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CHAP. 1 – SEC. 4: THE CELL IN ITS ENVIRONMENT
REVIEW:
The cell membrane separates the cell from the
outside environment.
The cell membrane allows needed materials into a
cell and allows wastes to leave the cell.
I. THE CELL MEMBRANE AS GATEKEEPER
The cell membrane is SELECTIVELY
PERMEABLE: it allows some things to enter the cell
and prohibits entry by other things.
The cell membrane is like a gatekeeper for a castle.
The gatekeeper decided who could enter a castle and
who could not enter.
Substances allowed to enter the cell are oxygen,
water, and carbon dioxide.
Substances not allowed to enter the cell are certain
large molecules and salts.
Movement in and out of the cell occur in three ways:
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1. DIFFUSION
2. OSMOSIS
3. ACTIVE TRANSPORT
II. DIFFUSION – MOLECULES IN MOTION
Diffusion is the main way substances pass in and out
of a cell.
Diffusion is the process by which molecules tend to
move from an area of higher concentration to an area
of lower concentration.
Example – Air Freshener: The air freshener contains
a high concentration of freshener molecules. Once
the freshener molecules were released from the can, it
spread around the room to areas of low
concentration of these types of molecules.
A. WHAT CAUSES DIFFUSION?
Generally, molecules are always moving and
bumping into one another.
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Molecules of a high concentration starting causing
more molecule collisions to occur in a certain area.
The collision causes the bumped molecules to push
farther out and bump other molecules.
Eventually, the molecules of high concentration will
be evenly spread around the room.
B. DIFFUSION IN CELLS
Our cells obtain oxygen from the area surrounding
the cell.
The area surrounding the cell has a high
concentration of oxygen molecules in the water. The
inside of the cell has a very low concentration of
oxygen molecules.
Since the cell membrane is permeable to oxygen
molecules, the oxygen molecules diffuse from an area
of high concentration (outside the cell) to an area of
lower concentration (inside the cell). SEE FIGURE
16 ON PAGE 41.
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III. OSMOSIS – THE DIFFUSION OF WATER
MOLECULES
Definition of OSMOSIS: the diffusion of water
molecules through a selectively permeable
membrane.
Osmosis helps cells properly function by getting the
water its needs in order for the cell to function.
In osmosis, water molecules diffuse from an area of
high concentration (outside the cell), through a
selectively permeable cell membrane, to an area of
low concentration (inside the cell).
LOOK AT FIGURE 17 ON PAGE 42.
Figure A shows the shape of a normal red blood cell.
The concentration of the water solution (water/salt)
molecules are equal both inside and outside the
cell.
Figure B shows a shrunken red blood cell. The
concentration of water is higher inside the cell than
outside the cell. The outside cell environment
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contains more salt than water. Therefore, water is
leaving the cell and not going back in it.
Figure C shoes a swollen red blood cell. Inside the
cell, there is a high concentration of salt. Outside the
cell, it is mainly just water. As a result, the water
pours into the cell and the cell cannot get rid of the
excess water molecules.
IV. ACTIVE TRANSPORT
Moving materials into the cell by diffusion and
osmosis does not require the cell to use energy. It is
like bicycling downhill.
The movement of materials through a cell membrane
without using energy is called PASSIVE
TRANSPORT.
ACTIVE TRANSPORT requires the cell to expend
energy to move materials in and out of the cell.
Usually, the area of high concentration is inside the
cell and the materials need to move outside the cell to
the area of low concentration.
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A. TRANSPORT PROTEINS
One type of active transport is when transport
proteins in the cell membrane “pick up” certain
molecules such as calcium, potassium, and sodium.
SEE FIGURE 18 ON PAGE 43.
The pick up can either have these types of molecules
enter or leave the cell.
B. TRANSPORT BY ENGULFING
Another type of active transport is called engulfing.
The cell membrane surrounding, or engulfs, a
particle that needs to enter the cell.
Once the particle is engulfed, the cell membrane
pinches off and forms a vacuole (with the particle in
it) inside the cell.
V. WHY ARE CELLS SMALL?
The reason cells are small because it is easier for
materials to enter and leave the cell.
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The cytoplasm in the cell gets the needed materials
that enter the cell to the necessary organelles that
need that the materials.
If the cell size increases, it becomes harder and
farther for the cytoplasm to carry the materials to the
necessary destination points within the cell.
In the reverse, it would take too long to remove
wastes if the cell becomes too big.
When a cell does get too large, it divides into two cells
by mitosis or meiosis.
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