CHAPTER 8

PHOTOSYNTHESIS

Photosynthesis – process by which plants use light to make food molecules from carbon dioxide and water

(chlorophyll)

6CO2 + 12H2O + Light C6H12O6 + 6O2 + 6H2O

Con. 8.1

Autotrophs – plants that make their own food (also known as producers)

Photoautotrophs—organisms that use light as a source of energy to make food

Chloroplasts

• All green parts of a plant have chloroplasts (leaves are major sites of photosynthesis)

• Green color of plants is due to green pigment w/in chloroplasts called chlorophyll

• Chloroplasts mainly found in mesophyll cells – green tissue in interior of leaf

p. 156

• Consists of double membrane surrounding a thick fluid (stroma) – sugars are made from CO2 here

--CO2 enters leaf and O2 exits by way of

tiny pores known as stomata

• Thylakoids (green sacs) are suspended in stroma – light energy is captured here

• Stack of these = Granum • Stacks of these = Grana

Thylakoid

Redox Reactions

6CO2 + 12H2O C6H12O6 + 6H2O +6O2

-Photosynthesis STORES energy

-Carbon dioxide is reduced to glucose

-Water is oxidized to oxygen

-Water is first consumed then produced

ENDERGONIC

GAINING ELECTRONS

LOSING ELECTRONS

-Plants make sugar from carbon and oxygen in CO2 and from some hydrogen in H2O

-O2 is released, coming from the H2O, not from CO2

- Cellular respiration harvests energy by _______ the sugar & _______ O2 to H2O (respiration is energy-releasing, going from potential to kinetic by traveling down energy levels)

- Photosynthesis goes uphill (gaining potential and traveling up energy levels)

oxidizing reducing

The Light Reactions

• Convert light energy to chemical energy & O2 gas is waste product

• Occur in thylakoid membranes

• Stored in ATP & NADPH

• No sugar produced

Con. 8.2

Visible Light

• Sunlight is a type of energy called radiation (AKA electromagnetic energy)

• Travels in rhythmic waves

• Light reactions of photosynthesis only use certain wavelengths/colors of Visible Light p. 160

• Light also behaves as discrete packets of energy called photons

• A photon is a fixed quantity of light energy (shorter the wavelength, the greater the energy)

• A photon of violet light packs almost twice as much energy as a photon of red light

• Light may be reflected, transmitted, or absorbed (pigments are substances that absorb light)

**Read Fig. 8.8 & Fig. 8.9 p. 161

• Pigments involved in photosynthesis are chlorophyll a and b and carotenoids

• The chlorophyll a is blue-green, chlorophyll b is yellow-green and carotenoids are shades of yellow and orange

spectrophotometer - measures the ability of a pigment to absorb various wavelengths

absorption spectrum – graph plotting a pigment’s light absorption vs. wavelength

action spectrum – looks @ effectiveness of different wavelengths of radiation driving photosynthesis (wavelength vs. rate of CO2 use or O2 release)

Photosystems

• Pigments are clustered in thylakoid membranes

• 2 Chlorophyll a molecule & a primary electron acceptor make up the reaction center of the pigment assembly

• Reaction center & other pigments function collectively as a light-gathering antenna that absorbs photons

• Energy is passed from pigment molecule to pigment molecule until it reaches the reaction center

• Combination of the antenna molecules, the reaction center, and the primary electron acceptor make up the photosystem

• This is the light-harvesting unit of the chloroplast’s thylakoid membrane

Photosystems (cont.)

Two Photosystems

• Photosystem I is called P700 because the light it absorbs best is red light w/a wavelength of 700 nm

• Photosystem II is called P680 because the light it absorbs best is orange shade of red light w/a wavelength of 680 nm

http://dopamine.chem.umn.edu/chempedia/index.php/Image:Thylocoid_membrane.png

ATP, NADPH, and O2

• First event in light reactions is the absorption of light energy

• Second event is the excitation of electrons by light energy

• Third event is formation of ATP & NADPH using energy made available by the cascade of energized electrons down electron transport chains

3 Steps of the Light Reaction

http://dopamine.chem.umn.edu/chempedia/index.php/Image:Calvin_cycle_1.png

Linear Electron Flow

photon relayed to PSII

P680 e- go to higher energy state

e- captured by primary e- acceptor

H2O is split into 2e-, 2p+, oxygen atom

e-’s go from PSII to PSI

e-’s fall down, helping to make ATP

excited PSI e-’s go down 2nd e- transport chain

NADPH made

Cyclic Electron Flow

uses PSI but not PSII

doesn’t make NADPH

possibly controlled by concentration of NADPH to help w/supply & demand

• Transport chains are similar to the one that functions in cellular respiration

• Consist of a series of electron-carrier molecules arranged in a membrane (the thylakoid of the chloroplast)

Chemiosmosis

• ATP is synthesized by chemiosmosis

• Electron transport chains associated w/the chloroplast’s photosystems are arranged in thylakoid membranes

• Electron transport chain in the chloroplast drives the transport of H+ through thylakoid membrane

• Flow of H+ back through the membrane is harnessed by ATP synthase to make ATP

• In photosynthesis this is called photophosphorylation

• H+ ions, along w/electrons from the electron transport chain, join w/NADP+ to form NADPH

p. 158

The Dark Reaction (Calvin Cycle)

• Cyclic series of reactions that assemble sugar molecules using CO2 and energy-containing products of light reaction

• Takes place in stroma

Con. 8.3

http://dopamine.chem.umn.edu/chempedia/index.php/Image:Calvin_cycle_2.png

3 Phases

1. Carbon Fixation

- cycle must occur 3 times (3 CO2) to get sugar

- CO2 molecule from air attaches to 5C RuBP (rubisco)

- unstable 6C molecule so splits forming 2 molecules of

3-phosphoglycerate

2. Reduction

- energy from ATP & high-energy electrons from NADPH

- help make one glyceraldehyde-3 phosphate (G3P)

- other 5 G3P are recycled

3. Regeneration of RuBP

- these 5 G3P are rearranged into 3 molecules of RuBP

- ATP is used

Totals for Calvin

1 G3P molecule

9 ATP consumed

6 NADPH used

- G3P can now be used for many organic molecules needed in the plant

Adaptation for Saving Water

• Most plants are C3 plants, which take carbon directly from CO2 in the air & use it in the Calvin cycle to build a 3-carbon molecule

• Stomata in leaf surface usually close when the weather is hot & dry

• Minimizes water loss

• But CO2 and O2 are not exchanged as normal

• Calvin cycle is diverted to an inefficient process called photorespiration

• Consumes ATP

• No sugar made

• Some plants have special adaptations that enable them to save water & avoid photorespiration ex: corn, sugarcane

• Special cells in C4 plants incorporate CO2 into a 4-carbon compound

• It’s broken down to release CO2 (this initiates the Calvin cycle)

Characteristic C3 Plants C4 Plants

Origin Temperate Tropical

Examples Rice, Soybean, many tree

species Corn, sorghum, sugarcane

Carbon dioxide fixation 3 carbon molecule 4 carbon molecule

Site of photosynthetic cycle Mesophyll cells Bundle sheath cells

Carbon dioxide

concentration Regulated by diffusion

Elevated high

concentrations

Stomatal behavior Open for longer periods Open for shorter periods

Water use efficiency* Not very efficient Very efficient

Climatic adaptation Mostly cooler, moderate

climate

Mostly warmer, drier

climate

Carbon dioxide saturation High Low

Light saturation Low High

Photorespiration High Low

*The ratio of carbon dioxide fixed to water used per unit area of the leaf.

CAM – Crassulacean Acid Metabolism • These plants are also adapted

to hot, dry climates ex: cacti, pineapples, succulents (aloe)

• Close stomata during day to prevent water loss & open at night

• Carbon compounds stored in vacuoles at night

• These compounds are broken down to release CO2 for photosynthesis during the day

Global Warming • In atmosphere, CO2

retains heat from sun that would otherwise radiate back into space

• Burning of fossil fuels & wood releases excess CO2, which may be causing global warming

Forest Replacement • Replace w/younger

growth of trees

• Increases photosynthesis, which reduces CO2, but burning at faster rate

• Older trees also remove CO2, but at slower rate