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Cellular Transport
Why must a cell control
materials moving into and
out of itself?
• The survival of a cell depends on its
ability to maintain proper conditions
within itself
• A cell has to be able to maintain
balance regardless of internal and
external conditions
• The process of maintaining the cells
environment is called homeostasis
Cell Membrane Structure
• What is it composed
of?
– 2 layers of
phospholipids
• What 3
macromolecules are
found in the cell
membrane?
– Lipids, carbohydrates
& proteins
• Cholesterol (not a
macromolecule)
• The phospholipid is composed
of 3 main parts:
– A charged phosphate group
– Glycerol
– 2 fatty acid chains
• Because the head bears a
charge it is polar
– This means the head forms
H bonds with water
molecules
• (likes water –hydrophyllic)
• fatty acid tails are nonpolar
– as a result they are
attracted to each other
and repel water
• (hates water –
hydrophobic)
* So if a bunch of phospholipids
were dropped in a container
of water they would always
form a cell membrane like
structure.
• The cell membrane has 2 major functions:
– Regulate what enters/leaves a cell
– Provide support
• Cell membrane is selectively permeable
(allows some things to cross, others cannot)
– Small substances, like water, carbon dioxide
and oxygen can pass through without help
• Why is cholesterol necessary in the cell
membrane?
– It helps stabilize the phospholipids
– Prevents the fatty acid chains of the
phospholipids from sticking together
– You want the plasma membrane to be flexible!!
Cholesterol
molecule
Cell Membrane Structure
• Proteins are also found within the
membrane
– Act as channels for bigger objects to pass
through
– They help create that selectively
Carbohydrate
permeable membrane that regulates what chains
enters and leaves the cell
cell membrane
Cholesterol
Cytoskeleton
Proteins
• Lastly, Carbohydrates also are
found within the membrane
– They attach to membrane proteins
and serve as identification tags
– This allows cells to
distinguish one type of
cell from another
cell membrane
Carbohydrate
chains
Description of the Cell Membrane
•Scientists have
developed a
model that
describes the
arrangement/mo
vement of the
cell membrane.
• The fluid mosaic model.
– It describes the membrane as
a flexible boundary of a cell
(fluid)
– Also, the variety of molecules
making up the membrane is
similar to the arrangement of
mosaic tiles.
Movement of materials
across the
membrane
• There are two types of
transport (movement) of
molecules across a cell
membrane:
– Passive Transport (no energy
from the cell required to move
molecules)
– Active Transport (energy from
the cell is required to move
molecules)
– First let’s learn why molecules
don’t stay in one place……..
Concentration Gradient
• Molecules have
natural motion
(kinetic energy),
which causes them to
collide and scatter.
• Molecules can be
concentrated in high
numbers or low
numbers in certain
areas
Concentration Gradient (cont..)
• Concentration
gradient is the name
for this difference
in the concentration
of a substance from
one location to
another.
• Molecules will always
diffuse down their
concentration
gradient – that is,
from a region of
high concentration
to low concentration.
Example:
•Dye molecules are initially at a
high concentration where they
are added to water.
•Random movements of the dye
and water molecules cause
them to bump into each other
and mix
–Thus the molecules are
moving to an area of High to
low concentration.
Eventually, they are evenly spread throughout the
solution.
This is referred to as dynamic equilibrium.
Equilibrium – molecules same throughout solution
Dynamic – molecules still moving
Diffusion
• Movement of molecules
OTHER THAN WATER
across a the cell membrane
• NO ENERGY REQUIRED
• Moves from areas of HIGH
concentration to areas of
LOW concentration
– Until dynamic
equilibrium is reached
• ***Equilibrium does
NOT mean that
movement across the
membrane STOPS
H
Dynamic
Equilibrium
H
L
= Dissolved
Molecule
L
Osmosis
• Movement of WATER across a selectively permeable
membrane (basically diffusion of water)
• NO ENERGY REQUIRED
• Water moves from HIGH concentration to LOW
concentration until equilibrium is reached
Higher Concentration
of Water
Water molecules
Cell
membrane
Lower Concentration
of Water
Sugar molecules
3 types of osmosis solutions
• Isotonic
• Hypertonic
• Hypotonic
ISOTONIC
•Isotonic = “equal strength” solution
–Same concentration of dissolved particles on both
inside and outside of cell
– Water molecules move into and out of the cell at an
equal rate, so size remains constant.
HYPERTONIC
• Hypertonic = “above strength” solution
– Higher concentration of dissolved particles in solution
– So water molecules are more concentrated inside the cell
– Thus water flows out of the cell, causing it to shrivel or
even die.
HYPOTONIC
• Hypotonic = “below strength” solution
• Lower concentration of dissolved
particles in solution
So water molecules are more
concentrated outside cell, causing water
to rush into the cell.
– Cell membrane could potentially expand
until it bursts.
Hyp- O - tonic
Recap
Osmosis
• Plant cells and bacteria have cell walls, so
they are rigid and CAN’T burst for the most
part unless the cell wall is not very strong
• Some cells can pump out water as it comes in
Facilitated Diffusion
• Many molecules are too
large to cross via diffusion
• They have a SPECIFIC
protein channel that lets
them pass thru the
membrane, this is called
Facilitated Diffusion.
• No energy required
• Moves from high to low
concentration
• Ex: ions, sugars, salts
Facilitated Diffusion
Facilitated Diffusion