (e.g. water) are solutes
25
How it works
26
Osmotic concentration
• When 2 solutions have different osmotic concentrations
– The hypertonic solution has a higher solute concentration ( the prefix “hyper” mean above or greater)
– The hypotonic solution has a lower solute concentration (the prefix “hypo” means below or less
• When two solutions have the same osmotic concentration
– the solutions are isotonic (the prefix “iso” means equal)
Osmotic pressure• Membrane strength determines
the effect on the cell
Human red blood cells
Plant cells
Maintaining osmotic balance• Some cells use extrusion
– water is ejected through contractile vacuoles
• Isosmotic regulation involves keeping cells isotonic with their environment– Water-dwelling organisms have developed ways to deal
with salt (marine) or water (fresh water) overload
– Terrestrial animals circulate isotonic fluid throughout their bodies
• Plant cells use turgor pressure to push the cell membrane against the cell wall and keep the cell rigid
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Active Transport
• Requires energy in the form of ATP
• Moves substances from low to high concentration
• Requires highly selective carrier proteins– Uniporters – move one molecule at a time
– Antiporters – move two molecules in opposite directions
– Symporters – move two molecules in the same direction
Na+/K+ ATPase (Na+-pump)
• Uses ATP to drive Na and K transport against their concentration gradients (i.e. uphill)
• Uses an antiporter to move 3 Na+ out of the cell and 2 K+ into the cell
• ATP energy is used to change the conformation of the carrier protein
• Affinity of the carrier protein for either Na+ or K+ changes so the ions can be carried across the membrane
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Coupled transport
• Uses ATP indirectly (secondary active transport)
• Kinetic energy released when a molecule moves by diffusion supplies energy to transport a different molecule
• Symporter is used
• Glucose–Na+ symporter – energy from Na+ diffusion
drives glucose uptake into the cell
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Na+
K+
6. Dephosphorylation of protein triggers
change back to original conformation,
with low affinity for K+. K+ diffuses into
the cell, and the cycle repeats.
1. Carrier in membrane binds
intracellular sodium.
2. ATP phosphorylates protein with
bound sodium.
5. Binding of potassium causes
dephosphorylation of protein.
4. This conformation has higher affinity
for K+. Extracellular potassium binds
to exposed sites.
3. Phosphorylation causes
conformational change in protein,
reducing its affinity for Na+. The Na+
then diffuses out.
P
P
P
P
P
ATP
+ADP
Extracellular
Intracellular
Bulk or Vesicular Transport• Endocytosis: Movement of substances into the cell
– Phagocytosis – (phagein-to eat) cell takes in particulate matter
– Pinocytosis – (pino-to drink) cell takes in only fluid
35
• Exocytosis: Movement of substances out of cell
•
– Requires energy– Used in plants to export cell wall material– Used in animals to secrete hormones, neurotransmitters, digestive enzymes
37
Bulk or Vesicular Transport
• Endocytosis: Movement of substances into the cell
– Receptor mediated endocytosis
38
Bulk or Vesicular Transport
What we’ve learned
• understand the role that cellular membranes,
concentration gradients and membrane proteins have on
cells exchanging molecules in their environment
• Principles of selectively permeable membrane
• Passive vs. Active Diffusion
• Channel vs. Carrier proteins
• Osmosis
• Na+/ K+ pump
• Coupled transport
• Bulk transport