Transport Across Cell
Membrane
By: Areeba Qamar
Lecturer (Physiology)
Why do Substances Transport..?
• Nutrient supply
• Maintains cells environment
• Helps in exchange of materials
• Who is the barrier for transport across cell??
• The Cell Membrane
Barrier can be….
• Permeable
• Impermeable
• Partially Permeable
• Cell membrane is partially permeable
Transporting substance can be….
• Classified by its nature
• Lipid soluble
• Can pass through the membrane.
• Uncharged or nonpolar molecules (such as O2, CO2,
and fatty acids) are also highly lipid soluble.
• Water soluble
• Charged particles (ions such as Na and K) and polar
molecules (such as glucose and proteins).
• For water-soluble ions less than 0.8 nm in dia, the
protein channels serve as an alternative route.
• Classified by its size
2 Types of Energy
• Passive Energy
• Active Energy
Membrane
Transport
Passive
Simple
Diffusion
Facilitated
Diffusion
Active
Primary
Active
Transport
Secondary
Active
Transport
Cotransport
Countertransport
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Diffusion
Molecules move
from area of
HIGH to LOW
concentration
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Diffusion
Diffusion is a
PASSIVE process
no energy is used to make
the molecules move.
Trying to reach
EQUILIBRIUM
Same amount of particles
inside as outside
Factors Influencing the Rate of
Diffusion
• FICK’S LAW OF DIFFUSION
• The magnitude (or steepness) of the concentration
gradient
• The surface area of the membrane across which
diffusion is taking place
• The lipid solubility of the substance
• The molecular weight of the substance
• The distance through which diffusion must take place
• Temperature
10
Two Types of Diffusion
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Simple Diffusion
Doesn’t require energy
Moves high to low
concentration
Example: Oxygen or water
diffusing into a cell and carbon
dioxide diffusing out.
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Facilitated Diffusion
Doesn’t require energy
Uses channel proteins to move
high to low concentration
Examples: Glucose or amino
acids moving from blood into a
cell.
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Carrier / Channel Protein Transporter
• Only ions fit through the narrow channels
• Small polar molecules, glucose and amino acids are
transported across the membrane by carriers
• Channels can be open or closed, but carriers are
always “open for business” (the number and kinds
can be regulated)
• Movement through channels is considerably faster
than carrier-mediated transport
• Carrier mediated transport exhibits
• Specificity
• Saturation
• Competition
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Osmosis
• Diffusion of water
across a membrane
• Moves from
• HIGH water potential
(low solute)
• to
• LOW water potential
(high solute)
Diffusion across a membrane
Semipermeab
le membrane
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Osmosis : H2O Across A Membrane
High H2O potential
Low solute concentration
Low H2O potential
High solute concentration
Tonicity
• Refers to the effect, the concentration of nonpenetrating solutes in a solution
has, on cell volume.
• Whether the cell remains the same size,
swells, or shrinks
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Cell in Isotonic Solution
Equal salt inside and outside the cell
10% NaCL
90% H2O
ENVIRONMENT
CELL
10% NaCL
90% H2O
What is the direction of water
movement?
equilibrium
The cell is at _______________.
NO NET
MOVEMENT
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Cell in Hypotonic Solution
Less salt outside the cell than inside the cell
10% NaCL
90% H2O
CELL
60% NaCL
40% H2O
What is the direction of water
movement?
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Cell in Hypertonic Solution
more salt outside the cell than inside the cell
20% NaCL
80% H2O
ENVIRONMENT
CELL
5% NaCL
95% H2O
What is the direction of water
movement?
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Isotonic Solution
NO NET MOVEMENT
OF H2O (equal
amounts entering &
leaving)
Hypotonic
Solution
CYTOLYSIS
Hypertonic
Solution
PLASMOLYSIS
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Osmosis in Red Blood Cells
Isotonic
Equilibrium
Hypotonic
Cytolysis
Hypertonic
Plasmolysis
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ACTIVE TRANSPORT
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Requires energy or ATP
Moves materials from Low to
High concentration
EXAMPLE: Sodium/Potassium
pump Pumping Na+ out & K+ in
against strong concentration
gradients.
Na/Glucose Co-transport
Vesicular Transport
• Substances which are much too big for channels, and
no carriers exist for them (they would not even fit into
a carrier molecule)
• These large particles are transferred between the ICF
and ECF not by crossing the membrane but by being
wrapped in a membrane-enclosed vesicle, a process
• known as vesicular transport.
• Vesicular transport requires energy expenditure by the
cell, so this is an active method of membrane
transport.
Vesicular
Transport
Endocytosis
Phagocytosis
Exocytosis
Pinocytosis
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Moving the “Big Stuff”
Large molecules move materials into the cell by one of two
forms of endocytosis.
Phagocytosis
Intake solid material
Pinocytosis
Intake particles suspended
in water
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Molecules are moved out of the cell by vesicles that
fuse with the plasma membrane.
This is how many hormones are secreted.
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Exocytosis
Large molecules that are manufactured in the cell are
released through the cell membrane.
Inside Cell
Outside the Cell
Thank You…