Photosynthesis, Cellular Metabolism, and Enzymes H. Biology RVC 9th Grade '26

Comprehensive practice flashcards covering Chapter 5: Photosynthesis, Cellular Metabolism, Thermodynamics, and Enzyme Function.

What are photoautotrophs, and which major groups of organisms are classified as photoautotrophs?

Photoautotrophs are producers that use light energy to synthesize chemical energy stored in organic molecules. They include plants, multicellular algae, unicellular protists, and cyanobacteria.

What is the overall balanced chemical equation for photosynthesis?

$$6\text{CO}_2 + 6\text{H}_2\text{O} + \text{Light Energy} \rightarrow \text{C}_6\text{H}_{12}\text{O}_6 + 6\text{O}_2$$

What role do stomata and guard cells play in a leaf?

Stomata are pores in the leaf's outer waxy cuticle that permit $\text{CO}_2$ to enter and $\text{O}_2$ to exit, with their opening and closing regulated by guard cells.

What organelle structure is depicted in this cross-section diagram, showing two outer membranes enclosing a dense fluid and stacked membranous sacs?

A chloroplast, featuring a dense fluid called the stroma and chlorophyll-containing thylakoids stacked into grana.

What is the evolutionary origin of the chloroplast according to the notes?

A photosynthetic prokaryote living inside a eukaryotic cell.

Tracking atoms through photosynthesis, what is the specific reactant origin of released oxygen ($\text{O}_2$)?

Water ($\text{H}_2\text{O}$) is split to release $\text{O}_2$; carbon dioxide ($\text{CO}_2$) does not create oxygen.

Which photosynthetic pigment directly converts solar energy into chemical energy during the light reactions, and what color is it?

Chlorophyll a, which is a blue-green pigment.

What wavelengths of light are absorbed and reflected by chlorophyll?

Chlorophyll absorbs violet-blue and red light, and reflects green light.

What are carotenoids, and what are the two specific types listed in the notes?

Carotenoids are yellow and orange accessory pigments. The two specific types are xanthophyll (yellow) and carotenes (orange).

What two main structural components make up a photosystem?

A reaction center complex and light-harvesting complexes.

During the linear electron flow of the light reactions, how are the electrons lost by the P680 reaction center chlorophyll a molecules replaced?

They are replaced by electrons released from the splitting of water ($\text{H}_2\text{O}$).

How does the cytochrome complex contribute to chemiosmosis during the light reactions?

As high-energy electrons pass down the electron transport chain, the cytochrome complex acts as an active transport protein that pumps $\text{H}^+$ from the stroma into the thylakoid space to build a proton gradient.

How is ATP synthesized via chemiosmosis in the chloroplast?

$\text{H}^+$ ions diffuse from high concentration inside the thylakoid space to low concentration in the stroma by facilitated transport through ATP synthase, generating ATP in the stroma.

What enzyme reduces $\text{NADP}^+$ to NADPH at the end of the second electron transport chain in the light reactions?

$\text{NADP}^+$ reductase.

What is the primary function of the light-dependent reactions?

To use solar energy to generate ATP and NADPH, which provide the chemical energy needed for the Calvin cycle.

Where does the Calvin cycle take place, and what are its three distinct phases?

It takes place in the stroma. Its three phases are: Phase 1: Carbon fixation, Phase 2: Reduction, and Phase 3: Regeneration of RuBP (the $\text{CO}_2$ acceptor).

What reaction occurs during Phase 1 (Carbon fixation) of the Calvin cycle?

The enzyme rubisco attaches $\text{CO}_2$ to a 5-carbon sugar (RuBP) to form an unstable 6-carbon intermediate that immediately splits into two 3-carbon molecules (3-phosphoglycerate).

What inputs are consumed during Phase 2 (Reduction) of the Calvin cycle to produce 1 net molecule of G3P?

Phase 2 uses $6\text{ ATP}$ and $6\text{ NADPH}$ to convert six 3-phosphoglycerate into six G3P, yielding 1 net G3P for sugar output.

How much ATP is used in Phase 3 of the Calvin cycle to regenerate RuBP?

$3\text{ ATP}$ are used to rearrange five 3-carbon G3P molecules into three 5-carbon RuBP molecules.

How many turns of the Calvin cycle and how many $\text{CO}_2$ molecules are required to net one molecule of glucose?

It requires $2\text{ turns}$ of the Calvin cycle and $6\text{ CO}_2$ molecules.

Which photosynthetic adaptations are illustrated here by sugarcane and pineapple to minimize water loss in hot, arid conditions?

Sugarcane uses $\text{C}_4$ photosynthesis with spatial separation of steps (carbon fixation in mesophyll cells, Calvin cycle in bundle-sheath cells), while pineapple uses CAM photosynthesis with temporal separation (carbon fixation at night, Calvin cycle during the day).

What are the dual benefits of photosynthesis for the plant itself versus ecosystems globally?

For the plant, it supplies glucose for cellular respiration and cellulose for structure; globally, it provides $\text{O}_2$ production and serves as the primary food source for ecosystems.

What is the First Law of Thermodynamics?

Energy cannot be created or destroyed, but it can be changed from one form to another (the Principle of Conservation of Energy).

What does the Second Law of Thermodynamics state regarding energy efficiency and entropy?

Energy conversions are not $100\%$ efficient; they increase disorder (entropy), and some energy is always lost as heat.

How do endergonic reactions differ from exergonic reactions in metabolism?

Endergonic reactions absorb and store energy (products have higher potential energy than reactants), whereas exergonic reactions release energy (products have lower potential energy than reactants).

What is the difference between anabolic and catabolic metabolic pathways?

Anabolic pathways require energy to build complex polymers from simpler monomers, while catabolic pathways release energy by breaking down complex polymers into simpler ones.

What molecule is shown in this diagram consisting of adenine, ribose, and three phosphate groups?

Adenosine triphosphate (ATP), the primary energy transfer molecule in cells.

What is energy coupling in cellular metabolism?

Using the energy released from exergonic reactions to drive endergonic reactions.

Based on this free energy reaction coordinate diagram, how does an enzyme alter activation energy ($E_A$) and the overall free energy change ($\Delta G$)?

The enzyme lowers the activation energy ($E_A$) to speed up the reaction, while the overall change in free energy ($\Delta G$) remains completely unaffected.

What determines an enzyme's substrate specificity?

Its unique 3-D shape and the specific arrangement of amino acids found within its active site.

What does the induced fit model describe regarding enzyme catalysis?

As the substrate molecules bind to the active site, the enzyme changes shape slightly to mold snugly around them, facilitating catalysis.

What happens when an enzyme undergoes denaturation?

The protein unfolds back to its primary structure due to environmental conditions outside its optimal range (such as extreme temperature, pH, or salinity), resulting in the loss of biological activity.

What form of enzyme inhibition is shown in this diagram, and how can it be overcome if reversible?

Competitive inhibition, where a chemical resembles the substrate and competes for the active site; it can be overcome by increasing substrate concentration.

What type of enzyme inhibition is depicted in this diagram, where the inhibitor binds to an allosteric site?

Noncompetitive inhibition, where an inhibitor binds to another site on the enzyme (the allosteric site), causing a conformational shape change that prevents the active site from binding the substrate.

Palisade mesophyll

Columnar photosynthetic cells located directly beneath the upper epidermis that contain a high concentration of chloroplasts for light capture.

Spongy mesophyll

Loosely arranged photosynthetic leaf tissue located beneath the palisade layer with prominent air spaces to facilitate gas exchange.

Leaf air spaces

Intercellular gaps within the spongy mesophyll that allow $\text{CO}_2$ and $\text{O}_2$ to diffuse readily throughout leaf tissue.

Leaf vascular bundle (vein)

A transport structure running through leaf tissue composed of xylem to import water and phloem to export sugars.

Xylem

Vascular plant tissue responsible for delivering water and dissolved minerals upward from roots to the photosynthetic cells of the leaf.

Phloem

Vascular plant tissue that distributes photosynthetic sugars, primarily sucrose, from mature leaves to non-photosynthetic organs.

Upper epidermis

A single protective outer layer of clear cells on the top surface of a leaf that secretes the hydrophobic waxy cuticle.

Lower epidermis

The protective epidermal cell layer on the bottom surface of a leaf containing the majority of stomata and guard cells.

Thylakoid lumen (thylakoid space)

The internal fluid compartment enclosed by the thylakoid membrane where protons accumulate to generate a chemiosmotic gradient.

Grana

Dense stacks of disc-shaped thylakoid sacs embedded within the stroma of a chloroplast.

Chlorophyll b

An accessory photosynthetic pigment that appears yellow-green and transfers absorbed light energy directly to chlorophyll a.

Anthocyanins

Water-soluble accessory pigments responsible for imparting red, purple, and blue coloration to plant organs.

Photons

Discrete packets or fixed quantities of light energy that behave as particles and are absorbed by pigment molecules.

Light-harvesting complex

A photosystem module composed of various pigment molecules bound to proteins that absorbs photons and passes resonance energy toward the reaction center.

Reaction center complex

The specialized association of proteins holding a special pair of chlorophyll a molecules and a primary electron acceptor within a photosystem.

Primary electron acceptor

A redox-active molecule in the reaction center complex that captures high-energy excited electrons transferred from chlorophyll a.

Photosystem II (PS II)

The pigment-protein complex that functions first in linear electron flow, featuring a P680 reaction center and an enzyme complex that splits water.

P680

The specialized pair of chlorophyll a molecules located in the reaction center of Photosystem II that absorbs light maximally at $680\,\text{nm}$.

Photosystem I (PS I)

The pigment-protein complex containing a P700 reaction center that absorbs light to re-energize electrons prior to reduction of $\text{NADP}^+$.

P700

The specialized pair of chlorophyll a molecules in the reaction center of Photosystem I that absorbs light maximally at $700\,\text{nm}$.

Plastoquinone (Pq)

A mobile electron carrier that transfers high-energy electrons from Photosystem II to the cytochrome complex during linear electron flow.

Plastocyanin (Pc)

A mobile copper-containing peripheral protein that delivers electrons from the cytochrome complex to Photosystem I.

Ferredoxin (Fd)

An iron-sulfur protein that accepts high-energy electrons from Photosystem I and shuttles them to $\text{NADP}^+$ reductase.

Photophosphorylation

The generation of ATP from ADP and inorganic phosphate driven by light-activated chemiosmosis across the thylakoid membrane.

Chloroplast chemiosmotic proton gradient direction

Protons are actively pumped from the stroma across the thylakoid membrane into the thylakoid space, establishing a high concentration inside.

ATP synthase orientation in chloroplasts

The catalytic knob projects into the stroma, releasing newly formed ATP molecules directly into the stroma where the Calvin cycle takes place.

Products resulting from the photolysis of water

Two water molecules split to generate four protons ($4\text{H}^+$), four electrons ($4e^-$), and one molecule of oxygen gas ($\text{O}_2$).

Second electron transport chain

The short linear pathway from Photosystem I through ferredoxin to $\text{NADP}^+$ reductase that yields NADPH without creating a proton gradient.

Glyceraldehyde 3-phosphate (G3P)

The three-carbon sugar directly synthesized by the Calvin cycle that acts as the starting building block for glucose and other carbohydrates.

Ribulose bisphosphate (RuBP)

A five-carbon phosphorylated sugar that serves as the initial carbon dioxide acceptor during the carbon fixation phase of the Calvin cycle.

Rubisco

The stromal enzyme that catalyzes the covalent attachment of atmospheric carbon dioxide to ribulose bisphosphate in carbon fixation.

3-Phosphoglycerate (3-PGA)

The three-carbon organic acid produced when the unstable six-carbon intermediate splits immediately following carbon fixation.

1,3-Bisphosphoglycerate

The high-energy phosphorylated intermediate formed in Phase 2 of the Calvin cycle after 3-phosphoglycerate receives a phosphate group from ATP.

Total ATP and NADPH required to net one molecule of G3P

The Calvin cycle consumes $9\,\text{ATP}$ ($6$ in reduction and $3$ in regeneration) and $6\,\text{NADPH}$ ($6$ in reduction).

Total ATP and NADPH consumed to synthesize one glucose molecule

The Calvin cycle requires $18\,\text{ATP}$ and $12\,\text{NADPH}$ across two net turns to assemble one six-carbon glucose molecule.

Starch synthesis in plants

The condensation of excess photosynthetic G3P into an insoluble branched glucose polymer stored inside chloroplasts as long-term energy.

Sucrose synthesis and transport

The conversion of cytoplasmic G3P into a transportable disaccharide that travels via the phloem to supply non-photosynthetic tissues.

Consequence of daytime stomatal closure in hot, dry environments

Transpirational water loss is curtailed, but internal $\text{CO}_2$ levels drop and $\text{O}_2$ levels rise, stopping photosynthetic sugar production.

Spatial separation in $\text{C}_4$ photosynthesis

Initial carbon fixation occurs in mesophyll cells to form a four-carbon compound, which is shuttled to bundle-sheath cells for the Calvin cycle.

Temporal separation in CAM photosynthesis

Stomata open at night to fix $\text{CO}_2$ into organic acids, which are stored until daytime when light reactions supply ATP and NADPH for the Calvin cycle.

Examples of $\text{C}_4$ plants

Corn, sugarcane, and crabgrass.

Examples of CAM plants

Pineapples, cacti, and jade plants.

Bundle-sheath cells in $\text{C}_4$ plants

Tightly packed photosynthetic cells arranged around leaf veins where $\text{CO}_2$ is released from organic acids to run the Calvin cycle efficiently.

Entropy ($S$)

A thermodynamic measure of the molecular disorder, randomness, or unusable thermal energy present in a system.

Free energy ($G$)

The portion of a biological system's internal energy capable of performing cellular work when temperature and pressure are uniform throughout.

Activation energy ($E_A$)

The initial threshold energy required to contort reactant molecules into a transition state so that chemical bonds can break.

Transition state

An unstable, high-energy intermediate configuration of reactants during a reaction where existing bonds are actively breaking and new bonds are forming.

Enzyme-substrate complex

A temporary non-covalent association formed when one or more substrate molecules bind directly to an enzyme's active site.

Allosteric site

A specific non-catalytic receptor site on an enzyme distinct from the active site where regulatory molecules or noncompetitive inhibitors bind.

Environmental factors that influence enzyme activity

Temperature, pH, salinity (ion concentration), and the presence of specific chemical inhibitors or activators.

Waxy cuticle

The hydrophobic lipid layer covering the leaf epidermis that prevents excessive water loss through evaporation.

Cyanobacterial chlorophyll location

Embedded directly within infoldings of the plasma membrane, as cyanobacteria lack membrane-bound chloroplasts.

Autotrophs

Organisms that sustain themselves without consuming other organisms by synthesizing organic molecules from inorganic raw materials.

Heterotrophs

Organisms that cannot synthesize their own food and must obtain organic molecules by consuming other organisms or organic byproducts.

Origin of glucose carbon atoms in photosynthesis

Derived entirely from atmospheric carbon dioxide ($\text{CO}_2$) fixed during the Calvin cycle.

Origin of glucose hydrogen atoms in photosynthesis

Derived from water ($\text{H}_2\text{O}$) absorbed by roots and delivered to leaves via the xylem.

Origin of glucose oxygen atoms in photosynthesis

Supplied by the oxygen atoms present in atmospheric carbon dioxide ($\text{CO}_2$).

Why leaves appear green to human eyes

Chlorophyll pigments absorb violet-blue and red light while transmitting and reflecting green light wavelengths.

Function of accessory pigments

To broaden the spectrum of light wavelengths absorbed by a leaf and transfer that captured resonance energy to chlorophyll $a$.

Resonance energy transfer

The relay of absorbed photon excitation energy from pigment molecule to pigment molecule across a light-harvesting complex to the reaction center.

Excited state of chlorophyll electrons

An unstable, high-energy state achieved when an absorbed photon boosts a valence electron to an orbital of higher potential energy.

Chloroplast double membrane

The outer and inner phospholipid bilayer envelopes of endosymbiotic origin that enclose the dense fluid stroma.

Linear electron flow products

The light reactions yield oxygen gas ($\text{O}_2$), reduced $\text{NADPH}$, and $\text{ATP}$ through photophosphorylation.

Source of luminal protons in chemiosmosis

Protons released directly inside the thylakoid space during water photolysis combined with protons pumped from the stroma by the cytochrome complex.

Chemical composition of ATP

A nitrogenous base (adenine), a five-carbon sugar (ribose), and a chain of three negatively charged phosphate groups.

ATP hydrolysis

An exergonic cleavage of the terminal phosphate bond in ATP by the addition of water, yielding ADP, inorganic phosphate ($\text{P}_i$), and free energy.