Cell Structure and Function

The cell is the basic structural and functional unit of life. All living organisms, from the smallest bacteria to complex multicellular organisms like humans, are made up of cells. Cells can exist as independent units of life (in unicellular organisms) or as building blocks of more complex structures (in multicellular organisms).

1.0Cell: The Unit of Life

• Unicellular organisms are capable of
• Independent existence and 
• performing the essential functions of life. 
• Anything less than a complete structure of a cell does not ensure independent living. Hence, the cell is the fundamental structural and functional unit of all living organisms.

Cell Theory 

• Proposed by Schleiden and Schwann Schleiden 
• All plants are made up of different types of cells, which form cells. 
• Schwann: The bodies of animals are composed of cells and cell products. 
• Schleiden and Schwann were not able to describe how new cells were formed.

Shape of Eukaryotic Cell

Prokaryotic Cells 

• The prokaryotic cells are represented by bacteria, blue-green algae, mycoplasma and PPLO (Pleuro Pneumonia Like Organisms). 
• They are generally smaller and multiply more rapidly than the eukaryotic cells. 
• They may vary greatly in shape and size. 
• The four basic shapes of bacteria are bacillus (rod like), coccus (spherical), vibrio (comma shaped) and spirillum (spiral). 

Eukaryotic Cells

• The eukaryotes include all the protists, plants, animals and fungi. 
• In eukaryotic cells there is an extensive compartmentalisation of cytoplasm through the presence of membrane bound organelles. 
• ,s possess an organised nucleus with a nuclear envelope. 
• In addition, eukaryotic cells have a variety of complex locomotory and cytoskeletal structures. Their genetic material is organised into chromosomes. 

Plasma Membrane/Cell Membrane

• Selective permeable 
• Structure of cell membrane : For study of cell membrane, mature RBCs of mammals are used 
• Fluid mosaic model : By singer and Nicolson Irregular (mosaic) arrangement of proteins 
• Lipid : Quasi fluid nature

Cell wall : Non living, rigid structure Outer covering in plant, fungi and bacteria

Functions :

1. Provide shape to the cell
2. Protection from mechanical damage and infection
3. Barrier to undesirable macromolecules
4. Cell-to-cell interaction

Endomembrane System 

Mitochondria 

• Number, shape, size and structure are variable  
• Sausage like or cylindrical shape  
• Site of aerobic respiration  
• Produce ATP, power house of cell  
• Divide by fission

Plastids 

• Plastids are found in all plant cells and in euglenoides. 
• These are easily observed under the microscope as they are large. 
• They bear some specific pigments, thus imparting specific colours to the plants. 
• Based on the type of pigments, plastids can be classified into chloroplasts, chromoplasts and leucoplasts.

Ribosomes 

• Ribosomes are the granular structures first observed under the electron microscope as dense particles by George Palade (1953). 
• They are composed of ribonucleic acid (RNA) and proteins and are not surrounded by any membrane.

Cytoskeleton 

• An elaborate network of filamentous proteinaceous structures consisting of microtubules, microfilaments and intermediate filaments present in the cytoplasm is collectively referred to as the cytoskeleton. 
• The cytoskeleton in a cell are involved in many functions such as mechanical support, motility, maintenance of the shape of the cell

Nucleus - Robert Brown : Discovery of the nucleus 

• Flemming : Gave the chromatin term, staining of nuclear material by basic dyes 
• Nuclear envelope : Two parallel discontinuous membranes with a perinuclear space of 10-50 nm 
• Barrier in between the cytoplasm and the nucleoplasm 
• Nuclear pore : Passage for movement of proteins and mRNA in both directions between the nucleus and the cytoplasm. 
• Nucleolus : Content of nucleolus is continuous with rest of the nucleoplasm because it is a membrane-less structure 
• Nucleoplasm : Nuclear matrix Contains Nucleolus and chromatin Site of active rRNA synthesis 
• Chromatin : Extended and elaborate network of nucleoprotein fibres in interphase nucleus.

2.0Biomolecules

• Biomolecules are the organic and inorganic molecules present in living organisms that are essential for life. 
• They are mostly carbon-based compounds.

Types of Biomolecules

Biomolecules are classified into 4 major types:

Carbohydrates (Sugars)

• Elements: C, H, O
• Function: Provide energy and structural support.
• Monosaccharides: Simple sugars (e.g., glucose, fructose)
• Disaccharides: Two monosaccharides joined (e.g., sucrose = glucose + fructose)
• Polysaccharides: Many monosaccharides (e.g., starch, cellulose, glycogen)

Proteins

• Proteins are polypeptides
• They are linear chains of amino acids linked by peptide bonds. Each protein is a polymer of amino acids. 
• As there are 20 types of amino acids (e.g., alanine, cysteine, proline, tryptophan, lysine, etc.), a protein is a heteropolymer and not a homopolymer. 
• A homopolymer has only one type of monomer repeating ‘n’ number of times. 
• Hence, dietary proteins are the source of essential amino acids. 
• Therefore, amino acids can be essential or non-essential. The latter are those which our body can make, while we get essential amino acids through our diet/food.

Lipids

• Fats: Glycerol + Fatty acids (saturated or unsaturated)
• Phospholipids: Found in cell membranes
• Steroids: Hormones like testosterone, cholesterol

Nucleic Acids

• Types: DNA and RNA
• Building Blocks: Nucleotides (Sugar + Phosphate + Nitrogenous base)
• DNA: A, T, G, C
• RNA: A, U, G, C

Enzymes

•  Enzymes are biocatalysts. 
• Almost all enzymes are proteins. 
• Some enzymes are nucleic acids (RNA), called Ribozymes. 
• Enzymes can be depicted by a line diagram. 
• Any enzyme like any protein has the primary, secondary & tertiary structure. 
• The catalytic structure of most of the enzymes is tertiary.  
• Inorganic catalysts work efficiently at high temperatures and high pressures 

Factors Affecting Enzyme Activity

3.0Cell Cycle and Cell Division

• The cell cycle and cell division are fundamental processes by which cells grow, replicate their DNA, and divide to produce new cells. 
• These processes are essential for growth, development, repair, and reproduction in living organisms.

Phases of Cell Cycle

• A typical eukaryotic cell cycle is illustrated by human cells in culture. 
• These cells divide once every 24 hours. 
• However, this duration of cell cycle can vary from organism to organism and also from cell type to cell type. 
• Yeast, for example, can progress through the cell cycle in only about 90 minutes. 
• The cell cycle is divided into two basic phases: 
• Interphase 
• M Phase (Mitosis phase) 
• G₁ phase (Gap 1) 
• S phase (Synthesis) 
• G₂ phase (Gap 2)

4.0Mitosis – Cell Division in Somatic (Body) Cells

• Definition:
  Mitosis is a type of cell division that results in two identical daughter cells, each having the same number of chromosomes as the parent cell.

Stages of Mitosis

Cytokinesis

• Division of the cytoplasm.
• In animal cells: cell membrane pinches in.
• In plant cells: a cell plate forms.

Meiosis – Cell Division in Sex Cells (Gametes)

• Meiosis is a type of cell division that reduces the chromosome number by half and produces 4 genetically different haploid cells (gametes like sperm and egg). It's used for:
• Sexual reproduction
• Maintaining chromosome number across generations
• Genetic variation

Meiosis Happens in Two Parts

Meiosis I – Reduction division (chromosome number is halved)

Meiosis II – Like mitosis, but with haploid cells

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