1. Photosynthesis •Plants and Oxygen•Plant Respiration•Parts of Photosynthesis•Light Reactions

Photosynthesis

Photosynthesis is essential to all life on earth; both plants and animals depend on it. It is the only biological process that can capture energy that originates in outer space (sunlight) and convert it into chemical compounds (carbohydrates) that every organism uses to power its metabolism.

Photosynthesis Photosynthesis uses carbon dioxide and water to assemble carbohydrate molecules and release oxygen as a waste product into the atmosphere.

The Plant CellNucleus

Cell Wall

Central Vacuole

Chloroplasts

Plants and Oxygen Production• Cyanobacteria, like these in

Yellowstone National Park, were the world’s first Oxygen Producers.

• This Oxygen Revolution transformed early earth’s atmosphere.

• It was good for some (allowing for increased efficiency with O2 respiration) but bad for others (in the form of mass extinction of anaerobes)

StromatolitesStromatolites are special rock-like structures that form in shallow water.

• They are formed by cyanobacteria that use water, carbon dioxide, and sunlight to create their food, and put out oxygen as a by-product.

• The Earliest Evidence of Stromatolites: 3.5 BYA!

This world map shows Earth’s distribution of photosynthesis as seen via chlorophyll a concentrations. On land, this is evident via terrestrial plants, and in oceanic zones, via phytoplankton.

Percentage of Earth's Surface Area

Open Ocean

Continental Shelf

Extreme Desert, Rock, Sand, Ice

Tropical Rainforest

Savanna

Cultivated land

Boreal forest

Temperate grassland

Woodland and shrubland

Tundra

Tropical seasonal forest

Temperate deciduous forest

Temperate evergreen forest

Swamp and marsh

Lake and stream

Estuaryu

Algal beds and reefs

Upwelling zones

0 10 20 30 40 50 60 70

Pretty Big!

HUGE!

Open OceanContinental Shelf

Extreme Desert, Rock, Sand, IceTropical Rainforest

SavannaCultivated land

Boreal forestTemperate grassland

Woodland and shrublandTundra

Tropical seasonal forestTemperate deciduous forestTemperate evergreen forest

Swamp and marshLake and stream

EstuaryuAlgal beds and reefs

Upwelling zones

0 500 1000 1500 2000 2500 3000

Average net primary production (g/m2/yr)

Low Productivity!

High Productivity!

Open OceanContinental Shelf

Extreme Desert, Rock, Sand, IceTropical Rainforest

SavannaCultivated land

Boreal forestTemperate grassland

Woodland and shrublandTundra

Tropical seasonal forestTemperate deciduous forestTemperate evergreen forest

Swamp and marshLake and stream

EstuaryuAlgal beds and reefs

Upwelling zones

0 5 10 15 20 25 30

Percentage of Earth's net primary production

Way higher than the others!

Oceans and Rainforest are vital to oxygen production on Earth! What will happen if they aren’t protected?

Photosynthesis/Respiration

Photosynthesis:• Plants use H2O and

carbon dioxide and produce starch and oxygen

H2O + CO2 = Starch/sugar + O2

Respiration• Animals use

starch/sugar and oxygen, and produce H2O and carbon dioxide

Starch/sugar + O2 = H2O + CO2

Photosynthesis is a multi-step process that requires sunlight, carbon dioxide (low in energy) and water as substrates.

Photosynthesis releases oxygen and produces simple carbohydrate molecules (which are high in energy) that can subsequently be converted dozens of other sugar molecules.

These sugar molecules contain energy and the energized carbon that all living things need to survive.

Stomata on the underside of a leaf

An open (left) and closed (right) stoma of a spider plant (Chlorophytum colosum) leaf. When guard cells are turgid, the stoma is open (left).

Stomata on the underside of a leaf

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The Chloroplast• Most of the living world

depends on chloroplasts for its energy!

• Two membranes on outside• Complex membrane structure

on inside

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Photosynthesis Summary

Photosynthesis SummaryPhotosynthesis takes place in two sequential stages

In the light-independent reactions, the chemical energy harvested during the light-dependent reactions drive the assembly of sugar molecules from carbon dioxide.

In the light-dependent reactions, energy from sunlight is absorbed by chlorophyll and that energy is converted into stored chemical energy.

Absorbed and Reflected Light

Absorbed Light

Reflected Light

Transmitted Light

Plants are green because chlorophyll reflects green light.

Photosystem

Excited electrons are the key to photosynthesis.

Grannum (stack of thylakoids)

Within the membrane of each thylakoid arecountless clusters of pigments. These pigments are inside Photosystems.

The pigments act as antenna, bouncing photons towards the Primary Electron Receptor

A photosystem consists of a light-harvesting complex and a reaction center. The first photosystem of photosynthesis is called photosystem 2.

Light harvesting complex

Reaction Center

Photosystem 2

Pigments in the light-harvesting complex pass light energy (in the form of photons) to two special chlorophyll a molecules in the reaction center.

Chlorophyll a molecules

The light excites an electron from the chlorophyll a pair, which passes to the primary electron acceptor.

Primary Electron Acceptor

Photosystem 2

The chlorophyll molecule must get a new electron from somewhere! It’s electron is replaced by the splitting of a water molecule. When a molecule of water is split energy (and oxygen) is released.

OH H

OH H

e-

For every two water molecules that are split, one molecule of O2, the oxygen we breathe, is produced.

Meanwhile, the excited electron that was raised to the primary electron acceptor is transferred to a mobile carrier protein, that moves it along the electron transfer chain.

As it moves along the electron transfer chain it releases ‘works’ to produce ATP, the currency of energy, that a cell uses. However, as it produces ATP becomes less and less excited.

ATP ATP ATP

Photosystem 2

ATP ATP ATP

At the end of the Electron Transfer Chain, the electron is no longer excited. It enter Photosystem 1 and is excited again by the photons boosting it back into it’s high energy state.

e-

Photosystem 1 Photosystem 2

ATP ATP ATP

The re-excited electron is again transferred to a mobile carrier protein, that moves it along the electron transfer chain, however this time it is combined with another electron, one proton, and a molecule of NADP+ to create a molecule of NADPH.

NADP+

H

NADPH

Photosystem 1

Active Transport: Proton pumps

The ATP released during the ETC transfer is used to drive proton pumps which sets up a concentration gradient of high H+ inside the thylakoid (lumen) and a low H+ outside the thylakoid (stroma)

H

H

Inside of Thylakoid

Outside of Thylakoid

ATP

ATP

H

HHH

ATPase Activity

H

Inside of Thylakoid

Outside of Thylakoid

ADP

Protons (H+) flow down concentration gradient through ATPase, an enzyme that synthesizes ATP.

PH

HHH

HHH

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Light Reactions• Light boosts electrons in Photosystem II, high energy

electrons passed along chain of carriers • Electrons replaced by splitting water• Passage of electrons down chain releases energy

used to fuel proton pumps to generate ATP• Chain ends in Photosystem I, electron energy

boosted again, passed on to NADPH• ATP, NADPH (fuel) produced by light reactions

provide energy to power Calvin Cycle (making sugar)