Cell: The Unit of Life – Class 11 NEET Free Notes | Easy & Detailed Explanation
1. Introduction to the Cell
When we look around, we notice both living and non-living things. You may have wondered what makes something living while another is non-living. The main difference is the presence of the basic unit of life, which is the cell, in all living organisms.
All organisms are made up of cells. Some organisms consist of a single cell and are called unicellular organisms, while others, like humans, consist of many cells and are called multicellular organisms. Unicellular organisms are capable of independent existence and can perform all essential life functions. Anything less than a complete cell structure cannot live independently. This is why the cell is called the fundamental structural and functional unit of all living beings.
The history of cell discovery began with Antonie Von Leeuwenhoek, who was the first to observe and describe a living cell. Later, Robert Brown discovered the nucleus. With the invention of the microscope and its advancement to the electron microscope, scientists were able to see all the structural details of the cell.
2. Cell Theory – Principles of Cell Structure and Function
In 1838, Matthias Schleiden, a German botanist, studied many plants and found that all plants are made up of different types of cells, which combine to form the tissues of the plant. Around the same time, Theodore Schwann, a German zoologist, studied animal cells and observed that each cell has a thin outer layer, now called the plasma membrane. Schwann also noted that the presence of a cell wall is a unique feature of plant cells. Based on these observations, Schwann proposed that both animals and plants are made of cells and the products of cells.
Together, Schleiden and Schwann formulated the first cell theory. However, this theory did not explain how new cells are formed. Later, in 1855, Rudolf Virchow discovered that cells divide and that new cells arise from pre-existing cells (Omnis cellula-e cellula). He refined the ideas of Schleiden and Schwann to complete the modern cell theory. According to today’s understanding of cell theory:
- All living organisms are made up of cells and the products of cells.
- All cells are made from pre-existing cells.
3. Overview of Cell – Structure and Functions
When you have observed onion peel cells or human cheek cells under a microscope, you notice the basic structure of cells. The onion cell, a typical plant cell, has a distinct cell wall as its outermost boundary, and just inside it is the cell membrane. The human cheek cells, like most animal cells, have only a cell membrane as their outer boundary. Inside both plant and animal cells is a dense, membrane-bound structure called the nucleus, which contains chromosomes carrying the genetic material DNA.
Cells that have a membrane-bound nucleus are called eukaryotic cells, while those that lack a membrane-bound nucleus are called prokaryotic cells. In both types, a semi-fluid matrix called cytoplasm fills the cell and serves as the main site for cellular activities. Many chemical reactions occur in the cytoplasm to maintain the living state of the cell.
Besides the nucleus, eukaryotic cells contain several membrane-bound organelles like the endoplasmic reticulum (ER), Golgi complex, lysosomes, mitochondria, microbodies, and vacuoles. Prokaryotic cells do not have these membrane-bound organelles. Ribosomes, which are non-membrane-bound, are found in all cells, in the cytoplasm, on rough ER, and inside organelles like chloroplasts (in plants) and mitochondria. In animal cells, another non-membrane-bound organelle called the centrosome helps in cell division.
Cells differ greatly in size, shape, and function. For example, Mycoplasmas are the smallest cells at about 0.3 µm, while bacteria range from 3–5 µm. The largest isolated single cell is the ostrich egg. Among multicellular organisms, human red blood cells are about 7 µm in diameter, whereas nerve cells can be extremely long. Cells also vary in shape, which may be disc-like, polygonal, columnar, cuboid, thread-like, or irregular, depending on the function they perform.
4. Prokaryotic Cells – Structure and Characteristics
Prokaryotic cells include organisms like bacteria, blue-green algae, mycoplasma, and PPLO (Pleuro Pneumonia Like Organisms). These cells are generally smaller than eukaryotic cells and can multiply rapidly. They also show variation in shape and size. The four main shapes of bacteria are: bacillus (rod-shaped), coccus (spherical), vibrio (comma-shaped), and spirillum (spiral-shaped).
Although prokaryotes have a wide variety of shapes and functions, their basic organization is similar. Most prokaryotic cells have a cell wall surrounding the cell membrane, except in mycoplasma. The cytoplasm, a semi-fluid matrix, fills the interior of the cell. Prokaryotic cells do not have a well-defined nucleus; their genetic material is naked DNA, not enclosed by a nuclear membrane. The main genomic DNA is usually a single circular chromosome, but many bacteria also have small circular DNA molecules called plasmids. Plasmids can give bacteria special traits, like resistance to antibiotics, and are used in biotechnology to monitor bacterial transformation with foreign DNA.
Unlike eukaryotic cells, prokaryotes lack membrane-bound organelles, except for ribosomes. They also have unique inclusions and a special structure called a mesosome, which is an infolding of the cell membrane. The mesosome is characteristic of prokaryotic cells and helps in cellular processes like respiration and cell division.
Cell Envelope – Structure and Modifications
Most prokaryotic cells, especially bacteria, have a chemically complex cell envelope. This envelope consists of three tightly bound layers: the outermost glycocalyx, followed by the cell wall, and then the plasma membrane. While each layer has its specific function, together they act as a single protective unit for the cell.
Bacteria can be classified into Gram-positive and Gram-negative based on how their cell envelope reacts to the Gram staining technique. Gram-positive bacteria retain the stain, while Gram-negative bacteria do not.
The glycocalyx varies in composition and thickness among bacteria. In some, it forms a loose sheath called a slime layer, while in others, it is thick and tough, forming a capsule. The cell wall determines the shape of the bacterium and provides structural support, preventing it from bursting or collapsing. The plasma membrane is selectively permeable, interacting with the external environment, and is structurally similar to that of eukaryotic cells.
A unique structure in prokaryotes is the mesosome, formed by extensions of the plasma membrane into the cytoplasm. These infoldings appear as vesicles, tubules, or lamellae and help in cell wall formation, DNA replication, distribution of DNA to daughter cells, respiration, secretion, and increasing surface area and enzymatic activity. In some prokaryotes like cyanobacteria, there are other membranous extensions called chromatophores, which contain pigments.
Bacteria can be motile or non-motile. Motile bacteria have flagella, which are thin filamentous extensions from the cell wall. The flagellum has three parts: filament, hook, and basal body. The filament is the longest part extending outside the cell. Other surface structures include pili and fimbriae, which are not used for movement. Pili are long, tubular structures made of special proteins, while fimbriae are small bristle-like fibres. These help bacteria attach to surfaces, such as rocks in streams or host tissues.
Ribosomes and Inclusion Bodies – Structure and Functions
In prokaryotic cells, ribosomes are associated with the plasma membrane. They are small, about 15 nm by 20 nm, and are made of two subunits: 50S and 30S, which together form 70S prokaryotic ribosomes. Ribosomes are the sites of protein synthesis. Often, several ribosomes attach to a single mRNA, forming a chain called polyribosome or polysome. The ribosomes in a polysome work together to translate the mRNA into proteins.
Inclusion bodies are storage forms of reserve materials in the cytoplasm of prokaryotic cells. These are not surrounded by membranes and lie freely in the cytoplasm. Examples include phosphate granules, cyanophycean granules, and glycogen granules. Some prokaryotes, like blue-green, purple, and green photosynthetic bacteria, also have gas vacuoles that help in buoyancy.
5. Eukaryotic Cells – Structure and Characteristics
Eukaryotic cells include all protists, plants, animals, and fungi. These cells show a high level of compartmentalisation because of the presence of membrane-bound organelles in the cytoplasm. Eukaryotic cells have a well-organised nucleus with a nuclear envelope, and their genetic material is arranged in chromosomes. They also possess complex locomotory structures and a cytoskeleton that helps in cell shape, movement, and intracellular transport.
Not all eukaryotic cells are identical. Plant and animal cells differ in some important ways. Plant cells have a cell wall, plastids, and a large central vacuole, which are absent in animal cells. On the other hand, animal cells have centrioles, which are absent in almost all plant cells. Understanding these differences helps in studying the structure and function of individual cell organelles in deeply.
Cell Membrane – Structure and Functions
The detailed structure of the cell membrane was studied more clearly after the invention of the electron microscope in the 1950s. Before that, chemical studies, especially on human red blood cells (RBCs), helped scientists understand the possible structure of the plasma membrane. These studies showed that the cell membrane is mainly made of lipids and proteins. The major lipids are phospholipids, which are arranged in a bilayer. In this bilayer, the polar heads face the outside, while the hydrophobic tails point inwards, protecting the nonpolar tails from the watery environment. In addition to phospholipids, the membrane also contains cholesterol, proteins, and carbohydrates. The ratio of protein and lipid varies in different cell types; for example, in human RBCs, the membrane has about 52% protein and 40% lipids.
Membrane proteins are classified based on their position: peripheral proteins lie on the surface, while integral proteins are partially or fully embedded in the membrane. An improved model of the cell membrane, proposed by Singer and Nicolson in 1972, is called the fluid mosaic model. This model describes the lipid bilayer as quasi-fluid, allowing lateral movement of proteins. This fluidity is important for processes like cell growth, formation of intercellular junctions, secretion, endocytosis, and cell division.
One of the key functions of the plasma membrane is transport. The membrane is selectively permeable, allowing some molecules to pass freely. Movement of molecules without energy is called passive transport. Simple diffusion allows neutral solutes to move from higher to lower concentration. Water also moves in this way through a process called osmosis. Polar molecules, which cannot pass through the nonpolar lipid bilayer, need carrier proteins to help them cross the membrane.
Some molecules move against their concentration gradient (from lower to higher concentration) using energy in the form of ATP. This is called active transport, for example, the Na⁺/K⁺ pump.
Cell Wall – Structure and Functions
A cell wall is a non-living, rigid structure that forms an outer covering for the plasma membrane in plants and fungi. The cell wall gives the cell its shape, protects it from mechanical damage and infections, and also helps in cell-to-cell interactions. It acts as a barrier, preventing unwanted macromolecules from entering the cell.
The composition of the cell wall varies among different organisms. In algae, the cell wall is made of cellulose, galactans, mannans, and sometimes minerals like calcium carbonate. In higher plants, it mainly contains cellulose, hemicellulose, pectins, and proteins.
In a young plant cell, the primary wall is flexible and capable of growth. As the cell matures, a secondary wall forms on the inner side of the primary wall, which is stronger and less flexible. Between adjacent cells, there is a layer called the middle lamella, made mostly of calcium pectate, which acts like glue, holding neighboring cells together.
The cell wall and middle lamella are often traversed by plasmodesmata, which are tiny channels that connect the cytoplasm of neighboring cells, allowing communication and transport of materials.
Endomembrane System – Organelles and Their Functions
In a eukaryotic cell, many membrane-bound organelles are distinct in their structure and function, but some of them work together in a coordinated way and are collectively called the endomembrane system. The endomembrane system includes the endoplasmic reticulum (ER), Golgi complex, lysosomes, and vacuoles. These organelles work together in processes like protein and lipid synthesis, modification, storage, and transport.
Other organelles like mitochondria, chloroplasts, and peroxisomes have specific functions that are independent of the endomembrane system. Therefore, they are not considered part of the endomembrane system.
Endoplasmic Reticulum (ER) – Structure and Functions
Electron microscopy of eukaryotic cells shows a network of tiny tubules scattered throughout the cytoplasm, known as the endoplasmic reticulum (ER). The ER divides the cell interior into two compartments: the luminal compartment (inside the ER) and the extra-luminal compartment (the cytoplasm outside the ER).
The ER can have ribosomes attached to its surface. When ribosomes are present, it is called rough endoplasmic reticulum (RER), and when ribosomes are absent, it is called smooth endoplasmic reticulum (SER).
RER is especially prominent in cells that are actively involved in protein synthesis and secretion. It is extensive and continuous with the outer membrane of the nucleus, allowing smooth transport of newly synthesized proteins.
On the other hand, SER is the main site for lipid synthesis. In animal cells, steroid hormones are also produced in the SER.
Golgi Apparatus – Structure and Functions
The Golgi apparatus was first observed by Camillo Golgi in 1898 as densely stained structures near the nucleus, and it is sometimes called Golgi bodies. It is made up of flat, disc-shaped sacs called cisternae, which are about 0.5 to 1.0 µm in diameter and are stacked parallel to each other. The number of cisternae can vary in different cells.
The Golgi cisternae are arranged near the nucleus in a concentric manner with two distinct faces: the cis face (convex, forming face) and the trans face (concave, maturing face). While these faces have different structures and functions, they are interconnected.
The main function of the Golgi apparatus is packaging and modifying materials for transport either within the cell or outside the cell. Vesicles carrying materials from the endoplasmic reticulum (ER) fuse with the cis face of the Golgi apparatus and move toward the trans face for secretion. Proteins synthesized on the ribosomes of the ER are modified in the Golgi cisternae before being released.
The Golgi apparatus is also the key site for the formation of glycoproteins and glycolipids, which are essential for various cellular functions.
Lysosomes – Structure and Functions
Lysosomes are membrane-bound vesicles that are formed through the packaging process in the Golgi apparatus. These vesicles are filled with a variety of hydrolytic enzymes, also called hydrolases, which include lipases (digest fats), proteases (digest proteins), and carbohydrases (digest carbohydrates). These enzymes work best in an acidic environment.
The main function of lysosomes is digestion. They can break down carbohydrates, proteins, lipids, and nucleic acids inside the cell. Because of these enzymes, lysosomes are often called the “suicide bags” of the cell, as they can digest worn-out organelles or foreign material that enters the cell.
Vacuoles – Structure and Functions
A vacuole is a membrane-bound space in the cytoplasm of a cell. It contains water, cell sap, waste products, and other materials that are not immediately useful for the cell. The vacuole is surrounded by a single membrane called the tonoplast. In plant cells, vacuoles can occupy up to 90% of the cell’s volume, making them very prominent.
The tonoplast plays an important role in transporting ions and other substances into the vacuole, often against concentration gradients. This allows the vacuole to maintain a higher concentration of certain substances compared to the cytoplasm.
In Amoeba and other protists, specialized vacuoles perform specific functions. The contractile vacuole helps in osmoregulation and excretion, while food vacuoles are formed by engulfing food particles to help in digestion.
Mitochondria – Structure and Functions
Mitochondria (singular: mitochondrion) are double membrane-bound organelles that are not easily seen under a microscope unless specially stained. The number, shape, and size of mitochondria vary depending on the activity of the cell. Typically, mitochondria are sausage-shaped or cylindrical, with a diameter of 0.2–1.0 µm and length of 1.0–4.1 µm.
Each mitochondrion has an outer membrane and an inner membrane, which divide its interior into two compartments: the outer compartment and the inner compartment. The inner compartment is filled with a dense substance called the matrix. The inner membrane forms infoldings known as cristae, which increase the surface area for chemical reactions. Both membranes contain specific enzymes essential for mitochondrial functions.
Mitochondria are the main site of aerobic respiration, where cellular energy (ATP) is produced, earning them the nickname “powerhouses of the cell.” The matrix also contains circular DNA, RNA, 70S ribosomes, and all the machinery needed for protein synthesis. Mitochondria replicate independently through a process called fission.
Plastids – Structure and Functions in Plant Cells
Plastids are organelles found in all plant cells and in some protists like euglenoides. They are generally large and visible under the microscope. Plastids contain specific pigments, giving colour to different parts of the plant. Based on the pigments, plastids are classified into chloroplasts, chromoplasts, and leucoplasts.
Chloroplasts contain chlorophyll and carotenoid pigments, which are essential for trapping light energy for photosynthesis. Chromoplasts have fat-soluble carotenoids like carotene and xanthophyll, which give yellow, orange, or red colours to flowers and fruits. Leucoplasts are colourless and store nutrients: amyloplasts store starch, elaioplasts store oils and fats, and aleuroplasts store proteins.
Most chloroplasts in green plants are found in the mesophyll cells of leaves. They can be lens-shaped, oval, spherical, discoid, or ribbon-like, measuring about 5–10 µm in length and 2–4 µm in width. Their number can vary from 1 per cell in Chlamydomonas to 20–40 per mesophyll cell.
Like mitochondria, chloroplasts are double membrane-bound. The inner membrane encloses a space called the stroma, which contains enzymes for carbohydrate and protein synthesis, as well as small circular DNA molecules and 70S ribosomes. Inside the stroma are flattened membranous sacs called thylakoids, which are stacked into grana (singular: granum). Thylakoids are connected by stroma lamellae, and the space inside the thylakoid is called the lumen. Chlorophyll pigments are embedded in the thylakoid membranes, enabling photosynthesis.
Ribosomes – Structure and Functions
Ribosomes are small granular structures in the cell first seen under an electron microscope by George Palade in 1953. They are made of ribonucleic acid (RNA) and proteins, and they are not surrounded by any membrane.
Ribosomes are the site of protein synthesis in both prokaryotic and eukaryotic cells. In eukaryotes, ribosomes are 80S, while in prokaryotes, they are 70S. Each ribosome is made of two subunits – a larger subunit and a smaller subunit. For 80S ribosomes, the subunits are 60S and 40S, and for 70S ribosomes, they are 50S and 30S.
Here, ‘S’ (Svedberg unit) measures the sedimentation rate, which indirectly tells about the size and density of the ribosome. Despite the difference in size, both 70S and 80S ribosomes have the same basic structure of two subunits working together.
Cytoskeleton – Structure and Functions
The cytoskeleton is a complex network of protein fibers present throughout the cytoplasm of a cell. It is made up of three main types of filamentous structures: microtubules, microfilaments, and intermediate filaments.
The cytoskeleton has several important functions. It provides mechanical support to the cell, helps in maintaining the cell’s shape, and is also involved in cell movement (motility). Additionally, the cytoskeleton plays a role in intracellular transport, helping to move organelles and vesicles within the cell.
In short, the cytoskeleton acts like a scaffolding and transport network inside the cell, giving it both structure and flexibility.
Cilia and Flagella – Structure and Functions
Cilia and flagella are hair-like projections from the cell membrane that help in movement. Cilia are short and numerous, working like oars to move either the cell or the fluid around it. Flagella are longer and primarily help in the movement of the entire cell. Prokaryotic bacteria also have flagella, but their structure is different from eukaryotic flagella.
Electron microscopy shows that both cilia and flagella are covered by the plasma membrane. Their core, called the axoneme, contains microtubules arranged along the length. Usually, there are nine doublets of microtubules around the periphery and two single microtubules in the center. This arrangement is called the 9+2 pattern.
The central microtubules are connected by a central sheath and linked to the peripheral doublets by radial spokes (total nine spokes). The peripheral doublets are also joined by linkers. Both cilia and flagella arise from basal bodies, which are similar to centrioles.
Centrosome and Centrioles – Structure and Functions
The centrosome is a cell organelle that usually contains two cylindrical structures called centrioles. These centrioles are surrounded by a gel-like material called pericentriolar material. The two centrioles in a centrosome are arranged perpendicular to each other. Each centriole has a cartwheel-like structure, made up of nine evenly spaced tubulin fibrils. Each of these fibrils is a triplet, and the triplets are connected to each other.
The central part of the proximal end of the centriole is called the hub, which is made of protein. The hub is connected to the peripheral triplets by radial spokes. The centrioles are important because they form the basal body of cilia and flagella, and also give rise to spindle fibres that form the spindle apparatus during cell division in animal cells.
Nucleus – Structure and Functions
The nucleus is an important cell organelle first observed by Robert Brown in 1831. Later, the material inside the nucleus that could be stained by basic dyes was named chromatin by Flemming. When a cell is not dividing, its nucleus is in the interphase stage and contains a network of long, thread-like nucleoprotein fibers called chromatin, along with a nuclear matrix and one or more round structures known as nucleoli. Using electron microscopy, scientists found that the nucleus is surrounded by a nuclear envelope, which consists of two parallel membranes separated by a small gap called the perinuclear space. This envelope acts as a barrier between the nucleus and the cytoplasm. The outer membrane of the envelope is usually connected to the endoplasmic reticulum and has ribosomes attached to it. The envelope also has tiny pores, called nuclear pores, which allow the movement of RNA and proteins between the nucleus and cytoplasm. Most cells have only one nucleus, though some cells may have more, while certain mature cells, like mammalian erythrocytes and sieve tube cells of plants, may completely lack a nucleus.
Inside the nucleus, the nuclear matrix or nucleoplasm contains chromatin and nucleoli. The nucleolus is a spherical structure where ribosomal RNA (rRNA) is actively produced. Cells that are actively making proteins often have larger and more nucleoli. During interphase, chromatin appears as a loose network, but during cell division, it organizes into distinct chromosomes. Chromatin is made of DNA, histone proteins, non-histone proteins, and RNA. A single human cell has around 2 meters of DNA, arranged into 46 chromosomes (23 pairs).
Each chromosome has a primary constriction called the centromere, which is flanked by disc-shaped kinetochores. The centromere holds the two chromatids of the chromosome together. Chromosomes are classified based on the position of the centromere. In metacentric chromosomes, the centromere is in the middle, forming two equal arms. Sub-metacentric chromosomes have the centromere slightly off-center, creating one short arm and one long arm. In acrocentric chromosomes, the centromere is near the end, forming one very short arm and one long arm, while telocentric chromosomes have a terminal centromere. Some chromosomes also have non-staining secondary constrictions, appearing as small fragments called satellites.
Microbodies – Structure and Functions
Both plant and animal cells contain many tiny membrane-bound vesicles called microbodies. These microbodies are small, round structures inside the cell that store and carry various enzymes. The enzymes present in these microbodies help in different chemical reactions that are essential for the normal functioning of the cell. Although they are very small in size, microbodies play an important role in cell metabolism and other biochemical processes.
6. Chapter Overview – Key Concepts and Highlights
All living organisms are made up of cells or groups of cells, and these cells can be very different in shape, size, and function. Based on the presence or absence of a membrane-bound nucleus and other organelles, cells can be classified as eukaryotic or prokaryotic. A typical eukaryotic cell has a cell membrane, nucleus, and cytoplasm, while plant cells also have a cell wall outside the membrane. The plasma membrane is selectively permeable, allowing certain molecules to pass in and out of the cell. The endomembrane system, which includes the endoplasmic reticulum (ER), Golgi complex, lysosomes, and vacuoles, helps in the transport, storage, and processing of various substances. Each organelle has a specific function. For example, the centrosome and centriole form the basal body of cilia and flagella, which help in cell movement, and in animal cells, centrioles also form the spindle apparatus during cell division.
The nucleus contains nucleoli and a chromatin network. It not only controls the activities of the organelles but also plays a major role in heredity. The endoplasmic reticulum is made of tubules or flattened sacs called cisternae, and it exists in two forms: rough ER (with ribosomes) and smooth ER (without ribosomes). ER helps in transporting substances, protein synthesis, and production of lipoproteins and glycogen. The Golgi body is made of flattened membranous sacs that package and transport secretions of the cell. Lysosomes are single-membrane vesicles containing digestive enzymes that break down all types of macromolecules. Ribosomes are responsible for protein synthesis and can be free in the cytoplasm or attached to the ER.
Mitochondria are known as the powerhouse of the cell because they produce ATP through oxidative phosphorylation. They have a double membrane, with the outer membrane smooth and the inner membrane forming folds called cristae. Plastids are pigment-containing organelles found only in plant cells. Chloroplasts are green plastids responsible for photosynthesis, with grana as the site of light reactions and stroma as the site of dark reactions. Other plastids, called chromoplasts, contain pigments like carotene and xanthophyll. The nucleus itself is surrounded by a nuclear envelope, a double membrane with nuclear pores, enclosing the nucleoplasm and chromatin.
In conclusion, the cell is the basic structural and functional unit of life, with each organelle performing its specific role to maintain cell survival and function.
