Photosynthesis and ATP Synthesis

Vocabulary practice flashcards covering leaf anatomy, chloroplast structure, photosynthetic pigments, light-dependent reactions, the Calvin cycle, and limiting factors of photosynthesis.

Heterotrophs

Organisms, including animals and fungi, that must consume food containing organic molecules (such as carbohydrates, fats, and proteins) as their only source of energy.

Autotrophs

Organisms that are self-sufficient and do not require organic molecules made by other organisms, using sunlight to synthesize their own organic molecules from simple inorganic substances.

Photosynthesis (Overall Equation)

The metabolic process by which plants produce carbohydrates and oxygen from carbon dioxide and water using sunlight: $6CO_2 + 6H_2O \rightarrow C_6H_{12}O_6 + 6O_2$.

Mesophyte

A plant adapted to normal terrestrial conditions, being adapted neither for living in water nor for withstanding excessive drought.

Cuticle

A waxy, transparent, watertight layer secreted by leaf epidermal cells that limits water loss to the stomata and forms a protective barrier against microorganisms and insects.

Stomata

Tiny pores located predominantly in the lower epidermis of leaves through which gases like carbon dioxide and oxygen diffuse, each bounded by two guard cells.

Guard Cells

Pairs of sausage-shaped epidermal cells with unevenly thickened walls and circular cellulose microfibrils that curve open as they become turgid via the active uptake of potassium ions and osmosis.

Palisade Mesophyll

The primary photosynthetic tissue of a leaf, consisting of cylindrical cells oriented at right angles to the upper epidermis that contain the highest number of chloroplasts.

Spongy Mesophyll

Leaf tissue beneath the palisade layer characterized by loosely and irregularly packed cells with large intercellular air spaces, primarily adapted for gaseous exchange.

Chloroplast Envelope

The double-membrane boundary consisting of an outer membrane and an inner membrane that encloses the interior of a chloroplast.

Thylakoids

Flattened, fluid-filled membrane sacs inside a chloroplast on which the light-dependent reactions of photosynthesis take place.

Grana

Stacks of disc-shaped thylakoids (singular: granum) inside the chloroplast that provide a large surface area for photosynthetic pigments.

Stroma

The fluid-filled background matrix surrounding the grana in a chloroplast, containing Calvin cycle enzymes, ribosomes, circular DNA, and starch grains.

Thylakoid Lumen

The interior fluid space enclosed within each thylakoid sac where hydrogen ions ($H^+$) accumulate to create a proton gradient for ATP production.

Photosynthetic Pigments

Coloured chemical compounds tightly embedded within thylakoid membranes that absorb light energy at specific wavelengths and reflect or transmit unabsorbed wavelengths.

Chlorophyll a and Chlorophyll b

The primary photosynthetic pigments; chlorophyll a possesses a $-CH_3$ group and absorbs slightly longer wavelengths, whereas chlorophyll b possesses a $-CHO$ group.

Absorption Spectrum

A graph showing the percentage or degree of light energy absorbed across different wavelengths (colours) by a pigment or mixture of pigments.

Carotenoids

Accessory pigments, including carotene and xanthophylls, that absorb short blue-green wavelengths, pass trapped energy to chlorophyll, and protect chlorophyll against photo-damage from intense light.

Photosystem

A membrane-spanning cluster of proteins and hundreds of pigment molecules (chlorophyll a, chlorophyll b, and carotenoids) that funnel light energy toward a reaction centre.

Reaction Centre

A special pair of chlorophyll a molecules situated at the core of a photosystem complex that emits high-energy electrons upon receiving light energy.

Photophosphorylation

The synthesis of ATP from ADP and inorganic phosphate ($P_i$) using energy originally derived from light and transferred via an electron transport chain.

Cyclic Photophosphorylation

A light-dependent pathway involving only photosystem I (PSI) in which photoexcited electrons pass down electron carriers to produce ATP before returning directly to PSI, producing no reduced NADP.

Non-cyclic Photophosphorylation

A light-dependent pathway involving both PSII and PSI where electrons travel from water to NADP, producing both ATP and reduced NADP and releasing oxygen.

Photolysis

The light-activated enzymatic splitting of water molecules associated with photosystem II: $2H_2O \rightarrow 4H^+ + 4e^- + O_2$.

Z-scheme

A diagrammatic representation of the light-dependent reactions of photosynthesis plotting the changes in electron energy levels as they move from PSII through PSI to NADP.

Ribulose Bisphosphate (RuBP)

A five-carbon ($5\text{C}$) sugar molecule that acts as the initial carbon dioxide acceptor during the light-independent stage of photosynthesis.

Rubisco

Ribulose bisphosphate carboxylase; the stroma enzyme that catalyzes the carboxylation of RuBP with carbon dioxide to yield an unstable six-carbon ($6\text{C}$) intermediate.

Glycerate 3-phosphate (GP)

A three-carbon ($3\text{C}$) intermediate (also known as phosphoglyceric acid, PGA) produced when the unstable six-carbon intermediate of carbon fixation splits.

Triose Phosphate (TP)

A three-carbon ($3\text{C}$) sugar (also known as GALP) formed by reducing GP using ATP and reduced NADP; it is the first carbohydrate synthesized in photosynthesis.

Limiting Factor

An environmental condition or resource (such as light intensity, temperature, or carbon dioxide concentration) that is in shortest supply and thereby restricts the rate of photosynthesis.

Light Saturation

The state reached when increasing light intensity no longer increases the rate of photosynthesis because another factor has become limiting.