Photosynthesis

Photosynthesis is a producer

• Photosynthesis nourishes most organisms on earth

• Plants and other autotrophs are producers

• Autotrophs produce their own food

• Heterotrophs feed on organic material

2 Types1. Anoxygenic: does

NOT produce O2

Bacteria

2. Oxygenic: produces O2

Cyanobacteria, algae, and land plants

Both use pigments but differ in function of pigments.

Chloroplast: photosynthetic

organelles

3 Processes occur here:

1. Capture light energy

2. Use energy to make ATP and reduce NADP+ to NADPH

3. Use ATP and NADPH to power the synthesis of organic molecules from CO2

2 Stages of Photosynthesis

1. Light-dependent reactions: Light energy stored in ATP and NADPH. Needs light.

2. Light-independent reactions: cycles that form organic molecules using CO2 in a process called carbon fixation. Does NOT need light.

Equation

6CO2 + 12H2O + light energy C6H12O2 + 6H2O + 6O2

Carbon Water Glucose Water Oxygen

Dioxide

Generally, opposite of cell respiration

The Chloroplast• The chloroplast is the site of

photosynthesis in a cell

• Contains a pigment called chlorophyll

• Chloroplasts are found mainly in the Mesophyll (leaf tissue)

• CO2 and O2 enter the leaf via the stomata

Structure of Chloroplast• Like mitochondria, chloroplasts have an

internal and external membrane.

• Thylakoids are structures made of the inner membranes. A stack of thylakoids is called granum (grana plural).

• Thylakoid membranes contain pigments like chlorophyll.

Structure of Chloroplast (cont’d)

• The photosynthetic pigments are clustered together to form photosystems which capture energy packets called photons.

• Surrounding the grana is a semiliquid called stroma which houses the enzymes that make organic molecules.

Discovery of Photosynthetic Processes

Some bacteria use hydrogen sulfide(H2S) instead of water for photosynthesisThus, the general formula for photosynthesis is:

CO2 + 2H2X CH2O + H2O +2X

By using O2 isotopes, C.B. Van Niel proved that the O2 produced came from the splitting of water

Tracking Atoms through Photosynthesis

Pigments

• Molecules that absorb light energy in the visible range.

Light is a form energy• Visible light is made of various colors which are

different due to their wavelength.

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

Photons

• Particle of light acting as a bundle of energy.

• Light has a dual nature as it is both waves and energy.

Photoelectric Effect

• When light or photons transfer energy to electrons, the electrons are removed from molecules and create an electric current.

• Chloroplasts act as photoelectric devices.

Absorption Spectrum

• The electrons that absorb energy jump to higher energy levels. The shorter the wavelength of light the greater the energy absorbed.

• To boost electrons to discrete energy levels, specific atoms can only absorb specific photons of light relative to the atoms energy levels. The range of photons a molecule can absorb is called the absorption spectrum.

Action spectrum

• Relative effectiveness of different wavelengths of light for photosynthesis.

Chlorophyll a and Chlorophyll b

• Chlorophyll a is the main pigment that can directly convert light energy to chemical energy.

• Chlorophyll b is a secondary or accessory pigment that helps to absorb a greater range of wavelengths of light.

• Both absorb red and blue-violet light. Thus they reflect green light giving a green appearance.

Structure of Chlorophylls

• Chlorophylls are made of a porphyrin ring (rings w/ single and double bonds) a Mg atom at the center, a hydrocarbon chain, and a side chain.

Carotenoid

• Pigment that absorbs mostly blue and green ranges of light capturing energy from wavelengths of light not absorbed by chlorophyll.

• Play protective role.

• May contain beta-carotene which is helpful for vision.

Phycobiloproteins• Pigment found in cyanobacteria and algae

which absorbs green light.

Photosystem Organization (8.4)

• 1 molecule of O2 for every 2500 molecules of chlorophyll.

• Light absorbed by a cluster of pigment molecules in a photosystem.

• Each photosystem had 2 parts: antenna complex and a reaction center.

Antenna Complex “Light-harvesting”

• Made of different pigments like chlorophyll which absorb photons of light and pass the energy from one pigment molecule to the next in the thylakoid membrane.

• Eventually the energy is passed to the reaction center.

Reaction Center

• When a chlorophyll molecule in the reaction center absorbs a photon of light, an electron is excited and moves the an electron acceptor quinone.

• Quinone then passes the e- to another acceptor.

• Water donates e- to chlorophyll and is oxidized to form H+ and O2 a product.

• http://www.sumanasinc.com/webcontent/animations/content/harvestinglight.html

LIGHT-DEPENDENT REACTIONS4 Parts to Reactions1. Primary photoevent: photon captured by

pigment and e- excited.2. Charge separation: energy of e- transferred to

acceptor molecule at reaction center.3. Electron transport: e- move along carrier

molecules until they reduce NADP+ to NADPH and H+ moves across membrane to generate a gradient.

4. Chemiosmosis: H+ diffuses back across membrane through ATP synthase to generate ATP.

Bacteria use 1 photosystem• In sulfur bacteria, e- absorb photons and are

boosted from the reaction center to an e- acceptor which happens to be H+ and together they form an H atom.

• e- are recycled as they are used to make ATP and return back to chlorophyll molecules. This is called cyclic photophosphorylation.

Chloroplasts have 2 Photosystems

1. Photosystem I (PS I): has absorption peak of 700 nm so we call it P700. Transfers e- to NADP+ to make NADPH.

2. Photosystem II (PS II): has an absorption peak of 680 nm, we call it P680. e- lost in PS I are replaced by e- from water generated in PS II.

** Systems are connected by cytochrome/b6-f complex of e- carriers.

Photosystem

How do they work together?

Noncyclic photophosphorylation

• e- do not return to photosystems, instead they are used to make NADPH. 2 main products of PS I and PSII are ATP and NADPH.

NOW DRAW IT

Light-Dependent Reactions

• http://highered.mcgraw-hill.com/olcweb/cgi/pluginpop.cgi?it=swf::535::535::/sites/dl/free/0072437316/120072/bio13.swf::Photosynthetic%20Electron%20Transport%20and%20ATP%20Synthesis

• Plants will carry out cyclic phosphorylation when they run low on ATP.

• Cyclic phosphorylation only generates ATP, (no NADPH or oxygen is made). The e- will leave PS I and return to the b6-f complex.

Carbon Fixation: Calvin Cycle (8.6)

• Occurs day or night but depends on NADPH and ATP for energy to make sugar.

• Produces PGAL (phosphoglycerate) a 3C molecule used to make glucose and other sugars. Thus it is called C3 photosynthesis.

• 6 turns of cycle produces 2 PGAL or 1 sugar.

• Occurs in the stroma.

NOW DRAW IT

• http://www.sinauer.com/cooper/4e/animations0305.html

Photorespiration and Other Processes (8.7)

Photorespiration• Normally rubisco uses CO2 in the Calvin

cycle. • However during photorespiration, the stomata

are closed increasing O2 levels and decreasing CO2 levels in the leaf.

• Rubisco will sometimes binds with O2 instead of CO2.

• ATP is used, CO2 is made and No sugar is formed.

• This generally occurs when temperatures are high or it is dray and the stomata are closed

C4 Photosynthesis• Assists plants that live in hot, dry areas such

as corn, crabgrass, and sugar cane.

• (Phosphoenolpyruvate) PEP carboxylase joins CO2 and PEP to produce oxaloacetate (OAA) in the mesophyll cells.

• Now in an organic form, CO2 is transferred to the bundle sheath cells and there it is used in the Calvin cycle.

• The advantage is reducing photorespiration.

• What is similar?

• What is different?

CAM plants

• CAM = crassulacean acid metabolism

• Adaptation to dry areas.

• At night plants open their stomata to capture CO2 using PEP carboxylase and store it in organic compounds.

• During the day, they use light energy and the CO2 to produce sugar.

• All reactions occur in the mesophyll cells.