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Cell Transport
Moving materials in and out of the cell.
Cell Membrane
• “Fluid Mosaic Model” (fluid with a mosaic of proteins)
• Phospholipid Bilayer- phosphate head, lipid tail
• Amphipathic – a molecule that’s hydrophobic and hydrophilic.
Selective Permeability
• Semi-permeable – allows only some materials
through**
• Small nonpolar molecules, such as hydrocarbons,
can dissolve in the lipid bilayer
• Large Polar molecules, such as sugars, do not
cross the membrane easily
Freeze Fracture
• Freeze-fracture is a specialized preparation technique
that splits a membrane along the middle of the
phospholipid bilayer
Maintaining Fluidity
• The phospholipids constantly trade places from side
to side. Every once in a while they will trade with a
phospholip from the other layer.
• High temperatures cause more movement, lower
temperatures cause less.
Observing Phospholipid Movement
• When placed together, the phopholipids of a mouse
and human cell ended up mixed due to all the
movement.
Cholesterol
• Cholesterol – a steroid hormone found in
membranes to help maintain fluidity.
• It restricts excessive movement at high temps. It
prevents sticking together at low temps.
Membrane Proteins
• Integral Proteins – go through the whole membrane
• Peripheral Proteins – sit on the outside of the
membrane (inside of the cell).
Protein Roles
• Cell membrane proteins do six main things
1. Transport materials in and out, 2. Act as Enzymes,
3. Pass signals.
Protein Roles
• 4. Identify other cells, 5. connect to other cells,
6. attach the cytoskeleton to the ECM fibers
Carbohydrate Roles
• Carbohydrates can bond to the lipids (glycolipids) or
more often proteins (glycoproteins) in the membrane
to aid in cell recognition, by protruding from the
membrane.
Passive Transport
• Passive Transport- requires no energy
Diffusion
• Simple Diffusion- molecules move from an area of
higher concentration to lower concentration (down
their concentration gradient)
• occurs until equilibrium is reached (molecules are
evenly spread out) Ex. cologne
Diffusion across a membrane
Osmosis
• Osmosis- diffusion of water molecules thru a
selectively permeable membrane (high to low water
concentration)
• the force exerted by osmosis is called osmotic
pressure
Osmosis continued
Cell in Solution
• Hypertonic solution – more dissolved material in
solution than in the cell (water flows out of the cell)
**
hypertonic
Cell in Solution
• Hypotonic solution -more dissolved material in the
cell than in the solution, causing (water flows into
the cell) (increases osmotic pressure, cells may
burst).
hypotonic
Isotonic
• Isotonic- same amount of dissolved material
in the solution and cell. (equilibrium is
reached)
Isotonic
Osmoregulation (control of water balance)
Why don’t cells burst?
• cells in organisms don’t
usually come into contact
with pure water
• plant cells have cell walls
that keep the cell from
expanding
• some cells use pumps
Facilitated Diffusion
• Facilitated diffusion - diffusion with the assistance
of integral transport proteins. (Still Passive
transport)
Channel Proteins
• Channel Proteins – allow certain materials to flow in and
out (down their concentration gradient.)
• Aquaporin – channel protein that only lets water in.
• Ion Channels – also called gated channels, open and close in
response to a chemical or electrical stimulus
Carrier Proteins
• Carrier Proteins – change shape to escort certain
material in and out of the cell. (down their
concentration gradients.)
Active Transport
• Active transport - requires energy
• Proteins act as pumps to move material from low to high
concentration. (often called electrogenic pumps)
Ex. Proton Pumps (Hydrogen Pumps)
Pumps continued
• Sodium-Potassium pump – pumps sodium out and
potassium in. (low to high concentration)
Electrochemical Gradient
• Pumps create a build up of ions on either side of the
membrane, which creates an electric charge.
• The charge causes an electrochemical gradient,
which moves the ions across the membrane like an
electric current to do work. (ex. Nerve signals)
Cotransport
• Cotransport – electrochemical gradient used to move
other molecules across the membrane
Moving large objects
• Endocytosis – a vesicle formed to take
materials into the cell**
• Exocytosis – a vesicle putting materials out
of the cell
3 Types of Endocytosis
• 1. Phagocytosis – engulfing solid particles
• 2. Pinocytosis – engulfing liquid substances
(extracellular fluid)
3 Types of Endocytosis
• Receptor mediated endocytosis – particles called
ligands bind to receptors, which signals the
engulfing of the particles.
You should now be able to:
1. Define the following terms: amphipathic
molecules, aquaporins, diffusion
2. Explain how membrane fluidity is influenced
by temperature and membrane composition
3. Distinguish between the following pairs or sets
of terms: peripheral and integral membrane
proteins; channel and carrier proteins; osmosis,
facilitated diffusion, and active transport;
hypertonic, hypotonic, and isotonic solutions
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings
4. Explain how transport proteins facilitate
diffusion
5. Explain how an electrogenic pump creates
voltage across a membrane, and name two
electrogenic pumps
6. Explain how large molecules are transported
across a cell membrane
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings