What Are Living Organisms Made Up Of?

Observations of various living materials, such as onion peel or cells from the human body, under a microscope reveal that living organisms are made up of fundamental units called cells.

These cells, regardless of the size of the organism or the part from which they are taken (e.g., onion bulbs of different sizes), exhibit similar basic structural features when viewed under a microscope.

Organisms can be classified based on the number of cells they contain:

• Unicellular Organisms: These organisms consist of a single cell that carries out all necessary life functions. Examples include Amoeba, Paramoecium, Chlamydomonas, and bacteria.
• Multicellular Organisms: These organisms are composed of many cells. In multicellular organisms, cells are often organised into tissues, organs, and organ systems, with different cells performing specialised functions (division of labour). Examples include fungi, plants, and animals.

Remarkably, every multicellular organism originates from a single cell, which divides and multiplies to form the complex organism. This supports the principle that all cells arise from pre-existing cells.

Cells within a single organism can have different shapes and sizes, which are related to their specific functions. For example, nerve cells are elongated for transmitting messages, while muscle cells are specialised for contraction. Some cells, like Amoeba, can even change their shape.

Despite variations in shape, size, and function, every living cell has the capacity to perform certain basic life functions (e.g., respiration, obtaining nutrition, removing waste). This is possible because of specific components within the cell called cell organelles.

Different cell organelles perform distinct functions necessary for the cell's survival and activity, such as synthesising substances, processing waste, or generating energy. These organelles work together to make the cell the basic functional unit of life. It is significant that many fundamental organelles are found in almost all types of cells, regardless of the organism or cell type, highlighting the common organisational principles of life.

The cell is considered the structural unit of life because all living organisms are composed of cells. It is the functional unit of life because all essential life processes take place within the cell.

What Is A Cell Made Up Of? What Is The Structural Organisation Of A Cell?

Under a microscope, most cells reveal three fundamental features:

1. Plasma membrane (Cell membrane)
2. Nucleus
3. Cytoplasm

These three components facilitate all activities inside the cell and enable interactions between the cell and its external environment.

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Plasma Membrane Or Cell Membrane

The plasma membrane, also called the cell membrane, is the outermost boundary of animal cells. In plant cells, it is located just inside the cell wall.

It separates the internal contents of the cell from the external environment.

The plasma membrane is a selectively permeable membrane. This means it regulates the movement of substances, allowing some materials to enter or exit the cell while preventing others.

The movement of substances across the plasma membrane can occur through different mechanisms:

• Diffusion: The spontaneous movement of substances (like gases, e.g., CO₂ and O₂) from a region of high concentration to a region of low concentration. This is a passive process and does not require cellular energy. For example, if the concentration of CO₂ is high inside the cell (due to metabolic processes) and low outside, CO₂ will move out by diffusion. Similarly, if O₂ concentration is low inside (used in respiration) and high outside, O₂ will move in.
• Osmosis: A special type of diffusion involving the movement of water molecules across a selectively permeable membrane from a region of high water concentration (low solute concentration) to a region of low water concentration (high solute concentration).

The behaviour of a cell placed in a solution depends on the solution's water concentration relative to the cell's cytoplasm:

• Hypotonic Solution: The solution outside the cell has a higher water concentration (lower solute concentration) than the cell cytoplasm. Water enters the cell by osmosis. The cell tends to swell up.
• Isotonic Solution: The solution outside the cell has the same water concentration (equal solute concentration) as the cell cytoplasm. There is no net movement of water; water moves in and out at equal rates. The cell remains the same size.
• Hypertonic Solution: The solution outside the cell has a lower water concentration (higher solute concentration) than the cell cytoplasm. Water leaves the cell by osmosis. The cell tends to shrink (plasmolysis occurs in plant cells).

Osmosis is crucial for processes like the absorption of water by plant roots and for single-celled freshwater organisms maintaining water balance.

The plasma membrane is flexible, being made up of organic molecules called lipids and proteins. Its flexible nature allows the cell to engulf food and other materials from its external environment. This process is called endocytosis. Amoeba obtains its food using endocytosis.

The structure of the plasma membrane is complex and can only be observed clearly using an electron microscope.

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Cell Wall

In addition to the plasma membrane, plant cells, fungi, and bacteria have an extra rigid outer layer called the cell wall. The cell wall lies outside the plasma membrane.

In plants, the cell wall is primarily composed of cellulose, a complex carbohydrate that provides structural strength and rigidity to the plant.

The cell wall is fully permeable, allowing substances to pass through freely.

When a living plant cell loses water through osmosis, the cell's contents shrink away from the rigid cell wall. This phenomenon is known as plasmolysis. Plasmolysis occurs when a plant cell is placed in a hypertonic solution.

The presence of the cell wall is vital for plant cells (and others with cell walls) because it allows them to withstand dilute (hypotonic) external media without bursting. When a plant cell is in a hypotonic solution, it takes up water by osmosis and swells. The cell contents build up pressure against the cell wall (turgor pressure). The cell wall exerts an equal and opposite pressure, preventing the cell from bursting. This turgidity helps maintain the plant's upright structure.

This ability to tolerate large changes in external media gives plant cells an advantage over animal cells, which lack a cell wall and would burst in hypotonic solutions.

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Nucleus

The nucleus is a prominent, usually spherical or oval, organelle often located near the centre of eukaryotic cells. It is typically visible under a light microscope, especially when stained with dyes like iodine, safranin, or methylene blue, which stain different parts of the cell differentially based on their chemical composition.

Structure of the Nucleus:

• It is enclosed by a double-layered membrane called the nuclear membrane.
• The nuclear membrane contains pores (nuclear pores) that regulate the passage of substances between the nucleus and the cytoplasm.
• Inside the nucleus are thread-like structures called chromatin material when the cell is not dividing.
• Before cell division, the chromatin material condenses and organises into distinct, rod-shaped structures called chromosomes.
• Chromosomes are composed of DNA (Deoxyribonucleic Acid) and protein.
• DNA molecules contain the genetic information required for building, organising, and maintaining the cell.
• Functional segments of DNA are called genes. Genes carry the information for inheriting characteristics from parents to offspring.

Functions of the Nucleus:

• It plays a central role in cellular reproduction, controlling how a cell divides to form new cells.
• It directs the cell's chemical activities, influencing the cell's development, maturity, and overall form, in conjunction with environmental factors.

Prokaryotic vs. Eukaryotic Cells (based on Nuclear region):

• Prokaryotic Cells: Found in organisms like bacteria. They lack a well-defined nuclear region because there is no nuclear membrane. The genetic material (nucleic acids, usually a single circular DNA) is located in an undefined region of the cytoplasm called the nucleoid. Prokaryotic cells also generally lack membrane-bound organelles.
• Eukaryotic Cells: Found in organisms like plants, animals, fungi, and protists. They have a well-defined nucleus enclosed by a nuclear membrane. Eukaryotic cells also contain various membrane-bound organelles in the cytoplasm.

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Cytoplasm

The cytoplasm is the gel-like, fluid substance found within the plasma membrane but outside the nucleus in eukaryotic cells. In prokaryotic cells, it fills the entire region inside the cell wall and plasma membrane (as there is no nucleus).

The cytoplasm is largely composed of water, salts, and organic molecules.

It is the region where many important cellular activities take place.

The cytoplasm contains numerous specialised, often membrane-bound, structures called cell organelles. These organelles perform specific functions necessary for the cell's life.

In prokaryotes, membrane-bound organelles are absent, and many cellular functions that occur in organelles in eukaryotes are carried out by less organised parts of the cytoplasm or simple membrane structures (like vesicles associated with photosynthesis in some bacteria).

The presence of membranes enclosing organelles in eukaryotic cells allows for the compartmentalisation of different cellular activities, which is crucial for the efficiency and complexity of larger cells.

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Cell Organelles

Complex eukaryotic cells require intricate chemical activities, which are separated and organised into distinct compartments within the cytoplasm. These compartments are the cell organelles, and they are typically enclosed by membranes.

While prokaryotic cells lack membrane-bound organelles, eukaryotic cells possess a variety of them, each performing a specific function.

Some major cell organelles include the endoplasmic reticulum, Golgi apparatus, lysosomes, mitochondria, and plastids (in plant cells).

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Endoplasmic Reticulum (Er)

The Endoplasmic Reticulum (ER) is a large, interconnected network of membrane-bound tubes and flattened sacs (vesicles). Its membrane structure is similar to that of the plasma membrane.

There are two types of ER:

1. Rough Endoplasmic Reticulum (RER): Looks "rough" under a microscope because it has ribosomes attached to its surface. Ribosomes are the sites where proteins are synthesised in the cell. Proteins made on the RER are often destined for secretion outside the cell, insertion into membranes, or delivery to other organelles.
2. Smooth Endoplasmic Reticulum (SER): Appears "smooth" because it lacks attached ribosomes. The SER is involved in the manufacture of fat molecules (lipids), which are important for cell function. It also plays a role in detoxifying poisons and drugs in liver cells of vertebrates.

Functions of the ER:

• It acts as a channel for transport of materials, particularly proteins and lipids, between different regions of the cytoplasm or between the cytoplasm and the nucleus.
• The SER helps in the synthesis of lipids and proteins. Some of these synthesised lipids and proteins are used to build the cell membrane (a process called membrane biogenesis). Others function as enzymes or hormones.
• It provides a surface for some biochemical reactions.
• The SER is involved in detoxification in certain cells (like liver cells).

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Golgi Apparatus

The Golgi apparatus (also called Golgi complex or Golgi body) is a cell organelle consisting of a system of membrane-bound vesicles, specifically flattened sacs called cisterns, arranged in parallel stacks. It was first described by Camillo Golgi.

The membranes of the Golgi apparatus often connect with the membranes of the ER, suggesting a functional relationship.

Functions of the Golgi Apparatus:

• It is involved in the storage, modification, and packaging of substances synthesised in the ER. These products are then dispatched to various locations within or outside the cell in vesicles.
• It is involved in the formation of complex sugars from simple sugars.
• It is also involved in the formation of lysosomes.

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Lysosomes

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

Lysosomes function as the cell's waste disposal system.

Functions of Lysosomes:

• They help keep the cell clean by digesting and breaking down foreign materials (like bacteria or food particles) that enter the cell.
• They also digest worn-out or damaged cell organelles, recycling their components.
• During disturbances in cellular metabolism or when the cell is damaged, lysosomes may burst and release their enzymes. These enzymes then digest the cell itself. Because of this function, lysosomes are often referred to as the 'suicide bags' of the cell.

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Mitochondria

Mitochondria are known as the powerhouses of the cell because they are responsible for releasing the energy required for various life activities.

Structure of Mitochondria:

• They have two membrane coverings: an outer membrane which is porous, and an inner membrane which is deeply folded.
• The folds in the inner membrane are called cristae. These folds significantly increase the surface area available for energy-generating chemical reactions.

Functions of Mitochondria:

• They release energy from food molecules (like glucose) through cellular respiration.
• This energy is stored in the form of ATP (Adenosine Triphosphate) molecules. ATP is considered the energy currency of the cell.
• The cell uses the energy stored in ATP to perform mechanical work and synthesise new chemical compounds.

Interestingly, mitochondria are unique among organelles because they possess their own DNA and ribosomes. This allows them to synthesise some of their own proteins, suggesting an evolutionary origin separate from the rest of the cell (Endosymbiotic Theory).

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Plastids

Plastids are large membrane-bound organelles found only in plant cells and some algae. They are also semi-autonomous, possessing their own DNA and ribosomes like mitochondria.

There are two main types of plastids:

1. Chromoplasts: Coloured plastids (excluding green). They contain pigments that give colour to flowers and fruits.
2. Leucoplasts: White or colourless plastids. Their primary function is storage of materials like starch, oils, and protein granules.

Chloroplasts: These are a type of chromoplast that contains the green pigment chlorophyll. They are crucial for performing photosynthesis in plants, the process by which light energy is converted into chemical energy (food).

Chloroplasts have a complex internal structure with numerous membrane layers (grana, stacks of thylakoids) embedded in a fluid substance called the stroma.

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Vacuoles

Vacuoles are membrane-bound sacs that serve as storage areas for various substances, including solid or liquid contents.

Vacuoles differ significantly between animal and plant cells:

• In animal cells, vacuoles are typically small and fewer in number.
• In plant cells, there is often a single, very large central vacuole that can occupy 50% to 90% of the cell volume.

Functions of Vacuoles:

• In plant cells: The large central vacuole is filled with cell sap, which contains important substances like amino acids, sugars, organic acids, and proteins. The vacuole helps maintain the turgidity and rigidity of the plant cell by exerting pressure against the cell wall.
• In single-celled organisms (e.g., Amoeba): Food vacuoles store ingested food items. Some specialised vacuoles (contractile vacuoles) help expel excess water and waste products from the cell, important for osmoregulation.

Each cell's ability to function and maintain its structure depends on the organised arrangement and interaction of its membrane system and various organelles. This complex internal organisation enables cells to carry out essential life processes like obtaining nutrition, respiration, waste removal, and synthesis of necessary molecules, solidifying the cell's role as the fundamental unit of life.

Cell Division

New cells are formed from pre-existing cells through a process called cell division. This process is essential for:

• Growth of organisms.
• Replacement of old, dead, or injured cells.
• Formation of gametes (sex cells) required for reproduction.

There are two main types of cell division:

1. Mitosis: This type of cell division occurs in most somatic (body) cells for growth and repair. In mitosis, a single 'mother cell' divides to produce two genetically identical 'daughter cells'. The daughter cells have the same number of chromosomes as the mother cell.
2. Meiosis: This type of cell division occurs in specific cells of reproductive organs or tissues in animals and plants to produce gametes (sperm and egg cells). Meiosis involves two consecutive divisions of a single mother cell, resulting in the formation of four new cells. These daughter cells (gametes) contain half the number of chromosomes compared to the mother cell.

The reduction in chromosome number during meiosis is crucial for sexual reproduction. When two gametes (each with half the chromosome number) fuse during fertilisation, the resulting offspring restores the original chromosome number characteristic of the species.

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