The Working Cell

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The Working Cell

• Chapter 5

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Membranes

• Membranes provide the structural basis for
  metabolic order
• Partition the cell into compartments (organelles)
  that contain enzymes in solution
• Enzymes are embedded into the membranes

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Plasma Membrane

• Forms a boundary between the living cell and its
  surroundings
• Controls the traffic of molecules into and out of
  the cell
• All membranes have selective permeability-allows
  some substances to cross more easily than others
  and blocks some substances all together

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Plasma Membrane

• Takes up substances the cell needs and disposes
  of cellular waste

Plasma Membrane
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Lipid Bilayer

• Membranes are mainly composed of lipids, mostly
  phosholipids
• Composed of
• a phosphorus head which is polar
• 2 Lipid tails which are non-polar

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Phospholipid Bilayer

• In water, phospholipids spontaneously form a
  stable two-layer sheet called a phospholipid
  bilayer
• Hydrophilic heads face out towards the water and
  the hydrophobic tails point inward and are
  shielded from the water
• The nature of the bilayer helps make the it
  semi-permeable

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Phospholipid Bilayer

• Polar molecules cannot pass through the non-polar
  region
• The types of proteins present in the membrane
  dictate which molecules will pass through

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A Fluid Mosaic Membrane

• A membrane is described as a mosaic because it
  is a mosaic of diverse proteins molecules
  embedded in a framework of phospholipids
• It is fluid because most of the proteins and
  phospholipids can drift laterally in the membrane
• Some proteins are linked on both sides of the
  membrane and/or to the cytoskeleton in order to
  maintain the integrity of the membrane

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A Fluid Mosaic Membrane
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Proteins in the Membrane

• Proteins perform many of the functions of
  membranes
• Some of these functions may include
• Attaching the membrane to the cytoskeleton
• Providing identification tags
• Forming junctions between adjacent cells

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Membrane Proteins

• Enzyme activity- act as catalysts in molecule
  assembly lines
• Receptors for a chain of reactions called signal
  transduction
• Transporting proteins move substances across the
  membrane

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Passive Transport

• In passive transport,
• substances diffuse
• through membranes
• without work by the
• cell
• Diffusion- tendency for particles of any kind to
  spread out spontaneously to regions where they
  are less concentrated
• They spread from areas of high concentration to
  areas of lower concentration

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Equilibrium

• A solution that is in equilibrium molecules
  continue to move back and forth but there is no
  net change in concentration on either side of the
  membrane

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Osmosis

• Diffusion of water molecules across a selectively
  permeable membrane
• Passive transport
• If a membrane is permeable to water but not the
  solute two solutions on either side of the
  membrane will be formed

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Osmosis

• Hypertonic solution- solution with the higher
  concentration of solute
• Hypotonic solution- the solution with the lower
  concentration of solute
• Water crosses the membrane until the solute
  concentrations are equal on both sides
• Solutions of equal concentration are said to be
  isotonic

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Osmoregulation

• Control of water balance
• Essential that water balance be maintained
  between the cell and its surroundings

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Transport Proteins

• Many substances cannot move across the membrane
  because of their size, polarity, or charge
• Some of these substances can move across with the
  help of transport proteins
• Facilitated diffusion-when a protein makes it
  possible to move a substance with the
  concentration gradient
• Makes diffusion faster

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Transport proteins

• Transport protein spans the membrane
• Provides pore for passage
• Does not expend energy, so it is passive transport

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Active Transport

• Energy is required to move substance across the
  membrane
• Transport protein actively pumps the solute
  across the membrane
• Usually against the concentration gradient
• Usually use ATP as their energy source

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Exocytosis and Endocytosis

• For transport of large molecules that are too big
  for transport across the membrane (proteins)
• Exocytosis- from inside the cell to outside the
  cell (like waste)
• Endocytosis- from outside the cell to inside the
  cell

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Exocytosis and Endocytosis

• Phagocytosis- cellular eating, engulfs prey
• Amoeba
• Pinocytosis- cellular drinking, takes droplets of
  fluid into vesicle

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Energy and the Cell

• Energy- the capacity to perform work
• The ability to move matter in a direction that it
  would not move if if left alone
• All organisms need energy to stay alive
• Energy is what allows change in an organism
• Two forms of energy
• Kinetic Energy- energy that is actually doing
  work
• Potential Energy- stored energy

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Forms of Energy

• Kinetic Energy-Energy that is actually doing work
• Heat- the energy associated with the movement of
  molecules in a body of matter
• Light
• Potential Energy- Stored energy
• The capacity to perform work that matter
  possesses as a result of its location or
  arrangement
• Chemical Energy is the potential energy of
  molecules

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The Laws of Energy

• Life depends on the conversion of energy
• Chemical energy is converted when a chemical
  reaction occurs in the body
• When the body breaks down sugars and converts
  them to other sugars, its harvests the chemical
  energy and uses it for cellular work
• These energy conversions are governed by two
  thermodynamic laws
• Thermodynamics is the study of energy
  transformations that occur in a collection of
  matter

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Laws of Thermodynamics

• First Law of Thermodynamics
• Aka Law of energy conservation
• Energy can be transferred and transformed, but it
  cannot be created or destroyed, the total amount
  of energy in the universe is constant.
• It can be converted from potential energy (stored
  energy) to kinetic energy, like heat or light,
  but it cannot be used up

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Laws of Thermodynamics

• Second Law of Thermodynamics
• Energy changes are not 100 efficient
• Energy conversions increase disorder, or entropy
  (the amt of disorder in a system)
• Some energy is always lost as heat

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Chemical Reactions

• There are two types of chemical reactions
• Endergonic Reactions- need an input of energy to
  to occur, produce products with a lot of
  potential energy
• Exergonic Reactions- chemical reaction that
  releases energy

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Endergonic Reactions

• Energy is absorbed from the surroundings as the
  reaction occurs
• Products store more energy than the reactants
• Example is photosynthetic reactions

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Exergonic Reactions

• Releases energy as the reaction occurs
• Releases potential energy from the covalent bonds
  that were broken
• Energy released is equal to the difference in
  energy between reactants and products
• An example is cellular respiration

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Exergonic Reactions

• Breakdown of glucose molecules in cells and
  storage of the energy released into a chemical
  form (ATP)
• Exergonic-releases energy to be stored as ATP
• Cells use ATP as an immediate source of fuel

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ATP Shuttles Chemical Energy within the Cell

• ATP powers nearly all forms of cellular work
• A cell does three main types of cellular work
• Chemical workdriving endergonic reactions
• Transport workpumping substances across
  membranes
• Mechanical workbeating of cilia

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Cellular Work

• When a cell uses chemical energy to perform work
  it couples an exergonic reaction with an
  endergonic one
• It takes the energy from the exergonic reaction
  and uses it to power the endergonic reaction,
  this is called energy coupling

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ATP

• Adenosine Triphosphate
• Made of 3 parts
• Adenine (nitrogenous base)
• Ribose (sugar)
• Chain of 3 phosphate groups

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Hydrolysis of ATP

• The covalent bonds between the 2nd and 3rd
  phosphate groups are week and can be easily
  broken
• 3 things happen when that bond breaks
• A phosphate is removed
• ATP becomes ADP
• Energy is released (3rd phosphate is energy
  shuttle

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Hydrolysis of ATP

• When the bond joining a phosphate group to the
  rest of an ATP molecule is broken by hydrolysis,
  the reaction supplies energy for cellular work

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• The hydrolysis of ATP to ADP powers cellular work

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Phosphorylation of ATP

• Phosphorylation is the transfer of a phosphate to
  a molecule
• Cellular work can be continuous because ATP is a
  renewable resource (by phosphorylation) that
  cells can generate

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Energy of Activation

• For a chemical reaction to begin, reactants must
  absorb some energy
• This energy is called the energy of activation
  (EA)
• This represents the energy barrier that prevents
  molecules from breaking down spontaneously
• A protein catalyst called an enzyme can decrease
  the energy barrier

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Energy of Activation
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ATP

• For ATP, the EA is the amount of energy needed to
  break the bond between the second and third
  phosphate groups
• Because this reaction requires energy, this rxn
  does not occur spontaneously
• Most other vital molecules in our cells require
  energy to react, this is a type of conservation
  for the cell

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Enzymes

• Protein molecule that serves as a biological
  catalyst, increasing the rate of a reaction
  without itself being changed into a different
  molecule
• Lowers the EA barrier
• Without enzymes, metabolic reactions would occur
  too slowly sustain life

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Enzymes are Specific

• The three dimensional shape of an enzyme
  determines which rxn the enzyme catalyzes
• Substrate- the substance the enzyme acts on
• How an enzyme works
• Enzyme available for bonding
• Enzyme bonds to substrate at active site
• Substrate changes into product
• Products are released

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How an enzyme works
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Enzyme activity and the cellular environment

• Enzyme activity is influenced by
• Temperature- rate of contact
• salt concentration- salt ions interfere with
  bonds
• pH- H ions interfere
• Each enzyme has its own set of optimal conditions
• Some enzymes require binding to non-protein
  cofactors in order to work
• Some cofactors are organic molecules called
  coenzymes

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Enzyme Inhibitors

• Chemical that interferes with an enzymes activity
• Competitive inhibitor- competes with the
  substrate for the active site
• Non-competitive inhibitor- binds outside the
  active site to change shape of the enzyme

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Converting Energy

• Chloroplast and mitochondria play central roles
  in harvesting energy and making it available for
  cellular work
• Chloroplasts use solar energy to convert carbon
  dioxide and water into glucose
• Mitochondria consume oxygen in cellular
  respiration and take the chemical energy stored
  in glucose to make ATP
• It does not matter whether the glucose comes from
  photosynthesis or from digestion with animals

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Converting Energy