Cellular Energy and Photosynthesis

Comprehensive vocabulary flashcards covering principles of cellular energy, chloroplast anatomy, light-dependent reactions, photophosphorylation, the Calvin cycle, pigment optics, and metabolic adaptations like C4 and CAM pathways.

Autotrophs

Organisms that capture energy from sunlight or chemical sources and transform it into energy sources usable by the cell.

Photoautotrophs

Organisms, such as plants, multicellular algae, unicellular protists, cyanobacteria, and purple sulfur bacteria, that utilize light energy to synthesize organic molecules and food (glucose).

Chemoautotrophs

Organisms that utilize chemical compounds (such as hydrogen sulfide) from their environment to synthesize organic nutrients through chemosynthesis.

Heterotrophs

Organisms that obtain energy by consuming organic molecules (such as glucose) manufactured by other organisms.

Sequential Reactions

Step-by-step pathways in biological systems where the product of one chemical reaction serves as the reactant for a subsequent reaction, allowing for more controlled and efficient transfer of energy.

Cyanobacteria

Prokaryotic photosynthetic organisms whose metabolic activity was responsible for the initial production and rise of oxygen in Earth's early atmosphere approximately $2.3\text{ billion years}$ ago.

Photosynthesis

The biological process that converts light energy into the chemical energy of food, represented by the overall equation: $6CO_2 + 6H_2O + \text{Light Energy} \rightarrow C_6H_{12}O_6 + 6O_2$.

Chloroplasts

Double-membraned plant cell organelles containing thylakoids, grana, and stroma that serve as the main sites of photosynthesis.

Mesophyll Cells

The interior leaf tissue cells in which chloroplasts are predominantly located.

Stomata

Microscopic pores on leaf surfaces that regulate gas exchange by allowing $CO_2$ to enter and $O_2$ to exit.

Chlorophyll

The primary green light-absorbing pigment situated within the thylakoid membranes of chloroplasts that captures sunlight and energizes electrons.

Oxidation

A chemical process characterized by the loss of electrons and/or hydrogen atoms.

Reduction

A chemical process characterized by the gain of electrons and/or hydrogen atoms.

Photosystem

A light-capturing unit embedded in a chloroplast's thylakoid membrane consisting of a reaction center complex and light-harvesting pigment-protein complexes (e.g., Photosystem II with $P680$ and Photosystem I with $P700$).

Photolysis of Water

The enzyme-assisted splitting of water ($H_2O \rightarrow 2H^+ + \frac{1}{2}O_2 + 2e^-$) at Photosystem II, which donates initial electrons to the transport chain and releases protons into the thylakoid lumen.

Cyclic Electron Flow

An alternate photosynthetic electron pathway involving only Photosystem I that generates ATP for the Calvin cycle via the cytochrome complex without producing NADPH or releasing oxygen.

Proton Gradient

A concentration difference of protons ($H^+$ ions) established across the thylakoid membrane, with higher concentration in the thylakoid lumen and lower concentration in the stroma.

Chemiosmosis

An energy-coupling mechanism that utilizes the potential energy stored within a transmembrane hydrogen ion ($H^+$) gradient to drive the synthesis of ATP.

ATP Synthase

A membrane-embedded transport enzyme that synthesizes ATP from ADP and inorganic phosphate ($P_i$) through passive proton translocation down an electrochemical gradient.

Calvin Cycle

The light-independent cyclic metabolic pathway occurring in the stroma that utilizes ATP and NADPH from the light reactions to fix $CO_2$ into glucose.

Rubisco

Ribulose-1,5-bisphosphate carboxylase-oxygenase; the enzyme that catalyzes the primary carbon fixation reaction by combining $CO_2$ with ribulose bisphosphate (RuBP).

Carbon Fixation

Phase 1 of the Calvin cycle, where inorganic carbon from $CO_2$ is covalently incorporated into an organic five-carbon molecule (RuBP) by rubisco to form an unstable intermediate that splits into 3-phosphoglycerate.

Glyceraldehyde 3-Phosphate (G3P)

The three-carbon carbohydrate produced during Phase 2 of the Calvin cycle; two net molecules of G3P (derived from six complete turns of the cycle) are required to synthesize one molecule of glucose.

Carotenoids

Accessory pigments in the thylakoid membrane that absorb blue-green light ($400\text{ nm} - 550\text{ nm}$) and reflect yellow, orange, or red wavelengths, becoming visibly apparent in leaves during autumn.

Visible Light Spectrum

The segment of the electromagnetic spectrum detectable by human vision, ranging from approximately $380\text{ nm}$ to $750\text{ nm}$ (or $400\text{ nm} - 700\text{ nm}$), where shorter wavelengths correlate with higher energy.

Absorption Spectrum

A graphical representation showing the specific wavelengths of light absorbed by individual photosynthetic pigments (such as chlorophyll a, chlorophyll b, and beta-carotene).

Action Spectrum

A graph profiling the relative effectiveness of different wavelengths of light in driving a physiological process, such as photosynthesis measured by oxygen ($O_2$) release.

Photorespiration

A wasteful metabolic pathway in plants where rubisco binds $O_2$ instead of $CO_2$, consuming ATP and producing $CO_2$ without producing sugars.

$C_3$ Plants

Plants (comprising roughly $85\%$ of species, including rice, wheat, and soybeans) that fix carbon directly into a 3-carbon compound (3-PGA) using rubisco, making them vulnerable to photorespiration under hot, dry conditions.

$C_4$ Plants

Plants (such as corn and sugarcane) that avoid photorespiration by spatially separating carbon fixation in mesophyll cells via PEP carboxylase into a 4-carbon compound from the Calvin cycle in bundle-sheath cells.

CAM Plants

Plants adapted to arid conditions (such as cacti and pineapples) that temporally separate carbon fixation by opening stomata at night to store carbon in a 4-carbon organic acid and closing them during the day to run the Calvin cycle.

$$ET_{50}$$

The estimated time required for $50\%$ of floating leaf discs to rise to the surface in a disc assay, serving as an indicator of photosynthetic rate.

Second Law of Thermodynamics

The physical law stating that the entropy (disorder) of an isolated system, such as the entire universe, will always increase over time; living cells maintain order by coupling energy-releasing processes to energy-storing processes.