Active and Passive Transport
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Diffusion is the random movement of
molecules from an area of higher
concentration to an area of lower
concentration.
WHY? All molecules are in constant motion;
they move and collide as they spread out into
the available space.
Even though molecules are moving about
randomly in all directions, there is a net
movement of molecules from and area of higher
concentration to an area of lower concentration.
Question: What happens at equilibrium?
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Osmosis is the diffusion of water across a
membrane.
A concentration gradient is a difference in
concentration across a distance.
Molecules only diffuse down a concentration
gradient. This does not require any energy.
Moving molecules against a concentration
gradient requires energy.
Diffusion is a main mechanism for cells to
receive nutrients and remove wastes.
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Cell membranes are barriers to the diffusion
of some substances.
A selectively permeable membrane allows
some substances to cross the membrane
more easily than others, and blocks others.
Passive transport does not require any energy
to move molecules across a membrane.
◦ Examples: diffusion, osmosis, facilitated diffusion
Oxygen (O2), carbon dioxide (CO2) and water (H2O) can
diffuse easily across the cell membrane.
Glucose (C6H12O6), amino acids, and ions cannot diffuse
through membranes.
They need to use special protein channels called
transport proteins.
Transport proteins move substances down their
concentration gradient, which does not require any
energy.
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Sometimes cells need to move substances
against their concentration gradient.
This requires energy and is called active
transport
During active transport, a specific transport
protein pumps a solute across a membrane,
usually against the concentration gradient
◦ Examples: moving Sodium and Potassium ions in animal
cells; Nitrogen, phosphorus, potassium, and calcium in
plant cells.
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The mitochondria supplies the energy for active
transport.
Active Transport
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When salt solution was added to the outside
environment of red onion cells, the concentration
of water molecules was ___________________ in
the solution than in the cells.
Molecules move from a region of ___________
concentration to a region of _______________
concentration.
A cell model that contains a 10% sugar solution is
placed in a beaker with a 40% sugar solution.
Water molecules will _________ from the
__________ into the _____________.
The “cell model” contains a
higher concentration of solutes
than the external environment.
(The cell model is hypertonic
compared to the external
environment).
Even though there was random
movement of glucose molecules
both into and out of the cell
model, there was a net
movement of glucose molecules
from the inside of the cell
model to the external
environment.
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Lipids (fats and oils) are
non-polar, which means
they are hydrophobic.
The polar heads of a
phospholipid associate
themselves with water
while the nonpolar (fatty
acid) tails arrange
themselves to exclude
water.
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1. Sketch or describe what happens
to ions and molecules when they are
in balance inside and outside the cell.
◦ Molecules are always in motion. Once
equilibrium is reached, molecules will
continue to move into and out of the cell
at equal rates in both directions.
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2. Membranes regulate the movement of
molecules into and out of cells. To show
what you know about how membranes
regulate movement, fill out a T-table with the
headings “Ions or Molecules” and “How They
Move Through Membranes”
Oxygen and carbon dioxide
Water
Glucose and amino acids
Nitrogen, Phosphorous, Potassium, and Calcium in
plant cells
◦ Sodium and Potassium in animal cells
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Why is it a bad idea to drink seawater if you
are stranded on a boat in the ocean? What do
you think would happen to your cells?
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Begin on page 297.
Read the introduction paragraph.
Follow the Process and Procedure (#1-3)
Answer all questions in complete sentences
Take notes on the reading, make sure you
sketch each phase of mitosis in your
notebook.
Answer Reflect & Connect #1-3 p. 303