TRANSPORT MECHANISM
AN INDTRODUCTION
TRANSPORT MECHANISM
Membrane transport refers to the collection of mechanisms that
regulate the passage of solutes such as ions and
small molecules through biological membranes, which are lipid
bilayers that contain proteins embedded in them. The regulation of
passage through the membrane is due to selective membrane
permeability - a characteristic of biological membranes which allows
them to separate substances of distinct chemical nature. In other
words, they can be permeable to certain substances but not to
others.
Two Types of Cellular Transport
Passive Transport: In passive transport is a spontaneous
phenomenon that increases the entropy of a system and decreases
the free energy. The transport process is influenced by the
characteristics of the transport substance and the nature of the
bilayer.
Active Transport: In active transport a solute is moved against a
concentration or electrochemical gradient; in doing so the transport
proteins involved consume metabolic energy, usually ATP. In primary
active transport the hydrolysis of the energy provider (e.g. ATP) takes
place directly in order to transport the solute in question, for
instance, when the transport proteins are ATPase enzymes.
Passive Transport
Diffusion – passive transport of substances DOWN a concentration
gradient (from high to low concentration).
Concentration gradient – one area has a higher concentration than another area
Equilibrium – when a space is filled evenly
Diffusion is simplest type of passive transport.
Some substance diffuse through lipid bilayer
Others though transport proteins
Lump of Sugar
Diffusion
What do you think will happen if you drop a lump of sugar into a beaker of
If you drop a lump of sugar into a beaker of water, the sugar particles
water?
will diffuse and become evenly distributed throughout the water.
Types of Passive Diffusion
1. Simple diffusion – movement of small or lipophilic
molecules
(e.g. O2, CO2, etc.)
2. Osmosis – movement of water molecules (dependent on
solute concentrations)
3. Facilitated diffusion – movement of large or charged
molecules via membrane proteins (e.g. ions, sucrose,
etc.)
1. Simple Diffusion
Moves small, nonpolar molecules directly through lipid bilayer.
O2
CO2
2. Osmosis
Definition – diffusion of water across selectively permeable
membrane
Type of facilitated diffusion (passive transport)
From high to low concentration
Allows cells to maintain water balance as environment changes
Pass through water channels
Protein channels specific to water..
Three possibilities for the direction of
water movement
Water moves out – (hypertonic solution)
Causes cell to shrink
Water moves in – (hypotonic solution)
Causes cell to swell
Cell could burst if it swells too much
Plant cells have rigid cell wall to prevent this
No net water movement – (isotonic solution)
No change in cell volume
Effects of Osmosis
Plants and fungi have cell walls to help regulate
Most plants are healthiest in a hypotonic environment
Some unicellular eukaryotes have contractile vacuoles
Force excess water in cell out
Animals cells do not have cell walls or contractile vacuoles
They actively remove solutes from cytoplasm
Facilitated Diffusions
Moves ions and polar molecules that simple diffusion cannot
Transport proteins help these substances diffuse through the cell
membrane
Two types:
Channel protein
Carrier protein
Channel Proteins
Ions, sugars, and amino acids can diffuse with help of channel
proteins.
Proteins, sometimes called pores, serve as tunnels through the lipid bilayer
Each channel allows the diffusion of a specific substance
Ex – only sodium ions can pass through sodium ion channels
Carrier Protein
Transport substances that fit within their binding site
1)Carrier protein binds to specific substances on one side of cell
membrane
(2)Binding causes protein to change shape
(3)As shape changes, the substance is moved across the cell
membrane and released on other side
Active Transport
Transports of substance against concentration gradient
Low to high concentration
Active Transport – requires energy
Most energy supplied directly or indirectly in the form of
ATP (adenosine triphosphate)
Pumps
Substances bind to carrier proteins in low concentrations, released in
higher concentration
Carrier proteins function as “pumps” – move substances against their
concentration gradient called membrane pumps.
Sodium-Potassium Pump
One of the most important membrane pumps in animal cells
In a complete cycle, the sodium-potassium pump transports three
sodium ions, Na+ , out of a cell and two potassium ions, K+ , into the
cell.
Pump actively transports against their concentration gradients
Energy supplied by ATP
Sodium-Potassium Pump
The sodium-potassium pump has four steps:
1. Three sodium ions inside the cell bind to the sodium potassium
pump.
2. The pump changes shape, transporting the three sodium ions across
the cell membrane and releasing them outside the cell.
3. Two potassium ions outside the cell bind to the pump.
4. The two potassium ions are transported across the cell membrane
and are released inside the cell.
Sodium-Potassium Pump
Important:
Pump prevents sodium from accumulating in the cell
Pump helps maintain the concentration gradient of sodium ions and potassium
ions across cell membrane
Many cells use this pump to help transport other substances (glucose) across the
cell membrane
Movement in Vesicles
Many substances, like proteins and polysaccharides, are too large to
be transported by carrier proteins.
So, they are moved across the cell membrane by vesicles
The movement of a substance into a cell by a vesicle is called
endocytosis.
The movement of a substance by a vesicle to the outside of a cell is
called exocytosis.
ENDOCYTOSIS
the taking in of matter by a living cell by invagination of its
membrane to form a vacuole.
EXOCYTOSIS
a process by which the contents of a cell vacuole are released to the
exterior through fusion of the vacuole membrane with the cell
membrane.
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