Topics

Characteristics of Prokaryotic Cells

Introduction

Prokaryotic cells are primitive cells that lack membrane-bound organelles. Bacteria and archaea are the primary examples of prokaryotic organisms.

Characteristics of Prokaryotic Cells:

1.     Lack of Nucleus: Prokaryotes have a nucleoid region where their circular DNA is located, but they lack a true membrane-bound nucleus.

2.   Plasmids: Small, circular DNA molecules that are separate from chromosomal DNA and often carry genes for antibiotic resistance.

3.   Cell Wall: Most prokaryotes have a rigid cell wall made of peptidoglycan (in bacteria), providing structural support and protection.

4.   Ribosomes: Prokaryotes have 70S ribosomes, which are smaller than eukaryotic ribosomes (80S).

5.    Lack of Membrane-Bound Organelles: Organelles like mitochondria, Golgi apparatus, and endoplasmic reticulum are absent.

6.   Binary Fission: Prokaryotes reproduce asexually through a process called binary fission.

NEET Focus: Characteristics of prokaryotic cells, especially structural components like the cell wall, ribosomes, and nucleoid, are commonly asked questions.

Characteristics of Eukaryotic Cells

Introduction

Eukaryotic cells are complex cells that contain membrane-bound organelles, including a true nucleus. All multicellular organisms and some unicellular organisms (like protists) are eukaryotic.

Characteristics of Eukaryotic Cells:

1.     Nucleus: Eukaryotic cells have a well-defined, membrane-bound nucleus that contains linear chromosomes made of DNA and histone proteins.

2.   Membrane-Bound Organelles: Organelles such as the mitochondria, Golgi apparatus, endoplasmic reticulum (ER), and lysosomes are present.

3.   Cell Wall: In plant cells and fungi, a cell wall is present (composed of cellulose in plants and chitin in fungi). Animal cells lack a cell wall.

4.   Cytoskeleton: A network of microfilaments, microtubules, and intermediate filaments that maintain cell shape and help in intracellular transport.

5.    80S Ribosomes: Larger ribosomes (80S) in the cytoplasm, with 70S ribosomes present in mitochondria and chloroplasts.

6.   Mitosis and Meiosis: Eukaryotic cells divide by mitosis (for growth and repair) and meiosis (for sexual reproduction).

NEET Focus: Comparisons between prokaryotic and eukaryotic cells are commonly tested. Focus on structural differences, organelles, and reproduction.

Structure and Function of Plasma Membrane

Introduction

The plasma membrane is a selectively permeable barrier that surrounds the cell, controlling the movement of substances in and out of the cell. It is composed primarily of lipids and proteins.

Structure of Plasma Membrane:

1.     Fluid Mosaic Model: Proposed by Singer and Nicolson in 1972, the plasma membrane is described as a dynamic structure with lipids and proteins that move laterally within the bilayer.

2.   Phospholipid Bilayer: The plasma membrane consists of two layers of phospholipids, with hydrophilic heads facing outwards and hydrophobic tails facing inwards.

3.   Proteins:

o    Integral Proteins: Span the entire membrane and are involved in transport and signaling.

o    Peripheral Proteins: Loosely attached to the outer or inner surface of the membrane, involved in cell signaling and maintaining cell shape.

4.   Carbohydrates: Attached to proteins (glycoproteins) or lipids (glycolipids), functioning in cell recognition and communication.

Functions of Plasma Membrane:

1.     Selective Permeability: Regulates the entry and exit of ions, nutrients, and waste products.

2.   Cell Signaling: Receptors in the membrane receive and transmit signals from other cells or the environment.

3.   Cell-Cell Recognition: Glycoproteins and glycolipids act as identification markers for cell recognition.

4.   Transport: Facilitates passive (diffusion, osmosis) and active (requiring energy) transport processes.

NEET Focus: The structure (fluid mosaic model) and function of the plasma membrane are critical topics. Questions on transport mechanisms (diffusion, facilitated diffusion, active transport) are frequently asked.

Structure and Function of Cell Wall

Introduction

The cell wall is an additional, rigid layer found in plant cells, fungi, and some bacteria. It provides structural support and protection.

Structure of Cell Wall:

1.     Composition in Plants: Made of cellulose, hemicellulose, and pectin.

2.   Middle Lamella: The region between adjacent plant cells, rich in pectin, helping to cement cells together.

3.   Primary Cell Wall: Thin, flexible, and forms first in young cells. Composed of cellulose and other polysaccharides.

4.   Secondary Cell Wall: Thicker and stronger, formed inside the primary wall in mature cells. Contains cellulose, lignin, and other strengthening materials.

Function of Cell Wall:

1.     Provides Mechanical Support: Maintains the shape of the cell and protects it from mechanical damage.

2.   Prevents Over-Expansion: Prevents excessive water uptake by maintaining turgor pressure.

3.   Acts as a Barrier: Protects against pathogens.

NEET Focus: Emphasize the differences between the primary and secondary cell walls, the role of the middle lamella, and the composition of the cell wall in plants and bacteria.

Structure and Functions of Ribosomes

Introduction

Ribosomes are the sites of protein synthesis in both prokaryotic and eukaryotic cells. They are non-membrane-bound organelles composed of RNA and proteins.

Structure of Ribosomes:

1.     Small and Large Subunits: Each ribosome consists of a large and a small subunit. In eukaryotes, the ribosome is 80S (with 60S and 40S subunits), while in prokaryotes, it is 70S (with 50S and 30S subunits).

2.   Location:

o    Free Ribosomes: Found in the cytoplasm and synthesize proteins that function within the cytoplasm.

o    Membrane-Bound Ribosomes: Attached to the endoplasmic reticulum (ER) and are involved in synthesizing proteins that are exported or embedded in membranes.

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Functions of Ribosomes:

1.     Protein Synthesis: Ribosomes translate mRNA into polypeptide chains (proteins) during the process of translation.

2.   Polyribosomes: Multiple ribosomes can attach to a single mRNA strand, forming a polyribosome, to increase the efficiency of protein synthesis.

NEET Focus: Understand the differences between 70S and 80S ribosomes, their locations, and roles in protein synthesis.

Structure and Function of Nucleus

Introduction

The nucleus is the largest organelle in eukaryotic cells, containing most of the cell’s genetic material. It acts as the control center for cellular activities.

Structure of Nucleus:

1.     Nuclear Envelope: A double membrane that surrounds the nucleus, separating it from the cytoplasm.

2.   Nuclear Pores: Openings in the nuclear envelope that regulate the exchange of materials (e.g., RNA and proteins) between the nucleus and the cytoplasm.

3.   Nucleoplasm: The gel-like substance inside the nucleus that contains chromatin and the nucleolus.

4.   Chromatin: A complex of DNA and histone proteins. It condenses to form chromosomes during cell division.

5.    Nucleolus: A dense, spherical region within the nucleus responsible for the synthesis of ribosomal RNA (rRNA) and the assembly of ribosomal subunits.

Functions of Nulceus

1.     Stores Genetic Material: The nucleus contains the cell’s DNA, which is organized into chromosomes. This genetic material holds instructions for the synthesis of proteins and is passed from one generation to the next during cell division.

2.   Controls Cellular Activities: The nucleus regulates gene expression and, therefore, controls the activities of the cell, including growth, metabolism, and protein synthesis.

3.   Ribosome Synthesis: The nucleolus within the nucleus is responsible for producing and assembling ribosomal RNA (rRNA), which combines with proteins to form ribosomes.

4.   Cell Division: The nucleus plays a key role in cell division (mitosis and meiosis) by ensuring the proper distribution of genetic material to daughter cells.

NEET Focus: The structure and function of the nucleus, including the nuclear envelope, chromatin, and nucleolus, are important topics. Be prepared for questions about the role of nuclear pores and how DNA is organized.

Structure and Function of Mitochondria

Introduction

Mitochondria are often referred to as the powerhouses of the cell because they generate most of the cell’s supply of adenosine triphosphate (ATP), the energy currency of the cell. These are double-membraned organelles found in most eukaryotic cells.

Structure of Mitochondria:

1.     Outer Membrane: A smooth, protective membrane that surrounds the mitochondrion.

2.   Inner Membrane: Folded into structures called cristae, which increase the surface area for chemical reactions involved in energy production.

3.   Matrix: The space enclosed by the inner membrane that contains enzymes, mitochondrial DNA (mtDNA), and ribosomes.

4.   Intermembrane Space: The space between the outer and inner membranes, which plays a role in the process of oxidative phosphorylation.

Functions of Mitochondria:

1.     ATP Production: Mitochondria are the site of aerobic respiration, where glucose is broken down in the presence of oxygen to produce ATP.

2.   Cellular Respiration: Mitochondria are involved in key steps of cellular respiration, including the Krebs cycle (in the matrix) and oxidative phosphorylation (along the cristae of the inner membrane).

3.   Regulation of Cellular Metabolism: Mitochondria regulate the metabolic activities of the cell by controlling the production of energy.

NEET Focus: Questions frequently revolve around the role of mitochondria in cellular respiration, structure (cristae, matrix), and the unique feature of mitochondrial DNA (mtDNA).

Structure and Functions of Golgi Apparatus

Introduction

The Golgi apparatus (or Golgi complex) is a membrane-bound organelle that plays a critical role in modifying, sorting, and packaging proteins and lipids for storage or transport out of the cell.

Structure of Golgi Apparatus:

1.     Cisternae: The Golgi apparatus is composed of flattened, membrane-bound sacs called cisternae. These are stacked on top of one another.

2.   Cis Face: The receiving side of the Golgi apparatus, where vesicles from the endoplasmic reticulum (ER) fuse and release their contents.

3.   Trans Face: The shipping side of the Golgi apparatus, where processed proteins and lipids are packaged into vesicles for distribution.

Functions of Golgi Apparatus:

1.     Protein Modification: The Golgi apparatus modifies proteins received from the ER by adding carbohydrate groups (glycosylation) or phosphate groups (phosphorylation).

2.   Lipid and Protein Sorting: It sorts and packages proteins and lipids into vesicles, which are then transported to their appropriate destinations (e.g., lysosomes, plasma membrane, or secretion).

3.   Formation of Lysosomes: The Golgi apparatus is involved in the formation of lysosomes, which contain digestive enzymes.

4.   Transport of Lipids: It helps in the transport of lipids and the formation of glycoproteins and glycolipids.

NEET Focus: Key questions may involve the structure and function of the Golgi apparatus, especially in the processing and transport of proteins and the formation of lysosomes.

Types and Functions of Endoplasmic Reticulum (ER)

Introduction

The endoplasmic reticulum (ER) is an extensive network of membranous tubules and sacs that plays a role in the synthesis of proteins and lipids, as well as other cellular processes.

Types of Endoplasmic Reticulum:

1.     Rough Endoplasmic Reticulum (RER):

o    Structure: The surface of the RER is studded with ribosomes, which give it a rough appearance.

o    Function: The RER is involved in the synthesis of proteins that are destined for export from the cell, insertion into the plasma membrane, or inclusion in lysosomes.

2.   Smooth Endoplasmic Reticulum (SER):

o    Structure: Lacks ribosomes, giving it a smooth appearance.

o    Function: The SER is involved in the synthesis of lipids (including steroids), metabolism of carbohydrates, detoxification of drugs and poisons, and storage of calcium ions.

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Functions of Endoplasmic Reticulum:

1.     Protein Synthesis: The RER synthesizes proteins that are transported into the ER lumen and further processed.

2.   Lipid Synthesis: The SER synthesizes lipids, including phospholipids and steroids, which are essential for cell membrane structure and function.

3.   Detoxification: The SER helps detoxify drugs and harmful substances, especially in liver cells.

4.   Calcium Storage: In muscle cells, the SER (known as the sarcoplasmic reticulum) stores calcium, which is essential for muscle contraction.