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.

– 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:

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

Cholesterollipid (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 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

• 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

• 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?

• 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