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CHAPTER 5: CELL
FUNCTIONS
5.1 Objectives
• Explain how an equilibrium is established as a
result of diffusion
• Distinguish between diffusion an osmosis
• Explain how substances cross the cell membrane
through facilitated diffusion
• Explain how ion channels assist the diffusion of
ions across the cell membrane
Selectively permeable: Allows
some molecules in and keeps other
molecules out.
Types of Cellular Transport
PASSIVE TRANSPORT: Cell DOES NOT use energy
1. Diffusion
Weee!!
2. Facilitated Diffusion
3. Osmosis
high
low
ACTIVE TRANSPORT: Cell USES energy
1. Protein Pumps
2. Endocytosis
high
3. Exocytosis
This is
gonna be
hard
work!!
low
TYPES OF
PASSIVE
TRANSPORT
DIFFUSION:
Movement of molecules
from an area of high
concentration to an area
of low concentration
Movement WITH the
concentration gradient
High
http://www.youtube.com/watch?v=gXJMBgyT_hk
Low
Diffusion occurs until
EQUILIBRIUM is reached
** When the concentration of molecules is the same throughout a
space they occupy!
Factors - Affect Rate of Diffusion
Pressure
diffusion will be faster
Temperature
Concentration
diffusion will increase
more diffusion
Factors involved in moving things in & out of
a cell:
• Size of the particles
• Whether they dissolve in water or not
• Condition in and/or outside cell
• Structure of the cell membrane
Osmosis
• The diffusion of water across
a membrane.
Water moves:
HIGH
LOW
http://www.youtube.com/watch?v=GbudKs-49jo
Isotonic Solutions (Iso = Same)
• The concentration of water molecules is the
same inside and outside the cell.
• Water molecules will move across the membrane at
the same rate in both directions = NO NET
MOVEMENT!
Hypertonic Solutions
• High solute concentration/lower water
concentration outside of the cell
• Water will move OUT of the cell
• Cell shrivels
Hypotonic Solution
• Lower concentration of solute on outside of
cell when compared to the inside of cell
• Water will move IN the cell
• Cell will swell or possibly burst
What type of solution are these cells in?
A
Hypertonic
B
Isotonic
C
Hypotonic
Crossing the Cell Membrane
• Why are most molecules and polar molecules not
able to pass through the cell membrane?
• Unable to pass through the nonpolar interior layer
of the lipid bilayer.
• How do these molecules get into the cell?
• Transport proteins
• These channels provide polar passageways
Facilitated Diffusion
• Movement of molecules across a membrane with
an intact protein
• No energy required!
• Carrier proteins assist these molecules with
facilitated diffusion
Carrier Proteins
• A transport protein that can bind to a specific
substance on one side of the cell membrane, carry it
across and release it on the other side.
Steps of Facilitated Diffusion
1. A molecule binds to the carrier protein on either
side of the cell
2. The carrier protein changes shape and transports
the molecule across the cell membrane
3. The molecule is released on the other side of the
cell
http://www.youtube.com/watch?v=I4123hUU8xo&NR=1
Diffusion Through Ion Channels
• ION CHANNEL: Membrane protein complexes that
facilitate the diffusion of ions across cell membranes.
• Some are always open, others are gated.
• Still passive transport because ions are moving
down their concentration gradient.
Electrical Charge and Ion Transport
• The inside of a cell is most often negatively charged
• Positively charged ions are more likely to diffuse
into the cell
• Negatively charged ions are more likely to diffuse
out of the cell
5.1 Objectives REVIEW
• Explain how an equilibrium is established as a result of
diffusion
• Distinguish between diffusion an osmosis
• Explain how substances cross the cell membrane through
facilitated diffusion
• Explain how ion channels assist the diffusion of ions across
the cell membrane
5.2 Objectives
• Distinguish between passive transport and active transport
• Explain how the sodium-potassium pump operates
• Compare endocytosis and exocytosis
Active Transport
• The transport of a substance across the cell membrane
AGAINST THE CONCENTRATION GRADIENT.
• Uses carrier proteins
• This process is fueled by ATP
• Referred to as “pumps”
Sodium Potassium Pump
• Transports 3 sodium ions
(Na+) out of the cell and 2
potassium ions (K+) into the
cell.
• Sodium ions are more
concentrated outside of the
cell.
• Potassium is more
concentrated inside the cell.
• Transported against their
concentration gradient!
http://www.youtube.com/watch?v=GTHWig1vOnY
4 Steps of Sodium-Potassium Pump
1.
3 sodium ions (Na+) and a phosphate group (P) from ATP
bind to the pump.
2.
Pump changes shape, transporting the 3 sodium ions
across the cell membrane.
3.
2 potassium (K+) ions bind to the pump and are
transported across the cell membrane
4.
The Phosphate group and 2 potassium ions are released
inside the cell.
Why is it Important?
• Prevents sodium ions from accumulating in the cell
• Too much sodium causes water to enter cell and possibly
burst.
• HYPOTONICITY
• Helps maintain the concentration gradient across the cell
membrane.
• Helps in transporting other substances across the cell
membrane
Endocytosis
Taking bulky material into a cell
• Uses energy
• Cell membrane folds in around food particle
• “Cell eating”
• Forms food vacuole & digests food.
**This is how white blood cells eat bacteria!
Exocytosis
•When substances are released
from the cell through a vesicle.
• Fuses with the plasma
membrane to let the
substance out
•Used to release large
molecules like proteins, waste
products, and toxins.
3 ways a Cell Membrane Works
1. Cellular Identity: Proteins give cell its identity.
2. Electrical Charge: given electrical charge, proteins
in membrane will open or close.
• Ex: nerves
3. Chemicals: Open/close based on chemicals
present.
Receptor Proteins
• Proteins the bind to a specific signal molecule, enabling the
cell to respond to the molecule.
• Ex. Exercising muscles need receptor proteins to know
when to contract or relax.
• Work with signal molecules
• May create SECOND MESSENGERS: Acts as a signal
molecule in the cytoplasm.
Second Messengers
• Formed by the receptor protein.
• Acts as a signal molecule in the cytoplasm.
• Change the function of the cell in many ways:
• Activate enzymes.
• Trigger a series of chemical reactions.
• Open other ion channels.
Receptor Protein
• A protein that binds to a specific signal molecule, enabling
the cell to respond to the signal molecule.
Functions of Receptor Proteins
• Changes permeability of the receiving cell.
• Triggers formation of second messengers inside the cell.
• Acts as a signal molecule in the cytoplasm.
• Activate enzymes inside the cell.
Changes in Permeability
• Binding of signal molecules with receptor proteins can open
the ion channel.
• Allows specific ions to cross the cell membrane.
• Very important for nervous system.
Enzyme Action
• Receptor proteins may act as enzymes.
• This can speed up chemical reactions.
• Certain drugs block receptor proteins.
• This causes cells to act in different ways
5.2 Objectives REVIEW
• Distinguish between passive transport and active transport.
• Explain how the sodium-potassium pump operates.
• Compare endocytosis and exocytosis.