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Chapter 4: Energy

From the sun to you in two easy steps

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Hello and welcome to the lecture corresponding to Chapter 4, Energy.

Energy Conversions

All life on Earth depends on capturing energy from the sun and converting it into a form that living organisms can use.

The sun is the source of the energy that powers all living organisms and other “machines.”

2 main processes: photosynthesis & cellular respiration

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All life processes require energy. And earth converts energy from the sun into various usable forms.

Photosynthesis & Cellular Respiration

Photosynthesis: The energy from sunlight is stored in the chemical bonds of molecules.

Cellular respiration: organisms use the energy stored in chemical bonds to fuel their lives

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This energy capture and conversion occurs in two important processes that mirror each other: (1) photosynthesis, the process by which plants capture energy from the sun and store it in the chemical bonds of sugars and other food molecules they make, and (2) cellular respiration, the process by which all living organisms release the energy stored in the chemical bonds of food molecules and use it to fuel their lives.

What is energy?

The capacity to do work

Work= Moving matter against an opposing force

2 forms of energy:

Kinetic

potential

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Humans, plants, and all other living organisms need energy for their activities, from thinking to moving to reproducing.

Kinetic Energy

The energy of moving objects, includes also:

Heat energy

Light energy

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Legs pushing bike pedals and the flapping wings of a bird are examples of kinetic energy.

Heat, which results from lots of molecules moving rapidly, is another form of kinetic energy. Because it comes from the movement of high-energy particles, light is also a form of kinetic energy—probably the most important form of kinetic energy on earth.

Figure 4-3 Kinetic and potential energy.

Potential Energy

A capacity to do work that results from the location or position of an object (think a person on a trampoline before jumping)

Concentration gradients

Food has potential energy

chemical bonds

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An object does not have to be moving to have the capacity to do work; it may have potential energy, which is stored energy or the capacity to do work that results from an object’s location or position.

Water behind a dam has potential energy, for example.

If a hole in the dam is opened, the water can flow through, and perhaps spin a water wheel or turbine.

Thermodynamics and its laws

The study of the transformation of energy from one type to another

1st Law: Energy is neither created nor destroyed but can change forms.

2nd Law: Each conversion of energy is inefficient, and some of the usable energy is converted to less useful heat energy.

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The study of the transformation of energy from one type to another, such as from potential energy to kinetic energy, is called thermodynamics. That fact that energy can change form but never disappear is an important feature of energy in the universe, whether we are looking at the sun and the earth or a human and her rice bowl. Although heat is certainly a form of energy, because it is not easily harnessed to do work, it is almost completely useless to living organisms for fueling their cellular activity.

Put another way, the second law of thermodynamics tells us that although the quantity of energy in the universe is not changing, its quality is.

Little by little, the amount of energy that is available to do work decreases.

Just as energy can never disappear or be destroyed, energy can never be created.

All the energy now present in the universe has been there since the universe began and everything that has happened since then has occurred by the transformation of one form of energy into another.

In all our eating and growing, driving and sleeping, we are simply transforming energy.

Energy Conversions

Only ~1% of the energy released by the sun that earth receives is captured and converted by plants.

Converted into chemical bond energy

What happens to the other 99%?

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Organisms on earth cannot capture every single bit of energy released by the sun; indeed, most plants capture only about 1% of the available energy.

What happens to the other 99%?

The rest of the energy from the sun is reflected back into space (probably about 30%) or absorbed by land, the oceans, and the atmosphere (about 70%), and mostly transformed into heat.

Figure 4-5 Energy efficiency and heat loss.

ATP: adenosine tri-phosphate

Cells temporarily store energy in the bonds of ATP molecules.

This potential energy can be converted to kinetic energy and used to fuel life-sustaining chemical reactions.

At other times, inputs of kinetic energy can be converted to the potential energy of the energy-rich but unstable bonds in the ATP molecule.

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

Breaking down a molecule of sugar, in a glass of orange juice for example, leads to a miniature burst of energy in your body.

The energy from the mini-explosion is put to work building the unstable high-energy bonds that attach phosphate groups to ADP molecules, creating new molecules of ATP.

Later—perhaps only a fraction of a second later—when an energy-consuming reaction is needed, your cells can release the energy stored in the new ATP molecules.

Structure of ATP

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ATP is a simple molecule with three components.

At the center of the ATP molecule are two of these components: a small sugar molecule attached to a molecule called adenine.

But it is the third component that that makes ATP so effective in carrying and storing energy for a short time: Attached to the sugar and adenine is a chain of three negatively charged phosphate molecule groups (hence the “tri” in triphosphate).

Because the bonds between these three phosphate groups must hold the groups together in the face of the three electrical charges that all repel one another, each of these bonds contains a large amount of energy and is stressed and unstable.

The instability of these high-energy bonds makes the three phosphate groups like a tightly coiled spring or a twig that is bent almost to the point of breaking.

With the slightest push, one of the phosphate groups will pop off, releasing a little burst of energy that the cell can use.

Figure 4-7 The structure of ATP and ADP.

How does ATP store energy?

Pop off the third phosphate group

ATP  ADP + Phosphate group + Energy release

Release a little burst of energy!

Use this energy to drive chemical reactions necessary for cellular functioning.

Building muscle tissue

Repairing a wound

Growing roots

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The instability of these high-energy bonds makes the three phosphate groups like a tightly coiled spring or a twig that is bent almost to the point of breaking.

With the slightest push, one of the phosphate groups will pop off, releasing a little burst of energy that the cell can use.

ATP Recycling in the Cell

ADP + Phosphate group + Energy = ATP

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Energy to make ATP comes from sunlight or from energy stored in chemical bonds.

Each time a cell expends one of its ATP molecules to pay for an energetically expensive reaction, a phosphate is broken off and energy is released.

What is left is a molecule with two phosphates, called ADP (adenosine diphosphate), and a separate phosphate group (labeled Pi).

An organism can then use ADP, a free-floating phosphate, and an input of kinetic energy to rebuild its ATP stocks.

The kinetic energy is converted to potential energy when the free phosphate group attaches to the ADP molecule and makes ATP.

In this manner, ATP functions like a rechargeable battery.

Figure 4-8 ATP is like a rechargeable battery.

Photosynthetic Organisms besides plants

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Although plants are the most visible organisms that have evolved the ability to capture light energy and convert it to organic matter, they are not the only organisms capable of photosynthesis.

Some bacteria and many other unicellular organisms are also capable of using the energy in sunlight to produce organic materials.

Figure 4-10 Plants are not the only photosynthesizes.

Photosynthesis: The Big Picture

Three inputs:

Light energy

CO2

water

Two products:

Glucose (sugar)

oxygen

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There are three inputs to the process of photosynthesis: light energy (from the sun), carbon dioxide (from the atmosphere), and water (from the ground). From these three inputs, the plant produces sugar and oxygen.

Photosynthesis is best understood as two separate events: a “photo” segment, during which light is captured, and a “synthesis” segment, during which sugar is synthesized.

Photosynthesis takes place in the chloroplasts.

Review chloroplasts from Ch.3

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The best way to know which parts of a plant are photosynthetic is simply to look for the green bits.

Leaves are green because the cells near the surface are packed full of chloroplasts, light-harvesting organelles found in plant cells, which make it possible for the plant to use the energy from sunlight to make sugars (their food) and other plant tissue (much of which animals use for food).

The sac-shaped organelle is filled with a fluid called the stroma.

Floating in the stroma is an elaborate system of interconnected membranous structures called thylakoids, which often look like stacks of pancakes.

Once inside the chloroplast, you can be in one of two places: in the stroma or inside the thylakoids.

The conversion of light energy to chemical energy—the “photo” part of photosynthesis—occurs inside the thylakoids.

The production of sugars—which are made in the “synthesis” part of photosynthesis—occurs within the stroma.

Light energy travels in waves

Light: A type of kinetic energy

Made up of little energy packets called photons

Different photons carry different amounts of energy, carried as waves.

Length of the wave = amount of energy the photon contains.

Plant pigments absorb specific wavelengths.

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Photosynthesis is powered by light energy, a type of kinetic energy made up of little energy packets called photons, which are organized into waves.

Photons can do work as they bombard surfaces such as your face (heating it) or a leaf (enabling it to build sugar from carbon dioxide and water). Photons have various amounts of energy and the length of the wave in which they travel corresponds to the amount of energy carried by the photon.

Electromagnetic Spectrum

Range of energy that is organized into waves of different lengths

The shorter the wavelength, the higher the energy

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The shorter the wavelength, the more energy the light carries.

Within a ray of light, there are super-high energy packets of photons (those with short wavelengths), relatively low-energy packets (those with longer wavelengths), and everything in between.

This range, which is called the electromagnetic spectrum, extends from extremely short, high-energy gamma rays and X-rays, with wavelengths as short as 1 nanometer (a human hair is about 50,000 nanometers in diameter) to very long, low-energy radio waves, with wavelengths as long as a mile.

Figure 4-14 A spectrum of energy.

Visible Spectrum

Range of energy humans see as light

ROYGBIV (from Red to Violet)

Pigments = molecules that absorb light

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Just as we can’t hear some super-high-pitched frequencies of sound (even though many dogs can), there are some wavelengths of light that are too short or too long for us to see.

The light that we can see, visible light, spans all the colors of the rainbow.

Humans (and some other animals) can see colors because our eyes contain light-absorbing molecules called pigments.

These pigments absorb wavelengths of light within the visible range.

The energy in these light waves excites electrons in the pigments, which in turn stimulates nerves in our eyes.

These nerves then transmit electrical signals to our brains.

We perceive different wavelengths within the visible spectrum as different colors.

Plant Pigments

Plant pigments can absorb light energy =>Absorbed energy excites electrons

Plant pigments can only absorb specific wavelengths of energy.

Therefore, plants produce several different types of pigments.

Chlorophyll a

Chlorophyll b

Carotenoids

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Unlike our eyes, however, plant pigments (the energy-capturing parts of a plant) absorb and use only a portion of visible light wavelengths. Other wavelengths pass through or bounce off.

Therefore, plants produce several different types of pigments in order to maximize their ability to absorb energy. Plants produce several different light-absorbing pigments:

Chlorophyll a

The primary photosynthetic pigment

Efficiently absorbs blue-violet and red wavelengths

Chlorophyll b

Absorbs blue and red-orange wavelengths

Reflects yellow-green wavelengths

Carotenoids

Absorb blue-violet and blue-green wavelengths

Reflect yellow, orange, and red wavelengths

Insert new Figure 4-15a

Absorption spectrum of pigments

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The primary photosynthetic pigment, called chlorophyll a, efficiently absorbs blue-violet and red wavelengths of light.

Every other wavelength generally travels through or bounces off this pigment.

Because chlorophyll a cannot efficiently absorb green light and instead reflects those wavelengths back, our eyes and brain perceive the reflected light waves as green, and so the pigment (and the leaves in which it is found) appears green.

Another pigment, chlorophyll b, is similar in structure but absorbs blue and red-orange wavelengths.

Chlorophyll b reflects back yellow-green wavelengths.

A related group of pigments called carotenoids absorbs blue-violet and blue-green wavelengths and reflects yellow, orange, and red wavelengths.

Figure 4-15 (part 1) Plant pigments.

Electron Excitation

Photons of specific wavelengths bump electrons up a quantum level into an excited state.

The excited electron can:

Return to its resting, unexcited state (some of that energy may excite other electrons in a nearby molecule)

Be passed to other molecules

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When chlorophyll is hit by photons of certain wavelengths, the light energy bumps an electron in the chlorophyll molecule to a higher energy level, an excited state. Upon absorbing the photon, the electron briefly gains energy, and the potential energy in the chlorophyll molecule increases. An electron in a photosynthetic pigment that is excited to a higher energy state generally has one of two fates (refer to the next two slides also): (1) The electron returns to its resting, unexcited state. In the process, energy is released, some of which may be transferred to a nearby molecule, bumping electrons on that molecule to a higher energy state (and the rest of the energy is dissipated as heat). Or (2) The excited electron itself is passed to another molecule.

Electron gain and loss

The passing of electrons in their excited state is the chief way energy moves through cells

Molecules that gain electrons always carry greater energy than before receiving them.

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

FOLLOW the electrons!

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The passing of electrons from molecule to molecule is one of the chief ways that energy moves through cells. Many molecules carry or accept electrons during cellular activities. All that is required is that the acceptor have a greater attraction for electrons than the molecule from which it accepts them. Photosynthesis is a complex process, but our understanding of this process can be greatly aided by remembering one phrase: FOLLOW THE ELECTRONS. In the passages that follow, if you feel that you are losing focus or getting lost, just remember to think about the electrons: Where are they coming from? What are they passing through? Where are they going? And what will happen to them when they get there?

Photosynthesis part 1: Photo

Light energy is transformed into chemical energy while splitting water molecules and producing oxygen.

Sunlight’s energy is first captured when an electron in chlorophyll is excited.

As this electron is passed from one molecule to another, energy is released at each transfer, some of which is used to build the energy-storage molecules ATP and NADPH.

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Pigments in the chloroplasts – chlorophyll a and b capture energy from the sunlight. This energy is transferred to energy storage molecules ATP and NADPH. This energy is then transferred to carbohydrates. Energy stored in the carbohydrates is extracted and used by heterotrophs (animals).

Photo reactions (need sunlight)

Occurs in the thylakoids

Require 2 photosystems (arrangements of pigments)

Electrons get excited & then return to rest

Electrons from chlorophyll a are passed to another pigment (primary electron acceptor) => 1st photosystem

To restore the electrons, water is split

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As these pigments absorb photons from the sunlight that hits the leaves, electrons in the pigments become excited and then return to their resting state.

As the electrons return to their resting state, energy (but not the electrons) is transferred to neighboring pigment molecules.

This process continues until the transferred energy from many pigment molecules excites the electrons in a chlorophyll a molecule at the center of the photosystem.

This is where the electron journey begins.

The special chlorophyll a continually loses its excited electrons to a nearby molecule, called the primary electron acceptor, which acts like an electron vacuum.

Why must plants get water for photosynthesis to occur?

As electrons keep getting taken away from the special chlorophyll a molecule, the electrons must be replaced. The replacement electrons come from water.

First photosystem produces ATP

Electrons are passed, & as some energy is released, a gradient of H+ (protons) is built

Concentration gradient help making of ATP

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Think of a pump pushing water into an elevated tank, creating a store of potential energy that can run out of the tank with great force and kinetic energy, which can be harnessed to do work, such as moving a large paddle wheel.

Similarly, the protons eventually rush out of the thylakoid sacs with great force—and that force is harnessed to build energy-storing ATP molecules, one of the two products of the “photo” portion of photosynthesis.

The Second Photosystem produces NADPH

NADPH (high energy electron carrier) is made

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The water-splitting photosystem is linked to the NADPH producing photosystem by the electron transport chain. In the second photosystem, electrons are excited by the sunlight, but are restored not from water splitting, but from the electron transport chain coming from the first photosystem. NADPH is a high-energy electron carrier.

NADPH is the second important product of the “photo” portion of photosynthesis.

Products from the “Photo” Portion

ATP and NADPH

Time for the “synthesis” part!

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In the next section, we cover the “synthesis” part of photosynthesis and see how plants use the energy in ATP and NADPH to produce sugar from carbon dioxide.

The Calvin Cycle (synthesis)

Series of chemical reactions

Occurs in stroma

Enzymes are recycled.

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The “synthesis” part of photosynthesis takes place in a series of chemical reactions called the Calvin cycle.

All the Calvin cycle reactions occur in the stroma of the leaves’ chloroplasts, outside the thylakoids. Plants carry out these reactions using the energy stored in the ATP and NADPH molecules that are built in the “photo” portion of photosynthesis. This dependency links the light-gathering (“photo”) reactions with the sugar-building (“synthesis”) reactions.

3 Steps: of the the Calvin Cycle

Fixation (of CO2)

Sugar creation

Regeneration

C3 plants

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Enzymes and coenzymes are recycled: rubisco (the most abundant protein on earth!), ADP, and NADP+.

Just as a magician seems to make a rabbit appear from thin air, the Calvin cycle takes invisible molecules of CO2 from the air and uses them to assemble visible—even edible—molecules of sugar.

Summary Calvin cycle

Carbon from CO2 in the atmosphere is attached (fixed) to molecules in chloroplasts, sugars are built, and molecules are regenerated to be used again in the Calvin cycle.

These processes consume energy from ATP and NADPH (the products of the “photo” part of photosynthesis.

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Evolutionary Adaptations

Plants that thrive in hot, dry conditions

Adaptations that reduce evaporative water loss

How do plants use water?

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But evolutionary adaptations in some plants enable them to thrive in hot, dry conditions.

Recent technological advances in agriculture use these innovative evolutionary solutions to battle the problem of world hunger.

Evolutionary adaptations: One method to combat water loss through evaporation is for plants to close their stomata, small pores usually on the underside of leaves.

Stomata: Pores for gas exchange

How to get CO2 when stomata are shut?

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These openings are the primary sites for gas exchange in plants: carbon dioxide for photosynthesis enters through these openings and oxygen generated as a by-product in photosynthesis exits through them.

When open, the stomata also allow water to evaporate from the plant.

Closing their stomata, however, solves one problem for plants (too much water evaporation) while it creates another: With the stomata shut, oxygen from the “photo” reactions of photosynthesis cannot be released from the chloroplasts and carbon dioxide cannot enter them.

If there are no carbon molecules for sugar production, the Calvin cycle tries to fix carbon but instead finds only oxygen. Plant growth comes to a standstill and crops fail.

C4 Photosynthesis

C4 plants produce ultimate “CO2-sticky tape” enzyme.

C4 photosynthesis adds an extra step.

Advantage: less water loss (corn, sugarcane)

Disadvantage: requires more energy

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Some plants, including corn and sugarcane, have evolved a process that minimizes water loss but still enables them to make sugar when the weather is hot and dry.

In the process called C4 photosynthesis, these plants add an extra set of steps to the usual process of photosynthesis .

C4 photosynthesis also adds additional energy expense. Outcompeted in mild climates by C3 plants.

CAM Photosynthesis

Stomata closed during hot, dry days

At night, stomata open, CO2 let in and temporarily binds to a holding molecule

During day, CO2 gradually released and used while stomata are closed

Disadvantage: more energy, slow growth

Cacti, pineapples

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Finally, a third and similar method of carbon fixation, called CAM (for crassulacean acid metabolism), is also found in hot, dry areas.

In this method, used by many cacti, pineapples, and other fleshy, juicy plants, the plants close their stomata during the hot, dry days.

At night, they open the stomata and let CO2 into the leaves, where it binds temporarily to a holding molecule.

In the day, when a carbon source is needed to make sugars in the Calvin cycle, the CO2 is gradually released from the holding molecule, enabling photosynthesis to proceed while keeping the stomata closed to reduce water loss

A disadvantage of CAM photosynthesis is that by completely closing their stomata during the day, CAM plants significantly reduce the total amount of CO2 they can take in. As a consequence, they have much slower growth rates and cannot compete well with non-CAM plants under any conditions other than extreme dryness.

All Three Photosynthetic Pathways

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It is believed that C4 and CAM photosynthesis originally evolved as successful adaptations to hot and dry regions.

Researchers are now using these adaptations to fight world hunger. Specifically, they have introduced from corn into rice several genes that code for the C4 photosynthesis enzymes. Once in the rice, these genes increase the rice plant’s ability to photosynthesize, thus leading to higher growth rates and food yields. The experiments are still in the early stages, and whether the addition of C4 photosynthesis enzymes will make it possible to grow new crops on a large scale in previously inhospitable environments is not certain. Early results suggest, however, that it is a promising approach.

Take-home message 4.11

C4 and CAM photosynthesis are evolutionary adaptations at the biochemical level that, although being more energetically expensive than C3 photosynthesis, allow plants to close their stomata and conserve water without shutting down photosynthesis.

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Cellular respiration

Process in which the high-energy bonds of sugar and other energy-rich molecules are broken, releasing the energy that went into creating them.

The cell captures the food molecules’ stored energy in the bonds of ATP molecules.

This process requires fuel molecules and oxygen and yields ATP molecules, water, and carbon dioxide.

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Cellular respiration is the process that breaks down high energy bonds in carbohydrates. This process generates ATP an energy rich molecule and releases water and carbon dioxide as metabolic wastes.

Cellular Respiration

Requires (1) fuel and (2) oxygen

Potential energy stored in chemical bonds of sugar, protein, and fat molecules

Breaks bonds to release the high-energy electrons captured in ATP

Oxygen is electron magnet

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This process is a bit like photosynthesis in reverse. In photosynthesis, the energy of the sun is captured and used to build molecules of sugars, such as glucose. In cell respiration, plants and animals break down the chemical bonds of sugar and other energy-rich food molecules (such as fats and proteins) to release the energy that went into creating them.

Figure 4-28 Cellular respiration: the big picture.

Three processes

Glycolysis

Krebs cycle

Electron transport chain

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The three processes of cellular respiration include:

Glycolysis: glyco = sugar; lysis = to break. Process where sugars (6C unit) are broken down to Pyruvate (3C unit).

Kreb cycle : generates electron carriers.

Energy in electron carriers is extracted and ATP is generated.

The first step of cellular respiration: Glycolysis.

The universal energy-releasing pathway

In the cytoplasm

Does not require oxygen

Glucose to pyruvate

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Used by all living organisms.

Glycolysis means the splitting (lysis) of sugar (glyco) and it is the first step all organisms on the planet take in breaking down food molecules; for many single-celled organisms this one step is sufficient to provide all of the energy they need. Glycolysis is ALWAYS the initial step in the process by which organisms break down sugar to generate fuel for their activities. Two distinct phases: an “uphill” preparatory phase and a “downhill” payoff phase.

As Figure 4-30 (Glycolysis up close) illustrates, glycolysis is a sequence of chemical reactions (there are ten in all) through which glucose is broken down to yield two molecules of a substance called pyruvate. Glycolysis has two distinct phases: an “uphill” preparatory phase and a “downhill” payoff phase.

Products of Glycolysis

Three of the ten steps yield energy.

Quickly harnessed to make ATP

High-energy electrons are transferred to NADH.

Net result:

Each glucose molecule broken down into two molecules of pyruvate

ATP molecules produced

NADH molecules store high-energy electrons

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In the absence of oxygen, only glycolysis occurs. This is sufficient to fuel the energy needs of single-celled organisms.

The Preparatory Phase to the Krebs Cycle

NADH I is produced

CO2 is released

Acetyl-Coenzyme A is made

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Acetyl-coAThe two pyruvate molecules move into the mitochondria and then undergo three quick modifications that prepare them to be broken down in the Krebs cycle:

Modification 1: Each pyruvate molecule passes some of its high-energy electrons to the energy-accepting molecule NAD+, building two molecules of NADH.

Modification 2: Next, a carbon atom and two oxygen atoms are removed from each pyruvate molecule and released as carbon dioxide. The CO2 molecules diffuse out of the cell and, eventually, out of the organism. In humans, for example, these CO2 molecules pass into the bloodstream and are transported to the lungs, from which they are eventually exhaled.

Modification 3: In the final step in the preparation for the Krebs cycle, a giant compound known as coenzyme A attaches itself to the remains of each pyruvate molecule, producing two molecules called acetyl-CoA. Each acetyl-CoA molecule is now ready to enter the Krebs cycle.

Figure 4-31 Preparations of pyruvate.

Krebs cycle

ATP

NADH

FADH2

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There are eight separate steps in the Krebs cycle. Three general outcomes are depicted here.

Outcome 1: A new molecule is formed. Acetyl-CoA adds its two-carbon acetyl group to a molecule of the starting material of the Krebs cycle, a four-carbon molecule called oxaloacetate. This process creates a six-carbon molecule.

Outcome 2: High-energy electron carriers (NADH) are made and carbon dioxide is exhaled. The six-carbon molecule then gives electrons to NAD+ to make the high-energy electron carrier NADH. The six-carbon molecule releases two carbon atoms along with four oxygen atoms to form two carbon dioxide molecules. This CO2 is carried by the bloodstream to the lungs from which it is exhaled into the atmosphere.

Outcome 3: The starting material of the Krebs cycle is re-formed, ATP is generated, and more high-energy electron carriers are formed. After the CO2 is released, the four-carbon molecule that remains from the original pyruvate-oxaloacetate molecule formed in Outcome 1 is modified and rearranged to once again form oxaloacetate, the starting material of the Krebs cycle. In the process of this reorganization, one ATP molecule is generated and more electrons are passed to one familiar high-energy electron carrier, NADH, and a new one, FADH2. The formation of these high-energy electron carriers increases the energy yield of the Krebs cycle. One oxaloacetate is reformed, the cycle is ready to break down the second molecule of acetyl-CoA. Two turns of the cycle are necessary to completely dismantle our original molecule of glucose.

Figure 4-32 Overview of the Krebs cycle.

Intermediate step and Krebs cycle

A huge amount of additional energy can be harvested by cells after glycolysis.

First, the end-product of glycolysis, pyruvate, is chemically modified.

Then in the Krebs cycle, the modified pyruvate is broken down step-by-step.

ATP is made

NADH and FADH2= high energy electron carriers

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We started this process of cellular respiration with a molecule of glucose, and now that we have seen the Krebs cycle in its entirety, let’s trace the path of the original six carbons in that glucose molecule.

Glycolysis: the six-carbon starting point. Glucose is broken down into two molecules of pyruvate. No carbons are removed.

Preparation for the Krebs cycle: Two carbons are released. Two pyruvate molecules are modified to enter the Krebs cycle and they each lose a carbon atom in the form of two molecules of carbon dioxide.

Krebs cycle: The last four carbons are released. A total of four carbon molecules enter the Krebs cycle in the form of Acetyl Co-A. For each turn of the Krebs cycle two molecules of carbon dioxide are released. So the two final carbons are released into the atmosphere during the second turn of the wheel. Poof! The six carbon atoms that were originally present in our single molecule of glucose are no longer present.

Mitochondria

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”

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Primary site of power generation

Feature 1: Mitochondria have a “bag-within-a-bag” structure, which makes it possible to harness the potential energy in the bonds of NADH and FADH2 molecules to produce ATP.

Feature 2: The inner “bag” of the mitochondria is studded with molecules. These molecules, mostly electron carriers, are sequentially arranged within the inner membrane of the mitochondrion, hence their description as a “chain.” This arrangement makes it possible for the molecules to hand off electrons in an orderly sequence. Material inside the mitochondrion can lie in one of two spaces: (1) the intermembrane space, which is outside of the inner bag, or (2) the mitochondrial matrix, which is inside the inner bag.

With two distinct regions separated by a membrane, the mitochondrion can create higher concentrations of molecules in one area or the other. And because a concentration gradient is a form of potential energy—molecules move from the high concentration area to the low concentration area the way water rushes down a hill—once a gradient is created, the energy released as the gradient equalizes itself can be used to do work. In the electron transport chain, this energy is used to build the energy-rich molecule ATP.

Follow the Electrons, as we did in Photosynthesis

Electrons come in the electron carriers NADH and FADH2

Electrons are passed in the chain

Energy released is used to from a H+ gradient

The Gradient provides energy to make ATP

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Start with the electrons carried by NADH and FADH2.

Over-the-counter NADH pills provide energy to sufferers of chronic fatigue syndrome. Why might this be?

Figure 4-34 The big energy payoff

Electron transport chain (ETC)

The largest energy payoff of cellular respiration comes as electrons from NADH and FADH2 produced during glycolysis and the Krebs cycle move along the electron transport chain.

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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Most of the energy in cellular respiration is generated in ETC. Electron carriers NADPH and FADH2 are oxidized the proton released run the ATP synthase pump to convert ADP to ATP an energy molecule.

Summary electron transport chain

The electrons are passed from one carrier to another and energy is released, pumping protons into the intermembrane space.

As the protons rush back to the inner mitochondrial matrix, the force of their flow fuels the production of large amounts of ATP.

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In anaerobic conditions

Oxygen deficiency limits the breakdown of fuel because the electron transport chain requires O2 as the final acceptor of the electrons generated during glycolysis and the Krebs cycle.

When O2 is unavailable, yeast resort to fermentation, in which they use a different electron acceptor, pyruvate, generating ethanol in the process, the alcohol in beer, wine, and spirits.

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Most cellular processes occur in the presence of oxygen. However, sometimes organisms are subjected to lack of oxygen a situation known as anaerobic condition. Some organisms like yeast can generate small amounts of energy by a process called fermentation.

Fermentation

Way to obtain energy from fuels without oxygen

After glycolysis, pyruvate is converted to another molecule (ethanol, lactic acid etc)

Other molecule (ex. NAD) is the electron acceptor

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With rapid, strenuous exertion, our bodies soon fall behind in delivering oxygen from the lungs to the bloodstream to the cells and finally to the mitochondria.

Oxygen deficiency then limits the rate at which the mitochondria can break down fuel and produce ATP.

This slowdown in ATP production occurs because the electron transport chain requires oxygen as the final acceptor of all the electrons that are generated during glycolysis and the Krebs cycle.

If oxygen is in short supply, the electrons from NADH (and FADH2) have nowhere to go.

Consequently, the regeneration of NAD+ (and FAD+) in the electron transport chain is halted, leaving no recipient for the high-energy electrons harvested from the breakdown of glucose and pyruvate, and the whole process of cellular respiration can grind to a stop.

Organisms don’t let this interruption last long, though; most have a back-up method for breaking down sugar.

Among animals, there is one willing acceptor for the NADH electrons in the absence of oxygen: pyruvate, the end product of glycolysis.

When pyruvate accepts the electrons, it forms lactic acid.

Cells can run on protein and fat as well as on glucose.

Humans and other organisms have metabolic machinery that allows them to extract energy and other valuable chemicals from proteins, fats, and carbohydrates in addition to glucose.

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4-17. Eating a complete diet: Cells can run on protein and fat as well as on glucose.

Evolution has built humans and other organisms with the metabolic machinery that allows them to extract energy and other valuable chemicals from proteins, fats, and a variety of carbohydrates.

For that reason, we are able to consume and efficiently utilize meals comprising various combinations of molecules:

Sugars: In the case of dietary sugars, many are polysaccharides—multiple simple sugars linked together—rather than solely the simple sugar glucose. Before they can be broken down by cellular respiration, the polysaccharides must first be separated by enzymes into glucose or related simple sugars that can be broken down by cellular respiration.

Lipids: Dietary lipids are broken down into their two constituent parts: a glycerol molecule and a fatty acid. The glycerol is chemically modified into one of the molecules produced during one of the ten steps of glycolysis. It then enters glycolysis at that step in the process and is broken down to yield energy. The fatty acids, meanwhile, are chemically modified into acetyl-CoA, at which point they enter the Krebs cycle.

Proteins: Proteins are chains of amino acids. Upon consumption they are broken down chemically into their constituent amino acids. Once that is done, each amino acid is broken down into (1) an amino group that may be used in the production of tissue or excreted in the urine and (2) a carbon compound that is converted into one of the intermediate compounds in glycolysis or the Krebs cycle, allowing the energy stored in its chemical bonds to be harnessed.