Photosynthesis

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Photosynthesis
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Overview

• Photosynthesis is the process that converts solar
  energy (sunlight) into chemical energy (glucose)
• Directly or indirectly, photosynthesis nourishes
  almost the entire living world

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• Autotrophs make their own food
• Live without eating anything from other organisms
• Are the producers of the biosphere, producing
  organic molecules from CO2 and other inorganic
  molecules
• Almost all plants are photoautotrophs, using the
  energy of sunlight to make organic molecules from
  H2O and CO2

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• Photosynthesis occurs in plants, algae, certain
  other protists, and some prokaryotes
• These organisms feed not only themselves but also
  most of the living world

Plants
Multicellular alga
Unicellular protist
Cyanobacteria
Purple sulfur bacteria
BioFlix Photosynthesis
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• Heterotrophs obtain their energy from the foods
  they consume
• Get their energy from organisms they eat
• Are the consumers of the biosphere
• Almost all heterotrophs, including humans, depend
  on photoautotrophs for food and O2

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• How is energy obtained?
• Highenergy bonds break
• Replaced by lowenergy bonds
• Extra energy is released
• Some stored in ATP
• Some released as heat
• ATP (adenosine triphosphate)
• Used to store and release energy in the cell
• Made of adenine, ribose (5-carbon sugar), and 3
  phosphate groups

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• ADP (adenosine diphosphate)
• When energy is available, small amounts are
  stored by adding a phosphate group to ADP
• Making ATP
• Releasing energy when needed is the reverse
  reaction
• The highenergy bond is broken
  between the second (2nd) and
  third (3rd) phosphates

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• How much ATP does a cell contain?
• Only a small amount of ATP is at hand
• Enough for a few seconds of activity
• Why?
• An ATP molecule is good for energy transfer and
  NOT for storing over long periods of time
• What is the energy storage molecule?
• Glucose is the storage molecule
• More than 90 times the chemical energy of ATP
• Cells can regenerate ATP from ADP as needed

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• Photosynthesis
• Plants use sunlight (solar energy) to convert
  water (H2O) and carbon dioxide (CO2) into
  highenergy carbohydrates (sugars mainly glucose
  C6H12O6 and starches complex sugars) and oxygen
  (O2)

6 CO2 12 H2O
Sunlight (Solar energy)
C6H12O6 6 O2 6 H2O
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• How do plants capture solar energy?
• Requires chloroplasts and the chlorophyll within
• Chlorophyll a green pigment that absorbs light
  energy
• Pigment lightabsorbing molecules

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• Our eyes see light as white light
• Light is actually a mixture of different
  wavelengths
• The different wavelengths your eyes can see make
  up the visible spectrum and produce different
  colors
• Different pigments absorb different wavelengths
• Wavelengths not absorbed are
  reflected/transmitted back

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• Chlorophyll is the major pigment of plants
• Two types of chlorophyll
• Chlorophyll a
• Major photosynthetic pigment
• Absorbs well in the blue-violet and red regions
• Little is absorbed in the green region
• Reflects back (this is why plants are green)
• Chlorophyll b
• Broadens the absorbing range
• Another pigment is carotene
• Red and orange pigments
• Absorbs light in other regions

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• Any compound that absorb light absorbs the energy
  from the light
• After chlorophyll absorbs light
• Much of the energy is transferred directly to
  electrons
• Energy levels are raised making them highenergy
  electrons

Animation Light and Pigments
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• A spectrophotometer measures a pigments ability
  to absorb various wavelengths
• This machine sends light through pigments and
  measures the fraction of light transmitted at
  each wavelength

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• Leaves are the major location of photosynthesis
• Their green color is from the chlorophyll
• Light energy absorbed by chlorophyll drives the
  synthesis of organic molecules (sugars)
• CO2 enters and O2 exits the leaf through
  microscopic pores called stomata

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• Chloroplasts are found mainly in cells of the
  mesophyll, the interior tissue of the leaf
• A typical mesophyll cell has 3040 chloroplasts
• Photosynthesis occurs inside chloroplasts
• The chlorophyll is in the membranes of thylakoids
  found in the chloroplast
• Saclike photosynthetic membranes
• Newspapers
• Thylakoids are arranged in columns called
  grana (granum singular)
• Stack of newspapers

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• Proteins within the thylakoid membrane organize
  pigments, including chlorophyll, into
  photosystems
• Photosystems are the lightcollecting units
• Chloroplasts also contain stroma
• A dense fluid

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Photosynthesis Reactions

• 2 types of reactions
• Lightdependent reactions
• Photo part
• Requires light
• Summary uses light to produce oxygen (O2) gas,
  ATP and NADPH
• Lightindependent reactions (Calvin cycle)
• Synthesis part
• Does not require light
• Summary uses CO2, ATP and NADPH to produce sugar

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• All reactions occur within the chloroplast
• Lightdependent reactions within the thylakoid
  membrane
• Calvin cycle in the stroma

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• Electron transport chain (ETC)
• When electrons (e-) gain energy from sunlight,
  they require a special carrier
• Highenergy electrons are too hot to handle
• Electron transport carrier molecules accept a
  pair of highenergy electrons and transfer
  (carry) them to other molecules
• Electron carriers are the electron transport
  chain
• NADP (nicotinamide adenine dinucleotide
  phosphate)
• Accepts and holds 2 highenergy e- s as well as a
  hydrogen ion (H)
• Converts NADP to NADPH
• Traps some sunlight into chemical form

NADP
NADPH
2 e- H
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H2O
CO2
Light
NADP
ADP
P

i
Calvin Cycle
Light Reactions
ATP
NADPH
Chloroplast
O2
(sugars)
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• Lightdependent reactions
• Uses light energy to produce O2 as a byproduct
  and ATP and NADPH for use later
• Step 1 pigments in photosytem II absorb light
  through electrons (increasing their energy)
• These highenergy electrons are transported by
  the ETC
• How are electrons available?
• Split H2O molecules into 2 electrons, 2
  hydrogen ions, and 1 oxygen
• Oxygen is released to the atmosphere
• Hydrogen ions are released inside
  the thylakoid membrane
• Electrons released replace those
  electrons lost to the ETC

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• Step 2 highenergy electrons move through the
  ETC from photosystem II to photosystem I
• This energy is used in the ETC to transport H
  ions into the inner thylakoid space

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• Step 3 pigments in photosystem I use light
  energy to reenergize electrons
• NADP picks up these highenergy electrons with
  H ions

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• Step 4 as electrons pass from chlorophyll to
  NADP
• H ions are pumped across the membrane
• Making the inside of the membrane fill with a
  positive () charge
• Making the outside of the membrane more
  negatively (-) charged
• Differences across the membrane provides energy
  to make ATP

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• Step 5 H cross the membrane only through ATP
  synthase
• As H crosses, ATP synthase rotates
• As it rotates, ATP synthase binds ADP and a
  phosphate group to make ATP

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• Calvin cycle
• Uses the energy of ATP and NADPH (shortterm
  storage) to build highenergy compounds (sugars)
  for long time storage
• Step 1 6 CO2 enter from the atmosphere combine
  with 3carbon (C) molecules to produce 12 3C
  molecules
• Step 2 12 3C molecules converted to high
  energy forms through ATP and NADPH
• Step 3 2 3C molecules are removed from the 12
  3C molecules
• Used to make sugars among other compounds need by
  the plant for growth and metabolism
• Step 4 10 3C molecules converted back to the 6
  5C molecules
• The cycle continues
• The sugars made
• Used to meet the energy needs of the plant
• Used to build complex macromolecules such as
  cellulose and starch

6 CO2
12 C-C-C
12 ATP
6 C-C-C-C-C
12 ADP
6 ADP
12 NADPH
6 ATP
12 NADP
12 C-C-C
10 C-C-C
2 C-C-C
C-C-C-C-C-C glucose (sugar)
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• Rate of photosynthesis
• Water shortage
• May slow or even stop photosynthesis
• Adaptations waxy coating to reduce water loss
• Temperature
• Too low or high slows down photosynthesis
• Enzymes function best between 0C and 35C
• Light intensity
• Increasing light increased photosynthesis
• Until photosynthesis rate is maximized (highest
  it can go)