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Chapter 4. How Cells Work. Energy. Energy is central to life Universal relationship between energy and work Ultimate energy source = SUN Plants transform light energy into chemical energy (C6H12O6) Photosynthesis. What is energy?. Energy = capacity to do work

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chapter 4

Chapter 4

How Cells Work

  • Energy is central to life
    • Universal relationship between energy and work
  • Ultimate energy source = SUN
    • Plants transform light energy into chemical energy (C6H12O6)
    • Photosynthesis
what is energy
What is energy?
  • Energy = capacity to do work
  • Work = movement against an opposing force
the energy currency molecule
The Energy Currency Molecule
  • Adenosine triphosphate, ATP
  • Structure of ATP
    • Adenosine
      • Ribose + adenine
    • 3 phosphate groups
      • Negatively charged molecules that repel each other
    • 2 HIGH ENERGY phosphate bonds
how do cells use atp
How do cells use ATP?
  • Breakage of the last bond (release of a phosphate group) releases energy and allows the cell to do work
  • ATP = STORED ENERGY = Potential
how cells use atp
How Cells Use ATP

ATP → ADP + Pi + energy

how do cells use atp1
How do cells use ATP?
  • From where does the energy to make ATP from ADP come?
    • Covalent bonds in macromolecules!
    • Cells recycle the ADP and phosphates
    • This process requires ENERGY!!
    • Analogous to recharging a battery:
      • The components are in the battery – energy needs to be added to the battery to make it useable
energy reactions and cycles
Energy Reactions and Cycles
  • Endergonic Reactions
    • Require energy
    • i.e. Synthesis of glucose from CO2 and water during photosynthesis
  • Exergonic Reactions
    • Release energy
    • i.e. Breakdown of glucose to CO2 and water by aerobic respiration



  • Reaction characteristics
    • Exergonic reactions in living things may not occur very quickly
      • Energy of Activation (high temperature, light) is needed to start the reaction
      • Amount of energy needed to start a reaction
    • Many different reactions are needed to complete a task
      • These reactions are linked together
ways to lower the energy of activation
Ways to Lower the Energy of Activation
  • Enzymes
    • Protein catalysts that lower the amount of energy needed to get the chemical reaction going
    • They maintain their original chemical composition while causing a change in the substrate (reactant)
    • The specific shape of the enzyme allows it to catalyze only one reaction
    • Active site = place on the enzyme that binds substrate
    • Since the enzyme does NOT change its shape, it is REUSABLE
altering the rate of an enzymatic reaction
Altering the Rate of an Enzymatic Reaction
  • One can alter the rate by altering two key factors:

1. Temperature

2. pH

3. Coenzymes and Cofactors

4. Allosteric Regulators

5. Salt Concentration

altering temperature
Altering Temperature
  • Gradual ↑ in temperature will INCREASE the rate of the reaction
    • How? By an increase in the speed at which the molecules are moving
    • This results in increased collisions of the enzyme and substrate
  • Extremely low temperatures will SLOW DOWN or STOP the reaction
    • Why? The enzyme and substrate are moving too slow to collide
  • Extremely high temperatures will STOP the reaction
    • Why? Because the enzyme will be denatured!
altering ph
Altering pH
  • Alterations in pH will STOP the reaction because the enzyme will be denatured!
  • Remember, a small pH change does NOT correlate with a small change in the pH of the environment!!
    • Why? pH scale is logarithmic
enzyme questions
Enzyme Questions
  • The presence of an enzyme _____ the required energy of activation of a chemical reaction.
  • Generally, as the amount of substrate is increased, the rate of the reaction _____.
  • Raising the temperature to over 50C ___ the rate of an enzymatic reaction.
  • Lowering the pH for an enzyme that works best in a highly acidic environment ___ the rate for the reaction.
ways that substances can move across the pm
Ways that substances can move across the PM
  • Passive
    • Process that does NOT require energy
    • Includes:
      • Diffusion
      • Osmosis
  • Active
    • Process that DOES REQUIRE energy!
    • Includes:
      • Endocytosis
        • Phagocytosis
        • Pinocytosis
        • Receptor-Mediated Endocytosis
      • Exocytosis
selective permeability
Selective Permeability
  • Protein channels located in the plasma membrane act as channels
  • Each channel passes only a certain kind of molecule (some are specific, some non-specific)
  • Types of selective permeability
    • Selective Diffusion
    • Facilitated Diffusion
    • Active Transport
selective diffusion
Selective Diffusion
  • Movement of molecules from high concentration to low concentration
  • Channels may act as ‘open doors’
  • Example includes ion channels
    • Ion channels allow passage of any ion that can fit in the channel
    • Essential roles in nervous system signaling
  • Oxygen, Carbon dioxide (CO2) and lipids can pass across the PM using diffusion
  • One way in which water and other substances can move across the PM
  • RANDOM movement of molecules in a solution from regions of HIGH concentration to regions of LOW concentration
    • HIGH low
  • Random movement occurs until equilibration occurs
    • Until there is NO NET MOVEMENT in any particular direction
    • NOTE: Individual molecules are still moving – but there is no overall directionality!
diffusion terms to know
DiffusionTerms to know:
  • Concentration gradient
    • A system that is imposed on a solution by molecules present in that solution.
    • Ex. Sugar in water
      • When sugar is dropped in water, the sugar molecules break up and dissolve over time. The individual sugar molecules moving into the water move DOWN their concentration gradient – they are moving from the cube of sugar to spread out in the water where there is no sugar.
  • Movement of WATER ONLY across the PM from the side with more water (less solute) to the side with less water (more solute)
  • Water passes into and out of a cell down its concentration gradient (DIFFUSION)
  • DIFFERENT from diffusion in that water movement depends upon the concentration of other substances in solution
osmosis terms to know
OsmosisTerms to know:
  • Osmotic concentration
    • Concentration of ALL molecules dissolved in a solution
  • Hypertonic
    • The solution with higher solute concentration
  • Hypotonic
    • The solution with lower solute concentration
  • Isotonic
    • Solutions are isotonic when the solute concentrations of both are equal
osmosis osmotic pressure
OsmosisOsmotic pressure
  • Generated by movement of water into a cell by osmosis
  • Ex. Red blood cell, Figure 4.28
what is another way that cells can take in food and liquids
What is another way that cells can take in food and liquids?
  • Diffusion
  • Osmosis
  • Endocytosis
    • Phagocytosis
    • Pinocytosis
    • Receptor-Mediated Endocytosis
  • Exocytosis
  • Allows for BULK PASSAGE of food and liquids INTO the cell
  • Two types:
    • Phagocytosis
      • “Cell eating”
    • Pinocytosis
      • “Cell drinking”
  • The PM engulfs the particle(s) forming a vesicle thus allowing a means of entry into the cell
endocytosis phagocytosis
  • “Cell eating”
  • Material that the cell takes in may include particulate, digested particles or other fragments of organic matter


  • “Cell drinking”
  • Material that the cell takes in is liquid
endocytosis continued
Endocytosis, continued
  • Rates of endocytosis vary among cells!
  • Ex. Muscle cells during exercise
  • Process by which material is discharged from the cell
  • Material to be discharged is packaged into vesicles inside the cell (by what organelle?)
  • Vesicles then make they way (along what?) to the plasma membrane for secretion into the cell exterior
problems with endocytosis
Problems with endocytosis
  • Expensive for the cell
    • Cell uses a lot of its membrane to form vesicles
  • Non selective
    • Anything can enter the cell through
everyday science hypercholesterolemia
Everyday ScienceHypercholesterolemia
  • Human genetic disease
  • Receptors are normally embedded in the PM
  • In patients with HC, the receptors are not help in place by clathrin
  • This results in a failure of cholesterol uptake into the cell (failure of the mouse-trap triggering mechanism) thus leaving the cholesterol to travel though the bloodstream and bind to arteries
discussion question
Discussion Question
  • Do you think muscle cells have a higher or lower rate of endocytosis during exercise?