🎓 edX E‑eshe · Ethiopia

HORT 202 · Plant Biochemistry

📚 Key Terms Glossary

Plant Biochemistry for Horticulture · HORT 202

📘 500+ terms 📖 6 modules 🔍 Searchable 🇪🇹 Ethiopian context

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🧬 Module I · Molecular Foundations

Cohesion

Attraction between water molecules due to hydrogen bonding; allows water columns to move up xylem vessels without breaking.

Module I 1.1.2

Adhesion

Attraction between water molecules and other surfaces (e.g., xylem walls); helps water rise against gravity through capillarity.

Module I 1.1.2

Surface Tension

Property of water that allows it to form a "skin" at the surface due to hydrogen bonding; affects water infiltration and supports small insects.

Module I 1.1.2

Heat Capacity

Water's ability to absorb or release heat slowly without changing temperature much; protects plants from temperature extremes.

Module I 1.1.2

Universal Solvent

Water's ability to dissolve more substances than any other liquid due to its polarity; essential for nutrient transport in plants.

Module I 1.1.2

Carbohydrate

Organic molecules composed of carbon, hydrogen, and oxygen (CH₂O)ₙ; include sugars, starches, and cellulose.

Module I 1.2.1

Monosaccharide

Single sugar unit; the simplest form of carbohydrate. Examples: glucose, fructose, galactose.

Module I 1.2.1

Disaccharide

Two sugar units linked by a glycosidic bond. Example: sucrose (glucose + fructose).

Module I 1.2.1

Polysaccharide

Many sugar units linked together; used for energy storage (starch) or structure (cellulose).

Module I 1.2.1

Starch

Primary energy storage polysaccharide in plants; consists of amylose (linear) and amylopectin (branched).

Module I 1.2.1

Cellulose

Structural polysaccharide in plant cell walls; consists of β-1,4 linked glucose units; indigestible by humans (dietary fiber).

Module I 1.2.1

Pectin

Complex polysaccharide in middle lamella; acts as glue holding plant cells together; important for fruit texture and jam making.

Module I 1.2.1

Brix (°Bx)

Measure of soluble solids (primarily sugars) in fruits and vegetables; 1°Bx = 1g sucrose per 100g solution.

Module I 1.2.2

Lipid

Hydrophobic organic molecules; include fats, oils, waxes, phospholipids, and sterols. Used for energy storage, membrane structure, and protection.

Module I 1.2.3

Fatty Acid

Long hydrocarbon chain with a carboxyl group; building block of many lipids. Can be saturated (no double bonds) or unsaturated (with double bonds).

Module I 1.2.3

Saturated Fatty Acid

Fatty acid with no double bonds; straight chain; solid at room temperature. Example: palmitic acid.

Module I 1.2.3

Unsaturated Fatty Acid

Fatty acid with one or more double bonds; bent chain; liquid at room temperature. Example: oleic acid.

Module I 1.2.3

Triglyceride

Glycerol + three fatty acids; main form of energy storage in plants (oils in seeds).

Module I 1.2.3

Phospholipid

Glycerol + two fatty acids + phosphate group; amphipathic molecule that forms bilayers in cell membranes.

Module I 1.2.3

Amphipathic

Having both hydrophilic (water-loving) and hydrophobic (water-fearing) regions; characteristic of phospholipids.

Module I 1.2.3

Wax

Ester of long-chain fatty acid and long-chain alcohol; forms protective coating (cuticle) on leaves and fruits.

Module I 1.2.3

Cuticle

Waxy layer covering leaves, fruits, and non-woody stems; reduces water loss and provides protection.

Module I 1.2.3

Carotenoid

Lipid-soluble pigments (yellow, orange, red); function in photosynthesis and as antioxidants. Examples: β-carotene, lycopene.

Module I 1.2.3

Amino Acid

Building block of proteins; contains amino group, carboxyl group, hydrogen, and variable R group.

Module I 1.3.1

Peptide Bond

Covalent bond formed between carboxyl group of one amino acid and amino group of another; releases water (condensation reaction).

Module I 1.3.1

Primary Structure

Linear sequence of amino acids in a protein; determined by DNA.

Module I 1.3.1

Secondary Structure

Local folding patterns (α-helix, β-sheet) stabilized by hydrogen bonds between backbone atoms.

Module I 1.3.1

Tertiary Structure

Overall three-dimensional shape of a protein; stabilized by interactions between R groups (hydrophobic, ionic, disulfide bridges).

Module I 1.3.1

Quaternary Structure

Arrangement of multiple polypeptide subunits; example: rubisco (16 subunits).

Module I 1.3.1

Enzyme

Protein catalyst that speeds up biochemical reactions by lowering activation energy.

Module I 1.3.2

Activation Energy

Energy barrier that must be overcome for a reaction to proceed; enzymes lower this barrier.

Module I 1.3.2

Active Site

Region of enzyme where substrate binds and catalysis occurs; has specific three-dimensional shape.

Module I 1.3.2

Substrate

Molecule upon which an enzyme acts.

Module I 1.3.2

Lock and Key Model

Model of enzyme action where active site is exactly complementary to substrate shape.

Module I 1.3.2

Induced Fit Model

Model where active site changes shape slightly upon substrate binding, becoming complementary after binding.

Module I 1.3.2

Denaturation

Loss of protein's three-dimensional structure (unfolding) due to heat, pH, or chemicals; loss of function.

Module I 1.3.3

Competitive Inhibition

Inhibitor resembles substrate and binds to active site; can be overcome by increasing substrate concentration.

Module I 1.3.3

Non-competitive Inhibition

Inhibitor binds to allosteric site, changing enzyme shape; cannot be overcome by adding more substrate.

Module I 1.3.3

Cofactor

Inorganic ion (e.g., Mg²⁺, Zn²⁺) required for enzyme activity.

Module I 1.3.2

Coenzyme

Organic molecule (often vitamin-derived) required for enzyme activity; e.g., NAD⁺, FAD.

Module I 1.3.2

⚡ Module II · Plant Metabolism & Energy Systems

Photosynthesis

Process by which plants convert light energy into chemical energy (sugars) using CO₂ and water.

Module II 2.1.1

Light Reactions

First stage of photosynthesis; occur in thylakoid membranes; produce ATP and NADPH using light energy.

Module II 2.1.1

Calvin Cycle

Light-independent reactions of photosynthesis; occur in stroma; fix CO₂ into sugars using ATP and NADPH.

Module II 2.1.2

Rubisco

Ribulose bisphosphate carboxylase/oxygenase; most abundant protein on Earth; fixes CO₂ in Calvin cycle but also has oxygenase activity.

Module II 2.1.2

Photorespiration

Wasteful process when rubisco fixes O₂ instead of CO₂; increases at high temperatures; reduces photosynthetic efficiency.

Module II 2.1.3

C3 Plant

Plant that fixes CO₂ directly via rubisco; first product is 3-PGA (3-carbon). Examples: wheat, rice, soybean.

Module II 2.1.3

C4 Plant

Plant that concentrates CO₂ using Kranz anatomy; first product is oxaloacetate (4-carbon). Examples: maize, sugarcane.

Module II 2.1.3

CAM Plant

Plant that fixes CO₂ at night (stomata open) and uses it during the day (stomata closed). Examples: cacti, pineapple.

Module II 2.1.3

Kranz Anatomy

Specialized leaf structure in C4 plants with bundle sheath cells surrounded by mesophyll cells; concentrates CO₂.

Module II 2.1.3

Glycolysis

First stage of cellular respiration; occurs in cytoplasm; breaks glucose (6C) into 2 pyruvate (3C); net gain 2 ATP, 2 NADH.

Module II 2.2.1

Pyruvate

Three-carbon end product of glycolysis; enters mitochondria for further oxidation.

Module II 2.2.1

Krebs Cycle

Second stage of respiration; occurs in mitochondrial matrix; oxidizes acetyl-CoA to produce NADH, FADH₂, and ATP.

Module II 2.2.2

Electron Transport Chain (ETC)

Series of protein complexes in inner mitochondrial membrane; transfers electrons from NADH/FADH₂ to O₂, pumping protons to create gradient for ATP synthesis.

Module II 2.2.3

Oxidative Phosphorylation

Process where ATP is synthesized using energy from electron transport chain and proton gradient.

Module II 2.2.3

Chemiosmosis

Movement of protons down their concentration gradient through ATP synthase, driving ATP synthesis.

Module II 2.2.3

ATP Synthase

Rotary enzyme that uses proton gradient to synthesize ATP from ADP and inorganic phosphate.

Module II 2.2.3

Fermentation

Anaerobic process that regenerates NAD⁺ from NADH to allow glycolysis to continue; produces ethanol or lactate.

Module II 2.2.1

Phloem

Living tissue that transports sugars, amino acids, and other organic compounds from source to sink.

Module II 2.3.1

Sieve Element

Conducting cell of phloem; living but enucleate at maturity; connected by sieve plates.

Module II 2.3.1

Companion Cell

Cell connected to sieve element via plasmodesmata; provides metabolic support and energy for phloem loading/unloading.

Module II 2.3.1

Source

Plant organ that produces or exports more photosynthates than it needs; e.g., mature leaves.

Module II 2.3.2

Sink

Plant organ that imports photosynthates for growth, storage, or respiration; e.g., roots, fruits, tubers.

Module II 2.3.2

Sink Strength

Ability of a sink to attract assimilates; determined by sink size × sink activity.

Module II 2.3.2

Nitrate Reductase

Enzyme that reduces nitrate (NO₃⁻) to nitrite (NO₂⁻) in cytoplasm; first step of nitrogen assimilation.

Module II 2.4.1

GS-GOGAT Pathway

Primary pathway for ammonium assimilation; incorporates NH₄⁺ into glutamine and glutamate.

Module II 2.4.1

Nitrogen Fixation

Conversion of atmospheric N₂ to ammonia (NH₃) by nitrogenase enzyme; occurs in bacteria (rhizobia in legume nodules).

Module II 2.4.2

Leghemoglobin

Oxygen-binding protein in legume nodules; maintains low O₂ concentration to protect nitrogenase while supplying O₂ for respiration.

Module II 2.4.2

Niger Seed (Nug)

Guizotia abyssinica; important oilseed crop in Ethiopia; oil content 40-50%, high in linoleic acid (up to 75%).

Module II 2.4.3

📈 Module III · Biochemical Basis of Productivity & Quality

Harvest Index (HI)

Ratio of economic yield to total biomass; measures how efficiently plant converts total dry matter into harvestable product.

Module III 3.1

Sugar/Acid Ratio

Brix divided by titratable acidity; key flavor quality index for fruits.

Module III 3.2

Ester

Aroma compound formed from alcohol and carboxylic acid; responsible for fruity smells (e.g., isoamyl acetate in bananas).

Module III 3.2

Terpene

Large class of volatile compounds built from isoprene units; contribute to floral and citrus aromas.

Module III 3.2

Anthocyanin

Water-soluble flavonoid pigment producing red, purple, and blue colors in fruits and flowers; pH-dependent color.

Module III 3.4

Lycopene

Red carotenoid pigment in tomatoes and watermelon; powerful antioxidant.

Module III 3.4

Chlorophyllase

Enzyme that breaks down chlorophyll during fruit ripening and leaf senescence.

Module III 3.4

🌵 Module IV · Stress & Environmental Biochemistry

Abiotic Stress

Stress from non-living factors (drought, salinity, temperature extremes, flooding, heavy metals).

Module IV 4.1

Compatible Solute

Small molecule (proline, glycine betaine, sugars) accumulated during stress to maintain cell turgor without interfering with metabolism.

Module IV 4.1

SOS Pathway

Salt Overly Sensitive pathway; regulates Na⁺ homeostasis and salt tolerance via SOS1 (Na⁺/H⁺ antiporter).

Module IV 4.1

Abscisic Acid (ABA)

Stress hormone that promotes stomatal closure during drought and regulates seed dormancy.

Module IV 4.3

Jasmonate

Lipid-derived hormone that regulates plant defense against herbivores and necrotrophic pathogens.

Module IV 4.3

Ethylene

Gaseous hormone involved in fruit ripening, senescence, and stress responses (flooding, wounding).

Module IV 4.3

Reactive Oxygen Species (ROS)

Partially reduced forms of oxygen (O₂⁻, H₂O₂, OH•) that can damage cells but also act as signaling molecules.

Module IV 4.4

Superoxide Dismutase (SOD)

Antioxidant enzyme that converts superoxide (O₂⁻) to hydrogen peroxide (H₂O₂).

Module IV 4.4

Catalase (CAT)

Antioxidant enzyme that converts H₂O₂ to water and oxygen.

Module IV 4.4

🔬 Module V · Analytical & Biochemical Techniques

Molarity (M)

Moles of solute per liter of solution.

Module V 5.1

Buffer

Solution that resists changes in pH when small amounts of acid or base are added.

Module V 5.1

Beer-Lambert Law

A = εcl; absorbance is directly proportional to concentration and path length.

Module V 5.2

Standard Curve

Graph of known concentrations vs. absorbance used to determine unknown concentrations.

Module V 5.2

Chromatography

Technique for separating mixtures based on differential partitioning between stationary and mobile phases.

Module V 5.3

Rf Value

Retention factor = distance moved by compound / distance moved by solvent front in chromatography.

Module V 5.3

HPLC

High-Performance Liquid Chromatography; advanced chromatography using high pressure for high resolution.

Module V 5.3

International Unit (U)

Amount of enzyme that catalyzes conversion of 1 μmol substrate per minute.

Module V 5.4

Specific Activity

Enzyme activity per mg protein; measure of enzyme purity.

Module V 5.4

🇪🇹 Module VI · Ethiopian Case Studies

Caffeine

Purine alkaloid in coffee; acts as natural pesticide and stimulant. Yirgacheffe coffee has highest caffeine (10.38 g/100g).

Module VI 6.1

Chlorogenic Acid (CGA)

Phenolic compound in coffee; antioxidant; degrades during roasting to form flavor compounds.

Module VI 6.1

Enset (Ensete ventricosum)

"False banana"; staple food for 20+ million Ethiopians; fermented to produce kocho and bulla.

Module VI 6.2

Kocho

Fermented enset product from pseudostem and corm; staple food in southern Ethiopia.

Module VI 6.2

Bulla

Premium enset product from squeezed pulp; fermented 5 days; used for porridge.

Module VI 6.2

Ersho

Traditional starter culture for teff (injera) fermentation; backslopping method ensures microbial continuity.

Module VI 6.4

Teff (Eragrostis tef)

Smallest cereal grain; Ethiopia's staple; gluten-free; used to make injera.

Module VI 6.4

Injera

Spongy, sour flatbread made from fermented teff; daily staple in Ethiopia.

Module VI 6.4

Silver Thiosulfate (STS)

Ethylene inhibitor used in cut flowers to delay senescence; binds to ethylene receptors.

Module VI 6.5

Vase Life

Duration cut flowers remain attractive after harvest; influenced by ethylene, water quality, and preservatives.

Module VI 6.5

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