Photosynthesis Section 3.1. Overview of Photosynthesis General Equation: Light Energy 6 CO 2 + 6 H 2...

Photosynthesis

Section 3.1

Overview of Photosynthesis

General Equation:

Light Energy

6 CO2 + 6 H2O C6H12O6 + 6 O2

The Leaf

• The leaf is the photosynthetic organ of green plants.

• mesophyll cells– The cells that perform

photosynthesis– they contain the

largest number of chloroplasts

Vein

Leaf cross section

Mesophyll

CO2 O2Stomata

Chloroplast

Mesophyll

5 µm

Outermembrane

IntermembranespaceInner

membrane

Thylakoidspace

ThylakoidGranumStroma

1 µm

Chloroplast

Mesophyll

5 µm

Outermembrane

Intermembranespace

Innermembrane

ThylakoidSpace (thylakoid lumen)

Thylakoid

GranumStroma

1 µm

Stomata (pores in leaves)

• Three functions:– Allow CO2 in

– Allow O2 out– When water goes

out, the leaves are cooled so they do not overheat (called evaporative cooling) CO2 in

O2 out

H2O out

H2O in

• Plants regulate size of stomata opening in response to environmental conditions in order to:– Maximize CO2 uptake– Minimize water loss

• In general, stomata reduce in size:– Sunny, warm, dry and windy

Guard cells• The guard cells

control the stomata by opening and closing.

• This is controlled by osmosis:– When water is scarce,

the guard cells lose H2O and deflate, closing the stomata.

– When water is abundant, the guard cells swell, opening the stomata.

• Also, in general, stomata are open in the daytime and closed at night

Two Types of Reactions in Photosynthesis

• Light Dependent Rxns (Light Reactions) – require light, which includes exciting electrons and

splitting H2O.

• Light Independent Rxns (Dark Reactions) – do not directly require sunlight, includes Carbon

fixation, the Calvin Cycle.

Light Reactions

• Occur in the grana• Split water, release

oxygen, produce ATP, and form NADPH

H2O CO2

Light

LIGHT REACTIONS

CALVINCYCLE

Chloroplast

Sucrose

NADPH

NADP

ADP+ P

O2

ATP

starch

NADH NADPH

Dark Reactions

• The Calvin cycle– Occurs in the

stroma– Forms sugar from

carbon dioxide, using ATP for energy and NADPH for reducing power

H2O CO2

Light

LIGHT REACTIONS

CALVINCYCLE

Chloroplast

Sucrose

NADPH

NADP

ADP+ P

O2

ATP

starch

Electromagnetic Radiation

Gammarays X-rays UV Infrared

Micro-waves

Radiowaves

10–5 nm 10–3 nm 1 nm 103 nm 106 nm1 m

106 nm 103 m

380 450 500 550 600 650 700 750 nm

Visible light

Shorter wavelength

Higher energy

Longer wavelength

Lower energy

Pigments• Pigments

– Are substances that absorb visible light

– The reflected light and transmitted include the colors we see

– Absorbed light is important in driving photosynthesis

Light

ReflectedLight

Chloroplast

Absorbedlight

Granum

Transmittedlight

Pigments

• Chlorophyll a– Is the main

photosynthetic pigment

• Chlorophyll b– Is an accessory

pigment

C

CH

CH2

CC

CC

C

CNNC

H3C

C

CC

C C

C

C

C

N

CC

C

C NMgH

H3C

H

C CH2 CH3

H

CH3C

HHCH2

CH2

CH2

H CH3

C O

O

O

O

O

CH3

CH3

CHO

in chlorophyll ain chlorophyll b

Porphyrin ring:Light-absorbing“head” of moleculenote magnesiumatom at center

Hydrocarbon tail:interacts with hydrophobicregions of proteins insidethylakoid membranes ofchloroplasts: H atoms notshown

Spectrophometer

Figure 10.8

Whitelight

Refractingprism

Chlorophyllsolution

Photoelectrictube

Galvanometer

Slit moves topass lightof selectedwavelength

Greenlight

The high transmittance(low absorption)reading indicates thatchlorophyll absorbsvery little green light.

The low transmittance(high absorption) readingchlorophyll absorbs most blue light.

Bluelight

1

2 3

40 100

0 100

Absorption Spectra

– Provide clues to the relative effectiveness of different wavelengths for driving photosynthesis

• They both have a double peak: in the red/orange and in the blue/violet areas.

• Between the two they absorb much of the V, B, O and R spectrums, leaving green and yellow (typical leaf colours).

Homework.

• Read Ch 3.1• Do p145 #4, 5, 6.