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Chapter 8 – Cells and Their Environment
Section 8.1 – Cell Membrane
Every cell has a cell membrane. It protects the cell and helps move things in and out of
the cell.
Homeostasis
Homeostasis is maintaining internal conditions regardless of surroundings. For example,
when you are hot, you sweat to maintain a normal body temperature.
A cell membrane controls this by being selectively permeable, which means that it only
lets certain things in/out.
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The cell membrane is made of lipids (fats) and has proteins in it to allow
things in or out.
The cell membrane also provides structural support for the cell.
It recognizes foreign material.
It communicates with other cells.
Lipid Bilayer
Phospholipid – a specialized lipid made of a phosphate “head” and two fatty acid “tails”.
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The head is attracted to water, which is why it is polar.
The tail repels water, which means that it is nonpolar.
Membrane Proteins
There are 4 types of proteins embedded in the cell membrane.
1.) Cell surface markers - "name tags"
- Has a chain of sugars on it to help it tell what cell it is.
Example:
2.) Receptor proteins - helps cell sense its surroundings
- Can bind to substances that cause a change within the cell
Example:
3.) Enzymes - help with chemical reactions
Example:
4.) Transport proteins - move substances into or out of the cell that would not otherwise
cross the membrane
Example:
Section 8.2 – Cell Transport
There are two ways in which cells must use energy to move material into and out of the
cell. They are called passive transport and active transport.
Passive Transport
In passive transport, substances cross the cell membrane down their concentration
gradient.
Passive transport does not use energy to move substances into and out of the cell.
- All things want to reach and be in equilibrium.
- Equilibrium is when all spaces are filled evenly.
- When one area has a higher concentration than another, it is known to have a
concentration gradient.
- In order to reach equilibrium the substance must move from an area of higher
concentration to lower concentration, which is known as diffusion.
Example:
There are three types of passive transport. They are…
- Diffusion
- Osmosis
- Facilitated Diffusion
Diffusion
Diffusion is the movement of particles from areas of higher concentration to lower
concentration.
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Simple diffusion is the movement of small, nonpolar molecules that pass
directly through the lipid bilayer.
Oxygen concentration is usually higher outside of the cell, therefore it moves
into the cell to an area of lower concentration.
Carbon dioxide has a higher concentration inside the cell, therefore it moves
out of the cell to an area of lower concentration.
Facilitated diffusion occurs when transport proteins help these substances diffuse
“move” through the cell.
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There are two types of transport proteins. They are channel proteins and
carrier proteins.
Ions, sugars, and amino acids can move through the cell membrane through
channel proteins. They are like tunnels through the lipid bilayer that allow
substances of specific size to diffuse through it.
Carrier proteins bind to a specific substance on one side of the cell, which
causes it to change shape. As the proteins shape changes, the substance moves
across the membrane. (Look at figure 6 on page 179.)
Osmosis
Water can diffuse across a selectively permeable membrane in a process called osmosis.
Osmosis allows cells to maintain water balance as their environment changes.
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Polar water molecules can not move straight through the bilayer. The cell
membrane contains channel proteins that only water can pass through. This
makes it a facilitated diffusion.
Predicting Water Movement
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The direction water moves in a cell depends on the concentration of a cell’s
environment.
There are 3 ways in which water can move in a cell.
1) Water moves out.
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If the solution is hypertonic, or has a higher solute concentration, water moves
out of the cell. The cell loses water and shrinks.
2) Water moves in.
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If the solution is hypotonic, or has a lower solute concentration, water moves
in the cell. The cell gains water and expands.
3) No change in water movement or equilibrium is reached.
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If the solution is isotonic, or has the same concentration, water moves into and
out of the cell at equal rates. The cell stays the same size.
Active Transport
Sometimes cells must transport substances against their concentration gradients. This
movement is called active transport because the cell must use energy to move these
substances. Active transport required energy to move substances against their
concentration gradient.
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In active transport, the carrier proteins do require energy to “pump”
substances against their concentration gradient.
One of the most important carrier proteins in animal cells is the sodium-potassium pump.
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The sodium-potassium pump pumps 2 K+ in and kicks 3 Na + out.
Look on page 182 for diagram.
Vesicles
Many substances are too large to be transported so they cross the cell in vesicles.
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The movement of a large substance into a cell by means of a vesicle is called
endocytosis.
The movement of a material out of a cell by means of a vesicle is called
exocytosis.
Section 8.3 – Cell Communication
Sending Signals
Cells communicate and coordinate activity by sending chemical signals that carry
information to other cells.
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A signaling cell produces a signal and that signal is detected by the target cell.
Targets
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Neighboring cells can communicate through direct contact. Just like talking to
the person beside you.
Short-distance signals may act a few cells away from originating one. Like
talking to a person across the room from you.
Long-distance signals are carried by nerve cells and hormones. Like calling
someone on the telephone.
Receiving Signals
Some signals are made possible by receptor proteins. A receptor protein binds only to
signals that match the specific shape of its binding site.
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Some cells may have receptor proteins that can detect and respond to light.
They enable the cell to detect its environment.
Once it binds to the signal protein, the receptor protein changes its shape and
relays information.
Responding to Signals
The cell can respond to a signal in three different ways.
1) Permeability Change – Transport proteins may open or close.
2) Enzyme Activation – Receptor proteins activate enzymes and trigger chemical
reactions.
3) Second Messenger – Acts as a signal molecule within the cell and causes changes
in the cytoplasm and nucleus.