I. Modes of Nutrition - acquisition of organic compounds
1. Autotrophs -“self feeders”
• make their own organic food compounds from inorganic compounds
• producers of the biosphere
Photoautotrophs - energy required to synthesize organic compounds comes from light.
e.g., plants, algae, cyanobacteria
Chemoautotrophs - oxidize inorganic compounds such as elemental S and NH to gener3te energy for
organic compound synthesis.
e.g., Sulfolobus oxidizes elemental S
2. Heterotrophs - "other-feeders"
• Consumers - unable to make their own food molecules and must consume other plants, animals,
carnivore - consumes other animals
herbivore - consume plants
omnivore - consumes plants and animals
saprobe - decomposers that absorb nutrients from dead organic matter
• site of photosynthesis in eukaryotic organisms (Review: structure of the chloroplast)
• chlorophyll, a green pigment in thylakoid membrane, absorbs the light energy that drives
photosynthesis, an anabolic process
• prokaryotes called cyanobacteria (blue-green algae) have thylakoid membranes (Figure 27.7)
• majority of chloroplasts found in mesophyll (interior tissue) of leaves
• Gas exchange (in with CO and2out with O ) occ2rs through microscopic pores called stomata
(stoma - singular) that are generally located on the lower side of the leaf
• xylem - transports water and minerals from the roots
• phloem - transports sugar and other organic compounds from the sites of synthesis to other parts of
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1. The Chemical Reaction for Photosynthesis
6 CO +212 H O + 2ight energy ▯ ▯ ▯ ▯ ▯ ▯ C H O 6 12O 6 6 H O 2 2
Redox reactions are involved in photosynthesis
Energy + 6 CO + 6 2 O ▯ ▯ 2 H O + 6 6 12 6 2
How does photosynthesis compare to cellular respiration?
• H O is split - source of H +
• H and e added to CO there2y reducing it CH O (carbo2ydrate)
• All oxygen from the 12 H O m2lecules consumed to produce 6 O (Figure 1025)
• The production of CH O r2sults in an increase in potential energy (stored energy)
Carbon is being fized into sugar during photosynthesis
Where does the energy come from to drive photosynthesis?
Transforms it from light energy
Light as an Energy Source
Electromagnetic radiation (EMR)
• EMR travels in rhythmic waves but also behaves like particles (photons)
• wavelength (λ) = distance between consecutive crests
• amount of energy carried by EMR is inversely related to the λ
• visible spectrum of EMR = 380-750 nm
• violet-blue and red light are most effectively absorbed by chlorophyll pigments (Figure 10.9)
2. The Two Stages of Photosynthesis (Figure 10.6)
i. Light reactions
ii. Calvin cycle (also known as dark reactions)
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i. Light Reactions
• convert light energy into chemical energy
• splits water and drives the transfer of a pair of electrons and protons from water to NADP
(Nicotinamide adenine dinucleotide phosphate) ▯ NADPH
generates ATP by a process known as photophosphorylation
• produces oxygen as the result of splitting water
• occurs in the thylakoid membranes
a) How is the energy in light captured?
• Substances that absorb visible light
• Different pigments absorb different λ of light
• participates directly in the light reactions
• accessory pigment
• transfers light energy to chlorophyll a
• shades of yellow and orange
• family of accessory pigments
• Broaden spectrum of light absorption
•also provide photoprotection; these pigments absorb and dissipate excessive light energy to
prevent damage to chlorophyll or the formation of reactive oxidative molecules
b) How is light energy transferred to chlorophyll?
e ground state ▯ e excited state (high energy state)
• The excited state is a transient (unstable) state for an electron
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• excited electrons represent stored or potential energy that is released as heat and free energy (i.e., that can be
used to perform cellular work).
How is the light energy harvested?
Photosystems are used to gather light energy
• Photosystems are antenna complexes of pigment molecules surrounding a reaction centre
• Photosystems consist of pigment molecules (may include chlorophyll a, chlorophyll b and carotenoids)
and certain proteins that together act as light harvesting units
• energy is transferred through the photosystem (from pigment molecule to pigment molecule) until it reaches
the reaction centre that contains two chlorophyll a molecules associated with a primary electron acceptor.
• The reaction centre is the site of the first light driven chemical reaction of Photosynthesis (i.e., a