Introduction to Photosynthesis (181-200)

Life on Earth is SOLAR powered Photosynthesis (Ps) nourishes almost all living organisms

Autotrophs

- mainly Ps organisms (

photoautotrophs

) that make their own food (using sun E, CO 2 , and H 2 O) Also called

producers

of the biosphere Exs = green plants and Ps protist groups (fig 10.2)

Heterotrophs

- get E from organic compounds produced by other organisms Also called

consumers

of the biosphere Exs = fungi, animals, & many protist groups Photosynthesis converts light E to chemical E of food

•

The Process That Feeds the Biosphere

Photosynthesis

– Is the process that converts solar (light) energy into chemical energy • Plants and other autotrophs – Are the

producers

of the biosphere

Plants are photoautotrophs

• They use the energy of sunlight to make organic molecules from water and carbon dioxide

Figure 10.1

•

Photosynthesis

Occurs in plants, algae, certain other protists, and some prokaryotes These organisms use light energy to drive the synthesis of organic molecules from carbon dioxide and (in most cases) water. They feed not only themselves, but the entire living world.

(a)

On land, plants are the predominant producers of food. In aquatic environments, photosynthetic organisms include

(b)

multicellular algae, such as this kelp;

(c)

some unicellular protists, such as Euglena;

(d)

the prokaryotes called cyanobacteria; and

(e)

other photosynthetic prokaryotes, such as these purple sulfur bacteria, which produce sulfur (spherical globules) (c, d, e: LMs).

(a) Plants Figure 10.2

(b) Multicellular algae (c) Unicellular protist

10  m

(d) Cyanobacteria

40  m

(e) Purple sulfur bacteria

1.5  m

•

Heterotrophs Heterotrophs

– Obtain their organic material from other organisms – Are the

consumers

of the biosphere – Includes fungi, animals, many protist groups and many bacteria

Chloroplasts – Sites of Ps within the cell

Primarily found in leaves (

mesophyll

= main part of a leaf)

Stomata

= regulated holes in leaves where gas exchange occurs (what gases does a plant need to exchange for Ps?) Organelles enclosed by a

double-membrane system

(endosymbiosis)

Stroma

= internal fluid-filled cavity

Thylakoids

= system of interconnected membrane sacs (separates the stroma from the

thylakoid space

)

Grana

= stacks of thylakoids

Chlorophyll

= green pigment that absorbs light E =

molecular bridge

sunlight and Ps activity between Molecules are embedded in the thylakoid membrane system

Chloroplasts: The Sites of Photosynthesis in Plants

• The leaves of plants – Are the major sites of photosynthesis

Leaf cross section

Vein

Mesophyll

Stomata CO 2 O 2

Figure 10.3

Chloroplasts

• Chloroplasts – Are the organelles in which photosynthesis occurs – Contain thylakoids and grana – Stroma is the fluid in the internal cavity – Chlorophyll is imbedded in the thylakoid membranes Mesophyll Chloroplast 5 µm Stroma Granum Thylakoid Thylakoid space Outer membrane Intermembrane Inner space membrane 1 µm

Tracking Atoms Through Photosynthesis:

• Photosynthesis is summarized as 6 CO 2 + 12 H 2 O + Light energy  OR CO 2 + H 2 O  [CH 2 C 6 H 12 O O] + O 2 6 + 6 O 2 + 6 H 2 O Overall Ps equation has been known since the 1800s The equation for Ps (fig 10.4) = reverse of respiration But carbohydrates are not made by simply reversing what happens in respiration BOTH processes occur in plant cells!

Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings

The Splitting of Water

• Chloroplasts split water into – Hydrogen and oxygen, incorporating the electrons of hydrogen into sugar molecules

Reactants: Figure 10.4

Products:

C 6 H 12 O 6 6 CO 2 12 H 2 O 6 H 2 O 6 O 2

•

Photosynthesis as a Redox Process

Photosynthesis is a redox process – Water is oxidized, carbon dioxide is reduced

Two Stages of Photosynthesis Two stages of Ps (fig 10.5): 1. Light rxns

: depend on light  make ATP & NADPH and give off O 2

NADPH

= very similar in structure to NADH (just add a phosphate group to NADH) = the e- carrier

Photophosphorylation

= how ATP is generated (using chemiosmosis again)

2. Calvin cycle

: use ATP and NADPH to fix C from the atmosphere into organic compounds

Carbon fixation

= initial incorporation of C into organic compounds

•

The Two Stages of Photosynthesis

Photosynthesis consists of two processes – The light reactions – The Calvin cycle

sunlight water carbon dioxide light-dependent rxns ATP NADPH NADP+ Calvin cycle oxygen P glucose new water

•

The Light Reactions

The light reactions – Occur in the

grana

– Split water, release oxygen, produce ATP, and form NADPH

•

The Calvin Cycle

The Calvin cycle – Occurs in the stroma – Forms sugar from carbon dioxide, using ATP for energy and NADPH for reducing power

An overview of photosynthesis

Figure 10.5

Light Chloroplast

H 2 O CO 2

LIGHT REACTIONS NADP  ADP + P ATP NADPH CALVIN CYCLE

O 2 [CH 2 O] (sugar)

•

Light Reactions

The light reactions convert solar energy to the chemical energy of ATP and NADPH

•

Light

Light

waves = electromagnetic energy, which travels in •

Wavelength

= distance between crests/troughs of waves (nm - km) – Smaller wavelengths = stronger light waves •

Electromagnetic spectrum

of light (fig 10.6) = entire range – • Different

pigments

absorb different wavelengths and reflect others (what we see that makes them colored) –

Visible light

systems (380-750 nm) important to biological What wavelength of light do plants reflect?

•

The Nature of Sunlight

Light – Is a form of electromagnetic energy, which travels in waves • Wavelength – Is the distance between the crests of waves – Determines the type of electromagnetic energy

•

The electromagnetic spectrum

The electromagnetic spectrum – Is the entire range of electromagnetic energy, or radiation

10 –5 nm 10 –3 nm 1 nm 10 3 nm 10 6 nm 1 m 10 6 nm 10 3 m

Gamma rays X-rays UV Infrared Micro waves Radio waves

Figure 10.6

380 450 500

Shorter wavelength Higher energy Visible light

550 600 650 700

Longer wavelength Lower energy

750 nm

•

The visible light spectrum

The visible light spectrum – Includes the colors of light we can see – Includes the wavelengths that drive photosynthesis

Photosynthetic Pigments

• •

Photosynthetic pigments

absorb specific wavelenths of light

Absorption spectrum

wavelength = a pigment’s light absorption vs. • •

Spectrophotometer

= instrument that measures absorbance of specific wavelengths (fig 10.8) Beam of light sent through solution  fraction of light transmitted at each wavelength measured

Photosynthetic Pigments: Light Receptors

• Photosynthetic Pigments – Are substances that absorb specific wavelengths within the visible light spectrum

Pigments

– Reflect some light, which include the colors we see Light Reflected Light Chloroplast

Figure 10.7

Absorbed light Transmitted light Granum

•

The spectrophotometer

The spectrophotometer – Is a machine that sends light through pigments and measures the fraction of light transmitted at each wavelength

Transmitted light is NOT absorbed by that particular pigment

•

An absorption spectrum

An absorption spectrum – Is a graph plotting light absorption versus wavelength White light Refracting prism Chlorophyll solution Photoelectric tube Galvanometer

2 3 1

0 100

4

Slit moves to pass light of selected wavelength

Green light

The high transmittance (low absorption) reading indicates that chlorophyll absorbs very little green light.

Figure 10.8

Blue light

0 100 The low transmittance (high absorption) reading chlorophyll absorbs most blue light.

• • • • • •

Photosynthetic Pigments

Chlorophyll a

(fig 10.10) absorption spectrum (fig 10.9a)

Chlorophyll b

= accessory pigment similar to chl. a When chlorophyll pigment is in its

excited state

)

absorbs light

 energy boosts an e- to an orbital of higher energy level (pigment If chlorophyll is isolated from chloroplast (fig 10.11) 

fluoresces

(emits light) in red-orange end of spectrum (E given off as heat)

Carotenoids

= other accessory pigments (hydrocarbons) reflecting various shades of orange/yellow/red (fig 10.9a) Most important function =

photoprotection

dissipate excess light E) (absorb &

•

Pigment Absorption Spectra

The absorption spectra of chloroplast pigments – Provide clues to the relative effectiveness of different wavelengths for driving photosynthesis

Absorption spectra of three pigments in chloroplasts EXPERIMENT

Three different experiments helped reveal which wavelengths of light are photosynthetically important. The results are shown below.

RESULTS

Chlorophyll

a

Chlorophyll

b

Carotenoids Wavelength of light (nm)

(a) Absorption spectra.

The three curves show the wavelengths of light best absorbed by three types of chloroplast pigments.

Figure 10.9

•

The action spectrum for photosynthesis

Profiles the relative effectiveness of different wavelengths of radiation in driving photosynthesis

(b) Action spectrum.

This graph plots the rate of photosynthesis versus wavelength. The resulting action spectrum resembles the absorption spectrum for chlorophyll

a

but does not match exactly (see part a). This is partly due to the absorption of light by accessory pigments such as chlorophyll b and carotenoids.

•

The action spectrum for photosynthesis

Was first demonstrated by Theodor W. Engelmann Aerobic bacteria Filament of alga 400 500 600 700

(c) Engelmann‘s experiment.

In 1883, Theodor W. Engelmann illuminated a filamentous alga with light that had been passed through a prism, exposing different segments of the alga to different wavelengths. He used aerobic bacteria, which concentrate near an oxygen source, to determine which segments of the alga were releasing the most O 2 and thus photosynthesizing most.

Bacteria congregated in greatest numbers around the parts of the alga illuminated with violet-blue or red light. Notice the close match of the bacterial distribution to the action spectrum in part b.

CONCLUSION

photosynthesis.

Light in the violet-blue and red portions of the spectrum are most effective in driving

Types of Chlorophyll

• Chlorophyll a – Is the main photosynthetic pigment • Chlorophyll b – Is an accessory pigment

Figure 10.10

C O CH 2 CH H CH 3 H 3 C C C C C C C N H C H 3 C C H C CH 2 C N C H H Mg C C N N C C C C C C C C C CH 2 O O O O CH 3 CH 2 CH 2 H CH 3 CH 3 CH 3 CHO in chlorophyll

a

in chlorophyll

b

Porphyrin ring:

Light-absorbing “head” of molecule note magnesium atom at center

Hydrocarbon tail:

interacts with hydrophobic regions of proteins inside thylakoid membranes of chloroplasts: H atoms not shown

•

Other Pigments

Other accessory pigments – Absorb different wavelengths of light and pass the energy to chlorophyll a

•

Excitation of Chlorophyll by Light

When a pigment absorbs light – It goes from a ground state to an excited state, which is unstable e – Excited state Heat Photon

Chlorophyll molecule

Photon (fluorescence) Ground state

Figure 10.11 A

•

Chlorophyll absorbs energy

If an isolated solution of chlorophyll is illuminated – It will fluoresce, giving off light and heat – The excited electron drops back to the ground-state orbital.

Figure 10.11 B

• Tomorrow, we will start with the different types of photosynthetic pigments, and which wavelengths of light each absorbs.

• We will also discuss the light reaction portion of photosynthesis. The light reaction produces ATP and NADPH which go to power the fixation and reduction of carbon dioxide into sugar by the Calvin Cycle.