Chapter 8

Photosynthesis

Photosynthesis: Overview

•  Photosynthesis converts solar energy into chemical energy

•  Organisms that carry on photosynthesis are called autotrophs

•  Heterotrophs are organisms that feed on other organisms

Light energy

PHOTOSYNTHESIS

6 CO2 6 + H2O

Carbon dioxide Water

C6H12O6 6 + O2

Glucose Oxygen gas

•  Photosynthesis occurs in chloroplasts –  In plants, photosynthesis occurs primarily in the

leaves, in the chloroplasts, which contain stroma, and stacks of thylakoids called grana

Leaf Cross Section

Leaf

Mesophyll Cell

Mesophyll

Vein Stoma

CO2 O2

Chloroplast

Chloroplast

Grana Stroma

TEM

9,7

50 ×

Stroma

Granum Thylakoid Thylakoid space

Outer membrane

Inner membrane

Intermembrane space

LM 2

,600

×

Grana

cuticle

stomata

stroma stroma

Leaf cross section upper epidermis

mesophyll

lower epidermis

leaf vein

CO2

O2

inner membrane outer membrane

Chloroplast

thylakoid space

thylakoid membrane

granum

channel between thylakoids

© Dr. George Chapman/Visuals Unlimited

Chloroplast, micrograph 37,000x

Overview of Photosynthesis

•  Glucose and oxygen are the products of photosynthesis

•  The oxygen given off comes from water •  CO2 gains hydrogen atoms and becomes

a carbohydrate

CO2 + 6 H2O C6H12O6 + 6 O2

solar energy

•  Photosynthesis is a redox process, as is cellular respiration

–  In photosynthesis •  H2O is oxidized and CO2 is reduced

Reduction

Oxidation

6 O2 6 H2O

Reduction

Oxidation

6 O2 6 CO2 + 6 H2O C6H12O6 +

C6H12O6 + 6 CO2 +

Photosynthesis

Cellular Respiration

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CH2O

H2O CO2

ADP + P NADP+

ATP

O2

thylakoid membrane

stroma

Calvin cycle

reactions Light reactions

solar energy

NADPH

•  Photosynthesis consists of two sets of reactions – Light Reactions (light-dependent) in Thylakoid

Membrane – Calvin Cycle Reactions (light-independent) in

Stroma

–  The Light Reactions •  Convert light energy to chemical energy and produce

O2 –  The Calvin Cycle assembles sugar molecules from CO2

•  Using ATP and NADPH from the light reactions

Light

CO2 H2O Chloroplast

LIGHT REACTIONS (in thylakoids)

CALVIN CYCLE

(in stroma)

NADP+

ADP + P

ATP

NADPH

O Sugar

Solar Energy Capture

•  Solar energy can be described in terms of its wavelength and its energy content

•  White or visible light is only a small portion of the spectrum

Increasing wavelength

Increasing energy

X rays UV Infrared

visible light

500 600 750

Gamma rays

Micro- waves

Radio waves

Wavelengths (nm)

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380

Solar Energy Capture

•  The photosynthetic pigments in chlorophylls a and b and the carotenoids can absorb specific portions of visible light

•  Green light is reflected and only minimally absorbed Wavelengths (nm)

380 500 600 750

Chlorophyll a Chlorophyll b carotenoids

Rel

ativ

e A

bsor

ptio

n

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Ene

rgy

of e

lect

ron

Photon

Excited state

Heat

Photon (fluorescence)

Ground state

Chlorophyll molecule

e–

Photosystem

Light-harvesting complexes

Reaction center

Primary electron acceptor

e–

To electron transport chain

Pigment molecules

Chlorophyll a molecule Transfer of energy

Photon

Thyl

akoi

d m

embr

ane

Photosystems capture solar power •  Thylakoid membranes contain multiple photosystems

–  That absorb light energy, which excites electrons

– The excited electrons •  Are passed from the primary electron acceptor to

electron transport chains

Thylakoid space

Photon

Stroma Th

ylak

oid

mem

bran

e 1

Photosystem II

e–

P680

2

H2O 1 2

+ 2 O2 H+

3 ATP Electron transport chain

Provides energy for synthesis of by chemiosmosis

4

Photosystem I

Photon

P700

e–

5

+ NADP+ H+ NADPH

6

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)

+

– The diffusion of H+ back across the membrane through ATP synthase •  Powers the phosphorylation of ADP to produce ATP

(photophosphorylation) photosystem II

Stroma

Pq

H+

A T P synthase complex

chemiosmosis

photosystem I

granum thylakoid membrane thylakoid space

stroma

H2O +

A TP + ADP P

2 NADPH

O2 1 2

thylakoid

e–

electron transport chain

e–

e– e–

e–

thylakoid space

NADP reductase

H+

H+

H+

H+

H+

H+

H+

H+

NADP+

Light Reactions

•  ATP Production – Thylakoid space acts as a reservoir for

hydrogen ions (H+) •  H+ from water being split •  Pumped in by electron transport chain

– More H+ in thylakoid space than stroma •  Electrochemical gradient

– H+ can only flow through ATP synthase – Energy powers making ATP by chemiosmosis

Calvin Cycle Reactions

•  The Calvin Cycle – Series of reactions that use CO2 from the

atmosphere to produce carbohydrate –  Includes

•  Carbon dioxide fixation •  Carbon dioxide reduction •  Ribulose-1,5-bisphosphate (RuBP) regeneration

THE CALVIN CYCLE: CONVERTING CO2 TO SUGARS •  ATP and NADPH power sugar synthesis in the

Calvin cycle – The Calvin cycle

• Occurs in the chloroplast’s stroma • Consists of carbon fixation, reduction, release of

G3P, and regeneration of RuBP

Input CO2 ATP NADPH

CALVIN CYCLE

G3P Output:

•  Photosynthesis uses light energy to make food molecules

Light

H2O CO2

NADP+

Photosystem II

Photosystem I

Electron transport chains

ADP P +

RUBP

CALVIN CYCLE

(in stroma) 3-PGA

Stroma

G3P NADPH

ATP

O2

LIGHT REACTIONS CALVIN CYCLE

Sugars

Cellular respiration

Cellulose

Starch Other organic compounds

Thylakoid membranes

Chloroplast

Photosynthesis: Summary

Calvin Cycle Reactions

•  Fixation of Carbon Dioxide – CO2 is attached to 5-carbon RuBP molecule

•  This results in a 6-carbon molecule that splits into two 3-carbon molecules (3PG)

– RuBP Carboxylase is the enzyme that makes this happen (RuBisCo)

Calvin Cycle Reactions

•  Reduction of Carbon Dioxide – Each 3PG molecules undergoes reduction to

G3P in two steps – Energy and electrons needed for this reaction

are supplied by ATP and NADPH (from light reaction)

Calvin Cycle Reactions

•  Importance of the Calvin Cycle –  G3P (glyceraldehyde-3-phosphate) can be converted

to many other molecules •  The hydrocarbon skeleton of G3P can form:

–  Fatty acids and glycerol to make plant oil –  Glucose phosphate (simple sugar) –  Fructose (+ glucose = sucrose) –  Starch and cellulose –  Amino acids

Alternate Pathways for Photosynthesis

•  C3 photosynthesis – First detectable molecule following fixation is

a 3-carbon molecule – Wheat, rice, oats – Mesophyll layers of leaves in parallel layers – Bundle sheath cells around the plant veins do

not contain chloroplasts

Alternate Pathways for Photosynthesis

•  C3 photosynthesis – RuBP can also bind with oxygen

•  Photorespiration •  Wasteful reaction because it uses oxygen and

releases carbon dioxide, decreasing the overall efficiency of the enzyme

•  Oxygen concentration in leaf rises when weather is hot and dry, because plant keeps stomata closed to conserve water

vein stoma

mesophyll cells

bundle sheath cell

a. C3 Plant

Calvin cycle

CO2

G3P

RuBP

mesophyll cell

3PG (C3)

a. CO2 fixation in a C3 plant, tulip

C3 Pathway

Alternate Pathways for Photosynthesis

•  C4 Pathway – Sugarcane and corn – Mesophyll cells are arranged in concentric

rings around the bundle sheath cells, which also contain chloroplasts

– CO2 is initially fixed into a four-carbon molecule

– The four-carbon molecules is later broken down into a three-carbon molecule and CO2

– CO2 enters the Calvin cycle

Alternate Pathways for Photosynthesis

•  C4 Pathway –  C4 plants tend to be found in hot, dry climates –  In these climates, stomata tend to close to conserve

water –  Oxygen then builds-up in the leaves –  But, RuBP carboxylase is not exposed to this oxygen

in C4 plants and photorespiration does not occur –  Instead in C4 plants, the carbon dioxide is delivered to

the Calvin cycle, which is located in bundle sheath cells that are sheltered from the leaf air spaces

vein stoma

mesophyll cells

bundle sheath cell

b. C4 Plant

CO2

CO2 C4

G3P

Calvin cycle

b. CO2 fixation in a C4 plant, corn

mesophyll cell

bundle sheath cell

C4 Pathway Alternate Pathways for Photosynthesis

•  When the weather is moderate, C3 plants ordinarily have the advantage

•  But when the weather becomes hot and dry, C4 plants have the advantage, and we can expect them to predominate

•  In the early summer, C3 plants such as Kentucky bluegrass predominate in lawns in the cooler parts of the United States, but by midsummer, crabgrass, a C4 plant, begins to take over

Alternate Pathways for Photosynthesis

•  CAM Pathway – Prevalent among most succulent plants that

grow in deserts, including the cacti – CAM plants partition carbon fixation by time – During the night CAM plants fix CO2 forming

C4 molecules, – The C4 molecules are stored in large vacuoles – During daylight – C4 molecules release CO2 to Calvin cycle

CO2

CO2

C4

G3P

night

day

c. CO2 fixation in a CAM plant, pineapple

Calvin cycle

CAM Pathway

Photosynthesis Versus Cellular Respiration

•  Both plant and animal cells carry out cell respiration –  In mitochondria –  Breaks glucose down –  Utilizes O2 and gives off CO2

•  Only plant cells photosynthesize –  In chloropalsts –  Builds glucose –  Utilizes CO2 and gives off O2

•  Both processes utilize an electron transport chain and chemiosmosis for ATP production

Thylakoid membrane H2O O2 Cristae O2 H2O

ADP ATP ADP ATP

solar energy

H2O CO2

NADH+H+

e–

e–

NADH+H+ e–

e–

e–

e–

Preparatory reaction Citric acid

cycle

NADH + H+

and FADH2

Electron transport chain

2 ATP 2 ADP

4 ADP 4 ATP total 2 ATP

net gair 2 ADP 2 32 ADP 32 or 34 or 34

ATP ATP

Matrix NAD+

CH2O CO2

Cellular Respiration

Light reactions

ADP + P

Calvin cycle

reactions

NADP+

NADPH

thylakoid membrane O2 CH2O

stroma

Stroma NADPH NADP+

CO2 CH2O

Photosynthesis

Glycolysis

glucose pyruvate

ATP

NADH

Photosynthesis Versus Cellular Respiration