Structure and Function of Cell Organelles | Botany Optional for UPSC PDF Download

Cell Organelles

Cell organelles are specialized structures found within both eukaryotic and prokaryotic cells, each with distinct functions crucial for the cell's survival and proper operation. These organelles are enclosed by membranes and play pivotal roles in various cellular processes, including energy production, protein synthesis, and waste management. This article delves into the fascinating world of cell organelles, exploring their diverse structures, functions, and compositions.

What are Cell Organelles?

Cell organelles are cellular structures responsible for various functions within a cell. These organelles can be categorized into two main types: membrane-bound organelles and non-membrane-bound organelles, each with distinct structures and functions. Collectively, these organelles work in coordination to ensure the proper functioning of the cell, including maintaining its shape, enabling cell motility, and supporting reproduction.
Organelles can further be classified into three types based on their association with membranes:

• Non-membrane organelles: These include the Cell Wall, Ribosomes, and Cytoskeleton. They are found in both prokaryotic and eukaryotic cells.
• Single-membrane organelles: Organelles covered by a single membrane layer are present in eukaryotic cells. Examples of these organelles include Vacuoles, Golgi Apparatus, Lysosomes, and the Endoplasmic Reticulum.
• Double-membrane organelles: These organelles are surrounded by two layers of membranes and are found in eukaryotic cells. Examples include the Nucleus, Mitochondria, and Chloroplasts.

Cell Organelle Diagram

Cell Organelles and Their Functions

Different types of cell organelles are present in the cell those are:

Plasma Membrane

The plasma membrane, also referred to as the cell membrane or cytoplasmic membrane, is a semipermeable barrier that surrounds both plant and animal cells. Composed of a lipid bilayer and proteins, this membrane selectively permits the passage of specific molecules. In animal cells, it plays a crucial role in maintaining cell shape, while in plant cells, this function is carried out by the cell wall.

Cytoplasm

Cytoplasm is a gel-like substance that exists in both plant and animal cells, occupying the space between the nucleus and the plasma membrane. It consists of a mixture of water, organic compounds, and inorganic substances. Cytoplasm serves as the location for numerous metabolic chemical reactions due to the presence of various cellular enzymes. Additionally, all of the cell's organelles are suspended within the cytoplasm.

Nucleus 

• The nucleus is a double-membraned structure that can be found in all eukaryotic cells. It holds the highest molecular weight within the cell and houses all of the cell's genetic material. The nucleus typically has a circular shape and appears dark in color due to its double membrane. These nuclear membranes are selectively permeable, playing a role in cellular transport. There are nuclear pores on the nuclear membrane that facilitate the movement of proteins and transcription factors.
• The presence of the nuclear membrane distinguishes the nucleoplasm from the cytoplasm. Within the nucleus, a small spherical body called the nucleolus can be found, and it contains the chromosomes. Chromosomes are thin, thread-like structures that carry genes. Genes are the hereditary units responsible for transferring genetic information from parent to offspring. The primary function of the nucleus is to oversee the maintenance of the cell and regulate its metabolic activities.

Endoplasmic Reticulum

The Endoplasmic Reticulum (ER) is a membranous structure characterized by its cristae, which are connected to tubules filled with a fluid. It serves as the cellular transport system, facilitating the movement of various proteins and enzymes within the cell or to locations outside the cell.
There are two distinct types of Endoplasmic Reticulum:

• Rough Endoplasmic Reticulum (Rough ER): This type is composed of cisternae, tubules, and vesicles that are distributed throughout the cell. Rough ER is primarily involved in protein synthesis.
• Smooth Endoplasmic Reticulum (Smooth ER): Smooth ER functions primarily as a storage organelle. It is also involved in synthesizing steroids and lipids and plays a role in detoxification processes.

Mitochondria

Mitochondria are semiautonomous organelles, meaning they possess some genes that code for their own proteins, but not all. They rely on the nucleus of the cell for the production of certain proteins. Mitochondria are often referred to as the "powerhouse of the cell." These organelles have a double membrane structure and are found in all eukaryotic cells. Mitochondria can vary in size and shape, typically appearing circular or oval. They serve as the primary site for aerobic respiration, which is the process that generates cellular energy.

Plastids

Plastids are large membrane-bound organelles that contain pigments. They are categorized into three types based on the type of pigments they contain:

• Chloroplasts: Chloroplasts are double-membrane organelles primarily found in the mesophyll cells of plant leaves. They contain the green pigment chlorophyll, which is essential for capturing and storing light energy during the process of photosynthesis.
• Chromoplasts: Chromoplasts contain fat-soluble pigments such as xanthophylls and carotene. These pigments give plants their vibrant colors, including shades of yellow, orange, and red.
• Leucoplasts: Leucoplasts are colorless plastids that serve as storage organelles for various nutrients. Different types of leucoplasts include amyloplasts (for storing carbohydrates like starch), aleuroplasts (for storing proteins), and elaioplasts (for storing oils and fats).

Ribosomes

• Ribosomes are organelles that are not membrane-bound and are primarily involved in protein synthesis. They consist of ribosomal RNA (rRNA) and ribosomal proteins. Ribosomes come in two main types: 70S ribosomes found in prokaryotic cells and 80S ribosomes found in eukaryotic cells. The "S" in these terms stands for Svedberg units, which are a measure of the rate at which particles sediment during centrifugation.
• Ribosomes are composed of two subunits, a large subunit, and a small subunit, which work together to read messenger RNA (mRNA) and assemble amino acids into a growing protein chain. These ribosomes can be found in two locations within a eukaryotic cell: attached to the endoplasmic reticulum (ER) or freely floating in the cytoplasm. Both types of ribosomes perform the essential function of protein synthesis.

Golgi Complex

The Golgi complex, also known as the Golgi apparatus, is a membrane-bound organelle consisting of flattened, stacked pouches called cisternae. This organelle is found in both plant and animal cells. The primary function of the Golgi apparatus is to assist in various cellular processes, including the transport, modification, targeting, and packaging of proteins and lipids to their respective target sites within or outside the cell. It plays a crucial role in the sorting and processing of molecules before they are delivered to their final destinations within the cell or secreted from the cell.

Microbodies

Microbodies are single membrane structures, small sac-like organelles that are present in both plant and animal cells. These microbodies contain various enzymes, which can be observed under an electron microscope. They play a role in specific metabolic processes within the cell, and their single-membrane structure helps isolate these enzymatic reactions from the rest of the cellular environment. 

Cytoskeleton

The cytoskeleton is a proteinaceous structure that extends from the cell membrane to the nucleus and is distributed throughout the entire cell. It plays a crucial role in providing shape to the cell and facilitating intracellular movement. The cytoskeleton is present in all types of cells and consists of various types of proteins that can rapidly assemble and disassemble based on the cell's needs. This dynamic nature of the cytoskeleton allows it to provide mechanical support to the cell and enables cellular movement through its contractile filaments.

Cilia and Flagella

• Cilia and flagella are cellular structures located on the outer part of the cell membrane that facilitate cell movement. Cilia are small, hair-like projections found on the cell surface, and they play a role in moving cells or extracellular fluid. Flagella, on the other hand, are longer and larger than cilia.
• Both cilia and flagella have a similar structural arrangement known as the "9+2" arrangement. This means they contain nine microtubule doublets arranged in a ring with two central microtubules in the center. The basal body of cilia and flagella is called the centriole, and it serves as the organizing center for the microtubules, providing the structural support needed for movement.

Centrosomes and Centrioles

• A centrosome is formed when two perpendicular centrioles come together and create a central organizing structure within the cell. Each centriole is composed of nine microtubule triplets, and three of these triplets are interlinked to form a single centriole. The centriole is also referred to as the Microtubule Organizing Center (MTOC) because it plays a crucial role in organizing microtubules within the cell.
• Centrioles are involved in connecting the peripheral fibrils with radial spokes, aiding in the organization of microtubules and providing structural support. In the context of cilia and flagella, the basal bodies from which these cellular structures extend are composed of centrioles.

Vacuoles

Vacuoles are single-membrane structures found within cells. In plant cells, vacuoles can occupy a significant portion of the cell's volume. These organelles serve several essential functions in the cell.

• Storage: Vacuoles are primarily used for storing various compounds. They can store nutrients, ions, sugars, and other essential molecules required for the cell's survival.
• Waste Storage: Vacuoles also play a role in storing waste products, helping to keep the cell's cytoplasm clean and free from harmful waste materials.
• Turgor Pressure: In plant cells, vacuoles are crucial for maintaining turgor pressure, which provides structural support to the cell and helps keep plant tissues rigid.

The number and size of vacuoles can vary depending on the type of cell and its specific functions. In plant cells, particularly, vacuoles are a prominent and distinctive feature.