Photosynthesis and Cellular Respiration

Photosynthesis Method of converting sun energy into chemical energy usable by cells Autotrophs: self feeders, organisms capable of making their own food Photosynthesis takes place in specialized structures inside plant cells called chloroplasts – Light absorbing pigment molecules (e.g. chlorophyll)

Oxidation and Reduction Oxidation means that a reactant lose electrons Reduction means that a reactant gains electrons

Overall Reaction 6CO H 2 O → C 6 H 12 O 6 + 6O 2 Carbohydrate made is glucose Water is split, resulting in the release of electrons and O 2 as a byproduct

Light-Dependent Reactions Overview: – Light energy is absorbed by electrons in the chlorophyll and boosts them to higher energy levels. – Electrons are “grabbed” by other molecules (electron acceptors) – The electrons “fall” to a lower energy state as they move from molecule to molecule, releasing energy that is harnessed to make ATP

Energy Shuttling ATP: cellular energy molecule with 3 phosphate groups bonded to it – When the third phosphate group is removed, lots of energy is released! Other nucleotide based molecules move electrons and protons around within the cell – NADP+, NADPH – NAD+, NADP – FAD, FADH 2

Light-dependent Reactions Photosystem: light capturing unit, contains chlorophyll, the light capturing pigment Electron Transport Chain: sequence of electron carrier molecules that shuttle electrons; energy released is used to make ATP Light reactions yield ATP and NADPH used to fuel the reactions of the Calvin Cycle

Step-by-Step… Light energy excites electrons in Photosystem II and water molecules are split to provide additional electrons The excited electrons move along a sequence of electron carrier molecules in the thylakoid called the Electron Transport Chain (ETC) As they move, they lose energy which is used to move protons (H+) into the thylakoid. This proton gradients lets ATP be made from ADP Electrons enter Photosystem I and are excited by more light energy The excited electrons move along another ETC This ETC moves the electrons close to the stroma where they combine with a proton and NADP+ to form NADPH Electrons from Photosystem II replace the ones used in Photosystem I. Water molecules are split to provide replacement electrons for Photosystem II.

Calvin Cycle ATP and NADPH generated in light reactions are used to fuel the Calvin Cycle, reactions which take CO 2 and break it apart, then reassemble the carbons into glucose. Carbon Fixation occurs – Taking carbon from an inorganic molecule (atmospheric CO 2 ) and making an organic molecule out of it (glucose)

Step-By-Step… CO 2 diffuses into the stroma and combines each CO 2 molecule with a five-carbon molecule (RuBP) The new six-carbon molecule is unstable and splits into two three- carbon molecules (3-PGA) Each three-carbon molecule is coverted into another three-carbon molecule (G3P). First, it receives a phosphate from ATP. Then it receives a proton from NADPH and releases a phosphate. One of the new three-carbon compounds leaves the cycle to make glucose. The remain three-carbon compounds are converted back into five carbon molecules (RuBP) through the addition of phosphates from ATP

Harvesting Chemical Energy 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 At this point life diverges into two forms and two pathways – Anaerobic cellular respiration (aka fermentation) – Aerobic cellular respiration

Cellular Respiration Reactions Glycolysis – Series of reactions which break the 6-carbon glucose molecule down into two 3-carbon molecules called pyruvate using ATP – Process is ancient -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

Step-By-Step… 2 ATP molecules attach to two phosphates to a glucose molecule, making a new six carbon compound The six-carbon compound is split into two three-carbon compounds (G3P) The two three-carbon compounds are oxidized and receive a phosphate and make a new three-carbon compound NAD+ is reduced to NADH The phosphate groups are removed, producing two molecules of pyruvic acid and 4 ATP are made

Anaerobic Cellular Respiration Some organisms thrive in environments with little or no oxygen – No oxygen used = anaerobic 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

Lactic Acid Production After glycolysis, a hydrogen atom is transferred from NADH (oxidizing to NAD+) and a free proton is added to pyruvic acid to form lactic acid NAD+ is used in glycolysis Fermentation can be used to produce cheese, yogurt, sour cream and more

Alcoholic Fermentation After glycolysis, a CO 2 molecule is removed from pyruvic acid, leaving a two-carbon compound Two hydrogen atoms, from NADH and a proton, are added to the two-carbon compound to form ethyl alcohol Alcoholic fermentation by yeast cells is the basis of the wine and beer industry Bread also rises due to CO 2 production

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

Step-By-Step A two-carbon compound (acetyl CoA) combines with a four-carbon compound (oxaloacetic acid) to make a six-carbon compound (citric acid) Citric acid releases a CO 2 molecule and a hydrogen atom to form a five-carbon compound – The hydrogen atom is transferred to NAD+ to make NADH The five-carbon compound releases a CO 2 molecule and a hydrogen atom to form a four-carbon compound. NAD+ becomes NADH The four-carbon compound releases a hydrogen atom to form another four-carbon compound – This hydrogen atom reduces FAD to FADH2 The four-carbon compound releases another hydrogen atom to regenerate oxaloacetic acid and reduces NAD+ to NADH

Krebs Cycle Outcome One glucose molecule is completely broken down after two turns of the Krebs Cycle Two turns produce four CO 2 molecules, two ATP molecules and hydrogen molecules used to make six NADH and two FADH 2 CO2 diffuses as waste ATP is used for energy Add in the four NADH from glycolysis and conversion to pyruvic acid, and we’re ready for the next step!

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

Step-By-Step Electrons in the hydrogen atoms from NADH and FADH2 are at a high energy NADH and FADH2 give up electrons to the ETC – NADH donates them at the beginning – FADH2 donates them midway The electrons move down the chain, loosing energy The energy creates a proton gradient and an electrical gradient from the positive charge ATP is generated from the gradients from ADP and phosphate Oxygen is the final acceptor of electrons that have passed down the chain. The protons, electrons, and oxygen combine to form 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