Transport
Across the
Plasma
Membrane
Plasma Membrane Transport
• Molecules move across the plasma
membrane by:
What are three types of
passive transport?
1) Diffusion
2) Facilitated Diffusion
3) Osmosis
ATP energy is not
needed to move the
molecules through.
Passive Transport 1: Diffusion
• Molecules can move directly
through the phospholipids of the
plasma membrane
This is called …
What is Diffusion?
•
•
Diffusion is the net
movement of molecules
from a high
concentration to a low
concentration until
equally distributed.
Diffusion rate is related
to temperature,
pressure, state of matter,
size of concentration
gradient, and surface
area of membrane.
http://www.biologycorner.com/resources/diffusion-animated.gif
What molecules pass through the
plasma membrane by diffusion?
•
•
•
•
•
Gases (oxygen, carbon
dioxide)
Water molecules (rate
slow due to polarity)
Lipids (steroid
hormones)
Lipid soluble molecules
(hydrocarbons, alcohols,
some vitamins)
Small noncharged
molecules (NH3)
Why is diffusion important to cells
and humans?
•
•
•
•
•
Cell respiration
Alveoli of lungs
Capillaries
Red Blood Cells
Medications: timerelease capsules
Passive Transport 2: Facilitated
Diffusion
• Molecules can move through the
plasma membrane with the aid of
transport proteins
This is called …
What is Facilitated Diffusion?
• Facilitated diffusion
is the net movement
of molecules from a
high concentration to
a low concentration
with the aid of
channel or carrier
proteins.
What molecules move through the plasma
membrane by facilitated diffusion?
• Ions
(Na+, K+, Cl-)
• Sugars
(Glucose)
• Amino Acids
• Small water
soluble molecules
• Water (faster rate)
How do molecules move through the
plasma membrane by facilitated diffusion?
• Channel and Carrier proteins are specific:
• Channel Proteins allow ions, small solutes, and water
to pass
• Carrier Proteins move glucose and amino acids
• Facilitated diffusion is rate limited, by the number of
proteins channels/carriers present in the membrane.
Specific Types of Facilitated
Diffusion
 Counter Transport – the transport
of two substances at the same time
in opposite directions, without ATP.
Protein carriers are called
Antiports.
 Co-transport – the transport of
two substances at the same time in
the same direction, without ATP.
Protein carriers are called
Symports.
 Gated Channels – receptors
combined with channel proteins.
When a chemical messenger binds
to a receptor, a gate opens to allow
ions to flow through the channel.
Why is facilitated diffusion important
to cells and humans?
• Cells obtain food for
cell respiration
• Neurons
communicate
• Small intestine cells
transport food to
bloodstream
• Muscle cells contract
Passive Transport 3: Osmosis
• Water Molecules can move directly
through the phospholipids of the
plasma membrane
This is called …
What is Osmosis?
• Osmosis is the diffusion of water through a
semipermeable membrane. Water molecules bound
to solutes cannot pass due to size, only unbound
molecules. Free water molecules collide, bump into
the membrane, and pass through.
Why is osmosis important to cells
and humans?
• Cells remove water
produced by cell
respiration.
• Large intestine cells
transport water to
bloodstream
• Kidney cells form
urine
Osmosis and Tonicity


Tonicity refers to the total solute
concentration of the solution outside the
cell.
What are the three types of tonicity?
1) Isotonic
2) Hypotonic
3) Hypertonic
Isotonic
 Solutions that have the same concentration of
solutes as the suspended cell.
 What will happen to a cell placed in an Isotonic
solution?
 The cell will have no net movement of water and
will stay the same size.
 Ex. Blood plasma has high concentration of
albumin molecules to make it isotonic to tissues.
Hypotonic
 Solutions that have a lower solute concentration
than the suspended cell.
 What will happen to a cell placed in a Hypotonic
solution?
 The cell will gain water and swell.
 If the cell bursts, then we call this lysis. (Red blood
cells = hemolysis)
 In plant cells with rigid cell walls, this creates
turgor pressure.
Hypertonic
 Solutions that have a higher solute concentration
than a suspended cell.
 What will happen to a cell placed in a Hypertonic
solution?
 The cell will lose water and shrink. (Red blood
cells = crenation)
 In plant cells, the central vacuole will shrink and
the plasma membrane will pull away from the cell
wall causing the cytoplasm to shrink called
plasmolysis.
Review: Passive Transport
• Diffusion – O2 moves in and CO2 moves out
during cell respiration
• Facilitated Diffusion – glucose and amino
acids enter cell for cell respiration
• Osmosis – cell removal or addition of water
Review Tonicity
 What will happen to a red blood cell in a
hypertonic solution?
 What will happen to a red blood cell in an
isotonic solution?
 What will happen to a red blood cell in a
hypotonic solution?
What are three types of
Active transport?
1) Active Transport
2) Exocytosis
3) Endocytosis
– Phagocytosis
– Pinocytosis
– Receptor-Mediated
endocytosis
ATP energy is
required to move the
molecules through.
Active Transport
 Molecules move from areas of low
concentration to areas of high concentration
with the aid of ATP energy.
 Requires protein carriers called Pumps.
The Importance of Active Transport
 Bring in essential molecules: ions,
amino acids, glucose, nucleotides
 Rid cell of unwanted molecules (Ex.
sodium from urine in kidneys)
 Maintain internal conditions different
from the environment
 Regulate the volume of cells by
controlling osmotic potential
 Control cellular pH
 Re-establish concentration
gradients to run facilitated diffusion.
(Ex. Sodium-Potassium pump and
Proton pumps)
The Sodium-Potassium Pump
 3 Sodium ions move out of
the cell and then 2
Potassium ions move into
the cell.
 Driven by the splitting of
ATP to provide energy and
conformational change to
proteins by adding and then
taking away a phosphate
group.
 Used to establish an
electrochemical gradient
across neuron cell
membranes.
http://www.biologie.uni-hamburg.de/b-online/library/biology107/bi107vc/fa99/terry/images/ATPpumA.gif
Active Transport 2: Exocytosis
 Movement of large
molecules bound in
vesicles out of the cell
with the aid of ATP
energy. Vesicle fuses
with the plasma
membrane to eject
macromolecules.
 Ex. Proteins,
polysaccharides,
polynucleotides, whole
cells, hormones, mucus,
neurotransmitters, waste
Active Transport 3: Endocytosis
 Movement of large molecules into the cell
by engulfing them in vesicles, using ATP
energy.
 Three types of Endocytosis:
– Phagocytosis
– Pinocytosis
– Receptor-mediated endocytosis
Phagocytosis
 “Cellular Eating” – engulfing large
molecules, whole cells, bacteria
 Ex. Macrophages ingesting bacteria or worn
out red blood cells.
 Ex. Unicellular organisms engulfing food
particles.
Pinocytosis
 “Cellular Drinking” – engulfing liquids and
small molecules dissolved in liquids;
unspecific what enters.
 Ex. Intestinal cells, Kidney cells, Plant root
cells