Photosynthesis: Where would we be without it?

Where’d we leave off? In cellular respiration, cells use the energy stored in _______ (and other biomolecules) to make ______, the universal energy molecule in living things! The big question is, where did all the energy in glucose come from in the first place? –Answer: _____________ New problem: How to capture the energy of ________ and convert it to the energy needed to make glucose…. glucose ATP the sun! the sun

So how did energy get stored in the form of glucose anyway? Photosynthesis! The reaction: 6 CO H 2 O + sunlight C 6 H 12 O O 2 (Look familiar? It should! Turn it around and replace light energy with ATP and what do you have?)

What organisms perform photosynthesis? Plants Some protists –Algae, Euglena Some bacteria –Cyanobacteria, Purple sulfur bacteria Because they make their own “food” they are called autotrophs

Where it all happens…

Where it all happens…LEAF ANATOMY

(fluid between grana) (a membrane-bound compartment) Details of Chloroplast Structure

Another player in the electron- carrier game… In cell respiration, NAD + and FAD were our “electron taxis” In photosynthesis, a different molecule will play this role… NADP + (nicotinamide adenine dinucleotide phosphate) a coenzyme derived from niacin, carries electrons in photosynthesis NADP + + H + + 2e - NADPH

There are two sets of reactions in photosynthesis

The light spectrum Light = electromagnetic radiation Travels in waves Distance between crests is the wavelength ( ) The entire range of radiation is the electromagnetic spectrum Wavelength of visible light is between 380 nm and 750 nm Light can also be thought of in terms of energy particles called photons The energy of a photon is inversely related to its wavelength

Light’s energy is captured by pigments in the chloroplasts The primary pigment, chlorophyll a, absorbs primarily blue and red light – and reflects green Accessory pigments, like chlorophyll b and carotenoids, widen the spectrum of light effectively absorbed by plants Violet / Blue / Green / Yellow / Orange / Red Action Spectrum (measured by O2 release) Absorpti on Spectru m

Evidence that chloroplast pigments participate in photosynthesis: absorption and action spectra for photosynthesis in an alga

The pigments are located in photosystems Photosystems are collections of light- absorbing pigments (primarily chlorophyll) in the thylakoid membranes There are two photosystems in chloroplasts (and beware -- #2 comes before #1) A photon of red or blue light zaps a chlorophyll molecule and one of the electrons in the chlorophyll is elevated to an excited state – it now has more potential energy than it had before! What happens next to this excited electron???

Moving electrons means moving energy! From Light energy to ATP and NADPH… There are two Electron Transport Chains in the Light Reactions Goal of E.T.C. following Photosystem II: ________________ Goal of E.T.C. following Photosystem I: ________________ make ATP make NADPH Splitting of water releases oxygen Cytochrome Complex NADP+ reductase

Cyclic electron flow

Light reactions: Making ATP and NADPH

The light reactions & chemiosmosis: the organization of the thylakoid membrane

Light reactions summary light energy absorbed by pigments (like cholorphyll) in photosystems in thykaloid membranes this energy used to raise electrons in cholorphyll to higher “excited” energy state excited electrons get passed along electron transport chains in series of redox rxns at end of ETC following Photosystem I, an electron pair and an H + are picked up by ________ to form _________. during ETC that follows Photosystem II, H + are actively pumped across thykaloid membrane into thykaloid space build up of H + creates an electric potential across membrane which is used to make ATP at the site of ATP Synthase (like chemiosmosis!) Problem: Chlorophyll has lost electrons…how will ETC start again? ______ molecules give up electrons to replace the ones eventually “lost” to NADPH, producing _________. (how useful for us!) NADP + NADPH H2OH2O oxygen

Now, how do we use that energy (ATP and NADPH) to make glucose?

Answer: The Calvin Cycle ! G3P (glyceraldehyde 3-phosphate) is a sugar molecule produced by the Calvin Cycle that is subsequently used to make Glucose and other compounds

Calvin cycle cliff notes Carbon enters the cycle in the form of CO 2 and is reduced to glucose. This is called carbon fixation. CO 2 is reduced when it gains electrons from NADPH. This cycle uses energy from ATP to transfer these high-energy electrons from NADPH to glucose. This first step, in which CO 2 is added to a 5-carbon sugar, is catalyzed by an enzyme called rubisco – probably the single most abundant protein on the planet! To make one glucose molecule from six CO 2, it costs 18 ATP and 12 NADPH

The Calvin cycle

The two reactions, summarized

Now what happens to the glucose? It can be stored as starch for energy later It can be used as cellulose for plant structure It can head off to the plant mitochondria to fuel cellular respiration!

C 4 leaf anatomy and the C 4 pathway

C 4 and CAM photosynthesis compared