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Diffusion and Osmosis
Importance of Water
Living things are
composed of more
than 50 percent water
and depend on water
to survive.
You can live for weeks
without food but only a
few days without
Why is water so
All the chemical reactions in living things take
place in water solutions.
Organisms use water to transport materials
through their bodies.
Plants use water to move minerals and sugars
between roots and leaves.
3. The large percentage of water in living things acts
like an insulator.
The water in a cell helps keep its temperature
constant, which allows life-sustaining chemical
reactions to take place.
Cell Membrane
Cells take in food,
and oxygen, from
their environment
and release waste
A cell has a
membrane around
it that works like a
window screen.
Selectively Permeable
A cell’s membrane is selectively permeable.
-Allows some things to enter or leave the cell
while keeping other things outside or inside the
– Cell membranes will allow small molecules like
O2, H20 and , CO2, to pass through.
– Cell membranes will not allow larger
molecules like Sucrose, and Starch. to pass
Passive Transport
How things move through a cell
membrane depends on the size of the
particles, the path taken through the
membrane, and whether or not energy
is used.
The movement of substances through
the cell membrane without the input
of energy is called passive
3 Types of Passive
Facilitated diffusion
1. Diffusion
Molecules in solids, liquids,
and gases move constantly.
This random movement of
molecules from one area
where there are more of
them into another area
where there are fewer of
them is called diffusion.
Example: Food Coloring and
Molecules of a substance will continue
to move from area into another until
the relative number of these molecules
is equal in the two areas.
When this occurs, equilibrium is
reached and diffusion stops.
2. Osmosis
Water molecules
move by diffusion
into and out of cells.
Osmosis is the
passage of water
from a region of high
water concentration
through a semipermeable
membrane to a
region of low water
If the area surrounding the cell
has a lower concentration of
water molecules than inside
the cell, the cell will lose water
by osmosis.
If the area surrounding the cell
has a higher concentration of
water molecules than inside
the cell, the cell will gain
water by osmosis.
Why do carrots in salt water
become limp?
When there is a lower
concentration of water
molecules surrounding
the carrot cells, water is
lost from inside the cells.
The cell membrane
comes away from its cell
The loss of water reduces
pressure against the cell
wall, and carrot cell
becomes limp.
Why do carrots stay crisp
in pure water?
When there is a
higher concentration
of water surrounding
the carrot cells than
inside the cell, water
moves into the cells.
The cells would fill with
Their cell membranes
press against their cell
Pressure increases and
the cells become firm.
3rd Type of Passive Transport:
Facilitated Diffusion
• Some substances pass easily through the
cell membrane by diffusion.
• Other substances are so large that they can
enter and leave the cell only with the help of
molecules in the cell membrane called
transport proteins. Glucose, Calcium,
• This process, a type of passive transport, is
known as facilitated diffusion.
Facilitated Diffusion
is a process of passive transport in which
molecules diffuse across membranes, with
the help of transport proteins.
Passive Transport: The movement of substances
through the cell membrane without the input of
Types of Passive Transport:
1. Diffusion is the process by which molecules move
from an area of higher concentration to an area of
lower concentration. O2 and CO2
2. Osmosis is the diffusion of water from a region
of high water concentration through a semipermeable membrane to a region of low water
3. Facilitated Diffusion is the transport of substances
through the cell membrane with the help of
Active Transport
• Sometimes a substance is
needed inside the cell
even though the amount
of that substance inside
the cell is already greater
than the amount outside
the cell.
• Active transport is the
movement of materials
through a cell membrane
using cellular energy.
Two Types of Active Transport
1. Endocytosis
2. Exocytosis
1. Endocytosis
• Large protein molecules and bacteria, can be
transported into a cell when they are
surrounded by the cell membrane.
• This process of taking substances into a cell by
surrounding it with the cell membrane is called
• Some one-celled
organisms take in
food this way.
• The cell membrane
surrounds and engulfs a
particle wraps around
the particle and forms
vesticle. (storage and
transport structures)
2. Exocytosis
• The contents of a vesicle can be released by
a cell using the process called exocytosis.
• Exocytosis occurs in the opposite way that
endocytosis does.
• Exocytosis occurs when a vesicle’s
membrane fuses with a cell’s membrane,
and the contents are released.