Biology Notes
Topic: Cell Processes / Achieving Homeostasis
AL Biology COS #2
Glencoe High School
Mrs. Barkley
Alabama Biology Course of Study
Objective #2
Describe cell processes necessary for achieving
homeostasis, including active and passive transport,
osmosis, diffusion, exocytosis, and endocytosis.
• Identifying functions of carbohydrates, lipids, proteins, and nucleic
acids in cellular activities
• Comparing the reaction of plant and animal cells in isotonic,
hypotonic, and hypertonic solutions
• Explaining how surface area, cell size, temperature, light, and pH
affect cellular activities
• Applying the concept of fluid pressure to biological systems
Examples: blood pressure, turgor pressure, bends, strokes
Essential Questions
(Cell Processes / Achieving Homeostasis)
 Explain why the cell membrane is the site of much biological
activity.
 Homeostasis is the maintenance of a stable internal
environment… What factors does your body work to
maintain and keep a constant check on???
 Explain the consequences that could occur if a patient was
given pure water instead of saline fluid within an IV.
 Explain why you should not drink ocean water, even if
appears to be your only source of water…
Key Concepts (Cell Processes)
#2-1 Word Parts
#2-2 Achieving Homeostasis
#2-3 Passive Transport
#2-4 Osmotic Effect of Solutions
#2-5 Active Transport
#2-6 Factors That Affect Cellular Activities
#2-7 Endothermic vs. Exothermic Reactions
#2-8 pH
#2-9 Enzyme Action
#2-10 Organic Compounds
#2-1 Word Parts
 homo- / homeo- same
 cyt- cell
 endo- inside
 exo- outside
 hyper- above, greater than
 hypo- below, less than
 iso- equal
 therm- temperature
#2-2 Achieving Homeostasis
Homeostasis
 maintenance of a stable internal environment
 Cell membranes help organisms maintain homeostasis by
controlling what substances may enter or leave cells
 What items must enter/exit the cell for it to survive?
 What factors does your body keep a constant check on??
2 Methods of Maintaining Homeostasis
1. Passive Transport
2. Active Transport
#2-3 Passive Transport
 Passive transport is a method cells use to move materials
across a cell membrane, in order to achieve homeostasis.
Question
Energy required?
Why does it occur?
Movement of molecules?
AKA:
When does it stop?
3 Examples
Answer
#2-3 Passive Transport
 Passive transport is a method cells use to move materials
across a cell membrane, in order to achieve homeostasis.
Question
Energy required?
Why does it occur?
Movement of molecules?
AKA:
When does it stop?
3 Examples
Answer
no
A difference in concentration (concentration
gradient) exists between two sides of a
membrane
From high concentration to low
concentration
Moving “down the concentration gradient”
When equilibrium is reached
Diffusion, facilitated diffusion, and osmosis
#2-3 Passive Transport
Example: Diffusion
 Movement of substances across
a cell membrane from a region
of high concentration to a
region of low concentration
 Driven by kinetic energy
 Occurs because a concentration
gradient exists (meaning there is
a difference in concentration)
 Water, Oxygen, and Carbon
Dioxide easily diffuse through
the membrane.
#2-3 Passive Transport
Example: Fascilitated
Diffusion
 occurs when substances cross
membranes from regions of
high concentration to regions
of low concentration with the
assistance of special transport
proteins
#2-3 Passive Transport
Example: Osmosis
 Diffusion of water across the
cell membrane (through
aquaporin)
 Osmosis occurs as cells are
exposed to various
solutions/fluids within the
body.
 Check out the next slide to
see the osmotic effect various
solutions have on cells.
#2-4 Osmotic Effect of Solutions
 Solutions that cells are exposed to can have a dramatic effect
on the movement of water (osmosis) through cell
membranes.
 Use the following terms to (ONLY) describe solutions, based
on the concentration level of solutes inside:
1. Hypotonic
2. Isotonic
3. Hypertonic
#2-4 Osmotic Effect of Solutions
Hypotonic Solutions
 A hypotonic solution has a lower
concentration of solutes than its
surrounding environment.
 This is also the area that the highest
water concentration will be found.
Therefore, water potential is high.
(Solute concentration and water
potential are inversely proportional
to one another!)
 A cell placed in a hypotonic
solution will have a net gain of
water.
#2-4 Osmotic Effect of Solutions
Isotonic Solutions
 Isotonic Solutions have a
water potential equal to that
of its surrounding
environment.
 Water will move into and out
of this solution, but in equal
amounts.
 Therefore, a cell placed in an
isotonic solution will not
experience a net loss or gain
of water.
#2-4 Osmotic Effect of Solutions
Hypertonic Solutions
 A hypertonic solution has a greater
concentration of solutes than its
surrounding environment.
 This is also the area that the lowest
water concentration will be found.
Therefore, water potential is low.
(Solute concentration and water
potential are inversely proportional
to one another!)
 A cell placed in a hypertonic
solution will have a net loss of
water.
#2-4 Osmotic Effect of Solutions
#2-4 Osmotic Effect of Solutions
Because plant cells have a cell wall not present within animal
cells, you will notice a couple of differences that plant cells
experience during extreme water gain and loss.
 Notice that an animal cell may burst (lyse) if too much water
enters. However, a plant cell has a cell wall that helps keep it
from bursting when placed in a similar solution. The pressure
that builds up within the plant cell during this process is
called turgor pressure, and the cell is said to be turgid.
 Notice than when a plant cell looses too much water, that its
cytoplasm will shrink away from its cell wall. This process is
known as plasmolysis. Animal cells do not have a cell wall and
do not undergo this process.
#2-4 Osmotic Effect of Solutions
These pictures illustrate red blood cells (RBC) exposed to various solutions.
Salt concentration within blood is 0.9% or 9 grams salt per liter blood.
#2-4 Osmotic Effect of Solutions
When given a problem concerning diffusion or osmosis, be able
to answer the following questions.
1. Does a concentration gradient exist?
2. Identify the solute within this problem.
3. Identify the solvent within this problem.
4. How would you classify the solution within the container
(cell’s outside environment)?
5. How would you classify the solution in the cell?
6. What direction will water molecules move / What is the
“NET” movement of water? (Be able to explain and draw an
arrow to indicate your answer.)
#2-5 Active Transport
Active transport is a method cells use to move materials
across a cell membrane, in order to achieve homeostasis.
Question
Answer
Energy required?
Yes
Why does it occur?
Cell is trying to reach homeostasis
Movement of molecules?
From low concentration to high concentration
AKA:
Moving up the concentration gradient
When does it stop?
When cell reaches homeostasis (NOT when molecules
are in equilibrium)
3 Examples
Endocytosis, Exocytosis, and the Sodium Potassium
Pump
#2-5 Active Transport
Endocytosis
 Particles too large to
enter a cell by other
methods are
transported within a
vesicle into the cell
through endocytosis.
#2-5 Active Transport
Exocytosis
Sodium Potassium Pumps
 A substance is secreted
 Removes 3 sodium ion out of
from a vesicle out of the
cell
a cell while bring 2 potassium
ion within a cell
#2-6 Factors That Affect Cellular
Activities
Characteristic of a Cell
Relationship to Cell
Processes
Cell Size
_______________ proportional to rate
of diffusion
Temperature
_______________ proportional to cell
processes
Concentration
The concentration of a substance will
alter the rate or a reaction. It may speed it
up or slow it down.
Amount of Enzymes Present
_______________ proportional to the
rate of a reaction
pH level
Certain compounds have an optimal pH
range.
#2-6 Factors That Affect Cellular
Activities
Characteristic of a Cell
Relationship to Cell
Processes
Cell Size
_Inversely _ proportional to rate of
diffusion
Temperature
__Directly__ proportional to cell
processes
Concentration
The concentration of a substance will
alter the rate or a reaction. It may speed it
up or slow it down.
Amount of Enzymes Present
__Directly__ proportional to the rate of
a reaction
pH level
Certain compounds have an optimal pH
range.
#2-7 Endothermic vs. Exothermic
Reactions
Endothermic Reactions
 absorbs energy in order
to proceed
 environmental
temperature drops
 examples: cold pack,
vinegar + baking soda
#2-7 Endothermic vs. Exothermic
Reactions
Exothermic Reactions
 release energy in the
form of heat or light
 environmental
temperature increases
 examples: hot-hands
#2-8 pH
 pH is a measurement of percent of hydrogen ions within a solution
 pH can be measured with an indicator (something that changes




color in the presence of various pH values) such as a dip stick,
litmus paper, cabbage juice, and even some flower colors.
Scale ranges from 0-14.
pH level below 7 are considered acidic.
pH levels around 7 are considered neutral.
pH levels above 7 are considered basic/alkaline.
#2-8 pH
Acids
Bases
 Electrolytes that release
 Electrolytes that release





hydrogen ions (H+) in water
Strong acids can be corrosive
Edible acids are often sour
Turns litmus paper red
Reacts with bases to form
water and a salt
Below 7 on the pH scale





hydroxide ions (OH-) in
water
Strong bases can be corrosive
Edible bases are often bitter
May be slippery
Reacts with acids to form
water and a salt
Above 7 on the pH scale
#2-9 Enzyme Action
Enzymes
 special proteins
 often end in –ase
 act on a certain substrate
 attach at an active site
 speed up chemical reactions
 examples: amylase, catalase
#2-9 Enzyme Action
 Enzyme and Substrate “Induced Fit” Theory Illustrated Below
#2-9 Enzyme Action
Catalyzed Reaction is
represented by dotted line.
Un-catalyzed reaction is
represented by solid line.
#2-10 Organic Compounds
 Organic compounds contain carbon and hydrogen.
 Organic compounds are generally referred to as non-
electrolytes.
 4 Main Types of Organic Compounds:
1. Carbohydrates
2. Lipids
3. Proteins
4. Nucleic Acids
#2-10 Organic Compounds
Carbohydrates
Lipids
 Chains of sugars
 “fats”
 Major sources of energy
 Found within cell
 Have C, H, and O in
membranes
 Store energy for cellular
activities
 Do not mix with water
(insoluble in water)
 Example: fats, oils
proportions 1:2:1
 Example: glucose
#2-10 Organic Compounds
Proteins
Nucleic Acids
 Made up of amino acids
 Contain genetic
 Enzymes are specialized
information
 Example: DNA and RNA
proteins.