Photosynthesis Chapter 8: Biology In Focus AP Bio 2014

The BIG energy transfers…

Remember the 1st Law of Thermodynamics – energy cannot be created or destroyed but only transferred. Energy from the sun is used to convert carbon dioxide and water to glucose (chemical energy) which is then converted to ATP (usable chemical energy), releasing carbon dioxide and water.

Some ecology terms… Autotrophs: Organisms capable of producing organic molecules from carbon dioxide (CO2) Also called producers Heterotrophs: must live on organic compounds produced by other organisms Also called consumers

What organisms perform photosynthesis? Plants, alga, some protists (single-celled organisms) and also some bacteria

Part 1: Overview and Leaf Structure

Leaf & chloroplast structure vocabulary Mesophyll Stomata Chloroplast: Double membrane Stroma Thylakoids Chlorophyll

Leaf structure 

FYI: Stoma = singular, stomata or stomates = plural

Stomates/stomata allow gas exchange 

Making a stomate peel for viewing

Location of the photosynthetic action -- the chloroplast

Chloroplast structure

Part 2: The biochemistry of photosynthesis

Where did we leaf off? In cellular respiration, cells use the energy stored in glucose to make ATP – the universal energy molecule in biology! They do this by moving energy-filled electrons from the bonds in glucose to the electron transport chain. Using the energy in these electrons, the electron transport chain and ATP synthase work together to make ATP by chemiosmosis.

So how did energy get stored in the form of glucose? Photosynthesis!

Photosynthesis – the reverse of cellular respiration?

Homework: Explain how the oxygen isotope 18O was used to demonstrate that the oxygen gas released by plants does NOT come from the CO2 used to make glucose

Tracing the atoms…

Photosynthesis occurs in two separate stages…

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

The take-home message is that light IS energy that can be transferred.

Light’s energy is captured by pigments in the chloroplasts The primary pigment, chlorophyll a, absorbs primarily blue and red light – and reflects green – these individual lines represent the pigment’s absorption spectrum Accessory pigments (chlorophyll b and the carotenoids) combine with chlorophyll’s absorption spectrum to produce a wider action spectrum

The molecular structure of chlorophyll

The pigments are located in BIG complexes called photosystems

A photon of light ZAPS the chlorophyll and… A photon of red or blue light zaps a chlorophyll molecule and one of the electrons in the chlorophyll is excited – it now has more potential energy than it had before! What can happen to this electron?

Moving electrons means moving energy Moving electrons means moving energy! From light to ATP and also to the electron carrier NADPH…

Light reaction: Making ATP and NADPH

The flow of energized electrons in the light reactions

Photophosphorylation: The production of ATP using the H+ gradient established by the splitting of water and the transport of H+ by the cytochrome complex via the ATP synthase found in the thylakoid membrane

Cyclic photophosphorylation Non-cyclic phosphorylation, which involves both photosystem II and I and produces both ATP and NADPH In cyclic phosphorylation, only PS I is involved. No NADPH is made but ATP is generated. Water isn’t split so no O2 is released. The Calvin cycle needs more ATP than NADPH.

Light reaction, the movie

How is ATP and NADPH used to make glucose How is ATP and NADPH used to make glucose? The Calvin Cycle (light-independent reactions)

Calvin cycle summary Carbon enters the cycle in the form of CO2 and is reduced to glucose. This is called carbon fixation: Phase 1. This first step, in which CO2 is added to a 5-carbon sugar, is catalyzed by an enzyme called Rubisco – probably the single most abundant protein on the planet. It is reduced when it gains electrons from NADPH (Phase 2: Reduction). This cycle uses energy from ATP to transfer these high-energy electrons from NADPH to glyceraldehyde 3-phosphate (G3P). Finally, the original CO2 acceptor, ribulose-1,5-bisphosphate (RuBP) is regenerated (Phase 3: Regeneration)

Calvin cycle, the movie

The two reactions, summarized…

Accounting To generate one G3P, it costs 9 ATP and 6 NADPH This G3P can then be used by the cell as an immediate energy source (to be metabolized in cellular respiration – G3P is an intermediate in glycolysis) or used to build glucose molecules that will be used by the plant to build cell walls, starches, and other polysaccharides.

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!

Different approaches: C4 and CAM plants Recall the mechanism of gas exchange – those stomata on the underside of the leaf must be open, which also results in water loss In arid conditions, plants close their stomata to prevent water loss – but this results in less CO2 available for the Calvin Cycle

C3 plants Photorespiration = the drawback. If it’s arid, stomata close and when the CO2 levels in the leaf decline, Rubisco actually takes O2 and sticks it into the Calvin cycle and then the product is broken down & released as CO2 These are the ones we’ve just been discussing – those that take CO2 and using Rubisco, add CO2 to ribulose-1,5-bisphosphate in the Calvin cycle Include rice, wheat, soybeans

C4 Plants – a spatial solution to arid conditions Called C4 because CO2 is fixed and a FOUR carbon compound is produced Mesophyll cells have an enzyme that has a very high affinity for CO2, so even when CO2 is low, it can bind and fix carbon This 4 carbon compound is then shuttled to bundle sheath cells and then releases the CO2 so these cells have enough CO2 for the Calvin cycle to keep running Include sugarcane, corn and grass.

PEP carboxylase Phosphoenolpyruvate carboxylase (also known as PEP carboxylase) is the enzyme that catalyzes the addition of bicarbonate (HCO3-) to phosphoenolpyruvate (PEP) to form the four-carbon compound oxaloacetate and inorganic phosphate PEP + HCO3- → oxaloacetate + Pi

CAM (crassulacean acid metabolism) plants – temporal solution During the day the energy needed is produced (ATP & NADPH) and the organic acids made the night before are used to generate CO2 for the Calvin cycle even though the stomates are closed Open stomata at night and close them during the day Include the succulents – jade, cacti, etc. At night these plants take in CO2 and convert the CO2 into a variety of organic acids in the mesophyll cells

Animated tutorials… http://www.pol2e.com/at06.03.html http://www.pol2e.com/at06.04.html http://www.pol2e.com/at06.05.html http://www.pol2e.com/ac06.07.html