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Cell Boundaries
Essential Questions
• What are the main functions of the cell
membrane and the cell wall?
• What happens during diffusion?
• What is osmosis?
• The CELL MEMBRANE is a thin, flexible barrier
around a cell that regulates materials entering
and leaving the cell; also provides protection
and support
• Nearly all cell membranes are composed of a
PHOSPHOLIPID BILAYER-two layers of lipids
with phosphate heads pointing away from the
membrane
Fluid-Mosiac Model
Outside
of cell
Carbohydrate
chains
Proteins
Cell
membrane
Inside
of cell
(cytoplasm)
Protein
channel
Lipid bilayer
• Phospholipid bilayer is made of a hydrophilic
(“water loving”) head and a hydrophobic
(“water fearing”) tail
• Cells are surrounded by extracellular fluid and
have cytoplasm inside (both watery solutions)
• Heads are outside and tails are inside
Phospholipid Bilayer
• CELL WALLS, present in plants, algae, fungi,
bacteria, surround the cell membrane and
provide support and protection for the cell; in
plants, typically composed of CELLULOSE
• Recall that a SOLUTION is a mixture of two or
more substances; two parts-SOLUTE and
SOLVENT. The CONCENTRATION of a solution
is the amount of solute in a given amount of
solvent
• Example: salt water; salt is the solute, water
is the solvent
• Because particles in a solution are in constant,
random motion, a solute will undergo
DIFFUSION-movement of solute particles from
area of high concentration to an area of lower
concentration; diffusion continues until it
reaches EQUILIBRIUM-concentrations are the
same throughout
• If a solute is at different concentrations on
either side of a cell membrane, the solute will
diffuse from the higher to lower concentration
side until equilibrium is achieved
Glucose
molecules
High
Concentration
Cell
Membrane
Low
Concentration
Protein
channel
• Because diffusion relies on random particle
motion, diffusion across a membrane does not
require energy
• Note that even after equilibrium is reached,
particles continue to move back and forth
across the membrane, but at equal rates
• A PERMEABLE MEMBRANE allows materials
to pass through;
• IMPERMEABLE MEMBRANES do not
• SEMIPERMEABLE (SELECTIVELY PERMEABLE)
MEMBRANES allow some materials to pass,
others not
Paper towel demo
Osmosis
• Water usually passes through most membranes,
while many solute particles cannot; OSMOSIS is
the diffusion of water through a selectively
permeable membrane
• If a membrane is water-permeable, but
impermeable to a solute at different
concentrations on either side of the membrane,
WATER DIFFUSES TO THE HIGHER
CONCENTRATION SIDE UNTIL EQUILIBRIUM IS
ACHIEVED
OSMOSIS
• Water will move across a membrane until
equilibrium is reached – isotonic “same
strength”
• When comparing 2 solutions, the solution with
the greater concentration of solutes –
Hypertonic “above strength”
• When comparing 2 solutions, the solution with
the lesser concentration of solutes –Hypotonic
“below strength”
• Draw Chart p. 186
Homework Questions
1. Describe the functions of the cell membrane
and cell wall.
2. What happens during diffusion?
3. Describe how water moves during osmosis.
4. What is the basic structure of a cell
membrane?
Osmotic Pressure
• As water diffuses toward the hypertonic side
of a membrane, this produces OSMOTIC
PRESSURE acting on the hypertonic side of
the membrane
• Cells have salts, sugars, proteins, and other
solutes dissolved in the cytoplasm, making
the inside of cells hypertonic to fresh/distilled
water; if not checked, water entering the cell
can make it swell to the point of
bursting
• Some molecules (glucose, proteins) appear to
be too large to diffuse through the cell
membrane, but still move across it through
FACILITATED DIFFUSION-diffusion of materials
through a PROTEIN CHANNEL imbedded in
the cell membrane
• Facilitated diffusion does not require cellular
energy
• A CONCENTRATION GRADIENT occurs when
there is an area at high concentration near an
area of lower concentration. Solutes diffuse
WITH THE CONCENTRATION GRADIENT-from
high to low concentration
• Sometimes a cell must move solute against
the concentration gradient-from lower to
higher concentration; this is done through
ACTIVE TRANSPORT-movement of solute
against conc. gradient via transport proteins,
which require energy
Molecule to
be carried
Energy
Molecule being
carried
• Larger molecules and solid clumps of material
may be transported by movements of the cell
membrane
1. ENDOCYTOSIS-taking material into cell by cell
membrane folding around material, pinching off
inside cell to form vacuole
Ex. PHAGOCYTOSIS-cell membrane engulfs
particle, pinches off into a vacuole;
PINOCYTOSIS-cell membrane forms pockets that
fill with liquid and pinch off into vacuoles
2. EXOCYTOSIS-membrane of vacuole merges
w/cell membrane, opens to outside, releasing
contents
Ex. contractile vacuole