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Announcements

• Problem Set #1 answers will be posted on website tomorrow. • Problem Set #2 will also be posted on website tomorrow and form the basis for discussion sections next week. • On Tuesday, we will watch “Life Story”. We will start it at the end of today’s class since it is longer than 80 minutes. Exam will feature questions related to film. • First midterm is Tuesday, January 28th. Scantron. Bring ID and #2 Pencils.

Chapter 4: Energy

From the sun to you in two easy steps

Lecture 4 Outline

I. Energy A. Potential vs. Kinetic B. ATP

II. Photosynthesis

A. Chloroplasts B. Light and Pigments C. “Photo” Reactions D. “Synthesis” Reactions E. Variations

III. Respiration

A. Glycolysis B. Citric Acid Cycle C. Oxidative Phosphorylation D. Various Inputs to System

Energy flows from the sun and through all life

on earth.

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Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings

Light energy

ECOSYSTEM

CO2 + H2O

Photosynthesis in chloroplasts

Cellular respiration in mitochondria

Organic molecules + O2

ATP

powers most cellular work Heat energy

Energy flow and chemical recycling in ecosystems

Chemical Energy Energy Conversions q  Only ~1% of the energy released by the sun that

earth receives is captured and converted by plants. • Converted into chemical bond energy

q  What happens to the other 99%?

Insert new fig 4-5

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How do cells directly fuel their chemical reactions?

q None of the light energy from the sun can be used directly to fuel cellular work.

q First, it must be captured in the bonds of a molecule called adenosine triphosphate (ATP).

Structure of ATP

Insert new fig 4-7

ATP molecules are like free-floating rechargeable batteries in all living cells.

Recycling in the Cell ADP + Phosphate group + Energy = ATP

Insert new fig 4-8

4.5 Where does plant matter come from? Photosynthesis: the big picture.

From a seed to a tree: Where does the mass come from?

Insert new fig 4-9

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PHOTOSYNTHESIS • Combination of two distinct processes:

PHOTO = harvesting of light energy --> Light Reactions

SYNTHESIS = Using E to build sugar --> Calvin Cycle • “Respiration in Reverse”:

6CO2 + 6H2O + Light E -----> C6H12O6 + 6O2

Photosynthesis: The Big Picture

q Three inputs

q Two products Insert  new  Figure  4-­‐11

Photosynthesis takes place in the chloroplasts.

Organelles Found in Plant Cells

Insert  new  Figure  4-­‐12  

A Closer Look at Chloroplasts

Insert  new  Figure  4-­‐13

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4.7 Light energy travels in waves: Plant pigments absorb specific wavelengths.

Light Energy q  A type of kinetic energy

q  Made up of little energy packets called photons

Electromagnetic Spectrum

q  Range of energy that is organized into waves of different lengths

q  The shorter the wavelength, the higher the energy

Insert  new  Figure  4-­‐14

Insert  new  Figure  4-­‐15a

Excitation of isolated chlorophyll by light

Excited state

Ene

rgy

of e

lect

ion

e–

Heat

Photon (fluorescence)

Chlorophyll molecule

Ground state Photon

(a) Excitation of isolated chlorophyll molecule (b) Fluorescence

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Fluorescent Fish

Insert  new  Figure  4-­‐16a Insert  new  Figure  4-­‐16b

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The Passing of Electrons in Their Excited State

q Chief way energy moves through cells q Molecules that gain electrons always carry

greater energy than before receiving them.

• Can view this as passing of potential energy from molecule to molecule

4.9 Photosynthesis in detail: The energy of sunlight is captured as chemical energy.

FOLLOW THE ELECTRONS!

Insert  new  Figure  4-­‐17

The “Photo” Part q  Sunlight q  ATP q  A high-energy electron carrier

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An Electron Transport Chain Connects the two photosystems Product #1 of the “Photo”

Portion of Photosynthesis:

ATP

The Second Photosystem q Follow the electrons Product #2 of the “Photo”

Portion of Photosynthesis:

NADPH

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“SYNTHESIS”

4.10 Photosynthesis in detail: The captured energy of sunlight is used to make

food.

The Calvin Cycle

q  Series of chemical reactions

q  Occurs in stroma

q  Enzymes are reCYCLEd.

The Processes in the Calvin Cycle Occur in Three Steps:

The battle against world hunger can use plants adapted to water

scarcity.

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Photosynthetic Adaptations to harsh environments

• On dry, hot days, plants must close stomata to conserve H2O. O2 from the light reactions then builds up inside the chloroplast. When [O2] gets high enough, it binds to the Rubisco enzyme (normally fixes CO2 into Calvin Cycle). This results in a 2C compound produced from the Calvin Cycle which is broken down to CO2 and H2O and is termed PHOTORESPIRATION. This generates no E and is very wasteful to the plant. Adaptations have evolved in tropical and desert plants to avoid photorespiration. Key to both are use of enzymes other than Rubisco to fix C from CO2.

Stomata Pores for gas exchange

How to get CO2 when stomata are shut?

C4 and CAM photosynthesis compared

Organic acids release CO2 to Calvin cycle

Spatial separation of steps. In C4 plants, carbon fixation and the Calvin cycle occur in different types of cells.

(a) Temporal separation of steps. In CAM plants, carbon fixation and the Calvin cycle occur in the same cells at different times.

(b)

Pineapple Sugarcane

Bundle- sheath cell

Mesophyll Cell

Organic acid

CALVIN CYCLE

Sugar

CO2 CO2

Organic acid

CALVIN CYCLE

Sugar

C4 CAM

CO2 incorporated into four-carbon organic acids (carbon fixation)

Night

Day

1

2 Organic acids release CO2 to Calvin cycle

CO2

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All Three Photosynthetic Pathways

http://indianapublicmedia.org/amomentofscience/solarpowered-sea-slugs/

Light energy

ECOSYSTEM

CO2 + H2O

Photosynthesis in chloroplasts

Cellular respiration

in mitochondria

Organic molecules + O2

ATP

powers most cellular work

Heat energy

Energy flow and chemical recycling in ecosystems The production of ATP by food breakdown • The oxidation of organic molecules releases energy. Exergonic; ΔG<0 • Organic molecules contain a great deal of energy, but unlike gasoline combustion, E must be extracted slowly such that the process releases usable E for the cell. RESPIRATION • Highly efficient catabolic pathway where enzymes and O2 extract the E from organic molecules like glucose. • Yields ATP, the energy currency of the cell.

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CELLULAR RESPIRATION • Respiration occurs in 3 different stages, using over 20 steps to efficiently harvest E present in chemical bonds. 1) Glycolysis: Splits Glucose into two pyruvate molecules Occurs in cytosol; does NOT require O2 2) Citric Acid Cycle (Krebs Cycle): Completes breakdown of glucose. Occurs in Mitochondrial matrix. 3) Oxidative Phosphorylation: Generates the vast majority of ATP. Occurs in Mitochondrial inner membrane. Driven by e- transport chain --> O2 ultimate acceptor of the e- derived from breakdown of glucose. Chemiosmosis couples ETC to ATP production.

Cellular Respiration

The big picture

INPUTS

OUTPUTS

A Human Example

q  Eat food q  Digest it q  Absorb nutrient molecules into bloodstream q  Deliver nutrient molecules to the cells

q  At this point, our cells can begin to extract some of the energy •  Stored in the bonds of the food molecules

The first step of cellular respiration: Glycolysis is the universal energy-releasing pathway.

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The second step of cellular respiration: The Krebs cycle extracts energy from sugar.

The Preparatory Phase to the Krebs Cycle

Energy Payoff of Krebs Cycle

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The third step in cellular respiration:

ATP is built in the electron transport chain.

Mitochondria

q Two key features of mitochondria are essential to their ability to harness energy from molecules:

• Feature 1: mitochondrial “bag-within-a-bag” structure

• Feature 2: electron carriers organized within the inner “bag”

The “Bag-within-a-Bag” Follow the Electrons, as We Did in Photosynthesis

2) This proton concentration gradient represents a significant source of potential energy!

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Proton Gradients and Potential Energy

The force of the flow of H+ ions fuels the attachment of free-floating phosphate

groups to ADP to produce ATP.

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q Oxygen deficiency limits the breakdown of fuel because the electron transport chain requires oxygen as the final acceptor of the electrons generated during glycolysis and the Krebs cycle.

Eating a complete diet: Cells can run on protein and fat as well as on glucose.

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Energy and Metabolism:

Reading: Chapter 4 Do PS #2 Answers to PS#2 posted next Friday Movie tonight and Tuesday