Plant Physiology 2

Photosynthetic Electron Transport Chain

A series of protein complexes involved in converting light energy into chemical energy in the form of ATP and NADPH.

PSII

2 H₂O + 4 photons + 2 PQ + H⁺ (from stroma) → O₂ + 4 H⁺ (from water, into lumen) + 2PQH₂

A major protein complex that absorbs light energy to initiate the electron transport chain. For one oxygen, you need four electrons, as in 2 waters. Generates 4 protons. Need 2 PQ. Mostly in stack regions (grana lamellae)

Cyt b6f

A protein complex that facilitates electron transfer between PSII and PSI.

2 PQH₂ + 4 PC(Cu²⁺) → 2 PQ + 4 PC(Cu⁺) + 8 H⁺ (deposited in lumen). Plastocyanin (PC) is a 1 electron carrier. Stacked and unstacked regions.

PSI

A protein complex that further processes electrons and contributes to NADPH production.

4 photons + 4 PC(Cu⁺) + 4 Fd(Fe³⁺) → 4 PC (CU²⁺) + 4 Fd(Fe²⁺). PC passing on ferredoxin through many components and it eventually passes it, making it negatively charged. Unstacked (stroma lamellae)

Ferredoxin-NADP reductase (FNR)

Ferredoxin is the ultimate donor, NADPH needs 2 electrons. Produce 2, need 4 ferredoxin. Need enzymes to catalyze.

4Fd(Fe²⁺) + 2 NADP⁺ + 2 H⁺ → 4 Fd(Fe³⁺) + 2 NADPH

FNR process

1) Water splits - H+ into lumen electron donor

2) PQ is reduced into PQH₂ on stroma (2 H+ required). PQ is oxidized to PQ on lumen (2 H+ deposited).

3) NADP⁺ + H⁺ → NADPH on stroma

ATP synthase

Enzyme that synthesizes ATP using the proton gradient generated by the electron transport chain. Mostly in unstacked regions (stroma lamellae).

CF₀ - Hydrophobic stalk, created H⁺ channels in the membrane

CF₁ - Catalytic part that generates ATP.

1 ATP / 4 H⁺ translocated.

Plastoquinone (PQ)

A mobile electron carrier that transports electrons from PSII to cyt b6f.

Plastocyanin (PC)

A small protein that carries electrons from cyt b6f to PSI. Helps out the lumen side of the membrane

Lumen

The inner compartment of the thylakoid membrane where protons accumulate during the electron transport chain.

Stroma

The fluid-filled space surrounding the thylakoids in the chloroplast, where the Calvin cycle takes place.

Z-scheme

A model describing the energetic changes of electrons as they move through the photosynthetic electron transport chain.

NADPH

A reduced form of nicotinamide adenine dinucleotide phosphate, used in the Calvin cycle for carbon fixation.

Photophosphorylation

The chemiosmotic hypothesis. An electrochemical gradient across a membrane generates a proton motive force, a source of free energy. This energy can be used to drive synthesis of ATP from ADP + P_i

Jagendorf (1960s)

Lumen side has a high H+ (low pH). In experiment, we are trying to simulate this. By changing pH - you generate the movement. Then they wanted to see if the protein gradient was enough to drive. They added a drop of ADP and phosphate into the buffer and measured how much ATP was produced. ATP was produced - proving the proton gradient is what drives it.

Light reaction

The phase of photosynthesis where light energy is converted into chemical energy (ATP and NADPH).

Calvin Cycle

The series of biochemical reactions in plants that convert carbon dioxide into glucose, using ATP and NADPH from light reactions. 18 ATP and 12 NADPH used to produce glucose.

RuBisCO

Ribulose-1,5-bisphosphate carboxylase/oxygenase, the enzyme that catalyzes the first step of the Calvin cycle. (5C).

Carboxylation

The initial step in the Calvin cycle where CO2 is fixed to form 3-PGA, adding carbon to sugar, leading to 3-carbon phosphatase.

3 RuBP (5 carbon components each RuBP) + 3 CO₂ → 6 3-PGA (1st stable carbon compound). This is driven by rubisco. We will see 18 carbon on each side. 18 C → 3 × 6 PGA.

Reduction

Reducing Phosphoglyceraldehyde.

Need 6 NADPH and 6 ATP. 6 PGA → 6TP (use NADPH and ATP to drive reaction). 1 triose phosphate is escorted out to cytosol, and eventually used for starch sucrose synthesis - main source of sugar in a cell

Regeneration

Get ADP back.

We use 5TP and regenerate it back into 3RuBP (15 C total for both) using 3 ATP. We need 9 ATP, 3 for regeneration and 6 in reduction. A single triose phosphate is 9.

Proton gradient

An electrochemical gradient across the thylakoid membrane that drives ATP synthesis.

Biochemical control of the Calvin Cycle

Light-dependent ion movement (stromal pH from 7→8, enzymes more active, NADPH passes electrons to ferredoxin and thioredoxin)

Redox regulation (FT system)

Triose phosphate made into starch (stroma) or sucrose (cytosol)

Amylose

Linear polymer of glucose molecules with 1-4 linkages, a component of starch.

Amylopectin

Branched polymer of glucose with both 1-4 and 1-6 linkages, a component of starch.

Starch synthesis

1. Synthesis of the sugar nucleotide ADP-glucose by ADP-glucose pyrophsphorylase

2. Elongation of the glucan chain by starch synthase

3. Branching enzymes are used to introduce 1-6 linkages

4. A range of other enzymes are used to process and pack the amylose and amylopectin into starch granule

Kinase

Adds P_i to substrate - granule is basically adding the phosphate group. It will open the entire granule. Other enzyme cannot work on it. Adding phosphate group into the starch granule (add 2 1-4 linkage). Starch made into maltose and glucose. Phosphatase catalyzes the reverse reaction (removing).

Water dikinases

Phosphorylate some of the glucosyl units within the starch granule.

Debranching enzyme

Release linear glucan chains

B-amylase

Hydrolyzes the glucans, releasing maltose (a disaccharide made up of two glucose units)

Maltose

(along with some glucose) is moved to cytosol by specific transporter. Is converted to glucose, which is then phosphorylated by hexokinase producing Glu 6-P.

Phloem

The vascular tissue responsible for the transport of photosynthates (e.g., sucrose) from sources to sinks.

Source to sink

The directional movement of photosynthates from areas of production (sources) to areas of consumption or storage (sinks). Xylem movement. Determined by Proximity, Development, and Vascular Connections.

Allocation

Indicate different fates of photosynthetic products. Sucrose is going to be separated into different plant cells. Whatever is allocated it can further be partitioned. Relative REM for partitioning mechanism (sink).

2 cell types in phloem

Sieve tube elements and associated companion cells

P-protein

phloem protein, used as main defense mechanism to fight against phloem leakage. if stem is broken or phloem gets damaged, they aggregate and plug really big pores at sieve plate. Blocks bottom and top of sieve tube elements. found in most angiosperms. Protects phloem against damage, such as herbivore feeding. Freely floating in the cytoplasm under normal conditions. It quickly aggregates to form a plug at the sieve plate, preventing excess loss of sap and blocking pathogen entry. Plugged with the polysaccharide callose.

Ordinary companion cell (OC)

Has plasmodesmatal connection with sieve tube elements but not with other surrounding cells (ie. bundle sheath cells). Apoplastic loading.

Transfer cells (TC)

Like OC, but with cell wall in-growths on the walls that face away the sieve tube element, increasing the area of PM on these surfaces. Apoplastic loading.

Intermediary cells (IC)

Plasmodesmatal connections with sieve tube element and with surrounding cells. Symplastic loading.

Phloem transports

Sucrose, non-reducing sugars and sugar alcohols, amino acids, some inorganic ions, most plant hormones, and specific proteins and RNAs.

Apoplastic loading

H⁺ sucrose symporter and proton ATPase. Make sure sucrose is continuously building up in companion cell.

Involved sugar into the apoplast and then back in the symplast of either an ordinary companion cell or transfer cell. Once in the companion cell or transfer cell it can enter the sieve tube element through plasmodesmata.

Symplastic loading

intermediary companion cells provide symplastic continuity between the surrounding cells and the phloem. These species don’t have an apoplast to symplast active transport step. Specific sugars are loaded and concentrated in phloem (not primarily sucrose). Make sugars bigger to avoid backflow.

Shade leaves

more total chlorophyll/reaction center, higher ratio of Chl b / Chl a, increased ratio of PS2 / PS1 (3:1) or more antennae chlorophyll for PS2, lower respiration rates.

Sun leaves

more Rubisco, larger pool of xanthophyll cycle components (later), lower ratio of PS2 / PS1 (2:1).

Controlling photodamage

1. Dissipate the excess light energy as heat

2. Scavenge the ROS that are produced (e.g. ROS-detoxifying enzymes such as superoxide dismutase and catalase)

3. Repair the photo-damage (e.g. active D1 replacement)

Zeaxanthin

Has a poorer ability to transfer absorbed light energy to chlorophyll. Instead, the absorbed energy is lost as heat.Violaxanthin would create ROS species with excess light, so mid-day has more of this (less stress on plant)