The Cell: Structure and Function
What is a Cell?
The cell is the basic structural and functional unit of all living organisms. It is the smallest unit of life that can perform all the necessary functions for survival, growth, and reproduction. Just like a building is made of bricks, a living organism is made of cells.
The term 'cell' was first used by Robert Hooke in 1665. He observed thin slices of cork (part of the bark of a tree) under a simple magnifying device. He saw small, empty compartments that resembled cells in a monastery, hence the name 'cell'. However, Hooke observed dead cells (plant cell walls).
Later, Anton von Leeuwenhoek (1674) was the first to observe living cells, such as bacteria, yeast, protozoa, and red blood cells, using improved microscopes.
Organisms can be classified based on the number of cells they possess:
- Unicellular Organisms: These organisms are made up of a single cell. This single cell performs all life processes, such as nutrition, respiration, excretion, growth, and reproduction. Examples include Amoeba, Paramecium, Bacteria, and some Algae.
- Multicellular Organisms: These organisms are composed of many cells. In multicellular organisms, cells are organised into tissues, tissues into organs, and organs into organ systems. Different cells perform different functions, leading to division of labour. Examples include plants, animals, and fungi.
Cell Theory
The Cell Theory is a fundamental concept in biology that describes the basic properties of cells and is considered the foundation of modern biology. It was developed through the work of several scientists.
Development of Cell Theory
In 1838, Matthias Schleiden, a German botanist, studied a large number of plants and observed that all plants are composed of different kinds of cells which form the tissues of the plant.
About the same time (1839), Theodor Schwann, a British zoologist, studied different types of animal cells and reported that cells had a thin outer layer which is today known as the 'plasma membrane'. He also concluded, based on his studies on plant tissues, that the presence of a cell wall is a unique character of plant cells. On the basis of these observations, Schwann proposed a hypothesis that the bodies of animals and plants are composed of cells and products of cells.
Schleiden and Schwann together formulated the initial Cell Theory.
Initial Postulates of Cell Theory (Schleiden and Schwann)
- All living organisms are composed of one or more cells.
- The cell is the basic unit of structure and organization in organisms.
Modification of Cell Theory (Rudolf Virchow)
However, the initial theory did not explain how new cells are formed. In 1855, Rudolf Virchow, a German physician, explained that cells divide and that new cells arise from pre-existing cells. He stated this principle in Latin: "$Omnis \: cellula \: e \: cellula$" (meaning "all cells arise from cells").
Modern Cell Theory
Virchow's postulate modified the cell theory, leading to the modern understanding. The generally accepted postulates of the modern cell theory are:
- All living organisms are composed of cells and products of cells.
- The cell is the basic structural and functional unit of all living organisms.
- All new cells arise from pre-existing cells ($Omnis \: cellula \: e \: cellula$).
- All cells are similar in basic chemical composition and metabolic activities.
- Hereditary information (DNA) is passed from cell to cell during cell division.
Example 1. If a cell undergoes 10 divisions, how many cells are produced?
Answer:
After 1 division: $1 \times 2 = 2$ cells
After 2 divisions: $2 \times 2 = 4$ cells
After 3 divisions: $4 \times 2 = 8$ cells
In general, after $n$ divisions, the number of cells produced from a single cell is $2^n$.
In this case, the cell undergoes 10 divisions. So, $n = 10$.
Number of cells = $2^{10} = 1024$ cells.
Thus, after 10 divisions, 1024 cells are produced from a single cell.
An Overview of Cell
Let's take a brief look at a typical cell to understand its basic structure and components before diving into details. Regardless of whether it's a prokaryotic or eukaryotic cell, some fundamental features are common.
Basic Components
Every cell is essentially defined by its outer boundary and the material contained within it.
- Cell Membrane (Plasma Membrane): This is the outermost boundary of the cell in animal cells and is located just inside the cell wall in plant cells and prokaryotes. It is a thin, flexible barrier that regulates the passage of substances into and out of the cell.
- Cytoplasm: This is the jelly-like substance that fills the cell and surrounds the organelles. It is the site of many metabolic reactions. In both prokaryotic and eukaryotic cells, the cytoplasm is the main arena of cellular activities.
- Genetic Material: All cells contain genetic material (DNA or RNA in some viruses) that carries the instructions for cell function and reproduction. In prokaryotes, this material is located in a region called the nucleoid; in eukaryotes, it is enclosed within a nucleus.
Prokaryotic vs. Eukaryotic Cells
Based on the presence or absence of a membrane-bound nucleus and other membrane-bound organelles, cells are broadly classified into two types:
- Prokaryotic Cells: Cells that lack a true nucleus (where the genetic material is enclosed by a nuclear membrane) and membrane-bound organelles (like mitochondria, endoplasmic reticulum, Golgi apparatus, etc.). Their genetic material (DNA) is typically a single, circular chromosome located in the cytoplasm in a region called the nucleoid. Examples include Bacteria and Archaea.
- Eukaryotic Cells: Cells that possess a true nucleus (with a nuclear membrane enclosing the genetic material) and various membrane-bound organelles. These organelles compartmentalize cellular activities, allowing for greater complexity and efficiency. Examples include cells of protists, fungi, plants, and animals.
Cell Size and Shape
Cells vary greatly in size and shape, depending on their function.
- Size:
- Mycoplasmas (a type of bacteria) are among the smallest cells, about $0.3 \:\mu\text{m}$ (micrometre) in length.
- Typical bacteria are about $1 \:\text{to} \: 2 \:\mu\text{m}$.
- The largest isolated single cell is the egg of an ostrich.
- Human red blood cells are about $7 \:\mu\text{m}$ in diameter.
- Nerve cells are among the longest cells in animals.
- Shape: Cells can be spherical (e.g., red blood cells), elongated (e.g., nerve cells), branched (e.g., nerve cells), discoidal, cuboidal, columnar, spindle-shaped, or irregular (e.g., Amoeba, white blood cells). The shape of a cell is often related to its specific function.
Prokaryotic Cells
Prokaryotic cells are fundamentally simpler in organisation than eukaryotic cells. They are represented by bacteria, blue-green algae (cyanobacteria), mycoplasma, and PPLOs (Pleuro Pneumonia Like Organisms).
General characteristics of prokaryotic cells:
- They are generally smaller than eukaryotic cells.
- They multiply more rapidly than eukaryotic cells.
- They lack a well-defined nucleus; the genetic material (usually a single, circular chromosome) is located in a region called the nucleoid.
- They lack membrane-bound organelles such as mitochondria, chloroplasts, Golgi apparatus, endoplasmic reticulum, lysosomes, and vacuoles.
- They have ribosomes, but they are 70S type (smaller than the 80S type found in eukaryotic cytoplasm).
- Some prokaryotes have plasmids, which are small, circular DNA molecules separate from the main chromosome, conferring unique traits (e.g., antibiotic resistance).
- Motile prokaryotes may have flagella.
*(Image shows a typical prokaryotic cell with features like cell wall, plasma membrane, cytoplasm, nucleoid, plasmids, ribosomes, flagellum, pilus, capsule/glycocalyx, inclusion bodies)*
Cell Envelope and its Modifications
Most prokaryotic cells, particularly bacterial cells, have a complex chemical cell envelope. This envelope is tightly bound and consists of three layers:
- The outermost glycocalyx
- The middle cell wall
- The innermost plasma membrane
Although each layer performs a distinct function, they act together as a single protective unit.
Glycocalyx
This is the outermost layer in many prokaryotic cells. Its composition varies among bacteria.
- It could be a loose sheath called the slime layer.
- Or it could be thick and tough, called the capsule.
Functions: Provides protection against desiccation (drying), helps in adhesion (to surfaces or to other cells), and in some cases, provides protection against phagocytosis by host cells.
Cell Wall
Located just inside the glycocalyx (if present), the cell wall is a rigid layer.
Functions:
- Provides structural support to the cell.
- Determines the shape of the cell.
- Protects the cell from mechanical stress and bursting if placed in a hypotonic solution (prevents osmolysis).
The chemical composition of the cell wall varies. In bacteria, it is commonly made of peptidoglycan (also called murein).
Plasma Membrane
This is the innermost layer of the cell envelope. It is a selectively permeable membrane made up of lipids and proteins.
Functions:
- Regulates the transport of substances into and out of the cell.
- It is metabolically active and involved in processes like cellular respiration (analogous to mitochondria in eukaryotes) and secretion.
Gram Staining
Based on the differences in the cell envelopes, particularly the cell wall composition, bacteria can be classified into two groups by the Gram staining procedure (developed by Hans Christian Gram):
- Gram positive bacteria: These bacteria take up the crystal violet stain and appear purple. They generally have a thick peptidoglycan layer and no outer membrane.
- Gram negative bacteria: These bacteria do not retain the crystal violet stain (it is washed away by alcohol) but are counterstained by safranin and appear pink or red. They have a thinner peptidoglycan layer and an additional outer membrane outside the peptidoglycan.
Modifications of Plasma Membrane
The plasma membrane in prokaryotes is not just a simple boundary; it undergoes certain modifications:
- Mesosomes: These are infoldings of the plasma membrane into the cell in the form of vesicles, tubules, or lamellae.
Functions of Mesosomes:
- Help in cell wall formation.
- Help in DNA replication and distribution to daughter cells.
- Help in respiration and secretion processes.
- Increase the surface area of the plasma membrane and enzymatic content.
- Chromatophores: In some prokaryotes like cyanobacteria (blue-green algae), the plasma membrane extends to form chromatophores. These are membranous extensions containing pigments like chlorophyll.
Function of Chromatophores: Involved in photosynthesis.
Ribosomes and Inclusion Bodies
Ribosomes
Ribosomes are the sites of protein synthesis. In prokaryotic cells:
- Ribosomes are 70S type. The 'S' stands for Svedberg's unit, which represents the sedimentation coefficient and indirectly indicates density and size.
- Each 70S ribosome is composed of two subunits: a large 50S subunit and a small 30S subunit. ($50S + 30S \neq 70S$ because Svedberg units are not additive).
- Ribosomes are found freely scattered in the cytoplasm.
- Several ribosomes may attach to a single mRNA molecule to form a chain called a polysome or polyribosome. The ribosomes of a polysome translate the mRNA into proteins.
Inclusion Bodies
Prokaryotic cells have no membrane-bound organelles. Reserve material in prokaryotic cells is stored in the cytoplasm in the form of inclusion bodies. These are not bound by any membrane system and lie freely in the cytoplasm.
Examples of inclusion bodies:
- Phosphate granules: Store phosphates, essential for nucleic acid synthesis and ATP.
- Cyanophycean granules: Store reserve proteins in cyanobacteria.
- Glycogen granules: Store carbohydrates (polysaccharide).
- Gas vacuoles: Found in some planktonic bacteria and cyanobacteria. They are hollow, gaseous spaces enclosed by a protein membrane (not lipid membrane).
Function of Gas Vacuoles: Provide buoyancy to the cell, allowing them to float in water bodies and regulate their position for optimal light exposure for photosynthesis.
Other Structures (Optional but Common)
- Flagella: Some motile bacteria have flagella, which are thin filamentous extensions from the cell wall. They help in motility. Bacterial flagellum is composed of three parts: filament, hook, and basal body.
- Pili and Fimbriae: These are surface structures (not involved in motility). Pili are elongated tubular structures made of a special protein (pilin) involved in bacterial conjugation (transfer of DNA). Fimbriae are small bristle-like fibres arising from the cell surface, helping bacteria to attach to rocks, host tissues, etc.
Eukaryotic Cells
Eukaryotic cells are characterized by the presence of a well-defined nucleus and membrane-bound organelles. They are generally larger and more complex than prokaryotic cells.
Eukaryotic cells are found in all protists, plants, fungi, and animals.
Key Differences Between Eukaryotic and Prokaryotic Cells
| Feature | Prokaryotic Cell | Eukaryotic Cell |
|---|---|---|
| Nucleus | Absent (Genetic material in nucleoid) | Present (Genetic material enclosed by nuclear membrane) |
| Membrane-bound Organelles | Absent (e.g., Mitochondria, ER, Golgi, Lysosomes, Vacuoles) | Present |
| Genetic Material | Usually single, circular chromosome; may have plasmids | Usually multiple, linear chromosomes; located in nucleus |
| Ribosomes | 70S type (50S + 30S) | 80S type (60S + 40S) in cytoplasm and on ER; 70S in mitochondria and chloroplasts |
| Cell Wall | Present in bacteria (peptidoglycan), absent in Mycoplasma. | Present in plant cells (cellulose), fungi (chitin), some protists; Absent in animal cells. |
| Size | Generally $0.1 - 5 \:\mu\text{m}$ | Generally $5 - 100 \:\mu\text{m}$ |
| Respiration | Plasma membrane/Mesosomes | Mitochondria |
| Cytoskeleton | Generally absent or rudimentary | Present (Microtubules, Microfilaments, Intermediate filaments) |
Differences Between Plant and Animal Cells
| Feature | Plant Cell | Animal Cell |
|---|---|---|
| Cell Wall | Present (made of cellulose) | Absent |
| Plastids (e.g., Chloroplasts) | Generally present (especially in photosynthetic parts) | Absent |
| Vacuoles | Usually a large central vacuole, occupying up to 90% of cell volume | Usually smaller, temporary vacuoles, or absent |
| Centrioles | Generally absent (except in some lower plant forms) | Present (involved in cell division) |
| Shape | Usually fixed, rectangular shape due to cell wall | Usually irregular or round shape |
| Storage Material | Starch | Glycogen and fat globules |
Cell Membrane
The cell membrane, also known as the plasma membrane, is the boundary of the cell. It is a living, dynamic structure.
Structure of Cell Membrane
The most widely accepted model for the structure of the cell membrane is the Fluid Mosaic Model, proposed by Singer and Nicolson in 1972.
According to this model:
- The membrane is a quasifluid mosaic of lipids and proteins.
- The main components are lipids and proteins. Carbohydrates are also present.
- The lipids are arranged in a bilayer. The most common lipids are phospholipids, which have a hydrophilic (water-attracting) head and two hydrophobic (water-repelling) tails.
- The hydrophobic tails face inwards, away from the aqueous environment.
- The hydrophilic heads face outwards towards the aqueous environment (both outside the cell and inside the cell in the cytoplasm).
- The fluid nature of the lipid bilayer allows for movement of proteins laterally within the membrane. This fluidity is important for many membrane functions.
- Proteins are also present in the membrane. Based on their location, they can be:
- Integral proteins: These are partially or totally buried in the lipid bilayer. Some span the entire membrane (transmembrane proteins).
- Peripheral proteins: These lie on the surface of the membrane or are partially embedded. They are not deeply inserted into the lipid bilayer.
- The ratio of lipids and proteins varies in different cell types. For example, in human erythrocyte (red blood cell) membrane, protein is about 52% and lipid about 40%.
*(Image shows the Fluid Mosaic Model with phospholipid bilayer, integral proteins, peripheral proteins, cholesterol, glycoproteins, glycolipids)*
Functions of Cell Membrane
- Selective Permeability: It regulates the passage of substances into and out of the cell. Some substances can pass through freely, some require transport proteins, and some cannot pass at all.
- Transport:
- Passive Transport: Movement of substances across the membrane without the expenditure of energy. This occurs down the concentration gradient (from a region of higher concentration to lower concentration). Examples: Simple diffusion (gases), Facilitated diffusion (using transport proteins), Osmosis (movement of water across a semipermeable membrane).
- Active Transport: Movement of substances across the membrane against the concentration gradient (from a region of lower concentration to higher concentration). This process requires energy (usually in the form of ATP) and specific membrane proteins (pumps). Example: Sodium-potassium pump.
- Endocytosis and Exocytosis:
- Endocytosis: The process by which the cell engulfs materials from the outside environment by forming vesicles from the plasma membrane. Example: Phagocytosis ("cell eating" - taking in solid particles) and Pinocytosis ("cell drinking" - taking in liquids).
- Exocytosis: The process by which materials (like waste products or secreted substances) are released from the cell by the fusion of vesicles with the plasma membrane.
- Cell recognition (glycoproteins and glycolipids on the outer surface).
- Cell adhesion.
- Transmission of signals (receptor proteins).
Cell Wall
The cell wall is a rigid outer layer found in plant cells, fungal cells, and some protists. It is absent in animal cells.
Composition and Layers
- In plant cells, the cell wall is primarily made of cellulose, hemicellulose, pectins, and proteins.
- In fungi, the cell wall is composed mainly of chitin.
- In algae, the cell wall is made of cellulose, galactans, mannans, and minerals like calcium carbonate.
In plant cells, the cell wall typically consists of:
- Primary wall: This is the outermost layer, formed during cell growth. It is relatively thin and flexible, allowing for cell expansion.
- Secondary wall: In some plant cells, as the cell matures, a secondary wall is laid down *inside* the primary wall. This layer is thicker, stronger, and more rigid (often contains lignin), providing mechanical strength.
- Middle lamella: This is a sticky layer primarily made of calcium pectate and magnesium pectate. It cements the neighbouring cells together.
*(Image shows adjacent plant cells highlighting cell wall layers and plasmodesmata)*
Functions of Cell Wall
- Provides shape to the cell.
- Protects the cell from mechanical damage and infection.
- Provides a barrier to undesirable macromolecules.
- Prevents the cell from bursting when placed in a hypotonic medium by withstanding the internal turgor pressure.
- Helps in cell-to-cell interaction (via plasmodesmata in plants).
Note: Plasmodesmata are cytoplasmic connections passing through the cell walls and middle lamella that connect the cytoplasm of adjacent plant cells.
Endomembrane System
Within a eukaryotic cell, there are many membrane-bound organelles. However, some organelles have functions that are coordinated with each other. These organelles are considered part of the endomembrane system.
Components of the Endomembrane System
The endomembrane system includes:
- Endoplasmic Reticulum (ER)
- Golgi apparatus
- Lysosomes
- Vacuoles
These organelles work together in the synthesis, modification, packaging, and transport of proteins and lipids.
Mitochondria, chloroplasts, and peroxisomes are *not* considered part of the endomembrane system because their functions are not coordinated with the above components, even though they are also membrane-bound.
The Endoplasmic Reticulum (ER)
The Endoplasmic Reticulum is a network of tiny tubular structures scattered in the cytoplasm. It extends from the nuclear envelope to the plasma membrane, forming a complex network.
Structure
The ER appears as a network of sacs and tubules. It forms a vast interconnected network of membranous channels and sacs called cisternae, tubules, and vesicles.
Types of ER
Based on the presence or absence of ribosomes on its surface, the ER is divided into two types:
- Rough Endoplasmic Reticulum (RER): It has ribosomes attached to its outer surface. RER is extensive and continuous with the outer membrane of the nucleus.
Functions of RER:
- Involved in protein synthesis (because of ribosomes).
- Involved in the folding and modification of proteins that are destined for secretion or insertion into membranes.
- Smooth Endoplasmic Reticulum (SER): It lacks ribosomes on its surface. It appears as tubules.
Functions of SER:
- Involved in the synthesis of lipids, including steroids and phospholipids.
- Involved in the detoxification of drugs and poisons in liver cells.
- In muscle cells, SER (called Sarcoplasmic Reticulum) stores calcium ions ($Ca^{2+}$) essential for muscle contraction.
- Metabolism of carbohydrates.
*(Image shows the nucleus with nuclear envelope, RER with ribosomes, and SER without ribosomes)*
Golgi Apparatus
The Golgi apparatus (also called Golgi complex or Golgi body) was first observed by Camillo Golgi in 1898.
Structure
It consists of flattened, disc-shaped sacs or cisternae stacked parallel to each other. These cisternae are about $0.5 \:\text{to} \: 1.0 \:\mu\text{m}$ in diameter. A stack of cisternae forms a Golgi body.
The Golgi cisternae are characteristically arranged with a convex cis or the forming face and a concave trans or the maturing face. The cis and trans faces are distinct but interconnected.
The cis face receives transport vesicles from the ER, and the trans face releases vesicles containing processed materials.
*(Image shows stack of Golgi cisternae, incoming vesicles from ER at cis face, and outgoing vesicles at trans face)*
Functions
The Golgi apparatus primarily functions as a packaging and dispatching station for materials synthesized in the ER.
- Modification and Packaging: Proteins and lipids synthesized in the ER are transported to the Golgi apparatus, where they are modified (e.g., glycosylation - addition of carbohydrates) and packaged into vesicles.
- Sorting and Transport: It sorts and directs the modified proteins and lipids to their correct destinations (e.g., secretion outside the cell, insertion into membranes, or delivery to other organelles like lysosomes).
- Formation of Lysosomes: Lysosomes are formed by budding off from the trans face of the Golgi apparatus.
- Formation of Glycoproteins and Glycolipids: The Golgi is the site for the synthesis of glycoproteins and glycolipids.
- In plant cells, the Golgi apparatus (often called dictyosomes) is involved in the synthesis of cell wall components like cellulose, hemicellulose, and pectins.
Lysosomes
Lysosomes are membrane-bound vesicular structures formed by the process of packaging in the Golgi apparatus. They contain hydrolytic enzymes (hydrolases, lipases, proteases, carbohydrases) that are active at acidic pH.
Structure and Function
- They are single membrane-bound sacs.
- The enzymes within lysosomes are capable of digesting lipids, carbohydrates, proteins, and nucleic acids.
- The lumen (interior) of lysosomes has an acidic pH (around 5.0), which is optimal for the activity of their hydrolytic enzymes.
Functions of Lysosomes:
- Intracellular digestion: They digest food particles taken into the cell through endocytosis (e.g., in Amoeba or white blood cells).
- Autophagy: They digest damaged or old organelles within the cell (self-eating). This process helps in cellular housekeeping and recycling.
- Autolysis: When a cell is severely damaged or dies, lysosomes may burst and release their enzymes, which digest the cell itself. Because of this, lysosomes are often called "suicidal bags" of the cell.
- Digestion of invading pathogens (like bacteria or viruses) in immune cells.
*(Image shows Golgi apparatus budding off a lysosome, and the lysosome fusing with a phagosome or old organelle for digestion)*
Vacuoles
Vacuoles are membrane-bound sacs found in the cytoplasm. They contain water, sap, excretory products, and other materials not useful to the cell.
Structure and Function
- Vacuoles are enclosed by a single membrane called the tonoplast.
- In plant cells, vacuoles are very prominent and can occupy up to 90% of the volume of the cell. The tonoplast in plant cells facilitates the transport of ions and other materials against the concentration gradient into the vacuole, thus its concentration is significantly higher in the vacuole than in the cytoplasm. This helps in maintaining turgor pressure.
- In animal cells, vacuoles are typically smaller and temporary.
- In Amoeba, there is a contractile vacuole, which is important for excretion and osmoregulation (removing excess water).
- In some protists, food vacuoles are formed by engulfing food particles.
Functions of Vacuoles:
- Storage of water, ions, nutrients, waste products, and pigments.
- Maintenance of turgor pressure in plant cells (keeps the plant rigid).
- Excretion of waste products (in some protists).
- Osmoregulation (in some protists).
- Providing support to the cell (in plants due to turgor).
*(Image shows a plant cell with large central vacuole and an animal cell with smaller vacuoles or none)*
Mitochondria
Mitochondria are membrane-bound organelles often called the "powerhouses of the cell" because they are the primary sites of aerobic respiration and ATP production (energy currency of the cell).
Structure
Mitochondria are typically cylindrical or sausage-shaped, about $0.2 \:\text{to} \: 1.0 \:\mu\text{m}$ in diameter and $1.0 \:\text{to} \: 4.1 \:\mu\text{m}$ in length. The number of mitochondria per cell varies depending on the physiological activity of the cell (e.g., liver cells have many mitochondria).
A mitochondrion is a double membrane-bound structure:
- Outer membrane: Smooth, freely permeable to small molecules.
- Inner membrane: Highly folded into finger-like or lamellae-like projections called cristae. The cristae increase the surface area for enzymatic activity. The inner membrane is selectively permeable.
The space between the outer and inner membrane is called the intermembrane space.
The space enclosed by the inner membrane is called the mitochondrial matrix. The matrix contains a single circular DNA molecule, a few RNA molecules, ribosomes (70S type), and enzymes required for aerobic respiration.
*(Image shows a cross-section of a mitochondrion highlighting outer membrane, inner membrane folded into cristae, and matrix containing ribosomes and circular DNA)*
Functions
- ATP Production: Mitochondria are the sites of aerobic respiration, where glucose and other food molecules are broken down in the presence of oxygen to produce ATP through oxidative phosphorylation. This ATP provides energy for most cellular activities.
- Storage of calcium ions (in some cell types).
- Involved in apoptosis (programmed cell death).
Semi-autonomous Nature
Mitochondria are considered semi-autonomous organelles because they have their own genetic material (circular DNA) and ribosomes (70S). This allows them to synthesize some of their own proteins independently of the nuclear DNA and cytoplasmic ribosomes, although most mitochondrial proteins are encoded by nuclear DNA and imported from the cytoplasm.
Plastids
Plastids are large membrane-bound organelles found in plant cells and euglenoids. They are absent in animal cells.
Types of Plastids
Based on the type of pigments present, plastids are classified into:
- Chloroplasts: Contain chlorophyll and carotenoid pigments. Responsible for capturing light energy for photosynthesis. They are typically green in colour. Found in the green parts of plants (leaves, stems).
- Chromoplasts: Contain carotenoid pigments like carotenes and xanthophylls. These pigments impart yellow, orange, or red colours to parts of the plant, such as flowers, fruits, and some roots (e.g., carrot root). Chlorophyll is absent.
- Leucoplasts: These are colourless plastids. They are involved in the storage of different types of nutrients.
- Amyloplasts: Store carbohydrates (starch). Found in potato tubers.
- Elaioplasts: Store oils and fats.
- Aleuroplasts (or Proteinoplasts): Store proteins.
Chloroplasts can develop into chromoplasts (e.g., ripening of tomatoes), and plastids can interconvert between forms depending on the needs of the cell.
Structure of Chloroplasts
Chloroplasts are typically lens-shaped, oval, or spherical, about $5 \:\text{to} \: 10 \:\mu\text{m}$ in length and $2 \:\text{to} \: 4 \:\mu\text{m}$ in width. Like mitochondria, they are double membrane-bound structures.
- Outer membrane: Permeable to small molecules.
- Inner membrane: Less permeable, enclosing the stroma.
Inside the inner membrane is the fluid-filled space called the stroma. The stroma contains the chloroplast's DNA (circular), 70S ribosomes, and enzymes required for the synthesis of carbohydrates and proteins (like those for the Calvin cycle).
Suspended in the stroma is a system of flattened membranous sacs called thylakoids. Thylakoids are typically stacked like piles of coins, forming structures called grana (singular: granum). Thylakoids in different grana are connected by flat membranous tubules called stromal lamellae.
The space inside a thylakoid is called the lumen. The thylakoid membranes contain chlorophyll and other pigments.
*(Image shows a cross-section of a chloroplast highlighting outer membrane, inner membrane, stroma, stacks of thylakoids forming grana, and connecting stromal lamellae)*
Function of Chloroplasts
Chloroplasts are the sites of photosynthesis. This process involves two main stages:
- Light-dependent reactions: Occur in the thylakoid membranes (grana), where light energy is captured and converted into chemical energy (ATP and NADPH).
- Light-independent reactions (Calvin cycle): Occur in the stroma, where the chemical energy from the light reactions is used to fix carbon dioxide and synthesize carbohydrates (sugars).
Semi-autonomous Nature
Similar to mitochondria, chloroplasts are also semi-autonomous organelles as they possess their own circular DNA and 70S ribosomes, enabling them to synthesize some of their own proteins.
Ribosomes
Ribosomes are responsible for protein synthesis. They are not membrane-bound organelles and are found in both prokaryotic and eukaryotic cells, as well as within mitochondria and chloroplasts.
Structure
Ribosomes are composed of ribonucleic acid (rRNA) and proteins. Each ribosome consists of two subunits: a large subunit and a small subunit. These subunits join together only when they are translating mRNA into protein.
Types of Ribosomes
There are two main types of ribosomes, classified by their sedimentation coefficient (Svedberg unit):
- 70S Ribosomes: Found in prokaryotes (cytoplasm), mitochondria, and chloroplasts (in eukaryotes). Composed of a 50S large subunit and a 30S small subunit.
- 80S Ribosomes: Found in the cytoplasm of eukaryotes. Composed of a 60S large subunit and a 40S small subunit.
Note: The Svedberg units are not directly additive because they depend on the size, shape, and density of the particle.
Location in Eukaryotic Cells
In eukaryotic cells, 80S ribosomes are found:
- Freely in the cytoplasm.
- Attached to the outer surface of the Rough Endoplasmic Reticulum (RER).
- Attached to the outer membrane of the nuclear envelope.
Additionally, 70S ribosomes are found within mitochondria and chloroplasts.
Function
The primary function of ribosomes is protein synthesis (translation). They read the genetic code on mRNA and assemble amino acids into polypeptide chains (proteins).
As mentioned earlier, multiple ribosomes can attach to a single mRNA molecule to form a polysome or polyribosome, simultaneously translating the same mRNA into multiple copies of the same protein.
Cytoskeleton
The cytoskeleton is an intricate network of protein filaments present in the cytoplasm of eukaryotic cells. It provides mechanical support, maintains cell shape, and is involved in various forms of cell movement.
Components
The cytoskeleton is made up of three main types of protein filaments:
- Microtubules: Hollow tubes made of the protein tubulin. They are the largest components of the cytoskeleton.
Functions: Maintain cell shape, provide tracks for organelle movement (e.g., vesicles transported by motor proteins), form spindle fibres during cell division, main components of cilia and flagella.
- Microfilaments (Actin Filaments): Solid rods made of the protein actin. They are the smallest components.
Functions: Muscle contraction, cell motility (amoeboid movement), changes in cell shape, cytokinesis (cell division), formation of microvilli.
- Intermediate Filaments: Fibrous proteins that are roped together. They are intermediate in size between microtubules and microfilaments. Composed of various proteins (e.g., keratin, vimentin).
Functions: Maintain cell shape, provide mechanical strength, anchor organelles (like the nucleus) in place.
*(Image shows simplified representations of microtubules, microfilaments, and intermediate filaments forming a network within the cytoplasm)*
Functions of Cytoskeleton
- Provides mechanical support to the cell.
- Helps maintain the shape of the cell.
- Involved in various types of cellular movement (e.g., cell crawling, muscle contraction, organelle transport).
- Helps in the organization of organelles within the cytoplasm.
- Forms the spindle apparatus during cell division.
Cilia and Flagella
Cilia and flagella are hair-like outgrowths of the cell membrane found in some eukaryotic cells. They are primarily involved in motility.
Comparison
- Cilia: Short, numerous, hair-like projections. They work like oars, causing the cell to move or moving the surrounding fluid/particles.
- Flagella: Longer, fewer (often only one or two per cell), whip-like structures. They propel the cell through the surrounding medium.
Despite their differences in size and number, both cilia and flagella have a remarkably similar internal structure.
Structure (Axoneme)
The core of a cilium or flagellum is called the axoneme. The axoneme is covered by the plasma membrane.
A typical eukaryotic axoneme shows a "$9+2$ array" of microtubules:
- There are nine pairs of radially arranged peripheral doublet microtubules.
- There are two single central microtubules.
The central tubules are connected by bridges and are also enclosed by a central sheath. One of the tubules of each peripheral doublet is also connected to the central sheath by a radial spoke.
Both the cilium and flagellum emerge from structures called basal bodies, which are similar in structure to centrioles.
*(Image shows a cross-section of a cilium/flagellum with the 9 peripheral doublets, 2 central singlets, radial spokes, central sheath, and nexin links)*
Functions
- Cell Motility: Enables single cells (e.g., sperm, some protozoa) to swim.
- Movement of Surroundings: Cilia lining the respiratory tract move mucus and trapped particles away from the lungs; cilia in the fallopian tubes help move the egg towards the uterus.
Centrosome and Centrioles
The centrosome is an organelle usually containing two cylindrical structures called centrioles. It is involved in cell division in animal cells.
Location and Structure
- The centrosome is typically located near the nucleus.
- Centrosomes are usually absent in plant cells, except in some lower forms.
- The centrosome matrix (amorphous pericentriolar material) surrounds the two centrioles.
- The two centrioles in a centrosome lie perpendicular to each other.
- Each centriole is made up of nine evenly spaced peripheral fibrils of tubulin protein. Each of the peripheral fibrils is a triplet (bundle of three microtubules). The triplets are linked together.
- The centre of the centriole is proteinaceous and called the hub, which is connected to the peripheral triplets by radial spokes. This arrangement of microtubules is referred to as the "$9+0$ array" (9 peripheral triplets, 0 central tubules).
*(Image shows a centrosome with two perpendicular centrioles and a cross-section of a centriole highlighting the 9 peripheral triplets and central hub)*
Functions
- Cell Division: Centrioles help organize the spindle fibres during animal cell division. They move to opposite poles of the cell and form the mitotic or meiotic spindle apparatus to separate chromosomes.
- Basal Body Formation: Centrioles form the basal body of cilia and flagella. The basal body gives rise to the axoneme structure ($9+2$).
Nucleus
The nucleus is a large, central, and prominent organelle in eukaryotic cells. It is often called the "control centre" of the cell because it contains the cell's genetic material (DNA) and controls most cellular activities.
Structure
The nucleus is typically spherical and bounded by a double membrane called the nuclear envelope.
- Nuclear Envelope: A double membrane separating the nucleus from the cytoplasm. It is punctuated by numerous nuclear pores, which are complex channels that regulate the passage of molecules (like mRNA and proteins) between the nucleus and the cytoplasm. The outer membrane is often continuous with the ER.
- Nucleoplasm (Karyoplasm): The fluid-filled space inside the nuclear envelope. It contains the chromatin and the nucleolus.
- Chromatin: A network of threads made of DNA and associated proteins (mainly histones). During cell division, the chromatin condenses to form visible chromosomes.
- Nucleolus: A dense, spherical structure located within the nucleoplasm. It is the primary site of ribosomal RNA (rRNA) synthesis and ribosome assembly. Cells actively involved in protein synthesis usually have larger and more numerous nucleoli. It is not membrane-bound.
*(Image shows a nucleus highlighting nuclear envelope with pores, nucleoplasm, chromatin threads, and nucleolus)*
Chromosomes
Chromosomes are condensed forms of chromatin visible during cell division. Each chromosome contains a single, long molecule of DNA coiled around proteins.
- Every eukaryotic species has a characteristic number of chromosomes in its somatic cells (e.g., human somatic cells have 46 chromosomes, or 23 pairs).
- Each chromosome consists of a primary constriction called the centromere, which holds together two sister chromatids (after DNA replication) during cell division.
- The centromere has a disc-shaped structure called the kinetochore, which serves as the attachment site for spindle fibres during cell division.
- Based on the position of the centromere, chromosomes are classified into four types:
- Metacentric: Centromere is in the middle, resulting in two equal arms.
- Sub-metacentric: Centromere is slightly away from the middle, resulting in one shorter arm and one longer arm.
- Acrocentric: Centromere is close to the end, resulting in one very short arm and one very long arm.
- Telocentric: Centromere is at the terminal end (not found in humans).
- Some chromosomes have non-staining secondary constrictions at a constant location, which gives the appearance of a small fragment called a satellite.
*(Image shows diagrams of metacentric, sub-metacentric, acrocentric, and telocentric chromosomes)*
Functions of Nucleus
- Contains the cell's genetic information (DNA) in the form of chromatin/chromosomes.
- Controls and regulates cell growth and reproduction.
- Governs the synthesis of proteins and enzymes through the expression of genes.
- Site of DNA replication and transcription (synthesis of RNA).
- The nucleolus is the site of rRNA synthesis and ribosome assembly.
Microbodies
Microbodies are small, membrane-bound organelles that contain various enzymes and are involved in diverse metabolic reactions.
Types and Functions
Two important types of microbodies are:
- Peroxisomes:
- Single membrane-bound vesicles.
- Contain enzymes like catalase and peroxidase, which are involved in breaking down hydrogen peroxide ($H_2O_2$), a toxic byproduct of metabolism, into water and oxygen.
- Involved in the breakdown of fatty acids (beta-oxidation).
- In plants, involved in photorespiration.
- Glyoxysomes:
- Found exclusively in plant cells (especially in the endosperm of germinating seeds) and fungi.
- Single membrane-bound.
- Contain enzymes of the glyoxylate cycle.
- Involved in converting stored fats into carbohydrates, which is essential for the growth of the seedling before it can perform photosynthesis.
Microbodies are thought to bud from the endoplasmic reticulum and Golgi apparatus.