Photosynthesis

Photosynthesis the process by which plants, some bacteria, and some protists use the energy from sunlight to produce sugar (glucose) which cellular respiration converts into ATP the "fuel" used by all living things. A biochemical pathway

Autotrophs Organisms that manufacture their own food Algae (protist) Bacteria (cyanobacteria) Plants

Heterotrophs Cannot manufacture food Take in nutrients Feed on autotrophs Primary Consumers Feed on other consumers Secondary consumers

ATP

Adenosine triphosphate major energy currency of the cell regulates many biochemical pathways like batteries for a cell

ATP/ADP and Energy ADP + P  ATP ATP  ADP + P + energy Energy is stored in the covalent bonds between phosphates the greatest amount of energy is in the bond between the second and third phosphate groups

Chlorophyll The conversion of unusable sunlight energy into usable chemical energy, is associated with the actions of the green pigment chlorophyll

Photosynthesis The photosynthetic process uses water and releases the oxygen that we absolutely must have to stay alive. Oh yes, we need the food as well!

Formula for photosynthesis 6 H2O + 6 CO2 (sunlight) C6H12O6 + 6 O2 six molecules of water plus six molecules of carbon dioxide (in the presence of sunlight) produce one molecule of sugar plus six molecules of oxygen

Photosynthesis vs. Respiration Photosynthesis uses carbon dioxide to make oxygen Respiration uses oxygen and produces carbon dioxide

Leaf cross-section

Leaves as food factories Plants are the only photosynthetic organisms to have leaves (and not all plants have leaves) 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 The products of photosynthesis, sugar and oxygen, leave the leaf

Important leaf structures Water enters the root and is transported up to the leaves through specialized plant cells known as xylem Land plants must guard against drying out (dessication) 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 (cuticle) but it can enter the leaf through an opening (stomata) Flanked by two guard cells.

Stomata

The Nature of Light White light is separated into the different colors (=wavelengths) of light by passing it through a prism. Wavelength is defined as the distance from peak to peak (or trough to trough). The energy is inversely proportional to the wavelength

The nature of light-wavelength

Wavelengths of light The order of colors is determined by the wavelength of light Visible light is one small part of the electromagnetic spectrum The longer the wavelength of visible light, the more red the color Wavelengths longer than red are referred to as infrared while those shorter than violet are ultraviolet

Characteristics of light Light behaves both as a wave and a particle. Wave properties of light include the bending of the wave path when passing from one material into another The particle properties are demonstrated by the photoelectric effect

Electromagnetic spectrum

Chlorophyll All photosynthetic organisms have chlorophyll a. Accessory pigments absorb energy that chlorophyll a does not absorb. Accessory pigments include chlorophyll b xanthophylls, and carotenoids Chlorophyll a absorbs its energy from the Violet-Blue and Reddish orange-Red wavelengths, and little from the intermediate (Green-Yellow-Orange) wavelengths.

Chlorophyll and Accessory Pigments A pigment is any substance that absorbs light The color of the pigment comes from the wavelengths of light reflected Chlorophyll, 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.

Absorption of chlorophyll

Structure of a chloroplast

Structure of a chloroplast The organelle is surrounded by a double membrane Inside the inner membrane is a complex mix of enzymes and water. This is called stroma and is important as the site of the dark reactions, more properly called the Calvin cycle. Embedded in the stroma is a complex network of stacked sacs. Each stack is called a granum and each of the flattened sacs which make up the granum is called a thylakoid.

Electron transport In the light reactions light strikes chlorophyll a in such a way as to excite electrons to a higher energy state In a series of reactions the energy is converted (along an electron transport process) into ATP and NADPH. Water is split in the process, releasing oxygen as a by-product of the reaction The ATP and NADPH are used to make C-C bonds in the Light Independent Process (Dark Reactions).

Electron transport chain

Photosystems Photosystems are arrangements of chlorophyll and other pigments packed into thylakoids Many Prokaryotes have only one photosystem, Photosystem II (so numbered because, while it was most likely the first to evolve, it was the second one discovered). Eukaryotes have Photosystem II plus Photosystem I.

Photosystem I

Photosystems I & II Photosystem I uses chlorophyll a, in the form referred to as P700 Photosystem II uses a form of chlorophyll a known as P680 Both "active" forms of chlorophyll a function in photosynthesis due to their association with proteins in the thylakoid membrane

Steps to photosystems 1. light forces electrons to enter a higher energy level Said to be “excited” 2. The excited electrons have enough energy to leave chlorophyll a LEO – chlorophyll a has been oxidized Leo says GER

Steps to photosystems If one substance is oxidized (loses electrons) then another must be reduced (gains electrons) These electrons are gained by the thylakoid membrane Primary electron receptor GER- gain of electrons is reduction

Primary electron receptor

Steps in photosystems 3. Primary electron acceptor donates electrons to a series of molecules in the thylakoid membrane Known as an electron transport chain As the electrons are passed from molecule to molecule they lose the energy gained when excited Energy lost is gained by protons in the thylakoid membrane

Electron transport chain

Steps of photosystems 4. Light is absorbed by photosystem I at the same time as photosystem II Electrons move from chlorophyll a to another electron acceptor molecule Lost electrons are replaced by those gained in photosystem II

Electron transport chain

Steps to photosystems 5. Primary electron acceptor of photosystem I donates electrons to a different electron transport chain Chain brings electrons to side of thylakoid membrane Electrons combine with a proton and NADP+ Causes NADP+ to become NADPH

Electron transport chain

Photosystem restoration Electrons from chlorophyll in PS II replace electrons that leave chlorophyll in PS I If not replaced both electron transport chains stop Photosynthesis does not occur

Net results of PS For every 2 molecules of water split, four electrons become available to replace electrons lost in PS II 2 H2O  4 H+ + 4 e- + O2 Protons stay inside thylakoid membrane Oxygen diffuses out- byproduct

Chemiosmosis Relies on a concentration gradient of protons across thylakoid membrane Build up of protons represents energy Energy is captured by ATP synthase Chemiosmosis

Chemiosmosis

ATP Synthase Acts as a protein carrier Acts as an enzyme Synthesizes ATP from ADP

The Calvin Cycle

Carbon fixation Carbon-Fixing Reactions are also known as the Dark Reactions (or Light Independent Reactions) The Calvin Cycle occurs in the stroma of chloroplasts Carbon dioxide is captured by the chemical ribulose biphosphate (RuBP). RuBP is a 5-C chemical. Six molecules of carbon dioxide enter the Calvin Cycle, eventually producing one molecule of glucose

Melvin Calvin . The reactions in this process were worked out by Melvin Calvin (shown below).

Steps of the Calvin Cycle 1. CO2 diffuses into the stroma Enzyme combines CO2 with RuBP Product is a 6 Carbon molecule which splits immediately into 2, 3-Carbon molecules of PGA (phosphoglycerate)

Calvin cycle calvin

Calvin Cycle 2. PGA is converted into another 3-C molecule , PGAL (phosphoglyceraldehyde) A. PGA receives a phosphate from ATP B. Receives a proton form NADPH and releases a phosphate group Net result: ADP & NADP+ are created

Calvin cycle calvin

Calvin cycle 3. Most PGAL is converted back to RuBP through a series of complex reactions Requires a phosphate from ATP By regenerating RuBP to allow Calvin cycle to continue

Calvin cycle calvin

Photosynthesis Balance Sheet Each turn of Calvin cycle fixes one CO2 3 turns to produce each PGAL Uses 3 ATP and 2 NADPH In total 3 turns of Calvin cycle uses 9 ATP and 6 NADPH

Rate of photosynthesis Effected by plant’s environment Light intensity CO2 concentration Temperature

Photosynthesis http://www.cnr.vt.edu/DENDRO/forestbiology/photosynthesis.swf