UNIT 5: CELLULAR TRANSPORT

Active and Passive Transport !!!

The Fluid Mosaic Model

The cell membrane is also called the plasma membrane and is made of a phospholipid bilayer.

The phospholipids have a hydrophillic (water attracting) head

And two hydrophobic (water repelling) tails.

The head of the phospholipid is made of an alcohol and a phosphate group.

While the tails are made of fatty acids.

Another substance in the membrane is cholesterol.

It maintains the membrane’s fluidity (makes it not too solid or not too liquid)

Proteins called integral proteins go all the way through the bilayer

While peripheral proteins are only on the side.

Integral proteins are also called channel proteins.

Large molecules use integral proteins to help move across cell membranes

TRANSPORT: THE TRAFFIC ACROSS MEMBRANES

The ability of molecules to move across the cell membrane depends on two things:

the semi-permeability of plasma membrane

and the size and charge of the particles that want to get through.

The plasma membrane helps a cell to maintain homeostasis, or a relatively stable internal environment.

It does this by being semi-permeable (it lets certain molecules inside of the cell, while keeping others out).

Generally speaking the membrane lets lipid molecules inside, while keeping non-lipid molecules out.

Water is a major exception to this rule!

Diffusion But what determines the

direction of the traffic across the membrane?

Most solutes (or particles dissolved in a solution) naturally want to spread out from each other.

Diffusion

So particles want to move from areas of high concentration to areas of low concentration.

This is known as diffusion. Ex: When you spray perfume, the perfume

particles will spread out in the room. If you drop food coloring into water, the

color will spread out.

Diffusion

The particles will keep spreading out until the concentration is the same throughout the solution.

Once the concentration is the same everywhere, the particles have reached what we call equilibrium.

Diffusion

So what does that have to do with the plasma membrane?

Well suppose a substance is present in unequal concentrations on either side of a cell membrane.

If the substance can move across the membrane, then its particles will move to the area of lower concentration, until equilibrium is reached.

Diffusion

This process of diffusion is how some molecules move into and out of the cell.

Since diffusion occurs naturally, the cell does not need to use energy and this is called PASSIVE transport.

Osmosis

Osmosis is the diffusion of water across a semi-permeable membrane.

It is also driven by the concentration of particles.

Some particles cannot move across the membrane, so water moves instead to even out the concentration.

A solution that has a high concentration of particles is said to be HYPERtonic

A solution that has low concentration of particles is said to be HYPOtonic.

Water will move from the hypotonic solution to the hypertonic solution, until the ratio of water to particles is the same on both sides.

The solution is then said to be ISOtonic (it has reached equilibrium).

Facilitated Diffusion Non-lipid materials cannot diffuse across the

membrane as easily as lipid molecules. These substances must rely on special

proteins called channel or carrier proteins that are embedded inside of the plasma membrane.

Facilitated Diffusion

The proteins pick up the substance from one side of the membrane and carry it across to the other.

Facilitated Diffusion

Facilitated Diffusion Since the substances are still travel

from a high concentration to a low concentration,

Facilitated diffusion does not require the cell to use ATP (energy)

Active Transport As powerful as diffusion is, cells

sometime have to use energy to move substances against the concentration gradient (from low concentrations to high concentrations).

This is called active transport.

Active Transport- Transport proteins Embedded in the plasma membrane

are special transport proteins, that use ATP (energy) to pump molecules in or out of the cell.

Transport Proteins The proteins are similar to the carrier

proteins that work in facilitated diffusion, BUT they pump molecules from areas of low concentration to high concentration

They require the use of energy (ATP) because they are going against the concentration gradient.

Active Transport - Transport Protein

Transport proteins are usually only involved in the transfer of smaller ions or molecules.

Endocytosis

Larger molecules or clumps of material can be actively transported across the membrane through a process called endocytosis.

Endocytosis Endocytosis occurs when the cell

membrane surrounds a substance and engulfs it.

The pocket that results breaks loose from the cell membrane and forms a vacuole within the cell’s cytoplasm.

Endocytosis There are two types of endocytosis:

phagocytosis and pinocytosis.

Phagocytosis In phagocytosis (cell eating), the

plasma membrane surrounds a particle and packages it in a food vacuole.

Pinocytosis In pinocytosis, tiny pockets form

along the plasma membrane, fill with liquid, and pinch off to form vacuoles within the cell.

Exocytosis Many cells also use active transport

to release large amounts of material from the cell. This is known as exocytosis.

The membrane of the vacuole surrounding the material fuses with the cell membrane, forcing the contents outside of the cell.