Plant Physiology

Photosynthesis, the light reaction

Photosynthesis is the process that converts solar energy into chemical energy• Nourishes almost the entire living world• Plants, algae

Autotrophs sustain themselves without eating anything derived from other organisms• Producers of the biosphere, make organic molecules

from CO2 and other inorganic molecules• Almost all plants are photoautotrophs, using the

energy of sunlight to make organic molecules from H2O and CO2

Photosynthesis

Heterotrophs obtain their organic material from other organisms• Consumers of biosphere• Almost all Heterotrophs depend on photoautotrophs

for food and O2

Photosynthesis converts light energy into chemical energy of food

•Chloroplasts are structurally similar to and likely evolved from photosynthetic bacteria•The structural organization of these cells allows for the chemical reactions of photosynthesis

Chloroplasts: The Sites of Photosynthesis in Plants

• Leaves are the major locations of Photosynthesis• Their green color is from Chlorophyll, the green

pigment within chloroplasts• Light energy absorbed by chlorophyll drives the

synthesis of organic molecules in the chloroplast• CO2 enters and O2 exits the leaf through

microscopic pores called Stomata

Chlorophylls are Leaf Pigments

• Chlorophylls collect light energy (absorbs it) in a resonant porphyrin group that hangs out like a kite on the surface of the thylakoid

• Chlorophyll a initiates the light-dependent reactions, primary pigment in plants and cyanobacteria

-absorbs violet-blue and red light • Chlorophyll b secondary pigment absorbing light

wavelengths that chlorophyll a does not absorb• Carotenoids are yellow and orange pigments that capture

light energy and pass electrons to chlorophyll

The Structure of Chlorophyll

• The positive charges on the Mg2+ ion attract electrons

• The electrons bounce around the porphyrin ring

Pigments (molecules that absorb visible light)

Each pigment has a characteristic absorption spectrum, the range and efficiency of photons it is capable of absorbing.

photon: a particle of light-acts as a discrete bundle of energy-energy content of a photon is inversely proportional to the wavelength of the light

photoelectric effect: removal of an electron from a molecule by light-occurs when photons transfer energy to electrons

Absorption of Light Energy• Light energy is absorbed by electrons

• The energy causes electrons to jump shells; the more energy absorbed, the further away electrons move from the nucleus

• The energy may be shed as fluorescence• Or transferred in the form of an electron to another

molecule

How Electrons Capture Energy• Electrons can absorb radiant energy.– Radiant energy comes in parcels called photons

• When electrons absorb energy, they hop to a higher shell.

• When electrons release energy, drop back to the lower shell.

• The energy released is a kind of light energy called fluorescence.

The Visible Spectrum of Light

• Photosynthesis uses only small visible portion of the electromagneticspectrum

• Wavelengths of visible light most important forphotosynthesis.

• The symbol for wavelength is λ

Energy for all life on Earth ultimately comes from photosynthesis.

6CO2 + 12H2O C6H12O6 + 6H2O + 6O2

Oxygenic photosynthesis is carried out by:cyanobacteria, 7 groups of algae, all land plants

sunlight

Internal Structure of a Leaf

Main site of Photosynthesis

The Chloroplast

1. The site of light harvesting or energy capture

2. The site of the start of carbohydrate synthesis

(Inside of thylakoid)

Photosynthesis Overview

• Photosynthesis takes place in chloroplasts.

• thylakoid membrane – internal membrane arranged in flattened sacs– contain chlorophyll and other pigments– Organized into photosystems

• Capture light and transfer energy (to pigment molecules)

• grana – stacks of thylakoid membranes• stroma – semiliquid substance surrounding thylakoid

membranes (houses the enzymes to make organic molecules)

Photosynthesis in the

Chloroplast

• The light-dependent reactions (the harvesting of light) occur on thylakoid membranes

• The carbon fixation reactions (formation of carbohydrate) occur in the stroma

Photosynthesis Overview

• Photosynthesis takes place in 3 stages:– Capturing energy from sunlight– Using the energy to make ATP and reduce NADP+

to NADPH• (nicotinamide adenine dinucleotide phosphate)

– Using the ATP and NADPH to synthesize organic molecules (glucose) from CO2

Photosynthesis Overview

Photosynthesis is divided into:light-dependent reactions

-capture energy from sunlight-make ATP and reduce NADP+ to NADPH

carbon fixation reactions (light-independent reactions)-use ATP and NADPH to synthesize organic molecules from CO2

Photosynthesis Overview

• Photosynthesis takes place in the green portions of plants– Leaf of flowering plant contains mesophyll tissue– Cells containing chloroplasts– Specialized to carry on photosynthesis

• CO2 enters leaf through stomata

– Diffuses into chloroplasts in mesophyll cells– In stroma, CO2 fixed to C6H12O6 (sugar)

– Energy supplied by light

Photosystem OrganizationA photosystem consists of1. an antenna complex (light harvesting

complex) of hundreds of accessory pigment molecules that gather photons and feeds energy to reaction center

2. a reaction center of one or more chlorophyll a molecules pass electrons out of photosystem (photochemical reactions)

In summary, energy of electrons is transferred through the antenna complex to the reaction center.

Photosystem OrganizationAt the reaction center (transmembrane

protein complex), the energy from the antenna complex is transferred to chlorophyll a.

This energy causes an electron from chlorophyll to become excited.

The excited electron is transferred from chlorophyll a to an electron acceptor.

Water donates an electron to chlorophyll a to replace the excited electron.

Converting light to chemical energy

pheophytin

Photosynthesis Overview

•Light Reactions• Two electron pathways operate in the

thylakoid membrane: the noncyclic pathway and the cyclic pathway.• Both pathways produce ATP; only the

noncyclic pathway also produces NADPH. • ATP production during photosynthesis is

called photophosphorylation; therefore these pathways are also known as cyclic and noncyclic photophosphorylation.

• Takes place in thylakoid membrane• Uses two photosystems, PS-I and PS-II (consists of pigment

complexes)• PS II captures light energy• Causes an electron to be ejected from the reaction center

(chlorophyll a)– Electron travels down electron transport chain to PS I– Replaced with an electron from water– causes H+ to concentrate in thylakoid chambers– causes ATP production

• PS I captures light energy (electrons and H)– Transferred permanently to a molecule of NADP+

– Causes NADPH production

Light Reactions:The Noncyclic Electron Pathway

Light Reactions:Noncyclic Electron Pathway

• Uses only photosystem I (PS-I)• Begins when PS I complex absorbs solar energy• Electron ejected from reaction center– Travels down electron transport chain– Causes H+ to concentrate in thylakoid chambers– Which causes ATP production– Electron returns to PS-I (cyclic)

• Pathway only results in ATP production

Light Reactions:The Cyclic Electron Pathway

Light Reactions:Cyclic Electron Pathway

•The Organization of the Thylakoid Membrane• PS II consists of a pigment complex and electron-acceptor

molecules; it oxidizes H2O and produces O2.• The electron transport system consists of cytochrome

complexes and transports electrons and pumps H+ ions into the thylakoid space.• PS I has a pigment complex and electron-acceptor

molecules; it is associated with an enzyme (oxidoreductases) that reduces NADP+ to NADPH.• ATP synthase complex has an H+ channel and ATP synthase; it

produces ATP.

• Thylakoid space acts as a reservoir for hydrogen ions (H+)• Each time water is oxidized, two H+ remain in the thylakoid space• Electrons yield energy– Used to pump H+ across thylakoid membrane– Move H+ from stroma into the thylakoid space

• Flow of H+ back across thylakoid membrane– Energizes ATP synthase– Enzymatically produces ATP from ADP + P

• This method of producing ATP is called chemiosmosis

ATP Production