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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.
?
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
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