Basics of Cell Biology: Key Concepts and Structures

Living things are built from tiny building blocks called cells. These come in all shapes and sizes, but all have a control center (nucleus) and a special wall (membrane) that lets things in and out.

Biology is the study of living organisms and is divided into many specialised fields that cover their morphology, physiology, anatomy, behaviour, origin, and distribution.

Cell Biology

The cell is the fundamental, structural and functional unit of all living organisms.

• Anton Van Leeuwenhoek first saw and described a live cell. 
• Robert Brown later discovered the nucleus.
• Dimension: Cells are characteristically microscopic in size (with exceptions-The largest isolated single cell is the egg of an ostrich). 
  • A typical eukaryotic cell is 10 to 100 µm in diameter, while most prokaryotic cells range only 1 to 10 μm in diameter.

 Components of a Cell

The general plan of cellular organisation varies between different organisms, but despite these

modifications, all cells resemble one another in certain fundamental ways. Features common to all cells are:

• Nucleoid/ Nucleus: Every cell contains DNA (deoxyribonucleic acid), the hereditary molecule. 
• In prokaryotes, the simplest organisms, most of the genetic material lies in a single circular molecule of DNA near the centre of the cell in a region called the Nucleoid. 
• The DNA of Eukaryotes, on the other hand, is contained in the nucleus, surrounded by a double membrane structure called the Nuclear Envelope
• Plasma Membrane: It is made up of phospholipids and proteins, is selectively permeable, regulates transport into and out of cells, and separates cellular contents from surroundings enclosing it.
• Cytoplasm: It is a semifluid matrix that fills the interior of the cell containing various organelles and micro/macro molecules which help in the functioning of the cell.
• Ribosomes: The ribosome is a complex molecule made of ribosomal RNA (Ribonucleic Acid) and proteins that form a factory for protein synthesis in cells. 
• The eukaryotic ribosomes are the 80S while the prokaryotic ribosomes are 70S.  
• Here S (Svedberg’s Unit) stands for the sedimentation coefficient; it is indirectly a measure of density and size.

CELL ORGANELLES

• About: These are specialised structures that serve specific functions to keep a cell alive  [e.g., mitochondria produce ATP (Adenosine Triphosphate), the energy currency of cells].

Mitochondria

• Structure: These are double membrane-bound structures.
• Features:Mitochondria are the sites of aerobic respiration producing cellular energy in the form of ATP; hence they are called power houses of the cell.
  • They also possess a single circular DNA molecule, a few RNA molecules, ribosomes (70S) and the components required for the synthesis of proteins.

Endo-Membrane System

• About: Many cell organelles are considered together as an endomembrane system because their functions are coordinated and it includes endoplasmic reticulum (ER), Golgi complex, lysosomes and vacuoles.
• Endoplasmic Reticulum (ER): It is a network of membranes inside a cell through which proteins and other molecules move.

• Golgi Apparatus: A Golgi body, also known as a Golgi apparatus, is a cell organelle that helps process and package proteins and lipid molecules.
• Lysosomes: These are membrane-bound cell organelles that contain various hydrolytic enzymes. These enzymes help in the destruction and breakdown of various worn-out cell parts, helping a cell in de-toxification. 
• They also destroy pathogens, such as bacteria and viruses and play a role in immunity. 
• They are called suicidal bags of the cell because they help in the self-destruction of the cells.
• Vacuoles: It is a membrane-bound space found in the cytoplasm containing water, sap, excretory products and other materials not useful for the cell. 
• Large vacuoles are bound by a single membrane called tonoplast.  
• In plant cells, the vacuoles can occupy up to 90 % of the volume of the cell.

Plastids

• About: Plastids are found in all plant cells and in euglenoids.
  • They bear some specific pigments, thus imparting specific colours to the plants.
• Classification: Based on the type of pigments, plastids can be classified into chloroplasts, chromoplasts and leucoplasts.
  • Like mitochondria, the chloroplasts are also double-membraned structures containing small, double-stranded circular DNA molecules and ribosomes.

Conclusion

Inside each cell, there are tiny machines (organelles) with specific jobs. Some make energy (mitochondria), some clean up waste (lysosomes), and plants even have special ones for color (plastids). These all work together to keep the cell alive.