Cell Membranes
Osmosis and Diffusion
Functions of Membranes
1. Protect cell
2. Control incoming and outgoing substances
3. Maintain ion concentrations of various
substances
4. Selectively permeable - allows some
molecules in, others are kept out
Phospholipid Bilayer
Fluid Mosaic Model
Blood-Brain Barrier
• Allows some substances into the
brain, but screens out toxins and
bacteria
• Substances allowed to cross include:
water, CO2, Glucose, O2, Amino Acids,
Alcohol, and antihistamines. HIV and
bacterial meningitis can cross the
barrier.
Solutions
• Solutions are made of solute and a solvent
• Solvent - the liquid into which the solute is poured and
dissolved. We will use water as our solvent today.
• Solute - substance that is dissolved or put into the
solvent. Salt and sucrose are solutes.
• Concentration Gradient- conentration diference across
the membrane.
• Equilibrium-
– concentration of molecules is equal throughout the space
– Molecule still move even in equalibrium
Methods of Transport Across
Membranes
1. Diffusion
2. Osmosis
3. Facilitated Diffusion
4. Active Transport
Methods of Transport Across
Membranes
1. Diffusion -passive transport - no energy
expended
2. Osmosis - Passive transport of water
across membrane
3. Facilitated Diffusion - Use of proteins to
carry polar molecules or ions across
4. Active Transport- requires energy to transport
molecules against a concentration
gradient – energy is in the form of
ATP
Diffusion
• Movement of molecules from an area of
high concentration to an area of low
concentration.
• Movement from one side of a membrane
to another, un-facilitated
Diffusion
• Solute molecules moving from an
area of high concentration to an (water)
area of low concentration
– Random motion drives diffusion
– Movement is based on kinetic
energy (speed), charge, and
mass of molecules
1
2 4
– Equilibrium is reached when
there is an even distribution of
solute molecules
3
Diffusion
Factors that affect diffusion
• Concentration gradient
• Size of molecules
• Type of molecules
• Chemical nature of the membrane
Osmosis
• WATER molecules difule across a
membrane- high to low conc. Of WATER
Osmosis
Tonicity is a relative term
• Hypotonic Solution - One solution has a
lower concentration of solute than
another.- water moves in
• Hypertonic Solution - one solution has a
higher concentration of solute than
another. - Water moves out
• Isotonic Solution - both solutions have
same concentrations of solute.- water
moves back and forth in equalibrium
Plant and Animal Cells put into
various solutions
Single Cell organisms
• Contractile Vacuoles collect excess water in
the cell and excrete it
• Animation
– http://www.thaigoodview.com/library/contest2551/science04/45/2/
cell/image/contractile_vacuole.gif
Osmosis in Red Blood Cells
Isotonic- Normal cells
equilibrium
movement of
water
Hypertonic-
higher conc inside- plasmolysis-
water moves out cells shrink
Hypotonic- higher Cytolysis-
conc outside- cells swell
water moves in and burst
Osmosis in Plant Cells
Plasmolysis- cells
are “wilted”
Hypertonic
Turgor pressure-
cells are rigid
Hypotonic
Types of Transport
Passive Transport
• When a cell uses no energy to move particles across a
membrane passive transport occurs
• Particles go DOWN their concentration gradient.
• Diffusion & osmosis are passive transport.
Plasma Concentration gradient
membrane
Passive Transport by proteins
• Passive transport of materials across the membrane using
transport (carrier) proteins specific for each moleule is
called facilitated diffusion.
• Beneficial because it can move both directions
Channel
Plasma proteins
membrane Concentration
gradient
Gated Ion Channels
Diffusion through Ion Channels
• Type of passive transport that involves membrane
proteins known as ion channels: sodium (Na+),
potassium (K+), calcium (Ca2+), chloride (Cl-)
• Ion channels provide small passageways through
which ions can diffuse. Each types of channel is
specific for one type of ion.
• Some are always open, others have gates which open
and close in response to stimuli (stretching of cell
membrane, electrical signals, chemicals in cytosol or
external environment)
Active Transport
• active transport :Movement of materials through a
membrane against a concentration gradient and
requires energy from the cell.
Cellular energy
Carrier
proteins
Plasma
membrane Concentration
gradient
Cellular
energy
• Sodium Potassium Pump animation
Animation:
http://highered.mcgraw-
hill.com/sites/0072495855/student_view0/chapter2/animation__how_the_sodium_
potassium_pump_works.html
Transport of Large Particles
• Endocytosis is a process by which a cell surrounds and takes in
material from its environment.
– The material is engulfed and enclosed by a portion of the cell’s plasma
membrane.
• Phagocytosis- “cell eating”
• Pinocytosis- “cell drinking”
– resulting vacuole with its contents moves to the inside of the cell
• Exocytosis is the expulsion or secretion of materials from a cell.
Endocytosis
Exocytose
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