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The Cell in its Environment
Cells and Heredity
Chapter 1 Section 4
The Cell Membrane
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The cell membrane is called selectively
permeable because some substances can
pass through it while others cannot.
The cell membrane is usually permeable to
substances such as oxygen, water, and
carbon dioxide.
It is not permeable to large molecules and
salts.
Substances pass the cell membrane by
diffusion, osmosis, or active transport.
Diffusion
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The main method by which substances
move into and out of cells is diffusion.
Diffusion is the process by which
molecules move from an area of higher
concentration to an area of lower
concentration.
Molecules are always in motion. As they move,
they bump into one another and spread out. Over
time, the molecules spread evenly in an area.
Diffusion
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In pond water organisms, there is more
oxygen in their watery surroundings than
within the cell. Therefore, oxygen diffuses
into the cell.
Look at figure 16, page 41. What would
happen if the concentration of oxygen
outside the cell was lower then the
concentration inside?
Oxygen would diffuse out of the cell.
Osmosis
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Like oxygen, water passes easily into and
out of cells through the cell membrane.
The diffusion of water is called osmosis.
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Remember: molecules tend to move from an area
of higher concentration to an area of lower
concentration.
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Through osmosis, water molecules diffuse
across the cell membrane to an area of
lower concentration.
Osmosis
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Look at Figure 17 A – 17 C on page 42,
which shows a red blood cell in different
watery environments. Figure 17 A shows a
normal RB cell.
Why did the cell in 17 B shrink?
Water moved out by osmosis because the
environment was salty and there was a higher
water concentration inside the cell than out.
Osmosis
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Why did the cell in 17 C become swollen?
Water diffused into the cell because the
environment had little salt and there was a
lower concentration of water inside the cell
than out.
Isotonic Solution – Water moves
in & out of cell at an equal rate
Hypotonic Solution – Water
moves into cell and cell swells
Hypertonic Solution – Water
moves out of cell & cell shrivels
Passive vs. Active Transport
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The movement of materials through a cell
membrane without using energy is called
passive transport.
Diffusion and osmosis are types of passive
transport.
Active transport is the movement of
materials through a cell membrane using
energy.
Types of Transport
Active Transport
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There are several ways a cell can move
materials by active transport.
The cell membrane has transport proteins
that pick up molecules and carry them
across (either into or out of the cell).
Cells can also engulf materials. The cell
membrane surrounds the molecule and
pinches off to form a vacuole.
Active Transport
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Name two scenarios in which a cell would
need to undergo active transport.
If the molecule that the cell needs to bring
in (or out) is too large to pass through the
membrane.
If the molecules are moving from an area
that has a low concentration to an area
that has a high concentration.
Movement of Molecules Against
the Concentration Gradient
Why Are Cells Small?
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As a cell’s size increases, more of its
cytoplasm is located farther from the cell
membrane.
The cytoplasm carries molecules to their
destination within the cell.
If the cell was too large, it would take too
long for molecules to travel within the cell.