Matthias Schleiden (1838) and Theodor Schwann (1839) together formulated the cell theory, stating that all plants and animals are made of cells.

Protoplasm is divided into two main components: cytoplasm (everything outside the nucleus) and the nucleus, which contains the genetic material.

Bacteria are prokaryotes and lack membrane-bound organelles including true mitochondria. They perform respiration using their plasma membrane instead.

Xanthophylls are yellow accessory pigments found in plant chloroplasts. They absorb blue-violet light and contribute to the autumn yellowing of leaves.

According to cell theory, the cell is both the structural (building block) and functional (metabolic) unit of all living organisms.

Carl Benda coined the term 'mitochondria' in 1897 from Greek words meaning thread (mitos) and granule (chondros), describing their appearance.

S.J. Singer and G.L. Nicholson proposed the Fluid Mosaic Model of the cell membrane in 1972, describing it as a fluid phospholipid bilayer with embedded proteins.

The rigid cell wall made of cellulose is unique to plant cells. It provides structural support and protection but is completely absent in animal cells.

Viruses are not cells — they have no cell membrane, no cytoplasm, and cannot carry out metabolism independently. They are therefore exceptions to cell theory.

Plasmodesmata are microscopic channels through plant cell walls that connect the cytoplasm of adjacent cells, allowing communication and transport between them.

Ribosomes are the only organelles present in both prokaryotes (Monera) and eukaryotes (Protista). All other complex organelles are absent in Monera.

The endoplasmic reticulum forms the internal scaffolding (endoskeleton) of the cell, providing mechanical support and a framework for the cytoplasm.

Prokaryotic cells have smaller 70S ribosomes (composed of 30S and 50S subunits), while eukaryotic cells have larger 80S ribosomes (composed of 40S and 60S subunits).

In plant cells, the individual units of the Golgi apparatus are called dictyosomes. The term Golgi complex is more commonly used for the whole system in animal cells.

Mitochondria are the powerhouses of the cell, playing a central role in cellular metabolism by producing ATP through cellular respiration.

Felice Fontana first observed and described the nucleolus in 1781 while studying the eel's skin cells under the microscope.

The primary function of the nucleolus is the synthesis and assembly of ribosomal RNA (rRNA) and ribosome subunits, which are then exported to the cytoplasm.

The centromere is a constricted region of a chromosome where sister chromatids are joined together and where spindle fibres attach during cell division.

A chromatid is one of the two identical halves of a replicated chromosome, joined at the centromere. Two sister chromatids together form one complete chromosome.

Rudolf Virchow in 1858 confirmed that all cells arise from pre-existing cells ('Omnis cellula e cellula'), describing the process of cell division as the basis of this principle.

Wilhelm von Waldeyer coined the term 'chromosomes' in 1888 to describe the deeply staining bodies seen in the nucleus during cell division.

Walther Flemming introduced the term 'mitosis' in 1882 after carefully observing and documenting the stages of cell division under the microscope.

Mitosis occurs in somatic (body) cells for growth and repair. It produces two identical daughter cells with the same chromosome number as the parent cell.

Spindle fibres are made of tubulin protein that polymerises to form microtubules. They attach to chromosomes at centromeres and pull them apart during cell division.

During late prophase (also called prometaphase), the nuclear envelope breaks down, allowing spindle fibres to access and attach to chromosomes.

Prophase I is the longest and most complex stage of meiosis, during which homologous chromosomes pair up (synapsis), crossing over occurs, and chromosomes condense.

Synapsis (pairing of homologous chromosomes) occurs during the zygotene substage of Prophase I of meiosis, forming bivalents held together by the synaptonemal complex.

Trisomy is a chromosomal abnormality where a cell has three copies of one chromosome instead of the normal two. Down syndrome (trisomy 21) is a common example.

During zygotene, homologous chromosomes begin to pair up (synapsis) and chromosomes shorten and condense. Leptotene is when chromosomes first become visible.

J.D. Robertson proposed the unit membrane model in 1959, describing the plasma membrane as a trilaminar structure of two dark protein layers sandwiching a light lipid layer.

Theodor Boveri named the centriole, a cylindrical organelle found in animal cells that plays a key role in organising the mitotic spindle during cell division.

Nerve cells (neurons) are the longest cells in the human body. Some motor neurons can extend over a metre in length from the spinal cord to the tips of the toes.

Cellular respiration (Krebs cycle and oxidative phosphorylation) takes place in the mitochondria, which generate ATP — the cell's energy currency.

Karyology is the branch of cell biology concerned specifically with the structure and function of the nucleus, including chromosomes and nuclear division.

Chloroplasts are green plastids that contain chlorophyll pigment, which captures light energy for photosynthesis. They also contain carotenoids as accessory pigments.

The G1 (Gap 1) phase typically lasts about 10 hours in a 24-hour cell cycle. During this phase the cell grows and prepares for DNA replication in S phase.

DNA replication occurs during the S (Synthesis) phase of Interphase, before mitosis begins. By the time the cell enters mitosis, all chromosomes have been duplicated.

Secretory vesicles bud off from the trans face of the Golgi complex, carrying processed proteins and lipids to the cell membrane or other destinations.

Cristae are the infoldings of the inner mitochondrial membrane. They greatly increase the surface area available for the enzymes of ATP synthesis (ATP synthase).

Rudolf Virchow (1858) described cell division and stated that all cells come from pre-existing cells, completing the modern cell theory.

The cell is both the structural unit (building block) and the functional unit (site of metabolic activities) of all living organisms, as stated by cell theory.

Robert Brown first described and named the nucleus in 1831 while studying orchid cells, establishing it as a consistent feature of plant cells.

Robert Hooke in 1665 observed dead cork cells under a microscope and named the empty spaces bounded by walls 'cells', because they resembled small rooms (cellula in Latin).

Cell theory — that all living things are composed of cells and all cells arise from pre-existing cells — is the most fundamental unifying principle in biology.

The cell is the smallest unit of life capable of carrying out all basic life functions including metabolism, growth, response to stimuli, and reproduction.

Electron microscopes have a resolution of 2-4 Angstroms, far greater than light microscopes (~2000 Angstroms), allowing visualisation of organelles and macromolecules.

Cell fractionation uses an ultracentrifuge to spin disrupted cells at different speeds, separating organelles by size and density for biochemical study.

Pinocytosis ('cell drinking') is the process by which cells engulf liquid material and dissolved substances by forming small vesicles from the plasma membrane.

Materials can cross a selectively permeable membrane by passive transport (diffusion, osmosis — no energy needed) or active transport (requires ATP energy against concentration gradient).

In plant cells, the rigid cell wall made of cellulose forms the outermost boundary, providing structural support. The cell membrane lies just inside it.

Bacterial cell walls are made of peptidoglycan (murein), a mesh-like polymer of sugars and amino acids, unlike plant cell walls which are made of cellulose.

Cytosol is the aqueous ground substance of the cytoplasm — a gel-like fluid containing dissolved proteins, ions, and metabolites in which organelles are suspended.

Mitochondria move through the cytoplasm via cytoplasmic streaming — the flow of cytosol driven by motor proteins along cytoskeletal filaments.

Rough endoplasmic reticulum (RER) has ribosomes attached to its outer surface, giving it a rough appearance. It is involved in synthesis and processing of proteins.

Cisternae are the flattened membrane-bound sacs of the endoplasmic reticulum that form internal compartments, separating ER contents from the cytosol.

George Palade first described ribosomes using electron microscopy in 1955, calling them 'small particulate component of the cytoplasm'. He won the Nobel Prize for this work.

A polysome (polyribosome) is a cluster of multiple ribosomes translating the same mRNA molecule simultaneously, increasing the efficiency of protein synthesis.

The Golgi complex consists of stacked, flattened membrane-bound sacs called cisternae. It functions to modify, sort, and package proteins for secretion or delivery to other organelles.

Lysosomes perform autophagy — the digestion of worn-out organelles and cellular debris within the cell using their hydrolytic enzymes.

Peroxisomes produce hydrogen peroxide during oxidation reactions and also contain catalase to decompose it, protecting the cell from this toxic byproduct.

Glyoxysomes are plant-specific organelles that contain enzymes of the glyoxylate cycle, converting fatty acids to succinate (a gluconeogenesis precursor) in germinating seeds.

Intermediate filaments provide mechanical strength and help determine and maintain cell shape. They are more stable than microtubules and microfilaments.

Microfilaments are thin filaments made of actin protein. They are involved in cell movement, muscle contraction, and cytokinesis during cell division.

Mitochondria are pleomorphic (variable in shape) and can appear as rods, vesicles, or filaments under a light microscope depending on the cell type and metabolic state.

Plastids are membrane-bound organelles unique to plant cells and algae. They include chloroplasts (green), chromoplasts (coloured), and leucoplasts (colourless).

Adjacent grana in chloroplasts are connected by intergranal lamellae (stroma lamellae), which are non-stacked thylakoid membranes running through the stroma.

A granum (plural: grana) is a stack of disc-shaped thylakoid membranes inside a chloroplast, resembling a stack of coins. This is where the light reactions of photosynthesis occur.

Centrosomes are organising centres for microtubules found near the nucleus in animal cells. They contain a pair of centrioles and are absent in most plant cells.

Karyolymph (nuclear sap or nucleoplasm) is the gel-like fluid inside the nucleus in which the chromatin and nucleolus are suspended.

A chromatid is an exact copy (replica) of a chromosome produced during DNA replication. Two identical sister chromatids are joined at the centromere.

Eukaryotes include all organisms with a true nucleus — animals, plants, fungi, and protists. Prokaryotes (bacteria and archaea) lack a membrane-bound nucleus.

Prokaryotic ribosomes are 70S (Svedberg units), made of a small 30S and large 50S subunit. Eukaryotic ribosomes are 80S (40S + 60S subunits).

The cell is universally recognised as the basic structural and functional unit of all living organisms, as established by cell theory.

Endocytosis is the process by which cells take in materials by engulfing them with the plasma membrane, forming intracellular vesicles. It includes phagocytosis and pinocytosis.

The primary cell wall in plants is composed mainly of cellulose microfibrils embedded in a matrix of pectin and hemicellulose. It is flexible, allowing cell growth.

The secondary cell wall is deposited inside the primary wall and contains cellulose, lignin, and sometimes silica, waxes, or inorganic salts depending on the cell type.

The endoplasmic reticulum forms a network throughout the cytoplasm providing mechanical support and also serves as a transport system for materials within the cell.

The nucleolus is the site of ribosomal RNA (rRNA) synthesis and ribosome subunit assembly. It is therefore called the 'factory' of ribosomes.

The Krebs cycle (citric acid cycle) occurs in the mitochondrial matrix. Glycolysis occurs in the cytosol. The Krebs cycle is central to aerobic respiration.

Plastids (including chloroplasts, chromoplasts, and leucoplasts) are present only in plant cells. Animal cells lack plastids entirely.

Higher (vascular) plants lack centrioles. Their spindle fibres form without centrioles during cell division. Centrioles are present in animal cells and lower plants.

Normal somatic (body) cells are diploid (2n), containing two sets of chromosomes — one from each parent. Human body cells have 46 chromosomes (23 pairs).

In mature plant cells, the large central vacuole pushes the cytoplasm and nucleus to the periphery of the cell, against the cell wall.

The frog (Rana pipiens) has 26 chromosomes (13 pairs) in its somatic cells. This is different from humans who have 46.

Ribosomal RNA (rRNA) is synthesised in the nucleolus. The nucleolus also assembles ribosomal subunits before they are exported to the cytoplasm.