Again, Dr. Britt will only quiz you on what goes into the Calvin cycle, its first step, and what comes out.

Six molecules of both ATP and NADPH are used up in the process, helping to drive the reactions and produce the electrons required to reduce the incoming CO2. The "spent" molecules (ADP and NADP+) return to the nearby thylakoids to be recycled back into ATP and NADPH.

Ribulose-1,5-bisphosphate (RuBP) is composed of five carbon atoms and includes two phosphates.

In the stroma of plant chloroplasts, in addition to CO2, two other components are present to initiate the light-independent reactions:

The biological pathway that leads to carbon fixation in green plants and cyanobacteria is called the Calvin Cycle and is a reductive pathway (consumes energy and electrons) which leads to the reduction of CO2 to G3P.

"Carbon Fixation", the incorporation of inorganic carbon (CO2) into organic molecules

while those organisms that require organic sources of carbon, such as glucose or amino acids, are referred to as heterotrophs

Organisms that can obtain all of their required carbon from an inorganic source (CO2) are referred to as autotrophs,

It is at this step that light energy is transformed into the more stable form of chemical energy. All of the subsequent redox reactions are involved in pumping protons or in delivering that e- to NADP.

The Calvin cycle takes place in the stroma, conveniently located vis a vis the ATP- and NADPH-producing thylakoids

The captured energy is transferred from chlorophyll to chlorophyll until ...eventually... (after about a millionth of a second), it is delivered to the reaction center. Up to this point, only energy has been transferred between molecules, not electrons. In other words, no new bonds have formed.

Both photosystems have the same basic structure; a number of antenna proteins to which chlorophyll molecules are bound surround the reaction center where the photochemistry takes place. Each photosystem is serviced by this light-harvesting complex, which passes energy from sunlight to the reaction center;

These two types are simply called photosystem II (PSII, carrying P680) and photosystem I (PSI, carrying P700), and were named (confusingly) in the order of their discovery. The two complexes differ on the basis of what they oxidize (that is, their source of electrons) and what they reduce (the place to which they deliver their energized electrons). Working in tandem, these two photosystems can power the production of both NADPH and ATP.

In oxygenic photosynthesis, two types of pigment are found embedded in the thylakoid membrane (in plants) or the bacterial inner membrane (in cyanobacteria).

oxygenic photosynthesis is more complex than the "sulfur-genic" photosynthesis described above, requiring two different reaction centers, with different reduction potentials.

The overall function of "light-dependent" reactions of photosynthesis is to transform solar energy into chemical compounds, in the form of NADPH and ATP. This energy supports the "light-independent" reactions and fuels the assembly of sugar molecules.

In non-cyclic photophosphorylation electrons are removed from the photosystem, as they end up in NADPH after moving through an electron transport chain

As in the green sulfur bacteria example above, the step that transfers light energy into the biomolecule takes place in a multiprotein, multipigment complex called a photosystem

This NADPH will be used for carbon fixation. Thus in order to keep generating NADPH there's needs to be a source of electrons to refill the "electron hole" in bacteriochlorophyll

In an electron transport chain analogous to that of respiration, the electron is passed exergonically from carrier to carrier. The redox reaction at one of the carriers powers a proton pump, pushing protons into a higher concentration compartment. Eventually the electron is used to reduce bacteriochlorophyllox (making a complete loop) and the whole process can start again.

This electron must come from an external source with a lower reduction potential than the (ground state) pigment and depending on the reduction potential of that pigment there are different possible sources that might be employed, including H2O, reduced sulfur compounds such as SH2, and even elemental S0

The visible light seen by humans as white light is composed of a rainbow of colors, each with a characteristic wavelength.

However, the electron energized by light might have an alternative fate: it might descend through a different series of carriers (without pumping protons) and instead be deposited onto a close relative of NAD+ called NADP. Addition of 2 e- generates NADPH, which is going to be used to build sugars from CO2

This electrochemical gradient generates a proton motive force whose concentration gradient can then be coupled to the endergonic production of ATP, via ATP synthase (again, just as in respiration).

For example, as you can see in the Table below, the ground state pigment at the reaction center of PSII (the "chlorophyll a" in P680) cannot reduce anything listed in the table- it is the weakest reducing agent described there (even weaker than H2O!).

While in the excited state, the pigment has a much lower reduction potential (E˚', it moves upward on our electron tower, it becomes a stronger reducing agent) and can donate these unstable, high potential energy electrons to carriers with greater E˚'

"unexcited", or "ground state"

The first step of the process involves the absorption of a photon by a pigment molecule. Light energy is transferred to the pigment and promotes electrons into an excited state.

Photophosphorylation probably evolved relatively shortly after electron transport chains and anaerobic respiration.

Photophosphorylation is the process of transferring the energy from light into ATP.

Because the energy changed the reduction potential such that the molecule is now a stronger e- donor, this high-energy e- can be transferred exergonically to an appropriate e- acceptor. In other words, the excited state can be involved in a redox reactions. This is a photochemical reaction

When an atom absorbs a photon of light, an electron acquires that energy, leaving its ground (lowest potential energy) orbital and moving up to a higher energy orbital. This is an unstable situation.

Each type of pigment can be identified by the specific pattern of wavelengths it absorbs from visible light. This characteristic is known as the pigment's absorption spectrum.

Carotenoids are the red/orange/yellow pigments found in nature. They are found in fruit

This ring structure is chemically related to the structure of heme compounds that also coordinate a metal and are involved in oxygen binding and/or transport in many organisms. Different chlorophylls are distinguished from one another by different "decorations"/chemical groups on the porphyrin ring

There are five major chlorophyll pigments named: a, b, c, d, and f. Chlorophyll a is related to a class of more ancient molecules found in bacteria called bacteriochlorophylls. Chlorophylls are structurally characterized by ring-like porphyrin group that coordinates a metal ion.

If a plant were able to absorb 100% of incident photons, what color would it be? What if a variant of the same species of plant lacked a pigment required to absorb red light?

green plants appear green because they reflect, rather than absorb, most of the photons at a combination of wavelengths that we perceive as green

At the center of the biological interactions with light are groups of molecules we call organic pigments.

Signaling interactions are largely responsible for perceiving changes in the environment

A specific "color" of light has a characteristic wavelength.

The distance between peaks in a wave is referred to as the wavelength and is abbreviated with the Greek letter lambda (Ⲗ

Like all plants, the bulrush requires oxygen to produce energy. One solution is obvious: Send shoots skyward like straws to suck down oxygen to the roots.

Plants store starch in the form of sugars.

. The dynamic response of stomata or gs to fluctuations in light intensity has been studied in several understorey forest-dwelling species, but relatively few reports have studied crop species (Chazdon and Pearcy, 1986; Chazdon, 1991; Tinoco-Ojanguren and Pearcy, 1993; Leakey et al., 2005; McAusland et al., 2016).

Efforts to develop a big data approach to photosynthetic phenomics by recruiting many researchers into online cloud-based initiatives (Kuhlgert et al., 2016) may be promising because not only can they assay many genotypes but they can also do so under the diverse conditions which plants experience in nature.

However, the importance of the dynamic responses of photosynthesis raises a key problem that has not been adequately addressed: it is difficult to capture photosynthetic responses within (rapidly) fluctuating environments, especially in the field.

Whilst morphological adaptations to such extreme temperature fluctuations are well documented, the physiological adjustments are not (Hedberg, 1970).

Indeed, it is particularly important that light is accurately tracked by the plant for optimal photosynthetic performance

‘photosynthome’

Indeed, we are beginning to understand that the way in which photosynthesis is regulated in response to fluctuations in the environment is perhaps a more important determinant of plant productivity than its performance under steady-state or temporarily steady-state conditions

How then can leaves cope?