Eukaryotic Cells
4. Mitochondria
- Mitochondria are clearly visible only when stained.
- Number, shape, and size vary depending on the cell’s physiological activity.
- Sausage-shaped or cylindrical, with a diameter of 0.2–1.0 µm (average 0.5 µm) and length 1.0–4.1 µm.
- A mitochondrion is a double membrane-bound structure with an outer membrane and an inner membrane, dividing the lumen into two compartments: the outer compartment and the inner compartment (matrix).
- The inner membrane forms infoldings (cristae) towards the matrix, increasing surface area.
- Both membranes have specific enzymes associated with mitochondrial function.
- The matrix contains circular DNA, a few RNA molecules, 70S ribosomes, and components for protein synthesis.
- Mitochondria divide by fission.
- Function: Sites of aerobic respiration, producing energy in the form of ATP. Hence, called the ‘power houses’ of the cell.
5. Plastids
- Found in all plant cells and euglenoides.
- Large-sized and easily observable under a microscope.
- Contain specific pigments.
- Based on pigment type, plastids are three types:
- Chloroplasts: Contain chlorophyll and carotenoid pigments, trapping light energy for photosynthesis.
- Chromoplasts: Contain fat-soluble carotenoid pigments (e.g., carotene, xanthophylls), giving yellow, orange, or red color.
- Leucoplasts: Colorless plastids of varied shapes and sizes with stored nutrients, including:
- Amyloplasts: Store starch (e.g., potato).
- Elaioplasts: Store oils and fats.
- Aleuroplasts: Store proteins.
Chloroplasts:
- Double membrane-bound organelles, mainly in mesophyll cells of leaves.
- Lens-shaped, oval, spherical, discoid, or ribbon-like, with length 5–10 µm and width 2–4 µm.
- Number varies from 1 (e.g., Chlamydomonas) to 20–40 per cell in mesophyll.
- Inner membrane is less permeable.
- The space within the inner membrane is called stroma, containing flattened membranous sacs called thylakoids.
- Thylakoid membranes enclose a lumen.
- Chlorophyll pigments are present in thylakoids.
- Thylakoids are stacked into grana or connected by intergranal thylakoids.
- Flat membranous tubules, stroma lamellae, connect thylakoids of different grana.
- Stroma contains double-stranded circular DNA, ribosomes, and enzymes for synthesizing carbohydrates and proteins.
- Chloroplast ribosomes are 70S, smaller than cytoplasmic 80S ribosomes.
6. Ribosomes
- Non-membranous granular structures composed of RNA and proteins.
- First observed by George Palade (1953).
- Eukaryotic ribosomes have two subunits: 60S (large) and 40S (small), forming 80S.
7. Cytoskeleton
- A network of filamentous proteinaceous structures in the cytoplasm.
- Provides mechanical support, motility, and maintains cell shape.
8. Cilia and Flagella
- Hair-like outgrowths of the cell membrane.
- Cilia: Small structures that work like oars, causing movement of the cell or surrounding fluid.
- Flagella: Longer, responsible for cell movement. Prokaryotic and eukaryotic flagella are structurally different.
- Cilia and flagella are covered with plasma membrane. Their core (axoneme) has microtubules in a 9+2 array: nine pairs of radially arranged peripheral microtubule doublets and a central pair.
- The central tubules are connected by bridges and enclosed by a central sheath, linked to one tubule of each peripheral doublet by a radial spoke. Peripheral doublets are interconnected by linkers.
- Emerge from basal bodies, centriole-like structures.
9. Centrosome and Centrioles
- Centrosome contains two non-membrane-bound cylindrical structures called centrioles.
- Surrounded by pericentriolar materials.
- Centrioles lie perpendicular to each other, made of nine evenly spaced peripheral fibrils of tubulin, each a triplet. Adjacent triplets are linked.
- The central part, called the hub, is proteinaceous and connected to peripheral triplets by radial spokes.
- Centrioles form the basal body of cilia or flagella and spindle fibers for the spindle apparatus during cell division in animal cells.
10. Nucleus
- First described by Robert Brown (1831).
- Stainable material named chromatin by Flemming.
- Typically, one nucleus per cell, but some cells have multiple (e.g., certain fungi), and some lack a nucleus (e.g., mammalian RBCs, sieve tube cells).
- The interphase nucleus contains:
- Nuclear envelope: Double-layered membrane with a perinuclear space (10–50 nm). Acts as a barrier between nucleus and cytoplasm. The outer membrane is continuous with ER and bears ribosomes. Has nuclear pores for RNA and protein movement.
- Nuclear matrix (nucleoplasm): Fluid within the nucleus.
- Chromatin: Nucleoprotein fibers containing DNA, histones, non-histone proteins, and RNA. Condenses into chromosomes during cell division.
- Nucleolus: Non-membranous spherical body, continuous with nucleoplasm, site of ribosomal RNA synthesis.
Chromosomes
- A human cell has 2 m of DNA distributed among 46 chromosomes (23 pairs).
- Each chromosome has a centromere (primary constriction) with disc-shaped kinetochores on its sides.
- Based on centromere position, chromosomes are four types:
- Metacentric: Middle centromere, forming two equal arms.
- Sub-metacentric: Centromere nearer one end, forming one shorter and one longer arm.
- Acrocentric: Centromere close to one end, forming one very short and one very long arm.
- Telocentric: Terminal centromere.
- Some chromosomes have a non-staining secondary constriction at a constant location, called a satellite.
11. Microbodies
- Membrane-bound minute vesicles containing various enzymes.
- Present in both plant and animal cells.
Differences Between Plant and Animal Cells
| Plant Cell | Animal Cell |
|---|---|
| Cell wall present | Absent |
| Plastids are present | Absent |
| A large central vacuole | Many small vacuoles |
| Centrioles are absent | Present |
Comparison Between Prokaryotic and Eukaryotic Cells
| Prokaryotic cells | Eukaryotic cells |
|---|---|
| Generally smaller | Larger |
| Genetic material is in the form of nucleoid | Genetic material is in the form of nucleus |
| Nuclear membrane absent | Present |
| Membrane bound organelles absent | Present |
| Circular DNA | Linear DNA |
| Ribosomes 70 S type | 80 S type (70 S in plastids and mitochondria) |