The Working Cell

1
The Working Cell

• Ch. 5

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A. Forms of Energy

• 1. Energy is capacity to do work cells
  continually use energy to develop, grow, repair,
  reproduce, etc.
• 2. Kinetic energy is energy of motion all moving
  objects have kinetic energy.
• 3. Potential energy is stored energy.
• 4. Food is chemical energy it contains potential
  energy.
• 5. Chemical energy can be converted into
  mechanical energy, e.g., muscle movement.

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Two Laws of Thermodynamics
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First law of thermodynamics (also called the law
of conservation of energy)

• a. Energy cannot be created or destroyed, but it
  can be changed from one form to another.
• b. In an ecosystem, solar energy is converted to
  chemical energy by the process of photosynthesis
  some of the chemical energy in the plant is
  converted to chemical energy in an animal, which
  in turn can become mechanical energy or heat loss.

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continued

• c. Neither the plant nor the animal create
  energy, they convert it from one form to another.
• d. Likewise, energy is not destroyed some
  becomes heat that dissipates into the environment.

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Second law of thermodynamics

• a. Energy cannot be changed from one form into
  another without a loss of usable energy.
• b. Heat is a form of energy that dissipates into
  the environment heat can never be converted back
  to another form of energy.

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Cells and Entropy

• 1. Every energy transformation makes the universe
  less organized and more disordered entropy is
  the term used to indicate the relative amount of
  disorganization.
• 2. When ions distribute randomly across a
  membrane, entropy has increased.
• 3. Organized/usable forms of energy (as in the
  glucose molecule) have relatively low entropy
  unorganized/less stable forms have relatively
  high entropy.

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continued

• 4. Energy conversions result in heat therefore,
  the entropy of the universe is always increasing.
• 5. Living things depend on a constant supply of
  energy from the sun, because the ultimate fate of
  all solar energy in the biosphere is to become
  randomized in the universe as heat the living
  cell is a temporary repository of order purchased
  at the cost of a constant flow of energy.

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Metabolic Reactions and Energy Transformations

• 1. Metabolism is the sum of all the biochemical
  reactions in a cell.
• 2. In the reaction A B C D, A and B are
  reactants and C and D are products.
• 3. Free energy (?G) is the amount of energy that
  is free to do work after a chemical reaction.

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continued

• 4. Change in free energy is noted as ?G a
  negative ?G means that products have less free
  energy than reactants the reaction occurs
  spontaneously.
• 5. Exergonic reactions have a negative ?G and
  energy is released.
• 6. Endergonic reactions have a positive ?G
  products have more energy than reactants such
  reactions can only occur with an input of energy.

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ATP Energy for Cells

• 1. Adenosine triphosphate (ATP) is the energy
  currency of cells when cells need energy, they
  spend ATP.
• 2. ATP is an energy carrier for many different
  types of reactions.
• 3. When ATP is converted into ADP P, the energy
  released is sufficient for biological reactions
  with little wasted.

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continued

• 4. ATP breakdown is coupled to endergonic
  reactions in a way that minimizes energy loss.
• 5. ATP is a nucleotide composed of the base
  adenine and the 5-carbon sugar ribose and three
  phosphate groups.
• 6. When one phosphate group is removed, about 7.3
  kcal of energy is released per mole.

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Coupled Reactions

• 1. Coupled reactions are reactions that occur in
  the same place, at the same time, and in a way
  that an exergonic reaction is used to drive an
  endergonic reaction.
• 2. ATP breakdown is often coupled to cellular
  reactions that require energy.
• 3. ATP supply is maintained by breakdown of
  glucose during cellular respiration.

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Metabolic Pathways and Enzymes

• 1. Enzymes are catalysts that speed chemical
  reactions without the enzyme being affected by
  the reaction.
• 2. Every enzyme is specific in its action and
  catalyzes only one reaction or one type of
  reaction.
• 3. Ribozymes are made of RNA rather than proteins
  and also serve as catalysts.

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• 4. A metabolic pathway is an orderly sequence of
  linked reactions each step is catalyzed by a
  specific enzyme.
• 5. Metabolic pathways begin with a particular
  reactant, end with a particular end product(s),
  and may have many intermediate steps.
• 6. In many instances, one pathway leads to the
  next since pathways often have one or more
  molecules in common, one pathway can lead to
  several others.

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• 7. Metabolic energy is captured more easily if it
  is released in small increments.
• 8. A reactant is the substance that is converted
  into a product by the reaction often many
  intermediate steps occur.

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Energy of Activation

• 1. A substrate is a reactant for an enzymatic
  reaction.
• 2. Enzymes speed chemical reactions by lowering
  the energy of activation (Ea) by forming a
  complex with their substrate(s) at the active
  site.
• a. An active site is a small region on the
  surface of the enzyme where the substrate(s)
  bind.
• b. When a substrate binds to an enzyme, the
  active site undergoes a slight change in shape
  that facilitates the reaction. This is called
  the induced fit model of enzyme catalysis.
• 3. Only a small amount of enzyme is needed in a
  cell because enzymes are not consumed during
  catalysis.

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• 4. Some enzymes (e.g., trypsin) actually
  participate in the reaction.
• 5. A particular reactant(s) may produce more than
  one type of product(s).
• a. Presence or absence of enzyme determines which
  reaction takes place.
• b. If reactants can form more than one product,
  the enzymes present determine which product is
  formed.

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Factors Affecting Enzymatic Speed

• 1. Substrate concentration.
• Because molecules must collide to react, enzyme
  activity increases as substrate concentration
  increases as more substrate molecules fill
  active sites, more product is produced per unit
  time.

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2. Optimal pH

• a. Every enzyme has optimal pH at which its rate
  of reaction is optimal.
• b. A change in pH can alter the ionization of
  the R groups of the amino acids in the enzyme,
  thereby disrupting the enzymes activity.

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3. Temperature

• As temperature rises, enzyme activity increases
  because there are more enzyme-substrate
  collisions.
• Enzyme activity declines rapidly when enzyme is
  denatured at a certain temperature, due to a
  change in shape of the enzyme.

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4. Enzyme cofactors

• a. Many enzymes require an inorganic ion or
  non-protein cofactor to function.
• b. Inorganic cofactors are ions of metals.
• c. A coenzyme is an organic cofactor, which
  assists the enzyme (i.e., it may actually
  contribute atoms to the reaction).
• d. Vitamins are small organic molecules required
  in trace amounts for synthesis of coenzymes they
  become part of a coenzymes molecular structure
  vitamin deficiency causes a lack of a specific
  coenzyme and therefore a lack of its enzymatic
  action.

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5. Enzyme inhibition

• a. Enzyme inhibition occurs when a substance
  (called an inhibitor) binds to an enzyme and
  decreases its activity normally, enzyme
  inhibition is reversible.
• b. In noncompetitive inhibition, the
  inhibitor binds to the enzyme at a location other
  than the active site (the allosteric site),
  changing the shape of the enzyme and rendering it
  unable to bind to its substrate.
• c. In competitive inhibition, the
  substrate and the inhibitor are both able to bind
  to the enzymes active site.

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Organelles and the Flow of Energy
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Photosynthesis

• 1. Photosynthesis uses energy to combine carbon
  dioxide and water to produce glucose in the
  formula
• 6 CO2 6 H2O energy C6H12O6 6 O2
• 2. Oxidation is the loss of electrons.
• 3. Reduction is the gain of electrons.
• 4. When hydrogen atoms are transferred to carbon
  dioxide from water, water has been oxidized and
  carbon dioxide has been reduced.

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• 5. Input of energy is needed to produce the
  high-energy glucose molecule.
• 6. Chloroplasts capture solar energy and convert
  it by way of an electron transport system into
  the chemical energy of ATP.
• 7. ATP is used along with hydrogen atoms to
  reduce glucose when NADP (nicotinamide adenine
  dinucleotide phosphate) donates hydrogen atoms
  (H e-) to a substrate during photosynthesis,
  the substrate has accepted electrons and is
  therefore reduced.
• 8. The reaction that reduces NADP is
• NADP 2e- H NADPH

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Cellular Respiration

• 1. The overall equation for cellular respiration
  is opposite that of photosynthesis
• C6H12O6 6 O2 6 CO2 6 H2O energy
• 2. When NAD removes hydrogen atoms (H e-)
  during cellular respiration, the substrate has
  lost electrons and is therefore oxidized.
• 3. At the end of cellular respiration, glucose
  has been oxidized to carbon dioxide and water and
  ATP molecules have been produced.
• In metabolic pathways, most oxidations involve
  the coenzyme NAD (nicotinamide adenine
  dinucleotide) the molecule accepts two electrons
  but only one hydrogen ion NAD 2e- H
  NADH

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Electron Transport Chain

• 1. Both photosynthesis and respiration use an
  electron transport chain consisting of
  membrane-bound carriers that pass electrons from
  one carrier to another.
• High-energy electrons are delivered to the system
  and low-energy electrons leave it.
• The overall effect is a series of redox
  reactions every time electrons transfer to a new
  carrier, energy is released for the production of
  ATP.

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ATP Production

• 1. ATP synthesis is coupled to the electron
  transport system.
• 2. Peter Mitchell received the 1978 Nobel Prize
  for his chemiosmotic theory of ATP production.
• 3. In both mitochondria and chloroplasts,
  carriers of electron transport systems are
  located within a membrane.
• 4. H ions (protons) collect on one side of the
  membrane because they are pumped there by
  specific proteins.

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• 5. The electrochemical gradient thus established
  across the membrane is used to provide energy for
  ATP production.
• 6. Enzymes and their carrier proteins, called ATP
  synthase complexes, span the membrane each
  complex contains a channel that allows H ions to
  flow down their electrochemical gradient.
• 7. In photosynthesis, energized electrons lead to
  the pumping of hydrogen ions across the thylakoid
  membrane as hydrogen ions flow through the ATP
  synthase complex, ATP is formed.
• 8. During cellular respiration, glucose breakdown
  provides energy for a hydrogen ion gradient on
  the inner membrane of the mitochondria that also
  couples hydrogen ion flow with ATP formation.

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Fluid-Mosaic Model

• 1. The fluid-mosaic model describes the plasma
  membrane.
• 2. The fluid component refers to the
  phospholipids bilayer of the plasma membrane.
• 3. Fluidity of the plasma membrane allows cells
  to be pliable.
• 4. Fluidity is affected by cholesterol molecules
  in the plasma membrane.
• 5. The mosaic component refers to the protein
  content in the plasma membrane.
• 6. Protins bond to the ECM and/or cytoskeleton to
  prevent movement in the fluid phospholipid bilayer

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. Permeability of the Plasma Membrane
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The plasma membrane is differentially
(selectively) permeable only certain molecules
can pass through.
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• a. Small non-charged lipid molecules (alcohol,
  oxygen) pass through the membrane freely.
• b. Small polar molecules (carbon dioxide, water)
  move down a concentration gradient, i.e., from
  high to low concentration.
• c. Ions and charged molecules cannot readily pass
  through the hydrophobic component of the bilayer
  and usually combine with carrier proteins.

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Both passive and active mechanisms move molecules
across membrane.

• a. Passive transport moves molecules across
  membrane without expenditure of energy includes
  diffusion and facilitated transport.

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• b. Active transport requires a carrier protein
  and uses energy (ATP) to move molecules across a
  plasma membrane includes active transport,
  exocytosis, endocytosis, and pinocytosis.

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• 3. The presence of a membrane channel protein
  called an aquaporin allows water to cross
  membranes quickly.
• 4. Substances enter or exit a cell through bulk
  transport.

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Passive Transport Across a Membrane

• 1. Diffusion is the movement of molecules from
  higher to lower concentration (i.e., down the
  concentration gradient).

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Diffusion continued

• a. A solution contains a solute, usually a solid,
  and a solvent, usually a liquid.
• b. In the case of a dye diffusing in water, the
  dye is a solute and water is the solvent.
• c. Once a solute is evenly distributed, random
  movement continues but with no net change.

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Diffusion continued

• d. Membrane chemical and physical properties
  allow only a few types of molecules to cross by
  diffusion.
• e. Gases readily diffuse through the lipid
  bilayer e.g., the movement of oxygen from air
  sacs (alveoli) to the blood in lung capillaries
  depends on the concentration of oxygen in
  alveoli.
• f. Temperature, pressure, electrical currents,
  and molecular size influence the rate of
  diffusion.

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1. Osmosis is the diffusion of water across a
differentially (selectively) permeable membrane

• a. Osmosis is illustrated by the thistle tube
  example
• 1) A differentially permeable membrane separates
  two solutions.
• 2) The beaker has more water (lower percentage of
  solute) and the thistle tube has less water
  (higher percentage of solute).
• 3) The membrane does not permit passage of the
  solute water enters but the solute does not
  exit.
• 4) The membrane permits passage of water with a
  net movement of water from the beaker to the
  inside of the thistle tube.

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• b. Osmotic pressure is the pressure that develops
  in such a system due to osmosis.
• c. Osmotic pressure results in water being
  absorbed by the kidneys and water being taken up
  from tissue fluid.

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2. Tonicity is strength of a solution with
respect to osmotic pressure.

• a. Isotonic solutions occur where the relative
  solute concentrations of two solutions are equal
  a 0.9 salt solution is used in injections
  because it is isotonic to red blood cells (RBCs).

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• b. A hypotonic solution has a solute
  concentration that is less than another solution
  when a cell is placed in a hypotonic solution,
  water enters the cell and it may undergo
  cytolysis (cell bursting).
• c. Swelling of a plant cell in a hypotonic
  solution creates turgor pressure this is how
  plants maintain an erect position.

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• When a plant cell is placed in a hypotonic
  solution, it is turgid.

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• A hypertonic solution has a solute concentration
  that is higher than another solution when a cell
  is placed in a hypertonic solution, it shrivels
  (a condition called crenation).
• Plasmolysis is shrinking of the cytoplasm due to
  osmosis in a hypertonic solution as the central
  vacuole loses water, the plasma membrane pulls
  away from the cell wall.

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• In a hypotonic solution, an animal cell will lyse.

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3. Facilitated Transport

• a. Facilitated transport is the transport of a
  specific solute down or with its
  concentration gradient (from high to low),
  facilitated by a carrier protein glucose and
  amino acids move across the membrane in this way.

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Active Transport Across a Membrane

• A. Active transport is transport of a specific
  solute across plasma membranes up or against
  (from low to high) its concentration gradient
  through use of cellular energy (ATP).

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• 1. Iodine is concentrated in cells of thyroid
  gland, glucose is completely absorbed into lining
  of digestive tract, and sodium is mostly
  reabsorbed by kidney tubule lining.
• 2. Active transport requires both carrier
  proteins and ATP therefore cells must have high
  number of mitochondria near membranes where
  active transport occurs.

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• 3. Proteins involved in active transport are
  often called pumps the sodium-potassium pump
  is an important carrier system in nerve and
  muscle cells.
• 4. Salt (NaCl) crosses a plasma membrane because
  sodium ions are pumped across, and the chloride
  ion is attracted to the sodium ion and simply
  diffuses across specific channels in the
  membrane.

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Bulk Transport

• 1. In exocytosis, a vesicle formed by the Golgi
  apparatus fuses with the plasma membrane as
  secretion occurs insulin leaves
  insulin-secreting cells by this method.
• 2. During endocytosis, cells take in substances
  by vesicle formation as plasma membrane pinches
  off by either phagocytosis, pinocytosis, or
  receptor-mediated endocytosis.

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• In phagocytosis, cells engulf large particles
  (e.g., bacteria), forming an endocytic vesicle.
• a. Phagocytosis is commonly performed by
  ameboid-type cells (e.g., amoebas and
  macrophages).
• b. When the endocytic vesicle fuses with a
  lysosome, digestion of the internalized substance
  occurs.

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• 4. Pinocytosis occurs when vesicles form around a
  liquid or very small particles this is only
  visible with electron microscopy.

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5. Receptor-mediated endocytosis, a form of
pinocytosis, occurs when specific macromolecules
bind to plasma membrane receptors.

• a. The receptor proteins are shaped to fit with
  specific substances (vitamin, hormone,
  lipoprotein molecule, etc.), and are found at one
  location in the plasma membrane.

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• b. This location is a coated pit with a layer of
  fibrous protein on the cytoplasmic side when the
  vesicle is uncoated, it may fuse with a lysosome.
• c. Pits are associated with exchange of
  substances between cells (e.g., maternal and
  fetal blood).
• d. This system is selective and more efficient
  than pinocytosis it is important in moving
  substances from maternal to fetal blood.

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• e. Cholesterol (transported in a molecule called
  a low-density lipoprotein, LDL) enters a cell
  from the bloodstream via receptors in coated
  pits in familial hypocholesterolemia, the LDL
  receptor cannot bind to the coated pit and the
  excess cholesterol accumulates in the circulatory
  system.

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Modification of Cell Surfaces

• A. Cell Surfaces in Animals
• 1. The extracellular matrix is a meshwork of
  polysaccharides and proteins produced by animal
  cells.

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• Collagen gives the matrix strength and elastin
  gives it resilience.
• Fibronectins and laminins bind to membrane
  receptors and permit communication between matrix
  and cytoplasm these proteins also form
  highways that direct the migration of cells
  during development.
• Proteoglycans are glycoproteins that provide a
  packing gel that joins the various proteins in
  matrix and most likely regulate signaling
  proteins that bind to receptors in the plasma
  protein.

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Junctions Between Cells are points of contact
between cells that allow them to behave in a
coordinated manner.

• Anchoring junctions mechanically attach adjacent
  cells.
• In adhesion junctions, internal cytoplasmic
  plaques, firmly attached to cytoskeleton within
  each cell are joined by intercellular filaments
  they hold cells together where tissues stretch
  (e.g., in heart, stomach, bladder).

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• In desmosomes, a single point of attachment
  between adjacent cells connects the cytoskeletons
  of adjacent cells.
• In tight junctions, plasma membrane proteins
  attach in zipper-like fastenings they hold cells
  together so tightly that the tissues are barriers
  (e.g., epithelial lining of stomach, kidney
  tubules, blood-brain barrier).

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A gap junction allows cells to communicate
formed when two identical plasma membrane
channels join

• They provide strength to the cells involved and
  allow the movement of small molecules and ions
  from the cytoplasm of one cell to the cytoplasm
  of the other cell.
• Gap junctions permit flow of ions for heart
  muscle and smooth muscle cells to contract.