(relaxing music)
- [Narrator] The smallest unit of life is the cell.
In order for life to be sustained,
nutrients and gasses must be able to pass into a cell.
Wastes and other products must be able
to pass out of the cell.
Many of the materials that pass in
or out of the cell do so by passive transport.
In this process, there is no energy,
no ATP expenditure by the cell.
Concentration gradients allow for the movement of materials.
Osmosis and dialysis,
both specific types of passive transport,
use this concentration gradient of high to low.
Another type of passive transport is facilitated diffusion.
Since it is passive, the high to low
concentration gradient is present.
However, the process is facilitated by a carrier protein.
The carrier protein is somewhat like a revolving door
allowing easy access across the membrane
and then rotating back to the other side.
All of these processes, osmosis, dialysis,
and facilitated diffusion are passive.
The cell does not have to use any energy
to move materials by these methods.
The concentration gradient is used.
However, there are materials that need to be moved
against their concentration gradient.
This process requires energy, ATP, and protein carriers
that are found in the plasma membrane.
Defined, active transport is material moved
from a low concentration to a high concentration
through a membrane using a protein carrier
that requires ATP.
An example of active transport is the mechanism
of uptake of iodine by the thyroid gland.
Iodine is essential for the manufacture
of hormones that influence body metabolism.
When you eat a meal containing iodine,
the iodine will pass from the small intestine
into the circulatory system.
As the capillaries pass through the thyroid gland,
iodine diffuses out of the circulatory system.
Iodine attaches to protein carriers
in the thyroid cell's plasma membrane.
Being proteins, they have a unique shape,
so only iodine will fit.
Glucose, which is also actively transported into the cell,
relies on a different protein receptor.
The energy required is somewhat analogous
to a rock being pushed up hill.
The processes of simple diffusion, osmosis, and dialysis,
facilitated diffusion, and active transport
allow for most all of the small material,
those dissolved in plasma and extracellular fluid,
to pass in or out of the cell.
Large particles, however, pass into the cell by endocytosis.
This is an active process.
It needs ATP.
In this process, the plasma membrane surrounds the particle,
such as bacteria or a food morsel,
and forms a vesicle that pinches off from the membrane.
The cell does this in two different ways.
One way is called phagocytosis.
In this method of cell eating,
large particles are surrounded
when the cell extends the plasma membrane
and engulfs the particle.
The result is a membrane-bound package of material.
Fluids that the cell wants to take in
enter by a process called pinocytosis.
In this type of endocytosis, the cell membrane sinks in,
the material sinks inward, and the membrane pinches off.
The cell uses energy for this movement.
In both types of endocytosis,
the cell ends up with a membrane-bound vesicle.
The material in the vesicle still has to pass
through the membrane in order to be used by the cell.
In this case, the lyososome joins with the vesicle,
and digestive enzymes break down the material
to particles small enough to pass through
the membrane and into the cytosol.
A specialized endocytosis called
receptor mediated endocytosis also occurs in cells.
In this process, molecules of a particular substance,
such as cholesterol, bind to receptors
in the plasma membrane.
The vesicle is formed as the membrane sinks in.
Then a lysosome joins the vesicle
and the digestive enzymes break down a substance,
releasing it from the receptors.
The substance diffuses into the cytoplasm,
the lysosome reforms, and the vesicle,
with now empty receptors,
becomes part of the plasma membrane again.
This allows for the cell to reuse or recycle the receptors.
A function of some cells is to manufacture
or synthesize materials for export.
In many cases, this material is made up
of large molecules like proteins
that cannot pass through a plasma membrane.
The process of exocytosis enables the cell
to pass this material out of the cell.
The cell is a very dynamic living entity.
It is essential that materials move in and out of the cell.
This is done by passive transport
using a concentration gradient,
and active transport, endocytosis and exocytosis using ATP.
(relaxing music)