Download An Interactive Lecture Guide to help you understand THE

An Interactive Lecture Guide to
help you understand
THE MOVEMENT OF MATERIALS
ACROSS A MEMBRANE
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Recall the definition of homeostasis
• The process of achieving a relatively stable
internal environment
CELLS MUST CONSTANTLY
RESPOND TO NATURAL
FORCES IN THEIR
ENVIRONMENT IN ORDER
TO MAINTAIN
HOMEOSTASIS
Therefore, materials like _Water, O2, ions, nutrients, &
hormones are needed to come into a cell and wastes need
to be let out of a cell.
Recall the structure of the cell membrane:
• Two thin layers of phospholipids and proteins.
▪ The membrane is not rigid but fluid.
– consistency of “a light grade of machine oil”
▪ This enables it to regulate flow of substances
into and out of the cell.
SELECTIVE PERMEABILITY:
– The membrane regulates the exchange of
materials into and out of a cell. Polarity, size,
and electric charge of molecules determine
whether they can pass through a membrane.
• SMALL HYDROPHOBIC molecules pass right
through (CO2, O2, and N2)
• SMALL, UNCHARGED or POLAR molecules
pass through as well (H2o, glycerol, ethanol)
• LARGER, UNCHARGED or POLAR molecules
have a harder time passing through (amino
acids, glucose, nucleotides)
• IONS can’t pass through (H+, Na+, K+,Ca+2,
and Cl-), they need the help of TRANSPORT
PROTEINS.
Similar to
selective hearing!
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WHY DOES THIS MATTER?
•
•
•
•
Cells survive only if they can maintain homeostasis.
Organelles (little organs) work to maintain this state.
Homeostasis is challenged because molecules move.
In order for cells to maintain internal conditions, they
need to be able to control the movement of “stuff”
into and out of them.
The cell membrane is the
biggest STAR of this unit - Why?
• It encourages the movement of good
stuff in and out.
• It discourages the movement of bad
stuff in and out.
Diffusion is the basic process underlying the
movement of molecules into and out of cells
• Diffusion– The passive movement of
molecules from regions of
[high] to [low]*
* [ ] = scientific symbol for
concentration
Examples of diffusion
• Student activity
• If I want the concentration of males to be
high in our classroom, what can be done?
• Dye in water
• Perfume or fresh bread!
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video
• When a difference in concentration exists across a
cell membrane a CONCENTRATION GRADIENT exists.
• Concentration– % of molecules in a certain place
In diffusion the rate of movement of molecules from [high]
to [low] exceeds the rate from [low] to [high] until the
concentration gradient no longer exists.
So, rate in = rate out!
Osmosis – Diffusion of water!
• The movement of water across a selectively
permeable membrane.
• Down the concentration gradient.
• Osmosis in the kitchen video
The rate of diffusion (including osmosis)
depends on several factors
•
• The size of the concentration
gradient
– The steeper the gradient, the
faster the molecules move
• The surface area of the
membrane
– A greater surface area relative to
the enclosed volume results in a
great rate of diffusion.
• PASSIVE TRANSPORT involves diffusion without any
input of energy. It moves substances down their
concentration gradients.
• FACILITATED DIFFUSION requires the help of transport
proteins in the membrane, but still move down their
concentration gradient.
The Fluid Mosaic Model
• The Cell Membrane is made of a combination
of phospholipids, proteins, and other
molecules that move around and collectively
behave as a “fluid” rather than a solid
structure
Parts of the Cell Membrane
• Phospholipids (bi-layer)
– A double layer of lipids that
provide a way to separate the
outside environment from
the interior of the cell
Cholesterol
lipid (steroid)
molecules located
sporadically I n-between
phospholipids that act to
adjust the fluidity of the
membrane
• Transport Proteins:
– Proteins that form
a “pore” for
molecules to travel
through in either
direction through
the cell membrane
Receptor Protein
Proteins with specifically-shaped binding sites on
the outside of the membrane that function to receive
molecular messages from other cells
Ligand: A chemical that bonds with a receptor protein and
carries a specific “message” from one cell to another
• Peripheral Proteins
– Proteins on the
interior of the cell
that relay “messages”
from the receptor
protein to the cell
nucleus.
• Recognition Proteins
– Proteins with
carbohydrate “flags”
that act to identify
the type of cell, and
help cells recognize
each other.
Build a Cell Membrane Activity
• Animation
• Activity
OSMOSIS IN THE CELL MEMBRANE
• Water moves towards the hypertonic place
•Tonic = dissolved substances
•Hyper = high
•Hypo = low
Hypertonic Solution:
high concentration of
dissolved stuff, low in
water - net water loss,
cell shrinks
Hypotonic Solution
• low concentration in
dissolved stuff and high in
water, net water gain, cell
swells
Isotonic
• equal amount of dissolved material inside and
outside of cell- no net loss
Review
Why not stop at diffusion?
• Cells don’t necessarily want to be isotonic, but
unless they fight it, they’ll go that way
(entropy).
Example:
•A frog in a freshwater pond will tend to
lose salts from his/her body to their
environment by diffusion.
•Therefore, the frog has a mechanism to
take back salts from the environmentagainst the [gradient]!
•This requires chemical energy which is
supplied by ATP.
• WHEN MOLECULES NEED TO BE IMPORTED OR
EXPORTED AGAINST THEIR [GRADIENT], CELLS
EXPEND ATP TO MOVE MOLECULES FROM [LOW] to
[HIGH]
– This is called ACTIVE TRANSPORT _
Where might active transport be utilized in our body?
• SODIUM - POTASSIUM PUMP– One of the most widely occurring active transport
proteins in eukaryotes.
– Used to transport sodium ions out of cells and
potassium ions into cells.
– Example: nerve cells have 30 times more
potassium in them than extracellular fluids.
– animation
• Sodium ions bind to the
protein on the inside of
the cell membrane;
• ATP is hydrolyzed and the
phosphate produced is
linked to the protein
• The shape of the protein is
changed in such a way that
the sodium ion can be
expelled out of the cell
• Potassium ions bind to the
protein
• Phosphate group is
removed causing the
protein to snap back to its
original shape
• Potassium ion moves into
the cell
MOVING BIG STUFF
• ENDOCYTOSIS– Large molecular materials are
enclosed within invaginations
(folding) of the plasma membrane,
subsequently pinching off to form
cytoplasmic vesicles.
– Phagosytosis (cell eating) - cell ingests
large particles such as bacteria or
pieces of debris
• Entrap, engulf, digest, absorb
– Pinocytosis (cell drinking) - cell
ingests liquid and/or dissolved solutes
and small suspended particles.
animation
Exocytosis
• Helps cells remove larger molecular waste
materials