Photosynthesis and Cellular Respiration. Photosynthesis Method of converting sun energy into...

Photosynthesis and Photosynthesis and Cellular RespirationCellular 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

6CO2 + 6 H2O → C6H12O6 + 6O2

Carbohydrate made is glucose Water is split, resulting in the release of electrons

and O2 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, FADH2

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 CO2 and break it apart, then reassemble the carbons into glucose.

Carbon Fixation occurs – Taking carbon from an inorganic molecule

(atmospheric CO2) and making an organic molecule out of it (glucose)

Step-By-Step…

CO2 diffuses into the stroma and combines each CO2 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:– C6H12O6 + 6O2 → 6CO2 + 6H2O

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 CO2 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 CO2 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 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

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 CO2 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 CO2 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 CO2 molecules, two ATP molecules and hydrogen molecules used to make six NADH and two FADH2

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 Transport 32 ATP 36 ATP

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