Discovery Of The Cell

The discovery of the cell is attributed to Robert Hooke in 1665. While examining a thin slice of cork (which is part of the bark of a tree) under a microscope he designed, Hooke observed a structure that resembled a honeycomb. It was made up of many small, distinct compartments or boxes.

Hooke named these boxes "cells," derived from the Latin word 'cella', meaning 'a little room'. Although Hooke was observing the dead cell walls of plant tissue, his observation was groundbreaking because it was the first time anyone had seen that living matter appeared to consist of discrete, separate units.

This initial discovery paved the way for further investigations. Later, in 1674, Leeuwenhoek, using an improved microscope, was the first to observe free-living cells, such as those found in pond water. In 1831, Robert Brown discovered the nucleus within the cell. The term 'protoplasm' for the fluid substance of the cell was coined by Purkinje in 1839.

The concept that all plants and animals are composed of cells and that the cell is the fundamental unit of life was formalised as the **Cell Theory** by two biologists, **Schleiden** (1838) and **Schwann** (1839). Rudolf Virchow (1855) further expanded this theory by stating that **all cells arise from pre-existing cells** (Omnis cellula e cellula).

The invention of the electron microscope around 1940 allowed scientists to see the complex internal structure of the cell and its various components, known as organelles.

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What are Living Organisms Made Up of?

Simple experiments, like preparing a temporary mount of onion peel and observing it under a microscope, reveal that living organisms, like the onion bulb, are made up of these small structural units called cells. Regardless of the size of the onion, its peel consists of similar small, box-like structures. These structures are the basic building units of the onion bulb.

This holds true for all organisms we observe. However, not all organisms are multicellular; some are composed of just a single cell that lives independently. Examples of **unicellular organisms** include Amoeba, Paramecium, Chlamydomonas, and bacteria.

**Multicellular organisms**, on the other hand, consist of many cells grouped together to form a single body. These cells specialise to perform different functions, forming various tissues and organs. Examples include many fungi, plants, and animals (like humans).

Every multicellular organism starts its life as a single cell. Growth and development occur through **cell division**, where existing cells divide to produce new cells. This process ensures that all cells originate from pre-existing cells, supporting Virchow's principle.

Observation of different cells from the human body (like nerve cells, blood cells, muscle cells, sperm, ovum, bone cells) shows that cells within a multicellular organism can have vastly different shapes and sizes. The shape and size of a cell are typically related to the specific function it performs (e.g., the elongated shape of a nerve cell for transmitting signals).

Despite their diversity in shape and function in multicellular organisms, each living cell performs certain basic functions essential for life. This capability arises from the presence of specific components within the cell called **cell organelles**. There is a division of labour within a single cell, with each organelle performing a specialized task (e.g., producing new materials, processing waste). The cell is able to live and function due to the collective activities of these organelles, which together constitute the basic unit of life.

Interestingly, despite the vast differences in cell types and the organisms they belong to, many fundamental cell organelles are conserved and found in almost all cells (especially eukaryotic cells).

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What Is A Cell Made Up Of? What is the Structural Organisation of a Cell?

Cells are highly organised structures containing specialized components called organelles. Observing cells under a microscope reveals three primary features common to almost all cells: the plasma membrane, the nucleus, and the cytoplasm. These structures are essential for the cell's activities and its interaction with the environment.

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Plasma Membrane Or Cell Membrane

The plasma membrane, also known as the cell membrane, is the outermost boundary of an animal cell. In plant cells, it lies just inside the cell wall. It is a thin, flexible membrane made primarily of **lipids and proteins**. Its structure is visible only with an electron microscope.

The plasma membrane plays a critical role in regulating the passage of substances into and out of the cell. It allows certain materials to enter or exit while preventing others. Due to this selective control over movement, it is called a **selectively permeable membrane** (or semi-permeable membrane).

Movement of substances across the plasma membrane can occur in several ways:

• **Diffusion:** Some substances, like carbon dioxide ($\text{CO}_2$) and oxygen ($\text{O}_2$), move across the membrane by diffusion. Diffusion is the spontaneous movement of a substance from a region of **high concentration** to a region of **low concentration**. When the concentration of $\text{CO}_2$ is high inside the cell (due to cellular respiration), it diffuses out into the external environment where the concentration is lower. Similarly, oxygen diffuses into the cell when its concentration is lower inside than outside.
• **Osmosis:** This is a special case of diffusion that involves the movement of **water molecules** across a selectively permeable membrane. Water moves from a region of **higher water concentration** (lower solute concentration) to a region of **lower water concentration** (higher solute concentration). The movement of water across the plasma membrane is affected by the amount of solutes dissolved in the surrounding solution.

What happens when a cell is placed in solutions of different concentrations?

• **Hypotonic Solution:** If the surrounding solution is **more dilute** than the cell's cytoplasm (higher water concentration outside), water will move **into** the cell by osmosis. More water enters than leaves, causing a net inflow. This can lead to the cell **swelling up** and potentially bursting (especially in animal cells).
• **Isotonic Solution:** If the surrounding solution has the **same water concentration** as the cell's cytoplasm, there is no net movement of water. Water moves in and out at equal rates. The cell retains its original **size and shape**.
• **Hypertonic Solution:** If the surrounding solution is **more concentrated** than the cell's cytoplasm (lower water concentration outside), water will move **out** of the cell by osmosis. More water leaves than enters, causing a net outflow. This leads to the cell **shrinking**.

Osmosis is crucial for processes like the absorption of water by plant roots and the maintenance of turgidity in plant cells. Unicellular freshwater organisms often face hypotonic environments and have mechanisms to deal with the water influx.

Beyond passive transport like diffusion and osmosis, cells can also move substances across the membrane against concentration gradients, requiring energy (active transport). The flexibility of the plasma membrane allows cells like Amoeba to engulf food and other materials from their environment. This process is called **endocytosis**.

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Cell Wall

Plant cells, in addition to the plasma membrane, have a rigid outer layer called the **cell wall**. This cell wall is located outside the plasma membrane. The primary component of the plant cell wall is **cellulose**, a complex carbohydrate that provides **structural strength** and rigidity to plants.

The cell wall is fully permeable to substances in solution, so it does not regulate the passage of materials like the plasma membrane does. However, it provides mechanical support and protection to the cell.

One key function of the cell wall is to protect the plant cell in hypotonic environments. When a plant cell is placed in a dilute solution, water enters by osmosis, causing the cell to swell. The cytoplasm contents press against the cell wall, creating **turgor pressure**. The rigid cell wall resists this pressure, preventing the cell from bursting (unlike animal cells which would lyse). The wall exerts an equal and opposite pressure against the swollen cell, which is called **wall pressure**. This turgidity is important for maintaining the plant's shape and structure.

When a living plant cell loses water in a hypertonic solution, the cell contents (including the plasma membrane) shrink away from the cell wall. This phenomenon is called **plasmolysis**. This only occurs in living cells; if the cells are killed (e.g., by boiling), the plasma membrane loses its selective permeability, and plasmolysis does not occur in the same way.

Cell walls are also present in the cells of fungi and bacteria, although their chemical composition differs from plant cell walls.

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Nucleus

The **nucleus** is a prominent, often spherical or oval, organelle usually located near the center of eukaryotic cells. It can be observed by staining cells with dyes like iodine, safranin, or methylene blue, which differentially colour parts of the cell, making the nucleus appear darker.

The nucleus is surrounded by a double-layered covering called the **nuclear membrane**. This membrane contains **pores** that regulate the passage of materials between the nucleus and the cytoplasm.

Inside the nucleus are thread-like structures called **chromatin material** in a non-dividing cell. When the cell is preparing to divide, the chromatin condenses and organises into distinct rod-shaped structures called **chromosomes**. Chromosomes are composed of **DNA (Deoxyribonucleic Acid)** and proteins. DNA molecules contain the genetic information necessary for cell construction, organisation, function, and reproduction. **Genes** are the functional segments of DNA.

The nucleus is vital for:

• **Cellular Reproduction:** It plays a central role in initiating and controlling cell division, ensuring that the genetic material is correctly passed to daughter cells.
• **Directing Chemical Activities:** The nucleus, through the information encoded in DNA, controls the synthesis of proteins and other molecules, thereby regulating all metabolic activities and determining the characteristics (development, form at maturity) of the cell and the organism.

Based on the presence or absence of a well-defined nucleus with a nuclear membrane, cells are classified into two types:

• **Prokaryotic cells:** These are simpler cells found in organisms like bacteria. They lack a well-defined nucleus; the genetic material (DNA) is present in an undefined region in the cytoplasm called the **nucleoid**. Prokaryotic cells also lack membrane-bound organelles. Their functions are carried out by poorly organised parts of the cytoplasm or associated with simple membrane structures (like photosynthetic pigments in some bacteria associated with vesicles).
• **Eukaryotic cells:** These are more complex cells found in plants, animals, fungi, and protists. They have a well-defined nucleus enclosed by a nuclear membrane and possess various membrane-bound organelles in the cytoplasm.

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Cytoplasm

The **cytoplasm** is the jelly-like fluid substance filling the cell, enclosed within the plasma membrane. In eukaryotic cells, it's the region between the nuclear membrane and the plasma membrane. It contains various specialized **cell organelles** suspended in the fluid. Each organelle performs a specific function necessary for the cell's life.

The key difference between prokaryotic and eukaryotic cells regarding cytoplasm is the presence of **membrane-bound organelles**. Eukaryotic cells have organelles like endoplasmic reticulum, Golgi apparatus, mitochondria, plastids, etc., each enclosed by its own membrane. Prokaryotic cells lack these membrane-bound organelles, though they do have ribosomes (which are not membrane-bound).

The presence of membranes is crucial for cellular function, especially in complex eukaryotic cells. Membranes help to keep different cellular activities separated within compartments (organelles), preventing interference. Viruses, for instance, lack any membrane structure and cannot carry out life processes independently; they can only function and multiply by invading host cells and utilizing the host's cellular machinery.

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Cell Organelles

Cell organelles are membrane-bound structures within the cytoplasm of eukaryotic cells that perform specific functions. They are essential for the complex activities required to maintain the cell's life and structure.

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Endoplasmic Reticulum (ER)

The **Endoplasmic Reticulum (ER)** is a vast network of interconnected membrane-bound tubes and sheets. It appears as tubules or flattened sacs called vesicles. The ER membrane structure is similar to that of the plasma membrane.

There are two types of ER:

• **Rough Endoplasmic Reticulum (RER):** It has a rough appearance under a microscope because it is studded with **ribosomes** on its surface. **Ribosomes** are the sites of **protein synthesis**. Proteins produced by ribosomes on the RER are then processed and transported within the ER to various destinations in or outside the cell.
• **Smooth Endoplasmic Reticulum (SER):** It lacks ribosomes and appears smooth. The SER is involved in the synthesis of **fat molecules (lipids)** important for cell function, including building cell membranes (a process called **membrane biogenesis**). SER also plays a role in detoxifying poisons and drugs in liver cells of vertebrates and synthesizes some enzymes and hormones (which can be protein or lipid based).

Functions of the ER:

• It serves as a network of **channels for transporting materials**, particularly proteins, between different regions of the cytoplasm or between the cytoplasm and the nucleus.
• It acts as a **cytoplasmic framework**, providing a surface for various biochemical reactions.
• SER is involved in lipid synthesis and detoxification.

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Golgi Apparatus

The **Golgi apparatus** (or Golgi complex/Golgi body), first described by Camillo Golgi, is an organelle composed of a series of flattened, membrane-bound sacs called **cisterns** arranged in parallel stacks. These cisterns are often connected to the ER membranes, forming part of the cell's complex membrane system.

Functions of the Golgi apparatus:

• It receives materials (like proteins and lipids) synthesized near the ER.
• It processes, **packages, and modifies** these materials.
• It then **dispatches** the packaged products in vesicles to various destinations within the cell or for secretion outside the cell.
• In some cases, it is involved in the synthesis of complex sugars from simple sugars.
• The Golgi apparatus is also involved in the formation of lysosomes.

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Lysosomes

Lysosomes are membrane-bound sacs containing powerful **digestive enzymes**. These enzymes are synthesised by the RER.

Lysosomes function as the **waste disposal system** of the cell. They digest and remove worn-out cell organelles, foreign materials (like bacteria or food particles) that enter the cell, and other cellular debris. They break down complex substances into simpler ones using their potent enzymes.

Under cellular stress or damage, lysosomes can burst, releasing their enzymes into the cytoplasm and digesting the cell itself. For this reason, lysosomes are sometimes referred to as the **"suicide bags"** of the cell.

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Mitochondria

Mitochondria are often called the **"powerhouses of the cell"** because they are responsible for generating most of the cell's supply of adenosine triphosphate (ATP), the main source of chemical energy for cellular activities.

Mitochondria have a unique structure with **two membranes**. The outer membrane is smooth and porous, while the inner membrane is extensively folded into finger-like projections called **cristae**. These folds significantly increase the surface area for the chemical reactions that produce ATP.

The energy released during respiration (breakdown of glucose) is used to synthesise ATP molecules. ATP is considered the **energy currency** of the cell; the energy stored in ATP is used for various cellular processes, including chemical synthesis (building new molecules) and mechanical work (like muscle contraction).

Mitochondria are also unique among organelles because they have their **own DNA and ribosomes**. This enables them to synthesize some of their own proteins, making them semi-autonomous organelles.

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Plastids

Plastids are large membrane-bound organelles found **only in plant cells** and some algae. There are different types of plastids, classified based on the pigments they contain:

• **Chromoplasts:** Coloured plastids (other than green), responsible for the colours of flowers, fruits, etc.
• **Leucoplasts:** White or colourless plastids primarily involved in **storage** of substances like starch, oils, and protein granules.
• **Chloroplasts:** A type of chromoplast that contains the green pigment **chlorophyll**. Chloroplasts are the sites of **photosynthesis**, the process by which plants convert light energy into chemical energy in the form of glucose. Chloroplasts also contain other pigments like yellow and orange.

Chloroplasts have an internal structure consisting of membrane layers embedded in a fluid matrix called the **stroma**. Like mitochondria, plastids also possess their **own DNA and ribosomes**, making them capable of synthesising some proteins and replicating independently.

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Vacuoles

Vacuoles are membrane-bound sacs that serve as **storage units** for various substances, including solid or liquid contents. The size and function of vacuoles differ significantly between plant and animal cells.

• In **animal cells**, vacuoles are generally **small** and may be involved in storage or waste removal.
• In **plant cells**, vacuoles are typically very **large**, and a single large central vacuole can occupy 50-90% of the cell's volume.

In plant cells, the large central vacuole is filled with **cell sap**, a solution containing sugars, amino acids, organic acids, proteins, and waste products. This cell sap contributes to the **turgidity and rigidity** of the plant cell by pushing against the cell wall (turgor pressure). Vacuoles also help in storing essential substances and disposing of waste.

In some unicellular organisms like Amoeba, **food vacuoles** are formed when the cell engulfs food particles through endocytosis. These vacuoles contain digestive enzymes to break down the food. Other specialised vacuoles in unicellular organisms, like contractile vacuoles in Paramecium, are important for expelling excess water and waste from the cell.

The specific organisation of membranes and organelles within a cell enables it to perform all its life functions, such as respiration, obtaining nutrients, removing waste, and synthesising new molecules. Thus, the cell is indeed the fundamental structural and functional unit of all living organisms.

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Cell Division

Living organisms grow, repair damaged tissues, replace old cells, and reproduce by forming new cells. The process by which new cells are produced from pre-existing cells is called **cell division**. There are two primary types of cell division: mitosis and meiosis.

• **Mitosis:** This type of cell division is responsible for the **growth and repair** of tissues in organisms. It is also involved in asexual reproduction in some organisms. In mitosis, a single parent cell (mother cell) divides to produce **two genetically identical daughter cells**. Each daughter cell has the **same number of chromosomes** as the mother cell. This ensures genetic continuity from one cell generation to the next for body cells (somatic cells).

• **Meiosis:** This specialized type of cell division occurs in specific cells of reproductive organs or tissues in sexually reproducing organisms (plants and animals). Its primary function is the formation of **gametes** (sex cells like sperm and egg cells), which are required for sexual reproduction. Meiosis involves **two consecutive divisions** of a single parent cell and results in the production of **four daughter cells**. Crucially, each daughter cell produced by meiosis has **half the number of chromosomes** compared to the parent cell. This reduction in chromosome number is essential so that when the male and female gametes fuse during fertilization, the resulting offspring has the correct (diploid) number of chromosomes, characteristic of the species.

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