CHAPTER 8 NOTES:
Cells and their environment
Lesson 1
HOMEOSTASIS is the maintenance of stable internal conditions in a changing environment. Examples: regulation of body
temperature, control of blood glucose levels, the regulation of salt and water balance, when you get hot, you sweat ...
when you get cold, and you shiver.
Homeostasis is about staying the same inside no matter what the outside does
CELL MEMBRANE is a double layer of phospholipids (lipid bilayer) that covers a cell’s surface and acts as a barrier
between the inside of a cell and the outside of a cell. Phospholipids are specialized lipids made of phosphate and fatty
acids.
Some of the functions performed by a cell membrane to maintain homeostasis within a cell:
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Control which substances pass into and out of the cell
Helps the cell keep its shape
Recognize foreign material
Communicate with other cells
Respond to chemical signals from other cells
Contains markers that give the cell its identity
Cell membrane is selectively permeable. That means that it allows only certain substances to pass through.
Cell membrane has many different kinds of proteins in it which perform important functions for the cell:
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Cell-surface markers- announces a cell’s identity, have protein portion and carbohydrate portions.
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Receptor proteins- recognize and bind to substances outside of the cell. Allow cell to detect and respond to its
surroundings.
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Enzymes- play a role in many chemical reactions inside the cell.
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Transport proteins- help move substances across the cell membrane.
Questions: 1. how does a cell membrane help a cell maintain homeostasis?
2. How does the cell membrane restrict the exchange of substances?
3. What are some functions of membrane proteins?
Lesson 2
CELL TRANSPORT
Concentration of a substance is the amount of that substance in a given volume. Concentration of sugar in a tea is the
amount of sugar (substance) in the cup of tea (given volume).
Molecules in a liquid or a gas tend to move from an area of high concentration to an area of low concentration. Just like
people in a crowded subway car tend to move to a car with fewer people inside.
A difference in the concentration of a substance across a distance is called Concentration Gradient.
Movement of substances down its concentration gradient (from high to low) is called Diffusion.
Diffusion tends to continue until the molecules are evenly distributed which means the solution is in Equilibrium.
Diffusion plays an important role n a cell’s ability to take in essential materials and get rid of harmful wastes.
Passive Transport- movement of molecules down the concentration gradient – does not require energy.
Check this website for the animations of the process:
http://highered.mcgraw-hill.com/sites/0072495855/student_view0/chapter2/animation__how_diffusion_works.html
Two main types of Passive Transport:
1) Simple Diffusion- small, uncharged, nonpolar molecules pass through lipid bilayer.
2) Facilitated Diffusion (assisted)- charged and polar molecules are assisted by transport proteins to pass through
cell membrane.
Check this website for the animations of the process:
http://highered.mcgrawhill.com/sites/0072495855/student_view0/chapter2/animation__how_facilitated_diffusion_works.html
Example of transport proteins – Carrier Proteins. Transport only molecules that fit its binding sites and change their
shape after the binding to release the molecule inside the cell.
Passive Transport of water molecules is called OSMOSIS.
Check this website for the animations of the process:
http://highered.mcgraw-hill.com/sites/0072495855/student_view0/chapter2/animation__how_osmosis_works.html
When cell is placed in a solution, the concentration of the solution is compared to that of the cell in a following way:
Hypertonic- the solution outside of the cell has a higher concentration of particles then the cell’s
cytoplasm. This
means that solution has a lower concentration of water, and water will move from the cell’s cytoplasm onto the solution
outside. The cell will shrink.
Hypotonic - the solution outside of the cell has a lower concentration of particles then the cell’s cytoplasm. This means
that solution has a higher concentration of water, and water will move into the cell from outside. The cell will swell.
Isotonic- the concentration of particles and water is equal inside and outside of the cell.
Active Transport
Often cells need to move molecules against the concentration gradient ( from an area of low concentration to an area of
high concentration). Cell requires energy for this type of transport which is called Active Transport.
Examples of Active Transport:
1) Carrier proteins (note that passive transport also uses certain carrier proteins) called Sodium-Potassium Pump –
powered by ATP, pumps sodium and potassium ions in and out of the cell against the concentration gradient.
http://highered.mcgrawhill.com/sites/0072495855/student_view0/chapter2/animation__how_the_sodium_potassium_pump_works.ht
ml
2) Endocytosis – cell uses vesicles to move molecules (nutrients) into the cell.
3) Exocytosis- cell uses vesicles to move molecules (wastes, pathogens, proteins) out of the cell.
Questions: 1) What determines the direction in which passive transport occurs?
2) Why is osmosis important?
3) How do molecules move against their concentration gradient?
CELL COMMUNICATION
To coordinate their activities cells must communicate. Cell use Signal Molecules- (something that is produced and
released by one cell to send a message to another)- to communicate.
Examples of Signal Molecules: hormones or nerve transmitter molecules.
How are the signals processed? Cells have Receptor Proteins on a cell’s membrane which has a specific shape that
allows them to bind to a signal molecule sent by another cell. In response to this binding the receptor protein changes
its shape and this change passes the information into the cytoplasm of the cell.
Check this website for the animations of the process:
http://highered.mcgrawhill.com/sites/0072495855/student_view0/chapter2/animation__receptors_linked_to_a_channel_protein.html
Cells may respond to a signal in three main ways:
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Change how permeable its membrane is
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Activate enzymes , which triggers chemical reactions
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Form a second messenger, which acts as a signal molecule within a cell (produces a change in cellular function)
Questions: 1) How do cells use signal molecules?
2)What are receptor proteins?
3)How do cells respond to signaling?