Photosynthesis

Chapter 9

Photosynthesis: The Big Picture

Source of BOTH matter and energy for most living organisms

Captures light energy from the sun and converts it into chemical energy

Synthesized organic molecules from inorganic molecule

BOTTOM LINE: Makes FOOD

Autotroph: Organisms that make their own food (energy-rich organic

molecules) from simple, inorganic molecules

Photoautotroph: Organisms that make their own food through

photosynthesis; obtain energy from the sun Type of autotroph

Heterotroph: Get carbon and energy by eating autotrophs or one

another

Definitions

Photoautotrophs

Capture sunlight energy and use it to carry out photosynthesisPlantsSome bacteria

cyanobacteriaMany protistans

algae

Plants

Algae (spirogyra)

Cyanobacteria

Algea (Kelp)

Linked Processes

Photosynthesis

Energy-storing pathway

Releases oxygen

Requires carbon dioxide

Aerobic Respiration

Energy-releasing pathway

Requires oxygen

Releases carbon dioxide

Photosynthesis Equation

6H2O + 6CO2 6O2 + C6H12O6

Water Carbon Dioxide

Oxygen Glucose

LIGHT ENERGY

In-text figurePage 115

Chloroplast Structure

Inner and outer membranes

Thylakoid

Stroma

Figure 7.3d, Page 116

Granum(Grana)

Light dependent reactions Converts light energy into chemical energy (ADP ATP) Gathers e- and H+ from water (NADP+ NADPH) Occurs in thylakoid membranesLight independent reactions (Calvin-Benson Cycle) Reduces CO2 to synthesize glucose using energy and

hydrogens (i.e. ATP and NADPH) generated in the light dependent reaction

Occurs in Stroma

Notice that these reactions do not create NADH, but rather NADPH

Two Stages of Photosynthesis

Two Stages of Photosynthesis

sunlight water uptake carbon dioxide uptake

ATP

ADP + Pi

NADPH

NADP+

glucoseP

oxygen release

LIGHT-INDEPENDENT

REACTIONS

LIGHT-DEPENDENT REACTIONS

new water

In-text figurePage 117

Electromagnetic Spectrum

Shortest Gamma rays

wavelength X-rays

UV radiation

Visible light

Infrared radiation

Microwaves

Longest Radio waves

wavelength

Visible Light Electromagnetic energy with a wavelength of

308-750nm Light energy is organized into packets called

photons The shorter the wavelength the greater the

energy carried by the photons

Properties of Light

White light (from the sun) contains all of the wavelengths of light

When light hits matter, it can be reflected (transmitted) or absorbedWhite substances reflect all lightBlack substances absorb all light

Pigments

A substance that absorbs light We see the color that is transmitted by

pigment The absorbed color disappears into

pigment

Plant pigments

Plant use a variety of pigments during photosynthesis:Chlorophylls a and bCarotenoidsAnthocyaninsPhycobilins

The main photosynthetic pigment is Chlorophyll a

ChlorophyllsW

avel

eng

th a

bso

rpti

on

(%

)

Wavelength (nanometers)

chlorophyll b

chlorophyll a

Chlorophyll a absorbs red and blue light, and reflects green light (what we see)

Note: The colors that are absorbed are used for photosynthesis

Figure 7.6a Page 119

Figure 7.7Page 120

Effect of Light on Pigments

What happens when light hits pigments? The color disappears, but the energy does not Absorbing photons of light excites electrons (e-),

thus adding potential energy Ground state: normal pigment Excited state: pigment absorbing light (e- excited)

e-e-

Photon of light:

Atom in pigment:Ground state

Atom in pigment:Excited state

Photosystems

In thylakoid membrane, pigments are organized in clusters called photosystems

These clusters contain several hundred pigment molecules

Two types of photosystems Photosystem I = P700 (absorbs light at 700nm)Photosystem II = P680 (absorbs light at 680nm)

Reaction Center Chlorophyll

One of the pigments in each photosystem is known as the reaction center chlorophyll (RCC)

if any pigment within the photosystem gets hit by a photon, the energy is transferred to the RCC

The RCC will then transfer its excited e-

into an electron transport chain

Pigments in a Photosystem

reaction center

Figure 7.11Page 122

Light Dependent Reactions

Location: the thylakoid membranes Function: to generate ATP (energy!) and

NADPH (reducing power!) that will be used in the light independent reaction

Two processes: Non-cyclic electron flow

Generates ATP and NADPH Cyclic electron flow

Generates only ATP

Noncyclic Electron Flow

Two-step pathway for light absorption and electron excitation

Uses two photosystems: type I and type II

Produces ATP and NADPH Involves photolysis - splitting of water

Machinery of Noncyclic Electron Flow

photolysis

H2O

NADP+ NADPH

e–

ATP

ATP SYNTHASE

PHOTOSYSTEM IPHOTOSYSTEM II ADP + Pi

e–

first electron transfer chain

second electron transfer chain

Figure 7.13aPage 123

Steps of Non-cyclic electron flow

Photosystem II gets hit by a photon; electron of RCC gets excited

The excited (high energy) e- gets picked up by an electron carrier and taken into an electron transfer chain (ETC)

The excited e- provides energy to pump protons (H+) into the thylakoid (tiny space)

Through chemiosmosis, ATP is generated

Chemiosmotic Model of ATP Formation Electrical and H+ concentration gradients

are created between thylakoid compartment and stroma

H+ flow down gradients into stroma through ATP synthase

The energy driven by the flow of H+ powers the formation of ATP from ADP and Pi

Chemiosmotic Model for ATP Formation

ADP + Pi

ATP SYNTHASE

Gradients propel H+ through ATP synthases;ATP forms by phosphate-group transfer

ATP

H+ is shunted across membrane by some components of the first electron transfer chain

PHOTOSYSTEM II

H2Oe–

acceptor

Photolysis in the thylakoid compartment splits water

Figure 7.15Page 124

Non-cyclic electron flow: Photolysis While Photosystem II gets hit by light, etc.,

water is split:

H2O ½ O2 + 2H+ + 2e-

This process is called photolysis The H+ are pumped into the thylakoid to

create the proton gradient The e- replace the excited e- that was

taken away from the RCC

Non-Cyclic Electron Flow:The saga continues Photosystem I gets excited at the same

time as photosystem II Its excited e- gets taken into a second

electron transfer chain that attaches the excited e- and the leftover H+ to NADP+ to make NADPH:

NADP+ + H+ + e- NADPH

Non-cyclic electron flow

The “electron hole” in photosystem I is then filled with the used up, low energy e- from photosystem II

Now everything is back to normal, and we can start all over again

Energy Changes in Non-cyclic electron flow

Figure 7.13bPage 123

Po

ten

tial

to

tra

nsf

er e

ner

gy

(vo

lts)

H2O 1/2O2 + 2H+

(Photosystem II)

(Photosystem I)

e– e–

e–e–

secondtransfer

chain

NADPHfirst

transferchain

Non-cyclic electron flow: Summary After two excited photosystems, two ETCs

and the splitting of water, both ATP and NADPH are generated!!!

Cyclic electron flow

The light independent reactions require more ATP than NADPH

Cyclic electron flow is like a short cut to making extra ATP

Involves only Photosystem I

Cyclic electron flow

Photosystem I gets excited Excited e- is carried into the first ETC;

energy goes to pump H+ into thylakoid compartment

Chemiosmosis powers formation of ATP The same e- (now low energy) replaces

itself in the “electron hole” in Photosystem I

Cyclic electron flow

photolysis

H2O

NADP+ NADPH

e–

ATP

ATP SYNTHASE

PHOTOSYSTEM IPHOTOSYSTEM II ADP + Pi

e–

first electron transfer chain

second electron transfer chain

Figure 7.13aPage 123

Light dependent reactions:Summary Non-cyclic electron flow

Generates ATP and NADPHUses both photosystems (P680 and P700)

and both electron transport chains Involves photolysis

Cyclic electron flowGenerates ATP onlyUses only P700 and only one ETC

Synthesis part of photosynthesis

Can proceed in the dark

Take place in the stroma

Also called Calvin-Benson cycle, or

Calvin Cycle, or Dark Reactions

Light-Independent Reactions

Calvin-Benson Cycle

Three Phases:

1. Carbon Fixation

2. Reduction

3. Regeneration of RUBP

Calvin-Benson Cycle: Carbon Fixation

Capturing atmospheric (gaseous) CO2 by attaching it to RuBP, a 5-carbon organic molecule

This process forms two 3-carbon molecules

The enzyme that catalyzes this process is called Rubisco

Calvin Benson Cycle:Reduction The captured CO2 has very little energy and no

hydrogens In order to make sugar, energy and hydrogens

need to be added to the molecules formed by Carbon fixation

ATP and NADPH (made in the light dependent reactions) break down to form ADP and NADP+ and, in the process, transfer energy and hydrogens to the 3-carbon compounds formed by carbon fixation, resulting in sugar formation

Calvin-Benson Cycle:Regeneration Some of the sugar created by reduction

leaves the Calvin cycle, and is used to build up glucose and other organic molecules

The rest of the sugar is used to remake (regenerate) RuBP

This process requires ATP (which was made in the light dependent reactions)

Calvin-Benson Cycle:Summary

The cycle proceeds 6 times to form each molecule of glucose

In the process, ATP and NADPH is used up 6CO2 are converted into C6H12O6 - glucose

In Calvin-Benson cycle, as described, the first stable intermediate is a three-carbon PGA

Because the first intermediate has three carbons, the pathway is called the C3 pathway

The C3 Pathway

Photorespiration in C3 Plants

On hot, dry days stomata (holes in the leaf) close to prevent evaporation of water

As a result, within the leaf oxygen levels rise, and Carbon dioxide levels drop

Rubisco attaches RuBP to oxygen instead of carbon dioxide

Results in a VERY wasteful process known as Photorespiration – uses up ATP without generating sugar

C4 and CAM Plants

To avoid photorespiration, plants that live in hot, dry climates evolved mechanisms to separate carbon fixation from the Calvin Cycle

The CO2 that enters the Calvin cycle is derived from the breakdown of previously synthesized organic acids

In this way, the enzyme that catalyzes the reaction that attaches CO2 to RuBP is not exposed to atmospheric oxygen

C4 and CAM Plants C4 plants (grasses) do carbon fixation in a different

location (cell type) than the Calvin cycle CAM plants (succulents and Cacti) do carbon fixation

at a different time (night) that the Calvin cycle (day)

Summary of Photosynthesis

Figure 7.21Page 129

light6O2

12H2O

CALVIN-BENSON CYCLE

C6H12O6

(phosphorylated glucose)

NADPHNADP+ATPADP + Pi

PGA PGAL

RuBP

P

6CO2

end product (e.g., sucrose, starch, cellulose)

LIGHT-DEPENDENT REACTIONS

6H2O

LIGHT-INDEPENDENT REACTIONS