The two fundamental categories of cellular organisation are prokaryotic cells and eukaryotic cells. Prokaryotic cells are found in the domains Bacteria and Archaea and lack a membrane bound nucleus and most membrane bound organelles. Eukaryotic cells are found in animals, plants, fungi, protists and several other groups and contain a membrane bound nucleus together with various specialised organelles. This distinction is based primarily on cellular architecture rather than on whether an organism is simple or complex.
Prokaryotic cells can perform highly sophisticated metabolic processes despite their relatively compact organisation, while some eukaryotic cells are extremely specialised. Within the eukaryotic category, animal, plant, fungal and protist cells have distinctive combinations of structures and functions. These categories are useful for classification, but real biological diversity is considerably richer than a simple two type scheme.
Prokaryotic cells generally have a plasma membrane, cytoplasm, ribosomes and DNA, usually including a main circular chromosome located in a nucleoid region. Many possess a cell wall, although the composition differs substantially between bacteria and archaea. Bacteria commonly have cell walls containing peptidoglycan, whereas archaeal cell walls do not contain peptidoglycan and can have distinctive molecular compositions. Some prokaryotes possess capsules, pili, fimbriae or flagella that assist with attachment, movement or interactions with environments.
Their ribosomes are smaller than typical eukaryotic cytoplasmic ribosomes, although mitochondrial and chloroplast ribosomes have evolutionary relationships with bacterial systems. Prokaryotic cells can be extremely metabolically diverse, including aerobic, anaerobic, photosynthetic, chemolithotrophic and fermentative lifestyles. Their small size and high surface area relative to volume can support rapid exchange of substances with the environment.
Eukaryotic cells possess a nucleus enclosed by a double membrane called the nuclear envelope. They also contain membrane bound organelles that create specialised intracellular compartments. Mitochondria perform major roles in aerobic energy metabolism, while chloroplasts in plants and algae perform photosynthesis. The endoplasmic reticulum participates in protein and lipid synthesis, the Golgi apparatus modifies and sorts many cellular products, lysosomes carry out degradation functions in many animal cells, and peroxisomes participate in oxidative reactions and lipid metabolism. Eukaryotic cells also possess a cytoskeleton composed of structures including microtubules, actin filaments and intermediate filaments. These components provide structural support, facilitate intracellular transport and contribute to cell movement and division. Compartmentalisation allows eukaryotic cells to conduct chemically different processes simultaneously within specialised environments.
Animal cells are eukaryotic cells lacking the cellulose cell wall and chloroplasts characteristic of plant cells. They generally possess a flexible plasma membrane, nucleus, mitochondria, endoplasmic reticulum, Golgi apparatus, ribosomes, cytoskeleton and other organelles. Different animal cell types have specialised structures. Neurons have long projections for electrical and chemical communication, muscle cells contain specialised contractile machinery, and intestinal epithelial cells can possess microvilli that increase surface area for absorption. Red blood cells in mammals lose their nuclei during maturation, allowing greater haemoglobin content and flexibility through narrow blood vessels. Animal cells therefore demonstrate how eukaryotic organisation can be adapted extensively to particular physiological functions.
Plant cells are eukaryotic cells characterised by structures including a cellulose containing cell wall, a large central vacuole and plastids such as chloroplasts in photosynthetic tissues. The cell wall provides mechanical support and helps maintain cell shape, while the central vacuole contributes to water balance, storage and turgor pressure. Chloroplasts contain chlorophyll and other pigments and provide the site of photosynthetic processes in plant cells. Plant cells also possess mitochondria, nuclei, ribosomes, endoplasmic reticulum, Golgi apparatus and cytoskeletal components. Not every plant cell contains chloroplasts because specialised tissues such as roots generally do not photosynthesise in the same way as green leaves. Plant cells consequently vary according to tissue, developmental stage and environmental conditions.
Fungal cells are eukaryotic and possess cell walls, but fungal walls are primarily composed of chitin and other materials rather than cellulose. Fungi generally obtain nutrients by secreting enzymes into their surroundings and absorbing the resulting products, rather than performing photosynthesis. Many fungi grow as networks of filamentous structures called hyphae, which can collectively form a mycelium. Yeasts are unicellular fungi and demonstrate that fungal organisation can occur in single cells as well as multicellular forms. Protists comprise an extremely diverse collection of eukaryotic organisms that do not fit conveniently into the traditional plant, animal or fungal categories. Some protists are unicellular, while others are multicellular or colonial, and their nutritional strategies include photosynthesis, ingestion and absorption.
Cell types can also be classified by specialised function rather than broad evolutionary category. Examples include immune cells, stem cells, reproductive cells, sensory cells, glandular cells and connective tissue cells. Stem cells are distinguished by their capacity for self renewal and differentiation, while differentiated cells express specialised molecular programmes suited to particular roles. In multicellular organisms, cells communicate and cooperate through chemical signals, cell adhesion and extracellular structures.
Tissue cells can therefore be highly dependent on neighbouring cells and environmental signals. This functional classification overlaps with the prokaryotic and eukaryotic distinction rather than replacing it. A complete description of a cell may require specifying its domain, organism, tissue, developmental state, structure, metabolism and function.
The distinction between prokaryotic and eukaryotic cells is one of the most important organisational divisions in biology, but it should not be interpreted as meaning that prokaryotes are primitive or incapable of complexity. Bacteria and archaea possess sophisticated molecular systems, diverse metabolic pathways and extensive regulatory networks. Eukaryotic complexity is strongly associated with compartmentalisation and endomembrane systems, but individual eukaryotic cells can also be highly specialised and dependent on multicellular organisation. Evolutionary evidence indicates that mitochondria and chloroplasts originated through ancient endosymbiotic relationships involving bacterial ancestors. This is supported by features including their double membranes, genomes, ribosomes and division mechanisms. Consequently, the major cell types reflect both fundamental structural differences and deep evolutionary history.
