What is a Cell? Cell Biology, Functions, Types of Cells & History Of Cell Theory

Each cell is equipped with different machineries with different functions necessary to maintain the cell. Most of the cell’s metabolic activities occur in the cytoplasm, which is the entire region between the nucleus and the plasma membrane and contains many organelles suspended in a semifluid medium, the cytosol. Let’s dive into the eukaryotic cell biology and different organelles in detail.

1. Nucleus

• The nucleus is the main controller of cellular activities and regulates gene expression by various mechanisms that are exclusive to eukaryotes. The nuclear envelope is a double lipid membrane bilayer that surrounds the nucleus and acts as a barrier to keep molecules from freely moving in and out. A protein structure called a pore complex lines each pore and regulates the entry and exit of certain large macromolecules and particles. Each pore complex has protein subunits called Nucleoporins arranged in a ring-like pattern. This structure facilitates the trafficking of RNA and proteins along with small polar and charged molecules.
• A fibrous network of lamin proteins is present below the inner membrane, called the Nuclear Lamina. This strengthens the structural framework of the nucleus.
• The Nucleopasm (also known as nuclear sap) is a clear fluid that’s enclosed in the nuclear envelope. It contains protein fibrils called the nuclear matrix, which maintains the shape of the nucleus. Nucleolus (involved in the synthesis of rRNA ribosomes) and Chromatin are suspended in the nucleoplasm.

The plasma membrane is the semipermeable membrane that forms the boundary of cytoplasm, guarded from outside by the extracellular matrix.

• The Fluid Mosaic Model: Proposed by Seymour Jonathan Singer and Garth L. Nicolson (1972), the Fluid Mosaic Model is the most accepted model for the organisation of the plasma membrane. This model suggests that the plasma membrane is a lipid bilayer surrounding the cell with a mosaic of globular proteins.
• The lipid bilayer makes the cell boundary in a quasifluid state, and it is dynamic. Due to the fluidic nature, lipids and proteins can freely diffuse laterally across the membrane.
• Two different types of proteins have been identified based on their location and association.

1. Peripheral Membrane Proteins.
2. Integral Membrane Proteins.

• A mesosome is a special membranous structure formed by the extension of the plasma membrane in a cell. It’s in the form of vesicles, tubules, and lamellae. It increases the surface of the plasma membrane.
• The plasma membrane, being selectively permeable, restricts molecular movement and maintains cell composition. Some molecules move across the membrane without any expenditure of energy along the concentration gradient, a process called Passive Transport. Passive movement of molecules occurs by the process of diffusion and osmosis. Carrier proteins facilitate the movement of other molecules.
• One of the most critical channel proteins for the transport of water in plant and animal cells is Aquaporin.

Even though cell walls are not found in animal cells, they are found in cells of bacteria, fungi, algae, and higher plants. The cell wall not only determines the cell shape but also prevents osmotic pressure from bursting the cell.

Some membrane-bound organelles work together in a system known as the Endomembrane System. Their functions are coordinated with each other for protein and lipid synthesis; their processing, packaging, and transport to respective locations in the cell. It includes four main parts, which are:

The endoplasmic reticulum (ER) is located near the nucleus and Golgi apparatus. Based on the presence or absence of ribosomes ER can be rough ER or smooth ER. The entire pathway of secretory protein in the cell is as follows: Rough ER > Golgi Apparatus > Secretory Vesicles > Cell Exterior Rough Endoplasmic Reticulum can be identified from smooth ER by the presence of ribosomes on its cytosolic surface. Smooth Endoplasmic reticulum (SER) does not have ribosomes on its surface and has a vital role in lipid metabolism. It’s also an essential site for cholesterol synthesis.

The Golgi Apparatus was first observed by Camillo Golgi, an Italian Biologist. It’s a dark-stained reticular structure that’s located near the cell nucleus. It’s a membrane-bound organelle consisting of a series of membranous sacs that appear like stacked pouches known as Cisternae. The main function of the Golgi Apparatus, or the Golgi Complex, is to pack the materials and prepare for secretions, which involves receiving and shipping.

Lysosomes are small spherical vesicles bound by a single membrane containing hydrolytic enzymes capable of breaking down macromolecules. They are either formed from the Golgi apparatus or directly from the endoplasmic reticulum. Lysosomes are enzymes capable of breaking down macromolecules. They carry out intracellular digestion and also recycle the cell’s organic material through a process called Autophagy.

The term “vacuole” means “empty”. Vacuoles are a lack of cytoplasmic material. Their major functions are storage, structural support, and recycling. It also protects the cell during biotic stresses.

An average mitochondrion is shaped like a sausage and sometimes appears like a rod or even cylindrical. Each mitochondrion is a double membrane-bound structure, i.e., outer membrane (smooth) and inner membrane (has infoldings called cristae). Mitochondria are also known as the powerhouse of the cell, as their primary purpose is to provide energy for the cell and its processes.

Plastids are found in the cytoplasm of plant cells, carrying certain pigments that give specific colours to the plant. Three different types of plastids are recognised based on their pigmentation.

1. Chromoplasts
2. Leucoplasts
3. Choloplasts

The Chloroplasts contain chlorophyll and carotenoid pigments, which are responsible for trapping light energy that’s vital for photosynthesis.

Ribosomes were first observed by George Palade in 1953 through an electron microscope. They are granular structures composed of Ribonucleic Acid (RNA) and proteins.

Svedberg’s unit, “S” stands for Sedimentation Coefficient, which is an indirect measure of density and size.

The Cytoskeleton gives the cell mechanical support, motility, and maintains the shape of the cell. It has three parts, which are:

• Microtubules
• Microfilaments
• Intermediate Filaments

Cilia and Flagella are hair-like outgrowths of a cell that help in the movement of the cell by functioning like oars. Flagella are comparatively longer than Cilia and are responsible for cell movement. Both the Cilia and Flagella emerge from a centriole-like structure called the basal bodies.

Controsome is an organelle that contains two cylindrical structures called centrioles and is surrounded by amorphous pericentriolar materials. The centrioles form the basal body of the cilia or flagella, and spindle fibres that give rise to the spindle apparatus during cell division in animal cells.

Inside the nucleus, you’ll find the cell’s genetic material (DNA) organised into Chromatin. Chromatin exists in two forms: euchromatin, which is less dense and active in transcription. The second form is heterochromatin, which is densely packed and typically inactive. During cell division, the Chromatin condenses into visible chromosomes.