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HOMEOSTASIS and TRANSPORT
A4
A4. Homeostasis and Transport
1. Describe how the structure of the plasma membrane
allows it to function as a regulatory structure and/or
protective barrier for a cell.
2. Compare the mechanisms that transport materials
across the plasma membrane.
3. Explain how organisms maintain homeostasis.
4. Describe how membrane-bound organelles facilitate
the transport of materials within a cell.
A4. Vocabulary
plasma membrane
phospholipid
selectively permeable
protein pump
diffusion
active transport
endocytosis
isotonic
hypertonic
hypotonic
lipid bilayer
impermeable
protein channel
osmosis
facilitated diffusion
passive transport
exocytosis
equilibrium
concentration gradient
homeostasis
Cell Membrane Structure and function
• Cell membranes are composed of phospholipids
• Phospholipids are arranged in two layers (bilayer)
• Each phospholipid has a “head” and two “tails”
– The head loves water (hydrophilic)
– The tails hate water (hydrophobic
• The lipid bilayer is flexible but strong
Cell Membrane Structure and Function
DRAW THIS
in your
NOTES!!
Cell Membrane Structure and Function
• The cell membrane acts as a fluid…
– It is not rigid
– Proteins and phospholipids can move around
• The cell membrane is a mosaic…
– Many different parts make up the cell membrane
Cell Membrane Structure and Function
• Barrier between the cytoplasm and the
environment
• Protects and supports the cell
• Controls everything that enters and leaves the
cell
Cell Membrane Structure and Function
Membranes are selectively permeable
• Some things can cross the membrane (permeable)
– Gases, water, small particles
• Some things can cross the membrane with the help
of a protein channel or pump
– Ions, proteins, large particles
• Some things can’t cross the membrane (impermeable)
Let’s Build a Model of a cell membrane!
• Cotton swabs represent phospholipids
• Other components are embedded within the
phospholipid bilayer
• Can you move the components back and
forth?
Can you do it?
• Give as many names for the boundary of the
cell as you can…
Mechanisms of Passive Transport
• requires NO energy
• Depends on a concentration gradient (difference)
– Concentration = relative amount of substance
• SUBSTANCE moves from HIGH concentration (of
substance) to LOW concentration (of substance)
• Some materials can pass freely, other materials
need a protein channel
Mechanisms of Passive Transport
Mechanisms of Passive Transport
• Passive transport
• Depends on a concentration gradient
– Concentration = relative amount of substance
– Gradient = difference
• MOVEMENT from HIGH concentration to LOW
concentration
– Down the concentration gradient
Mechanisms of Passive Transport
• Passive – requires NO energy
• Depends on a concentration gradient (difference)
– Concentration = relative amount of water
• MOVEMENT from HIGH concentration to LOW
concentration
• just like simple diffusion EXCEPT needs a protein
channel
Mechanisms of Passive Transport
• Diffusion of water
• WATER moves from HIGH concentration (of water) to
LOW concentration (of water)
• Aquaporins = protein channels for water to pass
• Plays a role in maintaining homeostasis (water balance)
Osmosis
Which way will the
water molecules
move?
Why wouldn’t the
green or purple
molecules move?
Mechanisms of Active Transport
• Requires energy
• Material flows AGAINST the concentration
gradient
– From LOW concentration to HIGH concentration
• Requires protein pumps (need energy to work)
Movement within a cell
• Use ATP energy to move or “pump” material
into an area that already has a high
concentration
• It’s like pumping air into a basketball
Movement within a cell
Movement within a cell
•
•
•
•
Used to transport very large molecules or food
Endocytosis = bringing materials into the cell
Exocytosis = taking materials out of the cell
Cell membrane/vesicles play a role
Movement within a cell
Homeostasis
• Maintaining stability in the cell or body
• Variables within the body can fluctuate
– Temperature, pH, water balance
• Important to maintain homeostasis near set point
• Cell membrane aids in maintaining homeostasis
Homeostasis
• Diffusion of water
• Passive (no energy required)
• Water will move across the membrane down its
concentration gradient
• Cell membrane regulates the amount of water
inside a cell
Homeostasis
• Amount of water inside = amount of water outside
• Water moves back and forth across the membrane
equally in both directions
• Equilibrium = equal concentrations of water on
both sides of the membrane
Homeostasis
• Amount of solute surrounding cell is high (high
solute = low water)
• Water moves out of the cell to balance
concentrations of water on both sides of
membrane
• Cell shrinks (dehydrates)
• EX: saltwater
Homeostasis
• Amount of solute surrounding cell is low (low
solute = high water)
• Water moves INTO the cell to balance
concentrations of water on both sides of
membrane
• Cell swells (animal cells could burst)
• EX: pure water
Homeostasis
Why would plant
cells be better
off than animal
cells?
What would
happen to the
cells of a
freshwater fish if
it was put into a
saltwater tank?
Cell Transport Review
• Divide your poster into six sections
• In each section, diagram one of the following
modes of transport
– Simple diffusion
– Facilitated diffusion
– Osmosis
– Active transport via protein pumps
– Exocytosis
– endocytosis
Keystone Questions
1. Carbon dioxide and oxygen are molecules that can
move freely across a plasma membrane. What
determines the direction that carbon dioxide and
oxygen move?
A.
B.
C.
D.
Orientation of cholesterol in the plasma membrane
Concentration gradient across the plasma membrane
Configuration of phospholipids in the plasma membrane
Location of receptors on the surface of the plasma
membrane
Keystone Questions
2. A sodium-potassium pump within a cell membrane requires
energy to move sodium and potassium ions out of a cell.
The movement of glucose into or out of a cell does not
require energy. Which statement best describes the
movement of these materials across a ac cell membrane?
A. Sodium and potassium ions move by active transport, and
glucose moves by osmosis
B. Sodium and potassium ions move by active transport and
glucose moves by facilitated diffusion.
C. Sodium and potassium ions move by facilitated diffusion and
glucose moves by osmosis
D. Sodium and potassium ions move by facilitated diffusion and
glucose moves by active transport
Keystone Questions
3. Which example is an activity that a fish most uses
to maintain homeostasis within its body?
A.
B.
C.
D.
Using camouflage to avoid predators
Feeding at night to regulate body temperature
Moving to deeper water to regulate metabolic wastes
Exchanging gases through its gills to regulate oxygen
levels.
Keystone Questions
4. Diffusion occurs because
A. Molecules are attracted to one another
B. Molecules constantly move and collide with one
another
C. Cellular energy forces molecules to collide with one
another
D. Cellular energy pumps molecules across the cell
membrane
Keystone Questions
5. During diffusion, when the concentration of
molecules on both sides of the membrane are
equal, molecules will
A. Move across the membrane to the outside of the cell
B. Stop moving across the membrane
C. Continue to move across the membrane in both
directions
D. Move across the membrane to the inside of the cell