Cell – The Unit Of Life

Learning Outcomes:

1. Understand the definition of a cell and its significance as the basic unit of life.
2. Identify and explain the cell theory and its modern interpretation.
3. Distinguish between prokaryotic and eukaryotic cells based on structure and function.
4. Describe the structural components of a eukaryotic cell, including membrane-bound organelles.
5. Understand the functions of various cell organelles and their role in maintaining life processes.

What is a Cell?

A cell is the basic unit of life, essential for independent existence and responsible for performing the vital functions that sustain life. The cell can be unicellular, with a single cell carrying out all necessary functions, or multicellular, where specialized cells form tissues and organs. Cells are the fundamental structural and functional units in all living organisms.

Anton Von Leeuwenhoek first observed living cells, while Robert Brown later discovered the nucleus, a crucial organelle. Advancements in microscopy have revealed intricate cellular details that were previously unknown.

Cell Theory

In 1838, Matthias Schleiden observed that plants consist of cells, while Theodore Schwann, a zoologist, concluded that animal cells also possess a plasma membrane and, in plant cells, a cell wall. This led to the proposition that all organisms are composed of cells and the products of cells. However, the theory lacked an explanation of how cells form.

In 1855, Rudolf Virchow proposed that cells arise from pre-existing cells (Omnis cellula-e cellula). Modern cell theory consists of two primary tenets:

1. All living organisms are made of cells and products of cells.
2. All cells arise from pre-existing cells.

An Overview of the Cell

Cells can differ in shape, size, and activities. Eukaryotic cells have membrane-bound nuclei and organelles, while prokaryotic cells lack these features. Both types of cells have cytoplasm, a semi-fluid matrix where essential chemical reactions occur. Eukaryotic cells are more complex, featuring organelles like the endoplasmic reticulum, Golgi apparatus, mitochondria, and lysosomes.

Ribosomes are non-membrane-bound organelles present in both cell types, contributing to protein synthesis. In eukaryotes, ribosomes can be found in the cytoplasm, on rough ER, and within mitochondria and chloroplasts. Additionally, animal cells have centrosomes involved in cell division.

Prokaryotic Cells

Prokaryotic cells, such as those found in bacteria, blue-green algae, and mycoplasmas, are generally smaller and simpler. These cells lack membrane-bound organelles and have naked genetic material without a nuclear envelope. Prokaryotic cells also possess plasmids, small circular DNA that can provide characteristics such as antibiotic resistance.

Key features of prokaryotic cells include:

1. Cell envelope: Consisting of glycocalyx, cell wall, and plasma membrane, it acts as a protective layer.
2. Mesosome: Formed by plasma membrane extensions, it plays a role in DNA replication and respiration.
3. Flagella, pili, and fimbriae: Surface structures used for motility and attachment.
4. Ribosomes: Smaller (70S) than those found in eukaryotes (80S), involved in protein synthesis.
5. Inclusion bodies: Reserve materials stored in the cytoplasm without a surrounding membrane.

Prokaryotic Cells vs. Eukaryotic Cells

Eukaryotic Cells

Eukaryotic cells, found in plants, animals, fungi, and protists, have a highly compartmentalized structure. The nucleus is bound by a nuclear envelope, containing chromosomes made of DNA. In addition to the nucleus, eukaryotic cells have several membrane-bound organelles that perform specific functions.

Key features of eukaryotic cells include:

1. Nucleus: Controls cell activities and houses genetic material.
2. Endomembrane system: Includes the endoplasmic reticulum (ER), Golgi apparatus, lysosomes, and vacuoles.
3. Mitochondria: Known as the powerhouses of the cell, they produce ATP through aerobic respiration.
4. Plastids: Found in plant cells, they are responsible for photosynthesis (chloroplasts), storage (leucoplasts), and coloration (chromoplasts).
5. Cytoskeleton: Provides structural support and is involved in cell movement and division.

The Cell Membrane

The cell membrane is a phospholipid bilayer that controls the movement of substances into and out of the cell. Proteins embedded in the membrane facilitate transport, while cholesterol helps maintain fluidity. The fluid mosaic model, proposed by Singer and Nicolson, describes the dynamic nature of the membrane, allowing lateral movement of proteins.

Key functions of the plasma membrane include:

1. Selective permeability: Controls the entry and exit of molecules.
2. Passive transport: Movement of molecules down their concentration gradient without energy input (e.g., diffusion, osmosis).
3. Active transport: Movement of molecules against their concentration gradient, requiring energy (e.g., Na+/K+ pump).

The Cell Wall

In plants, the cell wall provides structural support and protection. Composed of cellulose, hemicellulose, and pectins, it ensures the cell maintains its shape and prevents it from bursting. The middle lamella, rich in calcium pectate, helps glue neighboring cells together. Plasmodesmata are channels that connect the cytoplasm of adjacent cells, facilitating communication and transport.

Endomembrane System

The endomembrane system consists of the ER, Golgi apparatus, lysosomes, and vacuoles:

1. Endoplasmic reticulum (ER): A network of membranes that forms two types: rough ER (RER), involved in protein synthesis, and smooth ER (SER), responsible for lipid synthesis.
2. Golgi apparatus: Modifies and packages proteins for transport. It consists of cisternae, stacked membranous sacs.
3. Lysosomes: Contain hydrolytic enzymes for the digestion of macromolecules.
4. Vacuoles: Membrane-bound storage organelles, particularly large in plant cells, aiding in storage and maintaining cell turgor.

Mitochondria

Mitochondria are the energy centers of the cell, performing oxidative phosphorylation to generate ATP. They have a double membrane structure, with the inner membrane folding into cristae to increase surface area for enzymatic reactions. The matrix contains its own DNA, ribosomes, and enzymes necessary for cellular respiration.

Plastids

Plastids are unique to plants and some protists. The most important plastids include:

1. Chloroplasts: Contain chlorophyll for photosynthesis. They consist of stacked thylakoids called grana, where light-dependent reactions occur.
2. Chromoplasts: Provide color to plant parts with pigments like carotenoids.
3. Leucoplasts: Storage plastids for starch (amyloplasts), oils (elaioplasts), or proteins (aleuroplasts).

Ribosomes

Ribosomes are protein synthesis machines composed of RNA and proteins. Eukaryotic ribosomes (80S) are larger than prokaryotic ribosomes (70S), and they can be found freely in the cytoplasm or attached to the rough ER.

Cytoskeleton

The cytoskeleton is a network of microtubules, microfilaments, and intermediate filaments. It provides mechanical support, facilitates cell movement, and maintains cell shape.

Cilia and Flagella

Cilia and flagella are hair-like structures extending from the cell membrane that assist in movement. Cilia move in a coordinated manner, while flagella move in a whip-like motion. Both structures have a 9+2 arrangement of microtubules and originate from basal bodies.

Centrosome and Centrioles

The centrosome contains two centrioles, which are involved in forming spindle fibers during cell division. Centrioles also form the basal bodies of cilia and flagella.

Nucleus

The nucleus controls the activities of the

cell and contains the genetic material organized into chromatin. During cell division, chromatin condenses to form chromosomes. The nuclear envelope consists of a double membrane with pores that regulate the exchange of materials between the nucleus and cytoplasm.

Important Note: The nucleus plays a vital role in heredity, controlling the transmission of genetic information from one generation to the next.

Chromosomes

Chromosomes have a centromere that divides them into arms, which can vary in length depending on the type of chromosome. Chromosomes can be classified based on the position of the centromere as:

1. Metacentric: Centromere in the middle.
2. Submetacentric: Centromere slightly off-center.
3. Acrocentric: Centromere near one end.
4. Telocentric: Centromere at the terminal end.

Microbodies

Microbodies are small, membrane-bound vesicles containing enzymes. They play specialized roles in plant and animal cells, contributing to metabolic reactions.

MCQ: Which organelle is known as the “powerhouse of the cell”?
Answer: Mitochondria.