Download Lecture #10 - Suraj @ LUMS

Lecture 10
Transport In/Out of Cells
Transport
Materials are exchanged between the
cytoplasm and external cell environment
across the plasma membrane by several
different processes, some require energy
(active transport), some do not (passive
transport).
Keeping Things In!
• The cell membrane functions as a semipermeable barrier, allowing a very few
molecules across.
• Keeps the majority of organically produced
chemicals inside the cell.
The Cell Membrane
• Proteins are suspended in the inner layer.
• The more hydrophilic areas of these proteins "stick out"
into the cells interior as well as the outside of the cell.
• These integral proteins are sometimes known as gateway
proteins.
• Integral proteins function in:
1) cellular recognition,
2) are binding sites for substances to be brought into the
cell, through channels that will allow materials into the cell
via a passive transport mechanism.
3) as gates that open and close to facilitate active transport
of large molecules.
Diffusion
• The net movement of a substance (liquid or gas) from an
area of higher concentration to one of lower concentration.
• Molecules move down a concentration gradient.
• Does not require energy, happens spontaneously-an
example of passive diffusion.
• Eventually, if no energy is input into the system the
molecules will reach a state of equilibrium where they will
be distributed equally throughout the system.
• Factors that affect the rate of diffusion:
-The greater the difference in concentration the faster the rate of
diffusion. E.g when you exercise breathing becomes more rapid.
-The greater the surface area of a membrane the faster the rate of
diffusion. Microvilli on animal cells.
-The greater the distance the slower the rate of diffusion. Cells rely on
diffusion for internal transport of molecules.
Osmosis
• Osmosis is the diffusion of water across a semi-permeable
membrane.
• The presence of a solute decreases the water potential of a
substance. Thus there is more water per unit of volume in a glass
of fresh-water than there is in an equivalent volume of sea-water.
• Therefore in a cell, which has so many organelles and other large
molecules, the water flow is generally into the cell.
• Water, carbon dioxide, and oxygen are among the few simple
molecules that can cross the cell membrane by osmosis.
• It is one principle method of movement of substances within cells,
as well as the method for essential small molecules to cross the cell
membrane.
• Carbon dioxide is produced by all cells as a result of cellular
metabolic processes. Since the source is inside the cell, the
concentration gradient is constantly being replenished/re-elevated,
so it will move out of the cell to an area of low concentration.
• Hypertonic solutions are those in which more
solute is present - lower water potential.
• Hypotonic solutions are those with less solute higher water potential.
• Isotonic solutions have equal (iso-)
concentrations of substances. Water potentials
are thus equal, although there will still be equal
amounts of water movement in and out of the
cell, the net flow is zero.
Low water potential inside
Blood cells compared to
Distilled water so water
enters cause cells to burst
The salt solution has
an even lower water
Potential than the blood
Cells and so water moves out
Selective Permeability of Cell
Membranes
• The respiratory gases carbon dioxide and oxygen
diffuse rapidly through membranes.
• Water molecules are small enough to pass between
the hydrophobic phospholipid molecules.
• However ions and larger polar molecules e.g
amino acids, sugars, fatty acids and glycerol are
repelled by the hydrophobic region.
• Require alternative method of transport.
Carrier-assisted Transport
• Certain proteins integrated into the cell membrane are
able to move molecules into the cell through water filled
hydrophilic channels.
• The channels have a specific shape hence they are highly
selective about the chemicals they allow to cross.
• Some of these proteins can move materials across the
membrane only when assisted by the concentration
gradient, a type of carrier-assisted transport known as
facilitated diffusion.
• Both diffusion and facilitated diffusion are driven by the
potential energy differences of a concentration gradient.
• Glucose enters most cells by facilitated diffusion.
• Glycolysis maintains the concentration gradient.
Types of Passive Diffusion
Diffusion of Gases and Water Molecules Across The Lipid Bilayer
Facilitated Diffusion
Active Transport
• Is the movement against the concentration gradient.
• E.g the sodium-potassium pump in nerve cells.
• Na+ is maintained at low concentrations inside the cell and
K+ is at higher concentrations. The reverse is the case on
the outside of the cell.
• When a nerve message is propagated, the ions pass across
the membrane, thus sending the message.
• After the message has passed, the ions must be actively
transported back to their "starting positions" across the
membrane.
• Up to one-third of the ATP used by a resting animal is used
to reset the Na-K pump.
Active Transport
Types of Transport Proteins
• Uniport proteins transport one solute at a time.
• Symport proteins transport the solute and a
cotransported solute at the same time in the same
direction. E.g In the membrane of kidney cells
transporting glucose and sodium into the cell.
• Antiport proteins transport the solute in (or out)
and the co-transported solute goes in the opposite
direction. One goes in the other goes out or viceversa.E.g Sodium and Potassium Pump.
Vesicles/vacuoles
• Vesicles and vacuoles that fuse with the cell membrane
may be utilized to release or transport chemicals out of the
cell or to allow them to enter a cell.
• Exocytosis is the term applied when transport is out of the
cell.
• Endocytosis is the case when a molecule causes the cell
membrane to bulge inward, forming a vesicle.
• Phagocytosis is the type of endocytosis where an entire
cell is engulfed.
• Pinocytosis is when the external fluid is engulfed.
• Receptor-mediated endocytosis occurs when the material
to be transported binds to certain specific molecules in the
membrane. Examples include the transport of insulin and
cholesterol into animal cells.
Endocytosis and Exocytosis
Other Features of The Cell Membrane
• The outer surface of a membrane is usually rich in glycolipids.
• These, along with carbohydrates attached to the integral proteins, are
thought to function in the recognition of self.
• Multicellular organisms may have some mechanism to allow
recognition of those cells that belong to the organism and those that are
foreign.
• When a cell does not display the chemical markers that say "Made in
Mike", an immune system response may be triggered.
• This is the basis for immunity, allergies, and autoimmune diseases.
• Organ transplant recipients must have this response suppressed so the
new organ will not be attacked by the immune system, which would
cause rejection of the new organ.
• Allergies are in a sense an over reaction by the immune system.
• Autoimmune diseases, such as rheumatoid arthritis, the immune
system begins to attack certain cells and tissues in the body
Assignment 4
• Due Thursday 26th Dec
• Read pp196-220
• Q1. Plant cells are able to generate ATP in
chloroplasts, why then do they need
mitchondria?
• Q2. The level of CO2 is lower at higher
altitudes, how is it that plants are still able
to grow in such conditions?