Detailed Notes 3- Chapter 8 Cell: The Unit of Life

8.5.4 Mitochondria

1. Structure:

   • Mitochondria are double membrane-bound organelles, typically shaped like sausages or cylinders.
   • They are not easily visible under the microscope unless specifically stained.
   • Each mitochondrion consists of an outer membrane and an inner membrane, which divides its lumen into two aqueous compartments: the outer compartment and the inner compartment.
   • The inner compartment contains a dense homogeneous substance called the matrix.
   • The inner membrane forms infoldings called cristae, which increase the surface area for chemical reactions.
   • The two membranes have specific enzymes associated with mitochondrial function.

2. Size and Variability:

   • Mitochondria exhibit considerable variability in shape and size.
   • They typically have a diameter ranging from 0.2 to 1.0 µm and a length ranging from 1.0 to 4.1 µm.

3. Function:

   • Mitochondria are the sites of aerobic respiration, the process by which cells generate energy in the form of ATP (adenosine triphosphate).
   • They are often referred to as the “powerhouses” of the cell due to their role in ATP production.
   • The matrix of mitochondria contains DNA, RNA, ribosomes (70S), and components required for protein synthesis.
   • Mitochondria produce ATP through a series of metabolic pathways, including the citric acid cycle and oxidative phosphorylation.

4. Reproduction:

   • Mitochondria reproduce by a process called fission, where they divide to form new mitochondria.

8.5.5 Plastids

1. Presence and Observation:

   • Plastids are found in all plant cells and in some protists like euglenoids.
   • They are easily observed under the microscope due to their large size.

2. Types of Plastids:

   • Plastids can be classified into three main types based on the type of pigments they contain:
     • Chloroplasts: Contain chlorophyll and carotenoid pigments responsible for photosynthesis.
     • Chromoplasts: Contain fat-soluble carotenoid pigments like carotene and xanthophylls, giving plants yellow, orange, or red colors.
     • Leucoplasts: Colorless plastids that store various nutrients, such as amyloplasts (starch), elaioplasts (oils and fats), and aleuroplasts (proteins).

3. Structure of Chloroplasts:

   • Chloroplasts are lens-shaped, oval, spherical, discoid, or ribbon-like organelles with variable sizes (5-10µm in length and 2-4µm in width).
   • They are double membrane-bound organelles, with the inner membrane being relatively less permeable.
   • The space enclosed by the inner membrane is called the stroma.
   • Within the stroma, there are organized flattened membranous sacs called thylakoids, arranged in stacks called grana. The thylakoids contain chlorophyll pigments and are the site of light-dependent reactions of photosynthesis.
   • Stroma lamellae connect thylakoids of different grana.
   • The stroma contains enzymes required for carbohydrate and protein synthesis, as well as small, double-stranded circular DNA molecules and ribosomes.

4. Function of Chloroplasts:

   • Chloroplasts are the sites of photosynthesis, where light energy is captured and converted into chemical energy in the form of carbohydrates (such as glucose).
   • Thylakoids contain chlorophyll pigments that absorb light energy for photosynthesis.
   • The stroma contains enzymes necessary for the synthesis of carbohydrates and proteins.

8.5.6 Ribosomes

1. Observation and Composition:

   • Ribosomes were first observed under the electron microscope as dense particles by George Palade in 1953.
   • They are composed of ribonucleic acid (RNA) and proteins.
   • Ribosomes are not surrounded by any membrane and are found in the cytoplasm of cells.

2. Types of Ribosomes:

   • Eukaryotic ribosomes are larger and are denoted as 80S, while prokaryotic ribosomes are smaller and denoted as 70S.
   • Each ribosome consists of two subunits: a larger subunit and a smaller subunit.
   • The two subunits of eukaryotic ribosomes (80S) are 60S and 40S, while those of prokaryotic ribosomes (70S) are 50S and 30S.
   • The ‘S’ (Svedberg’s Unit) stands for the sedimentation coefficient, which indirectly measures density and size.

3. Function:

   • Ribosomes are the cellular machinery responsible for protein synthesis or translation.
   • They read the genetic information encoded in messenger RNA (mRNA) molecules and use it to assemble amino acids into polypeptide chains according to the sequence specified by the mRNA.
   • Ribosomes consist of a large and a small subunit that come together around mRNA during translation.
   • The ribosome moves along the mRNA molecule, synthesizing a polypeptide chain by linking together amino acids in the correct order dictated by the mRNA sequence.
   • Ribosomes can be found freely floating in the cytoplasm (free ribosomes) or attached to the endoplasmic reticulum (bound ribosomes), depending on the destination of the synthesized proteins.

8.5.7 Cytoskeleton

1. Composition:

   • The cytoskeleton is composed of three main types of filamentous protein structures:
     • Microtubules
     • Microfilaments (also known as actin filaments)
     • Intermediate filaments

2. Functions:

   • Mechanical Support: The cytoskeleton provides structural support to the cell, helping it maintain its shape and resist deformation.
   • Motility: Components of the cytoskeleton are involved in cell movement, including the movement of the cell itself (e.g., cell crawling) and the movement of organelles within the cell.
   • Maintenance of Cell Shape: The cytoskeleton helps determine and maintain the overall shape of the cell.
   • Intracellular Transport: Microtubules serve as tracks for the movement of vesicles, organelles, and other cellular components within the cell.
   • Cell Division: The cytoskeleton plays a crucial role in cell division by facilitating the assembly and function of the mitotic spindle, which helps separate chromosomes during cell division.
   • Cell Signaling: Certain components of the cytoskeleton are involved in cell signaling processes, allowing cells to respond to external stimuli and communicate with neighboring cells.

3. Dynamic Nature:

   • The cytoskeleton is a dynamic structure, meaning its components can rapidly assemble, disassemble, and reorganize in response to cellular needs and environmental cues.
   • This dynamic nature allows cells to quickly adapt to changing conditions and perform various cellular functions efficiently.

8.5.8 Cilia and Flagella

1. Definition and Function:

   • Cilia and flagella are hair-like outgrowths of the cell membrane.
   • Cilia are small structures that work like oars, causing movement either of the cell itself or of the surrounding fluid.
   • Flagella are longer and are primarily responsible for cell movement.

2. Structural Composition:

   • Both cilia and flagella have a similar structural organization.
   • They are covered with a plasma membrane.
   • The core of cilia and flagella is called the axoneme, which consists of microtubules arranged in a specific pattern.
   • The axoneme typically contains nine doublets of radially arranged peripheral microtubules and a pair of centrally located microtubules, forming a 9+2 array.
   • The central pair of microtubules is connected by bridges and enclosed by a central sheath. Nine radial spokes extend from the central sheath to the peripheral doublets.
   • The peripheral doublets are interconnected by linkers.
   • Both cilia and flagella emerge from centriole-like structures called basal bodies, which serve as the organizing centers for their assembly.

3. Comparison with Prokaryotic Flagella:

   • Prokaryotic bacteria also possess flagella, but their structure is different from that of eukaryotic flagella.
   • Eukaryotic flagella have a complex internal structure consisting of microtubules, whereas prokaryotic flagella are simpler in structure and are composed of protein filaments.

8.5.9 Centrosome and Centrioles

1. Centrosome:

   • The centrosome is an organelle typically containing two cylindrical structures known as centrioles.
   • It is surrounded by an amorphous region called pericentriolar material, which consists of proteins involved in microtubule organization and nucleation.
   • The centrosome plays a crucial role in organizing the microtubule network within the cell and is essential for various cellular processes, including cell division and cell motility.

2. Centrioles:

   • Centrioles are cylindrical structures found within the centrosome, typically in pairs.
   • Each centriole consists of nine evenly spaced peripheral fibrils made of tubulin protein. These fibrils form triplets, with adjacent triplets linked together.
   • The arrangement of these triplets resembles a cartwheel, with a central hub surrounded by radial spokes connecting to the peripheral fibrils.
   • The central part of the proximal region of the centriole, known as the hub, is also proteinaceous and plays a role in organizing the structure of the centriole.
   • Centrioles serve as the basal bodies for the formation of cilia or flagella in animal cells.
   • During cell division, centrioles play a crucial role in organizing spindle fibers, which are necessary for the formation of the mitotic spindle apparatus and proper chromosome segregation.

8.5.10 Nucleus

1. Discovery and Structure:

   • Robert Brown first described the nucleus in 1831, and Flemming named the stained material within it chromatin.
   • The nucleus is surrounded by a nuclear envelope consisting of two parallel membranes with a space between them, called the perinuclear space. It contains nuclear pores for the exchange of molecules between the nucleus and the cytoplasm.
   • Inside the nucleus, there are highly extended chromatin fibers, a nuclear matrix, and one or more nucleoli.

2. Chromatin and Chromosomes:

   • Chromatin is a combination of DNA, histone and non-histone proteins, and RNA. During cell division, chromatin condenses into structured chromosomes.
   • Chromosomes consist of DNA strands wrapped around histone proteins. A human cell typically contains 46 chromosomes (23 pairs).
   • Each chromosome has a primary constriction called the centromere, where two chromatids are held together. Kinetochores are disc-shaped structures present on either side of the centromere.

3. Variations and Functions:

   • Some organisms, like multinucleate fungi and some protozoans, have more than one nucleus per cell.
   • Some mature cells, such as mammalian erythrocytes and sieve tube cells of vascular plants, lack a nucleus.
   • Nucleoli are sites for active ribosomal RNA synthesis, and cells with high protein synthesis activity typically have larger and more numerous nucleoli.

8.5.11 Microbodies

Microbodies are membrane-bound organelles found in both plant and animal cells.

These organelles contain various enzymes and are involved in several metabolic processes within the cell.