Download Diffusion, Osmosis, And Some Others…

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.
Cell membrane 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
Cell Membrane Structure
• 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 permeable
membrane that regulates what enters and leaves
the cell
• Lastly, Carbohydrates also are found within
the membrane
– They attach to membrane proteins and serve as
identification tags
– This allows cells to
cell membrane
distinguish one type of
cell from another
Cholesterol
Cytoskeleton
Proteins
Carbohydrate
chains
Description of the Cell Membrane
•Scientists have developed
a model that describes the
arrangement/movement 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)
• Diffusion, Osmosis, Facilitated Diffusion
– 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 through the
membrane, this is called
Facilitated Diffusion.
• No energy required
• Moves from high to low
concentration
• Ex: ions, sugars, salts
Facilitated Diffusion
Facilitated Diffusion