Chapter 8 The Working Cell: Energy from Light

Ms. LevensailorHonors CS2

Brainstorm

• With your shoulder partner:– Brainstorm what you know about

photosynthesis.– Record this in your composition book.

Intro to Photosynthesis• Photosynthesis is the process by which

plants (also some bacteria and protists) use the energy from sunlight to produce sugar

• The conversion of unusable sunlight energy into usable chemical energy

• 6H2O + 6CO2 ---> C6H12O6+ 6O2

• Electrons from water are boosted "uphill" by the energy from sunlight. The chloroplast uses these "excited" electrons, along with carbon dioxide and hydrogen ions, to produce sugar molecules.

Intro to Photosynthesis

• A leaf may be viewed as a solar collector crammed full of photosynthetic cells

• The raw materials of photosynthesis, water and carbon dioxide, enter the cells of the leaf, and the products of photosynthesis, sugar and oxygen, leave the leaf.

Anatomy of a Leaf

Anatomy of a Leaf• Water enters the root and is transported up to the leaves

through specialized plant cells known as xylem.• Sugar can be transported throughout the plant through

other specialized plant cells known as phloem. • Land plants must guard against drying out (desiccation)

and so have evolved specialized structures known as stomata to allow gas to enter and leave the leaf. – Carbon dioxide cannot pass through the protective waxy layer

covering the leaf (cuticle), but it can enter the leaf through an opening flanked by two guard cells.

– Likewise, oxygen produced during photosynthesis can only pass out of the leaf through the opened stomata.

– Unfortunately for the plant, while these gases are moving between the inside and outside of the leaf, a great deal water is also lost.

Anatomy of a Leaf• The palisade mesophyll

– Consists of one or several layers of  elongated, narrow cells that are situated under the upper epidermis. 

– Chloroplasts are especially concentrated in the palisade mesophyll

– Much of the photosynthesis takes place here• The spongy mesophyll

– Consists of irregularly shaped cells which are located between the palisade mesophyll and the lower epidermis.

– Contain less chloroplast than the palisade mesophyll cells but photosynthesis take place in these cells as well.

– There are many intercellular air spaces between the spongy mesophyll cells which are interconnected and communicate with the stomata of the lower epidermis. 

– This allows the food-producing cells of the leaf to access the gases, which they need for photosynthesis and respiration.

Photosynthesis in Nature

• Photosynthesis nourishes all living things directly or indirectly.

• Organisms acquire organic compounds for energy as autotrophs or heterotrophs.

• Autotrophs: self-feeders, produce organic molecules from CO2 and inorganic raw materials (aka producers).

Autotrophs

– Photoautotrophs: organisms that use light as a source of energy to synthesize organic substances.

• Plants & algae– Chemoautotrophs: obtain energy by

oxidizing inorganic substances, such as sulfur and ammonia (without light).

Heterotrophs

• Obtain organic material by consuming compounds made by other organisms (aka consumers).– Animal eats plants or other animals

• Note: heterotrophs are dependent on photoautotrophs for food and oxygen!

Chloroplasts are the sites of photosynthesis!

All green parts of a plant have chloroplasts.-Leaves are the major site of photosynthesis.

Green color is due to the pigment, chlorophyll.– Chloroplasts are found mainly in the mesophyll

(tissue on the interior of the leaf).

*Put these terms in order from largest to smallest: thylakoid, leaf, chloroplast, mesophyll.

Anatomy of a Chloroplast

Pathways of photosynthesis

• 6CO2 + 12H2O + light C6H12O6 + 6O2 + 6H20• The carbohydrate produced is glucose.• Water appears on both sides because 12

molecules of water are consumed and 6 new molecules of water are produced during photosynthesis.

• We usually see the formula simplified:– 6CO2 + 6H2O + light > C6H12O6 + 6O2

The Splitting of Water

• The chloroplasts split water into hydrogen and oxygen.

• This means that hydrogen is extracted from water and incorporated into sugar.

6 CO2 12H2O

C6H12O6 6 H2O 6O2

Tracking Atoms

Reactants:

Products:

Check for Understanding

• Write the chemical equation for photosynthesis.

• Trace the atoms as they flow from reactant to product.

Photosynthesis

• Photosynthesis occurs in two main stages:– The light reactions (thylakoid)– The Calvin cycle (stroma)

Light Reactions

• Light reactions (the photo part of photosynthesis) convert solar energy to chemical energy.– Light absorbed by the chlorophyll drives a

transfer of electrons and hydrogen from water to an acceptor called NADP+.

• This temporarily stores the energized electron.– Water is split in the process (light rxn gives off

oxygen as a by product).

Light Reactions

• The solar power reduces the NADP+ to NADPH by adding a pair of electrons & a hydrogen nucleus (H+).

• ATP is generated in the light rxn by powering the addition of a phosphate group to ADP.– Photophosphorylation

• NOTE: No sugar is produced!

Calvin Cycle (light independent)

• Begins by incorporating CO2 from the air into organic molecules already present in the chloroplast. – Carbon fixation

• Reduces fixed carbon to carbohydrate by the addition of electrons.– Reducing power is provided by NADPH.– Also requires chemical energy in the form of

ATP.

Cooperation of Two Reactions

• Thylakoids of the chloroplast are sites of the light reactions.

• Stroma is the location of the Calvin cycle.

Check for Understanding

• The chemical products of the light reaction are oxygen, . . .

“Light” Review

• The energy of light is inversely proportional to the wavelength: longer wavelengths have less energy than shorter ones.

• The order of colors is determined by the wavelength of light.

• Visible light is one small part of the electromagnetic spectrum.

The light reaction converts light energy to chemical energy

Visible light consists of wavelengths from about 400 nanometers (nm), violet, to about 700 nm, red

• Short wavelengths of gamma rays to the very long wavelengths of radio waves

Pigment

• A pigment is any substance that absorbs light. • Chlorophyll, the green pigment common to all

photosynthetic cells, absorbs all wavelengths of visible light except green, which it reflects to be detected by our eyes.

• Pigments have their own characteristic absorption spectra, the absorption pattern of a given pigment (you graphed this in the lab).– Various pigments in the thylakoids to absorb the

maximum amount of light

Absorption Spectra vs. Action Spectrum

• Absorption Spectra: graph plotting a pigment’s light absorption vs. wavelength.

Absorption Spectra vs. Action Spectrum

• Action Spectrum: profiles the relative performance of the different wavelengths.– Graphs wavelength vs. photosynthetic rate.

Pigments• Several modifications of chlorophyll occur

among plants and other photosynthetic organisms.

• All photosynthetic organisms (plants) have chlorophyll a.

• Accessory pigments absorb energy that chlorophyll a does not absorb. – chlorophyll b (also c, d, and e in algae and protists)– xanthophylls– carotenoids (such as beta-carotene)

Pigments and Color• When light shines on a material that contains pigments,

three things can happen to the different wavelengths: – Absorbed– Transmitted– Reflected

• The pigments in the leaf's chloroplasts absorb blue-violet and red-orange light very well.

• Chloroplast pigments do not absorb green light well. Most of the green light passes through the leaf (is transmitted) or bounces back (is reflected). – Leaves look green because the green light is not absorbed.

Chromatography• Using a laboratory technique called paper chromatography,

you could observe the different pigments in a green leaf. – We did this in class

• Different pigments travel at different rates, depending on how easily they dissolve and how strongly they are attracted to the paper.

• Chlorophyll a, which absorbs mainly blue-violet and red light and reflects mainly green light

• Chlorophyll b, which absorbs mainly blue and orange light and reflects yellow-green

• Carotenoids, which absorb mainly blue-green light and reflect yellow-orange.

Ratio of Fronts

• Used to compare the rates of movement of different pigment bands.– Rf = distance solute traveled / distance solvent

front traveled.– Ratios written as n : 1

Solvent Front

Original Pigment Mark

Carotene

Xanthophyll

Clorophyll a

Clorophyll b

10 cm5 cm

Rf = 5 cm / 10 cm = 0.5

0.5 : 1

Check for Understanding

• Explain why a leaf containing chloroplasts looks green in color.

• Describe 3 things that could happen to light of a particular wavelength after it strikes a leaf.

Harvesting the Energy in Food

• Cellular respiration is a chemical process that converts stored energy into ATP (adenosine triphospate).

• ATP is the main energy source for plant and animal cells.

• ATP provides energy for cellular work.

How ATP Packs Energy

• Adenosine = consists of a nitrogen containing compound called adenine & a five carbon sugar called ribose.

Adenosine

How ATP Packs Energy

• The triphosphate tail consists of 3 phosphate groups.

• Tail = “business end” of ATP– The source of energy used for most cellular

work!I am the source!

Phosphate Groups

• Each group is negatively charged.– Like charges repel, contributing to the potential

energy stored in ATP.– Its like energy being compressed in spring.

• When ATP is involved in a chemical rxn that breaks a phosphate bond, potential energy is released.– Usually only one phosphate is lost from ATP.– The resulting molecule is ADP (adenosine

diphosphate.

ATP Cycle

• ATP is continuously converted to ADP as cells do work.

• ATP is recyclable:– ADP can be restored to ATP by adding a third

phosphate group.– This requires energy (from organic molecules

in food).– In muscle cells 10 million ATP molecules are

spent and regenerated per second!

ATP CYCLE

What happens when chlorophyll absorbs photons?

• When a molecule absorbs a photon an electron is elevated to an orbital of higher energy.– Results in more potential energy (excited

state).• Photons are absorbed by clusters of

pigment molecules in the thylakoid membrane.

Harvesting Light Energy• Within the thylakoid membrane, chlorophyll and other

molecules are arranged in clusters called photosystems – Energy "jumps" from molecule to molecule until it arrives at what

is called the reaction center of the photosystem • Alternating from ground to excited state and back again

• The reaction center consists of a chlorophyll a molecule located next to another molecule called a primary electron acceptor. – The primary electron acceptor is a molecule that traps the

excited electron from the chlorophyll a molecule. • Other teams of molecules built into the thylakoid

membrane can now use that trapped energy to make ATP and NADPH.

Photosystems

• Two photosystems are involved in the light reaction:

• Photosystem I is known as P700 (absorption peak at 700).

• Photosystem II is known as P680 (absorption peak at 680).– Identical chlorophyll a molecules.– Associated with different proteins in the

thylakoid membrane.

Noncyclic Electron Flow

• Two possible routes for electron flow:– Cyclic & noncyclic

• Noncyclic electron flow (predominant route) generates ATP & NADPH.– PHOTOSYSTEM I & II– Goes through photosystem II first

• Cyclic – PHOTOSYSTEM I

Photosystem II

• Read Pg. 186 Figure 10.12.• Fill out storyboard graphic organizer.

Photosystem II

• Traps light energy and transfers the light-excited electrons to an electron transport chain.

• "water-splitting photosystem" • electrons are replaced by splitting a molecule of

water• releases oxygen as a waste product, and also

releases hydrogen ions.

Electron Transport Chain

• The electron transport chain connecting the two photosystems releases energy, which the chloroplast uses to make ATP. – An electron transport chain pumps hydrogen

ions across a membrane (thylakoid membrane in photosynthesis).

– Light-excited electrons from chlorophyll travel down the chain.

Photosystem I

• The second photosystem – "NADPH-producing photosystem”– This photosystem produces NADPH by transferring

excited electrons and hydrogen ions to NADP+

• NOTE: So far no sugar has been produced. – Job of the Calvin cycle, which uses the ATP and

NADPH produced by the light reactions.

Photosystems & Electron Flow

Cyclic Electron Flow

• Use of only photosystem I.• Electrons cycle back from (Fd) to

cytochrome complex and continue onto P700 chlorophyll.

• No NADPH or release of oxygen.– Does generate ATP!

• WHY DO THIS?– Calvin cycle consumes more ATP than

NADPH.

Cyclic Electron Flow

Chemiosmosis: ATP Synthesis

• Electron transport chain in the thylakoid membrane pumps protons across the membrane.

• Potential energy is stored in the form of an H+ gradient.

• Also in the membrane is an ATP synthase complex.– Uses diffusion of H+ ions in the

phosphorylation of ADP.

Light Reactions

Calvin cycle makes sugars from carbon dioxide

• It is called a cycle because the starting material is regenerated each time the process occurs. – Starting material that gets regenerated is a compound called

ribulos bisposphate (RuBP), a sugar with five carbons. • With each turn of the Calvin cycle, there are chemical

inputs and outputs. – The inputs are:

• Carbon dioxide from the air • ATP and NADPH produced by the light reactions

– The cycle's output is • Energy-rich sugar molecule• Not yet glucose, but a smaller sugar named G3P

– The plant cell uses G3P as the raw material to make glucose and other organic molecules it needs

Calvin Cycle

• Important Notes:– The cycle spends ATP as an energy source.– It consumes NADPH as reducing power for

adding high energy electrons to make sugar.– For the net synthesis of one molecule of G3P,

the cycle must take place 3 times.• Fixing 3 molecules of CO2

Calvin Cycle

• Phase 1: Carbon Fixation– Each CO2 is attached to RuBP, catalyzed by

rubisco.– The product is a 6 carbon intermediate

(unstable!)– Immediately splits in half to form 2 molecules

of 3-phosphoglycerate (for each CO2)

Calvin Cycle

• Phase 2: Reduction– Each molecule of 3-phospholycerate receives an

additional phosphate group from ATP• Becomes 1,3-bisphosphoglycerate

– Then NADPH reduces 1,3-bisphosphoglycerate to G3P (glyceraldehyde-3-phosphate)

• Does this by donating a pair of electrons

– NOTE: for ever 3 molecules of CO2, there are 6 molecules of G3P BUT only 1 is counted as a net gain of carbohydrate.

Phase 2: Reduction

• 1 molecule exits the cycle to be used.• The 5 other molecules must be recycled to

regenerate 3 molecules of RuBP.

Calvin Cycle

• Phase 3: Regeneration of CO2 accetor (RUBP)– Series of complex rxn change the 5 molecules of G3P

into 3 molecules of RuBP.– To do this the cycle must spend 3 molecules of ATP.

• How many ATP are consumed?– 9

• How many NADPH molecules are consume?– 6