IB BIOLOGY
CELL MEMBRANES
PASSIVE TRANSPORT
There are two general means of cellular transport
Passive transport
Active transport
Passive transport
Passive transport occurs in
situations where there are
areas of different
concentration of a particular
substance.
Movement of substances occur
from a region of high
concentration to a region of
low concentration.
Movement occurs along a
concentration gradient.
Passive transport does not
require energy in the form of
ATP
Active transport
The movement of substances
occur against a concentraion
gradient.
Energy expenditure must
occur.
PASSIVE TRANSPORT:
OSMOSIS AND DIFFUSION:
DIFFUSION:
Diffusion is the passive transport of particles from a region of
higher concentration to a region of lower concentration, as a
result of random motion of particles.
In a living system diffusion often involves a membrane. For
example, oxygen gas moves from outside a cell to inside.
Oxygen is used by the cell when its mitochondria carry out
respiration, thus creating a lower concentration inside the cell
than outside. As a result oxygen diffuses into the cell. Carbon
dioxide diffuses in the opposite direction .
Membranes are not permeable to certain substances like
cellulose, so it does not diffuse across, even if there is a higher
concentration on one side of a membrane than the other.
Diffusion Google image
SIMPLE AND FACILITATED DIFFUSION:
Membranes allow some substances to diffuse through but not
others-they are partially permeable. Some of these move
between the phospholipid molecules in the membrane-this is
simple diffusion.
Other substances are unable to pass through membranes. To
allow these substances to diffuse through membranes, channel
proteins are needed. This is called facilitated diffusion.
Channel proteins are specific- they allow only one type of
substance to pass through. For example, chloride channel only
allow chloride ions to pass through. Cells cannot control the
direction of movement. Facilitated diffusion always causes
particles to move from a region of higher concentration to a
region of lower concentration.
There are sodium and potassium channels in the membranes of
neurons that open and close, depending upon the voltage
across the membrane. They are called voltage gated channels
and are used during the transmission of nerve impulses.
http://video.google.com/videoplay?docid=-5242394503257451479
OSMOSIS:
Animation: How Osmosis Works
Osmosis is the passive movement of water molecules from a
region of lower solute concentration to a region of higher
solute concentration, across a partially permeable membrane.
A partially permeable membrane allows certain substances to
pass through.
A hyperosmotic solution has a higher concentration of total
solutes than a hypoosmotic solution.
Water moves from a hypo-osmotic solution across a partially
permeable membrane.
If iso-osmotic solutions occur on either side of a partially
permeable membrane, no net movement of water will take
place.
Passive transport continues till there is an equal concentration
of the substance in both areas involved. This is called
equilibrium.
Simple diffusion
Facilitated diffusion
Osmosis
Substances other than water
move between phospholipid
molecules or through proteins
which possess channels.
Non-channel protein carriers
change shape to allow
movement of substances other
than water.
Only water moves through the
membrane using aquaporins
which are proteins with
specialized channels for water
movement
Substances that are small in size and non-polar move across
the membrane with ease.
Substances that are polar , large in size, or both do not.
Examples of small non-polar substances are gases such as
oxygen, carbon dioxide and nitrogen.
Ions such as chloride, potassium and sodium would have great
difficulty crossing membranes passively like large molecules
such as sucrose and glucose.
Molecules such as water and glycerol are small, uncharged
polar molecules that can fairly easily cross membranes.
Shockwave
Active and passive transport (animation)