BABS1201 Lecture Notes - Lecture 10: Light-Dependent Reactions, Anoxygenic Photosynthesis, Carbon Fixation
17 May 2018
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BABS1201
16TH APRIL 2018
PHOTOSYNTHESIS
× NOTE: In the final exam, if asked about photosynthesis or respiration, be sure to write
down the empirical equations
× Carbon fixation- where does the carbon in wood come from? The carbon dioxide in air
× Production of atmospheric oxygen- where does he oxygen we breathe come from? Non-
biological processes only contribute about 0.5% free oxygen as opposed to the current
proportion of roughly 21% in the atmosphere. The main sources of atmospheric oxygen are
cyanobacteria, plankton and plants in marine and terrestrial environments
× Why is photosynthesis important? It is responsible for:
1. Liberation of oxygen
2. Consumption of carbon dioxide
× It also directly or indirectly:
- Provides all of our food requirements
- Stores energy in petroleum, natural gas, coal and ethanol
- Supplies our needs for clothing and building materials
× Photosynthesis has transformed Earth from a toxic wasteland into the hospitable
environment we know today- it is probably the most important biological process on earth!
× What is photosynthesis? Photosynthesis is the physico-chemical process used by plants,
algae and photosynthetic bacteria, all of which use light energy to drive the synthesis of
organic compounds
× Photosynthesis can also be described as a light-dependent, anaerobic (not requiring oxygen)
form of metabolism. Conversion of unusable sunlight energy into usable chemical energy
× There are two forms of photosynthesis- oxygenic (creating oxygen) and anoxygenic (seen in
some bacterial forms, where light energy is captured and converted to ATP, without the
production of oxygen)
× The energy in photosynthesis is supplied by sunlight
× Empirical equation for photosynthesis- in plants, algae and certain (oxygenic)
photosynthetic bacteria, the photosynthetic process results in the release of molecular
oxygen and the removal of carbon dioxide from the atmosphere that is used to synthesise
carbohydrates (oxygenic photosynthesis). Some other types of photosynthetic bacteria sue
light energy to create organic compounds but do not produce oxygen (anoxygenic
photosynthesis)
× Photosynthesis is the reverse of respiration
× Photosynthesis- eukaryotes:
1. The photosynthetic process in plants and algae occurs in small organelles known as
chloroplasts, located inside cells
2. Chloroplasts (chloros = green. plast = form or entity) have envelopes consisting of
an inner and an outer phospholipid membrane
3. Inside the chloroplasts are thylakoids (thylakos = sac), the site of the light
dependent reactions- thylakoids consist of a thylakoid membrane surrounding a
thylakoid lumen
4. Chloroplast thylakoids frequently form stacks of flat discs (usually 2 to 10
micrometres in diameter and 1 micrometre thick), referred to as grana (Latin for
“stacks of coins”). Grana are connected by intergrana or stroma thylakoids, which
join granum stacks together as a single functional compartment
5. The stroma is the thick fluid found in between grana, where the carbohydrate
formation reactions (light independent) occur in the chloroplast of plant and algae
cells
6. Stages of photosynthesis:
1. Light dependent reactions- light is captured and used to make energy
carrying molecules (electron and proton transfer reactions). Light
dependent photosystems produce ATP and NADPH
2. Light independent (dark) reactions- the energy carrying molecules f ATP
and NADPH are then used in the capture of carbon dioxide (CO2) to make
glucose
× Light dependent reactions occur in the grana of the chloroplast (on the thylakoid
membrane)- chlorophyll is the pigment in chloroplasts which absorbs light, and green light
is reflected ® this is what makes plants green
× Photophosphorylation- the production of ATP using the energy of sunlight- all organisms
produce ATP, the universal energy currency of life. Only two sources of energy are
available to living organisms: 1. oxidation-reduction (redox) reactions, and 2. sunlight
(photons)
- A photon (light energy) strikes a pigment molecule, and its energy is
instantaneously changed from light energy into electrical energy (i.e. it boosts
chlorophyll electrons to a higher, more unstable orbital, where they are very
reactive)
- The electrical energy is “packaged” as chemical energy in the form of ATP and
another “energy courier” molecule called NADPH
- This process is performed by photosystems (protein complexes) which are found in
the thylakoid membranes of plants, algae and photosynthetic bacteria
- Photosystems- structures that contain chlorophyll molecules (pigments). The
pigments are bound to proteins to form antenna complexes that absorb photons and
transfer the resultant excitation energy to the reaction centre
× First, a photon of light hits a chlorophyll molecule surrounding the
Photosystem II complex. Photosystem II occurs first because it has a
wavelength of 680nm, which is shorter than Photosystem I which is 700nm-
therefore, light penetrates Photosystem II (P680) first
× Chlorophyll molecules transmit energy from excited electrons in the
antenna complex to a reaction centre
Document Summary
Note: in the final exam, if asked about photosynthesis or respiration, be sure to write down the empirical equations. Non- biological processes only contribute about 0. 5% free oxygen as opposed to the current proportion of roughly 21% in the atmosphere. The main sources of atmospheric oxygen are cyanobacteria, plankton and plants in marine and terrestrial environments. It is responsible for: liberation of oxygen, consumption of carbon dioxide. Stores energy in petroleum, natural gas, coal and ethanol. Supplies our needs for clothing and building materials. Photosynthesis has transformed earth from a toxic wasteland into the hospitable environment we know today- it is probably the most important biological process on earth! Photosynthesis is the physico-chemical process used by plants, algae and photosynthetic bacteria, all of which use light energy to drive the synthesis of organic compounds. Photosynthesis can also be described as a light-dependent, anaerobic (not requiring oxygen) form of metabolism. Conversion of unusable sunlight energy into usable chemical energy.
