Plant form and function

CHNOPS Elements

Carbon, Hydrogen, Nitrogen, Oxygen, Phosphorus, and Sulfur make up $99\%$ of plant mass.

Water & Minerals

Water ($H_2O$) makes up $>90\%$ of plant tissue; inorganic ions ($K^+$,

Dehydration Synthesis

Combines monomers into polymers by removing a water molecule.

Hydrolysis

Breaks polymers down into monomers by adding a water molecule.

Carbohydrates

Primary energy-storage and structural molecules made of carbon, hydrogen, and oxygen.

Monosaccharides

Single sugar units including pentoses (ribose) and hexoses (glucose).

Disaccharides

Two sugars linked by a glycosidic bond (e.g., sucrose, maltose, lactose).

Starch

Alpha-1,4 glucose polymer with helical/branched chains used for plant energy storage.

Cellulose

Beta-1,4 glucose linear chains linked by hydrogen bonds; builds plant cell walls.

Microfibrils

Bundles of cellulose chains cross-linked by hydrogen bonds to provide tensile strength.

Lipids

Hydrophobic molecules used for energy storage, cell membranes, and waterproofing.

Triglycerides

Fats and oils made of 1 glycerol + 3 fatty acids joined by ester linkages.

Phospholipids

Amphipathic lipids forming the cell membrane bilayer (hydrophilic head + hydrophobic tails).

Cutin & Suberin

Specialized plant lipids that form protective, waterproof layers like the cuticle.

Proteins

Functional polymers made of amino acids linked together by peptide bonds.

Enzymes

Protein catalysts that lower activation energy at active sites to speed up chemical reactions.

Nucleic Acids

Polymers of nucleotides (sugar, phosphate, base) linked by phosphodiester bonds (DNA/RNA).

Eukaryotic Plant Cells

Cells with a true membrane-bound nucleus enclosing linear genetic material.

Nucleus & Nucleolus

Nucleus stores the plant genome; nucleolus produces ribosomes for protein synthesis.

The Big Three Plant Features

Cell wall, central vacuole, and chloroplasts (defining structures of plant cells).

Plastids

Membrane-bound plant organelles originating from proplastids (chloroplasts, chromoplasts, leucoplasts) that manufacture and store chemical compounds. Name stems from Greek plastikos (molded) due to their ability to change shape and type.

Chloroplast Structure

Contains outer membranes, fluid stroma, and thylakoid sacs stacked into grana.

Endosymbiont Theory of Chloroplasts

Theory that a eukaryotic host cell engulfed a photosynthetic cyanobacterium (similar to Nostoc) without digesting it, which evolved into the chloroplast.

Mitochondria

Sites of cellular respiration that convert organic molecules into usable ATP.

Central Vacuole

Fluid-filled organelle bounded by the tonoplast; stores metabolites and regulates cell size.

Turgor Pressure

Internal water pressure against the cell wall that keeps plant tissue firm and upright.

Primary Cell Wall

Thin, flexible outer wall layer deposited while the plant cell is actively growing; made mostly of cellulose.

Secondary Cell Wall

Thick, rigid inner wall layer laid down inside the primary wall after growth stops; often contains lignin for extra strength.

Middle Lamella

Pectin-rich layer that cements the primary cell walls of adjacent plant cells together.

Plasmodesmata

Membrane-lined channels through cell walls connecting the cytoplasm of adjacent plant cells, allowing direct movement of water, nutrients, and signaling molecules.

Endomembrane System

Network including ER, Golgi (dictyosomes), tonoplast, and transport vesicles.

Dictyosomes

Plant Golgi apparatus units that package proteins and secrete cell wall polysaccharides.

Cytoskeleton & Microtubules

Tubulin protein helices (13 protofilaments) with dynamic (+/-) ends that guide wall assembly.

Epidermis

Outermost layer of cells of the primary plant body; protects, minimizes water loss via cuticle, allows aeration via stomata.

Periderm

Replaces epidermis in older stems/roots; made of cork cells, cork cambium, phelloderm; allows aeration via lenticels.

Parenchyma tissue

Polyhedral, variable shaped living cells with primary (or primary+secondary) walls; found throughout the plant; does photosynthesis, storage, respiration, secretion, wound healing.

Collenchyma tissue

Elongated living cells with unevenly thickened primary wall (no lignin); found under epidermis in young stems; provides flexible support.

Sclerenchyma tissue

Made of fibers and sclereids with thick lignified secondary walls; found in cortex, xylem, phloem; provides mechanical support and protection.

Fiber

Very long sclerenchyma cell with primary + thick secondary lignified wall; often dead when functional; provides support.

Sclereid

Variable-shaped sclerenchyma cell, shorter than fibers, thick lignified wall; may be living or dead when functional; provides mechanical protection.

Tracheid

Elongated tapering xylem cell with pits but no perforations; dead when functional; conducts water in gymnosperms and seedless plants.

Vessel element

Elongated xylem cell with pits AND perforations, joined end-to-end into a vessel; dead when functional; chief water conductor in angiosperms.

Sieve cell

Elongated tapering phloem cell with sieve areas; living at maturity, no P-protein; conducts food in gymnosperms.

Albuminous cell

Living phloem cell associated with sieve cells (not from same mother cell); delivers molecules and ATP to sieve cells in gymnosperms.

Sieve-tube element

Elongated phloem cell with sieve plates on end walls, contains P-protein; living at maturity, no functional nucleus; conducts food in angiosperms.

Companion cell

Living phloem cell derived from same mother cell as its sieve-tube element; delivers molecules and ATP to the sieve-tube element in angiosperms.

Dead when functional

When a cell's living contents die after building its wall, leaving behind an empty rigid structure (like a pipe or beam) that performs its function passively, with no living activity required.

Sieve cell vs sieve-tube element

Sieve cells occur only in gymnosperms (paired with albuminous cells); sieve-tube elements occur only in angiosperms (paired with companion cells).

Lateral vs vertical transfer

Two possible ways the chloroplast's genetic material could have been inherited/acquired during its evolutionary origin.

Heterocyst

Specialized cell found in some cyanobacteria (like Nostoc) associated with nitrogen fixation; relevant to the possible identity of the original chloroplast ancestor.

Kleptoplasty

The phenomenon of an organism stealing and retaining functional chloroplasts from another organism (algae) rather than inheriting them through normal endosymbiosis.

Paulinella chromatophora

An amoeba that acquired a photosynthetic symbiont relatively recently ($\sim 100\,\text{mya}$); shows gene reduction in the symbiont due to lateral gene transfer to the host, suggesting it may be evolving into a true chloroplast.

Proplastid

An undifferentiated plastid whose fate/type has not yet been determined; the starting point from which all other plastid types can develop.

Chloroplast envelope

The outer and inner membrane boundary surrounding a chloroplast.

Thylakoid

Internal membrane system inside the chloroplast where the light reactions of photosynthesis occur; embedded in the stroma.

Stroma

The fluid-filled space inside the chloroplast (outside the thylakoids) where the dark reaction (Calvin cycle) of photosynthesis occurs; doesn't require membranes since its reactions are enzyme-based, not membrane-based.

Granum

A stack of thylakoid membranes within the chloroplast.

Chloroplast orientation to light

Chloroplasts can reposition themselves within the cell depending on light intensity — spreading out in dim light to maximize light capture, and clustering/aligning edge-on in bright light to avoid photodamage.

Cp-actin filaments

Actin filaments associated with chloroplasts that help drive their movement/relocalization in response to light direction (e.g., evading intense blue light).

Etioplast

A plastid type that forms when a plant is grown without enough light (etiolated); contains a prolamellar body instead of thylakoid grana.

Prolamellar body

A crystalline, lattice-like membrane structure found inside etioplasts, formed in the absence of light.

Chromoplast

A plastid type that stores pigments (often red/orange/yellow carotenoids) instead of chlorophyll; chloroplasts can convert into chromoplasts (e.g., in ripening tomato fruit).

Leucoplast

A colorless plastid type that stores substances such as oils and volatile chemicals, often related to scents or chemical signaling.

Amyloplast

A plastid type that stores starch for later use by the plant; also functions in gravity detection (gravitropism) in roots.

Plastid pluripotency

The concept that plastids are not fixed in type — they can interconvert between different forms (proplastid, chloroplast, chromoplast, amyloplast, leucoplast, etioplast, etc.) depending on developmental and environmental cues.