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Notes on Homeostasis
Homeostasis – “biological balance”
cell(s) keeping a steady state with
or in spite of the environment
• homeo - same
• stat - steady
Homeostasis
Regulation of the internal environment to
maintain constant, favorable conditions
Every cell, tissue & organ in the human
body contributes toward total body
homeostasis
Ex: Body is cold (low temp), you
begin to shiver to raise body
temperature
Ex: Your blood oxygen level is low**,
your respiratory rate increases
to meet this O2 need
(raise blood O2)
**generalization
Ex: Blood collects & carries wastes from
all tissues toward kidneys, which then
filter blood & send waste materials
toward excretory system
Ex: Enzymes used in producing sperm
function best at 2-3° lower than
regular body temperature.
What determines a cell’s set of
homeostatic conditions?
The metabolic needs of the cell.
Homeostasis
• In the human body, these are the conditions
•
of homeostasis:
–pH of human blood is: 7.4
–Normal body temp is: 98.6 ºF (or 37 ºC)
–Saline concentration in blood is: 9g salt
per liter
–Regular blood glucose level is: 100-150
mg/dL
Clearly, homeostasis does NOT mean
conditions are equal to the conditions of the
environment
Write down the key ideas from the following video clip
dealing with metabolism and homeostasis; use these
for your Frayer Model in comp book!.
The BIG idea:
Because the cell membrane is
the interface between the cell
and the external environment,
the cell membrane plays a
MAJOR role in governing
homeostasis for the cell.
Review….
Solute: Any substance that is dissolved in water is a solute.
Example: salt
Solvent: The liquid in which a solute is dissolved
Example: water (of the saltwater )
Solution: A liquid that contains one or more solutes
Example: Water with sugar and/or salt
Concentration: The mass of solute in a given volume of
solution.
Concentration, then, just means how “salty” the water is.
What are two methods for changing a solution’s concentration?
New idea: Concentration Gradient
• A concentration gradient is a difference between
two solutions that are separated by space or a
barrier.
Notice these two “salt” solutions:
One solution is saltier than the other. What do you
think will happen to the salt particles over time?
Definition
K
A separation of two
different concentrations
of a solution.
K
Na
Na
K
K
K
K
K
KK K
K
K
K
K
K K
KK
K
K
K K
K
K
K
concentration gradient
Na
Na
Na
Na Na Na Na Na Na
Na Na
Na
Na
Na Na
Na
Na
Na
Na
Na
Na
Na
Na
Equilibrium /
Equal on both sides
In cells, concentration gradients
occur at the cell membrane—
between the intracellular and
extracellular environments.
Interacting with the concentration
gradient is how the cell membrane
helps to maintain homeostasis.
Cell membrane – regulates what enters
and leaves the cell
Semipermeable membrane – keeps out
some molecules but
allows others to enter
Structure of the Cell membrane
• composed of a bilayer of
phospholipids and proteins
Structure of
Nonpolar
• regulates what enters and
leaves the cell – selectively
permeable (semipermeable)
• Keeps out some molecules but
allows others to enter
Movement across the cell membrane:
2 types:
1. passive transport
2. active transport
1. Passive transport – does not
require cellular
energy
A. Diffusion – movement of molecules
from areas of greater
concentration to areas of
lesser concentration
Particles move WITH the
concentration gradient.
Diffusion with
no membrane:
Diffusion across
a membrane:
Once the concentration of the molecules
is the same, dynamic equilibrium has
been reached…
There is no NET change of concentration
Molecule of dye
Membrane
Equilibrium
Human body examples of Diffusion:
• Oxygen diffusing from lungs to blood
• Carbon dioxide diffusing from blood to
lungs
• Nitrogenous waste diffusing from blood
in the kidneys into urine
• Food particles absorbed in the small
intestine from the digestive tract.
What is the homeostatic role of the
cell membrane in processes of
diffusion?
B. Facilitated diffusion – molecules
that can not cross the cell
membrane’s lipid bilayer
directly are guided through
the protein channels
• example: glucose
Solute
Cell Membrane
Protein
Channel
What is the homeostatic role of the cell
membrane in processes of facilitated
diffusion?
C. Osmosis – diffusion of water
across a membrane
In osmosis, solute isn’t moving, but the
solute is the reason for water movement
Hypotonic – concentration of
solute molecules in the
environment is lower than
in the cell
HYPO could make a cell POP
• “hypo” means lower
• water comes in and the
cell swells
Hypertonic – concentration of
solute molecules in the
environment is greater
than in the cell
• “hyper” means higher
• water leaves and the cell
shrinks
HYPER will make a cell SHRINK
Isotonic – concentration of
solute molecules in the
environment is equal to
the inside of the cell
• “iso” means equal
• cells neither gain nor lose
water
Examples:
Salty water will be hypertonic to a cell
Distilled water will be hypotonic to a
cell
Plant cells swell until they are
restricted by the cell wall
• this sets up turgor pressure
and result in the cell becoming
rigid
Water leaves (loss of turgor
pressure) the plant cell wilts
• This condition is known as
plasmolysis
If too much water enters an animal
cell, the cell will burst
- this is called cytolysis
What is the homeostatic role of the
cell membrane in processes of
osmosis?
ISOTONIC SOLUTION
HYPOTONIC SOLUTION
HYPERTONIC SOLUTION
(1) Normal
(2) Lysing
(3) Shriveled
ANIMAL
CELL
Plasma
membrane
PLANT
CELL
(4) Flaccid
(5) Turgid
(6) Shriveled
Animal
Type of cell - ___________
hypertonic
Type of solution - _____________
8.5% NaCl
91.5% H2O
out
How will water move? _____
45% NaCl
55% H2O
osmosis
By what process? ______________
in
How will salt move? ______
diffusion
By what process? _______________
shrink in size
What will happen to the cell? _____________
plasmolysis
Biological term - __________________
Plant
Type of cell - ___________
hypotonic
Type of solution - _____________
8.5% NaCl
91.5% H2O
in
How will water move? _____
0% NaCl
Distilled water
osmosis
By what process? ______________ 100% H2O
out
How will salt move? ______
diffusion
By what process? _______________
swell and
become rigid
What will happen to the cell? _____________
turgor pressure
Biological term - __________________
Passive Transport
high
low
2. Active transport – requires cellular
energy
• materials move from an area of
lesser concentration to an area
of greater concentration
• In active transport, materials are moved
against the concentration gradient.
• Types of active transport are found in:
– protein pumps
– endocytosis
– exocytosis
Types of Active Transport
A. A Protein Pump pushes substances
against their concentration gradient in order
to keep the amounts that the cell prefers.
Example: Nerve cells use a sodium-potassium pump
to keep desirable concentrations inside the cell.
During a nerve impulse, protein channels OPEN, all
solutes diffuse with the concentration gradients,
but away from the desired levels…oh no!!!
That’s where the sodium-potassium pump comes
in…see next slide…--pumping things back so that
there IS the desired concentration gradient.
• example: the sodium-potassium
pump
• sodium (out) and potassium (in)
Proton Pump
Link to online video
Active Transport
Endocytosis vs. Exocytosis
Online link to video clip
B. Endocytosis – passage of large
molecules into the cell
through the cell membrane
• The material is enclosed in a
vesicle which pinches off from
the cell membrane
• 2. Phagocytosis is the process of large particles being
taken into the cell by means of extensions of
cytoplasm that go out and engulf the large particles.
• This is a leukocyte (white blood cell) in
• your blood that is shooting out extensions
• to get the large particles that you see.
a. pinocytosis – movement of
solutes or fluids
• pino = to drink
b. phagocytosis – movement of
large food
particles or
whole organisms
• phago = to eat
• example: WBCs, amoeba
C. Exocytosis – passage of
unwanted and/or large
molecules through the cell
membrane to the outside
• excretions and secretions;
Golgi body
• Excess water;
contractile
vacuole of
some protists.
What is the homeostatic role of the
cell membrane in processes of
active transport?
Active Transport
hig
h
ENERGY
low
True/False Discussion
“The cell membrane maintains homeostasis
by allowing solutes to reach equilibrium.”
“The cell membrane maintains homeostasis
by preventing solutes from reaching
equilibrium.”
“The cell membrane maintains homeostasis
by preventing the diffusion of large
molecules.”
W2W