Lecture 4: Lecture 4: PHOTOSYNTHESISPHOTOSYNTHESIS

Life’s grand deviceLife’s grand device

By By

Edgar MoctezumaEdgar Moctezuma

TODAY…• Photosynthesis

I. Intro

II. Properties of light and pigments

III. Chloroplast structure and function

IV. Light reactions

V. “Dark” or Carbon reactions

VI. Summary and conclusions

• RespirationI. Energy and food chainsII. Carbon Cycle

I. Introduction to photosynthesis• From the Greek

PHOTO = produced by light

SYNTHESIS = a whole made of parts put together.

Definition: PHOTOSYNTHESIS is the process whereby plants, algae, some bacteria, use the energy of the sun to synthesize organic compounds (sugars) from inorganic compounds (CO2 and water).

WHY IS PHOTOSYNTHESIS SO IMPORTANT?

PHOTOSYNTHESIS is one of the most important biological process on earth!

• Provides the oxygen we breathe

• Consumes much of the CO2

• Food

• Energy

• Fibers and materials

GENERAL FORMULA FOR PHOTOSYNTHESIS

light

6 CO2 + 12 H2O ---------> C6H12O6 + 6 O2 + 6 H2O

pigments, enzymes

* *

• Oxygen on earth allowed for the evolution of aerobic respiration and higher life-forms.

• Respiration: extracting energy from compounds (sugars)

C6H12O6 + O2 6 CO2 + ATP

II. PROPERTIES OF LIGHTVirtually all life depends on it!

• Light moves in waves, in energy units called PHOTONS

• Energy of a PHOTON inversely proportional

to its wavelength

• Visible light (between UV and IR) occurs in a spectrum of colors

Visible light contains just the right amount of energy for biological reactions

Light is absorbed by pigments

• The primary pigment for photosynthesis is chlorophyll a

• It absorbs blue and red light, not green (green light is reflected back!)

Absorption spectrumof chlorophyll a

• Absorption spectrum of chlorophyll a: BLUE & RED

• Action spectrum of photosynthesis closely matches absorption spectrum of chlorophyll a, but not perfectly (due to accessory pigments)

Accessory pigments like chlorophyll b and carotenoids (beta-carotene, lycopene):

• absorb light at different wavelengths, (extending the absorption range)

• help transfer some energy to chlorophyll a

• protect cell from harmful byproducts

Chlorophyll a is the primary photosynthetic pigment that drives photosynthesis.

Accessory pigments absorb at different wavelengths, extending the range of light useful for photosynthesis.

Where does photosynthesis occur?

The plant cell

III. Chloroplast structure and function: solar chemical factory

Chloroplast structure• Football shaped

• Double membrane

• Stroma

• Thylakoid membrane

• Grana (stacks)

• Lumen (inside thylakoid)

stroma

Grana

thylakoids

lumen

Inside a Chloroplast

• Remember: Structure correlates to function!

Overview of photosynthesis: Note: The Light and “Dark”or Carbon reactions happen at different sites in the chloroplast

LIGHT REACTIONS(Thylakoids)

“DARK” or CARBONREACTIONS

(Stroma)light

ATP

NADPH

(ENERGY)

H2O

O2

(OXYGEN GAS)

CO2

C6H12O6

(GLUCOSE)

IV. The Light Reactions

1. Light dependent

2. Occur in the thylakoid membrane of chloroplast

4. Use light energy (photons) to generate two chemical energy compounds: ATP & NADPH

3. Water is split into oxygen gas (O2) and H+

Chemical energy compounds made in the light reactions

ADP + Pi + Energy ATPadenosine inorganic adenosinediphosphate phosphate triphosphate

NADP+ + 2e- + H+ NADPH

Nicotinamide adenin dinucleotide phosphate

Sequence of events in the Light Reactions

STROMA

LUMEN (inside thylakoid)

PS II PS I ATPSe-

2 H2O O2 + 4 H+

(gas) (protons)

NADP+ + H+

NADPH ADP + Pi

ATP

H+

Summary of the Light reactions

2 H2O + 2 NADP+ + 3 ADP + 3 Pi

O2 + 2 NADPH + 3 ATP + 4 e- + 2 H+

(gas)

Light reactions: Chemical energy compounds are made from light energy, water is split intoO2 and protons

V. The“Dark” or Carbon Reactions

1. Light independent (can occur in light or dark; some enzymes require activation by light)

2. Occur in the stroma of chloroplasts

3. Use the chemical energy produced in Light Reactions (ATP; NADPH) to reduce CO2 to carbohydrate (sugar).

4. CO2 is converted to sugar by entering the Calvin Cycle

CO2RuBPRibulose bisphosphate

rubiscorubisco

3-PGA3-phosphoglycerate

ATP

ADP

NADPH

NADP+

Pi

ATP

ADP

carboxylation

reductionregeneration

GAPGlyceraldehyde 3-phos.

sugars

The Calvin Cycle • Named for M. Calvin

• 3 phases, 13 steps

• CO2 goes 6 cycles to produce 1 glucose

The Calvin Cycle

• CO2 enters the Calvin Cycle

• First product is a 3-carbon molecule: 3-PGA (phosphoglyceric acid). That’s why it’s also called C-3 cycle.

• Enzyme RUBISCO (ribulose bisphosphate carboxylase/oxygenase) is the main enzyme that catalyzes the first reactions of the Calvin Cycle.

• RUBISCO: Is the most abundant protein on earth!

Most plants use the Calvin Cycle to Convert CO2 into sugars.

These plants are called C-3 plants

Summary of Carbon Reactions

6 CO2 + 18 ATP + 12 NADPH + 12 H2O

C6H12O6 + 18 ADP + 18 Pi + 12 NADP+

+ 6 H2O + 6 O2

glucose

Carbon reactions: Use CO2 and chemical energy (ATP &NADPH) to produce sugars by means of the Calvin Cycle

Limitations on Photosynthesis

• Photosynthesis is not perfect in C-3 plants, it is only 1 - 4 % efficient

• Low efficiency due to photorespiration

• Photorespiration occurs when internal CO2

concentration becomes too low (drought);rubisco begins fixing oxygen.

C-4 plants are more efficient

• C-4 plants first product is a 4-carbon molecule

• The C-4 plants (sugar cane, corn, etc.), are more efficient than C-3 plants – they grow in

hotter climates with more light.• For example, sugar cane’s photosynthetic efficiency is 7%• C-4 plants have a different leaf anatomy

C-3 vs. C-4 leaf anatomy

Net venation Parallel venation

VI. Summary of Photosynthesis:

1. Light energy absorbed by chlorophyll a drives the reactions of photosynthesis.

2. Converts light energy into chemical energy to make organic compounds.

3. CO2 and H2O used to produce C6H12O6 (glucose) and O2 (gas).

4. Light Reactions occur in thylakoids of the chloroplasts; ATP and NADPH are formed; water is split to O2 (gas) and protons.

5. Carbon Reactions occur in stroma – Calvin Cycle fixes CO2 to produce C6H12O6 (glucose).

6. Low efficiency, about 1- 4% in C-3 plants.

7. Nevertheless, PHOTOSYNTHESIS is still the most important biological process on earth!

Importance of photosynthesis and the impact that it has in all our lives.

Without photosynthesis, virtually all plants and animals would become extinct.

Respiration, Energy & Carbon Cycle• Energy• Virtually all organisms require energy of food for:• Making chemicals

(proteins, carbs, etc.)• Movement• Cell division• Heat, electricity and light production

• The way living organisms obtain energy is throughCell respiration

RESPIRATION• Process of making energy of food available in

the cell…

• Involves breaking down• Complicated molecules into simple molecules

(C6H12O6, sugars) (CO2, water)

RESPIRATIONThe energy held by complicated molecules is held

temporarily as ATP (energy currency)

C6H12O6 + 6 O2 6CO2 + 6 H2O + 36 ATP

(glucose) (energy)

Respiration occurs mainly inMitochondria and Cytoplasm

Stages of Respiration

Cellular Respiration has three main stages:

• Glycolysis

• Krebs Cycle

• Electron transport system

3 Stages of cellular respiration• Glycolysis: Splitting of glucose – 2 net ATP generated

• Krebs Cycle: Energy of glucose molecule is harvested as ATP (2) – it occurs in the mitochondria (matrix)

• Electron Transport System: also happens in the mitochondria, more ATP are generated (32).

• For each glucose molecule, total ATP = 36• Only 39% efficient, rest is lost as heat.

Photosynthesis Respiration• Reaction: CO2+H2O+sunC6H12O6+O2+H2O C6H12O6+O2CO2+H2O+36ATP

• Reactants: Carbon dioxide, water, sun Glucose, oxygen

• Products: Glucose Energy

• By-products: Oxygen Carbon dioxide, water

• Cellular location: Chloroplasts Cytoplasm, mitochondria

• Energetics: Requires energy Releases energy

• Chemical paths: Light reactions & Glycolysis, Krebs cycle Calvin cycle & Electron Transport

Syst.

• Summary: Sugar synthesized using Energy released from energy from the sun sugar breakdown

Chapter 4: Table 4.1, p. 63

Photosynthesis and respiration

• Photosynthesis and respiration are complimentary reactions…

PHOTOSYNTHESIS RESPIRATION

CO2 + H2O O2 + SUGARS SUGARS + O2 H2O + CO2

PLANTS, PLANTS, ALGAE, ALGAE,

BACTERIABACTERIAMOST LIVING ORGANISMS

H2O H2O

O2O2CO2

CO2

SUGARS

Sunlightenergy

USEFUL CHEMICALUSEFUL CHEMICALENERGY (ATP)ENERGY (ATP)

ENERGY: ability to do workNewton’s First Law of Thermodynamics:

“Energy cannot be created or destroyed, it can only be transformed from one form

to another”

• Once a cell has used energy to do work, it cannot be used again by any organism.

(1701)

ENERGYENERGY FLOW IS LINEARSun Earth Producers 1o consumers 2o consum

heat resp, heat resp, heat resp, heat

Energy flows into ecosystem from the sun

Energy travels in a straight line by way of

food chains.

ENERGYHowever, much energy is lost as heat along the

way – as a result of respiration.

Approximately 90% energy is lost on each step!

• Newton’s Second Law of

Thermodynamics:

“In any transfer of energy there is always a loss of useful energy to the system, usually in

the form of heat”

Food Chains• (Not referring to SHOPPERS, SAFEWAY or GIANT !!!)

• Food chains demonstrate linear nature of energy

• Producers are the base of the food chain, they include photosynthetic organisms like:

• Plants

• Algae

• Certain bacteria

Food chains• Primary consumers – all plant

eaters (herbivores).

• Secondary consumers –

Eat primary consumers, (carnivores)

Food chains

• Decomposers – obtain energy by breaking down remaining organic material of the other members of the food chain.

• Fungi and bacteria.

Matter• All important elements move in cycles;

Environment Organisms

Cycles called biogeochemical cycles:

Water cycle

Carbon cycle

Nitrogen cycle

The Carbon Cycle• Carbon from the atmosphere (CO2) enters the

biosphere by way of plants!– CO2 used in photosynthesis

– Carbon moves into food chain

• Carbon is released to the physical environment by respiration – Release CO2 during respiration

– Amount CO2 fixed in photosynthesis = the amount released by respiration

Carbon Cycle• Carbon moves from atmosphere to plants to

animals and back to atmosphere.

“Look deep into nature, and then you will understand everything better.” Albert Einstein