Plants & Photosynthesis

1. Plants & Photosynthesis

2. Plant Tissues • Groundtissue – • Parenchyma– thin walled, used for storage, photosynthesis, or secretion. • Collenchyma – thick, flexible walls, used for support • Sclerenchyma– thickest walls, used for support.

3. Dermal - • Guard cells – surround stomata, turgor controls opening • Epidermis– thin protective layer, waxy cuticle secretion • Specialized cells & Cuticle – secretive, defensive

4. Vascular – • Xylem– conduction of water and minerals; may have both primary & secondary walls which lends itself to secondary function as support. Dead at maturity. • Tracheids– long,tapered, have openings at ends called pits (2◦ wall absent). • Vessel elements – shorter & wider; openings at ends have no walls.

5. Phloem– conduction of sugars. • Sieve tube elements  sieve tubes. Lack nuclei & • ribosomes,but are living. • Openings on end form sieve plates- allows contact between cells. • Companion cells – connected by plasmodesmata; provide physiological support.

6. Leaf Anatomy

7. Leaf Anatomy • Epidermis w/cuticle • Guard cells - stomata • Mesophyll – palisade layer, spongy layer, air spaces • Vascular bundles – xylem, phloem

8. Control of Stomata • Surrounded by 2 guard cells which swell w/water. Uneven construction of cell wall causes the cells to bulge outward, creating opening. • A diffusion of K+ ions creates a gradient for the movement of water, which leads to swelling.

9. Control of Stomata • Temperature, inner CO2 levels influence opening. • Most plants close stomata at night, probably due to high CO2 levels. • Water loss, or transpirationis minimized when stomata are closed.

10. Photosynthesis Light energy + 6H2O + 6CO2 C6H12O6 + 6O2

11. Overview • The process begins with light-absorbing pigments which are able to absorb the energy from light. In a series of reactions, this energy is eventually used to produce a molecule of glucose.

13. Chloroplasts • Similar to the mitochondria, chloroplasts are composed of an outer phospholipid membrane encasing a fluid interior (in this case stroma). • Pancakelike membranes called thylakoids are arranged in stacks called grana (granum, singular)

14. The thylakoids contain the light-absorbing pigments, such as chlorophyll a and b, and carotenoids and various enzymes. • The fluid stroma contains several enzymes involved in carbohydrate synthesis.

15. Chemical reactions of photosynthesis • The process of photosynthesis can be divided into two major series of reactions: • Light-dependent reactions • Light-independent or dark reactions

16. Light – dependent reactions H2O + ADP + Pi + NADP+ + light  ATP + NADPH + O2 + H+ • This equation sums up what occurs in this process. • Notice a photophosphorylation occurs, making ATP from ADP & Pi

18. The details of LDR • Noncyclic photophosphorylation occurs in two pigment clusters called Photosystems I and II. • Different pigments absorb different ranges of light wavelengths. • The absorbed light “excites” electrons.

19. These energized electrons are unstable and immediately re-emit the absorbed energy. • The energy is then reabsorbed by neighboring pigment molecules in a chainlike fashion, eventually being absorbed into a special chlorophyll a molecule called P680. (max. amt. of light absorbed nm)

20. This is actually photosystem II, beginning the energy transfer process. • A primary electron acceptor than accepts and passes two energized electrons through an electron transport chain of proteins, including ferredoxin and cytochrome.

21. As the electrons move “down” the chain, losing energy, the “lost” energy is used to phosphorylate ATP. (For every 2 electrons through the system, an average of 1.5 ATP molecules are made.) • The pigment cluster, Photosystem I, is the final recipient of the electrons, which are again energized by sunlight.

22. Energized electrons are passed off to P700 and then another primary electron acceptor. • Another, shorter, ETC, passes electrons along, terminating in the energy-rich coenzyme NADPH. (NADP+ and H+ are combined by the absorption of the electrons)

23. This entire process causes the removal of two electrons from PS II. • These electrons are replaced by splitting water in a process called photolysis. H2O  2H+ + ½ O2 + electrons

25. Cyclic photophosphorylation involves PS I and occurs simultaneously with noncyclic. • Energized electrons from PS I join with protein carriers and generate ATP. • Since electrons are not passed off to NADPH, they can be recycled. • Concentrations of ATP and NADPH probably influence whether or not the pathway is cyclic or noncyclic.

27. Light Dependent Reactions • What are the three main processes that make up LDR? • What’s needed? • What’s produced? • Where does it occur?

28. Light-independent reactions: Calvin-Benson cycle • “Fixes” CO2  organic molecule • Uses energy in ATP and NADPH • Six turns of the cycle are needed to generate 1 glucose molecule, however, glucose is NOT the DIRECT result of this cycle.

30. Calvin-Benson Cycle: A Closer Look • Carbon Fixation – CO2 attaches to a 5C sugar, RuBP (ribulose biphosphate) via the enzyme rubisco. • The 6C intermediary is unstable and quickly splits to form two 3C PGA (3-phosphoglycerate)

31. Reduction – Each PGA receives an additional phosphate group from ATP and electrons from NADPH reducing it to G3P aka PGAL (also an intermediate of glycolysis). Several turns of the cycle are needed to continue……

33. Regeneration – Once three turns of the cycle are achieved, the gain of 6 G3P is used to regenerate 3 molecules of RuBP, leaving one extra G3P to be used by the plant to make glucose. • Carbohydrate synthesis – Once six turns are achieved, there are enough molecules of G3P to make glucose.

35. Photorespiration • The enzyme rubisco is capable of “fixing” oxygen as well as carbon dioxide, thus it can be very inefficient. • The products made when oxygen is “fixed” to RuBP are not useful to make glucose. • This reduces the amount of carbon dioxide that is “fixed”, thus reducing glucose production and growth.

36. C4 & Cam Photosynthesis • Certain plants have evolved a means of reducing the amount of photorespiration that can occur. • C4 plants – carbon dioxide “fixes” to PEP (phosphoenolpyruvate) with the help of the enzyme PEP carboxylase to form OAA (oxaloacetate), a 4 C compound.

38. C4 & Cam Photosynthesis • OAA is then converted to malate and shuttled to special cells within the leaf which are not in direct contact with oxygen. • The malate then breaks down, forming carbon dioxide (& pyruvate) and the carbon fixation begins.

39. C4 & Cam Photosynthesis • Not only does this increase photosynthesis efficiency it also helps these plants conserve water because they do not need to have their stomata open as much to take in carbon dioxide. • These plants are often found thriving in hot, dry climates.

40. C4 & Cam Photosynthesis • Crassulacean acid metabolism – • Very similar to C4 but for a few differences. • Malic acid is made rather than malate. • Malic acid is shuttled to vacuole of cells. • Stomata are open at night and closed during the day to reduce water loss.