How things get into cells:
Principles of diffusion, osmosis, and the
nature of biological membranes.
 Diffusion
 Movement
of substances from an area of high
concentration to an area of low concentration
 Depends on 2nd law of thermodynamics
Osmosis: special case of diffusion of water
 Biological membranes

 Lipids
=barriers; proteins = channels.
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Movement of molecules depends on:
1. Kinetic energy
higher temperature = more kinetic energy
2. Concentration
gradient – more of something in
one area than another
Second Law of Thermodynamics: all things
tend toward entropy.
If there’s more of something in one area, it
will spread out.
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Diffusion

Passive process
 Depends on concentration and kinetic energy
 Does not require energy
 Moves substances from an area of high
concentration to an area of low concentration
 Down a concentration gradient
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Osmosis: a special instance of diffusion
The most concentrated form of water is
pure water.
To make water less concentrated, we
dissolve substances in it.
 Concentration of one solution relative to
another
 Isotonic – equal concentrations
 Hypertonic – more concentrated
 Hypotonic – less concentrated
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Tonicity – relative concentrations of solutions




Isotonic – two solutions contain the same
amount of substance dissolved in themequal concentrations
Hypertonic – a solution containing a greater
amount of dissolved substance- more
concentrated
Hypotonic – a solution containing a lesser
amount of dissolved substance – less
concentrated
Osmosis: The movement of water across
a selectively permeable membrane, down
a concentration gradient.
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Osmosis: special case of
the diffusion of water.
 Movement of water across
a semi permeable
membrane.
 If the environment is:
Isotonic: No NET flow.
Hypertonic: Water flows OUT
of cell.
Hypotonic: Water flows IN.

Water flows from where it (the water) is in high
concentration to where it is in low concentration.
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Osmosis is an important phenomenon in biology

Red blood cells must be collected carefully
 In
a hypotonic solution (distilled water), water
enters the RBCs and explodes them.

The action of penicillin depends on it
 Bacteria
are protected by a cell wall. When
drugs damage the wall, water rushes in and
explodes the bacterium, killing it.

Filtering action of the kidney depends on it
 Water
is drawn back out of urine.
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The physical law that molecules travel
down a concentration gradient (from a
region of high to low concentration)
drives most movement of molecules in
and out of cells.

Other movement of molecules and
particles e.g., movement “against” a
concentration gradient, requires some
form of metabolic energy.
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Entry of particles and molecules attached to
the cell membrane
Requires:
•Direct access of cell
membrane to outside
•Cytoskeleton
•Source of energy
Thus NOT done by cells with cell walls (plants, fungi,
bacteria) or cells without a cytoskeleton (bacteria).
http://bio.winona.msus.edu/bates/genbio/images/endocytosis.gif
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The type of molecule affects
how it gets through a membrane

Small molecules can pass through a membrane
 Water;

Gases such as O2 and CO2
Lipid molecules can
 Dissolve
in lipid bilayer, pass through membrane
 Many drugs, vitamins, hormones are lipid soluble

Larger, hydrophilic molecules cannot
 Ions,
sugars, amino acids cannot
 Transport proteins required
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Transport through membranes

Simple diffusion
 Molecules
travel down concentration gradient
 Membrane is not a barrier to their passage

Facilitated diffusion
 Molecules
travel down concentration gradient
 Cannot pass through lipid bilayer; their passage is
facilitated by protein transporters

Active transport
 Molecules
travel against concentration gradient
 Requires input of metabolic energy (ATP),
transporter
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How molecules get through the
membrane
http://www.rpi.edu/dept/chem-eng/Biotech-Environ/Membranes/bauerp/diff.gif
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Active transport: movement of
molecules against a concentration gradient.
Requires Energy.
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Transport into cells:
a step in the middle of a process

All cells need raw materials
 For

maintaining cells, building new ones
All cells need a source of energy
 For
continuing cell functions like movement
 For building new molecules, new cells

Chemical reactions that use or release
energy: Metabolism
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Food
preparation,
consumption,
digestion
(break down
into smaller
molecules.)
Small molecules
absorbed or transported into cells.
http://www.nlm.nih.gov/medlineplus/ency/images/ency/fullsize/8710.jpg
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Chemical reactions carried out by enzymes
Once inside the cell, molecules are used
as raw materials or for their energy.
 The chemical reactions involved are
carried out by protein molecules called
enzymes.
 Enzymes are the tools of the cell and have
several important properties.

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Enzymes are catalysts

Catalysts speed up the rate of a chemical
reaction
 2H2
+ O2 2H2O
 Reactions must occur quickly in a cell.

A catalyst is not used up
 Enzymes
are tools, so think “hammer”
 Used over and over to pound nails
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Enzymes overcome “activation energy”
Enzymes give an extra
push to reactions that don’t
require energy to finish.
Enzymes are facilitators:
they get all the reactants
together on the enzyme’s
surface so they can react.
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Enzymes are specific

There are thousands of different reactions
that take place in a cell, most of them
going on at the same time.
 To
speed up each one, there is a different
enzyme. Each type of enzyme can speed up
only 1 type of chemical reaction.
 Enzymes are proteins, and their 3D shape is
what makes them specific. Think “wrenches”
 Instructions for making enzymes thus found in
the DNA
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The next lecture(s) will discuss how cells take
molecules, break them down and release the
energy in them, and put them back together in
the unique way required by the cells.
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