cell transport: moving molecules in and out of the cell

Cell Transport: Moving molecules in and out of the cell

Plasma Membrane

Phospholipid bilayer that surrounds all cells and certain organelles within the cell

Phospholipid

Hydrophilic Phosphate head

Hydrophobic Fatty acid Tails

Remember the Triglycerides?What’s the difference?

In water what will happen?

Water on outside of cell

Water on outside of cell

Also Water on Inside of cell

Double layer allows:

• All hydrophobic parts to be away from water

• All hydrophilic parts to be near water

Membrane – more than just phospholipids

A little more than just phospholipids

Plasma membrane contains:

Proteins embedded in phospholipid bilayer;

Cholesterol tucked between fatty acids

Carbohydrate chains

Why proteins?

• Proteins increase contact with water and act as channels through which certain molecules can pass

Why cholesterol?

• Cholesterol affects fluidity. It stiffens the membrane and prevents solidification

Why Carbs?

Carbohydrate chains act as cell “ID” tags.

Fluid Mosaic Model: theory that describes the composition of the membrane

- Fluid because it is flexible; p-lipids and proteins can shift position

- Mosaic because it is made of many small molecules that work together as a whole

Membrane acts as a semi-permeable barrier (think bouncer!)

some stuff passes right through the phospholipids:

hydrophobic molecules (O2, N2, CO2)

small polar molecules (H2O)

some stuff stays out:

- ions (Na+, Cl-, K+, Ca++)

- small polar molecules (Sugars, Amino Acids, H2O)

But don’t we need these?!?! We’re getting there….

? ?

So what are the general factors that determine how a molecule enters the cell?

There are two general categories of transport:

Passive transport: The easy way; no energy required; molecules just flow

naturally across

Active Transport: the hard way; energy required; molecules need to

be pushed or carried across

BROWNIAN MOVEMENT

Molecules are ALWAYS moving

Passive Processes: No energy needed

• Diffusion: Movement of solutes from areas of high concentration to areas of low concentration until concentrations are equal.

• Concentration Gradient = differences in Concentration

Gaseous Diffusion

High Conc. Low Conc.

No membrane needed

Think of it as molecules spreading out!

Progression of Liquid Diffusion

High Conc. Low Conc.

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Facilitated Diffusion – movement of solute from high concentration to low concentration BUT requires

transport protein to move molecule that are too big or polar to get

through the bilayer on their own

Glucose is too big to fit through phospholipids and will be diffused through a transport protein to get

into the cell

Osmosis – A Passive Process where water moves across a cell membrane from an area that is hypotonic (lots of water, little solute) to an area that is hypertonic (little water, lots of solute)

think of the water as trying to dilute the concentrated side to make the concentration even

Weak Iced Tea Strong Iced Tea

Move water until you get

Two perfect Iced teas

Tonicity – term used to compare the concentration of one solution to another

- will determine where water goes

3 ConditionsHypotonic

Isotonic

Hypertonic

The solution with the higher concentration is called Hypertonic.

The solution with the lower concentration is called Hypotonic

If the concentrations are equal, they are called isotonic

20% Salt Solution

10% Salt Solution

Which is Hypertonic?

Inside is Hypertonic

10% Salt Solution

20% Salt Solution

Which is hypertonic?

?Outside is Hypertonic

20% Salt Solution

20% Salt Solution

Isotonic:

Isotonic

CrenationC

ytol

ysis

Comparing Cell types chart

PlasmolysisVs.

Turgidity in Plant cells

- Turgor pressure: Pressure that the water INSIDE the cell puts on

the cell wall. Supports plant

A B A B

Think about the cell membrane

• Transport proteins work a lot like enzymes. The correct molecule “seats” it self (even if no energy is required) and a conformational change takes place.

• This doesn’t happen with the phospholipid membrane. Things simply “pass through”. The higher the concentration difference the faster the rate of diffusion

Question:

• What will the graphs look like for the two substances that are going through the cell membrane?

• How is diffusion rate (facililated vs. simple) impacted by the concentration of the solute?

Active Transport

What is it: Cell must use energy to force molecules to move across the membrane from a low concentration to a high concentration.

When used: Usually used to move ions AGAINST the gradient (remember that ions can use facilitated diffusion with the gradient)

Active Transport: Ion Transfer

Example: Sodium:Potassium Pump Animation Goal: maintain NON-EQUILIBRIUM concentrations of ions!

Found in: nerve cellshttp://highered.mcgraw-hill.com/sites/0072495855/student_view0/chapter2/animation__how_the_sodium_potassium_pump_works.html

Vesicle TransportVesicle transport is different from

DiffusionOsmosisFacilitated diffusion

Active transport (using proteins) BECAUSE

We are moving large quantities (many molecules) rather than individual molecules. The cell must package the material in a vesicle and bring it in or out of the cell depending on the goal.

BULK Transport: the movement of large quantities at one time.

REQUIRES ATP DOES NOT REQUIRE A Concentration Gradient

Types of Vesicle Transport

• The two types of vesicle transport depend on direction –

• Moving materials into a cell in a vesicle is called endocytosis.

• Moving materials out of a cell is called exocytosis.

Bulk transport into cell

Endocytosis

PHAGOCYTOSIS

PINOCYTOSIS

(SOLIDS)

(LIQUIDS)

PHAGOCYTOSIS: “CELL EATING” PROTEINS, BACTERIA, DEAD CELLS ARE ENCLOSED IN A PL. MEMB. SAC.

NEEDS ATP

PINOCYTOSIS: “CELL DRINKING” PL. MEMB. SINKS BELOW A FLUID PARTICLE CONTAINING SMALL SOLUTES AND FUSES WITH PL. MEMB. FOMING A VESICLE

OCCURS IN MOST CELLS, EXP: KIDNEYS AND INTESTINES

Exocytosis – Bulk Transport out of cell

Way of releasing large quantities of stuff from the cell including : Hormones, mucus and cell wastes

NEEDS ATP