UNIT 4: HOMEOSTASIS
AND CELL TRANSPORT
BELL RINGER
• A driver traveling through the Lehigh Tunnel decided to hold their breath through
the tunnel as a “game” and ended up passing out and causing an accident. Write
down why this person passed out and what was their body “telling” them.
LEQ – EXPLAIN MECHANISMS THAT PERMIT
ORGANISMS TO MAINTAIN BIOLOGICAL
BALANCE BETWEEN THEIR INTERNAL AND
EXTERNAL ENVIRONMENTS
I. HOMEOSTASIS MECHANISMS
• A. Homeostasis – process by which organisms keep internal conditions relatively
constant despite changes in external environment
• 1. Requires constant communication among all organs systems
• 2. Homeostasis examples:
• a. Thermoregulation
• b. Water regulation (Osmoregulation)
• c. Oxygen regulation
• 3. Occurs when the concentration of molecules of a substance is the same across a space
• 4. Works to control what enters and exits the cell
I. HOMEOSTASIS MECHANISMS CONTINUED
• B. In order to keep all parts of the organism functional and working together, an organism
must be able keep internal conditions constant despite external pressures.
• 1. Examples of processes that help organisms maintain homeostasis
• a. Thermoregulation
• b. Oxygen regulation
• c. Proper water balance (Osmoregulation)
• C. Organisms accomplish these tasks by various feedback
mechanisms
• 1. Positive Feedback Mechanisms
• 2. Negative Feedback Mechanisms
II. POSITIVE FEEDBACK MECHANISM
• A. Positive feedback mechanism
• 1. Interaction that increases or amplifies the response of the system in which
it is incorporated
• B. Example: Coagulation (blood clotting)
• 1. Damage to a vessel wall will signal for the release of substances that
trigger the activation of blood platelets. As platelets begin to accumulate
around the damaged portion, they send chemical signals to activate more
platelets resulting in the formation of a clot, which acts as a temporary plug
until the vessel tissue can be repaired.
• C. Example: Child Labor
• Oxytocin released as uterus stretches  this causes more release of oxytocin
• Oxytocin stopped once baby is delivered  no more stretching uterus
III. NEGATIVE FEEDBACK MECHANISM
• A. Negative feedback mechanism
• 1. Interaction that reduces or dampens the response
of the system in which it is incorporated.
• B. Example: Insulin production
• C. Feedback Loops
• 1. Negative feedback loops  more common
mechanisms in maintaining homeostasis
• a. Responses in which a system works against change
• 2. Positive feedback loops  reinforce changes to a
system
QUICK REVIEW: WRITE YOUR RESPONSE
ON THE SLIDE
• Differentiate between positive and negative feedback mechanisms. Give an example
of each within your body.
IV. HOMEOSTASIS MECHANISMS:
THERMOREGULATION EXAMPLE
• A. Cells function properly in a narrow range of
temperatures
• B. Body temperature drops below 37 degrees Celsius 
nerve cells send signal to hypothalamus  signals cells
throughout body to speed up activities
• 1. Activity increase produces heat (SHIVERING)
• 2. Body goes back to normal temperature  nerve cells
inhibit hypothalamus from producing more chemicals
• C. Body temperature above 37 degrees Celsius 
hypothalamus slows down cell activities to reduce heat
(SWEATING)
• D. EXTREME HEAT/COLD  Homeostasis disturbed,
cells die-off, organ damage, and/or death
V. HOMEOSTASIS MECHANISMS:
OSMOREGULATION AND SOLUTE
CONCENTRATION
• A. Cell membrane – key in maintaining homeostasis because it regulates materials
entering/leaving
• B. Osmoregulation Example
• 1. Every cell needs water to carry out body’s processes (these take place in water)
• 2. Water lost through sweat, urine, etc.  need to drink water to replace water lost while
maintaining homeostasis
• C. Solute Concentration Example
• 1. Human cells must have a lower sodium ion concentration and a higher potassium ion
concentration than the blood.
• a. Cells have membrane pumps - move sodium out of the cell and move potassium into it
• b. Steady concentration of other solutes in the blood must also be maintained
• c. When solutes become imbalanced, diseases occur
• 2. Diabetes, for example, is the result of large fluctuations of blood glucose levels
VI. HOMEOSTASIS MECHANISMS: PH AND
OXYGEN REGULATION
• A. pH Balance and Oxygen Regulation
• 1. Cells rely on enzymes to perform many essential tasks.
• a. Enzymes can function only in a narrow range of temperatures and pHs.
• b. Necessary for the pH of the blood and the cytoplasm to remain constant at about a pH of 7.4.
• 2. If the pH rises due to illness or elevated carbon dioxide levels, the body increases the rate of
respiration.
• a. Removes excess carbon dioxide from the blood  pH balance restored
• 3. Some health conditions can cause the blood pH to be outside the normal range
• a. People with emphysema have lung damage  can no longer expel carbon dioxide normally  blood
pH is lower than normal  cells die-off
QUICK REVIEW: WRITE YOUR RESPONSE
ON THE SLIDE
• It is 100 degrees Fahrenheit outside and you just finished running 3 miles (you’re so
dedicated to track). All of a sudden you black out and drop to the floor. Someone sees
you and runs over with a glass of water and a cold towel. Which homeostasis
mechanisms are they trying to maintain within your body?
BELL RINGER
• How do U.S. borders filter people and vehicles coming in and out of the U.S.? If the
borders were taken down, what effects would this have on the U.S. and towns like near
the border?
LEQ – IDENTIFY AND DESCRIBE THE CELL
STRUCTURES INVOLVED IN TRANSPORT OF
MATERIALS INTO, OUT OF, AND
THROUGHOUT A CELL.
I. HOMEOSTASIS: FLUID MOSAIC MODEL
• A. Cell structures involved in transporting
molecules in, out, and through a cell
• 1. Fluid mosaic model
• a. Phospholipids
• 1.Polar hydrophilic phosphate head and 2 nonpolar hydrophobic
fatty acid tails
• 2. Line up in double layer with heads on outside and tails on inside
 Phospholipid Bilayer
• b. Cholesterol- small molecules between lipids
• 1. Make membrane firmer and prevent freezing at low T’s
• c. Membrane proteins- various types depending on
functions
• 1. Types:
• A. Transport materials
• B. Identify cell type
• C. Receptors to recognize substances
• D. Enzymes to regulate reactions
II. HOMEOSTASIS: CELL
STRUCTURES
• A. Proteins for transport
• 1. Made by ribosomes in Rough ER
• 2. Placed in sacs called vesicles
• a. Vesicles transported to Golgi where they are moved along and modified
• b. Vesicles move to cell membrane and release contents OR… move vesicle to another part
of cell where protein is needed for the cell to function properly
• B. Golgi apparatus and ER help maintain cellular homeostasis by assisting with
the transport of needed macromolecules
• C. In order to maintain homeostasis, cells must be able to sort and send
substances to the correct location
• 1. Golgi apparatus is key in sorting and packaging macromolecules, particularly
proteins
• 2. ER is the initial location of protein transport
III. HOMEOSTASIS: CELL MEMBRANE
PERMEABILITY
• A. Permeability – allowing a substance to pass through a membrane
• B. Cell membrane: Selectively permeable
• 1. Only allowing some substances to pass through a membrane
• 2. Based on:
• a. Size
• b. Shape
• c. Type
• 3. Important to maintain homeostasis!
QUICK REVIEW: WRITE YOUR RESPONSE
ON THE SLIDE
• List 4 cell structures within the cell membrane that allow it to maintain homeostasis.
• Why is it important for cell to be selectively permeable?
BELL RINGER
• It’s a cool day in the mid of winter and your mom decides to bake cookies to get the
house heated more. However, you’re in your room study for your next big biology test
(you’re so dedicated and look how happy you are!). Your room is down the hall far
from the kitchen. Soon, while studying so studiously something grabs your sniffer’s
attention. Write down what does it smell like and how did it get to your room.
LEQ – WHAT IS PASSIVE TRANSPORT AND
WHAT TYPES OF SUBSTANCES CAN BE
TRANSPORTED USING THE VARIOUS TYPES
OF PASSIVE TRANSPORT?
I. PASSIVE TRANSPORT
• A. Passive Transport
• 1. Substances move across membrane with NO CHEMICAL ENERGY REQUIRED
• 2. Occurs spontaneously
• 3. Examples:
• a. Diffusion
• b. Osmosis
• c. Facilitated diffusion
• d. Electrolytes
• e. Ion Channels
II. PASSIVE TRANSPORT – DIFFUSION AND
OSMOSIS
• A. Diffusion
• 1. Movement from high to low concentration
• 2. Concentration gradient
• a. Difference in concentration of a substance across a
space
• B. Osmosis – diffusion of water from high to low
concentration
• 1. Some aquatic organisms have contractile vacuoles
to pump out excess water that enters via osmosis
III. OSMOSIS: HYPERTONIC SOLUTIONS
• A. Hypertonic - Salt concentration inside the
cells is lower than the salt concentration outside
the cells.
• 1. Virtually all of the water in the cells will move
out of the cells into the surrounding solution, and
the cells will shrivel
• 2. Water will diffuse out of the cell
• B. Plasmolysis – plant cells shrink
• C. Crenation – animal cells shrink
IV. OSMOSIS: HYPOTONIC SOLUTIONS
• A. Hypotonic - Salt concentration inside the cell is
higher than the salt concentration outside the cell.
Water rushes into the cells and the cells can burst.
• 1. Water moves into the cell
• B. Cytolysis – animal cells
• 1. Bad  cells explode
• B. Turgid – plant cell
• 1. Helps maintain homeostasis – shape and structure
• C. Protists in water – contractile vacuole to pump out
excess water
V. OSMOSIS: ISOTONIC SOLUTIONS
• A. Isotonic - Salt concentration inside the
cells is the same as the salt concentration
outside the cells. (“iso” = same)
• 1. No net movement of water and salt
• B. Flaccid – plant cells
• 1. Can be fatal
• C. Animal cells – allows them to maintain
homeostasis
• D. Red blood cells (RBCs)  0.9% saline
(salt)
QUICK REVIEW: NEXT TO THIS IMAGE WRITE
DOWN THE TYPE OF SOLUTION THIS RBC IS IN.
ALSO DRAW A DIRECTIONAL ARROW SHOWING
WATER MOVEMENT.
QUICK REVIEW: NEXT TO THIS IMAGE WRITE
DOWN THE TYPE OF SOLUTION THIS RBC IS IN.
ALSO DRAW A DIRECTIONAL ARROW SHOWING
WATER MOVEMENT.
QUICK REVIEW: NEXT TO THIS IMAGE WRITE
DOWN THE TYPE OF SOLUTION THIS RBC IS IN.
ALSO DRAW A DIRECTIONAL ARROW SHOWING
WATER MOVEMENT.
DIFFUSION AND OSMOSIS REVIEW
VI. PASSIVE TRANSPORT: FACILITATED
DIFFUSION AND ION CHANNELS
• A. Facilitated Diffusion
• 1. Molecule binds to a carrier protein
• a. Carrier protein then changes its shape and transports the molecule down
its concentration gradient to the other side of the membrane
• 2. Molecules are too large or not soluble in lipids
• 3. Ex. Glucose and Insulin
• B. Ion Channels
• 1. Proteins, or groups of proteins, provide small passageways across the
cell membrane for specific ions to diffuse through
• 2. Solutes enter on “as needed” basis
• 3. Used for ions such as Na, K, Cl, and Ca
• a. Ion channel is SPECIFIC for that type of ion (i.e. Only Na can go through Na
ion channel)
• 4. Ex. Nerve Cells – use ion channels to generate a current (nerve
impulse)
DIFFUSION VS. FACILITATED DIFFUSION
QUICK REVIEW: WRITE YOUR RESPONSES
ON YOUR SLIDE
• List 3 types of transport required by the cell to maintain homeostasis.
• Name one molecule that doesn’t require any assistance moving into/out of a cell.
• Differentiate between facilitated diffusion and ion channels.
BELL RINGER
• Explain why a lawn chair would be easier to move through the door in this classroom
compared to a California king-sized bed.
LEQ – WHAT IS ACTIVE TRANSPORT AND
WHAT TYPES OF SUBSTANCES CAN BE
TRANSPORTED USING THE VARIOUS TYPES
OF PASSIVE TRANSPORT?
I. HOMEOSTASIS: ACTIVE TRANSPORT
• A. Active Transport - movement of molecules across the cell membrane from areas of
low to high concentration that REQUIRES CHEMICAL ENERGY (ATP)
• B. Examples:
• 1. Sodium-Potassium Pump (Na/K Pump)
• a. Pump made from carrier proteins
• 2. Endocytosis
• a. Pinocytosis
• b. Phagocytosis
• 3. Exocytosis
I. HOMEOSTASIS: ACTIVE TRANSPORT
II. ACTIVE TRANSPORT: SODIUM-POTASSIUM
PUMP
• A. Sodium-Potassium Pump (Na/K
Pump)
• 1. 3 Na+ ions leave and 2 K+ ions
enter AGAINST concentration
gradient
• 2. ATP supplies the energy needed
• 3. Carrier proteins - used as pump
• 4. Molecules cannot pass freely due
to charge and size
III. ACTIVE TRANSPORT: ENDOCYTOSIS
• A. Endocytosis
• 1. Cells ingest external materials by folding around them and forming a pouch
• 2. Pouch then pinches off and becomes a membrane-bound organelle called a vesicle
• 3. Allows large molecules to enter cell without passing through cell membrane
• 4. Phagocytosis
• a. Solid particles engulfed in vesicle
• b. Ex. White blood cells
• 5. Pinocytosis
• a. Liquid particles in vesicle
• b. Ex. Electrolytes entering cell
• 6. Lysosomes – bind with incoming food vacuoles and digest food
• a. Also release enzymes into vacuoles that have trapped bacteria
• b. Engulf damaged organelles, digest them, and release them from cell (endo/exocytosis)
IV. ACTIVE TRANSPORT: EXOCYTOSIS
• A. Exocytosis
• 1. Vesicles made by cell fuse with cell
membrane, releasing their contents into
external environment
• 2. Allows large molecules to exit the cell
without pass through the cell membrane
• 3. Uses vacuoles and vesicles
• a. Proteins made in ribosomes are
released this way after they travel to the
Golgi apparatus from the rough and
smooth ER  allows for packing into
vesicles
• b. Contractile vacuoles
• ENDOCYTOSIS
• EXOCYTOSIS
QUICK REVIEW: WRITE YOUR RESPONSES
ON YOUR SLIDE
• Differentiate between endocytosis and exocytosis.
• How are vesicles used in both processes?
• Differentiate between phagocytosis and pinocytosis.
ACTIVE VS. PASSIVE TRANSPORT REVIEW
QUICK REVIEW: WRITE YOUR RESPONSES
ON YOUR SLIDE
• Write down one main difference between active and passive transport.
• What type of proteins are used as the Na/K pump? Why is this pump needed for Na
and K ions?