Photosynthesis and Photosynthesis and Cellular RespirationCellular Respiration

Outline

I. PhotosynthesisA. IntroductionB. Reactions

II. Cellular RespirationA. IntroductionB. Reactions

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 + 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

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 cellNADP+, 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 (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:– C6H12O6 + 6O2 → 6CO2 + 6H2O

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 CO2 (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 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

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 Transport 32 ATP 36 ATP

Anaerobic organisms can’t be too energetic but are important for global recycling of carbon