THE MOVEMENT OF MATERIALS ACROSS A

MEMBRANE

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

HOMEOSTASISTherefore, 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.▪ 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 or 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.

WHY DOES THIS MATTER? Cells survive only if they can maintain

homeostasis. Organelles work to maintain this state. Homeostasis is challenged because

molecules move. In order for cells to maintain 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 AND OSMOSIS…TRAFFIC ACROSS A PLASMA

MEMBRANE

How materials move into or out of the cell

CONCENTRATION GRADIENT

The difference in concentration of a substance across an area.

To move “down” or “along” a concentration gradient means to move from HIGH to LOW concentration. To

move “up” or “against” a concentration gradient means to move

from LOW to HIGH concentration.

TWO TYPES OF TRANSPORT – PASSIVE VS. ACTIVE

Passive Transport- the movement of molecules along a

concentration gradient (high low)…it does NOT require energy.

Can you guess what Active transport means?

PASSIVE TRANSPORT involves diffusion without any input of energy. It moves substances down their concentration gradients. Three types: DIFFUSION-The natural movement of small molecules

from high to low concentration OSMOSIS- The diffusion of water FACILITATED DIFFUSION requires the help of transport

proteins in the membrane, but still move down their concentration gradient.

Passive Transport Type #1: DIFFUSION 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

WHY DOES DIFFUSION HAPPEN?

All molecules contain heat, causing them to be in constant random motion,

colliding with one another. As they move, they will naturally move (diffuse) into available space if they can easily

cross the membrane

When a difference in concentration exists across a cell membrane a CONCENTRATION GRADIENT exists.

  Concentration-

Total % of molecules in a certain place By diffusing, 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! People Molecule Example: Field Trip!!!!!!!!!

The rate of diffusion 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 Type #2: FACILITATED DIFFUSION

Requires the help of transport proteins in the membrane, but still move down their concentration gradient.

Passive Transport Type #3: OSMOSIS-

The movement of water across a selectively permeable membrane.

Down/Along the concentration gradient.

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

QUICK REVIEW OF PASSIVE TRANSPORT-DIFFUSION/OSMOSIS/

FACILITATED DIFFUSION

Cells don’t necessarily want to be isotonic, but unless they fight it, they’ll go that way.

Example:•A frog in a freshwater pond will tend to lose salts from his/her body to the 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.

This leads us to the “other” type of transport:ACTIVE TRANSPORT WHEN MOLECULES NEED TO BE IMPORTED OR

EXPORTED AGAINST THEIR [GRADIENT], CELLS EXPEND ATP TO MOVE MOLECULES FROM [LOW] to [HIGH] USING ENERGY!

 

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 THE BIG STUFF ENDOCYTOSIS-

Large molecular with materials are enclosed within invaginations of the plasma membrane, subsequently pinching off to form cytoplasmic vesicles.

Phagosytosis- cell ingests large particles such as bacteria or pieces of debris Entrap Engulf Digest Absorb

Pinocytosis- cell ingests liquid and/or dissolved solutes and small suspended particles. Also known as “receptor-mediated endocytosis”

animation

Exocytosis Helps cells remove larger molecular

waste materials