Chapter 8 Biology of Class 11 NCERT introduces one of the most fundamental units of biology: the cell. Everything in biology begins with understanding cells — their origin, structural organization, types, and functions. This chapter is critical for NEET aspirants as it establishes the base for cellular and molecular biology, which are major components of the NEET syllabus.

This chapter builds upon concepts discussed in Unit III: Cell – Structure and Functions, specifically delving into the microscopic unit that makes up every living organism — the cell.

What Is a Cell?

The cell is the basic structural and functional unit of life. All living organisms are made up of cells, which perform a wide array of biological functions. Cells vary in size, shape, structure, and function but share several common characteristics.

Discovery and Cell Theory

• Robert Hooke (1665): First observed dead cork cells under a microscope
• Antonie van Leeuwenhoek: First to observe living cells (bacteria and protozoa
• Cell Theory (Schleiden and Schwann): States that all living organisms are composed of cells and cell products.
• Rudolf Virchow added that all cells arise from pre-existing cells (Omnis cellula-e-cellula).

Types of Cells

Prokaryotic Cells

Prokaryotic cells are the most primitive and structurally simple types of cells. They are typically found in unicellular organisms such as bacteria and cyanobacteria.

• No membrane-bound organelles: Organelles such as mitochondria, endoplasmic reticulum, and nucleus are absent. Cellular functions are carried out in the cytoplasm or by specialized regions within the cell.
• No true nucleus: Genetic material (usually a single circular DNA molecule) is not enclosed in a nuclear membrane but is instead located in a region called the nucleoid.
• Cell envelope: Many prokaryotes have a cell envelope consisting of a glycocalyx, cell wall, and plasma membrane, which provides shape and protection.
• Ribosomes: Present but smaller (70S type) than those in eukaryotic cells. They function in protein synthesis.
• Cell division: Occurs through a simple process called binary fission.
• Plasmids: Extrachromosomal DNA that often carries genes for antibiotic resistance or other functions.
• Flagella (if present): Aid in locomotion and are structurally simpler than eukaryotic flagella.

Prokaryotic cells are remarkably efficient and capable of surviving in extreme environments. Despite their simplicity, they carry out all necessary life processes and play vital roles in ecological cycles.

Eukaryotic Cells

Eukaryotic cells are structurally more complex and are the building blocks of multicellular organisms such as plants, animals, fungi, and protists.

• Well-defined nucleus enclosed by nuclear membrane: The genetic material (DNA) is organized into chromosomes and enclosed within a double membrane called the nuclear envelope. This compartmentalization allows for better regulation of gene expression and cellular activities.
• Presence of membrane-bound organelles: Organelles like mitochondria, endoplasmic reticulum (ER), Golgi apparatus, lysosomes, and peroxisomes carry out specialized functions. This compartmentalization increases cellular efficiency and specialization.
• Cytoskeleton: Eukaryotic cells have a dynamic cytoskeleton composed of microtubules, microfilaments, and intermediate filaments, aiding in cell shape, motility, and intracellular transport.
• Larger cell size: Typically ranging from 10–100 µm in diameter, allowing for greater functional complexity.
• Mode of division: Eukaryotic cells divide via mitosis (for growth and repair) or meiosis (for gamete formation in sexually reproducing organisms).

Eukaryotic cells form tissues and organs, and their advanced internal organization supports the development of complex life forms. These characteristics make eukaryotic cell structures a critical focus for NEET preparation.

Cell Organelles and Their Functions

Plasma Membrane

The plasma membrane, also known as the cell membrane, is a vital component of all living cells. It serves as a dynamic barrier that separates the internal contents of the cell from its external environment.

• Structure: The membrane follows the Fluid Mosaic Model, proposed by Singer and Nicolson, consisting of a phospholipid bilayer interspersed with proteins, cholesterol, and carbohydrates. This model describes the fluid nature of the lipids and the mosaic-like arrangement of proteins.
• Selective Permeability: The membrane is selectively permeable, allowing only specific substances to pass through, thereby maintaining homeostasis. Small, non-polar molecules can diffuse freely, while larger or charged molecules require transport proteins.
• Transport Mechanisms: Includes passive transport (diffusion, facilitated diffusion, osmosis), active transport (requiring ATP), and bulk transport processes like endocytosis (phagocytosis and pinocytosis) and exocytosis.
• Proteins and Carbohydrates: Integral proteins function as channels or carriers, while peripheral proteins are involved in signaling and anchoring. Glycoproteins and glycolipids play roles in cell recognition and immune response.
• Function in Cell Signaling: Receptor proteins on the membrane surface detect external signals (like hormones or neurotransmitters), triggering intracellular responses.

Understanding the structure and function of the plasma membrane is crucial for NEET as it connects directly to human physiology, cellular processes, and pathophysiology.

Cell Wall (Plant cells)

The cell wall is a rigid, non-living outer covering found only in plant cells, fungi, and some protists. It is located outside the plasma membrane and plays a crucial role in maintaining the shape, structure, and integrity of plant cells.

• Composition: In plants, the cell wall is primarily made of cellulose, a complex carbohydrate (polysaccharide). It may also contain hemicellulose, pectin, and lignin depending on the plant type and cell maturity.
• Structure: The plant cell wall has three layers:
  • Middle lamella: The outermost layer made primarily of calcium pectate, which helps in cementing adjacent cells together.
  • Primary cell wall: Flexible and extensible, present in young growing cells.
  • Secondary cell wall: Formed inside the primary wall after the cell stops growing; rich in cellulose and lignin, providing additional rigidity.
• Functions:
  • Provides mechanical strength and structural support to plant cells.
  • Protects the protoplasm against mechanical injury and pathogen invasion.
  • Maintains cell shape and prevents excessive water uptake (prevents bursting in hypotonic solutions).
  • Facilitates transport and communication between cells via plasmodesmata — cytoplasmic connections through the wall.

For NEET, understanding the structure, composition, and functions of the cell wall is crucial, especially while differentiating between plant and animal cells and analyzing cell integrity and osmoregulation.

Endomembrane System

The endomembrane system refers to a set of interrelated membrane-bound organelles in eukaryotic cells that work together to synthesize, modify, package, and transport lipids and proteins. These membranes are either physically connected or communicate through the transfer of vesicles.

• Key Components:
  • Endoplasmic Reticulum (ER): Exists in two forms:
    • Rough ER (RER): Studded with ribosomes; involved in the synthesis of proteins that are usually secreted or transported to organelles.
    • Smooth ER (SER): Lacks ribosomes; responsible for lipid synthesis, detoxification, and carbohydrate metabolism.
  • Golgi Apparatus: A stack of flattened cisternae that modifies, sorts, and packages proteins and lipids received from the ER. It is especially important in the formation of lysosomes and secretory vesicles.
  • Lysosomes: Membrane-bound vesicles filled with hydrolytic enzymes. These are formed by the Golgi apparatus and are involved in intracellular digestion, recycling of cellular components (autophagy), and defense against pathogens.
  • Vacuoles:
    • In plant cells, a large central vacuole maintains turgor pressure, stores nutrients, and manages waste.
    • In animal cells, vacuoles are smaller and involved in transport and storage.
• Functions:
  • Coordination of biosynthetic, secretory, and endocytic pathways.
  • Regulation of lipid and protein trafficking within the cell.
  • Detoxification and recycling of cellular components.

From the NEET perspective, questions frequently test the functions of each component, particularly the ER, Golgi complex, and lysosomes. Focus on diagram-based and concept clarification questions for these organelles.

Mitochondria

Mitochondria are double-membraned, cylindrical organelles prominently known as the “powerhouses of the cell” due to their vital role in energy production through aerobic respiration.

• Outer Membrane: Smooth and permeable to small molecules and ions.
• Inner Membrane: Folded into cristae to increase surface area; houses proteins of the electron transport chain and ATP synthase.
• Matrix: The innermost compartment that contains enzymes of the Krebs cycle, mitochondrial DNA (mtDNA), RNA, and ribosomes.

Key Functions:

• ATP Production: Mitochondria generate adenosine triphosphate (ATP), the cellular energy currency, via oxidative phosphorylation.
• Krebs Cycle Site: The matrix hosts enzymes of the citric acid cycle.
• Thermogenesis: In brown adipose tissue, mitochondria help in heat production.
• Calcium Storage and Apoptosis: Also involved in cellular calcium regulation and programmed cell death (apoptosis).

Plastids (Only in plant cells)

Plastids are double-membraned organelles unique to plant cells and some protists. They originate from proplastids and perform varied functions depending on their type and pigmentation.

• Chloroplasts: These plastids contain green pigment chlorophyll and are the site of photosynthesis, where light energy is converted into chemical energy. Chloroplasts also contain thylakoids, grana, and stroma, and have their own DNA and ribosomes, making them semi-autonomous.
• Chromoplasts: Rich in pigments like carotenoids (yellow, orange, and red), chromoplasts impart color to flowers and fruits, aiding in pollination and seed dispersal by attracting pollinators.
• Leucoplasts: These are colorless plastids and primarily function in the storage of nutrients. They are further classified into:
  • Amyloplasts: Store starch
  • Elaioplasts: Store oils and lipids
  • Aleuroplasts: Store proteins

Plastids are essential not just for photosynthesis but also for storage, synthesis of fatty acids, and amino acids. Their semi-autonomous nature, similar to mitochondria, is an important concept frequently tested in NEET.

Ribosomes

• Ribosomes are non-membranous and composed of ribosomal RNA (rRNA) and proteins. They serve as the molecular machines responsible for protein synthesis.
• Found in both prokaryotic and eukaryotic cells, ribosomes can either be freely floating in the cytoplasm or attached to the endoplasmic reticulum, forming the rough ER (RER).
• Each ribosome consists of two subunits: a large subunit and a small subunit. In eukaryotes, these are referred to as 80S ribosomes (composed of 60S and 40S), while in prokaryotes, they are 70S ribosomes (composed of 50S and 30S).
• Ribosomes read the messenger RNA (mRNA) and assemble amino acids into polypeptide chains in a process called translation.

Nucleus

• The nucleus is the largest membrane-bound organelle in eukaryotic cells, surrounded by a double membrane called the nuclear envelope, which contains nuclear pores to facilitate the regulated exchange of substances between the nucleus and cytoplasm.
• Inside, the nucleus contains the genetic material (DNA) organized into chromatin — loosely packed during interphase and condensing into chromosomes during cell division.
• The nucleolus, a dense, spherical structure within the nucleus, is responsible for the synthesis of ribosomal RNA (rRNA) and the assembly of ribosomal subunits.
• The nucleus regulates cell metabolism, growth, protein synthesis, and heredity, functioning as the control center of the cell.
• It contains nucleoplasm (karyolymph), which provides a medium for the diffusion of nuclear components and supports various nuclear processes.

Cilia and Flagella

Cilia and flagella are slender, hair-like projections extending from the surface of certain eukaryotic cells. Both structures are composed of microtubules arranged in a characteristic “9+2” pattern — nine outer doublets of microtubules surrounding a central pair. This configuration is covered by the plasma membrane and is anchored to the cell by a basal body (9+0 arrangement), which is structurally similar to a centriole.

• Cilia are short and usually numerous on the cell surface. They beat in a coordinated, rhythmic fashion to move fluids or mucus over the cell surface. For example, ciliated epithelial cells in the respiratory tract help clear mucus and trapped dust.
• Flagella are longer and usually occur singly or in pairs. They are used for the locomotion of cells, such as in human sperm cells where the flagellum enables the sperm to swim toward the ovum.
• Movement Mechanism: The sliding of microtubule doublets against each other, powered by the motor protein dynein, generates bending movements.

Cilia and flagella are essential structures in understanding cell motility, and diagram- or structure-based MCQs on these are frequently asked in NEET.

Cytoskeleton

• The cytoskeleton is a dynamic and adaptable framework composed of three major types of protein filaments: microtubules, microfilaments (actin filaments), and intermediate filaments. Each component plays a unique and crucial role in cellular architecture and function.
• Microtubules are hollow tubes made of tubulin proteins that provide tracks for the movement of organelles and vesicles within the cell. They are also involved in forming structures like centrioles, cilia, and flagella.
• Microfilaments, composed of actin, are thin and flexible strands that help in maintaining cell shape, enabling movement, and supporting cellular processes like cytokinesis and endocytosis.
• Intermediate filaments provide tensile strength, helping cells withstand mechanical stress. These filaments vary in protein composition depending on the cell type (e.g., keratin in epithelial cells).
• The cytoskeleton not only maintains the cell’s shape and provides internal organization but also facilitates intracellular transport, chromosome movement during mitosis, and signal transduction pathways.

Centrosome and Centrioles

• Centrosome is a microtubule-organizing center (MTOC) unique to animal cells and some protists. It is crucial for maintaining cell organization and facilitating cell division.
• It consists of two centrioles, each made up of nine triplets of microtubules arranged in a cylindrical structure. These centrioles are positioned perpendicular to each other within the centrosome.
• During cell division, the centrosome duplicates, and each pair of centrioles migrates to opposite poles of the cell. They form the spindle apparatus, which ensures accurate segregation of chromosomes during mitosis and meiosis.
• Centrioles also help in the formation of basal bodies, which give rise to cilia and flagella in eukaryotic cells.

Microbodies

Microbodies are small, spherical, membrane-bound organelles found in both plant and animal cells. They play vital roles in various metabolic processes including detoxification, lipid metabolism, and photorespiration. These organelles are especially important from a NEET perspective due to their unique biochemical pathways and enzymatic roles.

• Peroxisomes: Present in both plant and animal cells, peroxisomes contain oxidative enzymes like catalase and urate oxidase. They are responsible for the detoxification of hydrogen peroxide (H₂O₂) by converting it into water and oxygen. They are also involved in β-oxidation of very long-chain fatty acids and the metabolism of reactive oxygen species (ROS). In plant cells, peroxisomes participate in photorespiration — a process associated with the Calvin cycle in photosynthesis.
• Glyoxysomes: These are a specialized type of peroxisome found predominantly in germinating seeds of plants, especially oilseeds. Glyoxysomes contain key enzymes of the glyoxylate cycle, including isocitrate lyase and malate synthase, which allow the conversion of stored lipids into sugars (gluconeogenesis) — an essential process during seed germination when photosynthesis is not yet functional.
• Additional Characteristics:
  • Enclosed by a single membrane.
  • Contain enzymes that perform redox reactions.
  • Involved in metabolic pathways that intersect with mitochondria and chloroplasts in plant cells.

These organelles are important in understanding plant biochemistry, energy metabolism, and detoxification — areas frequently covered in NEET questions.

Differences Between Plant and Animal Cells

NEET Prep Tips for Chapter 8 Biology

• Memorize the differences between prokaryotic and eukaryotic cells.
• Practice labeling diagrams of mitochondria, chloroplast, and the nucleus.
• Use tables for memorizing differences between plant and animal cells.
• Focus on organelle functions; these are frequently tested.
• Revise NCERT line by line—diagrams and text-based questions are common in NEET.

FAQs

1. What is the most important function of mitochondria?

Mitochondria are responsible for producing ATP through aerobic respiration. They play a key role in cellular energy metabolism, which is essential for all physiological processes.

2. How does the plasma membrane regulate transport?

The plasma membrane facilitates transport through passive mechanisms such as diffusion and osmosis, and active mechanisms that require ATP. It also engages in bulk transport like endocytosis and exocytosis, enabling precise regulation of cellular intake and output.

3. Why is the nucleus considered the brain of the cell?

The nucleus governs cellular activities by controlling gene expression and harboring the cell’s hereditary information in the form of DNA. It directs protein synthesis, cellular growth, and replication.

4. What are lysosomes and what do they do?

Lysosomes are membrane-bound vesicles that contain hydrolytic enzymes. They digest worn-out organelles, engulf foreign substances, and play a crucial role in autophagy and apoptosis.

5. What is the role of ribosomes in protein synthesis?

Ribosomes read mRNA sequences and translate them into amino acid chains (proteins) via a process called translation. They are found either free in the cytoplasm or bound to the rough ER.

6. How are plant cells structurally different from animal cells?

Plant cells have a rigid cell wall, chloroplasts, and a large central vacuole, while animal cells have centrioles and more lysosomes but lack a cell wall and chloroplasts.

Conclusion

Understanding the cell is crucial for success in NEET. From structure to function, organelles to processes, every topic in this chapter serves as a building block for advanced biology in Class 12 and in medical entrance exams. Now that you’ve mastered the basics of cells, you’re ready to explore the biochemical makeup in Chapter 9: Biomolecules.