Cell-Biology on emsenn.net

Arbuscule

Fri, 06 Mar 2026 00:00:00 +0000

An arbuscule is a finely branched structure formed by an endomycorrhizal fungus inside a plant root cell. The name comes from the Latin arbusculum (little tree), describing its shape: a hyphal tip that has penetrated the plant cell wall and branched repeatedly within the cell, creating a dense, tree-like structure with an enormous surface area packed into a tiny space. The arbuscule is the primary site of nutrient exchange between fungus and plant in arbuscular mycorrhizal (AM) associations.

Cell

Fri, 06 Mar 2026 00:00:00 +0000

The cell is the basic structural and functional unit of life. All known living organisms are composed of one or more cells, and all cells arise from preexisting cells by division. This is cell theory, one of the foundational principles of biology, established by Matthias Schleiden and Theodor Schwann in the 1830s and extended by Rudolf Virchow in 1855 with the principle omnis cellula e cellula — every cell from a cell.

Cells and Metabolism

Fri, 06 Mar 2026 00:00:00 +0000

¶Assumed audience

General adult who has completed What Is Life.

¶Cell theory

All organisms are composed of cells; all cells come from preexisting cells.

¶Prokaryotes and eukaryotes

Prokaryotic cells (bacteria, archaea) lack a membrane-bound nucleus. Eukaryotic cells (plants, animals, fungi, protists) have a nucleus containing DNA, plus membrane-bound organelles (mitochondria, endoplasmic reticulum, and in plants, chloroplasts).

¶Cell components

DNA in the nucleus (or nucleoid) encodes the organism’s genetic information (DNA). Ribosomes translate genetic information into proteins. The cell membrane controls what enters and exits. Mitochondria generate energy through cellular respiration. Chloroplasts (in plants) capture light energy through photosynthesis.

Chloroplast

Fri, 06 Mar 2026 00:00:00 +0000

Chloroplast: the organelle in plant cells (and algae) where photosynthesis occurs. Chloroplasts contain chlorophyll — the pigment that absorbs light energy — organized in flattened membrane structures called thylakoids, which are often stacked into grana. The thylakoid membranes are the site of the light-dependent reactions, where light energy drives the splitting of water molecules and the generation of ATP and NADPH. These energy carriers then fuel the Calvin cycle in the surrounding stroma, fixing atmospheric carbon dioxide into organic sugars. The entire process converts light energy into chemical energy, providing the foundation for nearly all food webs on Earth.

Coenocytic

Fri, 06 Mar 2026 00:00:00 +0000

Coenocytic (from Greek koinos, shared, and kytos, container) describes a hyphal organization in which the filament is a continuous tube containing many nuclei in a shared cytoplasm, undivided by cross-walls. Where septate hyphae are partitioned into cells by septa, coenocytic hyphae are open tubes — sometimes enormously long — in which nuclei, organelles, and cytoplasm move freely from end to end.

This organization is characteristic of several fungal lineages, including many Zygomycota (Rhizopus, Mucor, the common bread molds) and Chytridiomycota (aquatic fungi that also produce zoospores). The coenocytic condition is generally considered ancestral in fungi — septate construction evolved later, in the lineages leading to Ascomycota and Basidiomycota, the two largest phyla. See fungal taxonomy for the broader classification context.

Cytoplasm

Fri, 06 Mar 2026 00:00:00 +0000

Cytoplasm is the gel-like material that fills the interior of a cell, enclosed by the cell membrane and surrounding the nucleus and organelles. It consists of water, dissolved ions, small molecules, enzymes, and a network of protein filaments (the cytoskeleton) that provides structural support and serves as tracks for intracellular transport. In a fungal hypha, the cytoplasm contains mitochondria (for energy production), ribosomes (for protein synthesis), endoplasmic reticulum and Golgi apparatus (for protein processing and vesicle production), vacuoles (for storage and turgor), and the nucleus or nuclei that house the organism’s DNA.

Fungal Cell Biology

Fri, 06 Mar 2026 00:00:00 +0000

Fungi are eukaryotes — their cells have a membrane-bound nucleus containing DNA, plus organelles including mitochondria, endoplasmic reticulum, Golgi apparatus, and vacuoles. But the fungal cell differs from animal and plant cells in ways that define how fungi live, feed, grow, and relate to their environments.

¶Cell wall

The most distinctive feature of a fungal cell is its wall. Where plant cell walls are made of cellulose, fungal cell walls are made of chitin — the same polymer found in arthropod exoskeletons. Chitin is a tough, flexible polysaccharide of N-acetylglucosamine units. It gives hyphae their structural rigidity while remaining flexible enough to allow the continuous tip growth that drives mycelial expansion. The presence of chitin rather than cellulose is one of the features that distinguishes fungi from plants and places them closer to animals in the eukaryotic tree — both fungi and animals belong to the clade Opisthokonta. The cell wall also contains glucans and glycoproteins that contribute to structure, cell-cell recognition, and environmental sensing.

Septum

Fri, 06 Mar 2026 00:00:00 +0000

A septum (plural: septa) is a cross-wall that divides a hypha into individual cells. In septate fungi — which include most Ascomycota and Basidiomycota — septa occur at regular intervals along the hypha, creating a chain of cells. Each septum typically has a central pore that allows cytoplasm, organelles, and even nuclei to flow between cells, maintaining the hypha as a functionally connected unit despite the physical partitions.

Not all fungi have septa. Coenocytic fungi — including many Zygomycota and Chytridiomycota — grow as continuous tubes with no cross-walls, their hyphae containing many nuclei in a shared cytoplasm. The distinction between septate and coenocytic growth is one of the basic structural differences among fungal groups (see fungal taxonomy). Septate hyphae can seal their pores in response to damage, isolating injured cells and preventing cytoplasmic loss from spreading through the network. This gives septate fungi a damage-control mechanism that coenocytic fungi lack.