Endosymbiotic Theory

� The oldest known fossils are 3.5 bya = stromatolites which are rock like layers of bacteria and sediment.

� Earliest life forms may have emerged as early as 3.9 bya.

� Earliest prokaryotes were chemoheterotrophs� Used chemicals in the environment

for food and energy� There was still little oxygen in the

atmosphere and so they must have been anaerobic

Evolution of Prokaryotes

� Some prokaryotes developed the ability to convert light energy into chemical energy = photosynthesis

� These organisms released oxygen as a product of photosynthesis� Oxygen gas accumulated in the atmosphere

� Increase in oxygen created a “crisis”� Some organisms died

� Other organisms evolved more efficient metabolic pathways that used oxygen for respiration

Evolution of Photosynthetic Organisms

� Eukaryotic cells differ from and are much more complex than prokaryotes

� Fossil evidence suggests that eukaryotic cells may have been present 2.7 bya

Evolution of Eukaryotes

� Endosymbiotic Theory - suggests that mitochondria and chloroplasts were formerly small prokaryotes living within larger cells

� Prokaryotic ancestors of organelles probably lived as internal parasites within a larger prokaryotic host cell.

Eukaryotes Arose from Endosymbiosis

What Exactly Happened?

Heterotrophic bacteria

Ancient Prokaryotes

Ancient Heterotrophic Prokaryote

Primitive Heterotrophic Eukaryote

Primitive Autotrophic (Photosynthetic) Eukaryote

Chloroplast

Photosynthetic bacteria

Nuclear envelope evolving Mitochondrion

Plants and plant-like protists

Animals, fungi, and animal-like protists

Origin of Eukaryotes1 Endosymbiosis to explain the origin of mitochondria

and chloroplasts2 Invagination of the plasma membrane to form the

endomembrane system

Origin of Eukaryotes1 Endosymbiosis to explain the origin of mitochondria

and chloroplasts2 Invagination of the plasma membrane to form the

endomembrane system

Mitochondria

Origin of Eukaryotes1 Endosymbiosis to explain the origin of mitochondria

and chloroplasts2 Invagination of the plasma membrane to form the

endomembrane system

Nucleus

Endoplasmic Reticulum

Golgi Body

Mitochondria

Chloroplast

Origin of Eukaryotes1 Endosymbiosis to explain the origin of mitochondria

and chloroplasts2 Invagination of the plasma membrane to form the

endomembrane system

Chloroplast

Endoplasmic Reticulum

Nucleus

Golgi Body

Mitochondria

Membrane-Bound Organelles

� Mitochondria = membrane-bound organelle that produces energy for the cell

� Chloroplast = membrane-bound organelle that captures sunlight and uses it to make food for the cell

� Inner membranes of both organelles have enzymes and transport systems that are homologous to those in the plasma membranes of modern prokaryotes

� Both organelles replicate by a process similar to binary fission

� Each organelle has a single circular chromosome similar to prokaryotes

� Ribosomes of both organelles are similar to prokaryotic ribosomes in terms of size and nucleotide sequence.

Evidence for Endosymbiosis

Cellular Respiration

You feel weak when you are hungry because food serves as a source of energy. How does the food you eat get converted

into a usable form of energy for your cells?

Whenyouexercise,yourbodyusesoxygentogetenergyfromglucose,a6-carbonsugar.

1.Howdoesyourbodyfeelatthestartofexercise,suchatalongslowrun?Howdoyoufeel1minuteintotherun;10minutesintotherun?

2.Whatdoyouthinkishappeninginyourcellstocausethechangesinhowyoufeel?

3.Thinkaboutrunningasfastasyoucanfor100meters.Couldyoukeepupthispaceforamuchlongerdistance?Explainyouranswer.

Chemical Energy and Food

� Calorie – amount of energy needed to raise a temperature of 1 gram of water by 1 degree Celsius

� Cells use all sorts of molecules for food, including fats, proteins, and carbohydrates. The energy stored in each of these molecules varies because their chemical structures, and therefore their energy-storing bonds, differ.

� Cells break down food molecules gradually and use the energy stored in the chemical bonds to produce compounds such as ATP that power the activities of the cell.

Where do organisms get energy?

� Food = chemical energy� It provides living

organisms with chemical building blocks they need to grow and reproduce

� ATP = organic molecule containing high energy bonds -powers most cell activities (cell energy)

Cellular Respiration Overview

� Transformation of chemical energy in food into chemical energy cells can use: ATP à cellular respiration

� Overall Reaction:� C6H12O6 + 6O2 → 6CO2 + 6H2O

Glucose + Oxygen à Carbon dioxide + Water + Energy

Stages of Cellular Respiration

1) Glycolysis2) Krebs cycle3) Electron transport chain

Stages of Cellular Respiration

1) Glycolysis – produces small amount energy� Glucose is broken down to

pyruvate acid during glycolysis making some ATP

� Most of the glucose’s energy (90%) remains locked in the chemical bonds of pyruvic acid at the end of glycolysis

Stages of Cellular Respiration

2) Krebs Cycle – little more energy is generated from pyruvic acid

Stages of Cellular Respiration

3) Electron transport chain–produces a bulk of the energy in cellular respiration by using oxygen, a powerful electron acceptor

Energy Totals

Efficiency of Cellular Respiration

� The36ATPmoleculesthecellmakesperglucoserepresentsonlyabout38%ofthetotalenergythatwasintheglucosemolecule.

� Therestoftheenergyisreleasedasheat.

Oxygen and Energy� Aerobic- process that

requires oxygen� Krebs cycle and electron

transport chain are aerobic

� Both take place in mitochondria

� Anaerobic- process that does not require oxygen� Glycolysis is anaerobic� Takes place in cytoplasm

Photosynthesis vs. Cellular Respiration

� The reactants of cellular respiration are the products of photosynthesis and vice versa.

� The release of energy by cellular respiration takes place in plants, animals, fungi, protists, and most bacteria.

� Energy capture by photosynthesis occurs only in plants, algae, and some bacteria.

Fermentation

We are air-breathing organisms, and we use oxygen to release chemical energy from the food we eat. But what if oxygen isn’t around? What happens when you hold your breath and dive under water, or use up

oxygen so quickly that you cannot replace it fast enough?

Fermentation� Fermentation - in the absence

of oxygen, fermentation releases energy from food molecules by producing ATP

� Electron transport chain doesn’t run à WHY?

� Alcoholic Fermentation –yeasts are other organisms use alcoholic fermentation produces ethyl alcohol and carbon dioxide

� Lactic Fermentation – MOST organisms carry out fermentation using a chemical reaction that converts pyruvic acid to lactic acid

Energy and Exercise• To obtain energy for

exercise the body uses:

• Stored ATP• ATP formed

through lactic acid fermentation

• ATP formed through cellular respiration

Energy use during intense exercise

� StoredATPcansupportonlyafewsecondsofintenseexercise(ex:50metersinarace)

� LacticacidfermentationcanusuallysupplyenoughATPtolastabout90seconds(ex:200metersprint)

� Cellularrespirationisnecessaryforsustainedexercise

Role of cellular respiration inexercise

� CellularrespirationproducesATPmoreslowlythanfermentation

� Glycogen storedinmusclesisbrokendownintoglucose forcellularrespiration

� Usuallytheamountofstoredglycogenisenoughtolastforabout15to20minutesofactivity

� Tocontinueexercisingthebodywillbreakdownotherstoredmoleculesincludingfats.