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Warm up
Complete the sentence stem:
Things go in and out of the cell
by……
Cells and Their Environment
Chapter 4 Section 1
The Plasma Membrane -
Gateway to the Cell
TAKS
• (4) Science concepts. The student knows that
cells are the basic structures of all living things
and have specialized parts that perform specific
functions, and that viruses are different from cells
and have different properties and functions. The
student is expected to:
– (B) investigate and identify cellular processes
including homeostasis, permeability, energy
production, transportation of molecules, disposal of
wastes, function of cellular parts, and synthesis of new
molecules.
Objectives
• Relate concentration gradients, diffusion,
and equilibrium.
• Predict the direction of water movement
into and out of cells.
• Describe the importance of ion channels in
passive transport.
Key Terms
• Passive Transport
• Concentration
Gradient
• Equilibrium
• Diffusion
• Osmosis
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Hypertonic Solution
Hypotonic Solution
Isotonic Solution
Ion Channel
Carrier Protein
Facilitated Diffusion
Structure of the Cell
Membrane
Cell Membranes
• Cell membranes help organisms maintain
homeostasis by controlling what substances
may enter or leave cells.
• To stay alive, a cell must exchange
materials such as food, water, & wastes
with its environment.
• These materials must cross the cell or
plasma membrane.
Cell Membrane
Cell Membrane
• Small molecules like
water, oxygen, &
carbon dioxide can
move in and out
freely.
• Large molecules like
proteins &
carbohydrates cannot
move easily across the
plasma membrane.
Cell Membrane
• The Cell Membrane is semi-permeable or
selectively permeable only allowing certain
molecules to pass through.
FLUID MOSAIC MODEL
FLUID- because individual phospholipids and proteins
can move around freely within the layer, like it’s a
liquid.
MOSAIC- because of the pattern produced by the
scattered protein molecules when the membrane is
viewed from above.
The cell membrane is made of
2 layers of phospholipids
called the lipid bilayer Hydrophobic molecules pass easily;
hydrophilic DO NOT
Ions, hydrophilic molecules larger than
water, and large molecules such as proteins
do not move through the membrane on their
own.
Concentration
• The mass of solute in a given volume of
solution or mass/volume.
• Concentration Gradient is a difference in
the concentration of a substance across a
distance
Equilibrium
• Is a condition in which the concentration of
a substance is equal throughout a space.
Passive Transport
• Some substances can cross the cell membrane
without any input of energy by the cell.
• Diffusion: The movement of a substances from an
area of high to an area of low concentration cause
by the random motion of particles across the
membrane is known as passive transport.
• If diffusiion is allowed to continue cytolysis(cell
bursts) eventually results.
3 Types of Passive Transport
• 1. Diffusion (simple) is the movement of
molecules from an area of higher concentration to
an area of lower concentration.
• Small molecules can pass through the cell
membrane by diffusion
• Diffusion across a membrane is a type of passive
transport because it does not require energy.
Diffusion
Solute moves DOWN concentration gradient (HIGH to
LOW)
Diffusion
• Diffusion is driven by the kinetic energy of
the molecules.
• Kinetic energy keeps molecules in constant
motion causing the molecules to move
randomly away from each other in a liquid
or a gas.
The Rate of Diffusion
• The rate of diffusion depends on temperature, size
of the molecules, & type of molecules diffusing
• Molecules diffuse faster at higher temperatures
than at lower temperatures
• Smaller molecules diffuse faster than larger
molecules
• Most short-distance transport of materials into &
out of cells occurs by diffusion
Diffusion & the Concentration
Gradient
• Diffusion always occurs down a concentration
gradient (water moves from an area where it is
more concentrated to an area where it is less
concentrated)
• Diffusion continues until the concentration of the
molecules is the same on both sides of a
membrane
• When a concentration gradient no longer exists,
equilibrium has been reached but molecules will
continue to move equally back & forth across a
membrane
2. Osmosis
• The diffusion of water across a semipermeable membrane is called osmosis.
• Diffusion occurs from an area of high water
concentration (less solute) to an area of
lower water concentration (more solute)
• Movement of water is down its
concentration gradient & doesn’t require
extra energy.
Water
• Cytoplasm is mostly water containing dissolved
solutes
• Concentrated solutions have many solute
molecules & fewer water molecules
• Water moves from areas of low solute
concentration to areas of high solute concentration
• Water molecules will cross membranes until the
concentrations of water & solutes is equal on both
sides of the membrane; called equilibrium
• At equilibrium, molecules continue to move
across membranes evenly so there is no net
movement
Osmosis
• Diffusion of water
across a membrane
• Moves from HIGH
water potential (low
solute) to LOW
water potential (high
solute)
Diffusion of H2O Across A
Membrane
High H2O potential
Low solute concentration
Low H2O potential
High solute concentration
Hypertonic Solutions
• Solute concentration
outside the cell is higher
(less water)
• Water diffuses out of the
cell until equilibrium is
reached
• Cells will shrink & die if
too much water is lost
• Plant cells become
flaccid (wilt); called
plasmolysis
Hypotonic Solutions
• Solute concentration
greater inside the cell
(less water)
• Water moves into the
cell until equilibrium
is reached
Hypotonic Solutions
• Animal cells swell & burst if they take in
too much water
• Plant cells become turgid due to water
pressing outward against cell wall
• Turgor pressure in plant cells helps them
keep their shape
• Plant cells do best in hypotonic solutions
Isotonic Solutions
• Concentration of
solutes same inside &
outside the cell.
• Water moves into &
out of cell at an equal
rate so there is no net
movement of water.
• Animal cells do best in
isotonic solutions.
hypotonic
hypertonic
isotonic
hypertonic
isotonic
hypotonic
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3. Facilitated Diffusion
• Faster than simple diffusion
• Considered passive transport because extra energy
not used
• Occurs down a concentration gradient
• Involves carrier proteins embedded in a cell’s
membrane to help move across certain solutes
such as glucose
• Carrier molecules change shape when solute
attaches to them
• Change in carrier protein shape helps move solute
across the membrane
Facilitated Diffusion
Molecules will randomly move through the
pores in Channel Proteins.
copyright cmassengale
Facilitated Diffusion
• Some Carrier
proteins do not
extend through the
membrane.
• They bond and drag
molecules through
the lipid bilayer and
release them on the
opposite side.
Carrier Proteins
• Carrier proteins in the cell membrane form tunnels
across the membrane to move materials
• Channel proteins may always be open or have
gates that open & close to control the movement
of materials; called gated channels
• Gates open & close in response to concentration
inside & outside the cell
• Ion Channel is a transport protein with a polar
pore through which ions can pass.
Carrier Proteins
Facilitated Diffusion
Active Transport
• Requires the use of ATP or energy.
• Moves materials against their concentration
gradient from an area of lower to higher
concentration.
• May also involve membrane proteins
• Used to move ions such as Na+, Ca+, and K+
across the cell membrane.
Objectives
• Compare active transport with passive
transport.
• Describe the importance of the sodiumpotassium pump.
• Distinguish between endocytosis and
exocytosis.
Key Terms
• Active Transport
• Sodium-Potassium
Pump
• Endocytosis
• Exocytosis
• Receptor Protein
• Second Messenger
Active Transport
• Plants use active transport to help roots
absorb nutrients from the soil (plant
nutrients are more concentrated inside the
root than outside)
• The movement of a substance across the
cell membrane against its concentration
gradient.
Types of Active Transport
1. Sodium-Potassium Pump
• Sodium-Potassium pump moves 3 Na+ out
for every 2 K+ into the cell
– Causes a difference in charge inside and outside
the cell
– Difference in charge is called membrane
potential
• Ion pumps help muscle & nerve cells work
• Figure 5 pg. 82
2. Bulk Transport
• Moves large, complex molecules such as
proteins across the cell membrane
• Large molecules, food, or fluid droplets are
packaged in membrane-bound sacs called
vesicles
3. Endocytosis
• Endocytosis moves large particles into a
cell
Types of Endocytosis
a. Phagocytosis
• Phagocytosis is one type of endocytosis.
– Cell membrane extends out forming
pseudopods (fingerlike projections) that
surround the particle
Membrane pouch encloses the material &
pinches off inside the cell making a vesicle
– Vesicle can fuse with lysosomes (digestive
organelles) or release their contents in the
cytoplasm
– Known as "cell eating"
b. Pinocytosis
• Pinocytosis most common form of
endocytosis
– Cell membrane surrounds fluid droplets
– Fluids taken into membrane-bound vesicle
– Known as "cell drinking"
Example of Pinocytosis
pinocytic vesicles forming
mature transport vesicle
Transport across a capillary cell (blue).
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4. Exocytosis
• Exocytosis is used to remove large products
from the cell such as wastes, mucus, & cell
products
Exocytic vesicle
immediately after
fusion with plasma
membrane.