PHOTOSYNTHESIS

Fundamental biological processes

for making and using energy

Photosynthesis: process by

which plants convert radiant

energy to chemical energy

Respiration: process by which

glucose molecules are broken

down and stored energy is

released

Photosynthesis - autotrophs make glucose

Respiration – organisms break down glucose

TYPES OF ORGANISMS BY ENERGY

PRODUCTION

Autotrophs- organisms that produce organic

molecules from inorganic substances (photosynthesis)

- Photoautotrophs- use light energy to make food (plants, algae, cyanobacteria)

- Chemiautotrophs-oxidize inorganic chemicals to drive food making reactions (bacteria, fungi)

Heterotrophs

- organisms that obtain energy

from other organisms

(heterotrophs or autotrophs)

- do not make own food

Location of photosynthesisChloroplast- double membrane organelle

Thylakoid discs (photosystem: 200-300

thylakoids)

- Harvest sunlight

- Contains chlorophyll and

accessory pigments

- Photosystem I and II are linked

structurally and functionally

Grana (stacks of thylakoid discs)

location of light reactions

Stroma (protein rich solution, outside grana)

location of Calvin Cycle

Mesophyll: location of chloroplasts

Stomata: pores in leaf CO2 enters/ O2 exits

Chlorophyll: pigiment in thylakoids

PHOTOSYNTHESIS

6 CO2 + 6 H2O + light energy → C6H12O6 + 6 O2

process whereby autotrophs (plants) take in light energy and convert it to chemical energy (sugar)

Redox Reactions

water is split → e- transferred with H+ to CO2 → sugar

biochemical pathway- series of linked redox reactions where product of one reaction is consumed by the next reaction

Endergonic- absorbs solar energy

Exergonic- releases energy for organism

Tracking Atoms through Photosynthesis

Evidence that chloroplasts split water molecules enabled

researchers to track atoms through photosynthesis

(C.B. van Niel)

Reactants:

Products:

6 CO2 12 H2O

C6H12O6 6 H2O 6 O2

6 CO2

Photosynthesis = Light Reactions + Calvin Cycle

“photo” “synthesis”

energy building sugar building

reactions reactions

Light Energy and PigmentsComes from radiation (energy that travels in waves) from the sun

photon- particles which have energy

wavelength- crest to crest of wave

sunlight- mixture of all visible wavelengths

white light- all wavelengths reflected equally so looks white

visible spectrum- all colors of white light

Pigment: substance that absorbs light

- photosynthesis: absorbed light energy is used to make chemical

bond energy

- wavelengths not absorbed are reflected (color we see)

Absorption spectrum

graph plotting pigment light absorption vs wavelength

representation of how well particular pigment absorbs different

wavelengths of white light

Photosynthetic pigments

chlorophyll a (blue green)

- primary photosynthetic pigment

- directly involved in converting light → chemical energy

- hides other pigments

chlorophyll b (yellow green)

- accessory pigment

- absorbs light and transfers energy to chlorophyll a

carotenoids (orange, yellow)

- xanthophylls (yellow) / carotenes (orange)

- accessory pigments

- converts energy to chloro. a

- seen in autumn when chloro. breaks down

- photoprotection for chlorophyll

anthocyanin (red, purple, blue): antioxidants

- non photosynthetic parts of plant (flowers/fruits)

- absorb different pigments so we see other colors

Determining Absorption Spectrum

Action SpectrumAction spectrum

plots rate of photosynthesis of

different wavelengths,

i.e. CO2 consumption , O2 release

(different than absorption

spectrum)

Englemann’s experiment:

Used alga and bacteria

Measured O2 output

Result: violet-blue and red

wavelengths caused

most photosynthesis

Electron Excitement

-light is made of photons

(particles which carry fixed amount of energy)

-when light strikes chlorophyll , some of its atoms absorb the photons

-energy is transferred to the atoms

electrons and excites them to jump to next level

* *Move from ground state to excited state**

- excess energy is released as light or heat

Photosystems

Photosystem: reaction center (proteins that hold special pair of

chlorophyll a molecules) + light harvesting complexes (cloro. a & b,

carotenoids bound to proteins)

- located in thylakoid discs

- absorb light energy

Primary electron acceptor: accepts electrons and becomes reduced

(electrons move to higher energy level)

Photosystem I: chlorophyll a absorbs at 700 nm- far red (p700)

Photosystem II: chlorophyll a absorbs at 680 nm- red (p680)

Overview of Stages of Photosynthesis

2 Stage Process

1. light reactions (needs light)

- occurs in thylakoid membranes

4 basic steps

- sun’s energy is trapped by chlorophyll

- electron transport (linear/cyclical)

- water is split and oxygen is released (O2 production)

- ATP and NADPH are formed and released into stroma

- purpose: to make ATP and NADPH(energy carrier molecule)

2. Calvin Cycle:

- occurs after light reactions

- can occur in light or dark

3 basic steps

- carbon fixation to glucose

- reduction of NADP to

NADPH

- regeneration of RuBP

to start cycle over again

Light Reactions Electron Flow

Two routes for electron flow:

A. Non-cyclic (linear) electron flow

B. Cyclic electron flow

animation

STEPS OF LIGHT REACTION

1. photosystem II absorbs light and excites electrons of chlorophyll a

- molecules and electrons are forced to higher energy level(reaction center)

***purpose of photosystem II is to generate ATP and supply electrons to photosystem I ***

2. excited electrons leave chlorophyll a molecule (oxidation reaction)

3. primary electron acceptor sends electrons into ETC

- reduction reaction

- chain uses energy of electrons to make ATP

- water is split (photolysis) and O2 is released into atmosphere

- electrons from water replace those lost in Photosystem II

- pumps H+ ions (from splitting of water) to interior of grana (lumen)

- inside grana , there is a high concentration (proton gradient) of H+ ions

- chemiosmosis occurs (making of ATP)

- H+ ions back move across grana membranes (ATPsynthase)

LINEAR ELECTRON FLOW

NON-CYCLICAL PHOTOPHOYPHORYLATION

animation

4. at end of ETC, electrons are passed to photosystem I thru the

(cytochrome complex = plastiquinone Pq (e- carrier) and plastocyanin Pc (protein)

- photosystem also absorbs light to excite electrons in chloro. A

- electrons go thru separate electron transport chain in photosystem I to a different primary electron acceptor

ferradoxin Fd (protein) facilitates movement of e-

- purpose of photosystem I is to generate NADPH

5. NADP+

- accepts electrons and H+ ions (reduces it to NADPH)

- NADPH and ATP move into stroma

LINEAR ELECTRON FLOW

NON-CYCLICAL PHOTOPHOYPHORYLATION

Cyclic Electron Flow:

• uses PSI only

• produces ATP for Calvin Cycle

animation

Non cyclic

•PS II and I

•Reaction center is P680

•Both ATP and NADPH are produced

•Photolysis (splitting) of water occurs

•O2 is by-product

•Predominant in green plants

Cyclic

•PS I only

•Reaction center is P700

•Electrons travel back to PS I

•Only ATP is produced

•No photolysis of water

•No O2 involved

•Predominant in bacteria

End products of light

reactions

1. ATP and NADPH:

needed to power dark

reactions

2. O2:

by product

released into atmosphere

Chemiosmosis in Chloroplasts and Mitochondria

Respiration and photosynthesis use chemiosmosis to generate ATP

ETCs pump protons (H+)

across membrane from

areas of low concentration

to high concentration

Protons then diffuse back

across membrane thru

ATPsynthase to make ATP

H+ reserviors for each organelle

mitochondria- matrix

chloroplast – lumen

Mitochondria:high energy e- come from organic molecules

Chloroplasts: high energy e- come from water

Thylakoid Membrane Organization

Proton motive force (H+ gradient) generated by:

(1) H+ from water

(2) H+ pumped across by cytochrome

(3) Removal of H+ from stroma when NADP+ is reduced

animation

Calvin Cycle- light independent: can occur in light or darkness, always

after light rxns

- occurs in stroma

- purpose: Carbon fixation to glucose molecule (from

CO2 in atmosphere)

- Uses ATP and NADPH

- 3 Phases

1. carbon fixation

2. reduction

3. regeneration of RuBP (CO2 acceptor)

calvin cycle animation

Steps of Calvin Cycle

1. Carbon fixation

- 3 CO2 enters plant from

atmosphere and binds with

RuBP ribulose biphosphate

(5 C sugar)

- catalyzed by rubisco enzyme

( RuBP carboxylase)

- forms unstable 6 C intermediate

sugar

- this splits into 2 PGA per CO2

net: 6 PGA

2. Reduction (PGA to G3P)

2 steps

- each PGA gets phosphate

from ATP

- then each molecule reacts with H

from NADPH and breaks

phosphate bond

- net gain: 1 molecule of G3P/PGAL

(glyceraldehydre 3 phosphate)

- 6 G3P formed, but

- 1 molecule used to make sugar

- 5 molecules used to regenerate RuBP

6 ATP and 6 NADPH needed to produce 1 net G3P/PGAL

3. Regeneration of RuBP from G3P

- 1 G3P/PGAL placed on glucose

- 5 G3P/PGAL used to regenerate RuBP

(cyclical – continues over and over again)

3 ATP needed to regenerate RuBP

End product of Calvin Cycle

Glucose

*6 turns of cycle needed to make 1 molecule of glucose*

Calvin Cycle uses:

3 CO2, 9 ATP, 6 NADPH

animation

1 C6H12O6 molecule = 6 CO2, 18 ATP, 12 NADPH

Vcell photosynthesis video

Photorespiration

• wasteful pathway that competes with Calvin cycle

• takes place at the same time that photosynthesis does

• occurs under low CO2/ dry conditions (closed/partially closed stomata)

• rubisco acts on O2 instead of CO2

• 25% less glucose produced

• C4 pathway is a result of evolution

- early plants had high CO2 levels that dropped over time so

plants evolved a more efficient type of photosynthesis to cope

• C4 and CAM help minimize photorespiration

photorespiration video

Evolutionary Advantages

- Calvin cycle is most common pathway for carbon fixation

- C3 plants: plants that fix C thru calvin cycle (because of 3 C PGA that is initially formed)

- other plants fix C through alternative pathways and then release it into Calvin cycle

- alternative pathways found in plants in dry hot climates

- these plants use STOMATA (pores on undersurface of leaves)

- major passageways thru which O2 and CO2 goes in and out- major passageways of water loss

Alternative Pathways of Carbon Fixation1. C4 Pathway

- during hottest part of day, stomata are partially closed

- plants fix CO2 into 4 C compounds when CO2 is low

- mesophyll: PEP carboxylase fixes CO2 (4-C)

pumps CO2 to bundle sheath

- bundle sheath: CO2 used in Calvin Cycle

- lose less water than C3 plants

- corn, sugarcane, crabgrass

advantage in hot sunny areas

Alternative Pathways of Carbon Fixation,

cont.2. CAM Pathway

- Crassulacean acid metabolism (CAM)

- stomata open at night, closed during day

- night: plants take in CO2 and fix into many

compounds, stored in mesophyll cells

- day: light reactions supply ATP, NADPH; CO2

released from organic acids for Calvin cycle

- lose less water than C3 and C4 plants

- cactus, pineapples

advantage in arid areas

C fixation and Calvin

Cycle together

C fixation and Calvin

Cycle in different

CELLS

C fixation and Calvin

Cycle at different

TIMES

Factors Affecting Rate of Photosynthesis

Environmental Variables

1. Light intensity/ direction of incoming light

- high intensity = high rate

- saturation point: levels off after certain

intensity because pigments can only

absorb so much light

2. Light color

3. CO2 levels

- same mechanism as light

4. Temperature

- higher temp = higher rate unless enzymes denature

5. pH of leaf

Factors Affecting Rate of Photosynthesis

Plant Variables

1. leaf color/ variegation- amount of chlorophyll

2. leaf size

3. stomata density and distribution

4. leaf age