Vaccari D.A., Strom P.F., Alleman J.E. Environmental Biology for Engineers and Scientists

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where T is the temperature in degrees Celsius, r

the reaction rate at 2 0



C, and y an

empirical coefﬁcient. This expression is commonly used to describe the effect of tempera-

ture on biological growth rates in biological waste treatment processes. In the example of

Figure 5.4, the denominator of equation (5.47) is very close to 1.0 up to a temperature of



C. Thus, the numerator carries most of the effect of temperature and the exponential

form of (5.48) holds.

5.3.5 Other Considerations

Keep in mind that the deactivation due to temperature described above is reversible. Irre-

versible denaturation can be modeled in a number of ways, depending on the mechanism.

In the simplest case, ﬁrst-order decay may sufﬁce. Irreversible denaturation by heat may

be one of the principal mechanisms of heat sterilization.

It is also important to know that many enzymes function when associa ted with mem-

branes or other biological structures and may not function at all when extracted into solu-

tion. Some, such as pancreatic lipase, function only when absorbed at the interface

between a lipid droplet and an aqueous solution.

5.4 BIOCHEMICAL PATHWAYS

With the preceding discussion of the types of compounds that form living things (Chapter 4)

and of the enzymes that facilitate and control their chemical transformations and of ther-

modynamic relationships governing individual reactions, we can now examine some of

the most important of these transformations. We are usually interested not in individual

reactions but in a sequence of reactions, called a pathway, leading from initial reactants to

ﬁnal products. For example, the oxidation of the carbon in glucose to carbon dioxide

involves some 21 reactions, usually divided into two pathways: glycolysis, which forms

an intermediate called pyruvate, and the Krebs cycle, which produces the CO

The detailed reactions of each pathway are not given here. Instead, we examine the

overall reactions for several major pathways, with a few details about what occurs within

the pathway. The rate of an overall reaction pathway is often governed by a single limiting

step in the sequence of reactions. In the case of biodegradation of xenobiotic compounds,

it is often the ﬁrst step that limits the overall rate. Biochemical pathways can be classiﬁed

in several ways. One of the simpl er ways is to divide them into the following: Catabolic

pathways are those that break down organic compounds, usually to provide energy;

anabolic pathways are those that synthesize complex organics, such as to form new

cell material, from simpler precursors. However, many metabolic pathways do not ﬁt

easily into these two categories.

5.4.1 Glycolysis

First we consider ways of extracting energy from organics without external oxidizers such

as oxygen. Essentially, this means producing ATP from ADP (phosphorylation) for

short-term use (on the order of seconds or minutes). Phosphor ylation of ADP by reaction

with organic molecules, and without an electron acceptor such as oxygen, is called sub-

strate-level phosphorylation. For longer-term energy supply, cells store glucose (as

starch in plants or as glycogen in animals) or lipids. Thus, our starting point is glycolysis,

98 ENERGY AN D METABOLISM

which provides a ready source of ATP by partial oxidation of glucose. It incorporates sub-

strate-level phosphorylation. Its products feed into respiration pathways, for more energy

extraction, or to fermentation pathways for waste elimination.

Glycolysis is the conversion of glucose to pyruvic acid, an important metabolic inter-

mediate. It is the ﬁrst of several pathways for the production of ATP from glucose

(Figure 5.5). Glycolysis consists of nine reactions involving nine intermediate com-

pounds. The six-carbon glucose ﬁrst enters into several reactions involving ATP, forming

a fructose with two phosphate groups. Since this consumes ATP, it represents an invest-

ment by the cell to get the reaction going, forming an activated intermediate compound.

This splits into two three-carbon sugars, each with one phosphate, which is oxidized by an

NAD to form NADH

. Both three-carbon sugars then go through a series of steps that

generate two ATPs each, for a total of four. Thus, the net reaction of glycolysis is

glucose þ 2ADP þ 2P

þ 2NAD ) 2 pyruvic acid þ 2ATP þ 2NADH

ð5:49Þ

Since each ATP has a standard Gibbs free energy of 7.3 kcal/mol and the complete

oxidation of glucose has 686 kcal/mol, this reaction has an efﬁciency of only 2.1%. Addi-

tional energy is contained in the pyruvic acid and the NADH

, but these are unavailable

without an oxidizing agent. This is why anaerobic bacteria yield less cell mass than aero-

bes do, since they do not harvest as much energy for synthesis. In the aerobic process of

respiration, described below, both the pyruvate and the NADH

can be made to yield more

ATP.

Pyruvate is a link to numerous other biochemical pathways, besides feeding into the

respiration pathway. It can be made into alanine, leading to synthesis of amino acids.

Pyruvate is also the starting point for many fermentation products described below.

Some of the other glycolysis intermediates are also used to synthesize other sugars or

amino acids. There also are other less important pathways for glucose catabolism. The

cell shifts to them to control the level of ATP in the cell or to form other compounds.

5.4.2 Fermentation

Besides ATP and pyruvate, glycolysis produces the reducing agent NADH

. Unless this

were reoxidized to NAD, the cell’s supply of NAD would be depleted. Also, pyruvate is

retained by the cell. A buildup of pyruvate would decrease the rate of the glycolysis by the

law of mass action. Many cells use the pyruvate and NADH

in respiration with oxygen

(described in the next section) to produce more ATP. In the absence of oxygen, cells use a

different pathway, in which the NADH

is used to reduce the pyruvate to various products

that can leak out of the cell as waste products. This process, which forms such partially

oxidized by-products to regenerate NAD, is called fermentation. More generally, fermen-

tation is an anaerobic biochemical process in which organic compounds serve as both

electron donors and acceptors. Thus, some organics become reduced (e.g., NAD) and

others are oxidized (e.g., ethanol to acetic acid), and no inorganic electron acceptor

(such as oxygen) is needed.

In animals, the rapid generation of ATP needed for muscle power can only be gener-

ated by glycolysis. Animal cells eliminate p yruvate and regenerate NAD by a single-step

reaction that forms lactic acid. However, lactic acid is a ‘‘blind alley’’ in animals and can-

not be used further. But unlike pyruvate, it can diffuse out of the cell, where the blood

transports it to the liver. There it is converted back to pyruvate to enter other pathways.

BIOCHEMICAL PATHWAYS 99

The accumulation of lactic acid in the muscles causes the pain that results from vigorous

exercise. Rest allows time for the elimination and conversion of the lactic acid.

Bacteria and yeast can produce other products. The most important is ethanol, which is

formed by yeast in two steps, with acetaldehyde as an intermediate. Speciﬁc organism and

cultivation conditions yield speciﬁc end products. Yeast can produce glycerol in addition

Glucose (C-6)

Pyruvate x 2 (C-3)

2 ATP

4 ATP

2 NADH

Glycolysis

Lactic Acid (C-3)

NADH

Fermentation

Acetaldehyde (C-2)

Each pyruvate

Acetyl-CoA

Oxaloacetic

Acid (C-4)

Citric

Acid (C-6)

CoA-SH

Transition to Krebs Cycle

NADH

KREBS CYCLE

Isocitric acid

NADH

α-Ketoglutaric

acid (C-5)

Succinyl-CoA

(C-4)

GTP

ATP

NADH

Malic Acid

Fumaric acid

Succinic acid

FADH

NADH

Each

pyruvate

Glycogen

or starch

Fats

Amino

Acids

All NADH

and FADH

produced go to the CYTOCHROME SYSTEM

(Nine

steps)

Figure 5.5 Glycolysis, fermentation, and the Krebs cycle. Inputs and outputs from each cycle are

shown, as well as connections with the metabolism of other biochemical compounds.

100

ENERGY AND METABOLISM

to ethanol, although it is derived from intermediates of glycolysis and not from pyruvate.

Clostridium produces acetone, isopropanol, butyrate, and butanol. Proprionic acid bac-

teria produce proprionate. Coliforms produce formic acid, acetic acid, hydrogen, and

. Enterobacter produces ethanol, 2,3-butanediol, formic acid, and lactic acid.

Fermentation produces other compounds of commercial importance, some of which

may originate with pyruvate, such as penicillin. However, although they are produced dur-

ing fermentation, they are not end products used by the cell. Thus, thes e are properly

referred to as secondary metabolites, not fermentation products.

5.4.3 Respiration

Respiration is a process in which organic compounds or reduced inorganics (such as

hydrogen or ferrous iron) are oxidized by inorganic electron acceptors for the produc tion

of energy. Eukaryotes can only use oxygen as a ﬁnal electron acceptor, in what is called

aerobic respiration. Microorganisms can also use nitrate, sulfate, some metals, and even

carbon dioxide, in what is generally called anaerobic respiration. In environmental

applications the term anoxic respiration is generally used for nitrate reduction, and anae-

robic respiration is limited to the other forms.

Strictly speaking, glycolysis is not a part of respiration, although it is the ﬁrst step lead-

ing to respiration for glucose. Nevertheless, when discussing respiration, many scientists

and engineers are referring to the overall conversion of glucose or other carbohydrates to

carbon dioxide and water:

þ 6O

) 6CO

þ 6H

O ð5:50Þ

Respiration occurs in two phases. The ﬁrst is the Krebs cycle [also called the citric

acid cycle or tricarboxylic acid (TCA) cycle]. The Krebs cycle completes the job of oxi-

dizing the carbon that originated with glucose, forming CO

and ATP. However, much of

the energy is left in the reducing power of NADH

or FADH

. These are converted to ATP

in the second phase, called the electron transport system or cytochrome system. The

cytochrome system is also responsible for reducing oxygen to water, the other product

of the overall reaction for the oxidation of glucose. Both the Krebs cycle and the cyto-

chrome system are cyclic because intermediates involved in each reaction are regenerated

by other reactions.

Both of these process are mediated by enzymes bound to membranes and require the

presence of the membranes in order to function. In eukaryotes this occurs within the mito-

chondria. Pyruvate must diffuse into the mitochondria to enter the process. Prokaryotes do

not have internal membrane structures. Their respiratory enzymes are bound to their cell

membrane. The discussion here focuses on eukaryotes, but the process is similar in pro-

karyotes. Mitochondria consist of an inner and an outer membrane, forming an inner and

an outer compartment. The inner membrane is folded extensively (Figure 5.6). The Krebs

cycle occurs within the inner compartment. The cytochrome system is integral to the inner

membrane and involves reactants in both compartments.

Glycolysis forms two three-carbon pyruvate molecules for each glucose. When oxygen

or another suitable electron acceptor is available, the pyruvate enters into respiration

instead of fermentation. Respiration begins when the pyruvate diffuses into the inner com-

partment of the mitochondria. There, each pyruvate becomes covalently bonded through a

sulfhydryl bond with a coenzyme, called coenzyme A CoA, which itself is a derivative of

BIOCHEMICAL PATHWAYS 101

ADP. The process is an oxidation and requires an NAD. It resu lts in a compound called

acetyl-CoA:

COOH

O + CoA

SH + NAD

S-CoA

O + CO

+ NADH

pyruvate

acetyl-CoA

ð5:51Þ

The removal of one CO

for each pyruvate accounts for two of the six carbons originating

in the glucose. The NADH

carries muc h of the energy of oxidation and will be used to

generate ATP in the electron transport system.

Acetyl-CoA is another important metabo lic intermediate. It is a point of connection

between a number of pathways, including the breakdown of lipids and amino acids for

energy. But for purposes of the present discussion, a pair of acetyl-CoAs serve the purpose

of carrying four carbons from the original glucose molecule into the Krebs cycle.

Krebs Cycle The Krebs cycle oxidizes the two carbons remaining from each pyruvate to

. One ATP is formed as a result, and the remaining four pairs of hydrogens (with their

energetic electrons) reduce the cofactors NAD or FAD. The overall reaction is

acetyl-CoA þ 3NAD þ FAD þ ADP

) 2CO

þ CoA þ 3NADH

þ FADH

þ ATP

ð5:52Þ

Since two such reactions occur for each glucose, all of the original carbon atoms are

accounted for. Only two additional ATPs are formed, for a total of four when combined

with glycolysis. Thus, the efﬁciency so far, in terms of standard Gibbs free energy, is four

times 7.3 kcal/mol for the ATPs, divided by 686 kcal/mol glucose, or 4.3%. However,

Figure 5.6 Mitochondrion structure, showing membrane structure and division into inner and

outer compartments. (From Smith et al., 1983.)

102

ENERGY AND METABOLISM

there are a total of 10 pairs of energetic hydrogen electrons that have been captured by

cofactors.

Some details of the cyclical reactions of the Krebs cycle may help clarify the picture.

The cycle starts when the C

oxaloacetate combines with two carbons of the acetyl-CoA

that originated with pyruvate. The products of this ﬁrst reaction regenerate the CoA and

form the C

tricarboxylic acid citric acid:

COOH

HO C COOH

COOH

acetyl-CoA

oxaloacetate citric acid

CoA

ð5:53Þ

A series of eight reactions complete the cycle, in which the two carbons added by the

ﬁrst step are effectively removed, along with the four pairs of hydrogen. (Some of the

hydrogen originates in H

O, which is incorporated at several steps.) Some of the inter-

mediates may be used to synthesize other compounds needed by the cell. The reducing

power of the NADH

may be used in synthesis reactions of other pathways. Both of these

reduce the energy yield of the Krebs cycle; however, this is not necessarily inefﬁcient

since useful products are made. However, the cell must maintain a supply of oxaloacetate

so that the cycle can continue. If intermediates are drawn off, oxaloacetate can be formed

from pyruvate and CO

The Krebs cycle is normally controlled by feedback inhibition by the products of the

cycle. High levels of the ratios NADH

/NAD, ATP/ADP, or acetyl-CoA/CoA indicate that

the cell has ample energy, and slow the cycle.

The Cytochrome System The six carbons from the glucose have been disposed of, but

the substantial reducing power in the form of NADH

and FADH

has yet to be converted

to energy in a useful form: namely, as ATP. In eukaryotes this conversion occurs with a

series of enzymes and other compounds bound to the inner mitochondrial membrane,

which comprise the cytochrome system (Figure 5.7). NADH

reduces the ﬁrst of these

compounds, ﬂavin mononucleotide (FMN), which spans the membrane from one side to

the other. FMN takes the two hydrogens, passes their electrons to the next membrane

compound (at a lower energy), and releases the protons to the outside of the membrane.

It may be useful to think of the electrons as dropping to a lower voltage, to use an elec-

tronic analogy, each time they pass from one electron carrier to the next. Thus, the net

effect is to use some of the energy from the electrons to move a pair of protons to the

outer compartment of the mitochondrion.

The compound that receives the electrons does a similar thing; it passes them on at a

yet lower energy, which may or may not result in moving protons to the outer compart-

ment. In all, six protons are transported. Consider the result: Energy is used to transport

protons, creating a pH gradient. This pumping of protons across a membrane into an area

of increasing proton concentration stores energy, just as to pumping air into a tank pro-

duces a pressure difference that also stores energy. But there isn’t only a pH gradient.

Since the protons are transported without their electrons or complementary anions, an

electric charge gradient, an actual voltage, is also accumulated. The combined chemical

and electrical potential across the inner membrane of the mitochondrion is called the

chemiosmotic potential.

BIOCHEMICAL PATHWAYS 103

The energy of the chemiosmotic potential is used when the protons ﬂow back across

the membrane to the inne r compartment through a complex of transmembrane proteins

called ATP synthase, producing ATP from ADP. Think of air that has been pumped into

a pressure vessel then ﬂowing back out through a turbine to produce electricity.

Altogether, 3 mol of ATP is produced for each mole of NADH

.TheFADH

acts similarly,

except that its electrons have a lower energy to start with, so it enters the chain farther

along. It provides only enough energy to produce 2 mol of ATP per mole of FADH

Finally, what becomes of the energy-depleted electrons in the transport chain? The

ﬁnal components of the transport chain are a series of membrane-bound compounds called

cytochromes. The ﬁnal cytochrome performs one last reduction, that of oxygen. Each

mol of O

receives 2 mol of electrons and 2 mol of H

, forming H

O and completing

the process of respiration. Because of the use of an electron acceptor, ATP production in

this system is called oxidative phosphorylation.

Assuming that none of the intermediate compounds or NADH

have been shunted off

for other cellular purpos es, the ﬁnal tally for ATP is a maximum of 36 mol per mole of

glucose oxidized. Now the efﬁciency based on standard Gibbs free energy is 38%, a big

improvement over glycolysis or the Krebs cycle. Since organisms will use every oppor-

tunity for growth, a more typical number of ATPs actually produced by glycolysis and

respiration is 20 to 25.

In prokaryotes, the electron transport chain is located in the cell membrane. Ba cteria

perform the function of the electron transport system without mitochondria by pumping

protons outside the cell, depleting it within to create the chemiosmotic potential .

Alternative Electron Acceptors Some bacteria can switch to other electron acceptors,

such as nitrate, when oxygen is absent. The nitrate becomes reduced ultimately to

Outer mitochondrial membrane

Inner mitochondrial membrane

GlucosePyruvate

Glycolysis

Krebs

Cycle

Krebs

Cycle

FMN

CoQ

NADH

NAD

FADH

FAD

Cyt

ATP

synthase

ADP

ATP

Cytoplasm

Outer compartment

Inner compartment

Figure 5.7 Cytochrome electron transport system, with a relationship to glycolysis and the Krebs

cycle.

104

ENERGY AND METABOLISM

nitrogen by the process of denitriﬁcation. It is thought that this occurs in a series of steps

as follows:



) NO



) NO ) N

O ) N

However, the nitrate requires a higher-energy electron for its reduction. It gets it at an

earlier point in the electron transport system, so that only 2 mol of ATP is formed per

mole of NADH

. This is why organisms that can will always use oxygen when it is pre-

sent and switch to nitrate only in the absence of oxygen.

Sulfate and carbon dioxide can also function as electron acceptors for certain micro-

organisms. However, the organisms that do so cannot also use oxygen. When sulfate is

reduced, the product is hydrogen sulﬁde, a poisonous gas. Carbon dioxide is reduced to

methane by molecular hydrogen by a special group of organisms wi thin the archaea. Oxi-

dized metal ions such as ferric (III) iron can also be an electron acceptor. This occurs, for

instance, in the sediments of wetlands, where oxygen is limiting. The iron becomes

reduced to ferrous (II) iron. If an environment contains oxygen, nitrate, sulfate, and car-

bon dioxide, the oxygen will tend to be used up ﬁrst, producing H

O. Then the nitrate,

then sulfate, and then CO

will be used. In homogeneous environments, one electron

acceptor will not be utilized until the energetically more favorable one is depleted. How-

ever, in some situations microenvironments may form, allowing several of these reactions

to proceed simultaneously. For example, in biologi cal slime layers in aquatic systems,

microorganisms near the slime layer surface may have access to oxygen while organisms

below the surface may be depleted of oxygen and will use nitrate.

5.4.4 Oxidation of Fats and Amino Acids

Obviously, glucose is not the only fuel used by living things. Our foods contain other

sugars, such as lactose in milk and fructose in the disaccharide sucrose (table sugar).

The success of dieters hinges on the body’s ability to use fat as fuel; of course, this is

why the body stores fat in the ﬁrst place. Under starvation conditions, the body obtains

its energy for basic cell function by cannibalizing itself, by oxidizing its proteins.

The sugars are converted fairly easily into either glucose or another intermediate in the

glycolysis pathway. In the case of sucrose and glycogen, these polys accharides are split

into simple sugars by phosphorolysis (splitting by phosphate) instead of hydrolysis. This

results in glucose-6-phosphate, the intermediate in glycolysis that just follows the point

where an ATP is reacted with glucose to get things going. Thus, the extra ATP is not

needed, and glycolysis yields one more ATP than for glucose itself.

The human body maintains only enough stored glycogen to last about a day. Then it

must switch to fats as a fuel. Most fats are stored in the body as triglycerides. Because

they are not soluble in water, they must be transported in the blood as lipoproteins. After a

fatty meal the concentration in the blood may be sufﬁcient to give it a milky opalescence.

Lipase enzymes in the blood or in fatty tissues hydrolyze the lipoproteins to glycerol and

to fatty acids bound to blood proteins. The glycerol enters glycolysis after a few steps

involving ATP. The fatty acids enter the cell and react with CoA similar to the way pyr-

uvate does at the start of the Krebs cycle. The fatty acid-CoA compounds are then trans-

ported into the inner compartment of the mitochondria, where a series of reactions split

off the last two carbons of the fatty acid, with the CoA, forming acetyl-CoA and a shorter

fatty acid-CoA. The process, called beta oxidation, is repeated until the fatty aci d has

BIOCHEMICAL PATHWAYS 105

been consumed. The acetyl-CoAs produced then enter the Krebs cycle, where they are

further oxidized. In addition, for every acetyl-CoA formed, one NADH

and one

FADH

are formed, feeding the electron transport system production of ATP. Th is

accounts for the high-energy yield of fats in comparison to carbohydrates.

Proteins are ﬁrst hydrolyzed into their component amino acids, followed by deamina-

tion, removal of the amino group. Finally, each of the 20 amino acids is converted to

either pyruvate, acetyl-CoA, or one of the other intermediates in the Krebs cycle, for

further oxidation. Free amino acids are not stored in the body. Excess proteins in the

diet thus must be eliminated by the mechanism just described. Deamination releases

ammonia to the blood, whi ch can be toxic and must be rapidl y removed. This can be

accomplished by incorporation into new amino acids or by excretion either directly as

ammonia (ﬁsh), uric acid (bi rds and reptiles), or urea (mam mals).

5.4.5 Photosynthesi s

Virtually all the organic carbon in our environment, from the carbon in a person’s ﬁnger-

nails to the carbon in a plastic pen, was formed by plants from CO

in the air. The energy

for this conversion comes entirely from sunlight. Among the few known exceptions in

nature are ecosystems found on the ocean ﬂoor and hot springs that obtain their energy

from oxidation of reduced inorganic compounds issuing from deep below the ocean ﬂoor

in hot-water vents. Most human-made sources of energy, such as fossil fuels and weather-

driven electric plants (wind and hydroelectric), ultimately come from the sun. Only

nuclear, geothermal, and tidal electrical facilities, plus a portion of wind energy driven

by Earth’s rotation, do not derive their energy from the sun.

Use of the sun’s energy to synthesize carbohydrates is called photosynthesis. Only cer-

tain bacteria, algal protists, and green plants are capable of photosynthesis. Organisms

that can synthesize their own carbohydrates from inorganic precursors are called auto-

trophs. Those that use sunlight to provide the energy for this are called photoautotrophs.

Some bacteria can use inorganic energy sources, such as H

S, NH

, or reduced metal-

lic salts such as manganese or ferrous iron, to form carbohydrates from CO

. These are

called chemoautotrophs or lithoautotrophs. All other organisms, including all animals

and fungi, and most bacteria, depend ultimately on the autotrophs for organic carbon and

energy. Organisms such as animals, fungi, and many bacteria that must obtain their

organic carbon ultimately from autotrophs are called heterotrophs.

Photosynthesis is somewhat more complicated to describe than respiration. However,

once we have understood respiration, there are enough similarities to respiration in reverse

to describe it in those terms. Recall that in respiration, there is a separation between the

oxidation of organic carbon to CO

(glycolysis and the Krebs cycle), which produces

reducing power as NADH

, and the electron transport system, which consumes the redu-

cing power and reduces oxygen to water.

In photosynthesis, the CO

gets reduced, produc ing glucose. The reducing power

comes from photons of light. The overall net reaction of photosynthesis is

6CO

þ 6H

O ) C

þ 6O

ð5:54Þ

The two major parts of photosynthesis are the light reactions, which are analogous to

electron transport in respiration, and the dark reactions, which can be compared to the

reverse of the Krebs cycle and glyc olysis. Several basic experimental facts support the

106 ENERGY AND METABOLISM

division of photosynthesis into light and dark reactions: (1) Plants give off oxygen only in

the light; and (2) if a suspension of algae is illuminated for some time in the absence of

, then placed in the dark with CO

, the CO

is incorporated into carbohydrate for a

brief time. Additional work with tracer elements further supports the idea.

In the light reactions, energy from light is used to oxidize H

OtoO

and produce ATP

and/or NADPH

. As in respiration, electron transport is involved and even uses cyto-

chromes. In fact, one type of organism, the purple nonsulfur bacteria, use the same elec-

tron transport chain for both respiration and photo synthesis. In algae and plants there are

actually two electron transport systems just for photosynthesis, which act in concert. The

light reactions require a membrane structure, the chloroplast (except in cyanobacter),

which is similar to the mitochondrion. Furthermore, ATPs are formed as a result of the

generation of a chemiosmotic potential by the electron transport system. In the dark reac-

tions, the ATP and NADPH

are used in a cyclic reaction to form sugars from CO

.Some

of the reactions involved are the reverse of parts of glycolysis.

Chloroplasts are the cel lular organelles in plants and alga e where all of the light reac-

tions and some of the dark reactions are located. Bacteria perform the function of chlor-

oplasts using their cytoplasmic membrane, as they do for the respiratory electron transport

system. Like mitochondria, they contain an internal membrane structure that divides the

interior into stacks of ﬂattened hollow disks, thylakoids (Figure 5.8). The inner compart-

ment of the thylakoid is called the lumen, the outer compartment is called the stroma.

The membranes of the thylakoids are studded with three groups of proteins and other

compounds (Figure 5.9). Two of these groups are photosystem I and photosystem II,

which perform the principal task of capturing the light energy and transporting the elec-

trons. Each photosystem consists of pigments, proteins, and electron transport com-

pounds, such as cytochromes. The third group is actually a single complex, called the

CF1 particle. Like the ATP synthase particle in the mitochondrion, The CF1 particle

uses the energy stored in the chemiosmotic potential created by the electron transport sys-

tems (in this case of the photosystems) to generate ATP.

The photosystems consist of an association of membrane-bound particles, one of which

is called the antenna. The antenna is a complex of numerous chlorophyll molecules, most

bound to proteins. Chlorophyll is the green pigment of plants, which absorbs much of the

light energy for photosynthesis. The antenna then transfers the energy to a ‘‘special pair’’

Figure 5.8 Chloroplast structure. (From Fried, 1990. # The McGraw-Hill Companies, Inc. Used

with permission.)

BIOCHEMICAL PATHWAYS 107