CELL MEMBRANE &CELL TRANSPORT

Homeostasis:Maintaining a Balance

Organisms must adjust to changes in their environment.

If not…DEATH!

A formal definition is—maintaining a stable internal state despite what is going on externally.

What Maintains Homeostasis? The PLASMA or CELL

MEMBRANE maintains the proper concentrations of materials by controlling the passage of molecules in and out of the cell.

Therefore, the cell membrane’s function is to maintain HOMEOSTASIS through passive transport, active transport and cell communication!

Cell Membrane

X 53, 260

This electron micrograph of the cell membrane shows the appearance of the phospholipid bilayer using several staining processes. The magnification on this structure is 53,260 X

Characteristics of the Cell

Membrane

The cell membrane is Selectively Permeable

aka Semi-permeableAllows some things in/out and not

others… Oxygen, nitrogen,carbon dioxide,

and other small, nonpolar molecules can diffuse directly

Water was once thought to move directly through, but it is now understood to travel through aquaporins (a type of transport protein).

Ions, sugars, and larger molecules move through transport proteins or in vesicles.

Plasma/Cell Membrane-Structure

Phospholipid Bilayer with proteins embedded/floating in it

Phospholipid Structure: Polar Head (hydrophilic

—”water loving”) Nonpolar Tails—

(hydrophobic—”water fearing”)

PhospholipidPolar Head

Nonpolar Tails

Bilayer ArrangementOutside of cell

Inside cell

Proteins are embedded/floating in the lipid bilayer.

Protein in membrane

Cell Membrane Structure-ProteinsThese proteins are needed for the membrane to function properly.3 Types of Membrane Proteins

1. Transport—regulate what enters or leaves cell2. Marker—identify the cell3. Receptor—allow cells to communicate

Transport Proteins—Channel Proteins• Function as

“gates/passageways”• Allow polar sugars, amino

acids, and ions to cross the membrane.

• Special channel proteins:– Gated ion channels—gates

that open/close– Carrier proteins—change

shape to allow specific molecule to pass

– Aquaporins—allow water to diffuse through (called osmosis)

Channel Protein

Carrier Protein

http://personal.tmlp.com/Jimr57/textbook/chapter3/cms2.htm

Transport Protein

Marker Proteins• Cell’s “Name Tag”• Protein sticks out of

phospholipid layer• Often has carbohydrates

attached to outside end• Functions in cell

identification to identify the cell to other cells and molecules

• Important in – immunity—so various

white blood cells in your body do not mistake your cells for foreign cells

- blood typing – so you can’t receive just any ol’ type of blood

http://personal.tmlp.com/Jimr57/textbook/chapter3/cms2.htm

Marker Protein

Receptor Proteins• Function as

“messenger/receiver”• Receive information from the

environment (extracellular fluid, blood, interstitial fluid) and transmit that info to the inside of the cell

• Protein has specific shape/charge to only allow certain molecules (like hormones) to bond

• Triggers a response in cell• Ex) epinephrine can bind to a

receptor protein and send a message inside that says “break down glycogen”

http://personal.tmlp.com/Jimr57/textbook/chapter3/cms2.htm

Receptor Protein

CELL TRANSPORT• Concentration gradient - the

difference the concentration of a particular substance across a space. .

• Equilibrium is reached when the molecules become even throughout a space.

Cell Transport Types• Passive Transport—no energy required,

molecules move from high to low concentration (down or with concentration gradient)– Diffusion– Osmosis– Facilitated Diffusion

• Active Transport—energy (ATP) required, molecules move from low to high concentration (up or against the concentration gradient)– Pumps– Vesicles

Passive Transport

Diffusion, Osmosis, Facilitated Diffusion

Diffusion Diffusion - the process by which molecules spread from areas of high concentration, to areas of low concentration Molecules are said to go

“Down” or “with” the concentration gradient.

Passive Transport-requires no energy

Osmosis• Osmosis - the diffusion of watermolecules through

a semi-permeable membrane requires

no energyEx. Water will move in the

direction where there is a higher

concentration of solute (and hence a lower concentration of water).

About this diagram: this represents a cell in a solution. The cell will not let the red particles pass through the membrane. The green molecules can pass through, as can water molecules. The arrows show the direction of particle movement. The green particles are moving in to the cell where their concentration is lower, and water is moving out of the cell because its concentration is higher inside

osmosis

diffusion

Isotonic Solution• Concentration of

solute and solvent outside the cell is equal to/the same as the solution you are comparing it to (often the cytoplasm in cell)

• Water is moving in and out at equal rates, no net movement into or out of cell since

• Cell size would stay the same

Red blood cells in isotonic solutionX 1000Note that all the cells appear normal.

Hypertonic Solution There are more solute

(ex. salt) molecules in solution outside the cell when compared to the inside of cell

Water will move out of the cell

Cell would shrink Plants cells shrink

(plasmolysis) because cell membrane pulls away from cell wall, so plant wilts because water has left the central vacuole. Animal cell shrink (crenate). In both cases, the cell may die.

Crenated red blood cells in hypertonic salt solution X 1000Notice that the cells have shrunk.

Hypotonic Solution There are less solute

(ex. salt) molecules in the solution outside the cell than inside the cell.

Water will move into the cell.

Cell will swell Cell could burst

(lyse) Plant cells have

vacuoles to collect extra water

Red blood cells in hypotonic solutionX 1000Note that the pinkish cells have swollen (the little dip in the middle of a normal rbc is not visible and one side bows out).

Osmosis Links

Osmosis Examples

http://www.tvdsb.on.ca/westmin/science/sbi3a1/Cells/Osmosis.htm

Hypotonic, Hypertonic, Isotonic Interactive Quiz

http://www2.nl.edu/jste/osmosis.htm#Osmosis

Facilitated Diffusion (uses Transport Proteins,

passive)• Moves substances

(from high to low concentration) down the concentration gradient without using cell’s energy using channel/carrier proteins located in membrane

Active Transport in Cells

Why do cells need active transport?

• Cells must transport certain amino acids, sugars, etc. into their cytoplasm from the surrounding fluid.

• Some of these substances, however, are already in higher concentrations inside versus outside.

How does active transport work?

• Some types involve carrier proteins that function as “pumps”.

• Other types use vesicles.• Energy is provided by ATP.

Active Transport Using Pumps

Sodium-Potassium PumpProton Pumps

Example of active transport types using pumps

Sodium/Potassium Pump•Na+ pumped out of a cell•K+ pumped into a cell• Important because it prevents cells from bursting by lowering the sodium inside causing less water to enter through osmosis.

•Used by many cells, including nerve cells, to send a chemo-electric message.

Sodium-Potassium Pump

Sodium/Potassium pump:

3 Na+ out of cell (yellow diamonds)

2 K + into cell (red/purple squares)

Here the energy of a phosphate from ATP (shown in pink) is used to exchange sodium atoms for potassium atoms.

Proton PumpProton (H+) pump – forces protons out of a

membrane enclosed space (organelle or cell), often to create a proton gradient down which the protons can flow back in

Why would the cell “waste” energy on a proton pump? *Because the cell needs isolated areas of the cell with different pH for particular functions; ex) lysosomes – have proton pumps to maintain a pH=5*Because the cell only uses one ATP to pump a proton out, and that proton can be used in co-transport

Co-transport – process cells use to bring large molecules, such as sugars, into a cell with a minimum amount of energy used; usually a proton and a sugar enter a double tunneled protein at the same time; the tunnel only “works” when both molecules are present

Active Transport Using Vesicles

Endocytosis & Exocytosis

EndocytosisSubstances are moved into a

cell by a vesicle that pinches off from the cell membrane

Requires energy (ATP)

Types of Endocytosis• Pinocytosis—when the nutrient

particles are dissolved in a liquid; “cellular drinking”

• Phagocytosis—when the nutrient particles are solids; “cellular eating”

ExocytosisExocytosis- substances inside a vesicle are

released from a cell as the vesicle fuses with the cell membrane

Involves the cell getting rid of waste or the cell secreting cell products (ex. Hormones, insulin)

• Requires Energy (ATP)

Animations—Exocytosis, Endocytosis

McGraw-Hill: Cotransport, Na-K pump, Endo/Exocytosis, & Proton Pump

http://highered.mcgraw-hill.com/sites/0072437316/student_view0/chapter6/animations.html#

Exocytosis using ER and Golgi:

http://www.wisc-online.com/objects/index_tj.asp?objid=AP11203

Cell Membrane Physiology--Interactive

http://www.getbodysmart.com/ap/nervoussystem/neurophysiology/membranephys/menu/menu.html