Transcript Types of Transport

Transport Across the
Cell Membrane
• maintains homeostasis of cell
• membrane is selectively permeable –
some things can pass through but others
can’t
• 3 types: active transport, passive
transport, and bulk transport
1. Passive Transport
• passive
= no energy req’d
• move through membrane due
to differences in concentration
gradient
• 3 different types
1. Diffusion
• Movement of molecules from an area of
high concentration to an area of low
concentration across a concentration
gradient
• Used transport small molecules like CO2
H2O, and O2
• Animation: How Diffusion Works
2. Osmosis
• Diffusion of water from a region of high
concentration to a region of low
concentration
• Water can diffuse into or out of a cell, it
depends on the concentration on either side
of the cell membrane
• Animation: How Osmosis Works
Water potentials
• Water moves from a place with a LESS
NEGATIVE (higher) water potential to a place
with a MORE NEGATIVE (lower) water
potential
• The water potential of pure water is 0 (zero).
• Solutions have negative water potentials – the
more concentrated the solution, the more negative
the water potential.
3 Types of Osmosis
1. Hypotonic
• contain a low concentration of
solute relative to another
solution (e.g. the cell's
cytoplasm).
• cell is placed in a hypotonic
solution, the water diffuses
into the cell, causing the cell to
swell and possibly explode.
2. Hypertonic
• contain a high concentration
of solute relative to another
solution (e.g. the cell's
cytoplasm).
• when cell is placed in a
hypertonic solution, water
diffuses out of the cell,
causing the cell to shrivel.
3. Isotonic
• contain the same
concentration of solute as an
another solution (e.g. the
cell's cytoplasm).
• when cell placed in an
isotonic solution, the water
diffuses into and out of the
cell at the same rate.
• fluid that surrounds the body
cells is isotonic.
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3. Facilitated diffusion
Key features
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how glucose/charged ions moves into cells
passive – does not require energy
uses carrier proteins
solute molecules “combine” with carrier
proteins in the membrane.
• carrier molecules speed (or facilitate) the
passage of the solute molecules across the
membrane.
Spot the difference!
2 kinds of proteins involved:
1.CARRIER PROTEINS
• bind to a specific type of diffusing molecule.
• have a highly specific hydrophilic region to
which the solute molecule binds.
• binding cause the protein to undergo a change
in shape that moves the solute across the
bilayer and release it on the other side
Carrier proteins
Animation: How Facilitated Diffusion Works
Carrier proteins
2. ION CHANNELS
• formed by proteins with a
central pore that is lined
with charged groups.
• help the diffusion of
charged particles such as
Ca2+, Na+, K+, HCO3- and
Cl ions.
• Some channels are gated
and allow cells to regulate
the flow of ions from one
cell to another.
Ion channel
Factors affecting Rate of
Diffusion
1. Concentration Difference
• happens ONLY when a concentration gradient is
present and solute travels ALONG (down) a
concentration gradient
2. Saturation
• there are only a limited number of carrier
molecules per unit area of membrane.
• rate of movement reaches a max. when all carrier
molecules are fully loaded with solute molecules
2. Active Transport
• the transport of molecules or ions
across a membrane by carrier
proteins against a concentration
gradient.
• requires energy
• involves carrier proteins in the membrane.
• hydrolysis of ATP releases the energy
required for active transport.
• Cells involved in active transport have a large
number of mitochondria to provide the ATP
required
Spot the difference
Sodium-Potassium Pump
• ex. of active transport
• Exists in most cell
membranes.
• Actively removes sodium
ions from the cell while
actively accumulating
potassium ions into them
from their surroundings
• Animation: How the
Sodium Potassium Pump
Works
3. Bulk Transport
• used for materials to large to enter via
passive or active transport
• vesicles created by folding of cell
membrane onto itself to either engulf or
expel materials
• 2 types: endocytosis & exocytosis
Endocytosis
• the transport of large particles into the cell in
vesicles formed by folding in of the cell
surface membrane
• 3 types:
1. Pinocytosis (cell drinking)
• intake of small droplet of
extracellular fluid along
with solute particles
• occurs in all cells often
2. Phagocytosis (cell eating)
• intake of large droplet of extracellular fluid
including particulate matter (bacteria or
organic matter)
• occurs only in specialised cells like amoeba or
macrophages (bacteria fighting immune cells)
• Animation: Phagocytosis
3. Receptor-assisted endocytosis
• involves intake of specific molecules that
attach to special protiens in cell membrane
that serve as receptors
• have a unique shape that fit only to one
specific molecule
• ex. animal cells use this to bring cholesterol
into cell
Exocytosis
• the reverse process and is used to secrete
proteins, e.g digestive enzymes, out of the cells.
• vesicle forms inside cell moves to membrane
and empties contents outside of cell
• ex. pancreas secretes insulin