Photosynthesis and Cellular Respiration

How do cells obtain organic compounds for energy? Heterotrophs: Cannot make their own food Autotrophs: Can make their own food – Photoautotrophs: Use energy from the sun (photosynthesis) to produce organic compounds (glucose) Plants, algae and some bacteria – Chemoautotrophs: Use energy stored in inorganic compounds (chemosynthesis) to produce organic compounds Some bacteria found at the hydrothermal vents of the seafloor

Photosynthesis Method of converting light energy from the sun into chemical energy that cells can use – Divided into the light-dependent and light-independent reactions Photosynthesis takes place in chloroplasts – The parts of a chloroplast are as follows: Thylakoids- disk-shaped structures that contain the pigment chlorophyll (absorbs the sunlight) Grana- A stack of thylakoids Stroma- Liquid between grana

Draw a Chloroplast

Overall Photosynthesis Reaction 6CO H 2 O + light energy → C 6 H 12 O 6 + 6O 2 On the left of the arrow are the reactants (the components that “react” together). On the right of the arrow are the products. Identify how a plant obtains the reactants.

Light-dependent Reactions Chlorophyll (in thylakoids) absorbs the light energy – Plants have 2 types: Chlorophyll A and Chlorophyll B Water molecules are split apart producing H and O 2 Electrons flow throughout the thylakoid membrane (electron transport chain) Energy compounds ATP and NADPH are produced

Light-independent Reactions (Dark Reactions) Occur in the stroma ATP and NADPH from the light reactions are used to fuel the break down of CO 2 and the reassembling of the atoms to produce glucose. This reassembling is called “carbon fixation”. Carbon fixation occurs in a series of reactions called the Calvin Cycle.

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

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: – C 6 H 12 O 6 + 6O 2 → 6CO 2 + 6H 2 O

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

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

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 CO 2 (single cell fungi (yeast) in beer/bread) or lactic acid (muscle cells)

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

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 CO 2 and H 2 O – 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 3 rd stage

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

Energy Tally 36 ATP for aerobic vs. 2 ATP for anaerobic – Glycolysis 2 ATP – Kreb’s 2 ATP – Electron Transport32 ATP 36 ATP Anaerobic organisms can’t be too energetic but are important for global recycling of carbon