Chapter 7

How Cells Acquire Energy

• Self nourishing• Obtain carbon from carbon dioxide• Photosynthetic autotrophs (plants,

protistians, and bacterial membranes) harness light energy

• Chemosynthetic autotrophs (few bacteria) extract energy from chemical reactions involving inorganic substances (e.g.Sulfur compounds)

Autotrophs

• Obtain carbon and energy from the autotrophs

• Include protistans, bacteria, animals, and fungi

• Carbon and energy enter the web of life by photosynthesis and in turn are released by glycolysis and aerobic respiration

Heterotrophs

• A. Photosynthesis Transforms Solar EnergyB. Organic molecules built by photosynthesis provide both the building blocks and energy for cells.C. Plants use the raw materials: carbon dioxide and waterD. Chloroplasts carry out photosynthesis

• 2 stages of photosynthesis takes place in the chloroplast– Which has two layers (membranes) stroma and

thylakoids• E. Chlorophylls and other pigments involved in

absorption of solar energy reside within thylakoid membranes of chloroplasts

Overview of Photosynthesis

Plants as Solar Energy Converters

• A. Solar Radiation - Only 42% of solar radiation that hits the earth’s atmosphere reaches surface; most is visible light.

Continue…• B. Photosynthetic Pigments - Pigments found in

chlorophyll absorb various portions of visible light; absorption spectrum.

• 1. Two major photosynthetic pigments are chlorophyll a and chlorophyll b.2. Both chlorophylls absorb violet, blue, and red wavelengths best.3. Very little green light is absorbed; most is reflected back; this is why leaves appear green.4. Carotenoids are yellow-orange pigments which absorb light in violet, blue, and green regions.5. When chlorophyll breaks down in fall, the yellow-orange pigments in leaves show through.

• Carontenoid: absorbs blue-green wavelengths but reflect yellow, orange, and red

• Anthocyanins: Pigments found in flowers

• Phycobilins: are the blue and red pigments of red algae and cyanobacteria

Secondary Pigments

• Electromagnetic Spectrum: light energy travels in waves through space from gamma rays to radio waves

• The shorter the wavelength the more energy –Example Sun’s radiation

Properties of Light

• Photoautotrophs use a small range (400-750 nm) of wavelength for photosynthesis -- which is the range for visible light

• Light energy is packaged as photons, which vary in energy as a function of wavelength – Blue violet light : most energetic – Red light: least energetic

Continue…

• Photosynthetic bacteria pigment is found at the plasma membrane

• In thylakoid membrane systems of cholorplast the pigments are organized in clusters called photosystems consisting of 200 to 300 pigment molecules

Where are photosynthetic pigments located?

• Light-dependent reactions convert light energy to chemical energy (which is then stored into ATP)– Liberated electrons are picked up by NADPH

• Light-independent reaction assemble sugars and other organic molecules using ATP, NADPH, and CO2

12 H20 + 6CO2 602 + C6H12O6 + 6H20

Things that Don’t need Glucose

• Light Dependent Reactions occur in the thylakoid– Thylakoid are folded into grana (stacks of

disks) and channels – The interior spaces of the thylakoid disks and

channels are coninuous and are filled with H+ needed for ATP synthesis

• Carbohydrates formation occurs in the stroma (semifluid) area that surrounds the grana

Light Dependent Reactions

• First reaction of photosynthesis

• Three events occur

• 1- Pigment absorb sunlight energy and give up excited electrons

• 2- Electron and hydrogen transfer lead to ATP and NADPH formation

• 3- Pigments that gave up the electrons in the first place get electron replacements

Light Dependent Reactions

• The pigments “harvest” photon energy from sunlight– Absorbed photons of energy boost electrons

to a higher level.– Electrons return to lower level– Released energy is trapped by cholorphylls

located in the photsystem’s reaction center– The trapped energy is then used to transfer a

chlorophyll electron to an acceptor molecule

What Happens to the Absorbed Energy?

• Is an organized array of enzymes, coenzymes, and other proteins embedded in or anchored to a cell membrane– Accept electrons which are then processed

through a step-by-step array to produce ATP and NADPH

Electron Transfer Chain

• Oldest mean of ATP production being used by early bacteria– Excited electrons leave the P700 reaction

center, pass through an electron transport system, and then return to the original photosystem I

– Energy associated with the electron flow drives the formation of ATP from ADP

– Figure 7.12

Cyclic Pathway

• ATP Formation transfer through two photosystems and two electron transport systems (ETS) in the thylakoid membrane– Boosted electrons moves through a transport

system that releases energy for

ADP + PiATP

– Electrons fills “hole” left by electron boost in P700 of photosystem I

Noncyclic Pathway SPLITS WATER, PRODUCES NADPH & ATP

– Electron from photolysis of water fills “electron hole” left in p689 and produces oxygen byproduct

• Pathway continues when chlorophyll P700 of photosystemI is absorbs energy– Energy hole is filled by elctron from P680– Boosted electron from P700 passes to

acceptor, then ETS is finally joins NADP to form NADPH (which along with the ATP can be used in synthesis of organic compounds

• Turn to page 123 (Figure 7.13)• Hyperlink\Light reactions.mht

Continue…

• Watch movie here…

• You can watch this movie going to my portaportal.com website

• Guest Name Mssweikle

• Go to the AP Biology Folder

• Find Photosynthesis movie

• Oxygen is a by-product of the noncyclic pathway

• Beginning about 1.5 billion years ago, large amounts of oxygen began accumulating in the atmosphere, which at the time was oxygen-free

The New Atmosphere

• These reactions (Calvin-Benson Cycle) are the “synthesis” of phytosynthesis– Handout: Slide 27

• The participants and their roles in the synthesis of carbohydrates are– ATP, which provides energy– NADPH provides the hydrogen atoms and electrons– Atmosphere provides the Carbon dioxide

• The reactions are dependent on sunlight

Light-Independent Reactions

Fixation of Carbon Dioxide1. CO2 fixation is the attachment of CO2 to

an organic compound called RuBP.

2. RuBP (ribulose bisphosphate) is a five-carbon molecule that combines with carbon dioxide.

3. The enzyme RuBP carboxylase (rubisco) speeds this reaction; this enzyme comprises 20–50% of theprotein content of chloroplasts, probably since it is a slow enzyme.

1. With reduction of carbon dioxide, a PGA (3-phosphoglycerate[C3]) molecule forms.

2. Each of two PGA molecules undergoes reduction to PGAL in two steps.

3. Light-dependent reactions provide NADPH (electrons) and ATP (energy) to reduce PGA to PGAL.

Reduction of Carbon dioxide

Regeneration of RuBP

• 1. Every three turns of Calvin cycle, five molecules of PGAL are used to re-form three molecules of RuBP.

• 2. Every three turns of Calvin cycle, there is net gain of one PGAL molecule; five PGAL regenerate threemolecules of RuBP.

Importance of the Calvin Cycle1. PGAL, the product of the Calvin Cycle can be

converted into all sorts of other molecules.

2. Glucose phosphate is one result of PGAL metabolism; it is a common energy molecule

3. Glucose phosphate is combined with fructose to form sucrose used by plants.

4. Glucose phosphate is the starting pint for synthesis of starch and cellulose.

5. The hydrocarbon skeleton of PGAL is used to form fatty acids and glycerol; the addition of nitrogen forms various amino acids.

Calvin Cycle

• Each PGA then receives a phosphate group from ATP plus H+ and electrons from NADPH to form PGAL – Most PGAL molecules will continue in the

cycle to fix more carbon dioxide, but two PGAL join to form a sugar phosphate, which will be modified to sucrose, starch, and cellulose.

– Final Tally:

How Do Plants Build Glucose?

• Sugar phosphate are used as cellular fuel and as building blocks in synthesis of sucrose or starch.– Sucrose is the most easily transportable– Starch is the main storage form, but will be

converted by to sucrose for distribution to leaves, stems, and roots

– Photosynthesis yields intermediates and products that can be used in lipid and amino acid synthesis

Continue…

Factors that affect photosynthesis

1. Light Quality (color)

2. Light intensity

3. Light Period

4. Carbon Dioxide Availability

5. Water Availability

How gases enter and leave plants?

• 1. The Calvin Cycle is the MOST Common Pathway for Carbon Fixation. Plant Species that fix Carbon EXCLUSIVELY through the Calvin Cycle are known as C3 PLANTS.

• 2. Other Plant Species Fix Carbon through alternative Pathways and then Release it to enter the Calvin Cycle.

C3 Plants

• 3. When a plant's Stomata are partly CLOSED, the level of CO2 FALLS (Used in Calvin Cycle), and the Level of O2 RISES (as Light reactions Split Water Molecules).

• 4. A LOW CO2 and HIGH O2 Level inhibits Carbon Fixing by the Calvin Cycle. Plants with alternative pathways of Carbon fixing have Evolved ways to deal with this problem.

Continue…

C4 Plants

• C4 PLANTS - Allows certain plants to fix CO2 into FOUR-Carbon Compounds. During the Hottest part of the day, C4 plants have their Stomata Partially Closed. C4 plants include corn, sugar cane and crabgrass. Such plants Lose only about Half as much Water as C3 plants when producing the same amount of Carbohydrate.

• Cactus, pineapples, and other succlents have different adaptations to Hot, Dry Climates. They Fix Carbon through a pathway called CAM.

• Plants that use the CAM Pathway Open their Stomata at NIGHT and Close during the DAY, the opposite of what other plants do. At NIGHT, CAM Plants take in CO2 and fix into Organic Compounds.

• During the DAY, CO2 is released from these Compounds and enters the Calvin Cycle.

• Because CAM Plants have their Stomata open at night, they grow very Slowly, But they lose LESS Water than C3 or C4 Plants.

CAM Plants

• The ocean host a vast number of photoautotrophic prokaryotic cells and protistans – They shape the global climate

Ocean Photoautotrophs