Energy Flow, Ecosystems and the Environment - Flashcards (Photosynthesis and ATP)

ATP: The energy source for the cell

  • Energy must be readily available; ATP provides an immediate energy supply for cellular processes.

  • ATP hydrolysis: ATP \rightarrow ADP + P_i\quad \Delta G \approx -34\ \text{kJ mol}^{-1}

  • ATP synthesis: ADP + P_i \rightarrow ATP\quad \Delta G \approx +34\ \text{kJ mol}^{-1}

  • Energy to drive ATP synthesis comes from catabolic (and redox) reactions; ATP acts as an energy store linked to these processes.

  • ATP is the universal energy carrier in all living organisms; its production/breakdown is essential for life.

  • Common uses of ATP include active transport, anabolic reactions, and muscle contraction.

  • Pneumonic for redox changes: OIL RIG (Oxidation Is Loss, Reduction Is Gain).

Chloroplasts and chlorophyll

  • Photosynthesis overview: 6CO2 + 6H2O \xrightarrow{light} C6H{12}O6 + 6O2\quad \Delta H = +2880\ \text{kJ}

  • Chloroplast structure

    • Outer and inner membranes forming the chloroplast envelope

    • Stroma: fluid matrix containing enzymes for the Calvin cycle

    • Grana: stacks of thylakoids where chlorophyll is located

    • Thylakoids: membrane discs where light reactions occur

    • Lamellae: connects grana; provide structural organization

  • Chloroplast pigments and photosynthesis

    • Chlorophyll pigments absorb light; arranged on thylakoid membranes to maximize light capture

    • Chloroplast pigments can be separated by chromatography; pigments identified by R values

  • Chloroplast roles in photosynthesis and respiration linkage

    • Photosynthesis occurs in chloroplasts; glucose formed can be respired in mitochondria

    • Photosynthesis is endothermic; aerobic respiration is exothermic

Absorption and action spectra; chromatography

  • Absorption spectrum: amount of light absorbed by a pigment across wavelengths

  • Action spectrum: rate of photosynthesis vs wavelength; shows effectiveness of different wavelengths

  • Multi-pigment advantage: different pigments absorb different wavelengths, broadening overall light capture

  • Photosystems absorb at specific wavelengths

    • Photosystem I (PSI): peak at \lambda_{max} = 700\ \text{nm}

    • Photosystem II (PSII): peak at \lambda_{max} = 680\ \text{nm}

  • Chromatography and Rf values

    • Rf value: Rf = \frac{\text{distance travelled by pigment}}{\text{distance travelled by solvent}}

    • R_f values range between 0 and 1 (solvent-dependent)

  • Example pigment R_f values (solvent: 3:1:1 petroleum ether–propanone–chloroform on silica)

    • Carotene: R_f \approx 0.98

    • Chlorophyll a: R_f \approx 0.59

    • Chlorophyll b: R_f \approx 0.42

    • Phaeophytin: R_f \approx 0.81

    • Xanthophylls: R_f \approx 0.28, 0.15

Photosystems and the light-dependent stage

  • Two main functions of the light-dependent stage

    • Photolysis of water to release oxygen

    • Production of ATP (and NADPH) for use in the Calvin cycle

  • Electron flow and ATP production

    • Light energy excites chlorophyll; electrons flow through an electron transport chain

    • Proton gradient drives ATP synthase (chemiosmosis)

  • Cyclic and non-cyclic photophosphorylation

    • Cyclic: ATP only (no net NADPH or O2)

    • Non-cyclic: ATP and NADPH produced, with O2 released from water

  • Location in the chloroplast

    • PSII and associated electron transport chains are mainly on grana

    • PSI is mainly on the intergranal lamellae

Chloroplast structure and the role of chlorophyll

  • Chloroplast envelope: outer and inner membranes

  • Grana: stacks of thylakoids; site of PSII activity

  • Thylakoids: membrane discs hosting chlorophyll and reaction centers

  • Lamellae: connect grana; aid light capture efficiency

  • Stroma: fluid surrounding grana; contains enzymes for the light-independent reactions (Calvin cycle)

  • Chlorophyll a vs b; carotenoids

    • Chlorophyll a: blue-green pigment

    • Chlorophyll b: yellow-green pigment

    • Carotenoids (carotene, xanthophylls): absorb other wavelengths; protective roles

  • Overall photosynthesis context

    • Light-dependent reactions occur in the thylakoid membranes; Calvin cycle occurs in the stroma

The overall photosynthesis equation and energy concepts

  • Overall process: 6CO2 + 6H2O \xrightarrow{light} C6H{12}O6 + 6O2

  • Energy input is required to split water and form glucose; energy is stored in chemical bonds (glucose)

  • Photosynthesis and respiration are tightly linked in energy metabolism

Quick reference terms

  • Photosystem I (PSI): absorbs at \lambda_{max} = 700\ \text{nm}

  • Photosystem II (PSII): absorbs at \lambda_{max} = 680\ \text{nm}

  • Stroma: fluid matrix where Calvin cycle occurs

  • Granum: stack of thylakoids

  • Lamellae: connections between grana

  • R_f value: pigment migration measure in chromatography

  • RUBISCO: enzyme fixing CO$_2$ in the Calvin cycle (mentioned as part of later content)

  • Calvin cycle (light-independent): fixation of CO$_2$ to form sugars in the stroma