Photosynthesis and Cellular Respiration

1. Photosynthesis and Cellular Respiration

2. Outline I. Photosynthesis A. Introduction B. Reactions II. Cellular Respiration A. Introduction B. Reactions

3. Photosynthesis • Method of converting sun energy into chemical energy usable by cells • Autotrophs: self feeders, organisms capable of making their own food • Photoautotrophs: use sun energy e.g. plants photosynthesis-makes organic compounds (glucose) from light • Chemoautotrophs: use chemical energy e.g. bacteria that use sulfide or methane chemosynthesis-makes organic compounds from chemical energy contained in sulfide or methane

4. Photosynthesis • Photosynthesis takes place in specialized structures inside plant cells called chloroplasts • Light absorbing pigment molecules e.g. chlorophyll

5. Overall Reaction • 6CO2 + 12 H2O + light energy → C6H12O6 + 6O2+ 6H2O • Carbohydrate made is glucose • Water appears on both sides because 12 H2O molecules are required and 6 new H2O molecules are made • Water is split as a source of electrons from hydrogen atoms releasing O2 as a byproduct • Electrons increase potential energy when moved from water to sugar therefore energy is required

6. Light-dependent Reactions • Overview: light energy is absorbed by chlorophyll molecules-this light energy excites electrons and boosts them to higher energy levels. They are trapped by electron acceptor molecules that are poised at the start of a neighboring transport system. The electrons “fall” to a lower energy state, releasing energy that is harnessed to make ATP

7. Energy Shuttling • Recall ATP: cellular energy-nucleotide based molecule with 3 phosphate groups bonded to it, when removing the third phosphate group, lots of energy liberated= superb molecule for shuttling energy around within cells. • Other energy shuttles-coenzymes (nucleotide based molecules): move electrons and protons around within the cell NADP+, NADPH NAD+, NADP FAD, FADH2

8. Light-dependent Reactions • Photosystem: light capturing unit, contains chlorophyll, the light capturing pigment • Electron transport system: sequence of electron carrier molecules that shuttle electrons, energy released to make ATP • Electrons in chlorophyll must be replaced so that cycle may continue-these electrons come from water molecules, Oxygen is liberated from the light reactions • Light reactions yield ATP and NADPH used to fuel the reactions of the Calvin cycle (light independent or dark reactions)

11. Calvin Cycle (light independent or “dark” reactions) • ATP and NADPH generated in light reactions used to fuel the reactions which take CO2 and break it apart, then reassemble the carbons into glucose. • Called carbon fixation: taking carbon from an inorganic molecule (atmospheric CO2) and making an organic molecule out of it (glucose) • Simplified version of how carbon and energy enter the food chain

13. Harvesting Chemical Energy • So we see how energy enters food chains (via autotrophs) we can look at how organisms use that energy to fuel their bodies. • Plants and animals both use products of photosynthesis (glucose) for metabolic fuel • Heterotrophs: must take in energy from outside sources, cannot make their own e.g. animals • When we take in glucose (or other carbs), proteins, and fats-these foods don’t come to us the way our cells can use them

14. Cellular Respiration Overview • Transformation of chemical energy in food into chemical energy cells can use: ATP • These reactions proceed the same way in plants and animals. Process is called cellular respiration • Overall Reaction: • C6H12O6 + 6O2 → 6CO2 + 6H2O

15. Cellular Respiration Overview • Breakdown of glucose begins in the cytoplasm: the liquid matrix inside the cell • At this point life diverges into two forms and two pathways • Anaerobic cellular respiration (aka fermentation) • Aerobic cellular respiration

16. C.R. Reactions • Glycolysis • Series of reactions which break the 6-carbon glucose molecule down into two 3-carbon molecules called pyruvate • Process is an ancient one-all organisms from simple bacteria to humans perform it the same way • Yields 2 ATP molecules for every one glucose molecule broken down • Yields 2 NADH per glucose molecule

18. Anaerobic Cellular Respiration • Some organisms thrive in environments with little or no oxygen • Marshes, bogs, gut of animals, sewage treatment ponds • No oxygen used= ‘an’aerobic • Results in no more ATP, final steps in these pathways serve ONLY to regenerate NAD+ so it can return to pick up more electrons and hydrogens in glycolysis. • End products such as ethanol and CO2 (single cell fungi (yeast) in beer/bread) or lactic acid (muscle cells)

20. Aerobic Cellular Respiration • Oxygen required=aerobic • 2 more sets of reactions which occur in a specialized structure within the cell called the mitochondria • 1. Kreb’s Cycle • 2. Electron Transport Chain

21. Kreb’s Cycle • Completes the breakdown of glucose • Takes the pyruvate (3-carbons) and breaks it down, the carbon and oxygen atoms end up in CO2 and H2O • Hydrogens and electrons are stripped and loaded onto NAD+ and FAD to produce NADH and FADH2 • Production of only 2 more ATP but loads up the coenzymes with H+ and electrons which move to the 3rd stage

23. Electron Transport Chain • Electron carriers loaded with electrons and protons from the Kreb’s cycle move to this chain-like a series of steps (staircase). • As electrons drop down stairs, energy released to form a total of 32 ATP • Oxygen waits at bottom of staircase, picks up electrons and protons and in doing so becomes water

25. Energy Tally • 36 ATP for aerobic vs. 2 ATP for anaerobic • Glycolysis 2 ATP • Kreb’s 2 ATP • Electron Transport 32 ATP 36 ATP • Anaerobic organisms can’t be too energetic but are important for global recycling of carbon