Structure, Components, and Functions of Cell: A Complete Overview

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The cell is the basic building block of life. Inside, a cell has various parts that help it function properly. Each part has a special job. Our body has different types of cells like muscle cells and nerve cells. Even though they have different shapes and functions, all cells share the same basic parts. In these notes on Biology, we will explore the different parts of cells and what they do.

Our body has muscle cells, nerve cells, and many other types. While their shapes and functions vary, all these cells share common components. In the following Biology notes, we will discover the various cell components and their roles.

By understanding the basic components of cells, we can better appreciate how our body works. These essential parts, such as the cell membrane, nucleus, and mitochondria, perform unique functions that keep cells healthy and active. Whether it’s the nerve cells sending signals or muscle cells helping us move, each cell type relies on its parts to work correctly.

Explore our comprehensive guide to cell components and their functions, and learn how each part contributes to the overall health and efficiency of cells. This guide is designed to enhance your understanding of Biology and cell functions, perfect for students and curious minds seeking to unlock the secrets of life’s building blocks.

Cell Membrane (Plasma Membrane)

Structure and Model

  • Composition: Made of proteins and lipids.
  • Fluid Mosaic Model: Proposed by Singer and Nicholson in 1972. Describes the plasma membrane as a mosaic of components, including phospholipids, proteins, cholesterol, and carbohydrates, giving the membrane a fluid character.
  • Examples: Human red blood cells, bone cells, and nerve cells exhibit characteristic shapes due to the plasma membrane.

Selective Permeability

  • Function: Allows selective substances to enter and exit the cell, hence it is known as ‘selectively permeable.’

Functions of Plasma Membrane

  • Enclosure: Encloses the entire cell contents.
  • Shape: Provides shape to animal cells.
  • Transport: Helps transport materials in and out of the cell.

Transport Methods

  • Diffusion: Molecules move from a region of higher concentration to a region of lower concentration without energy. Example: Absorption of glucose in a cell.
  • Osmosis: Movement of water molecules from a region of higher concentration to a region of lower concentration through a membrane.
  • Active Transport: Movement of molecules from a region of lower concentration to a region of higher concentration, requiring energy provided by ATP. Example: Na+/K+ Pump.
Components of Cell
Components of Cell

Cell Wall

Structure and Composition

  • Location: Outermost layer of a cell.
  • Presence: Only found in plant cells.
  • Composition: Made of cellulose.
  • Protection: Protects the inner parts of the cell.
  • Rigidity: Provides shape and prevents distension, leading to cell turgidity.

Functions of Cell Wall

  • Movement: Freely allows the movement of water and other chemicals in and out of the cells.
  • Plasmodesmata: Breaks in the primary wall of adjacent cells connect the cytoplasm of one cell to another through cytoplasmic strands called plasmodesmata.

Endoplasmic Reticulum

Discovered by Keith R. Porter in 1954

The endoplasmic reticulum (ER) is a crucial cellular structure discovered by Keith R. Porter in 1954. It is notably absent in human red blood cells (RBCs), blue-green algae, and bacteria.

Functions of the Endoplasmic Reticulum

  • Protein Production: The ER assists in the production and storage of proteins.
  • Protein Storage: It plays a key role in storing proteins within cells.

Types of Endoplasmic Reticulum

The endoplasmic reticulum is categorized into two types:

  • Rough Endoplasmic Reticulum (RER): Characterized by the presence of ribosomes on its surface, which gives it a “rough” appearance under a microscope.
  • Smooth Endoplasmic Reticulum (SER): Lacks ribosomes, resulting in a “smooth” appearance, and is involved in lipid synthesis and detoxification processes.

Functions of Endoplasmic Reticulum

Rough Endoplasmic Reticulum (RER)

  • Ribosome Presence: The surface of the rough endoplasmic reticulum is studded with ribosomes.
  • Protein Assembly: Involved in the synthesis and packaging of essential proteins.

Smooth Endoplasmic Reticulum (SER)

  • Ribosome Absence: Lacks ribosomes on its surface.
  • Lipid and Steroid Synthesis: Synthesizes lipids and steroids.
  • Carbohydrate Storage: Plays a role in the storage of carbohydrates.

Ribosomes

Location of Ribosomes

  • Cytoplasm: Ribosomes are present in the cytoplasm of a cell.
  • Rough Endoplasmic Reticulum (RER): They are also found on the surface of the rough endoplasmic reticulum.

Composition of Ribosomes

  • RNA: Ribosomes are composed of ribonucleic acid (RNA).
  • Proteins: They also consist of proteins.

Functions of Ribosomes

  • Protein Synthesis: Ribosomes are nicknamed the “protein factory of the cell” due to their role in synthesizing protein molecules.
  • Amino Acid Collection: They collect essential amino acids and produce protein chains according to the cell’s needs.

Golgi Apparatus

Discovery and Presence

  • Discovery: Discovered by Camillo Golgi in 1898.
  • Animal Cells: Typically, there are 3 to 7 Golgi body organelles present in an animal cell. They are located near the nucleus.
  • Plant Cells: Known as dictyosomes in plants, these organelles are dispersed throughout the cell. Many dictyosomes are present in plant cells.
  • Blue-Green Algae: Not present in blue-green algae.

Functions of Golgi Apparatus

  • Storage and Transport: Stores and transports proteins and lipids that have been synthesized.
  • Synthesis and Secretion: Assists in the synthesis and secretion of essential molecules.
  • Wall Element Synthesis: Synthesizes cell wall elements like pectin and mucilage.
  • Temporary Vacuole: Stores synthesized proteins in a temporary vacuole known as the “lipochondria.”

Lysosomes

Discovery and Structure

  • Discovery: Discovered by Christian de Duve in 1955.
  • Structure: Pouch-like structures found only in animal cells. These special vesicles are pinched off from the Golgi body.
  • Enzymes: Contain various digestive enzymes such as protease, phosphatase, glycosidases, and sulfatases.
  • Nickname: Known as the “suicidal bags of cells” because they contain lytic enzymes that can digest their own cell.

Functions of Lysosomes

  • Intracellular Digestion: The primary function is intracellular digestion.
  • Digestion of Macromolecules: Digest nucleic acids, polysaccharides, fats, and proteins.
  • Autolysis: Involved in autolysis, where they can break down a dying cell or unwanted substances, leading to cell death.
  • Cell Maintenance: Digest food, break down aging or diseased cells, and eliminate unwanted materials.

Plastids

  • The term, “Plastid” was first coined by Ernst Haeckel in 1866.
  • Plastids are absent in Animals, Blue-Green Algae and Bacteria. 
  • There are three types of plastids, based on their colour:-
    • Leucoplast – white or colourless. For Example Roots with the exception of carrots. 
    • Chromoplast – blue, red, pink, orange, yellow. For Example:- Petals of Flowers.
    • Chloroplast:– green in colour. 

Functions of Plastids

Plastids are organelles found in the cells of plants and algae, each with distinct functions crucial for the organism’s survival and development. Here’s a breakdown of their functions:

Chloroplasts

  • Chloroplasts are perhaps the most well-known type of plastid, responsible for photosynthesis.
  • Within chloroplasts, chlorophyll pigments capture light energy, converting it into chemical energy in the form of glucose, which is used as a source of energy for the plant.
  • Photosynthesis also produces oxygen as a byproduct, which is vital for the survival of many organisms on Earth.

Chromoplasts

  • Chromoplasts are responsible for the synthesis and storage of pigments other than chlorophyll, giving fruits and flowers their vibrant colors.
  • These pigments attract pollinators and seed dispersers, aiding in reproduction and the dispersal of seeds.

Leucoplasts

  • Leucoplasts are colorless plastids primarily involved in the storage of starch, lipids, and proteins.
  • Amyloplasts, a type of leucoplast, store starch in roots, tubers, and seeds, providing a reserve of energy for the plant.
  • Elaioplasts store lipids, while protein-storing leucoplasts are known as proteinoplasts.

✅ Also Read: Types of Pigment in Plants with examples: Complete Details

Other Functions

  • Plastids are involved in various metabolic processes, including the synthesis of fatty acids, amino acids, and certain hormones.
  • Some plastids, like etioplasts, are intermediates between proplastids and chloroplasts, undergoing differentiation into chloroplasts upon exposure to light.
  • Plastids can also participate in the detoxification of harmful compounds within plant cells.

Overall, plastids play indispensable roles in plant metabolism, energy production, pigment synthesis, and storage, contributing significantly to plant growth, development, and environmental adaptation.

Cytoplasm

Structure and Composition

  • Location: Present inside the cell, within the cell membrane.
  • Exclusion: Not found in the nucleus of the cell.
  • Composition: Mainly consists of water, but also contains enzymes, salts, organelles, and various organic molecules.

Functions of Cytoplasm

  • Organelles: Houses all the cell’s organelles, such as the nucleus, ribosomes, mitochondria, Golgi bodies, plastids, lysosomes, and endoplasmic reticulum.
  • Support and Nourishment: Supports and nourishes the cell’s organelles, ensuring they function properly.

Mitochondria

Overview

  • Nickname: Known as the “Powerhouse of the Cell.”
  • Presence: Found in both plant and animal cells.
  • Function: Acts as the energy generator and releaser in cells.
  • Appearance: Appears as tiny thread-like structures under a light microscope, approximately 0.5 – 1.00 μm in size.
Components of Mitochondria
Components of Mitochondria

Structure of Mitochondria

  • Double Membrane: Composed of a double membrane with five distinct compartments:
  • Outer Mitochondrial Membrane: The outer boundary of the mitochondrion.
  • Intermembrane Space: The space between the outer and inner membranes.
  • Inner Mitochondrial Membrane: Contains infoldings called cristae.
  • Cristae Space: Formed by the infoldings of the inner membrane.
  • Matrix: The space within the inner membrane.

Functions of Mitochondria

  • ATP Production: Converts glucose into ATP through processes such as glycolysis, the citric acid cycle, and ATP synthesis.
  • Cellular Respiration: Oxidizes pyruvate in its matrix to form pyruvic acid, capturing energy stored in ATP.
  • Energy Supply: Provides ATP, which is used by cell organelles to produce energy for their functioning.

Nucleus

Discovery and Structure

  • Discovery: Robert Brown discovered the nucleus in 1831.
  • Shape and Staining: The nucleus is spherical and stains deeply. White blood cells (WBCs) have lobed nuclei.
  • Number: Typically, each cell contains one nucleus.
  • Nuclear Membrane: A double-layered membrane with fine nuclear pores encloses the nucleoplasm, containing a chromatin network and a nucleolus.
Components of Nucleolus
Components of Nucleolus

Components of Nucleus

  • Nuclear Envelope: A double-layered membrane made of lipids and proteins, similar to the plasma membrane, with ribosomes attached to the outer membrane. The nuclear pores facilitate the transport of large molecules in and out of the nucleus.
  • Nucleoplasm: A liquid found within the nucleus, supporting and surrounding all nuclear components like DNA, RNA, chromosomes, and nucleoli.

Chromatin

  • Composition: Made of histone and nucleic acid.
  • Structure: The fibrillar structures form a network called chromatin fibrils, which condense into chromosomes during cell division. Genes are point-like structures on chromosomes.

Types

  • Heterochromatin: Dark, highly coiled DNA, less genetically active.
  • Euchromatin: Light, uncoiled DNA, more genetically active.
  • Chromosome Count: Each cell contains 23 pairs of chromosomes, totaling 46.

Nucleolus

  • Discovery: Felice Fontana discovered the nucleolus in 1774.
  • Sub-organelle: Found within the nucleus of eukaryotic cells, except in sperm and some algae.
  • Composition: Made of DNA, RNA, and proteins (85% protein, 10% RNA, 5% DNA).
  • Function: Acts as a storehouse for RNA and proteins. Disappears briefly during the early phase of the cell cycle and reappears after telophase in the daughter nuclei. Regulates the synthetic activity of the nucleus.
  • Nucleocytoplasmic Interaction: The nucleus and cytoplasm are interdependent.

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