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 condition 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 (or) Semi-permeable

It allows 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”)

Phospholipid

Polar Head

Nonpolar Tails

Bilayer Arrangement Outside of cell

Inside cell

Proteins are embedded/floating in the lipid bilayer.

Protein in membrane

Cell Membrane Structure-Proteins

These proteins are needed

for the membrane to function

properly.

3 Types of Membrane Proteins

1. Transport - regulate what enters or leaves cell

2. Marker - identify the cell

3. 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)

Marker Proteins • Cell’s “Name Tag”

• Protein sticks out of phospholipid layer

• Often has carbohydrates attached to outside end

• Functions in cell identification; identifies 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 anytype of blood

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”

Solutions

• Solution: a mixture of a solute and a solvent – water and sugar (koolaid :-)

• Solute: the substance in a solution that is dissolved by the solvent – sugar

• Solvent: the substance in a solution that is dissolving the solute – water

Isotonic Solution • Concentration of

solute and solvent molecules on one side of the membrane is equal to/the same as the solution on the other side (often the cytoplasm in cell)

• Water moves 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 solution

X 1000

Note that all the cells appear normal.

Hypertonic Solution More solute molecules on one

side of the membrane than the

other

When a cell is in a hypertonic

solution, water moves out of the

cell shrinks (dehydrates)

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 1000

Notice that the cells have shrunk.

Hypotonic Solution

Fewer/less molecules on one side of the membrane than the other

When a cell is in a hypotonic solution, water moves into the cell

Cell will swell, and could burst (lyse)

Plant cells have vacuoles to collect extra water

Red blood cells in hypotonic solution

X 1000

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

CELL TRANSPORT

• Concentration gradient - a difference in the concentration of a particular substance across a space.

• Equilibrium is reached when the molecules are distributed evenly throughout a space.

Types of Transport

• Passive Transport—no energy required, molecules move from high to low concentration (down or with concentration gradient) – Diffusion – Osmosis – Facilitated Diffusion

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

Passive Transport

Diffusion, Osmosis, Facilitated Diffusion

Diffusion

is 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

is the diffusion of water

molecules through a

semi-permeable

membrane requires no

energy

Ex. Water will move

toward the higher

concentration of solute

(and hence a lower

concentration of water).

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

Requires energy from the cell because materials are being moved against the

concentration gradient

Why do cells need active transport?

• Cells must maintain a certain concentration of molecules (amino acids, sugars, etc.) inside their cytoplasm from the surrounding fluid

• Some of these must be moved against the concentration gradient (there are already more molecules inside the cell)

How does active transport work?

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

• Other types use vesicles

• Energy is provided by ATP (more on ATP later)

Active Transport 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 (H+) Pump

• Forces protons out of a membrane enclosed space to create a proton gradient down which protons flow back in

• The cell uses one ATP to pump a proton out; that proton can be used to bring in large molecules with minimal energy output

Active Transport with Vesicles

Exocytosis & Endocytosis

Endocytosis

Substances 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”

Exocytosis Exocytosis- substances inside a vesicle are released

from a cell as the vesicle fuses with the cell membrane

Involves the cell expelling waste or the cell secreting cell products (ex. Hormones, insulin)

• Requires Energy (ATP)

Cell Transport Animations

• http://highered.mcgraw-hill.com/sites/0072495855/student_view0/chapter2/animation__phagocytosis.html

• http://highered.mcgraw-hill.com/olcweb/cgi/pluginpop.cgi?it=swf::535::535::/sites/dl/free/0072437316/120068/bio03.swf::Sodium-Potassium%20Exchange%20Pump

• http://highered.mcgraw-hill.com/olcweb/cgi/pluginpop.cgi?it=swf::535::535::/sites/dl/free/0072437316/120068/bio02.swf::Endocytosis%20and%20Exocytosis