Chapter 10

photosynthetic reaction center

Multiple light-harvesting complexes surround a single reaction center within the photosystem. Light energy is captured by the light-harvesting complexes and transferred to the reaction center, where chlorophyll a molecules participate in redox reactions that result in the con- version of the light energy to chemical energy.

Adsorption Spectrum

A graph of light absorption versus wavelength of light; shows how much light is absorbed at each wavelength. If we plot light absorbed by a purified pigment against wavelength, the result is an absorption spectrum for that pigment.

3PG

RuBP is the initial CO2 acceptor, and 3PG is the first stable prod- uct of CO2 fixation. The enzyme rubisco catalyzes the reaction of CO2 and RuBP to form 3PG.

NADP reductase

gets passed electrons to napd reductase. a proton and NADP+ molecule come together to form NADPH

chlorophyll

harvest light energy for photosynthesis. For example, the pigment chlorophyll absorbs both blue and red light, and we see the remaining light, which is primarily green. read pg 190

grana

multiple stacks of thylakoids

granum

one stack of thylakoids

RubisCO

ribulose bisphosphate carboxylase/oxygenase (rubisco), is the most abundant protein in the world! It constitutes up to 50 percent of all the protein in every plant leaf. Fixation of CO2. As we have seen, this reaction is cata- lyzed by rubisco, and its stable product is 3PG. The enzyme rubisco catalyzes the reaction of CO2 with RuBP. As its full name indicates, rubisco (ribulose bisphosphate carboxylase/oxygenase) is an oxygenase as well as a car- boxylase—it can add O2 to the acceptor molecule RuBP in- stead of CO2. The affinity of rubisco for CO2 is about ten times stronger than its affinity for O2. This means that inside a leaf with a normal exchange of air with the outside, CO2 fixation is favored even though the concentration of CO2 in the air is far less than that of O . Contraction of ribulose bisphosphate carboxylase/oxygenase, the enzyme that combines carbon dioxide or oxygen with ribulose bisphosphate to catalyze the first step of photosynthetic carbon fixation or photorespiration, respectively.

carbon fixation

The initial product of CO2 fixation is 3PG. Later, the carbon from CO2 ends up in many molecules. Fixation of CO2. As we have seen, this reaction is cata- lyzed by rubisco, and its stable product is 3PG. photorespiration, lowers the overall rate of CO2 fixation The phase of photosynthesis in which chemical energy captured in the light reactions is used to drive the reduction of CO2 to form carbohydrates.

Rubp

The initial reaction in the Calvin cycle adds the one- carbon CO2 to the five-carbon acceptor molecule ribulose 1,5-bisphosphate (RuBP). In a typical leaf, five-sixths of the G3P is re- cycled into RuBP. 5 RuMP is converted to RuBP in a reaction requiring ATP. RuBP is ready to accept another CO2.

Thylakoid

The internal membranes of chloroplasts look like stacks of flat, hollow pita bread. Each stack is called a granum (plural grana) and the pita bread-like compartments are called thylakoids (see Figure 5.12). Thylakoid lipids are distinctive: only 10 percent are phospholipids, whereas the rest are galactose-substituted diglycerides and sulfolipids. Because of the abundance of chloroplasts, these are the most abundant lipids in the biosphere. A flattened sac within a chloroplast. Thylakoid membranes contain all of the chlorophyll in a plant, in addition to the electron carriers of photophosphorylation. Thylakoids stack to form grana.

G3P

The product of this cycle is glyceraldehyde 3-phosphate (G3P), which is a three-carbon sugar phosphate, also called triose phosphate. A phosphorylated three-carbon sugar; an intermediate in glycolysis and photosynthetic carbon fixation.

photosystem 2

absorbs light at 680 nm and passes electrons to the electron transport chain in the chloroplast. absorbs light energy best at 680 nm, oxidizes water molecules, and passes its energized electrons through a series of carriers to produce ATP. After an excited chlorophyll in the reaction center (Chl*) gives up its energetic electron to reduce a chemical acceptor molecule, the chlorophyll lacks an electron and is very unstable. It has a strong tendency to "grab" an electron from another molecule to replace the one it lost—in chemical terms, it is a strong oxidizing agent. The replenishing electrons PHOTOSYSTEM II come from water, splitting the H—O—H bonds:

Ferredoxin

From photosystem I the electrons are passed to ferredoxin (Fd). Passes electrons to NADP reductase. Electrons from ferredoxin then reduce thioredoxin.

cyclic electron transport

If the pathway we just de- scribed—the linear or noncyclic pathway—were the only set of light reactions operating, there might not be sufficient ATP for carbon fixation. Cyclic electron transport makes up for this imbalance. This pathway uses photosystem I and the electron transport system to produce ATP but not NADPH; it is cyclic because an electron is passed from an excited chlorophyll and recycles back to the same chlorophyll

photosystem 1

In photosystem I, an excited electron from the Chl* at the reaction center reduces an acceptor. The oxidized chlorophyll (Chl+) now "grabs" an electron, but in this case the electron comes from the last carrier in the electron transport system. This links the two photosystems chemically. They are also linked spatially, with the two photosystems adjacent to one another in the thylakoid membrane. The en- ergetic electrons from photosystem I pass through several molecules and end up reducing NADP+ to NADPH. The Chl* in the reaction center of photosystem I passes electrons to an electron carrier, ferredoxin (Fd), leaving positively charged chlorophyll (Chl+). Photosystem I absorbs light at 700 nm, passing electrons to ferrodoxin and from there to NADPH.

plastocyanin

In the thylakoid membrane, electrons are passed from photosystem II to photosystem I via a series of electron carriers, including plastoquinone (PQ), cytochrome (Cyt), and plastocyanin (PC).

calvin cycle

The Calvin cycle uses the ATP and NADPH made in the light to reduce CO2 in the stroma to a carbohydrate. Like all biochemical pathways, each reaction is catalyzed by a specific enzyme. The cycle is composed of three distinct processes (Figure 10.13): • Fixation of CO2. As we have seen, this reaction is cata- lyzed by rubisco, and its stable product is 3PG. • Reduction of 3PG to form glyceraldehyde 3-phosphate (G3P). This series of reactions involves a phosphoryla- tion (using the ATP made in the light reactions) and a reduction (coupled to the oxidation of NADPH made in the light reactions). • Regeneration of the CO2 acceptor, RuBP. Most of the G3P ends up as ribulose monophosphate (RuMP), and ATP is used to convert this compound into RuBP. So for every "turn" of the cycle, one CO2 is fixed and one CO2 acceptor is regenerated The stage of photosynthesis in which CO2 reacts with RuBP to form 3PG, 3PG is reduced to a sugar, and RuBP is regenerated, while other products are released to the rest of the plant. Also known as the Calvin-Benson cycle.

carotenoid

The carotenoids are a family of light-absorbing pigments found in plants and animals. Beta-carotene (β-caro- tene) is one of the pigments that traps light energy in leaves during photosynthesis. In humans, a molecule of β-carotene can be broken down into two vitamin A molecules. Vitamin A is used to make the pigment cis-retinal, which is required CAROTENOIDS for vision. yellow, orange, or red lipid pigment commonly found as an accessory pigment in photosynthesis; also found in fungi.

Chloroplast stroma

The fluid in which the grana are suspended is called the stroma. Like the mitochondrial matrix, the chloroplast stroma contains ribosomes and DNA, which are used to synthesize some, but not all, of the proteins that make up the chloroplast. The fluid contents of an organelle such as a chloroplast or mitochondrion.

Action Spectrum

action spectrum is a plot of the rate of photosynthesis carried out by an organism against the wavelengths of light to which it is exposed. An action spectrum for photosynthesis can be determined as follows: 1. Place the organism in a closed container. 2. Expose it to light of a certain wavelength for a period of time. 3. Measure the rate of photosynthesis by the amount of O2 released. 4. Repeat with light of other wavelengths.

plastoquinone

an electron transporter, takes electrons from photosystem 2 to cytochrome bf.