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Cell Transport
Movement through the Membrane
Crossing the Cell Membrane
• To stay alive, a cell must exchange materials such as
food and wastes with its environment.
• These materials must cross the cell membrane.
• The plasma membrane must regulate the movement
of dissolved molecules on one side of the membrane
to the liquid on the other side.
• Small molecules go in and out freely while large
molecules such as proteins and carbohydrates
cannot.
• Semi-permeable membrane. Only allows certain
molecules to pass in and out of cell.
Substances That Are Let IN & OUT
IN
• Water
• Glucose (a type of sugar)
• Lipids (fats)
• Amino Acids (the building blocks of protein)
OUT
• Water
• Byproducts (waste) of glucose, lipids, and amino
acids
• Other waste products
Fluid Mosaic Model
• The composition of all cell membranes is typically a
double-layered sheet called a lipid bilayer.
• A phospholipid molecule has two parts:
– 1) Phosphate group (PO4)
– 2) Glycerol (the lipid)
Carbohydrate Chain
Outside of Cell
Protein Channel
Inside of Cell (Cytoplasm)
Lipid Bilayer
Phospholipid Molecule (Cont’d)
• The bilayer of phospholipid molecules are arranged:
– With the polar phosphate groups touching the external
environment and inside of the cell (the cytoplasm)
– With the two fatty acid tails facing each other
• The plasma membrane is not just a bilayer of pure
phospholipds
– Inside of the bilayer:
• Cholesterol – prevents fatty acid chains from sticking together
• Transmembrane proteins – act as transport channels to move
substances into and out of the cell
Simple Diffusion (Passive Transport)
• Passing of particles through a cell membrane
Diffusion. Particles move from higher concentration  lower
concentration. “It does not require energy”
Concentration. The mass of solute in a given volume of
solution. (12g of salt in 3 liters of water, concentration would be 4g/L)
Diffusion ends when the molecules are dispersed evenly
• Homeostasis or equilibrium is reached
Homeostasis. A process at which organisms maintain an
internal stable environment.
Explaining Concentration
• Solute
– A substance that is dissolved in solvent to make a
solution.
• Solvent
– Substance in which a solute is dissolved in to form
a solution.
• Solution
– Mixture of 2 or more substances in which
molecules of the substances are evenly
distributed.
Concentration Gradient
• Difference between the concentration of a
particular molecule in one area and the
concentration in an adjacent area
• Diffusion always occurs down a gradient
– From higher concentration to lower concentration
• At equilibrium the gradient is no longer
present
Osmosis (Passive Transport)
• The diffusion of WATER across a selectivelypermeable membrane.
• Occurs in response to the solutes dissolved in the
water.
• Water moves from where there is more water to
where there is lower water.
• Another thought- water moves from where there
are less particles (solutes) to where there are
more particles.
Ex: Contractile vacuoles. Cavities in cytoplasm
that are specialized to collect water.
Hypertonic Solution
• Concentration of solutes in the solution is higher
than the concentration of solutes inside the cell
“Above Strength”
• Water will diffuse out of the cell
• Cell will shrivel and shrink
• Cell will die
Ex. Lettuce in salt water. Lettuce wilts in a few
minutes because cells have lost their water to
the salty, hypertonic environment
Hypotonic solution
• Concentration of solutes is lower in the
solution than the concentration of solutes
inside the cell. “Below Strength”
• Water flows into the cell
• Causes the cell to swell
• Will not occur in plant cells due to cell wall
Ex. Distilled water has no solutes
Ex. Lettuce in distilled water stays crisp
Isotonic Solution
• Equal amount of solutes
• IV in hospitals is isotonic, replenishes fluids
only (osmosis does not occur)
Facilitated Diffusion (Passive Transport)
• Used when molecules can’t cross by simple
diffusion
• Some molecules must be facilitated with other
molecules.
• “Carrier Proteins” must carry molecules across
the membrane
Ex: Molecules such as glucose, that cannot diffuse
across the cell membrane’s lipid bilayer on their
own move through protein channels instead.
Active Transport
•
•
•
•
•
•
Moving against a concentration gradient
Low concentration to high concentration
Requires Energy!
Often involves carrier proteins
Important in animals (homeostasis)
Important in plants to absorb nutrients from
soil
Bulk Transport (Active Transport)
•
Large molecules, food and other substances
are packaged in membrane bound sacs and
moved across the membrane
4 Types:
1.
2.
3.
4.
Endocytosis- bringing into the cell
Pinosytosis- drinking (endocytosis)
Phagocytosis- eating (endocytosis)
Exocytosis- opposite of endocytosis, wastes
leave the cell
http://www.maxanim.com/physiology/Endocytosis%20and%20Exocytosis/ee4.swf
Additional Terms
• Transport Protein
– Assist in the movement of substances by facilitated diffusion or active
transport
• Types: channels, carriers, and pumps
• Turgor Pressure
– The pressure exerted by water inside the cell against the cell wall.
• Vesicle
– Vesicles are small organelles present in cells. Are membrane enclosed
sacs which store and transport substances to and from.
• Permeable
– Capable of being permeated or passed through
Ex. Wood is permeable to oil
• Impermeable
– Not permeable or passable.