Chapter 6: Cell Energy: Photosynthesis and Respiration

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Chapter 6Cell Energy Photosynthesis and
Respiration

• Section 1
• Photosynthesis Capturing and Converting Energy

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Photosynthesis

• In the process of photosynthesis, plants convert
  the energy of sunlight into the energy in the
  chemical bonds of carbohydrates sugars, and
  starches
• Put more simply, plants use the energy of
  sunlight to produce carbohydrates in a process
  called photosynthesis

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Photosynthesis

• An understanding of photosynthesis was developed
  from studies of plant growth
• Dutch physician Jan Van Helmont devised an
  experiment to determine how plant growth actually
  works
• Found the mass of a pot of dry soil and a small
  seedling
• Planted the seedling in the pot of soil
• Watered regularly for 5 years
• Gained about 75 kilograms
• Mass of soil was unchanged
• Concluded that most of the mass must come from
  water because that is all he added to the pot

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Photosynthesis

• Although Van Helmont did not realize it, carbon
  dioxide in the air made a major contribution to
  the mass of his tree
• It is the carbon in carbon dioxide that is used
  to make carbohydrates in photosynthesis

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Photosynthesis

• Almost 100 years after Van Helmonts experiment,
  Joseph Priestley performed an experiment that
  would give another insight into the process of
  photosynthesis
• Took a candle, placed a glass jar over it, and
  watched as the flame gradually died out
• Something in the air was necessary to keep a
  candle burning
• When that substance was used up, the candle went
  out
• oxygen

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Photosynthesis

• Priestley then placed a spring of mint under the
  jar and allowed a few days to pass, the candle
  could be relighted and would remain lighted for
  awhile
• The mint had produced the substance required for
  burning
• oxygen
• Later, Dutch scientist Jan Ingenhousz showed that
  this only occurred when the plant was exposed to
  light

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Requirements for Photosynthesis

• These experiments reveal that in the presence of
  light, plants transform carbon dioxide and water
  into carbohydrates and release oxygen
• Usually produces the sugar glucose
• 6CO2 6H2O C6H12O6 6O2

light
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Sunlight

• Nearly all organisms on Earth depend on the sun
  for energy
• Autotroph organisms that are able to use a
  source of energy, such as sunlight, to produce
  food directly from simple inorganic substances in
  the environment
• Heterotroph organisms that obtain energy from
  the foods they eat
• The sun bathes the Earth in a steady stream of
  light
• We see colorless white light but it is actually
  a mixture of different wavelengths of light
• Visible spectrum

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Pigments

• Process of photosynthesis begins when light is
  absorbed by pigments in the plant cell
• Colored substances that absorb or reflect light
• Principal pigment in green plants is chlorophyll
• Absorbs red and blue light but does not absorb
  light in the middle region of the spectrum very
  well
• These wavelengths are reflected

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Energy-Storing Compounds

• In a green plant, the energy of sunlight is
  transferred to electrons, raising them to a
  higher energy level
• The electrons belong to the pigment chlorophyll
• High-energy electrons are trapped in chemical
  bonds
• Two ways in which energy of sunlight is trapped
  in chemical bonds

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Energy-Storing Compounds

• First way sunlight is trapped in chemical bonds
• Simpler of the two
• A pair of high-energy electrons are passed
  directly to an electron carrier
• A molecule that can accept a pair of electrons
  and later transfer them along with most of their
  energy to another compound
• Plants use the electron carrier NADP
• When NADP accepts a pair of high-energy
  electrons, it is converted to NADPH
• ONE WAY IN WHICH SOME OF THE ENERGY OF SUNLIGHT
  CAN BE TRAPPED IN CHEMICAL FORM

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Energy-Storing Compounds

• Second way sunlight is trapped in chemical bonds
• Involves adenosine triphosphate (ATP)
• Consists of adenine, a 5-carbon sugar called
  ribose, and three phosphate groups
• During photosynthesis, green plants produce ATP,
  which is an energy-storing compound used by every
  living cell

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• As one might suspect,
• there are 3 phosphate
• groups.

• There is a high E bond
• between the 2nd and 3rd
• P group.

• When cells need E this
• high E bond is broken and
• E is released. Its not ATP
• anymore. What is the
• new molecule formed??

ADP

• Notice the other two components of the the ATP
  molecule.

Adenine and Ribose
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• E from the food a cell takes in is used to
• convert ADP back to ATP.

ADP phosphate    ATP by the enzyme
ATP synthetase
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Chapter 6Cell Energy Photosynthesis and
Respiration

• Section 2
• Photosynthesis The Light and Dark Reactions

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Photosynthesis The Light and Dark Reactions

• The production of NADPH and ATP requires sunlight
• Light reactions the energy of sunlight is
  captured and used to make energy-storing
  compounds
• Another set of reactions called the dark
  reactions uses the energy stored in NADPH and ATP
  to produce glucose
• Do not require light
• However, they can and do occur in the light also

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The Light Reactions

• Photosynthesis takes place in the chloroplast
• Within the chloroplast are saclike photosynthetic
  membranes that contain chlorophyll
• Light reactions take place in these membranes
• Can be divided into four basic processes light
  absorption, electron transport, oxygen
  production, and ATP formation

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Light Absorption

• Photosynthetic membranes contain clusters of
  pigment molecules, or photosystems, that are able
  to capture the energy of sunlight
• Two photosystems in plants
• Photosystem I
• Photosystem II
• Each contains several hundred chlorophyll
  molecules as well as other accessory pigments
• Absorb light in the regions of the spectrum where
  chlorophyll does not

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Light Absorption

• After light energy is absorbed by one of the
  pigment molecules in a photosystem, the energy is
  passed from one pigment molecule to the next
  until it reaches a special pair of chlorophyll
  molecules in the reaction center of the
  photosystem
• In the reaction center, high-energy electrons are
  released and are passed to the first of many
  electron carriers

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Electron Transport

• High-energy electrons are transferred along a
  series of electron carriers
• Electron transport?the electron carriers
  themselves are known as the electron transport
  chain
• At the end of the chain, the electrons are passed
  to NADP, converting it to NADPH

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Oxygen Production

• The photosynthetic membrane contains a system
  that provides new electrons to chlorophyll to
  replace the ones that wound up in NADPH
• Four electrons are removed from two water
  molecules
• 4 H ions
• 2 O atoms
• Form a single molecule of oxygen gas
• Released into the air

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ATP Formation

• H ions are released inside the photosynthetic
  membrane as well as being pumped across the
  membrane
• The inside of the membrane fills up with H ions
• Makes the outside negatively charged and the
  inside positively charged
• Forms ATP

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A Summary of the Light Reactions

• Use water, ADP, NADP
• Produce O2, ATP and NADPH
• The dark reactions will convert these
  energy-storing molecules to a more convenient form

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The Dark Reactions

• Light does not play a role in the dark reactions
• The series of chemical changes that make up the
  dark reactions is critical to living things
• Carbon dioxide is used to make organic compounds
• The dark reactions form a cycle called the Calvin
  cycle

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The Calvin Cycle

• 5 carbon sugar (C5) combines with CO2 to form two
  3 carbon compounds (C3)
• Relatively slow
• Uses the enzyme rubisco to speed up the process
• Using ATP and NADPH, the 3 carbon compounds are
  converted to PGAL (phosphoglyceraldehyde)
• 6 turns of the cycle to make one molecule of
  glucose

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Chapter 6 Cell Energy Photosynthesis and
Respiration

• Section 3
• Glycolysis and Respiration

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Glycolysis Breaking Down Glucose

• C6H12O6 6O2 ? 6CO2 6H2O
• Gives off 3811 calories
• Glycolysis takes place in the cytoplasm of a cell
• In glycolysis, a series of enzymes catalyzes
  chemical reactions that change glucose, one step
  at a time, into different molecules

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Respiration

• If oxygen is available, respiration can take
  place
• Aerobic process
• Respiration is the process that involves oxygen
  and breaks down food molecules to release energy
• Uses the pyruvic acid formed in glycolysis
• Often used as a synonym for breathing
• Takes place in the cells mitochondria

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The Krebs Cycle

• First set of reactions in respiration
• Krebs cycle
• 2 carbon atoms added (from the breakdown of
  pyruvic acid)
• 2 carbon atoms removed (in 2 molecules of CO2)
• 3 molecules of NAD converted to NADH
• 1 molecule of FAD converted to FADH2
• 1 molecule of GDP converted to GTP

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Electron Transport in the Mitochondrion

• High energy electrons from NADH and FADH2 are
  passed to electron transport enzymes in the
  mitochondrion
• Form an ETC along which electrons are passed
• Enzyme at the end of the chain combines e- from
  ETC, H ions from fluid inside the cell, and O2
  to form H2O
• Oxygen is the final electron acceptor in
  respiration
• Is essential for obtaining energy from both NADH
  and FADH2

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ATP Formation

• Electron transport involves the movement of
  hydrogen ions
• As enzymes accept electrons, they pump a hydrogen
  from the inside to the outside
• This movement powers the formation of ATP
• On average, the movement of a pair of electrons
  down the ETC produces enough energy to form 3 ATP
  from ADP
• More H ions outside
• This imbalance supplies the energy to make ATP
  from ADP

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The Totals

• Glycolysis and respiration together produce a
  total of 36 ATP molecules

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Obtaining Energy From Food

• Complex carbohydrates are broken down into simple
  sugars that are then converted into glucose
• The pathways we have discussed can be used to
  produce energy
• The cell can generate chemical energy in the form
  of ATP from just about any source

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Breathing and Respiration

• Final acceptor for all electrons in respiration
  is oxygen
• Without oxygen, electron transport cannot
  operate, Krebs cycle stops, and ATP production
  stops
• With each breath we take, air flows into our
  lungs
• Oxygen has a critical role to play in the
  mitochondria of every cell

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Energy in Balance

• Photosynthesis and respiration can be thought of
  as opposite processes
• Photosynthesis deposits energy
• Respiration withdraws energy

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Chapter 6 Cell Energy Photosynthesis and
Respiration

• Section 4
• Fermentation

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Fermentation

• Fermentation is a process that enables cells to
  carry out energy production in the absence of
  oxygen
• Breakdown of glucose and release of energy in
  which organic substances are the final electron
  acceptors
• Fermentation is anaerobicit does not require
  oxygen
• Fermentation enables cells to carry out energy
  production in the absence of oxygen
• Produces 2 ATP

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Lactic Acid Fermentation

• In many cells, the pyruvic acid that accumulates
  as a result of glycolysis can be converted to
  lactic acid
• Lactic acid fermentation
• Pyruvic acid NADH ? lactic acid NAD
• Lactic acid is produced in muscles during rapid
  exercise when the body cannot supply enough
  oxygen to tissues to produce all of the ATP that
  is required
• Causes a burning, painful sensation
• Large muscles quickly run out of oxygen
• Muscle cells begin to rapidly produce ATP by
  fermentation

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Alcoholic Fermentation

• Another type of fermentation occurs in yeasts and
  a few other microorganisms
• Pyruvic acid is broken down to produce a 2 carbon
  alcohol and carbon dioxide
• Alcoholic fermentation
• Pyruvic acid NADH ? alcohol CO2 NAD

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Alcoholic Fermentation

• Particularly important to bakers and brewers
• Causes dough to rise and forms bubbles in beer
  and wine
• To brewers, alcohol is a welcomed byproduct of
  fermentation
• However, it is not desirable from a yeast cells
  point of view
• Alcohol is toxic
• When the level of alcohol reaches about 12
  percent, yeast cells die
• Thus alcoholic beverages must be processed if
  higher concentrations of alcohol are desired