Movement of Materials
• The transport of water and other types of
molecules across membranes is the key to many
processes in living organisms.
• Without the input of oxygen, water, and
nutrients, cells would not be able to maintain
themselves.
• Likewise, the elimination of waste products is
crucial to the continued activities of the cell.
• The cell membrane regulates the transport of
materials through its chemical and structural
composition
Passive Transport
• Diffusion
• Facilitated diffusion
• Osmosis
DO NOT REQUIRE ATP ENERGY.
Active Transport
• Active transport
• Endocytosis
– Phagocytosis
– Pinocytosis
– Receptor mediated endocytosis
• Exocytosis
THESE ALL REQUIRE ATP ENERGY
Diffusion
• One of the basic ways substances can get in
and out of cells is by simple diffusion.
• Diffusion is the movement of molecules from
a region of high concentration to a region of
low concentration by means of random
molecular motion.
• The difference between the high
concentration and low concentration is called
the concentration gradient.
• In cells, small non-polar molecules can easily
diffuse through the cell membrane.
• Oxygen and Carbon dioxide gases diffuse
between the cytoplasm and the extracellular
fluid by following the concentration gradient
created and maintained by cellular respiration.
• Factors that affect the rate of diffusion are:
1. Temperature. Temperature measures the average
kinetic energy of the molecules. As the kinetic
energy increases, the temperature increases. At
higher temperatures the molecules move faster and
diffuse faster.
2. Size of the solvent molecules. Generally, smaller
molecules diffuse faster than larger molecules.
3. “Steepness” of the concentration gradient.
The rate of diffusion is proportional to the
gradient.
Osmosis
• Water, though polar, is small enough that it
too can move across the cell membrane.
• Osmosis is the special name given to the
movement of water molecules, across a
semipermeable membrane, along a
concentration gradient.
• Water is the usual solvent in cells. The solutes
( ions, minerals, glucose, amino acids) are
dissolved in water.
• Most solutes cannot freely cross the cell
membrane. This creates concentration
gradients which will lead to the movement of
water molecules.
• Osmotic pressure is created by the difference
in water concentration.
• HYPERTONIC solutions have more SOLUTE
molecules than the cytoplasm.
• HYPERTONIC solution: water molecules are
drawn OUT of the cell.
• Effect on Animal cells:
Cells shrink:
Crenation
• Effect on Plant Cells:
– Plasmolysis
• In a HYPOTONIC solution, there are less
solutes than in the cell cytoplasm.
• Water moves INTO the cell
• Effect of Hypotonic solution on Animal cells:
The cells will swell and may burst. (Lysis)
• Effect of Hypotonic solution on Plant cells
Cell will fill with water, but the cell wall will
stop it from bursting.
Cells are said to be turgid
• In an ISOTONIC solution, the concentrations of
solute are equal on both sides of the
semipermeable membrane.
• There is no NET movement or water
molecules across the membrane.
Facilitated Diffusion
• Movement of specific molecules down a
concentration gradient, passing through the
membrane via a specific carrier protein.
• Each carrier protein has its own shape and only
allows one molecule (or one group of closely
related molecules) to pass through.
• Common molecules entering/leaving cells this
way include glucose and amino-acids.
• It is passive and requires no energy from the cell.
• Most cells are exposed to extracellular glucose
concentrations that are higher than those
inside the cell, so facilitated diffusion results in
the net inward transport of glucose.
• Once glucose is taken up by these cells it is
rapidly metabolized, so intracellular glucose
concentrations remain low and glucose
continues to be transported into the cell from
the extracellular fluids.
• The glucose carriers are
“reversible” so glucose
can be transported in
the opposite direction.
This occurs in liver cells
when glucose is
synthesized from
glycogen and released
into the circulation.
Ions
• Channel proteins (porins)form open pores in the
membrane, allowing small molecules of the
appropriate size and charge to pass freely
through the lipid bilayer.
• Ion channels allow the passage of ions across
plasma membranes. They are present in all cells
but are especially important in nerve and muscle
cells. Their regulated opening and closing is
responsible for the transmission of electric
signals.
Open and Shut Potassium
channels
• Three properties of ion channels are central to
their function .
1. Transport through channels is extremely rapid. More
than a million ions per second flow through open
2. Ion channels are highly selective because narrow
pores in the channel restrict passage to ions of the
appropriate size and charge. There are specific channel
proteins for the passage of Na+, K+, Ca2+, and Cl- across
the membrane.
3. Most ion channels are not permanently open. Instead,
the opening of ion channels is regulated by “gates” that
transiently open in response to specific stimuli.
• The plasma membranes of many cells also
contain water channel proteins (aquaporins),
through which water molecules are able to
cross the membrane much more rapidly than
they can diffuse through the cell membrane.
Active Transport
• Requires ATP energy
• Transports Substances AGAINST the
concentration gradient
• Proteins act as PUMPS
• Sodium- Potassium pump is typical example of
symport transport (two substances being
transported at the same time, in opposite
directions)
Endocytosis
• Transport of materials into cell by means of
vesicles
• Requires energy to change the shape of cell
membrane
• 3 types of endocytosis
– Phagocytosis: for “bulky” substances
– Pinocytosis: cell “drinking”
– Receptor mediated pinocytosis: very specific
Phagocytosis
• Amoeba, White blood cells of immune system
Pinocytosis
Receptor mediated pinocytosis
Exocytosis
• Vesicles fuse with cell membrane and release
contents to the extra cellular environment
• Used to excrete metabolic wastes, and export
substances, such as hormones.
• Can be regulated by the cell
• Vesicles often created by the Golgi Apparatus