Chapter 7:PHOTOSYNTHESIS

2. The “Light” Reactions

1. Overview of Photosynthesis

3. The “Dark” Reactions

1. Overview of Photosynthesis

What is Photosynthesis?The process of converting light energy (kinetic)into energy stored in the covalent bonds of glucose molecules (potential).

• carried out by photoautotrophs• plants, phytoplankton, cyanobacteria (any

photosynthetic organism)

• the basis of almost all ecosystems• all “food energy” ultimately comes from the sun• source of all atmospheric oxygen (O2)

Lightenergy

PHOTOSYNTHESIS

6 CO2 6+ H2O

Carbon dioxide Water

C6H12O6 6+ O2

Glucose Oxygen gas

Photosynthesis vs Respirationessentially the

reverseof each other

Leaf Cross Section

Leaf

Mesophyll Cell

CO2 O2

Chloroplast

Chloroplast

Photosynthesis occurs in Chloroplasts

The Chloroplastouter membrane

inner membrane

granum

thylakoid

stroma

Photosynthesis consists of 2 sets of Reactions

The light-dependent or “Light” Reactions:• convert sunlightenergy intochemical energy(stored in ATP &NADPH)

“Dark” Reactions(Calvin cycle):

• use chemicalenergy fromlight reactionsto make glucose

Light

CO2H2OChloroplast

LIGHTREACTIONS(in thylakoids)

CALVINCYCLE

(in stroma)

NADP+

ADP+ P

ATP

NADPH

O Sugar

Electrons

2. Light-dependent(“Light”) Reactions

Light Reactions occur in Thylakoids

A variety of light-absorbing

pigments &electron transport

proteins areembedded within

the thylakoidmembrane

The Pigments absorb “Visible” Light

Chlorophyll a & b:• the major pigments (absorb red, blue…, reflect green)

Carotenoids (e.g., β-carotene)• accessory pigments (absorb green, blue, reflect red, yellow)

blue red

Absorption Range for each Pigment

Transmittedlight

Absorbedlight

Reflectedlight

Light

Chloroplast

Chlorophyll absorbs “non-green” light energy

• green lightpasses onthrough oris reflected,causing theleaves to appear green

1H2O ½ O2 + 2 H+ + 2 *e-

2e- transport chain(ETC) pumps H+

into thylakoid

PS I

PS II

3 ATP Synthaseuses H+ flow tomake ATP

4 2 e- toNADPHPS II

PS I

Light Energy absorbed by Pigments Fuels 4 General Steps of the “Light Reactions”:

1) H2O split to O, 2 H+ & 2 high energy e- (*e-) in PS II

H2O O2 + H+ + *e-sunlight

2) Energy released by a series of *e- transfers is used to generate H+ gradient• H+ accumulates inside the thylakoid membrane

• *e- ends up in NADPH (an electron carrier)

3) H+ gradient used to make ATP (chemiosmosis)

4) *e- “re-energized” in PS I, passed on to NADP+

e–

ATP

MillmakesATP

Phot

on

Phot

on

Photosystem II Photosystem I

NADPHe–

e–e–

e–

e–

e–

Analogy of Light reactions

Chloroplast

Stroma (low H+ concentration)

Light Light

NADP+ + H+ NADPH

H+

H+

H+

H+

ATPPADP +

Thylakoidmembrane

H2O 1

2O2 + 2 H+ H+

H+

H+ H+

H+

H+

H+ H+

H+

H+

Photosystem II Electrontransport chain

Photosystem I ATP synthase

Thylakoid space(high H+ concentration)

Summary of the“Light” Reactions

3. Light-independent(“Dark”) Reactions

The “Dark” ReactionsA series of reactions called the Calvin cycle that synthesize glucose from CO2 and H2O:

CO2 + H2O C6H12O6 (glucose)ATP, NADPH

• can occur in dark (doesn’t require light directly)• also occurs during daylight!

• takes place in the stroma of chloroplasts• outside the thylakoids

• uses energy stored in ATP and NADPH• produced by the light reactions

“Dark” Reactions

Don’tmemorize

this!!

Involves an anabolicpathway known as theCalvin cycle:

• endergonic reactions ofthis pathway are fueled byATP & NADPH from the“light” reactions

• resulting sugars can beused as a source ofenergy or to build otherorganic molecules

Calvincycle

Summary of Photosynthesis

stroma

Key Terms for Chapter 7

• chlorophyll, carotenoids

Relevant Review Questions: 1-6, 8-10, 12

• ATP synthase

• ATP, NADPH

• electron transport chain (ETC)

• photoautotroph

• chloroplast, thylakoid, stroma

• Light reactions, dark reactions, Calvin cycle