Cell Biology:
The Cell Membrane
Lesson 2 – Transport Across the Cell Membrane
(Inquiry into Life pg. 69-80)
Today’s Objectives
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Analyze the structure and function of the cell membrane,
including:
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Describe passive transport processes
Explain factors that affect the rate of diffusion across a cell
membrane
Predict the effects of hypertonic, isotonic, and hypotonic
environments on osmosis in animal cells
Describe active transport processes
Compare specific transport processes
Explain why cells divide when they reach a particular surface
area-to-volume ratio, including:
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Differentiate between cells that have a high or low surface areato-volume ratio
Demonstrate an understanding of the significance of surface areato-volume ratio in cell size
Transport across Cell Membranes
4 main ways:
 Passive Transport:
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1) Diffusion
2) Osmosis
3) Facilitated Transport
Active Transport:
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4) Active Transport
1) Diffusion (passive transport)
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Diffusion is the movement of a solute from an area of
high concentration to an area of low concentration until
evenly distributed
Solute is some solid particles or molecules suspended in
air or liquid
Example: a foul odor in the corner of a room will spread
out until it is evenly distributed
Example: cream in coffee will diffuse until concentrations
are balanced
1) Diffusion (passive transport)
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Diffusion refers to the process by
which molecules intermingle as a
result of their kinetic energy of
random motion
Consider two containers of gas A and
gas B separated by a partition; the
molecules of both gases are in
constant motion and make numerous
collisions with the partition
If the partition is removed as in the
illustration, the gases will mix
because of the random velocities of
their molecules
In time, a uniform mixture of A and B
molecules will be produced in the
container
2) Osmosis (passive transport)
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Osmosis is a special type of diffusion in which water
moves from an area of high water concentration to an
area of low water concentration across a membrane
2) Osmosis (passive transport)
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Pressure caused by the concentration gradient between
two different solutions is called osmotic pressure
Hydrostatic pressure (gravity) offsets osmotic pressure
The solute cannot spread out because it is too big to pass
through the membrane. Therefore, water moves across
the membrane from [high water] to [low water].
Water moves through the protein-lined pores of the cell
membrane
Water will move from side A to side B. Level on A
will fall. Level on B will rise. Until gravity stops it =
hydrostatic pressure)
3) Facilitated Transport (passive transport)
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In facilitated transport, solutes move across the
membrane from an area of [high solute] to [low
solute] with the help of a carrier protein
Example: molecules needed by the cell such as glucose
enter through facilitated transport from the blood
Examples of solutes: gases, molecules
4) Active Transport
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In active transport, solutes move from an area of [low
solute] to an area of [high solute] across a membrane with
the aid of a carrier protein
Example of solute: ions
Since the movement is against the concentration gradient,
energy is required (ATP)
Example: Sugar is removed from urine by active transport into
the blood. Since there is already a lot of glucose in the blood, it
is traveling against the concentration gradient
Example: Na/K Pump. Found in nerve and muscle cells. Carrier
protein changes shape to fit Na and K
4) Active Transport
Summary
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Diffusion
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Solute moves from [high solute] to [low solute]
No membrane required
No carrier protein required
No energy required
Osmosis
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[high water] to [low water]
Membrane required
No carrier protein required
No energy required
Summary
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Facilitated Transport
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[high solute] to [low
solute]
Membrane required
Carrier protein required
No energy required
Gas/molecule movement
Summary
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Active Transport
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[low solute] to [high
solute]
Membrane required
Carrier protein required
Energy required (ATP)
Ion movement
Factors affecting Diffusion
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Surface area vs.Volume
Cell
A
B
C
D
Surface Area Volume
LxWx6
6 units2
24 units2
96 units2
384 units2
S.A. :Volume Ratio
LxWxH
1 units3
8 units3
64 units3
512 units3
6:1
3:1
1.5:1
0.75:1
Nutrients (oxygen and glucose) enter a cell while wastes (carbon
dioxide and urea) exit a cell across the cell membrane.
The amount of cell membrane = surface area
Surface Area vs. Volume
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Inside the cell, organelles use up nutrients and produce
wastes
The larger the cell, the more nutrients needed and the
more waste produced
The size of the cell (# of organelles) = volume
Small cells have a high SA : Volume ratio
They can supply the organelles with plenty of nutrients
and remove wastes
Surface Area vs. Volume
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Large cells have a low SA : Volume ratio because volume
increases faster than surface area
If a cell gets too big, wastes will increase and nutrients
able to enter the cell will decrease
Therefore, cells are limited in size: active cells must be
smaller than less active cells because they need more
nutrients and produce more waste
Some cells can increase their surface area without
increasing volume by producing folds
Factors that will Increase Diffusion
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A) Concentration Gradient
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B) The size and shape of the molecules
C) Temperature
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The higher the temperature, the faster the rate of diffusion
D) Type of Medium
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The difference in concentration between two areas
The greater the [] gradient, the faster the rate of diffusion
Rate of diffusion is faster in air than in liquid
E) Movement of the Medium
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Water or air currents increase the rate of diffusion
Example: stirring while adding cream to coffee
Endocytosis and Exocytosis
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Endocytosis:
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Process in which large materials enter a cell
Vesicles form as a way to transport molecules into a cell
Two types: phagocytosis and pinocytosis
Phagocytosis
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A type of endocytosis in which whole cells, bacteria, or
cell fragments are taken into the cell
Referred to as “cell eating”
Transports very large pieces
Pinocytosis
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A type of endocytosis in which molecules such as
proteins are taken into the cell
Referred to as “cell drinking”
Transports smaller “large” pieces and liquids
Exocytosis
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A process by which products or wastes exit a cell
Reverse of endocytosis
Exocytosis
Hypertonic, Hypotonic, Isotonic Solutions
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Use your textbook to find the following definitions:
Hypertonic solution:
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Hypotonic solution:
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Solution with a higher concentration of solute than its surroundings
If a cell is placed in a hypertonic solution, water will rush out of the cell in order
to balance the concentration of solute causing the cell to shrivel
Solution with a lower concentration of solute than its surroundings
If a cell is placed in a hypotonic solution, water will rush into the cell in order to
balance the concentration of solute causing the cell to swell and possibly burst
Isotonic solution:
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Solution in which the osmotic concentration is the same as the solute
concentration of its surroundings
If a cell is placed in a isotonic solution, water diffuses into and out of the cell at
the same rate
Hypertonic, Hypotonic, Isotonic Solutions
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Water will move from a hypotonic solution to a
hypertonic solution
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No net movement of water between isotonic solutions
Tonicity
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Tonicity refers to concentration of solute in a solution
Below are the effects of tonicity on animal cells
RED BLOOD CELLS IN DIFFERENT SOLUTIONS
NORMAL RED
BLOOD CELLS
RBC PLACED
IN A
HYPERTONIC
SOLUTION
RBC PLACED
IN A
HYPOTONIC
SOLUTION
Tonicity Experiment – Thistle Tube
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Inside tube is hypertonic, outside tube is hypotonic
Water moves from [high water] (hypotonic) to [low
water] (hypertonic)