Chapter 7: PHOTOSYNTHESIS 1. Overview of Photosynthesis 2. The - - PowerPoint PPT Presentation

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)

Light energy PHOTOSYNTHESIS 6 CO2 6 + H2O Carbon dioxide Water C6H12O6 6 + O2 Glucose Oxygen gas

Photosynthesis vs Respiration

essentially the reverse

• f each other

Leaf Cross Section Leaf Mesophyll Cell

CO2 O2 Chloroplast

Chloroplast

Photosynthesis occurs in Chloroplasts

The Chloroplast

• uter membrane

inner membrane granum thylakoid stroma

Photosynthesis consists of 2 sets of Reactions

The light-dependent or “Light” Reactions:

• convert sunlight

energy into chemical energy (stored in ATP & NADPH)

“Dark” Reactions (Calvin cycle):

• use chemical

energy from light reactions to make glucose

Light CO2 H2O Chloroplast LIGHT REACTIONS (in thylakoids) CALVIN CYCLE (in stroma) NADP+ ADP + P ATP NADPH O Sugar E l e c t r

• n

s

• 2. Light-dependent

(“Light”) Reactions

Light Reactions occur in Thylakoids

A variety of light-absorbing pigments & electron transport proteins are embedded within the thylakoid membrane

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

Transmitted light Absorbed light Reflected light Light Chloroplast

Chlorophyll absorbs “non-green” light energy

• green light

passes on through or is reflected, causing the leaves to appear green

1

H2O ½ O2 + 2 H+ + 2 *e-

2

e- transport chain (ETC) pumps H+ into thylakoid

PS I PS II

3

ATP Synthase uses H+ flow to make ATP

4

2 e- to NADPH

PS 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 Mill makes ATP

P h

• t
• n

Photon

Photosystem II Photosystem I NADPH e– e– e– e– e– e–

Analogy of Light reactions

Chloroplast

Stroma (low H+ concentration)

Light Light NADP+

+ H+

NADPH H+ H+ H+ H+

ATP

P ADP +

Thylakoid membrane

H2O 1 2 O2 + 2 H+ H+ H+ H+ H+ H+ H+ H+ H+ H+ H+ Photosystem II Electron transport chain Photosystem I ATP synthase Thylakoid space (high H+ concentration)

Summary of the “Light” Reactions

• 3. Light-independent

(“Dark”) Reactions

The “Dark” Reactions

A 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’t memorize this!!

Involves an anabolic pathway known as the Calvin cycle:

• endergonic reactions of

this pathway are fueled by ATP & NADPH from the “light” reactions

• resulting sugars can be

used as a source of energy or to build other

• rganic molecules

Calvin cycle

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