Chapter 5

Question 1

A photoautotroph is

Answer 1

an organism that makes its own food using energy from the Sun
- primary producers on Earth
- convert light energy to chemical energy & using it to assemble complex organic molecules from simple
inorganic raw materials

Question 2

Producers are oganisms that…

Answer 2

assemble complex organic molecules from simple inorganic materials and use these molecules for energy and building blocks
- serve directly or indirectly as food sources for consumers

Question 3

Consumers are

Answer 3

the organisms that live by eating other organisms.
- use organic molecules obtained from producers to obtain energy, & as building materials for their cells & body parts

Question 4

bodies of both producers and consumers provide…

Answer 4

chemical energy & building materials for decomposers, which feed on dead and decaying organisms.
- Decomposers return simple inorganic molecules back into the soil, air, or water, making them available once again to photosynthetic organisms

Question 5

cycling of matter through producers, consumers, and
decomposers can continue indefinitely with no further input. HOWEVER…

Answer 5

This cycle of matter requires constant input of free energy from the Sun, as energy is lost as heat at each stage due to the second law of thermodynamics: : entropy increases with every energy transformation or chem reaction

Question 6

There are The Two Stages of Photosynthesis.

Answer 6

1) light-dependent reactions
2) Calvin cycle

light-dependent reactions = directly associated with the absorption of photons of light.

The light-independent reactions (Calvin cycle) = don’t require light themselves, but are dependent on
products of the light-dependent reactions

Question 7

light-dependent reactions

Answer 7

• 1st stage of photosynthesis,
• Light energy is captured by pigment molecules & used to synthesize nicotinamide adenine dinucleotide phosphate (NADPH) & ATP.
• H2O are split, releasing O2 into the environment.
• High-energy e- are transferred via an ETC to NADP+ to form NADPH.
• A p+ gradient is established across a membrane, & the energy from this gradient used to make ATP.

Question 8

Calvin cycle

Answer 8

• 2nd stage of the photosynthesis
• high-energy e-s from NADPH & E from ATP used to convert CO2 into organic compounds = CO2 fixation.
• CO2 is first converted into a simple 3-carbon carb by adding e-s & p+s, which is then used to form larger molecules like glucose.
• Carbs= primary end products of photosynthesis, but reduced C produced is also used to form other molecule backbones like in lipids & proteins.
• All organic molecules of plants are direct or indirect products of photosynthesis.

Question 9

The general chemical equation for photosynthesis is

Answer 9

6 CO2 + 6 H2O C6H12O6 + 6 O2
- equation is reversal aerobic cellular respiration

Question 10

In photosynthetic eukaryotes, both photosynthesis stages occur in __________________ .

Answer 10

the chloroplast

Question 11

Chloroplast membranes

Answer 11

chloroplast= an organelle formed from 3 membranes
that define 3 distinct compartments
- Outer membrane covers the entire surface of the organelle, & the 2nd one, the inner membrane lies just outer 1. –> it has an intermembrane space
- in the stoma is the 3rd membrane system, the thylakoid membranes, or thylakoids.

Question 12

stroma is

Answer 12

• the aqueous environment within the inner membrane around the thylakoids
• is where Calvin cycle takes place
• enzymes that catalyze the reactions of the Calvin cycle r found in the stroma

Question 13

Thylakoid lumen is

Answer 13

the space enclosed by a thylakoid

Question 14

A Stoma is

Answer 14

• plural: Stomata
• is a minute opening through which O2 and CO2 are exchanged with the surrounding atmosphere
• Typically, most of the stomata (thousands per square centimeter) are located in the lower epidermis. GOOGLE

Question 15

Thylakoid’s job

Answer 15

• light absorption by chlorophylls and other accessory pigments –> this is where the light-dependent reactions of photosynthesis occur.
• electron transfer
• ATP synthesis by ATP synthase
• also gives leaves their green colour, cuz the chlorophyll & other accessory pigments r embedded within it

Question 16

Th ere are two points about light and pigment molecules that are important in photosynthesis

Answer 16

1) the absorption of a photon by a pigment molecule excites a single electron, moving it from a low energy, or ground state, to a higher energy, or excited state.

2) the diff between the energy level of the ground state & the energy level of the excited state must be equivalent to the energy of the photon of light that was absorbed. –> If the energies are not equivalent, the photon cannot be absorbed by the pigment.

Question 17

After light absorption, an excited electron in a pigment molecule can follow one of three pathways:

Answer 17

Outcome 1: The excited e- returns to its ground state, releasing energy as thermal energy or fluorescence (light of lower energy and longer wavelength).

Outcome 2: The excited e- transfers its energy to a neighboring pigment molecule, exciting that molecule’s e- while the original e- returns to its ground state. This requires close alignment of molecules.

Outcome 3: The excited e- is transferred to a nearby electron-accepting molecule. is one of the most
important steps in photosynthesis—the energizing & transferring of an e-. In photosynthesis, the key electron-accepting molecule is called a primary electron acceptor

Question 18

Fluorescence is

Answer 18

the emission of light of a longer wavelength (lower energy) than the absorbed light.
- Fluorescence emits lower energy because a small amount of the energy of the photon that was initially absorbed is always lost as thermal energy.

Question 19

primary electron acceptor is

Answer 19

a molecule capable of accepting electrons &
becoming reduced during photosynthesis

Question 20

The most dominant types of chlorophylls are

Answer 20

chlorophyll a & chlorophyll b,
- have slightly different structures

Question 21

In photosynthetic prokaryotes other than ________________, closely related molecules
called ___________________ carry out the same functions as chlorophylls.

Answer 21

cyanobacteria, bacteriochlorophylls

Question 22

The 2 major photosynthetic pigments in plants

Answer 22

1st= chlorophyll
2nd= carotenoids –> red, orange, yellow pigments

• Photosynthesis depends on the absorption of light by chlorophylls & carotenoids acting in combination

Question 23

What happens to chlorophyll a during photosynthesis?

Answer 23

Chlorophyll a becomes oxidized and donates an electron to a primary electron acceptor.

Question 24

Why are carotenoids and chlorophyll b called accessory pigments?

Answer 24

they are referred to as accessory pigments cuz after light absorption, they transfer this excitation
energy to molecules of chlorophyll a.
-This set of accessory pigments= antenna complex, captures light energy & transfers it to a chlorophyll a molecule & the primary electron acceptor in the reaction centre

Question 25

An antenna complex is

Answer 25

a cluster of light-absorbing pigments embedded in the
thylakoid membrane able to capture & transfer E to special chlorophyll a molecules in the reaction centre

Question 26

A reaction centre is

Answer 26

a complex of proteins & pigments that contains the primary electron acceptor

Question 27

A pigment molecule does not absorb all wavelengths of light. The wavelengths that are not absorbed are

Answer 27

transmitted (pass through the object) or reflected.
- This reflected light is what gives the pigment its distinctive colour

Question 28

You can determine which wavelengths of light a pigment absorbs by producing an _____________________ which is…

Answer 28

absorption spectrum
a plot of the amount of light energy of various
wavelengths that a substance absorbs
- is produced using an instrument called a spectrophotometer, which analyzes a sample of the pigment

Question 29

Leaves appear green because

Answer 29

chlorophylls/ chlorophyll a absorbs strongly blue and red light but does not absorb green or yellow light.

Question 30

Note that carotenoids do not absorb light waves in…

Answer 30

the red and yellow range
- These light waves are
reflected back to your eyes as the colour orange.
- carotenoids are found in carrots

Question 31

The vivid colours of fall foliage appear when

Answer 31

the dominant green chlorophyll degrades from the leaves & other pigments are revealed.
- These pigments reflect colours other than green.

Question 32

An action spectrum is

Answer 32

a plot of the effectiveness of light energy of different
wavelengths in driving a chemical process
- is usually determined by using a suspension of
chloroplasts or algal cells & measuring the amount of O2 released by photosynthesis at different wavelengths of visible light. –> cuz more light obsorbed, more O2 made by plants (i think)

• Note if an action spectrum for a physiological phenomenon matches the absorption spectrum of a pigment, it is very likely that the two are linked.

Question 33

Engelmann’s Experiment

Answer 33

• in 1882, Theodor Engelmann conducted one of the 1st action spectra studies using a microscope & glass prism.
• Setup: He placed strand of green alga, Spirogyra, on a slide with water containing aerobic bacteria and used a prism to split light into a spectrum across the algal strand.
• Observation: Aerobic bacteria clustered around areas of the algal strand where the most oxygen was released, indicating high photosynthetic activity.
• Results: The bacteria clustered most under blue, violet, and red light, and sparsely under green light.
• Conclusion: This demonstrated that certain wavelengths of light (blue, violet, and red) are more effective in driving photosynthesis (are absorbed most effectively), while green light is less effective.

Question 34

Do pigment molecules float freely within the thylakoid membranes

Answer 34

No
- they are bound very precisely to a number of diff proteins
- These pigment proteins are organized into photosystems

Question 35

Photosystem composition

Answer 35

• composed of the large antenna complex (AKA a light-harvesting complex) of proteins & about 250-400 pigment molecules surrounding a central reaction centre
• reaction centre of photosystem has a few proteins, each bound to a pair of chlorophyll a molecules, as well as the primary electron acceptor.
  –> Light E absorbed by the antenna complex is transferred to specialized chlorophyll molecules in the reaction centre. – it’s converted to chemical E when
  a reaction centre chlorophyll donates an e- to the primary electron acceptor. –> This electron is passed along an electron transport chain.

Question 36

There are two kinds of photosystems.

Answer 36

• Photosystem I (PSI)
• Photosystem II (PSII)
• are numbered based on the order they were
  discovered. However, in photosynthesis, the actions of photosystem II occur before those of photosystem I

Question 37

Photosystem I vs II: reaction centre

Answer 37

• Photosystem I reaction centre: contains specialized chlorophyll a molecules called P700 molecules (P = pigment) cuz they absorb light optimally 700 nm wavelength.
• Photosystem II reaction centre: contains specialized P680 chlorophyll a molecules, absorb light optimally at 680 nm wavelength.
• P700 & P680 r structurally identical to other chlorophyll a molecules. –> Their specific patterns of energy absorption result from interactions with proteins in the photosystem.

Question 38

______________ is the most abundant and widespread source of energy on Earth?

Answer 38

Solar energy
In 1 day, enough solar energy strikes Earth’s surface for human energy needs for 55 years

Question 39

Keep in mind that that the same chemical processes that take place in photosynthetic eukaryotes, occur in…

Answer 39

in photosynthetic prokaryotes, even though they do not have chloroplasts and other specialized structures that eukaryotes have.
–> In prokaryotes, the photosynthesis reactions occur both in the cytosol & along folds in the cell membrane

Question 40

As in all electron transport systems, the electron carriers of the photosynthetic system consist of

Answer 40

non-protein organic groups that alternate between being oxidized and being reduced as e- move through the system.
–> The carriers include compounds that are similar in structure and function to those in mitochondrial ETC

Question 41

As life emerged on Earth, it began to substantially change the environment. Elaborate

Answer 41

• early atmosphere may have = 3 % O2 gas & as much
  as 10 % CO2.
• today’s atmosphere = 21 % O2 gas & only 0.04 % CO2
• When simple organisms began using photosynthesis as a way to store energy in chemical compounds, they used H2O as their source of H atoms and their e-s.–> A by-product of this process was O2 gas.
• As early photosynthetic organisms flourished, they
  made an abundance of O2 gas that accumulated in the atmosphere. –> This oxygen enabled the evolution of the rich diversity of aerobic life forms that we see
  on Earth today

Question 42

What happens in photosystem II?

Answer 42

• Water is split into electrons, protons, and oxygen gas within photosystem II.
• a photon of light is absorbed by the antenna complex, & its E is transferred to the molecule P680, & 1 of its e-s goes from ground state to an excited state, resulting in the energized molecule P680*
• The excited e- is then transferred to the primary acceptor molecule, which becomes - charged, while the P680 now carries a + charge
  –> P680+ is now extremely electronegative and can exert forces strong enough to remove an e- from water making it the strongest oxidant known in biology.
  –> reduction of P680+ to P680 by e- from water is facilitated by an enzyme subunit of photosystem II called the water-splitting complex. –> is inside the thylakoid membrane, facing the lumen.
  -due to the strong electronegative pull, the water-splitting complex oxidizes H2O, passing an e- to the P680+ to make it neutral again
• The acceptor molecule also transfers an e- to a molecule of plastoquinone (PQ) and becomes neutral, allowing the photon absorption process to start all over again.
• Note: this entire process occurs twice for each
  water molecule that is completely oxidized.

Question 43

What is the strongest oxidant known in biology?

Answer 43

P680+
- has the ability to remove e- from water

Question 44

What is the water-splitting complex?

Answer 44

• it’s an enzyme in photosystem II, reduces P680+ by donating an e- from water, which neutralizes P680.
• is inside the thylakoid membrane, facing the lumen.

Question 45

Overview of ETC

Answer 45

1st steps in the ETC within the thylakoid membrane
involve the transfer of the high-energy e- to the primary acceptor & then to plastoquinone (PQ).
- Here, the light energy that is transferred to the e- is used to generate a proton gradient.
- At the end of the ETC, a second photosystem energizes the e-s a second time. The high-energy e-s can then be transferred to NADPH carrier molecules.

Question 46

The major steps of the linear electron transport system

Answer 46

1) Oxidation of P680: Light absorption in photosystem II excites P680 (forming P680*), which then donates a high-energy e- to the primary acceptor.

2) Oxidation-reduction of plastoquinone: e-s are then transferred to PQ, which shuttles e-s between photosystem II & the cytochrome complex through lipid bilayer. PQ also takes up p+ from the stroma & releases them into the lumen when it gives e-s to cytochrome complex, increasing lumen p+ conc.

3) Electron transfer from the cytochrome complex and shuttling by plastocyanin: From the cytochrome complex, electrons are transferred to plastocyanin, which shuttles them to photosystem I.

4) Oxidation-Reduction of P700: In photosystem I, photon of light absorbed excites P700, forming P700*, which transfers an e- to its primary acceptor, becoming P700+. P700+ then accepts e- from plastocyanin to return to its neutral state.

5) Electron transfer to NADP+ by ferredoxin: The 1st e- from P700* is transported by a short sequence of carriers within photosystem I. It is then transferred to ferredoxin, an iron-sulfur protein. The oxidation of ferredoxin transfers the electron to NADP+, reducing it to NADP.

6) Formation of NADPH: A 2nd e- is transferred to NADP by another molecule of ferredoxin. This 2nd e- & a p+ (H+) from the stroma are added to NADP by the NADP+ reductase to form NADPH. –> NADPH is now carrying two high-energy e- .
- The conc of p+ in the stroma decreases as a result of this NADPH formation. Along with the movement of p+ from stroma to lumen by plastoquinone and the splitting of H2O into p+, these 3 processes create a much higher p+ conc inside the lumen than outside in the stroma

• This pathway is referred to as linear (or non-cyclic) to distinguish it from an alternative process in which e-s r not passed on to NADPH. The p+ gradient is used to make ATP with the same kind of ATP synthase complexes found in mitochondrial membranes.

Question 47

In the electron transport of photosynthesis just described, a proton gradient like in Cellular Respiration is established across the ____________________

Answer 47

Thylakoid Membrane

Question 48

the p+ gradient in photosynthesis is established by 3 mechanisms: Describe them

Answer 48

1. p+ are added to the lumen by the reduction & oxidation of plastoquinone as it moves from photosystem II to the cytochrome complex and back again. –> rmr it picks up p+ from stroma & releases them like the the mitochondria carriers
2. The conc of p+ inside the lumen is increased by adding 2 p+ for each water molecule that is split in the lumen.
3. The removal of one p+ from the stroma for each NADPH molecule formed decreases the conc of p+ in the stroma outside the thylakoid.

• higher conc of p+ inside the membrane creates
  a proton-motive force that drives p+ out of the lumen, back into the stroma. –> the thylakoid membrane allows p+ to pass out into the
  stroma only through the pores in the protein complexes of ATP synthase, embedded in the membrane = chemiosmosis, is the same
  process that occurs in cellular respiration

Question 49

Are chloroplast ATP synthase identical to mitochondria ATP synthase?

Answer 49

Yes

Question 50

All electron transport chains operate by electrons being pulled …

Answer 50

pulled spontaneously “downhill” from molecules with high-energy electrons (molecules that are easily oxidized) to molecules that are progressively more
electronegative.
- MITOCHONDRIAL ETC: the e- flow is “downhill” from high-energy NADH to very electronegative O2.
- PHOTOSYNTHETIC ETC: The opposite occurs in the
ETC in photosynthesis, which begins with low-energy H2O & ends with high-energy NADPH. –> to drive photosynthetic electron transport, low-energy e-s in H2O must be given enough PE to establish a p+ gradient and enough energy to form NADPH.

Question 51

How are the low-energy e-s in H2O getting enough PE to establish a p+ gradient and enough energy to form NADPH?

Answer 51

-in photosynthetic ETC, the e- flow is essentially the opposite, from H2O to NADP+ –> this flow does not occur spontaneously but must be boosted twice through the absorption of light energy

ESSENTIALLY WHAT HAPPENS IS…
- A photon excites an e- in P680 chlorophyll in Photosystem II, moving it further from the nucleus and weakening its attachment. –> The excited electron transfers to a primary electron acceptor, beginning a series of redox reactions.
- Through a series of redox reactions, the e- travels from plastoquinone to plastocyanin, progressively losing free energy. –>cuz we are releasing E, this part happens spontaneously. This energy release helps form a proton gradient across the thylakoid membrane.
- The e- reaches the strongly electronegative P700 in Photosystem I but remains tightly held until a photon re-excites it.
- This new high-energy e- transfers to the primary acceptor in Photosystem I and then to NADP+, forming NADPH.
- Two photons of light are required to transfer 1e-, one for Photosystem II and one for Photosystem I, to complete e- transfer from H₂O to NADPH, following an energy path = the “Z scheme.”
- Note that 2 e- are transferred per molecule of NADPH produced.
- In photosynthesis, e- flow from H₂O to NADP⁺, requiring energy input from light, unlike the spontaneous flow from NADH to O₂ in respiration.
- The process resembles a cyclist climbing two hills with “uphill” sections (energy input needed) and “downhill” coasting (spontaneous energy release), reflecting energy requirements at each stage in photosynthetic electron transport.

Question 52

How many photons need to be absorbed by the photosynthetic apparatus to produce a single molecule of O2?

Answer 52

First, write out a balanced equation. This shows that two molecules of H2O must be oxidized to energize and remove four electrons:
2 H2O –> 4 H+ + 4 e– + O2

To move 1e- down the chain requires the absorption of 2 p+ . Therefore, to get 4e- (and yield one O2 molecule), the photosynthetic apparatus needs to absorb 8 photons of light, 4 by each photosystem.
- also per 8 photons, 2 NADPH are formed.

Question 53

Cyclic Electron Transport

Answer 53

• Photosystem I can operate independently of Photosystem II in a process called cyclic electron transport –> e- are transferred from Photosystem I to ferredoxin but do not proceed to reduce NADP⁺.
• Instead, reduced ferredoxin donates e-s back to plastoquinone.
• Plastoquinone is continuously reduced and oxidized, enabling p+ movement across the thylakoid membrane without involving Photosystem II.
• Cyclic electron transport AKA cyclic photophosphorylation, converts light energy into ATP but does not produce NADPH or involve water oxidation.
• Cyclic electron transport is crucial for balancing energy needs in photosynthesis, providing additional ATP required for reactions that need more ATP than NADPH like the Calvin cycle and other chloroplast reactions.

Question 54

Calvin Cycle background info

Answer 54

• last chapter= CO2 is fully oxidized carbon molecule & thus contains no usable energy.
• sugar molecules such as glucose & sucrose are highly reduced –> have many C–H bonds & thus are an abundant source of energy.
• In chloroplast stroma, a series of 11 reactions uses
  NADPH to reduce CO2 into sugar. –> process= endergonic –> energy from hydrolysis of ATP.

These 11 enzyme-catalyzed (or light-independent) reactions= the Calvin cycle.
–> The Calvin cycle is by far the most dominant pathway on Earth by which CO2 is fixed into carbs.

• many plants use a small number of additional light-independent steps immediately prior to the Calvin cycle.

Question 55

The Calvin cycle can be divided into three phases: List them.

Answer 55

1) fixation (of CO2)
2) reduction (of 3-phosphoglycerate to G3P)
3) regeneration (of RuBP from G3P)

Question 56

Calvin Cycle- Phase 1: Carbon fixation

Answer 56

• Carbon fixation is the conversion of inorganic carbon (CO₂) into an organic form
• CO₂ reacts with ribulose-1,5-bisphosphate (RuBP), a 5-carbon sugar, to form two 3-carbon molecules of 3-phosphoglycerate.
• This single step has monumental significance for life on Earth –> Every C atom in every cell of almost all living things has taken part in this chemical reaction
• This type of photosynthesis is called C3 metabolism, named for the two 3-carbon molecules produced.

Question 57

Calvin Cycle- Phase 2: Reduction

Answer 57

• each molecule of 3-phosphoglycerate gets an
  additional phosphate added from the hydrolysis of ATP.
• This molecule is then reduced by high-energy e- from 1NADPH & one phosphate is removed, producing glyceraldehyde-3-phosphate (G3P)
• NADPH is now been oxidized to NADP+ again

Question 58

Calvin Cycle- Phase 3: Regeneration

Answer 58

• Some G3P molecules are rearranged to regenerate RuBP, allowing the Calvin cycle to restart.
• Each cycle converts 1 CO₂ into 1 reduced carbon (basically CH₂O unit), but three cycles are needed to produce an extra 3-carbon G3P molecule.
• In 3 complete turns of the cycle, 3 CO2 (3 C) are combined with 3 molecules of RuBP (15 C), to produce 6 molecules of 3-phosphoglycerate (18 C). These go
  on to yield 6 molecules of G3P (totalling 18 C). Five of the 6 molecules of G3P (totalling 15 C) are used to regenerate the 3 RuBP molecules (15 C). Thus, the cycle generates 1 molecule of G3P (3 C) after 3 turns
• The production of this one molecule of G3P is the ultimate goal of photosynthesis.
• To produce 1 G3P, the Calvin cycle needs 9 ATP and 6 NADPH, regenerated by light reactions.
• For one glucose molecule (2 G3P), 18 ATP and 12 NADPH are required.

Question 59

Rubisco is

Answer 59

RuBisCO (ribulose-1,5-bisphosphate carboxylase oxygenase) catalyzes the very first reaction of the Calvin cycle, fixing CO₂.
- arguably the most important enzyme of the biosphere. –> By catalyzing CO2 fixation in all photoautotrophs, it provides the source of organic carbon molecules for most of the world’s organisms
- RuBisCO helps convert around 100 billion tonnes of CO₂ into carbs each year.
- RuBisCO is the most abundant protein on Earth, comprising over 50% of the protein in plant leaves and totaling about 40 million tonnes worldwide ( about 6 kg per person on Earth.)

Question 60

How is the G3P produced used?

Answer 60

• serves as the starting point for various organic molecules.
• G3P can form monosaccharides like glucose through reactions that are the reverse of the 1st half of glycolysis, which may further combine into disaccharides (e.g., sucrose) & polysaccharides (e.g., starch, cellulose).
• G3P also leads to the synthesis of amino acids, fatty acids, lipids, and nucleic acids through other pathways that take place in chloroplasts and in the surrounding cytosol and nucleus.

Question 61

Role of sucrose

Answer 61

In higher plants, sucrose is the primary form in which photosynthetic products move between cells and is a major storage form alongside starch.
–> Once glucose is produced during photosynthesis, it is often converted into sucrose within the leaf cells. (CHATGPT)

Question 62

Melvin Calvin’s Experiment

Answer 62

• In the 1940s, Melvin Calvin at the University of California at Berkeley Calvin used carbon-14 (¹⁴C), a radioactive isotope, to trace carbon in photosynthesis.
• He replaced CO₂ with ¹⁴C-labeled CO₂ in photosynthetic algae to track carbon movement.
• Calvin exposed algae to light for varying times, then killed and analyzed them.
• He used two-dimensional chromatography (a process for separating components of a mixture) to separate organic compounds in algae.
• Chromatograms on photographic film revealed which compounds contained ¹⁴C, mapping the steps of carbon fixation.
• This work allowed him to outline the stages of photosynthesis, showing how CO₂ becomes complex organic molecules.

Question 63

Calvin published his paper that consists of his experiment in ________

Answer 63

1948

Question 64

Why did Calvin use ¹⁴C

Answer 64

• Calvin had to develop a method that would separate the photosynthesis reactions from other reactions in the plant, isolate the stages of photosynthesis, & track the movement of carbon through these
  stages.
• Calvin was able to track carbon atoms through the reaction pathway by using a radioactive isotope C-14 cuz it works just like C-12 but emits small amounts of radiation

Question 65

For his experiment, Calvin chose a simple unicellular
algae called _____________, in which each cell contained only _________________. ELABORATE

Answer 65

Chlorella, one chloroplast
- Chlorella was kept in a thin layer in a glass container, exposed to ¹⁴CO₂ in the dark, and then to light to begin photosynthesis.
- Calvin exposed cells to light for short intervals (few seconds to 30 seconds) and then killed the cells in boiling alcohol to stop the reaction.
- Organic compounds were extracted and separated using two-dimensional chromatography with two different solvents.
- Calvin used photographic film to detect ¹⁴C in compounds, where radioactive spots appeared on the film.
- Sequential chromatograms allowed Calvin to identify the order and formation of ¹⁴C-labeled compounds during photosynthesis.
- cuz reactions were fast, evidence was hard to observe but by comparing the chromatogram images from each trial and considering a variety of factors, Calvin eventually determined the steps in the cycle.

Question 66

Between _______ and _______, Calvin and his team published ____ papers describing their research on the ________________ of photosynthesis.

Answer 66

1948, 1953, 21, cyclic pathway

Question 67

Aftermath of Calvin’s discovery

Answer 67

• Calvin’s findings and the ingenuity of his approach
  allowed fast advances, not only in the field of photosynthesis but also in many other biochemical studies.
• He was awarded the 1961 Nobel Prize in Chemistry for his research.
• It had been almost 60 years since the 1902 Nobel Prize winner, Emil Fischer, was rewarded for his research on the structure of glucose—the end product of the Calvin cycle.

Question 68

Atmospheric CO2 is available to every land plant and to aquatic plants whose leaves float on the surface of the water. For submerged aquatic plants, carbon dioxide is available in the form of _____________________.

Answer 68

dissolved carbonic acid

Question 69

Carbon dioxide, however, makes up only_______ of the atmosphere.

Answer 69

0.04 % –> a concentration 1/500 that of the
O2 gas we breathe! This ratio has significant consequences for photosynthesis.

Question 70

Preventing Water Loss

Answer 70

Water, abundant in the cytosol, is always available for photosynthesis. –> However, land plants have adaptations to reduce water loss, which can also limit gas exchange.

• A waterproof cuticle covers leaf surfaces, sealing off interior of leaf, preventing rapid water loss through evaporation. –> Perhaps the most important adaptation for land plants
  –> To enable controlled gas exchange with the atmosphere, each leaf has many microscopic stomata that can be opened and closed by the surrounding pair of guard cells

Question 71

A stoma is

Answer 71

• Plural: stomata
  small pores in the surface of a leaf that can be opened and closed to control the exchange of gases between
  the atmosphere and the leaf interior

Question 72

When are stomata open and when are they closed?

Answer 72

• The stomata open during the day, allowing carbon dioxide to enter and be used for photosynthesis.
  –> When the stomata are open, some water is lost through transpiration (loss of water vapour through stomata), but this water is replaced by water taken up by the plant’s roots.
• At night, photosynthesis stops and the stomata close to conserve water.
• When a plant is at risk of losing too much water due to high temperatures or there is a shortage of water in the soil, the stomata remain closed even during the day.

Question 73

The Problem with Rubisco

Answer 73

• Rubisco, the most abundant enzyme, is slow, catalyzing only about three CO₂ molecules per second. –> this slow rate is countered by its abundance within the cells.
• rubisco’s active site occasionally binds with O2 gas instead of CO2 & catalyzes a reaction between a molecule of O2 and RuBP.
• This is why it’s called ribulose-1,5-bisphosphate carboxylase oxygenase. One of the products of the reaction between O2 and RuBP is a molecule that is not useful to the cell.
• One of the products of the reaction between O2 and RuBP is a molecule that is not useful to the cell. –> This molecule must be converted back into a useful molecule to prevent too much
  RuBP from being wasted.
• The recovery pathway is long and involves reactions within the chloroplast, peroxisomes, and mitochondria. –> It consumes ATP and releases a
  molecule of CO2.
• This means that, instead of fixing CO2, the oxygenase activity of rubisco does the opposite.
• Since O2 is a reactant in the recovery pathway and CO2 is produced at later steps, the entire process is termed photorespiration.

Question 74

Photorespiration is

Answer 74

the catalysis of O2 instead of CO2 by rubisco into RuBP, which slows the Calvin cycle, consumes ATP, and results in a release of carbon (CO2 Im pretty sure)

Question 75

Under laboratory conditions, when concentrations of both O2 and CO2 are equal…

Answer 75

the binding of CO2 will happen more frequently because the active site of rubisco has a greater attraction for CO2 than for O2.
–> In fact, the binding with CO2 will occur about
80 times as fast as the binding with O2.

• In nature, however, the atmosphere does not
  contain equal amounts of the two gases—it contains about 21 % O2 and about 0.04% CO2.
• Since the amount of O2 in the atmosphere is much greater than the amount of CO2, under normal atmospheric concentrations and at moderate temperatures rubisco will bind with CO2 about 75 % of the time. –> 25 % of the time, rubisco binds with O2 and releases rather than fixes a molecule of CO2.
  –> This is a drain on cell resources, but the plant can still fix enough carbon to meet its normal demands for the production of energy-rich carbohydrates.

Question 76

Many terrestrial plants, especially those living in hot, dry climates, face the problems of photorespiration and water loss. ELABORATE

Answer 76

• They need to open their stomata to let in CO2 for the Calvin cycle, but they need to keep the stomata closed to conserve water.
• When stomata are closed, or partly closed, less CO2 can enter the leaf, and, as the CO2 that is present is consumed in the Calvin cycle, its concentration drops and photorespiration increases.
• This is even worse in warm climates cuz the solubility of O2 and CO2 decreases as the temperature increases. (The stroma, where the Calvin cycle occurs, is an aqueous environment.)
• solubility of CO2 decreases more rapidly than that of O2 as the temperature increases, resulting in a decrease in the CO2:O2 ratio.
• As this ratio decreases, photorespiration increases. In high temperatures, as much as 50% of the plant’s E could be wasted by photorespiration

Question 77

C4 Plants

Answer 77

• Some plants in hot, dry climates have specialized leaf structures and carbon fixation processes to reduce photorespiration.
• In these plants, the Calvin cycle occurs in bundle-sheath cells, which are surrounded by mesophyll cells that separate them from the air spaces within the leaf.
• This separation reduces the exposure of the rubisco-containing bundle-sheath cells to O2 gas and therefore reduces the rate of photorespiration.
• The mesophyll cells also reduce access to CO2, but this is not a problem because they operate a second carbon fixation pathway called the C4 cycle

Question 78

The C4 cycle is

Answer 78

an alternative form of C fixation that some plants use, particularly in hot weather, to increase the conc of CO2 available for the Calvin cycle reactions.
- In the C4 cycle, CO2 combines with a 3-carbon molecule, phosphoenolpyruvate (PEP), to produce the 4-carbon oxaloacetate.
- Oxaloacetate is then reduced to malate by e-s
transferred from NADPH. The malate diffuses into the bundle-sheath cells, where it enters chloroplasts and is oxidized to pyruvate, releasing CO2.
- The combined effect of the physical arrangement of cells and the C4 pathway establishes a high conc of CO2 around the rubisco while reducing its exposure to O2.

Question 79

A key distinction between C4 and C3 metabolism is related to the binding of CO2. ELABORATE

Answer 79

• In the C4 cycle, the initial binding of CO2, which incorporates CO2 into phosphoenolpyruvate, is catalyzed by enzyme PEP carboxylase.
• Unlike rubisco, PEP carboxylase has a much greater affinity for CO2 than for O2, so it can efficiently catalyze the binding of PEP regardless of the O2 conc near the enzyme.
• Many tropical plants & several temperate crop species, including corn and sugar cane, have C4 metabolism.
• Remember that C4 plants use the Calvin cycle as well.

Question 80

Even though C4 metabolism helps prevent photorespiration, it is not widespread among plants. Why?

Answer 80

• For each turn of the C4 cycle, the double hydrolysis of ATP to AMP (adenosine monophosphate) is required to regenerate PEP from pyruvate. –> means that there is an additional energy requirement, equivalent to 6 ATP for each G3P produced by the Calvin cycle.
• However, in hot climates, photorespiration can decrease carbon fixation efficiency by over 50 %, so the C4 pathway is worth the energy cost.
• Hot climates also tend to receive a lot of sunshine, so the additional ATP requirement is easily met by the cyclic light reactions.
• In temperate climates, the lower temps mean that photorespiration is less of a problem, and the additional ATP requirement is harder to meet with less sunshine. –> In Florida, for example, 70 % of
  native species are C4 plants, but there are no C4 species in Manitoba.

Question 81

The enhanced efficiency of C4 metabolism has a number of implications.

Answer 81

1) C4 plants can open their stomata less than C3 plants, enabling them to survive better in arid environments.
2) C4 plants also require one-third to one-sixth
as much rubisco, and so have a much lower nitrogen demand. This enables them to survive in more nutrient-poor soil conditions

Question 82

CAM plants

Answer 82

• AKA Crassulacean Acid Metabolism –> derived from the Crassulaceae family in which the metabolic pathway was first observed, and from the plants’ nighttime accumulation of malic acid.
• C4 plants run their Calvin & C4 cycles simultaneously but in different locations.
• Other plants, mostly succulents, run their Calvin cycle & C4 cycle in the same cells, but do so at different times of the day= CAM plants

Question 83

Characteristics of CAM plants

Answer 83

• they live in regions that r hot & dry during the day & cool at night.
• These cacti & succulent species, with fleshy leaves or stems, have a low surface-to-volume ratio, & fewer stomata.
• Furthermore, their stomata open only at night, when they release the O2 that accumulates from photosynthesis during the day and allow CO2 to enter. - The CO2 that enters is fixed by a C4 pathway into malate, which accumulates throughout the night and is stored in the form of malic acid in cell vacuoles.
• As the Sun rises & the temp increases, the stomata close, reducing H2O loss and cutting off the exchange of gases with the atmosphere.
• Malic acid diffuses from vacuoles into cytosol, where the malate is oxidized to pyruvate, and a high conc of CO2 is released.
• The high conc of CO2 favours the carboxylase activity of rubisco, allowing the Calvin cycle to proceed efficiently with little loss of CO2 from photorespiration.
• The pyruvate produced by malate breakdown accumulates during the day, but is converted back to malate during the night. As in other C4 plants, this
  step requires an expenditure of ATP.

Question 84

The Artificial Leaf

Answer 84

March of 2011, scientists from the Massachusetts Institute of Technology announced the development of the first practical “artificial leaf.”
- size of playing card
- When in water and exposed to sunlight, it is able
to generate electricity using a process that begins by splitting water into H &1/2 O2 gas .
- goal is to enable homes to generate their own power in a green way.

Question 85

Scientists are working to copy the energy-trapping power of photosynthesis itself. ELABORATE

Answer 85

Unlike plants, which use sunlight to synthesize sugars & other complex organic molecules, scientists are hoping to use the energy-capturing ability of photosystems more directly.
- rmr photosystem II is able to split water molecules, separating the hydrogen electrons and protons from oxygen atoms.
-If the free energized e-s or hydrogen gas could be captured using artificial photosystems, they could be used to generate electricity or to provide a high-energy, clean-burning fuel.

Question 86

Photosynthesis VS Aerobic Cellular Respiration: Organisms that do these processes

Answer 86

• Photosynthesis occurs in the cells of autotrophs such as plants, algae, and cyanobacteria.
• aerobic cellular respiration used by majority of eukaryotic & many prokaryotic heterotrophs, & by all photosynthesizing organisms.

Question 87

Photosynthesis VS Aerobic Cellular Respiration: Purpose

Answer 87

• photosynthesis converts sunlight into chemical E for use as food by the organisms themselves and by organisms at higher trophic levels
• Aerobic cellular respiration extracts chemical E from food & converts it into chemical PE within ATP. This PE of ATP then supplies the free E that is needed to drive all other cellular metabolic activities

Question 88

Photosynthesis VS Aerobic Cellular Respiration: Location

Answer 88

• photosynthesis occurs in the chloroplast, while aerobic cellular respiration begins in the cytosol & is completed within the mitochondrion

Question 89

Photosynthesis & Aerobic Cellular Respiration are complementary. why is this so?

Answer 89

• Photosynthesis produces O2 & sugars, which are the reactants for cellular respiration.
• The waste products of cellular respiration, H2O & CO2 are the reactants for photosynthesis.

Question 90

In addition to being complementary, photosynthesis & aerobic cellular respiration share many similar chemical steps & processes. ELABORATE

Answer 90

• Both use ETCs, along with ATP synthase complexes, to generate ATP by chemiosmosis.
• Both use & regenerate carrier molecules.
  –> photosynthesis= NADP+ molecules r reduced during the light-dependent reactions to NADPH molecules, which then deliver hydrogens and their high-energy e-s to the Calvin cycle for use in carb synthesis
  –> aerobic cellular respiration= molecules of both NAD+ & FAD r used to carry hydrogens & their high-energy e-s to an ETC . There, they are oxidized using oxygen to produce H2O & ATP.

Question 91

are all plant cells green?

Answer 91

• Most plant cells are not green &, like animal cells, are incapable of performing photosynthesis.
• These non-green cells form the roots, inner parts of stems, & various reproductive parts of a plant

Question 92

The key organelles used in Photosynthesis & Aerobic Cellular Respiration share structural features.

Answer 92

• chloroplasts and mitochondria all have inner folded membranes that create separated fluid-filled spaces that allow p+ gradients to be established
• Chloroplasts & mitochondria even possess their own unique DNA and replicate independently of cell division
• Both have complementary carbon-fixing (Calvin) and carbon-releasing (citric acid) cycles.