Botanica 2

ATP

ADP + P Calvin

cycle denominate piante C3.

ADP + P

ATP

grano, la segale

é le patate e i

zzano il ciclo

denominate +

NADPH + H

PGAL (3C)

From light

reactions PGAL (3C) +

NADP 3 molecules CO

2

(3 × 1C = 3) 6 molecules PGA

3 molecules RuBP (6 × 3C = 18)

PGAL (3C) (3 × 5C = 15)

[rearrangements]

FIGURE 6.17 Calvin cycle In the Calvin cycle, the rubisco

enzyme combines carbon dioxide with the 5-carbon RuBP to Sugars

yield two 3-carbon molecules (PGA). ATP and NADPH from the Starch

light reactions are used to convert PGA to the 3-carbon molecule 5 molecules PGAL

Amino acids (5 × 3C = 15)

PGAL, some of which is used by the cell to make sugars and 6 molecules PGAL

Fatty acids

other carbon-containing compounds. With energy from ATP, (6 × 3C = 18)

the remaining PGAL is converted back to the 5-carbon RuBP,

completing the cycle. The inset shows that three turns of the 1 molecule PGAL

(1 × 3C = 3)

cycle are required to produce one molecule of PGAL (see text).

or RuBP (Figure 6.17). A single molecule of carbon dioxide is at-

tached or fixed to it through the operation of the enzyme ribulose

Quando le piante C3 sono soggette a stress durante la stagione calda e secca, chiudono i

1,5-bisphosphate carboxylase/oxygenase, which is commonly called

The final terms indicate that the enzyme rubisco can act

rubisco.

as either a carboxylase, and combine with carbon dioxide, or as

loro stomi per ridurre la perdita d'acqua. Di conseguenza, la concentrazione di anidride

an oxygenase, and combine with oxygen. When rubisco combines

carbon dioxide with RuBP, the resulting 6-carbon sugar is imme-

diately split into two 3-carbon molecules of 3-phosphoglycerate, or

carbonica all'interno delle foglie diminuisce.

PGA. Because the first detectable product of the Calvin cycle is the

3-carbon molecule PGA, this metabolic pathway is called the C 3

The C pathway is the most widespread carbon fixation

pathway. 3

pathway, and about 85% of known plant species use it exclusively.

The remaining 15% of plants have an additional pathway for as-

L'enzima rubisco combina quindi l'ossigeno, anziché l'anidride carbonica, con lo zucchero a

similating and fixing carbon dioxide. Common cereal grasses such

as wheat, rye, and oats as well as potatoes and soybeans use the C

3

pathway and are therefore referred to as C plants.

5 atomi di carbonio RuBP.

3

Rubisco is the most abundant enzyme in the world and one

of the most important. All plants and photosynthetic protists use

some form of rubisco to fix carbon dioxide into simple sugars.

6–18

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Photosynthesis H O Calvin

2 2 PGA (3C) cycle

CO

2 Carboxylase

Rubisco

RuBP (5C) CO 2

Oxygenase

O 2 Calvin

1 PGA (3C) cycle

Photorespiration

The recent trend of increasing levels of atmospheric carbon FIGURE 6.18 Photorespiration When the rubisco enzyme

Circa il 15% delle specie vegetali conosciute ha sviluppato meccanismi alternativi per

behaves as was described for the Calvin cycle (i.e., as a carboxylase),

dioxide has actually increased the efficiency of C plants grown

3 RuBP is combined with CO giving rise to two PGA molecules,

as crops. But other plants have evolved mechanisms to overcome 2

which continue along in the Calvin cycle (top pathway). When

garantire che la rubisco nelle loro cellule fotosintetiche non sia esposta a bassi livelli di

photorespiration. CO levels are low, however, rubisco can act as an oxygenase

2

(bottom pathway), combining RuBP with oxygen, which yields

anidride carbonica.

C plants and CAM plants have mechanisms to reduce only one PGA molecule and, following a series of reactions, CO .

2

4 By causing the loss of carbon that might otherwise have been

photorespiration used to construct sugars or other molecules, photorespiration can

CICLO C4- Le piante C4 riducono la foto respirazione preconcentrando la CO .

C plants rely on the Calvin cycle alone for the fixation of carbon considerably reduce the efficiency of photosynthesis.

3 2

dioxide into simple sugars. Consequently, if the level of carbon

dioxide around their mesophyll cells drops to low levels, rubisco

reacts with oxygen rather than carbon dioxide and photorespiration

Ciclo CAM- Le piante CAM riducono la foto respirazione fissando la CO di notte.

2

occurs. About 15% of known plant species have evolved alternative

mechanisms to ensure that the rubisco in their photosynthetic cells

is not exposed to low levels of carbon dioxide.

Nei deserti, piante come cactus e agavi sono frequentemente esposte a temperature elevate

C plants reduce photorespiration by preconcentrating CO

e lunghi periodi di stress idrico. Per ridurre la perdita d'acqua, le piante del deserto devono

4 2

Did you ever wonder why crabgrass is so rampant in your lawn, or

why crops such as corn and sugarcane are so productive? To a large

chiudere i loro stomi durante il giorno. Queste condizioni porterebbero a livelli elevati di

degree these plants are successful in warm weather because they

fotorespirazione nelle piante C3.

have an additional photosynthetic pathway, called the C pathway. Ciclo CAM-Le piante CAM rid

4

In these plants the first product of carbon fixation is a compound

called oxaloacetate. Since oxaloacetate has four carbons rather than

three, this pathway is called the C pathway, and plants that use it

Molte piante del deserto, in particolare piante CO

4 2

are C plants.

4

succulente con steli o foglie spessi e carnosi, hanno

In the mesophyll cells of the leaves of C plants, there is no

4

Calvin cycle and carbon dioxide combines with a 3-carbon com-

sviluppato un meccanismo diverso rispetto alle piante Chloroplast

pound called phosphoenolpyruvate (PEP) to form oxaloacetate in a Nucleus Stomata o

reaction catalyzed by the enzyme PEP carboxylase (Figure 6.19).

C4 per ridurre la fotorespirazione.

Oxaloacetate is then very rapidly reduced to malate. The unusual CO 2

feature of C plants is that the mesophyll cells do not carry out PEP Malate

4 a

the remaining steps of photosynthesis, because there is no Calvin

Poiché questo meccanismo è stato riconosciuto per la

cycle. They transport the malate to special cells called bundle- Oxaloacetate a

Malic acid

sheath cells, which surround the veins of the leaf. There the malate

prima volta nelle piante chiamate «stonecrops: nella Stomata open

is converted into carbon dioxide and the 3-carbon pyruvate. The FIGURE 6.21 Exam

Vacuole

famiglia delle Crassulaceae, è chiamato metabolismo

carbon dioxide enters the Calvin cycle, and rubisco fixes it into (Sansevieria) and (b)

PGA, just as in C plants. Carbon dioxide is fixed twice, once in

acido delle crassulacee (CAM).

3 a

6–20 Chloroplast Nucleus Stomata cl

CO

2 b

Malate

AMP ATP

+

2 P Pyruvate FIGURE 6.20 Crassulac

are open, CO enters the

2

Malic acid

PEP 4-carbon malate, which

Stomata closed stomata close (conservin

Vacuole the vacuole and is conve

be fixed in the chloropla

molecule is, through the

b cost is less than tha

FIGURE 6.20 Crassulacean acid metabolism At night, when the stomata

are open, CO enters the plant and, through a series of steps, is converted to the

Ezioplasti high temperatures,

2

4-carbon malate, which is stored in the vacuole as malic acid. In the daytime, are even better suite

stomata close (conserving water for the plant) and the malic acid is removed from plants generally can

the vacuole and is converted to malate and a CO molecule is split off, which can of the high water co

2

be fixed in the chloroplast by rubisco in the Calvin cycle. The 3-carbon pyruvate

In assenza dello stimolo luminoso i proplastidi della foglia si trasformano in ezioplasti. Lo freeze readily, and t

molecule is, through the input of energy, converted back to PEP. plants are restricted

stimolo luminoso determina la conversione dell’ezioplasto in cloroplasto. not exposed to freez

cost is less than that of photorespiration, which is the case at pathway of CAM p

plants will be favored. CAM plants

high temperatures, then C 6.19). They grow m

4

are even better suited to hot, dry climates, but succulent CAM plants can displace

plants generally cannot tolerate freezing temperatures because and CAM plants pe

of the high water content of their cells. Their water-filled cells ent set of climatic c

freeze readily, and the plants are damaged. Most succulent CAM

plants are restricted to latitudes and elevations where they are

not exposed to freezing temperatures. The carbon-concentrating

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Gli ezioplasti sintetizzamo molti lipidi, che sono componenti delle membrane interne dei

cloroplasti, mentre è ridotta la sintesi delle proteine di membrana. La sintesi di lipidi porta

alla formazione di un sistema di membrane tubulari molto ramificato, che forma un corpo

paracristallino, il corpo prolamellare.

Se la plantula viene esposta alla luce i lipidi del corpo prolamellare si disperdono andando a

formare le membrane tilacoidali e l’ezioplasto si trasforma in cloroplasto.

Cromoplasti

Possono avere membrane interne ma mancano di un vero e proprio sistema tilacoidale. La

conversione cloroplasto-cromoplasto comporta la degradazione delle clorofille.

Accumulano caretenoidi (pigmenti):

- In gocciole lipidiche giallo arancio (plastoglobuli)

- In cristalli

- LegaB a membrane interne

- La conversione cloroplasto – cromoplasto dipende da faCori endogeni (ormoni e

nutrienB) ed ambientali (fotoperiodo e temperatura)

- Il processo può essere reversibile

I cromoplasti possono derivare da plastidi non fotosintetici (barbabietola e carota) o dai

cloroplasti (pomodoro, peperone).

La presenza dei cromoplasti è responsabile della colorazione di alcuni fiori (ranuncolo), frutti

(pomodoro) e radici (carota).

I cromoplasti possono ridifferenziarsi in cloroplasti. La luce è probabilmente il fattore più

importante del reinverdimento, via fitocromo, tuttavia anche fattori nutrizionali sono

coinvolti.

Alte temperature, fertilizzazione azotata e gibberelline stimolano il reinverdimento delle

arance e delle clementine (varietà di agrumi che deriva dall'ibridazione tra mandarino ed

arancia).

Ancora sono scarse le informazioni sul meccanismo molecolare, ma evidenze sperimentali

suggeriscono che l’acido gibberellico stimoli il reinverdimento e riduca l’espressione dei geni

biosintetici dei carotenoidi, come fitoene sintasi e desaturasi e β-carotene idrossilasi.

I Gerontoplasti

Nelle foglie senescenti si osservano plastidi che, in seguito a processi degradativi

(demolizione delle clorofille e del sistema interno dei tilacoidi e accumulo di carotenoi