Lecture #9

Photosynthesis and Plant Nutrition

Plant nutrition• bulk of the plants organic material is derived

from CO2– photosynthesis converts this C02 to carbohydrates– carbohydrates used to make ATP

• BUT other chemicals are required for the plant to complete its life cycle

• some chemicals are obtained via extraction of mineral nutrients from the soil– e.g. NO3-/nitrate ions from the soil

• H+ ions and some O2 are obtained from water– 80-90% of the plant cell is water – growth of the plant

cell is through the accumulation of water – but most water is lost via transpiration – controlled by

stomata• water that is retained has three functions

– 1. acts as a solvent– 2. provides most of the water for cell elongation– 3. helps maintain the form of soft tissue by

keeping the cells turgid

Minerals

H2O

H2O

O2

O2

CO2

CO2

Nutrients• more than 50 chemical elements have been

identified among the inorganic substances in a plant

• chemical elements within a plant reflect the soil conditions

• but many of these chemicals may not be essential elements – those essential for the viability of the plant (to

complete its life cycle or produce another generation)• three criteria to be an essential element:

– 1. the element must be necessary for complete, normal plant development through a full life cycle

– 2. the element itself must be necessary and no substitute can be effective

– 3. the element must act within the plant not outside of it

• to determine essential elements – researchers grow plants under hydroponic conditions (soil-less) in which minerals replace the soil• hydroponic culture can ensure optimal mineral nutrition by

using specifically constructed mineral solutions - expensive

Control: Solutioncontaining all minerals

Experimental: Solutionwithout potassium

Nutrients• nutrients can be divided into

macronutrients and micronutrients– macro – plants require these in large

quantities• carbon, oxygen, sulfur, hydrogen, nitrogen

and phosphorus• these components form the organic

compounds that are the structure of the plant:

– micro – plants require these in very small amounts

• function mainly as cofactors in the plants enzymatic reactions

• e.g. iron – metallic component of the cytochrome complexes in photosynthesis

– due to their catalytic role – required in very small quantities

Mineral Deficiency Healthy

Phosphate-deficient

Potassium-deficient

Nitrogen-deficient

• plants suffering from an overabundance of minerals is rare– many plants crystalize the excess in vacuoles

or the unnecessary ion is not absorbed • BUT desert soils frequently have an excess

of many minerals = PROBLEM– plant are unable to grow not because of

mineral toxicity but because of osmotic drought

– roots are unable to extract water from the soil• mineral deficiencies are not seen in natural

populations• more frequent in non-native crop plants

– artificial selection for rapid growth and high fruit/seed yield requires large amounts of minerals – must be provided through fertilizers (changes soil composition)

Mineral Deficiency Healthy

Phosphate-deficient

Potassium-deficient

Nitrogen-deficient

• virtually all soils are deficient in nitrogen• the act of harvesting crops can also induce soil

depletion – fruits, storage roots, seeds and tubers have the greatest concentrations minerals– the remaining part of the plant does not contribute

much to the re-enriching of the soil• diagnosis is usually obvious – symptoms are often

distinctive• symptoms of nutrient deficiency depend partly on the

nutrient’s function– e.g. deficiency of magnesium – component of

chloroplasts – yellowing of leaves• symptoms also depend on its mobility within the

plant• deficiencies of nitrogen, phosphorus and

potassium are most common• shortages of micronutrients are less common

– amount of micronutrient required to correct the problem must be carefully considered

Soil• soil’s origin – weathering of rock– two types of weathering: physical and

chemical– initial rock may be:

• a. volcanic/igneous (basalt, granite)• b. metamorphosed (marble, slate)• c. sedimentary (sandstone, limestone) • d. or other types

– all rocks have a crystalline structure – numerous contaminating ions trapped within this structure

• called the crystal matrix– so weathering releases these ions into the

forming soil– physical weathering: water seeps into the

cracks and crevices – mechanically fractures rocks upon expansion

– chemical weathering: • 1. acids help dissolve the rock • 2. organisms can also break down rock

• weathering produces a variety of soil particles from the largest (sand), to clay particles to the smallest (silt)

• eventual result is topsoil – mixture of particles derived from rock, organisms and partially decayed organic material (humus)– humus – consists of decomposing organic material

formed by bacteria• builds a crumbly soil that retains water but is still porous

enough for adequate aeration of roots– organic materials – one teaspoon of topsoil contains 5

billion bacteria plus fungus, algae, insects, earthworms, nematodes and plant roots

– organisms – affects the soil’s physical and chemical properties

• e.g. earthworms – turn over and aerate the soil by burrowing and adding mucus that holds fine particles together

• bacterial metabolism – alters mineral composition• plant roots also affect soil composition and texture by

releasing organic acids• topsoil’s function depends on its particle sizes

– e.g. larger sand particles do not fit together tightly and have numerous spaces between them

– these spaces permit gas diffusion – roots in soil are never starved for oxygen

– the spaces fill with water after rains – e.g. silt – absorbs water well and is used to prevent

erosion

layers of soil = horizons

Soil

(O horizon)

• soil pH is critical to the health of the plant• if the soil becomes too acidic, certain ions like

manganese, aluminum can become so soluble as to become toxic to the plant

• in alkaline soils, iron and zinc are insoluble and unavailable to the plant

• so each plant has an optimal pH range• acidic (pH 4.5 to 5.5): azalea, blueberry, cranberry,

fennel, gardenias, potatoes, rhododendrons, sweet potatoes

• neutral (pH 5.5 to 6.5): carrots, chrysanthemums, corn, cucumbers, peas, pointsettias, radishes, strawberries, tomatoes

• alkaline (pH 6.6 to 7.5): apples, asparagus, beets cabbage, cauliflower, lettuce, onions, soybeans, spinach

Plant growth and nitrogen• for plants to use nitrogen - gaseous N2 must be converted into ammonium or

nitrate• ammonium and nitrate in the soil is NOT derived from rock• main source is the decomposition of humus by microbes

– nitrogen fixing bacteria – in plants and in the soil– work on organic material in the soil – e.g. feces, decomposing leaves etc…

• the nitrogen from proteins and other organic compounds is repackaged into inorganic materials and then absorbed as minerals in the soil

• some nitrogen is lost when soil microbes (dentrifying bacteria) converts nitrate ions to nitrogen gas – returns to the atmosphere

Nitrogen Fixation• three steps in N2 metabolism: nitrogen fixation,

nitrogen reduction and nitrogen assimilation• nitrogen fixation: conversion of N2 gas into nitrate

(NO3-), nitrite (NO2-) or ammonium (NH4+)– performed by living organisms and natural processes

• e.g. lightning can fix over 150 million tons of nitrogen annually– energy of a lightning strike in the air converts elemental

nitrogen to a useful form that dissolves in the water in the atmosphere

– falls to the ground as rain• fixation also performed by plants, nitrogen-fixing

bacteria and cyanobacteria (e.g. Nostoc, Anabaena)– both bacteria and cyanobacteria convert 130 million tons

of N2 into forms that animals and plants can use – e.g. NH4+

– plants: fix nitrogen in association with bacteria• these prokaryotes fix nitrogen for their own use but allow

the excess to the taken up by the plant – or becomes available when the prokaryote dies

• enzyme nitrogenase – converts N2 to NH3 where it picks up an extra H+ in the cell’s water = NH4+

• rate of nitrogen fixation depends on the stage of development of the plant

• as much as 90% of the total nitrogen fixed can take place during seed development

Alders are called pioneer plants because they are the first to grow in poor, nitrogen-deficient soils such as bogs, dunes and glacial rubble- obtain their N2 from a symbiosis with the prokaryote Frankia

• Nitrogen reduction: process of reducing nitrate ions (NO3-) to NH4+• if NH4+ is not used by the plant – it will be used by soil bacteria to make ATP• this converts the NH4+ to NO3-• must be converted back• NO3- first converted into NO2- (nitrite) then to NH4+• done by enzymes nitrate reductase and then nitrite reductase• expensive process – requires numerous electrons (from FAHD2 and NADH) that are no

longer available to make ATP

• Nitrogen assimilation: actual incorporation of ammonium into organic molecules (e.g. amino acids)

• usually occurs in roots – assimilated molecules then travel via vascular tissues

• similar to the electron transport chain – NH4+ gets transferred through a series of amino acid “carriers” to generater a new amino acid

• NH4+ passes to glutamate to create glutamine• glutamine transfers the NH4+ to a-ketoglutarate re-generating glutamate• glutamate transfers the NH4+ to a final acceptor – generating a new amino

acid– e.g. NH4+ transferred to pyruvate = alanine is produced

Nitrogen Reduction & Assimilation

Bacteria and nitrogenBacteroidswithinvesicle

Nodules

Roots

Pea plant root.

5 µm

Infectedroot hair

Infectionthread

Rhizobiumbacteria

FormingBacteroids

Bacteroid

Developingroot nodule

NodulevasculartissueBacteroid

• nitrogen-fixing bacteria and symbiotic nitrogen fixation:– most efficient symbioses occur in the legume family– cells of the root are infected by nitrogen-fixing Rhizobium bacteria - in the form of bacteroids (bacteria contained within vesicles in the root cell)

– the bacteroids fix atmospheric N2 and supply it to the root as ammonium

– roots emit chemical signals that attract the bacteria & the bacteria emit signals which cause the root hairs to elongate and form an infection thread within the hair

– the bacteria enter via infection thread and form bacteroids

– the root develops swellings as a result = root nodules

– the nodule develops vascular tissue to supply the NH4+ to the rest of the plant

• symbiotic nitrogen fixation and agriculture:– the basis for crop rotation to restore nitrogen content in the

soil– first year: plant a legume crop– year 2: planting of a non-legume such as corn– year 3: an alfalfa– year 4: plant a legume crop to restore nitrogen

concentrations

Mycorrhizae and Plant Nutrition• Mycorrhizae are mutualistic associations of fungi and roots• The fungus benefits from a steady supply of sugar from the host plant• The host plant benefits because the fungus increases the surface area for water

uptake and mineral absorption• In ectomycorrhizae, the mycelium of the fungus forms a dense sheath over the

surface of the root and in between the root cells• In endomycorrhizae, microscopic fungal hyphae extend into the root• farmers and foresters often inoculate seeds with fungal spores to promote formation

of mycorrhizae

Epidermis

Fungalhyphae

Cortex

Endomycorrhizae.

Roothair

Endodermis

Vesicle

Casparianstrip

Arbuscules

Cortical cells10 µm

(LM, stained specimen)

Epidermis

Mantle(fungal sheath)

Fungalhyphaebetweencorticalcells

Endodermis

Mantle(fungalsheath)

Cortex

Ectomycorrhizae.

100 µm

(colorized SEM)

Epiphytes, Parasitic Plants, and Carnivorous Plants

• Some plants have nutritional adaptations that use other organisms in nonmutualistic ways

Staghorn fern, and epiphyte. This tropical fern (genus Platycerium) grows on large rocks, cliffs, and trees. It has two types of fronds: branched fronds resembling antlers and circular fronds that form a collar around the base of the fern.

Mistletoe, a photosynthetic parasite. Dodder, a nonphotosynthetic

parasite.Indian pipe, a nonphotosynthetic parasite.

DodderHost’s phloem

Haustoria

Venus’ flytrap.

Pitcher plants. Sundews.

Photosynthesis

6 CO2 + 12 H2O + Light energy C6H12O6 + 6 O2 + 6 H2 O

1. Light Reactions: light + water = O22. Stroma Reactions - Calvin Cycle: CO2 + ATP + NADPH = sugar

H2O

LIGHTREACTIONS

Chloroplast

Light

ATP

NADPH

O2

NADP+

CO2

ADPP+ i

CALVINCYCLE

[CH2O](sugar)

Photosynthesis• 6 CO2 + 12 H2O + Light energy C6H12O6 + 6 O2 + 6 H2 O

• requires the reduction of carbon – converting it into carbohydrate

• this will require 4 electrons and a good source of energy to reduce the carbon

• electrons come from water• energy comes from light• water and light do not act directly on CO2

– rather they create the intermediates ATP and NAPDH via light-dependent reactions

– the ATP and NADPH then interact with CO2 in the stroma reactions (formerly the dark reactions) to produce carbohydrates

Light• light is a small segment of the electromagnetic radiation spectrum

– from gamma rays to radio waves• the radiation can be thought of as a set of waves or as a set of energized

particles called photons– each wave has a specific wavelength and photons with specific energy levels

• in photosynthesis – specialized pigments are present to absorb wavelengths of radiation in the visible range

Visible light

Gammarays

X-rays UV Infrared Micro-waves

Radiowaves

10–5 nm 10–3 nm 1 nm 103 nm 106 nm1 m

(109 nm) 103 m

380 450 500 550 600 650 700 750 nm

Longer wavelength

Lower energy

Shorter wavelength

Higher energy

Photosynthetic Pigments: The Light Receptors

• Pigments are substances that absorb visible light• different pigments absorb different wavelengths• wavelengths that are not absorbed are reflected or transmitted

– Leaves appear green because chlorophyll reflects and transmits green light

• the pigments of photosynthesis are located in the chloroplast

Chloroplast

LightReflected light

Absorbed light

Transmitted light

Granum

• photosynthetic pigments: chlorophylls & carotenoids– chlorophyll a & chlorophyll b

• transfer absorbed light energy to electrons that then enter chemical reactions

Chlorophyll a

Chlorophyll b

Carotenoids

Wavelength of light (nm)

Absorption spectra

Abs

orp

tion

of

light

by

chlo

rop

last

pig

men

ts

400 500 600 700

• chlorophylls do not absorb light at short wavelengths (e.g. 400nm or less) – and little photosynthesis occurs at those wavelengths

• as wavelengths get longer – absorption increases and so does photosynthesis• chlorophyll a: peak absorptions at 425nm and 650nm• the accessory pigments – the carotenoids and chlorophyll b– absorb in wavelengths

not covered by the chlorophyll a– the absorbed energy is then passed on to chlorophyll a – broadens the absorption spectrum of

chlorophyll a• carotenoid: peak absorption from 480nm – 500nm• chlorophyll b: peak absorption at 480nm and 680nm• the shorter wavelengths of light have more energy to transfer to the electron in the

chlorophylls – they excite the electron to a higher “state”• and the electrons emit more energy as they return to the “ground” state

Photosynthesis:The Chloroplast

• Plastids: group of organelles that perform many functions– synthesis, storage and export– storage plastids for sugar = amyloplasts– plastids with bright red and yellow pigments =

chromoplasts• like mitochondria – comprised of an outer

and inner membrane– plus an inner fluid = stroma– also have ribosomes and DNA

• plastids that undergo photosynthesis – chloroplasts– known as the green plastids due to the presence

of chlorophylls• earliest chloroplasts are called proplastids

– once exposed to light – mature into chloroplasts• like mitochondria – the inner membrane of

the chloroplast is extensively folded to increase surface area for the enzymes of photosynthesis– these folded membranes are called thylakoid

membranes– a stack of thylakoid membranes = granum

• photosynthetic pigments are located in the thylakoid membranes

• thylakoid membrane of the chloroplast is the site for the pigments and enzymes of photosynthesis

• pigments & enzymes make up two photosystems (named in order of the discovery NOT their functional order)– photosystem I – occurs after PSII– photosystem II– each has a characteristic reaction center,

special chlorophyll a molecules and specific associated proteins

– PSII chlorophyll a = P680– PSI chlorophyll a = P700– absorbed light energizes these two

photosystems and induces a flow of electrons through the photosystems and other molecules built into the thylakoid membrane

– during the light reactions – there are two possible routes for this electron flow:

• noncyclic• cyclic

Thylakoid Membranes

CH3

CHO

in chlorophyll a

in chlorophyll b

Porphyrin ring:light-absorbing“head” of molecule; note magnesium atom at center

Hydrocarbon tail:interacts with hydrophobicregions of proteins insidethylakoid membranes of chloroplasts; H atoms not shown

Photosystems

Thylakoid

Photon

Light-harvestingcomplexes

Photosystem

Reactioncenter

STROMA

Primary electronacceptor

e–

Transferof energy

Specialchlorophyll amolecules

Pigmentmolecules

THYLAKOID SPACE(INTERIOR OF THYLAKOID)

Th

yla

koid

me

mb

rane

• pigments are located in light-harvesting complexes

• when light strikes any pigment – either chlorophyll a or an accessory pigment – the energy is transferred to a reaction center

• the reaction center contains a pair of chlorophyll a molecules that are different from the light harvesting complexes– in photosystem II = P680– in photosystem I = P700

• the energy excites the electrons of P680 or P700

• electrons are transferred to a series of electron acceptors located in the thylakoid membrane

• electrons are eventually transferred to a final acceptor = NADP+ reducing it to NADPH

Light Reactions: Non cyclic electron flow

• 1. a photon of light strikes the pigments in the thylakoid membrane (i.e. light-harvesting complex) and the energy is relayed via excited electrons to the two P680 chlorophyll a molecules in the reaction center of PSII– the electrons of P680 are excited to a higher

energy state (P680+)• 2. each excited electron from P680+ is captured

by a primary electron acceptor in the reaction center– called phaeophytin

• 3. water is split into two H+, two electrons and an oxygen atom– these electrons are transferred to P680 to replace

the electrons it has lost to the primary electron acceptor

– oxygen atoms combine to form O2• 4. each excited electron passes from the primary

electron acceptor of PSII to the reaction center of PSI via an electron transport chain comprised of a cytochrome complex and two cofactors called Pq (plastoquinone) and Pc (plastocyanin)

LightP680

e–

Photosystem II(PS II)

Primaryacceptor

[CH2O] (sugar)

NADPH

ATP

ADP

CALVINCYCLE

LIGHTREACTIONS

NADP+

Light

H2O CO2

En

erg

y o

f el

ectr

on

s

O2

e–

e–

+2 H+

H2O

O21/2

Pq

Cytochromecomplex

Electron transport chain

Pc

ATP

• 5. the exergonic “fall” of electrons to their lower energy state through the electron transport chain provides energy for the creation of ATP

• 6. light energy is also transferred to the PSI complex – photons are absorbed by the light-harvesting complex of the PSI system and this excites an electron within P700 (P700+)– this electron is captured by the primary acceptor of PSI & creates a “hole”

in p700– this hole in P700 is filled by one of the electrons that has reached the bottom

of the ETC of PSII• 7. the photoexcited electrons are passed from PSI down a second ETC

through a cofactor called ferredoxin (Fd) and ultimately to NADP+reductase

• 8. NADP+ reductase takes the electrons from Fd and passes them to NADP+ (2 electrons) reducing it to NADPH

LightP680

e–

Photosystem II(PS II)

Primaryacceptor

Ene

rgy

of e

lect

rons

e–

e–

+2 H+

H2O

O21/2

Pq

Cytochromecomplex

Electron transport chain

Pc

ATP

P700

e–

Primaryacceptor

Photosystem I(PS I)

e–e–

ElectronTransportchain

NADP+

reductase

Fd

NADP+

NADPH+ H+

+ 2 H+

Light

LE 10-14

ATP

Photosystem II

e–

e–

e–e–

Millmakes

ATP

e–

e–

e–

Ph

oto

n

Photosystem I

Ph

oto

n

NADPH

STROMA(Low H+ concentration)

Light

Photosystem II Cytochromecomplex

2 H+

LightPhotosystem I

NADP+

reductaseFd

PcPq

H2O O2

+2 H+

1/22 H+

NADP+ + 2H+

+ H+NADPH

ToCalvincycle

THYLAKOID SPACE(High H+ concentration)

STROMA(Low H+ concentration)

Thylakoidmembrane ATP

synthase

ATPADP

+P

H+i

[CH2O] (sugar)O2

NADPH

ATP

ADPNADP+

CO2H2O

LIGHTREACTIONS

CALVINCYCLE

Light

• as electrons pass from one carrier to another, H+ ions are pumped from the stroma and are deposited in the thylakoid space

• these H+ ions stored in the thylakoid space create a proton gradient• when H+ flows back down its gradient – an enzyme (ATP synthase) uses this

energy to create ATP from ADP

Chemiosmosis

SOUND FAMILIAR?

MITOCHONDRIONSTRUCTURE

Intermembranespace

MembraneElectrontransport

chain

Mitochondrion Chloroplast

CHLOROPLASTSTRUCTURE

Thylakoidspace

Stroma

ATP

Matrix

ATPsynthase

Key

H+ Diffusion

ADP + P

H+

i

Higher [H+]

Lower [H+]

http://highered.mcgraw-hill.com/olcweb/cgi/pluginpop.cgi?it=swf::535::535::/sites/dl/free/0072437316/120072/bio13.swf::Photosynthetic%20Electron%20Transport%20and%20ATP%20Synthesis

Cyclic Electron flow• under certain conditions – the cyclic electron flow path is an

alternative – short-circuit path• uses PSI but not PSII• electrons cycle back from ferroredoxin/Fd to the cytochrome complex

and continue on to the P700 chlorophyll• no production of NADPH and no release of O2• but cyclic flow does generate ATP• function??

– noncyclic flow produces NADPH and ATP is roughly equal amounts– the Calvin cycle consumes more ATP than NADPH – creates an ATP

“debt”– cyclic electron flow makes up the difference in the ATP– concentration of NADPH may regulate which pathway is taken

Photosystem IPhotosystem II ATP

Pc

Fd

Cytochromecomplex

Pq

Primaryacceptor

Fd

NADP+

reductase

NADP+

NADPH

Primaryacceptor

Non-cyclic and cyclic flow animations

• http://www.mcgrawhill.ca/school/applets/abbio/ch05/phothospo_cyclic_and_no.swf

Stroma Reactions

• light reactions – electron flow pushes electrons from water (low potential energy) to NAPDH (high potential energy)

• so at the end of the light reactions – produced two potential energy sources– ATP– NADPH

• NADPH and ATP shuttle this energy to the Calvin cycle for the production of sugar

• reactions are performed in the stroma of the chloroplast

• used to be called the dark reactions – no involvement of light – happens in the dark

Calvin cycle

sugar produced = glyceraldehyde-3-phosphate

Light

CO2H2O

Light reactions Calvin cycle

NADP+

RuBP

G3PATP

Photosystem IIElectron transport

chainPhotosystem I

O2

Chloroplast

NADPH

ADP+ P i

3-Phosphoglycerate

Starch(storage)

Amino acidsFatty acids

Sucrose (export)

• similar to the citric acid cycle – starting material is regenerated after molecules enter and leave the cycle– citric acid cycle is catabolic: breakdown

– oxidizes acetyl CoA and releases energy– Calvin cycle is anabolic: synthesizes

– builds sugar from smaller molecules and requires energy• spends ATP as a energy source and consumes NAPDH as an electron source• has three phases:

– Carbon fixation– Carbon reduction– Regeneration of the CO2 acceptor

Calvin cycle

– performed by C3 plants – since the first organic product made is a 3 carbon sugar

– 1. Carbon Fixation: incorporation of CO2 into a 5-carbon sugar called ribulose bisphosphate (RuBP)

• CO2 molecules are attached one at a time to RuBP

• done by the enzyme rubisco – the most abundant protein on Earth??

• produces a 6 carbon intermediate that is immediately broken down into two molecules of a 3 carbon sugar called 3-phosphogylcerate

[CH2O] (sugar)

NADPH

ATP

ADPNADP+

CO2

CALVINCYCLE

Input

CO2

(Entering oneat a time)

Rubisco

3 P PShort-lived

intermediate

Phase 1: Carbon fixation

6 P

3-Phosphoglycerate6 ATP

6 ADP

CALVINCYCLE

3

P P

Ribulose bisphosphate(RuBP)

3

6 NADP+

6

6 NADPH

Pi

6 P

1,3-BisphosphoglycerateP

6 P

Glyceraldehyde-3-phosphate(G3P)

P1G3P

(a sugar)Output

Phase 2:Reduction

Glucose andother organiccompounds

3

3 ADP

ATP

Phase 3:Regeneration ofthe CO2 acceptor(RuBP) P5

G3P

Calvin cycle

– 2. Carbon Reduction: each 3-phosphoglycerate receives an addition phosphate group from ATP (i.e. phosphoryalted) = 1,3-bisphosphoglycerate

• requires 6 molecules of ATP• next - a pair of electrons from NADPH

reduces 1,3-bisPG to make the 3 carbon end-product called glyceraldehye 3-phosphate (G3P)

• this consumes 6 molecules of NADPH• the aldehyde group stores more potential

energy than the bonds of 1,3-bisPG• G3P is the same intermediate produced

upon the splitting of glucose during glycolysis

– 3. Regeneration of the CO2 acceptor: a series of complex steps converts the carbon skeletons of 5 molecules of G3P into three molecules of ribulose bisphosphate

• RuBP is the carbon acceptor of carbon fixation

• cycle spends three more molecules of ATP

[CH2O] (sugar)

NADPH

ATP

ADP

NADP+

CO2

CALVINCYCLE

Input

CO2

(Entering oneat a time)

Rubisco

3 P P

Short-livedintermediate

Phase 1: Carbon fixation

6 P

3-Phosphoglycerate6 ATP

6 ADP

CALVINCYCLE

3

P P

Ribulose bisphosphate(RuBP)

3

6 NADP+

6

6 NADPH

Pi

6 P

1,3-BisphosphoglycerateP

6 P

Glyceraldehyde-3-phosphate(G3P)

P1G3P

(a sugar)Output

Phase 2:Reduction

Glucose andother organiccompounds

3

3 ADP

ATP

Phase 3:Regeneration ofthe CO2 acceptor(RuBP) P5

G3P

-for the net synthesis of one G3P – the Calvin cycle consumes 9 ATP and 6 molecules of NAPDH and makes 1 molecule of sugar

http://www.science.smith.edu/departments/Biology/Bio231/calvin.html

Arid plants and photosynthesis

• in most plants the initial fixation of carbon occurs by rubisco = C3 plants– e.g. rice, wheat and corn– during a dry, hot day - their stomata are partially closed

• these plants produce less sugar due to declining levels of CO2 in the leaf (starves the Calvin cycle)

• in addition, rubisco can bind O2 in place of CO2 – results in a two carbon compound that exits the chloroplast

• the peroxisomes and mitochondria rearrange this 2 carbon compound to regenerate CO2 = photorespiration

– photorespiration – consumes O2 and produces CO2 & occurs in the light

– photorespiration in C3 does NOT generate ATP and does NOT produce sugar – so why do it???

• may be evolutionary baggage – relic from an earlier time when the atmosphere has less O2 and more CO2 than it does today

• not known currently whether photorespiration benefits the plant

Arid plants and photosynthesis: C4 plants

• in C4 plants the Calvin cycle is prefaced with an alternate mode of carbon fixation and this results in a 4-carbon product– C4 plants have a unique leaf anatomy– two distinct types of photosynthetic cells: bundle-sheath cells and

mesophyll cells– bundle-sheath cells are arranged as sheaths around the vascular bundles

with mesophyll cells in between these BS cells and the leaf surface– sugar is produced in a three step process:

Arid plants and photosynthesis: C4 plants• 3 step process in C4 plants:

– 1. CO2 enters the mesophyll cells of the leaf and is added to a 3 carbon substrate called PEP (phosphophenolpyruvate) to eventually generate a 4 carbon sugar (malate)

• done by the enzyme called PEP carboxylase • CO2 addition to PEP produces a 4 carbon compound called oxaloacetate which is

then converted into a 4 carbon sugar called malate– 2. malate enters the bundle sheath cells & is converted back into a 3 carbon

sugar called pyruvate– 3. this results in the liberation of CO2 which then enters the Calvin cycle for

the production of 3-glyceraldehyde phosphate– in arid climates the BS sheath cells essentially pump CO2 into the cell to keep

the CO2 levels high in the leaf and ensure an efficient Calvin cycle

Photosyntheticcells of C4 plantleaf

Mesophyll cell

Bundle-sheathcell

Vein(vascular tissue)

C4 leaf anatomy

StomaBundle-sheathcell

Pyruvate (3 C)

CO2

Sugar

Vasculartissue

CALVINCYCLE

PEP (3 C)

ATP

ADP

Malate (4 C)

Oxaloacetate (4 C)

CO2PEP carboxylaseMesophyllcell

• CAM plants – succulents, many cacti, pineapples– open their stomata at

night only– incorporate the CO2 at night

into a variety of organic acids through the crassulacean acid metabolic (CAM) pathway

– the mesophyll cells store the organic acids they make during the night in vacuoles

– in the morning when the stomata close – the light reactions in CAM plants supply ATP and NADPH and the organic acids release and supply the CO2 so they can enter the Calvin cycle

Bundle-sheathcell

Mesophyllcell Organic acid

C4

CO2

CO2

CALVINCYCLE

Sugarcane Pineapple

Organic acidsrelease CO2 toCalvin cycle

CO2 incorporatedinto four-carbonorganic acids(carbon fixation)

Organic acid

CAM

CO2

CO2

CALVINCYCLE

Sugar

Spatial separation of steps Temporal separation of steps

Sugar

Day

Night

Virtual Lab – Plant Transpiration

• http://www.mcgrawhill.ca/school/applets/abbio/ch05/phothospo_cyclic_and_no.swf