Photosynthesis and Cellular Respiration

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

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

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

Overall Reaction 6CO2 + 6H2O → C6H12O6 + 6O2 carbon dioxide water light energy sugar oxygen Carbohydrate made is glucose 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

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

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

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)

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

Harvesting Chemical Energy So we see how energy enters food chains… (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

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 cellular respiration Overall Reaction: C6H12O6 + 6O2 → 6CO2 + 6H2O sugar oxygen energy carbon dioxide water out

Where Does Cellular Respiration Takes Place? takes place in two parts of the cell: Glycolysis occurs in the Cytoplasm Krebs Cycle & ETC Take place in the Mitochondria 16

Review of Mitochondria Structure Smooth outer Membrane Folded inner membrane Folds called Cristae Space inside cristae called the Matrix 17

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 (fermentation) Aerobic cellular respiration

Cellular Respiration Reactions Glycolysis Series of reactions which break 6-carbon glucose molecule into two 3-carbon molecules “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

Glycolysis Summary 3. Requires input of 2 ATP 1. Takes place in the Cytoplasm 2. Anaerobic (Doesn’t Use Oxygen) 3. Requires input of 2 ATP 4. Glucose split into two molecules of Pyruvate or Pyruvic Acid 5. Produces 2 NADH and 4 ATP 6. Pyruvate is oxidized to Acetyl CoA and CO2 is removed 21

Anaerobic Cellular Respiration Some organisms thrive in environments with little or no oxygen Marshes, bogs, gut of animals, sewage treatment ponds No oxygen used = anaerobic “not 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)

Fermentation (2 forms) Occurs when O2 NOT present (anaerobic) 1. Called Lactic Acid fermentation in muscle cells (makes muscles tired) 2. Called Alcoholic fermentation in yeast (produces ethanol and CO2) Nets only 2 ATP 24

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

A Little Krebs Cycle History Discovered by Hans Krebs in 1937 He received the Nobel Prize in physiology or medicine in 1953 for his discovery Forced to leave Germany prior to WWII because he was Jewish 26

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

Krebs Cycle Summary Requires Oxygen (Aerobic) Cyclical series of oxidation reactions that give off CO2 and produce one ATP per cycle Turns twice per glucose molecule Produces two ATP Takes place in matrix of mitochondria 28

Krebs Cycle Summary Each turn of the Krebs Cycle also produces 3NADH, 1FADH2, and 2CO2 Therefore, For each Glucose molecule, the Krebs Cycle produces 6NADH, 2FADH2, 4CO2, and 2ATP 29

Kreb Cycle ATP NETS: 3NADH, 1ATP, 1FADH2, & 2CO2 30

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

Electron Transport Summary 34 ATP Produced H2O Produced Occurs Across Inner Mitochondrial membrane Uses coenzymes NAD+ and FAD+ to accept e- from glucose NADH = 3 ATP’s FADH2 = 2 ATP’s 32

Electron Transport Chain Animation 34

Glycolysis Diagram 35

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