BIOLOGY 2 E BIOL 1441 Photosynthesis Power Point

BIOLOGY 2 E BIOL 1441 Photosynthesis Power. Point Image Slideshow This work is licensed under a Creative Commons Attribution. Non. Commercial-Share. Alike 4. 0 International License.

AUTOTROPHS and HETEROTROPHS There are two types of autotrophs: • Photoautotrophs use sunlight to make food. They include plants (a), algae (b), and cyanobacteria (c). Chemoautotrophs, such as thermophilic bacteria (d, e), capture energy from inorganic compounds to make food.

Autotrophs and Heterotrophs • Heterotrophs, including animals, fungi, and most bacteria, rely on the sugars produced by autotrophs for their energy needs. • The predator that eats the deer receives a portion of the energy that originated in the photosynthetic vegetation that the deer consumed.

8. 1 Overview of photosynthesis • Photosynthesis uses solar energy to produce sugar from carbon dioxide and water. Oxygen is a waste product.

Photosynthetic reactants • Before exploring the chemical nature of photosynthesis, let’s establish the sources of its components: • H 2 O is absorbed by the roots from the soil. • CO 2 is acquired from the air as a result of gas exchange through the stomata in plants (small pores on the leaf underside). • Likewise, the O 2 waste product exits through the stomata. • Sunlight. Stoma (singular) Guard cells

Leaf structure • https: //www. youtube. com/watch? v=Bf-RFPa. Ze. AM • Animation from California Academy of Sciences

Photosynthesis equation • Just like cellular respiration, photosynthesis involves complex metabolic pathways. • The two metabolic pathways of photosynthesis are: - The Light reactions - The Calvin cycle

REACTION OUTCOMES AND LOCATIONS • The light reactions convert light energy to chemical energy, making ATP and NADPH (NADPH is an e- carrier, like NADH in respiration). • It occurs in the thylakoid membranes of chloroplasts. • The Calvin cycle uses the ATP and NADPH to make sugar (food) • It occurs in the stroma of chloroplasts.

Chloroplast structure • Double membrane (outer & inner) • Stroma (chloroplast fluid, not to be confused with stoma, a single leaf pore) • Grana (stacks of thylakoids) • Lumen is inside thylakoid Notes: • Only plant cells that conduct photosynthesis have chloroplasts. • High densities of chloroplasts occur in mesophyll (middle leaf) cells of leaves near the leaf surface (lots of light exposure).

What is light energy? • Light energy is electromagnetic energy, composed of photon energy particles that travel as waves. • Longer wavelengths (crests farther apart) carry less energy than shorter wavelengths • We can see only a fraction of this energy (visible range)

What is light energy? • Wavelengths are measured in nm (nanometers; billionths of a meter. • In the visible range (700 -400 nm), the violets have the shortest wavelengths (and most energy) and the reds have the longest.

Absorption of light • Pigments (colored molecules) absorb specific wavelengths of light; • each has a unique absorbance spectrum. • The main pigments of thylakoid membranes are: chlorophyll a (a) chlorophyll b (b) β-carotene (c), a type of carotenoid. Chlorophyll a and b capture light for photosynthesis; reason leaves are green (green wavelengths are reflected so we see the green). β-carotene helps to protect photosystems by dissipating excess energy; also found in cells of carrots & oranges Other carotenoids include lycopene (red color of tomato) and zeaxanthin (yellow of corn seeds) Carotenoids help extend the range of light that can be absorbed • •

Absorption spectrum: Wavelengths of light that chlorophyll (and other pigments such as carotenoids) can absorb & capture E. Action spectrum: Range of wavelengths at which the plant (because of its pigments) can carry out photosynthesis: needs both ends of the visible spectrum. One way to measure: amount of O 2 released under different light conditions.

Figure 10. 8 Evidence that chloroplast pigments participate in photosynthesis: absorption and action spectra for photosynthesis in an alga

The light reactions https: //www. youtube. com/watch? v=hj_WKgn. L 6 MI&t=72 s

Components of thylakoid membranes • Components of the thylakoid membranes include: • Photosystems II and I; sites of light absorption • Molecules that make up an Electron Transport Chain (ETC) • Two enzyme complexes, NADP reductase and ATP synthase

Components of thylakoid membranes

The photosystems • Photosystem II and I consists of a lightharvesting complex and a reaction center. • Pigments in the light-harvesting complex pass light energy to two special chlorophyll a molecules in the reaction center. • In the reaction center, the light excites an efrom the chlorophyll a pair, passing it to the primary electron acceptor and then to the ETC. • This is a light driven redox reaction • The lost electron is then replaced. • In photosystem II (a), the e- comes from the splitting of water, which releases oxygen as a waste product • In photosystem I (b), the e- comes from photosystem II via the ETC

Note that Photosystem II (PS II) comes “before” Photosystem I (PS I), because they were discovered in that order.

The electron transport chains • There are two parts of the ETC in the light reaction: • The first transports e- from PS II to PS I via • • • Plastoquinone Qb Cytochrome b 6 f Plastocyanin • The second transports efrom PS I to NADP reductase via • Ferredoxin • The final e- acceptor of the light reaction is NADP+ yielding NADPH

The electron transport chains • Just like the ETC of cellular respiration: • an H+ gradient is created as e- “fall” down the chain, releasing energy • H+ pumped into the lumen space • ATP synthase uses the gradient to generate ATP (chemiosmosis) • How do you think the p. H of the lumen would compare to the p. H of the stroma when a plant is photosynthesizing?

Figure 10. 12 How noncyclic electron flow during the light reactions generates ATP and NADPH (Layer 5)

The Calvin cycle • The ATP and NADPH from the light-dependent reactions are made in the stroma for use in the Calvin cycle.

The LIGHT-INDEPENDENT REACTIONS Three stages to the Calvin Cycle: 1. Fixation: CO 2 added to Ru. BP by enzyme Rubisco to generate 2 3 -PGA molecules 2. Reduction: ATP and NADPH used to add e- and make sugar (GA 3 P, also called G 3 P) • Note 2 GA 3 P glucose 3. Regeneration: of Ru. BP from GA 3 P Three cycles are required to make one GA 3 P (three CO 2 needed for each one GA 3 P that is saved to make glucose and other sugars).

The Calvin cycle

The Calvin cycle • Ru. Bis. CO: ribulose bisphosphate carboxylase oxygenase. • Ru. Bis. CO can function as a carboxylase (Calvin cycle) or an oxygenase (photorespiration – which does not result in sugar production; O 2 is accepted instead of CO 2). • Some plants, like this cactus, have evolved mechanisms to reduce the chances of photorespiration.

Figure 10. 17 The Calvin cycle (Layer 3)

Animation of photosythesis • https: //www. youtube. com/watch? v=gz 5 L 1 GUaa. XA

What about plants in hot, dry climates? Stomata (leaf pores) have to be open to let CO 2 in, O 2 out. But this also allows a lot of water to evaporate: better to keep them at least partly closed, especially in the day.

1) C 4 plants (includes corn, sugarcane). Leaves have mesophyll cells closest to surface: these carry out light reactions AND capture CO 2 and incorporate it into a 4 -C organic compound. The enzyme that does this (PEP carboxylase) is much better at fixing C than rubisco (rubisco can actually bind oxygen as well as CO 2, an evolutionary leftover from when there was very little free oxygen in the atmosphere).

So: PEP carboxylase can fix C from CO 2 even when the stomata are partly closed and CO 2 concentrations are low. This 4 -C compound is sent to bundle-sheath cells deeper in the leaf, via plasmodesmata. The 4 -C compound releases CO 2 into the bundle-sheath cells, keeping its concentration high enough for rubisco to work in the Calvin Cycle. (There’s more, but this is the basic idea).

Figure 10. 18 C 4 leaf anatomy and the C 4 pathway

2) CAM plants: Cacti, pineapples, etc. Close stomata during day, open at night (prevent evaporation). SO: Take up CO 2 at night and store it in organic acids in vacuoles. Then in the day, light reactions produce ATP and NADPH to power the Calvin Cycle, and CO 2 is released from the stored organic acids.

Figure 10. 19 C 4 and CAM photosynthesis compared

Photosynthesis: Probably the most important biochemical process on Earth. And, one of the oldest: goes back billions of years, and is the reason that there’s so much free oxygen in the atmosphere. Besides food, wood, forests etc. , provides a huge store of carbon at a time when CO 2 levels are rising dangerously.

Figure 10. 20 A review of photosynthesis

10. 1 The greenhouse effect (Part 1)

10. 1 The greenhouse effect (Part 2)

Carbon dioxide (and other greenhouse gases) trap heat (everyone agrees). Levels of carbon dioxide have been increasing exponentially since the Industrial Revolution (everyone agrees). Increases in global temperature track rises in CO 2 very closely (everyone agrees). Likeliest explanation: human activities are causing rising temperatures (97% of experts agree).