Concept 10.1: Photosynthesis converts light energy to the chemical energy of food

Question 1

Photosynthetic enzymes and other molecules are grouped together in a

Answer 1

biological membrane

Question 2

The process of photosynthesis most likely originated in a group of bacteria that had infolded regions of the

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plasma membrane containing clusters of such molecules.

Question 3

the original chloroplast was a photosynthetic prokaryote that lived inside an ancestor of eukaryotic cells

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what has come to be known as the endosymbiont

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the tissue in the interior of the leaf

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mesophyll

Question 5

Carbon dioxide enters the leaf, and oxygen exits, by way of microscopic pores called

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stomata

Question 6

Water absorbed by the roots is delivered to the leaves in

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veins.

Question 7

A typical mesophyll cell has

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about 30–40 chloroplasts, each measuring about 2–4 µm by 4–7 µm.

Question 8

A chloroplast has two membranes surrounding a dense fluid called the

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stroma

Question 9

Suspended within the stroma is a third membrane system, made up of sacs called

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thylakoids

Question 10

which segregates the stroma from the

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thylakoid space inside these sacs.

Question 11

In some places, thylakoid sacs are stacked in columns called

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grana (singular, granum).

Question 12

the green pigment that gives leaves their color, resides in the thylakoid membranes of the chloroplast.

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Chlorophyll

Question 13

The internal photosynthetic membranes of some prokaryotes are also called

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thylakoid membranes

Question 14

In the presence of light, the green parts of plants produce organic compounds and oxygen from carbon dioxide and water. Using molecular formulas, we can summarize the complex series of chemical reactions in photosynthesis with this chemical equation:

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6 CO2 + 12 H2 O + light energy ּּ→ C6 H12 O6 + 6 O2 + 6 H2 O

Question 15

We use glucose (C6 H12 O6) here to simplify the relationship between photosynthesis and respiration, but the direct product of photosynthesis is actually a

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three-carbon sugar that can be used to make glucose

Question 16

Water appears on both sides of the equation because 12 molecules are consumed and 6 molecules are newly formed during

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photosynthesis.

Question 17

We can simplify the equation by indicating only the net consumption of water:

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6CO2 + 6H2O + Light energyּּ→ C6H12 O6 + 6O2

Question 18

Writing the equation in this form, we can see that the overall chemical change during photosynthesis is the reverse of the one that occurs during

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cellular respiration

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Now let’s divide the photosynthetic equation by 6 to put it in its simplest possible form:

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CO2 + H2O ּּ→ [ CH2O] + O2

Question 20

Here, the brackets indicate that CH2O is not an actual sugar but represents the general formula for a

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carbohydrate

Question 21

One of the first clues to the mechanism of photosynthesis came from the discovery that the O2 given off by plants is derived from H2O and not from .

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CO2

Question 22

Here is the chemical equation for photosynthesis in these sulfur bacteria:

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CO2 + 2H2S ּּ→ [CH2O] + H2O +2S

Question 23

Learn van Neil formulas

Question 24

Let’s briefly compare photosynthesis with cellular respiration. Both processes involve

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redox reactions

Question 25

During cellular respiration, energy is released from sugar when electrons associated with hydrogen are transported by carriers to oxygen, forming

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water as a by-product

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The electrons lose potential energy as they “fall” down the electron transport chain toward electronegative oxygen, and the

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mitochondrion harnesses that energy to synthesize ATP

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Photosynthesis reverses the direction of

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electron flow.

Question 28

Water is split, and its electrons are transferred along with hydrogen ions (H+) from the water to carbon dioxide, reducing it to

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sugar.

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Because the electrons increase in potential energy as they move from water to sugar, this process requires

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energy—in other words, it is endergonic

Question 30

Actually, photosynthesis is not a single process, but

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two processes, each with multiple steps

Question 31

These two stages of photosynthesis are known as the

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light reactions and the Calvin cycle

Question 32

The light reactions are the steps of photosynthesis that convert solar energy to

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chemical energy

Question 33

Water is split, providing a source of electrons and protons (hydrogen ions, H+) and giving off .

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O2 as a by-product

Question 34

Light absorbed by chlorophyll drives a transfer of the electrons and hydrogen ions from water to an acceptor called

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NADP+ (nicotinamide adenine dinucleotide phosphate) where they are temporarily stored.

Question 35

The light reactions use solar energy to reduce NADP+ to _________ by adding a pair of electrons along with an H+.

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NADPH

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

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photophosphorylation

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Thus, light energy is initially converted to chemical energy in the form of two compounds:

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NADPH and ATP. NADPH, a source of electrons, acts as “reducing power” that can be passed along to an electron acceptor, reducing it, while ATP is the versatile energy currency of cells.

Question 38

Notice that the light reactions produce no sugar; that happens in the second stage of photosynthesis, the.

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Calvin cycle

Question 39

The Calvin cycle is named for ________________, who, along with his colleagues James Bassham and Andrew Benson, began to elucidate its steps in the late 1940s.

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Melvin Calvin

Question 40

The cycle begins by incorporating from the air into organic molecules already present in the

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chloroplast.

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This initial incorporation of carbon into organic compounds is known as

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carbon fixation

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The Calvin cycle then reduces the fixed carbon to carbohydrate by the addition of

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electrons.

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To convert to carbohydrate, the Calvin cycle also requires chemical energy in the form of

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ATP, which is also generated by the light reactions.

Question 44

Thus, it is the Calvin cycle that makes sugar, but it can do so only with the help of the

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NADPH and ATP produced by the light reactions.

Question 45

The metabolic steps of the Calvin cycle are sometimes referred to as the _________________________________, because none of the steps requires light directly.

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dark reactions, or light-independent reactions

Question 46

As Figure 10.6 indicates, the thylakoids of the chloroplast are the sites of the light reactions, while the Calvin cycle occurs in the

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stroma.

Question 47

On the outside of the thylakoids, molecules of NADP+ and ADP pick up electrons and phosphate, respectively, and NADPH and ATP are then released to the .

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stroma, where they play crucial roles in the Calvin cycle