Chapter 4a Autothrophy

1
Chapter 4a - Autothrophy

• Autotrophs obtain energy from non-living sources
  (Sun and mineral resources), and use them to
  synthetise complex, energy-rich compounds.
• Through Autotrophy, producers produce the energy
  required by all other organisms.
• 2 types of autotrophs (living in environments
  without or with access to sunlight)
• - Photoautotrophs - Use E from sunlight to
  convert CO2 to sugars (plants, algae, bacteria)
• - Chemoautotrophs Use E from oxidation of
  substances with Fe, S, etc. to make sugars from
  CO2 (all are bacteria).
• 2 types of heterotrophs (Use E produced by
  autotrophs)
• - Photoheterotrophs Capable of Using both E
  from light or organic compounds at will,
  depending on availability (some bacteria, some
  algae)
• - Chemoheterotrophs Use E from organic
  compounds made by autotrophs (animals, fungi,
  some bacteria, unicellulars).

2
Photosynthesis The basics

• Energy from the Sun forms a continuous series of
  waves The Spectrum.
• Shorter wavelengths (blue light) have more E than
  longer wavelengths (red light)
• Within the spectrum, a narrow and specific range
  of wavelengths called Visible Light, is roughly
  the same portion that plants use for
  photosynthesis, and that animals see.
• Photo-autotrophs have light-absorbing substances
  (Pigments) that absorb visible light.
• The pigments for photosynthesis and some enzymes
  are contained within sacs called thylakoids that
  occur in stacks called grana (singular- granum).
  These in turn are within Chloroplasts (structures
  containing chlorophyll, a green pigment)
• (? Chloroplast grana thylakoid - pigments)
• - bacteria thylakoids float inside cell
• - plants and algae thylakoids in grana inside
  chloroplasts.

3
Pigments

• Within the chloroplast, the space around the
  thylakoids, called the stroma contains the
  chloroplasts DNA, RNA, and enzymes needed for
  protein synthesis.
• Chlorophyll in thylakoids is responsible for most
  of the photosynthesis, but not all.
• Chlorophyll absorbs violet/blue and orange/red
  light, but does not absorb the green light range
  of wavelengths (this is why leaves are largely
  green).
• Two main types Chlorophyll a and b.
• - Chl a CH3 (carboxyl) group in light
  sentitive portion of molecule.
• - Chl b CHO group instead (at same
  place).
• There are other pigments as well, with different
  capabilities of absorption of light (different
  wavelengths, thus colors). Some bacteria contain
  rhodopsin instead of chlorophyll.
• Chorophyll content in leaves declines during
  certain seasons, and other pigments (carotenoids
  yellow and xanthophylls red become evident)

4
Photosynthesis Energy conversions (E)

• - 3 Energy (E) conversions
• 1- Absorption of light E
• 2- Conversion of Light E into Chemical Energy
• 3- Storage of Chemical E as sugars
• - Light reactions
• 1) Absorption of light E by pigments in
  thylakoids
• 2) conversion of Light E to Chemical E (short
  lived E-rich molecules)
• - Calvin Cycle (once called Dark reactions)
• 3) Chemical E are used to make 3-carbon sugars
  from CO2. These become available for future plant
  growth
• - Overall photosynthesis equation
• ( Light Energy)
• 3CO2 3H2O -----------------?
  C3H6O3 3O2
• Carbon water (Pigments)
  3-Carbon Oxygen gas
• dioxide gas sugar
  
• (reduced gains e-)
  (oxidizedlooses e-)

5
Photosynthesis The light reactions

• Overall process
• 1- Thylakoid pigments (Chlorophyll, others)
  absorb light E
• 2- Water is split into H O
• 3- Light E converted into Chemical E
• ______________________________________________
• Structures/molecules involved
• 2 types of clusters of light-absorbing pigments
  (Photosystems I and II, PSI, PSII)
• Reaction center The only Chlorophyll-a molecule
  that can participate directly in e- flow
• e- carriers Proteins other molecules in
  thylakoid membrane - form an e- transport system
  between PSI and PSII
• NADP molecule (Nicotinamide adenine dinucleotide
  phosphate) gets reduced to NADPH, which
  provides protons and e- needed in Calvin cycle.
• ATP synthetase Enzyme in thylakoid membrane -
  Synthetises ATP from ADP and phosphate

6
Light reactions- PSI PSII, Reaction Centers
7
Light reactions in detail
8
Light reactions Full details

• 1) Pigments in PSI PSII absorb light E and
  transfer it to their reaction centers
• 2) e-carriers carry e- from PSIIs reaction
  center to PSIs reaction center. PSII receives
  more e- from enzyme that splits H20 into H, e-,
  and O2 gas e- carriers continue moving them
  along.
• 3) When e- and protons from water reach PSI, they
  reduce NADP to NADPH. This is the end of e- flow
  in Light Reactions.
• 4) A high concentration of protons ()
  accumulates in the thylakoid membrane. The
  protons transfer E to ATP-synthetase, which makes
  ATP from ADP phosphate.
• __________________________________________________
  ______
• E from light forces e- from water to NADP in
  chloroplasts (actually in thylakoids). NADPH is
  formed, which is used to synthetize ATP.
• ? The overall products of Light Reactions
• 1- NADPH 2- ATP 3- O2 gas
• ? Photosynthesis continues with the Calvin Cycle
  Energy from NADPH and ATP will be used to make
  sugars from CO2.
• - ATP and NADPH are not long-term E storage
  molecules
• - New Carbon compounds (Sugars) needed for
  growth have not yet been made.

9
Bacterial Photosynthesis w/o H2O splitting

• Water-splitting (oxidation) reaction in plants
  (Photoautotrophs)
• 2H2O -----? 4H 4 e- O2 (gas)
• Oxidation of hydrogen sulfide in photosynthetic
  bacteria Chromatium (Photoautotroph)
• 2H2S -----? 4H 4 e- 2So (solid)

10
Photosynthesis The Calvin Cycle

• Overall process
• 1) CO2 is reduced to sugars that can be used for
  growth
• 2) Takes place in the stroma of chloroplasts.
• 3) Requires ATP and NADPH formed during Light
  Reactions.
• 4) ADP and NADPH are returned to the Light
  Reactions.
• _______________________________________________
• Structures/molecules involved
• Stroma of chloroplasts
• RuBP (ribulose biphosphate, a 5-C
  sugar-phosphate)
• 6-C molecule (unstable) ? PGA (3-C
  phosphoglyceric acid) ? PGAL (phosphoglyceraldehy
  de, a 3-C sugar-phosphate)
• ATP and NADPH from Light Reactions.

11
Photosynthesis Calvin Cycle in detail

• CO2 fixation CO2 combines with RuBP forming 6-C
  molecule. It splits into 2 PGA
• Enzymes using ATP and NADPH reduce each PGA to
  PGAL
• Enzymatic reactions convert PGAL to 5-Carbon
  sugar-phosphate
• ATP adds another P to 5-C molecule, producing
  RuBP again.
• - Cycle is repeated 3 times, producing 6 PGAL, 5
  used for RuBP, the other used for growth.

12
Photosynthesis More of Calvin Cycle

• Rubisco (enzyme) catalyzes the fixation of CO2.
  The product is the 3-C acid PGA. ? C3 plants are
  those that use only the Calvin cycle to fix CO2.
• The Calvin Cycle cannot operate in the dark (i.e,
  at night) because
• - Light activates Rubisco and other enzymes
• - Light provides E for synthesis of ATP and
  NADPH
• - In the dark, little CO2 comes in through
  closed stomata
• Plants use Sugar-phosphates from the Calvin cycle
  in other cellular processes that occur in the
  Chloroplast or elsewhere in the plant.
• 1- making larger Carbohydrates
• - Sucrose (12-C disacharide) in cytoplasm
  of leaves
• - Starch (mediated by enzymes in
  chloroplasts). Broken down at night as source
  of E and Carbon skeletons
• 2- making aminoacids and then proteins
• 3- making Carbon skeletons
• 4- Providing a source of chemical E
• 3- Making lipids
• When consumers eat plants, they use products of
  photosynthesis as source of E and Carbon
  skeletons.