Transport In/Out of Cells

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Lecture 10

• Transport In/Out of Cells

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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).

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Keeping Things In!

• The cell membrane functions as a semi-permeable
  barrier, allowing a very few molecules across.
• Keeps the majority of organically produced
  chemicals inside the cell.

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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.

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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.

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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-elevat
  ed, so it will move out of the cell to an area of
  low concentration.

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• 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.

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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
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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.

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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.

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Types of Passive Diffusion
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Diffusion of Gases and Water Molecules Across The
Lipid Bilayer
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Facilitated Diffusion
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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.

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Active Transport
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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 vice-versa.E.g Sodium and Potassium Pump.

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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.

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Endocytosis and Exocytosis
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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

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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?