Chapter 10

Question 1

y. Almost

all heterotrophs, including humans, are completely dependent, either directly or indirectly, on p

Answer 1

photoautotrophs for

food—and also for oxygen, a by-product of photosynthesis.

Question 2

In

a sense, then, fossil fuels represent

Answer 2

stores of the sun’s energy

from the distant past

Question 3

where does the ability to photosynthesize even come from? describe how photosynthetic enymes and other molecules are in a biological membrane and how this allows for psynth

Answer 3

The remarkable ability of an organism to harness light
energy and use it to drive the synthesis of organic compounds emerges from structural organization in the cell:
Photosynthetic enzymes and other molecules are grouped
together in a biological membrane, enabling the necessary
series of chemical reactions to be carried out efficiently.

Question 4

The process of photosynthesis most likely originated in a

Answer 4

group of bacteria that had infolded regions of the plasma

membrane containing clusters of such molecules. I

Question 5

n existing photosynthetic bacteria

Answer 5

a, infolded photosynthetic membranes function similarly to the internal membranes of the
chloroplast, a eukaryotic organelle.

Question 6

what is the purpose of veins in leaves

Answer 6

Leaves also use veins to export

sugar to roots and other nonphotosynthetic parts of the plant

Question 7

Let’s briefly compare photosynthesis with cellular respiration (not so brief lmao)

Answer 7

Both processes involve redox reactions. During cellular respiration, energy is released from sugar when electrons associated
with hydrogen are transported by carriers to oxygen, forming
water as a by-product (see Concept 9.1). The electrons lose
potential energy as they “fall” down the electron transport
chain toward electronegative oxygen, and the mitochondrion
harnesses that energy to synthesize ATP (see Figure 9.15).
Photosynthesis reverses the direction of electron flow. Water is
split, and its electrons are transferred along with hydrogen ions
(H+
) from the water to carbon dioxide, reducing it to sugar.

Question 8

Light absorbed by chlorophyll

drives a transfer of

Answer 8

the electrons and hydrogen ions from
water to an acceptor called NADP1 (nicotinamide adenine
dinucleotide phosphate), where they are temporarily stored.

Question 9

(The electron acceptor NADP+

is first cousin to

Answer 9

NAD+
, which
functions as an electron carrier in cellular respiration; the two
molecules differ only by the presence of an extra phosphate
group in the NADP+
molecule.)

Question 10

The light reactions use solar
energy to reduce NADP+
to

Answer 10

NADPH by adding a pair of electrons along with an H+

Question 11

photophosphorylation

Answer 11

The light reactions also generate ATP,
using chemiosmosis to power the addition of a phosphate
group to ADP,

Question 12

Thus, it is the Calvin cycle that makes sugar, but it can

do so only with

Answer 12

the help of the NADPH and ATP produced by

the light reactions.

Question 13

Nevertheless, the Calvin cycle in most plants

occurs during when and why

Answer 13

daylight, for only then can the light reactions

provide the NADPH and ATP that the Calvin cycle requires

Question 14

Chloroplasts are c what and what do their thylakoids do

Answer 14

Chloroplasts are chemical factories powered by the sun.
Their thylakoids transform light energy into the chemical
energy of ATP and NADPH, which will be used to synthesize glucose and other molecules that can be used as energy
sources.

Question 15

wavlength

Answer 15

The distance between the crests of electromagnetic waves
is called the wavelength. Wavelengths range from less
than a nanometer (for gamma rays) to more than a kilometer (for radio waves).

Question 16

emagnetic spectrum and what is visible light and why is it called that

Answer 16

The distance between the crests of electromagnetic waves
is called the wavelength. Wavelengths range from less
than a nanometer (for gamma rays) to more than a kilometer (for radio waves). This entire range of radiation is known
as the electromagnetic spectrum (Figure 10.7). The segment most important to life is the narrow band from about
380 nm to 750 nm in wavelength. This radiation is known
as visible light because it can be detected as various colors
by the human eye.

Question 17

different pigments do what and what happens if a pigment absorbs all wavelengths

Answer 17

. Different pigments absorb light of different wavelengths, and the wavelengths that are absorbed disappear.
If a pigment is illuminated with white light, the color we
see is the color most reflected or transmitted by the pigment.
(If a pigment absorbs all wavelengths, it appears black.)

Question 18

spectrophotometer

Answer 18

spectrophotometer. This machine directs
beams of light of different wavelengths through a solution of
the pigment and measures the fraction of the light transmitted at each wavelength

Question 19

absorption spectrum

Answer 19

A graph plotting a pigment’s light
absorption versus wavelength is called an absorption
spectrum (Figure 10.9)

Question 20

the three types of chloroplasts and describe them

Answer 20

chlorophyll a, the key light-capturing pigment
that participates directly in the light reactions; the accessory
pigment chlorophyll b; and a separate group of accessory
pigments called carotenoids. The spectrum of chlorophyll a
suggests that violet-blue and red light work best for photosynthesis, since they are absorbed, while green is the least
effective color. This is confirmed by an action spectrum for psynth (figure 10.10b)

Question 21

action spectrum what is it and how is it made

Answer 21

which profiles the relative
effectiveness of different wavelengths of radiation in driving
the process. An action spectrum is prepared by illuminating
chloroplasts with light of different colors and then plotting
wavelength against some measure of photosynthetic rate,
such as CO2 consumption or O2 release.

Question 22

brief overview of englemanns experiment where he made an action spectrum (page 241)

Answer 22

Before equipment
for measuring O2 levels had even been invented, Engelmann performed a clever experiment in which he used bacteria
to measure rates of photosynthesis in filamentous algae
(Figure 10.10c). His results are a striking match to the modern action spectrum shown in Figure 10.10b

Question 23

carotenoids

Answer 23

Other accessory pigments include carotenoids, hydrocarbons that are various shades of yellow and orange because
they absorb violet and blue-green light (

Question 24

Carotenoids may broaden the spectrum of colors that can do what and what is a more imp fxn of at least some o them

Answer 24

drive photosynthesis. However, a more important function
of at least some carotenoids seems to be photoprotection: These
compounds absorb and dissipate excessive light energy that
would otherwise damage chlorophyll or interact with oxygen, forming reactive oxidative molecules that are dangerous
to the cell

Question 25

Interestingly, carotenoids similar to the photoprotective ones in chloroplasts have a

Answer 25

photoprotective role in the human eye. (Carrots, known for aiding night vision, are rich in carotenoids.)

Question 26

what is a photosystem made of and define its two parts mentioned

Answer 26

A photosystem is composed of a reaction-center complex surrounded by several light-harvesting complexes (Figure 10.13). The reaction-center complex is an organized association of proteins holding a special pair of chlorophyll a molecules and a primary electron acceptor. Each light-harvesting complex consists of various pigment molecules (which may include chlorophyll a, chlorophyll b, and multiple carotenoids) bound to proteins. T

Question 27

The pair of chlorophyll a molecules in the reaction-center complex are special because t

Answer 27

heir molecular environment—their location and the other molecules with which they are associated—enables them to use the energy from light not only to boost one of their electrons to a higher energy level, but also to transfer it to a different molecule—the primary electron acceptor, which is a molecule capable of accepting electrons and becoming reduced.

Question 28

what do ps 1 and ps 2 each have and explain the significance of p680 and p700

Answer 28

) Each has a characteristic reaction-center complex—a particular kind of primary electron acceptor next to a special pair of chlorophyll a molecules associated with specific proteins. The reaction-center chlorophyll a of photosystem II is known as P680 because this pigment is best at absorbing light having a wavelength of 680 nm (in the red part of the spectrum). The chlorophyll a at the reaction-center complex of photosystem I is called P700 because it most effectively absorbs light of wavelength 700 nm (in the far-red part of the spectrum). These two pigments, P680 and P700, are nearly identical chlorophyll a molecules

Question 29

However, their association | with different proteins in the thylakoid membrane affects

Answer 29

the electron distribution in the two pigments and accounts for the slight differences in their light-absorbing properties. Now let’s see how the two types of photosystems work together in using light energy to generate ATP and NADPH, the two main products of the light reactions

Question 30

p680+ (the oxidized form of p680 ig? check page 245)

Answer 30

electron acceptor. (P680+ is the strongest biological oxidizing agent known; its electron “hole” must be filled. This greatly facilitates the transfer of electrons from the split water molecule.)

Question 31

Cyclic electron flow can also occur in

Answer 31

n photosynthetic species that possess both photosystems; this includes some prokaryotes, such as the cyanobacteria shown in Figure 10.2d, as well as the eukaryotic photosynthetic species that have been tested thus far

Question 32

Although the process is probably in part | an “evolutionary leftover,” research suggests

Answer 32

it plays at least one beneficial role for these organisms. Plants with mutations that render them unable to carry out cyclic electron flow are capable of growing well in low light, but do not grow well where light is intense. This is evidence for the idea that cyclic electron flow may be photoprotective.

Question 33

Whether ATP synthesis is driven by linear or cyclic electron flow, what stays the same

Answer 33

the actual mechanism

Question 34

the mito's intermembrane space is analogous to what

Answer 34

thylakoid space. If you imagine the cristae of mitochondria pinching off from the inner membrane, this may help you see how the thylakoid space and the intermembrane space are comparable spaces in the two organelles, while the mitochondrial matrix is analogous to the stroma of the chloroplast.

Question 35

The proton (H+ ) gradient, or pH gradient, across the thylakoid membrane is substantial. - explain it and explain the pHs and what an increase of three pH units mean, and what was supported by experiments that illuminated thylakoid spaces

Answer 35

When chloroplasts in an experimental setting are illuminated, the pH in the thylakoid space drops to about 5 (the H+ concentration increases), and the pH in the stroma increases to about 8 (the H+ concentration decreases). This gradient of three pH units corresponds to a thousandfold difference in H+ concentration. If the lights are then turned off, the pH gradient is abolished, but it can quickly be restored by turning the lights back on. Experiments such as this provided strong evidence in support of the chemiosmotic model

Question 36

photoresp - is it energetically goood? what happens and why is it diff than normal cell resp

Answer 36

The process is called photorespiration because it occurs in the light (photo) and consumes O2 while producing CO2 (respiration). However, unlike normal cellular respiration, photorespiration uses ATP rather than generating it. And unlike photosynthesis, photorespiration produces no sugar. In fact, photorespiration decreases photosynthetic output by siphoning organic material from the Calvin cycle and releasing CO2 that would otherwise be fixed. This CO2 can eventually be fixed if it is still in the leaf once the CO2 concentration builds up to a high enough level. In the meantime, though, the process is energetically costly, much like a hamster running on its wheel.

Question 37

what is photorespiration

Answer 37

As CO2 becomes scarce within the air spaces of the leaf and O2 builds up, rubisco adds O2 to the Calvin cycle instead of CO2. The product splits, and a two-carbon compound leaves the chloroplast. Peroxisomes and mitochondria within the plant cell rearrange and split this compound, releasing CO2

Question 38

In the | ancient atmosphere that prevailed when rubisco first evolved,

Answer 38

the ability of the enzyme’s active site to bind O2 would have made little difference.

Question 39

? According | to one hypothesis, photorespiration is evolutionary baggage—

Answer 39

a metabolic relic from a much earlier time when the atmosphere had less O2 and more CO2 than it does today. I

Question 40

The hypothesis suggests that (abt photoresp being evolutionary baggage) also what is there some evidence for

Answer 40

modern rubisco retains some of its chance affinity for O2, which is now so concentrated in the atmosphere that a certain amount of photorespiration is inevitable. There is also some evidence that photorespiration may provide protection against the damaging products of the light reactions, which build up when the Calvin cycle slows due to low CO2.

Question 41

s. In C4 plants, there are two distinct types | of photosynthetic cells:

Answer 41

: bundle-sheath cells and mesophyll | cells

Question 42

loosely arranged mesophyl cells in a c4 plant

Answer 42

Between the bundle sheath and the leaf surface are the more loosely arranged mesophyll cells, which, in C4 leaves, are closely associated and never more than two to three cells away from the bundle-sheath cells.

Question 43

The Calvin cycle is confined to the chloroplasts of the bundle-sheath cells. However, the Calvin cycle is preceded by i

Answer 43

incorporation of CO2 into organic compounds in | the mesophyll cells

Question 44

Recall that in C3 plants, the binding of O2 rather than CO2 by rubisco leads to photorespiration, lowering the efficiency of photosynthesis. C4 plants overcome this problem by concentrating C

Answer 44

CO2 in the bundle-sheath cells at the cost of | ATP.

Question 45

how should rising co2 levels and temp affect c4 plants? what happens in these cases

Answer 45

Rising CO2 levels should benefit C3 plants by lowering the amount of photorespiration that occurs. At the same time, rising temperatures have the opposite effect, increasing photorespiration. (Other factors such as water availability may also come into play.) In contrast, many C4 plants could be largely unaffected by increasing CO2 levels or temperature

Question 46

what is likely to alter the | balance of C3 and C4 plants in varying way

Answer 46

. In different regions, the particular combination of CO2 concentration and temperature. The effects of such a widespread and variable change in community structure are unpredictable and thus a cause of legitimate concern.

Question 47

what do cam plants do and what is their process

Answer 47

A second photosynthetic adaptation to arid conditions has evolved in many succulent (water-storing) plants, numerous cacti, pineapples, and representatives of several other plant families. These plants open their stomata during the night and close them during the day, just the reverse of how other plants behave. Closing stomata during the day helps desert plants conserve water, but it also prevents CO2 from entering the leaves. During the night, when their stomata are open, these plants take up CO2 and incorporate it into a variety of organic acids

Question 48

hloroplasts. Notice in Figure 10.21 that the CAM pathway is similar to the C4 pathway in that and whats the difference

Answer 48

CO2 is first incorporated into organic intermediates before it enters the Calvin cycle. The difference is that in C4 plants, the initial steps of carbon fixation are separated structurally from the Calvin cycle, whereas in CAM plants, the two steps occur within the same cell but at separate times. (Keep in mind that CAM, C4, and C3 plants all eventually use the Calvin cycle to make sugar from carbon dioxide.)

Question 49

Technically, green cells are the only autotrophic parts of | the plant. The rest of the plant depends on

Answer 49

organic molecules | exported from leaves via veins

Question 50

Most plants and other photosynthesizers make more | organic material each day than

Answer 50

they need to use as respiratory fuel and precursors for biosynthesis

Question 51

In accounting for the consumption of the food molecules produced by photosynthesis, let’s not forget that most plants lose

Answer 51

leaves, roots, stems, fruits, and sometimes | their entire bodies to heterotrophs, including humans

Question 52

Furthermore, although each chloroplast is minuscule, their collective productivity in terms of food production is

Answer 52

prodigious: Photosynthesis makes an estimated 150 billion metric tons of carbohydrate per year (a metric ton is 1,000 kg, about 1.1 tons). That’s organic matter equivalent in mass to a stack of about 60 trillion biology textbooks—and the stack would reach 17 times the distance from Earth to the sun! No chemical process is more important than photosynthesis to the welfare of life on Earth.

Question 53

englemanss experiment

Answer 53

. In 1883, Theodor W. Engelmann illuminated a filamentous alga with light that had been passed through a prism, exposing different segments of the alga to different wavelengths. He used aerobic bacteria, which congregate near an oxygen source, to determine which segments of the alga were releasing the most O2 and thus photosynthesizing most. Bacteria congregated in greatest numbers around the parts of the alga illuminated with violet-blue or red light.