Eukaryotic Cell Organelles: Nucleus, Mitochondria, Chloroplasts
Core: The Cell’s Control Center
The core, or nucleus, is the main organelle in eukaryotic cells (both animal and plant). It houses the cell’s genome, where DNA replication and RNA synthesis occur. Most cells have a single nucleus, although some, like mammalian erythrocytes, lack nuclei, while others, like hepatocytes, can have two, and osteoclasts can be multinucleated.
Interphase Nucleus
In animal cells, the resting nucleus usually occupies a central position, while in plant cells, it’s often eccentric due to large vacuoles. The interphase nucleus comprises:
- Nuclear Membrane: Encloses the nucleus, separating it from the cytoplasm. It consists of:
- Outer Nuclear Membrane: Has ribosomes on its cytoplasmic face and is continuous with the endoplasmic reticulum (ER).
- Inner Nuclear Membrane: Lined with a protein fiber network for structural support.
- Nucleoplasm: An aqueous medium containing molecules like enzymes and proteins involved in nuclear activities.
- Chromatin: The genetic material of the interphase nucleus, appearing as scattered lumps and fibers. Each chromatin fiber is a DNA molecule associated with proteins, mainly histones.
- Heterochromatin: Densely packed and transcriptionally inactive.
- Euchromatin: Extended and transcriptionally active.
- Nucleolus: A non-membrane-bound spherical structure within the nucleus, visible as granules and fibers. It’s involved in ribosomal subunit synthesis.
Nucleolus Location and Function
The nucleolus is located near specific chromosome regions called nucleolus organizer regions (NORs), where ribosomal RNA (rRNA) is transcribed. It synthesizes rRNA types (except a small portion of the large subunit) and forms ribosome subunits by binding with proteins from the cytoplasm.
Nucleoli in Cell Division
During mitosis, the nucleolus disorganizes, and chromatin condenses into chromosomes. The nuclear envelope disintegrates, releasing the nuclear contents into the cytoplasm. After division, chromosomes decondense, and the nuclear envelope reforms.
Chromosome Structure
- Chromatids: Two longitudinal halves of a chromosome, each containing a DNA molecule.
- Centromere: The constricted region joining sister chromatids, where spindle fibers attach.
- Arms: Portions of the chromosome divided by the centromere.
- Kinetochores: Protein discs on either side of the centromere, attaching to spindle microtubules.
- Satellite: A terminal portion in some chromosomes, separated by a secondary constriction.
- Telomeres: Chromosome ends, preventing structural changes.
- Bands: Segments of chromosomes with distinct staining patterns.
Energy-Generating Organelles
Mitochondria and chloroplasts are membrane-bound organelles responsible for energy metabolism in eukaryotic cells.
Mitochondria
Mitochondria produce energy for cellular activities through chemical reactions.
Features
- Form: Variable, from spherical to elongated cylindrical.
- Number: Varies based on cell energy needs.
Ultrastructure
Mitochondria have a double membrane (outer and inner), an intermembrane space, and a matrix.
Functions
The main function is energy production through cellular respiration.
Composition
- Matrix: Contains mitochondrial DNA, ribosomes, ions, water, and enzymes for the Krebs cycle.
- Inner Membrane: Contains proteins for the electron transport chain and ATP synthesis.
- Outer Membrane: Permeable due to large aqueous channels.
- Intermembrane Space: Similar to cytosol, containing various enzymes.
Plastids: Chloroplasts
Chloroplasts are green plastids found in photosynthetic plant cells.
Features
- Form: Ovoid or lenticular in higher plants.
- Number: Varies.
Structure
Chloroplasts have a double membrane, stroma, and thylakoid membranes forming grana.
Role
Responsible for photosynthesis, converting light energy into glucose.
Chemical Composition
- Stroma: Contains chloroplast DNA, ribosomes, starch grains, lipid inclusions, and enzymes for the Calvin cycle.
- Membranes: Contain lipids, proteins, and pigments for energy transport and generation.
Genesis of Mitochondria and Chloroplasts
The endosymbiotic theory suggests that mitochondria and chloroplasts originated from endocytosed bacteria and cyanobacteria, respectively, forming a symbiotic relationship with eukaryotic cells.