Introduction to Photosynthesis

Chapter 10p. 181-188

Autotrophs: Producers of the Biosphere

Autotroph: “self-feeding”; produce own organic molecules from CO2 & inorg. molec. in environment i.e.: plants, algae, some

bacteria Heterotroph: “feeds on

others”; must consume other orgs to obtain nutrients, O2, & energy i.e.: animals, fungi, most

bacteria

Chloroplasts: Sites of Photosynthesis

Plants appear green due to pigment chlorophyll inside thylakoid space of chloroplasts Found in mesophyll tissue

of leaves 30-40 per cell Stomata: pores through

which CO2 enters & O2

leaves

Stomata

Chloroplast Structure

Tracking Atoms through Photosynthesis

O2 given off by plants comes from H2O; NOT CO2 Chloroplast splits H2O → 2H + O

C.B. van Niel: proved 2H from split H2O goes to glucose; O released as atmospheric O2

Photosynthesis is a Redox Reaction

Unlike cell respir., photosynthesis is endergonic Energy comes from sun Reverses direction of e- flow from H2O →

CO2 (oxidized)

CO2 is reduced to glucose

CO2 + H2O + energy → C6H12O6 + O2

The 2 Stages of Photosynthesis

1) Light Reactions (“photo”) e- + H+ transferred to NADP+ (cousin of

NAD+) O2 given off as byproduct Produces 1 ATP

(“photophosphorylation”) Occurs in the thylakoid

The 2 Stages of Photosynthesis

2) Calvin Cycle (“synthesis”) CO2 incorporated into organic molecules

already present (“carbon fixation”) “Fixed” C is reduced to glucose (add e-)

Powered by NADPH & ATP from light rxns Occurs during day in most plants; relies

on light rxns Occurs in stroma

Photosynthesis Overview

Nature of Sunlight

Light: electromagnetic energy (“radiation”) Travels in waves; distance

between called wavelength Also acts as photons:

particles of light energy Electromagnetic

Spectrum: entire radiation spectrum

Visible light = 380-750nm Amt energy inversely

proportional to wavelength Purple photon > red photon

Photosynthetic Pigments

Pigment: substance that absorbs visible light at different wavelengths Reflected/transmitted

wavelength is color we see

Spectrophotometer: measures absorbed wavelengths

Photosynthetic Pigments

Chlorophyll a: main photosynthetic pigment Absorbs red & blue

photons; reflects green Only pigment directly

involved in light rxns Other pigments

(Chlorophyll B & Carotenoids) transfer photons to chlorophyll a & provide photoprotection

Excitation of Chlorophyll by Light

When molecules absorb light energy (photons), e- “jump” to next orbital Ground state → excited

state Specific to wavelength Unstable e- will “fall” back

quickly, releasing energy (heat)

Fluorescence: energy released as light

Reactions of Photosynthesis

Chapter 10p. 189-198

Photosystems Consists of 3 sections: 1) Light-Harvesting Complex:

contain all 3 types pigments; ↑ surface area to absorb more light

2) Reaction-Center: at center; receives energy from light-harvesting complex & becomes excited Contains special chlorophyll a molecules whose e-’s

move to higher energy level 3) Primary Electron Acceptor: receives e-s

from excited chlorophyll a molecules & “catches” them e-’s then enter into Noncyclic Electron Flow

Types of Photosystems

Photosystem II: absorbs 680nm best (“P680”) P700 & P680 identical, but surrounded

by diff. proteins Work together to make ATP & NADPH

for Calvin Cycle Photosystem I: Reaction center

chlorophyll a absorbs 700nm best (“P700”)

Noncyclic Electron Flow Predominant route for e-s Steps:

1) Photots. II absorbs light, P680 excited 2) e-s captured by Primary e- Acceptor

P680 now has e- “hole” 3) e-s replaced in P680 by split H2O

molecule; O2 released inside thylakoid 4) ETC takes e-s from Primary e- Acceptor to

Photosystem I Composed of plastoquinone (Pq), 2

cytochromes, & plastocyanin (Pc)

Noncyclic Electron Flow

5) As e-s “fall” down chain, energy is harvested to make ATP by chemiosmosis outside thylakoid “Noncyclic Photophosphorylation”

6) Final e- acceptor is P700 P700 e-’s excited by light energy are captured by

Primary e- Acceptor Fills “hole” created by Primary e- Acceptor of Photo II

7) Primary e- Acceptor passes e-s to 2nd ETC → ferredoxin (Fd)

8) NADP+ reductase transfers e-’s from Fd to NADP+→ makes NADPH in stroma

Summary of Noncyclic Electron Flow

P680 → Primary e- Acceptor → 1st ETC → P700 → Primary e-

Acceptor → Fd → NADP+ reductase → NADPH

Cyclic Electron Flow

Calvin cycle uses more ATP than NADPH If ATP runs low,

chloroplast switches to cyclic

Involves Photosystem I (P700) only Fd takes e-s to

cytochrome complex of 1st ETC & returns them to P700

No NADPH produced; no O2 released

Calvin Cycle

C enters as CO2, leaves as sugar (G3P) Cycle must turn 3 x’s to make glucose

Must “fix” 3 C’s into org. molecules Occurs in 3 phases:

1) C Fixation: 3C’s from 3CO2 are incorporated into RuBP, catalyzed by rubisco

2) Reduction: e-’s from NADPH reduce 6 1,3 biphosphate → 6 G3P (↑ energy)

Spends 6 ATP 3) Regeneration of RuBP: G3P rearranged → RuBP

(can pick up CO2 again) Spends 3 ATP

Calvin Cycle - Summarized

For each turn of Calvin: In: Out:

9 ATP 9 ADP 6 NADPH 6 NADP+

3 CO2 1 G3P (will become

glucose)

Alternate Methods of C Fixation

In hot, dry climates, stomata remain closed to prevent H2O loss Also prevents CO2

in & O2 out Result is

Photorespiration

Photorespiration Most plants are called C3 because C

fixation creates a 3-C compound Closed stomata ↓ [CO2] inside leaf, and ↑

[O2] O2 will be picked up by rubisco (instead of

CO2) Photorespiration: uses light (“photo”)

to consume O2 (“respiration”) No ATP produced; no sugar made May be ancient evolutionary adaptation

C4 Plants

C fixed by PEP carboxylase to form 4-C compound (oxaloacetate → malate) PEP carbox. has ↑↑ affinity for CO2; can

“fix” CO2 when rubisco can’t 4-C cmpnd (malate) enters Bundle Sheath

cells where CO2 breaks off & enters Calvin Keeps CO2 levels ↑ for rubisco Minimizes photorespiration & ↑ sugar

production

CAM Plants

Water-storing plants (cacti, pineapple, etc.) close stomata during day, open at night Store org. molec.

until day when light rxns can provide ATP & NADPH