Cell – The Unit of Life | Prokaryotic & Eukaryotic Cells + Cell Organelles

Introduction to Cells

The concept of the cell as the basic unit of life was proposed by scientists such as Robert Hooke and Matthias Schleiden in the 17th century. All living organisms, from the smallest bacteria to the largest mammals, are made up of cells. Cells perform all the necessary functions for the organism's survival, including metabolism, energy production, growth, and reproduction. Despite their diversity in structure and function, all cells share certain features.

Types of Cells: Prokaryotic vs Eukaryotic

Cells are broadly classified into two categories based on their internal structure: prokaryotic cells and eukaryotic cells.

Prokaryotic Cells

Prokaryotic cells are simpler and smaller in structure. They are found in organisms such as bacteria and archaea.

Characteristics of Prokaryotic Cells:

• Size: Small, typically 0.1 to 5 micrometers in diameter.

• Structure: Lack membrane-bound organelles, including a true nucleus.

• Genetic Material: The genetic material is a single circular DNA molecule located in the nucleoid region, not enclosed by a membrane.

• Cell Wall: Most prokaryotes have a cell wall made of peptidoglycan (in bacteria) or pseudomurein (in archaea).

• Ribosomes: Ribosomes are smaller (70S) compared to eukaryotic cells.

• Reproduction: Asexual reproduction through binary fission (a process where the cell splits into two identical cells).

• Examples: Bacteria, Archaea.

Mnemonic to Remember:
Prokaryotes are Primitive, Plain, and Plural (meaning they have multiple forms like bacteria and archaea).

Eukaryotic Cells

Eukaryotic cells are more complex and larger than prokaryotic cells. They are found in plants, animals, fungi, and protists.

Characteristics of Eukaryotic Cells:

• Size: Larger, typically 10 to 100 micrometers in diameter.

• Structure: Eukaryotic cells have membrane-bound organelles, including a nucleus.

• Genetic Material: The DNA is enclosed within a membrane-bound nucleus and is organized into chromosomes.

• Cell Wall: Present in plant and fungal cells (made of cellulose in plants and chitin in fungi), but absent in animal cells.

• Ribosomes: Ribosomes are larger (80S).

• Reproduction: Eukaryotes can reproduce sexually or asexually, with processes like mitosis and meiosis.

• Examples: Plants, animals, fungi, protists.

Mnemonic to Remember:
Eukaryotes are Evolved, Extra-complex, and have a Kernel (referring to the nucleus).

Cell Organelles: The Specialized Structures of Eukaryotic Cells

Eukaryotic cells have various organelles, each responsible for carrying out specific functions. Here is a detailed look at the major organelles found in these cells.

Nucleus

The nucleus is often considered the control center of the cell. It contains the cell’s genetic material (DNA) and coordinates activities like growth, metabolism, protein synthesis, and cell division.

• Structure: Surrounded by a double membrane called the nuclear envelope, which has pores for material exchange between the nucleus and the cytoplasm.

• Function: Stores and protects the cell's genetic information. It also regulates gene expression and cell division.

Mnemonic to Remember:
The Nucleus is the Nerve center of the cell, controlling activities.

Endoplasmic Reticulum (ER)

The ER is a network of membranous tubules involved in the synthesis of proteins and lipids.

• Rough ER: Has ribosomes attached to its surface and is involved in protein synthesis and modification.

• Smooth ER: Lacks ribosomes and is involved in lipid synthesis, detoxification, and calcium storage.

Mnemonic to Remember:
The Rough ER makes Ribosomes and proteins, while the Smooth ER helps with Synthesis of lipids.

Golgi Apparatus

The Golgi apparatus is responsible for modifying, sorting, and packaging proteins and lipids for transport to different parts of the cell or secretion out of the cell.

• Structure: A series of flattened, membrane-bound sacs.

• Function: Modifies proteins (such as adding sugar groups to form glycoproteins) and packages them into vesicles for transport.

Mnemonic to Remember:
The Golgi is the Gift-packaging center of the cell.

Mitochondria

Mitochondria are known as the powerhouses of the cell because they generate the energy required for various cellular functions through cellular respiration.

• Structure: Have a double membrane structure; the inner membrane is folded to form cristae, which increases surface area for energy production.

• Function: ATP production (energy) through aerobic respiration.

Mnemonic to Remember:
Mitochondria are the Mighty energy producers of the cell.

Lysosomes

Lysosomes are membrane-bound organelles containing enzymes that break down waste materials, cellular debris, and foreign invaders.

• Function: Digestion and waste removal.

• Structure: Spherical vesicles containing digestive enzymes.

Mnemonic to Remember:
Lysosomes act like Little cleaners that digest waste in the cell.

Chloroplasts (in plant cells)

Chloroplasts are responsible for photosynthesis in plant cells.

• Structure: Contains chlorophyll and other pigments that capture light energy, with a double membrane and internal thylakoids.

• Function: Converts light energy into chemical energy in the form of glucose.

Mnemonic to Remember:
Chloroplasts help plants Capture energy from the sun for photosynthesis.

Cytoskeleton

The cytoskeleton provides structure and support to the cell and facilitates cell movement.

• Components: Microtubules, microfilaments, and intermediate filaments.

• Function: Maintains cell shape, helps in intracellular transport, and aids in cell division.

Mnemonic to Remember:
The cytoskeleton is the cell's Construction worker, giving structure and shape.

Prokaryotic Cell Organelles

Although prokaryotic cells lack many of the membrane-bound organelles found in eukaryotic cells, they still possess essential structures for life:

• Nucleoid: A region where the DNA is located.

• Ribosomes: Carry out protein synthesis, smaller than eukaryotic ribosomes (70S).

• Cell Wall: Provides structural support and protection.

• Plasma Membrane: Encloses the cell, controlling the movement of materials in and out.

• Flagella and Pili: Structures used for movement and attachment to surfaces.

Comparison: Prokaryotic vs Eukaryotic Cells

Conclusion

Cells are the fundamental unit of life, and understanding the differences between prokaryotic and eukaryotic cells is essential to biology. While both types of cells share some common features, eukaryotic cells are far more complex and specialized. The organelles within eukaryotic cells enable them to perform various life-sustaining functions. From the control center of the nucleus to the energy production in the mitochondria, each organelle plays a critical role in maintaining the cell's functions.