An Interactive Lecture Guide to help you understand THE MOVEMENT OF MATERIALS ACROSS A MEMBRANE .
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Transcript of An Interactive Lecture Guide to help you understand THE MOVEMENT OF MATERIALS ACROSS A MEMBRANE .
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An Interactive Lecture Guide to help you understand
THE MOVEMENT OF MATERIALS ACROSS A MEMBRANE
www.tripod.com
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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.
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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.
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– 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.
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Similar to selective hearing!
SELECTIVE PERMEABILITY:
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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.
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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.
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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
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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
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• 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!
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Osmosis – Diffusion of water!• The movement of water across a selectively
permeable membrane. • Down the concentration gradient.• Osmosis in the kitchen video
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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.
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• 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.
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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
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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
Cholesterollipid (steroid)
molecules located sporadically I n-between phospholipids that act to adjust the fluidity of the membrane
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• Transport Proteins:– Proteins that form
a “pore” for molecules to travel through in either direction through the cell membrane
Receptor ProteinProteins with specifically-shaped binding sites on
the outside of the membrane that function to receive molecular messages from other cellsLigand: A chemical that bonds with a receptor protein and carries a specific “message” from one cell to another
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• 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.
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Build a Cell Membrane Activity
• Animation• Activity
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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
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Hypotonic Solution
• low concentration in dissolved stuff and high in water, net water gain, cell swells
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Isotonic
• equal amount of dissolved material inside and outside of cell- no net loss
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Review
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• 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 environment- against the [gradient]! •This requires chemical energy which is supplied by ATP.
Why not stop at diffusion?
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• 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 _
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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
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• 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
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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
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Exocytosis• Helps cells remove larger molecular waste
materials