Cell Organelles and Chromosomes
Nucleus
The Nucleus was first reported by Robert Brown in 1831. It controls the life and activities of the cell and is the repository of hereditary information. In animal cells, it generally occupies a central position, while in plant cells it is pushed towards the periphery by the large central Vacuole. A cell with one nucleus is called Mononucleate; cells with two nuclei are Binucleate, and those with more than two are Multinucleate. The nucleus is only visible when the cell is in the non-dividing stage, when it contains the Chromatin network and a soluble fluid called Nucleoplasm.
Central Control: The nucleus directs all cellular activities and houses the hereditary material
Position: Central in animal cells; peripheral in plant cells due to the large vacuole
Nucleate States: Mononucleate (one nucleus), binucleate (two nuclei), multinucleate (more than two)
The nucleus is enclosed by a Nuclear Membrane, which is actually a double-membrane structure called the nuclear envelope. The outer membrane is continuous with the Endoplasmic Reticulum at certain points. The outer and inner membranes are fused at intervals, creating openings called Nuclear Pores. These pores allow the exchange of materials between the nucleus and the cytoplasm. Undifferentiated cells (such as egg cells) possess about 30,000 nuclear pores per nucleus, while differentiated cells like Erythrocytes have only 3–4 pores per nucleus. Each pore has a definite structure that regulates the traffic of substances passing through it.
Double Membrane: The nuclear envelope consists of an outer membrane (continuous with ER) and an inner membrane enclosing nuclear contents
Nuclear Pores: Openings where the two membranes fuse, allowing controlled exchange of materials between nucleus and cytoplasm
Pore Number Variation: Undifferentiated cells (e.g., eggs) have ~30,000 pores; differentiated cells (e.g., red blood cells) have only 3–4
ER Continuity: The outer nuclear membrane connects directly with the endoplasmic reticulum
Functions of Nuclear Pores
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Allow mRNA to exit the nucleus to the cytoplasm for protein synthesis
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Permit ribosomal subunits assembled in the nucleolus to reach the cytoplasm
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Allow proteins (e.g., histones, transcription factors) to enter the nucleus
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Enable exchange of small molecules and ions between nucleoplasm and cytoplasm
The Nucleolus is a darkly stained body within the nucleus that lacks its own membrane boundary. It is the site of Ribosomal RNA (rRNA) synthesis and storage. The nucleolus consists of two regions: a peripheral granular area composed of precursors of ribosomal subunits, and a central fibrillar area containing large molecular weight RNA and rDNA. Ribosomes are assembled in the nucleolus and then exported to the cytoplasm through nuclear pores, making the nucleolus the 'factory of ribosomes'.
No Membrane: The nucleolus is a dense, non-membrane-bound structure inside the nucleus
rRNA Synthesis: Ribosomal RNA is transcribed and stored in the nucleolus
Peripheral Granular Area: Contains precursors of ribosomal subunits
Central Fibrillar Area: Contains high molecular weight RNA and ribosomal DNA (rDNA)
Ribosome Assembly: Complete ribosomes are assembled here and exported to the cytoplasm via nuclear pores
The Nucleoplasm is the soluble, semi-fluid material enclosed within the nuclear envelope. Along with the cytoplasm, it forms the Protoplasm. The nucleoplasm contains the chromatin network, the nucleolus, and various dissolved molecules including nucleotides, enzymes, and proteins necessary for DNA and RNA synthesis.
Composition: Semi-fluid matrix containing nucleotides, enzymes, proteins, and other molecules needed for nuclear functions
Protoplasm Component: Together with cytoplasm, the nucleoplasm constitutes the protoplasm
Chromatin Network: The diffuse, thread-like genetic material dispersed within the nucleoplasm
Endoplasmic Reticulum
The Endoplasmic Reticulum (ER) is a network of membrane-bound channels extending throughout the cytoplasm. Under the electron microscope, these channels appear continuous with the Plasma Membrane and also in contact with the Nuclear Membrane. The material inside the channels is separated from the cytoplasm by spherical or tubular membranes called Cisternae. ER exists in two morphological forms that differ in structure and function.
Network of Channels: Membrane-enclosed tubules and sacs that extend throughout the cytoplasm
Cisternae: Spherical or tubular membranes forming the internal spaces of ER, separating ER contents from the cytoplasm
Continuity with Other Membranes: ER channels are continuous with the plasma membrane and the nuclear membrane
The Rough Endoplasmic Reticulum (RER) is distinguished by the presence of Ribosomes attached to its outer surface. RER is primarily involved in the synthesis of proteins. After synthesis, these proteins are either stored in the cytoplasm or exported out of the cell through the ER channels, passed on to the Golgi Apparatus for modification, and then shipped to their final destination via vesicles that bud off from the Golgi.
Ribosome Attachment: The rough appearance is due to ribosomes studded on the cytoplasmic face of the ER membrane
Protein Synthesis: RER is the site of synthesis of proteins destined for secretion, membrane insertion, or lysosomal incorporation
Transport Role: Newly synthesised proteins enter the ER lumen and are transported to the Golgi apparatus for processing
The Smooth Endoplasmic Reticulum (SER) lacks ribosomes on its surface, giving it a smooth appearance. SER is involved in the metabolism of various types of molecules, particularly lipids. It helps detoxify harmful drugs and poisons. In muscle cells and nerve cells, SER plays a role in the transmission of impulses. SER is also important for transporting materials from one part of the cell to another.
No Ribosomes: The absence of ribosomes gives SER its smooth appearance under the electron microscope
Lipid Metabolism: SER synthesises lipids and lipoproteins
Detoxification: SER contains enzymes that break down harmful drugs and poisons — it is especially abundant in liver cells
Calcium Storage: In muscle cells, SER (called the sarcoplasmic reticulum) stores and releases calcium ions for muscle contraction
Impulse Transmission: SER assists in transmitting impulses in nerve cells
RER vs SER Comparison
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RER — has ribosomes on surface; SER — no ribosomes
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RER — synthesises proteins; SER — synthesises lipids and metabolises drugs
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RER — transports proteins to Golgi; SER — transports materials within the cell
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RER — abundant in secretory cells (pancreas, plasma cells); SER — abundant in liver cells and steroid-producing cells
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Both — provide mechanical support and maintain cell shape
Golgi Apparatus
The Golgi Apparatus was discovered by Camillo Golgi in 1898. It is found in virtually all eukaryotic cells. The apparatus consists of stacks of flattened, membrane-bound sacs called Cisternae, along with associated vesicles — together these form the Golgi Complex. It is a complex system of interconnected tubules surrounding the central stacks. Vesicles derived from the budding of SER fuse to form new cisternae at the outer convex surface (the Forming Face). The inner concave surface is the Maturing Face, where cisternae break up into vesicles that carry products to their destinations.
Discovery: Camillo Golgi, 1898
Cisternae Stacks: Flattened, membrane-bound sacs arranged in parallel stacks
Forming Face (Cis Face): Outer convex surface where vesicles from SER fuse to form new cisternae
Maturing Face (Trans Face): Inner concave surface where cisternae break up into secretory vesicles
Directional Flow: Materials move from the forming face through the cisternae to the maturing face
The Golgi Apparatus is primarily concerned with cell secretions. Proteins synthesised on ribosomes are passed through the Endoplasmic Reticulum to the Golgi, where they are modified by the addition of carbohydrates and converted into Glycoproteins or Glycolipids. The finished products are packed inside membrane-bound vesicles before export. For example, in mammals, the pancreas secretes enzyme-containing granules that aid digestion — the Golgi apparatus plays a key role in forming these granules. The most important function of the Golgi is the modification, sorting, and packaging of proteins and lipids.
Protein Modification: Adds carbohydrate groups to proteins, converting them into glycoproteins
Lipid Modification: Converts lipids into glycolipids
Sorting and Packaging: Categorises modified molecules and packages them into vesicles for transport to their correct destinations
Secretion: Forms secretory granules (e.g., digestive enzymes in pancreatic cells, hormones in endocrine cells)
Products Processed by the Golgi Apparatus
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Secretory proteins — enzymes, hormones, extracellular matrix components
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Lysosomes — vesicles containing hydrolytic enzymes budded off from the Golgi
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Glycoproteins — proteins with added carbohydrate groups
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Glycolipids — lipids with added carbohydrate groups
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Cell wall components — in plant cells, the Golgi helps assemble cellulose and pectin for the cell wall
The Endoplasmic Reticulum and the Golgi Apparatus work together as an integrated secretory pathway. Proteins synthesised on ribosomes attached to the RER enter the ER lumen. Transport vesicles bud off from the ER and carry these proteins to the forming face of the Golgi. After modification within the Golgi cisternae, secretory vesicles pinch off from the maturing face and travel to the Plasma Membrane for export, or to other destinations within the cell. This coordinated pathway ensures that cellular products are properly processed before reaching their final destination.
RER to Golgi Transport: Vesicles budding from ER carry proteins to the forming face of the Golgi
Golgi Processing: Proteins are modified, sorted, and packaged as they pass through the Golgi cisternae
Golgi to Destination: Vesicles from the maturing face deliver products to the plasma membrane (for secretion), to lysosomes, or to other organelles
Mitochondria
Mitochondria are among the most important organelles in eukaryotic cells, responsible for manufacturing and supplying energy. Often called the 'powerhouse of the cell', they appear as vesicles, rods, or filaments under a compound microscope, and show complex morphology under the electron microscope. Each mitochondrion is bound by two membranes: a smooth outer membrane and an inner membrane that forms infoldings called Cristae into the inner chamber, which contains the Mitochondrial Matrix. The mitochondrial membranes share structural similarity with other cell membranes.
Double Membrane: Outer membrane is smooth; inner membrane is folded into cristae projecting into the matrix
Cristae: Infoldings of the inner membrane that increase the surface area for the chemical reactions of respiration
Mitochondrial Matrix: The fluid-filled inner compartment containing enzymes, coenzymes, organic salts, inorganic salts, circular DNA, and ribosomes
Variable Morphology: Number, shape, and size of mitochondria vary depending on the physiological activity of the cell
The inner mitochondrial membrane is the site of the electron transport chain and oxidative phosphorylation. Its surface is covered with small knob-like structures known as $F_1$ Particles (also called oxysomes or ATP synthase). These particles are embedded in the cristae membranes and are responsible for synthesising ATP from ADP and inorganic phosphate during aerobic respiration. The cristae greatly increase the surface area of the inner membrane, maximising the capacity for ATP production.
F₁ Particles: Small knob-like projections on the inner surface of cristae; these are ATP synthase enzymes that catalyse ATP formation
Electron Transport Chain: The series of protein complexes embedded in the inner membrane that transfer electrons and pump protons
Surface Area: Cristae folds maximise the inner membrane surface area for the reactions of aerobic respiration
Oxidative Phosphorylation: The process by which ATP is generated using the proton gradient established by the electron transport chain
Mitochondria extract energy from organic food molecules and convert it into ATP (adenosine triphosphate), the universal energy currency of the cell. The mitochondrial matrix contains a large number of enzymes and coenzymes that drive vital metabolic processes including Kreb's Cycle, aerobic respiration, and fatty acid metabolism. Spent energy in the form of ADP is regenerated into ATP by the mitochondria. The number of mitochondria in a cell correlates with its energy demands — highly active cells like muscle cells and liver cells contain many mitochondria.
Energy Extraction: Organic food molecules are broken down through Kreb's cycle and the electron transport chain to release energy
ATP Production: The released energy is captured and stored as ATP, which powers all cellular activities
Self-Replicating: Mitochondria contain their own circular DNA and ribosomes, allowing them to replicate independently
Energy Demand Correlation: Cells with high metabolic activity (muscle, liver, sperm cells) have more mitochondria than less active cells
Mitochondria contain their own circular DNA and ribosomes, which is evidence that they were once free-living prokaryotic organisms that entered into a symbiotic relationship with eukaryotic cells (endosymbiotic theory). The presence of DNA and ribosomes means mitochondria can synthesise some of their own proteins and replicate independently of the cell nucleus. This semi-autonomous nature is a key feature distinguishing mitochondria from other organelles.
Circular DNA: Mitochondria possess their own small, circular DNA molecule, distinct from nuclear DNA
Own Ribosomes: Mitochondrial ribosomes (55S) are different from cytoplasmic ribosomes (80S) and resemble prokaryotic ribosomes
Semi-Autonomous: Can replicate independently, but still depend on nuclear genes for most proteins
Chromosomes
The nucleus is deeply stained by basic dyes because of the Chromatin material within it. In non-dividing cells, chromatin appears as a diffuse, thread-like network dispersed in the Nucleoplasm. During cell division, this chromatin condenses and coils into distinct, darkly stained, thread-like structures called Chromosomes. This condensation makes the genetic material easier to distribute evenly between daughter cells.
Chromatin (Interphase): Diffuse, loosely coiled form of DNA and proteins visible in non-dividing cells
Chromosomes (Dividing): Highly condensed, tightly coiled structures formed from chromatin during cell division
Staining: Both chromatin and chromosomes stain deeply with basic dyes because DNA is acidic and binds basic stains
Each Chromosome consists of two identical copies called Chromatids, held together at a central constriction called the Centromere. A chromatid is an exact replica of the chromosome. The centromere is the site where Spindle Fibres attach during cell division to pull chromatids apart. Each chromosome is composed of DNA and proteins. All the information necessary to control cell activities is located on chromosomes in the form of Genes, which are transmitted from one generation to the next.
Chromatids: Two identical copies of a chromosome, joined at the centromere; they separate during cell division
Centromere: The constriction point where spindle fibres attach; holds sister chromatids together
DNA and Protein Composition: Chromosomes are made of DNA wrapped around histone proteins
Genes: Segments of DNA on chromosomes that carry the instructions for specific traits
Chromosome Structure Summary
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Chromatid — one of the two identical copies of a replicated chromosome
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Centromere — the constriction point joining sister chromatids; attachment site for spindle fibres
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Chromatin — loosely organised DNA + protein in non-dividing cells
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Gene — a segment of DNA on a chromosome that encodes a specific trait or function
The number of chromosomes is constant within a species across all individuals and generations. Normal body (somatic) cells contain a Diploid number of chromosomes ($2n$), while germ cells (Gametes) contain a Haploid number ($n$). In diploid cells, chromosomes exist in homologous pairs — one inherited from each parent.
$$2n \; \text{(diploid)} \longrightarrow n \; \text{(haploid)}$$
Reduction from diploid to haploid chromosome number during gamete formation (meiosis)
$2n$=diploid chromosome number in somatic cells(chromosomes per cell)
$n$=haploid chromosome number in gametes(chromosomes per cell)
$2n = 46$ (human somatic cells)
→$n = 23$ (human gametes)
Diploid ($2n$): The complete set of paired chromosomes in somatic (body) cells
Haploid ($n$): Half the diploid number; found in gametes (sperm and egg cells)
Species-Specific Constant: Every species has a fixed chromosome number maintained across generations
Chromosome Numbers in Selected Organisms
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Human — 46 (2n), 23 (n)
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Chimpanzee — 48 (2n), 24 (n)
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Frog — 26 (2n), 13 (n)
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Fruit fly (Drosophila melanogaster) — 8 (2n), 4 (n)
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Onion — 16 (2n), 8 (n)
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Potato — 48 (2n), 24 (n)
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Garden pea — 14 (2n), 7 (n)