Download 02 Transport Across the Cell Membrane

Transport Across the Cell Membrane
What is the Function of a Cell
Membrane?
• Acts as a barrier from the external
environment
• Controls substances that enter and exit the
cell
– Selectively permeable: movement across the cell
membrane depends on the chemical composition
of a substance
• EX: molecule size, structure, charge, etc.
Transport
Across the Cell
Membrane
Passive
Transport
Active Transport
Does not
require input of
energy
Requires input
of energy
Passive Transport
• Does NOT require the input of energy from the cell
• Involves movement down a concentration gradient.
– Difference in concentration (# of molecules) of a given substance
between adjoining areas.
• Number of substances that can pass through using this
method is limited due to low energy requirements.
• Ex: Oxygen, Nitrogen, Carbon Dioxide
Passive Transport
Types of
Passive
Transport
Diffusion
Osmosis
Facilitated
Diffusion
Diffusion
• What is diffusion?
– Spontaneous movement of molecules from an
area of high concentration to an area of low
concentration
• What types of molecules can do this?
• Small uncharged molecules
– Ex: oxygen, carbon dioxide, nitrogen, water
• How do they do it?
• By passing through the phospholipid bilayer or through
channels formed by transmembrane proteins
Osmosis
• Movement of WATER across a selectively
permeable membrane
• Water moves across the membrane to
maintain equilibrium
– Concentration of solute (proteins, minerals…) is
equal on both sides of the membrane
• Once equilibrium is obtained, water will
continue to move across the membrane but
there is no net gain on either side.
Osmosis
Osmosis
• HYPOtonic solution
– A solution where the concentration of
solute molecules is lower outside the cell
than inside the cell
– There is a higher concentration of water
outside of the cell
– Water moves into the cell and the cell
expands (or bursts)
• Called hemolysis
Osmosis
• ISOtonic solution
– A solution where the
concentration of solute
molecules outside the cell is
equal to the concentration of
solutes inside the cell
– Concentration of water is the
same on both sides of the
membrane
Osmosis
• HYPERtonic solution
– A solution where the concentration
of solute molecules is higher
outside of the cell than inside
– There is a higher concentration of
water inside the cell
– Water moves out of the cell and
the cell shrinks
• Called crenation
Osmosis
Facilitated Diffusion
• Some proteins cannot pass through the cell
membrane’s hydrophobic middle layer.
• Carrier proteins can help to pass some
molecules through the membrane.
• Molecules that use this mode of transport
include:
– Large polar molecules
– Large ions
Simple Vs. Facilitated Diffusion
Simple Diffusion
Facilitated Diffusion
Molecules pass through the
phospholipid bilayer or protein
channels
Molecules pass through the cell
membrane via carrier proteins
Carrier proteins typically only carry 1
type of molecule
Active Transport
Types of
Active
Transport
Bulk Transport
Endocytosis
Transport
through Gates
or Channels
Exocytosis
Active Transport
• Used to move substances in or out of a cell
AGAINST the concentration gradient
• Requires ENERGY in order to move substances
across the membrane
– Energy is used to open a channel or a gate in the
protein of the membrane to assist in the binding
of a substance to the protein
– Energy can also be used to assist in a shape
change that pushes the substance through the
membrane (bulk transport)
Bulk Transport
• Some molecules are too large to cross the
membrane through proteins. They must travel
in and out of cells through bulk transport.
• 2 Types
– Endocytosis
– Exocytosis
Endocytosis
• Process by which cells ingest large materials
• The cell membrane engulfs material from outside of
the cell.
• This portion of the membrane pinches off, forming a
membrane bound vesicle inside the cell called a
vacuole.
Exocytosis
• Reverse process of Endocytosis
• Molecules are carried by vacuoles inside the cell and
expelled into the extracellular space (outside of the
cell).
videos
• Passive and Active Transport
• Endocytosis and Exocytosis