Table of contents | |
What is Cell? | |
What is Cell Theory? | |
Overview of Cells | |
Prokaryotic Cell | |
Eukaryotic Cells |
Our world is full of many different things. Some of these are living things like animals and plants that grow and survive. Others are not alive, like rocks and chairs, which don't have the qualities of living beings.
What is a difference Between living and non living organisms ?
The answer lies in the fundamental unit of life – the cell. Cells are the building blocks of all living organisms, and their presence is what sets living things apart from non-living things.
Fundamental Unit of Life - Cell
A cell is the basic structural and functional unit of life in all living organisms. Cells are the smallest entities that can carry out the processes necessary for life, such as metabolism, growth, reproduction, and responding to environmental stimuli. They are often referred to as the building blocks of life.
Note: Cytology: (G.k. kyios = cell ; logas = study) is the branch of biology which comprises the study of cell structure and function. Cell is the structural and functional unit of all living beings.
Cell theory states that "All living organisms are composed of cells and their products".
Matthias Schleiden (1838) observed different plant cells forming plant tissues.
Theodore Schwann (1839) identified the plasma membrane in animal cells and recognized cell walls as unique to plant cells.
Schleiden and Schwann proposed that both animals and plants are made of cells and their products, However this theory lacked an explanation for the process of cell formation.
In 1855 Rudolf Virchow provided an explanation by stating that cells divide and give rise to new cells from pre-existing ones
Therefore cell theory states that:
(i) All living organisms are made up of cells and their products.
(ii) Cells can only arise from the division of pre-existing cells.
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An overview of a typical cell involves understanding its fundamental components, structures, and functions. Cells are the basic building blocks of life and can vary in size, shape, and function depending on the organism and its specialized roles. Broadly, cells can be categorized into two main types: prokaryotic cells and eukaryotic cells.
(a) Prokaryotic Cell (b) Eukaryotic Cell
Cells are also classified on the basis of their shape and function they perform some the different types with different shapes are given below
Cells can vary in shape and structure depending on their functions and the organisms they belong to. While the basic structural components of cells are similar, there can be variations.
Shapes of Cell
Prokaryotic cells exhibit a less complex structure when compared to eukaryotic cells, as they do not possess a genuine nucleus or membrane-bound organelles.
Prokaryotic Cell
(i) Prokaryotic cells, especially bacteria, feature a complex cell envelope comprising three tightly bound layers:
(ii) These layers, while distinct, function together as a protective unit.
(iii) Bacteria are categorized as Gram-positive (retain Gram stain) or Gram-negative (do not retain Gram stain) based on their cell envelope differences.
Note: Bacteria are categorized as either Gram-positive or Gram-negative based on their cell envelope structure and response to a staining procedure called the Gram stain:
Gram-positive & Gram-negative
Location: Prokaryotic ribosomes are found at the cell's plasma membrane.
Size: They are about 15 nm by 20 nm in size and consist of 50S and 30S subunits, forming 70S ribosomes.
Function: These ribosomes are where protein synthesis takes place, and multiple ribosomes can attach to one mRNA, forming polysomes.
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Characteristics: Inclusion bodies are not enclosed by membranes and include materials like phosphate, cyanophycean, and glycogen granules.
(c) Gas Vacuoles
Presence: Gas vacuoles are found in specific photosynthetic bacteria, such as blue-green and purple-green varieties.
Complex cells with a true nucleus and membrane-bound organelles.
Eukaryotic Cell
Now, let's examine specific cellular organelles to gain insight into their structures and roles in the cells.
The cell wall is a rigid, protective structure that surrounds the plasma membrane of plant cells, fungi, and some bacteria. It was first discovered by Robert Hooke in 1665.
The composition of cell walls varies depending on the type of organism. In plant cells, the primary component of the cell wall is cellulose, a complex carbohydrate made up of glucose molecules. Fungal cell walls contain chitin, while bacterial cell walls can be composed of peptidoglycan or other materials.
Components of Cell Wall
The cell membrane, also known as the plasma membrane, is a fundamental structural component of all cells in living organisms. It is a selectively permeable barrier that separates the interior of the cell from its external environment. The cell membrane plays several crucial roles in maintaining the integrity and function of the cell.
Every living cell is externally covered by a thin transparent electron microscopic, elastic regenerative and selective permeable membrane called plasma membrane.
Cell Membrane
According to Singer and Nicolson, it resembles a "protein iceberg floating in a lipid sea."
The term "cell membrane" was initially coined by C. Nageli and C. Cramer in 1855 to describe the outer membrane covering of the protoplast. However, the term "plasmalemma" or "plasma membrane" was introduced by Plowe in 1931 to replace "cell membrane."
Functions of Plasma Membrane
Functions of Plasma Membrane
(i) Involved in Membrane Functions: Supports cell growth, intercellular junctions, secretion, endocytosis, and cell division. Selectively permeable for molecules on both sides. passive transport takes place.
(ii) Transfer of Molecules without energy: Simple diffusion for neutral solutes (from high to low concentration). Osmosis for water (movement by diffusion).
(iii) Active Transport: Requires energy (typically ATP). Moves ions or molecules against their concentration gradient. Example: Na+/K+ Pump.
The Endomembrane system is a important component within eukaryotic cells, serving as a complex network of membranes and organelles that work together to perform various essential functions. It includes a range of organelles like the endoplasmic reticulum (ER), Golgi apparatus, lysosomes, and vacuoles. These organelles are interconnected by membranes and function together to carry out tasks such as protein synthesis, modification, and transport, lipid metabolism, and cellular waste management.
The Endomembrane System
The endomembrane system encompasses organelles such as the endoplasmic reticulum (ER), Golgi complex, lysosomes, and vacuoles. These organelles work together to perform specific tasks.
Note: The mitochondria, chloroplasts, and peroxisomes do not operate in coordination with the components of the endomembrane system. Therefore, they are not classified as part of this system.
Let's study these organelles in detail:
Electron microscopy studies conducted on eukaryotic cells reveal the presence of a network or reticulum made up of small tubular structures scattered throughout the cytoplasm. This network is known as the endoplasmic reticulum (ER).
Endoplasmic Reticulum
The ER often shows ribosomes attached to their outer surface. The endoplasmic reticulum bearing ribosomes on their surface is called rough endoplasmic reticulum (RER). In the absence of ribosomes they appear smooth and are called smooth endoplasmic reticulum (SER).
[Question: 891010]
Test: Prokaryotic & Eukaryotic Cells
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Camillo Golgi discovered reticular structures near the nucleus in 1898, and these structures were later named Golgi bodies after him. Golgi bodies consist of flat, disk-shaped sacs or cisternae, typically measuring 0.5µm to 1.0µm in diameter.
Golgi Apparatus
These cisternae are stacked parallel to each other and can vary in number within a Golgi complex. Golgi cisternae are arranged concentrically near the nucleus, featuring a distinct convex cis face and a concave trans face. Despite their differences, the cis and trans faces of the Golgi apparatus are interconnected.
Functions:
Lysosomes are vesicular structures enclosed by membranes, and they are created through the packaging process in the Golgi apparatus. These isolated lysosomal vesicles have been discovered to contain a wide range of hydrolytic enzymes (known as hydrolases), including lipases, proteases, and carbohydrases.
Lysosomes
Function: These enzymes are most effective when the environment is acidic. They have the capacity to break down carbohydrates, proteins, lipids, and nucleic acids.
Vacuoles are membrane-bound compartments present in the cytoplasm of cells. They contain water, sap, waste products, and other substances that aren't essential for the cell's functioning. A single membrane known as the tonoplast encloses the vacuole. In plant cells, vacuoles can occupy a significant portion, up to 90 percent, of the cell's volume.
Vacuole - Plant Cell
Function:
Mitochondria are semi autonomous having hollow sac like structures present in all eukaryotes except mature RBCs of mammals and sieve tubes of phloem. These are absent in all prokaryotes like bacteria and cyanobacteria. Mitochondria are organelles found in the cells that produce energy. They are not easily visible under the microscope and the number of mitochondria per cell varies depending on the cells' physiological activity.
Mitochondria
Functions of Mitochondria:
[Question: 891011]
Plastids are present in all plant cells and in euglenoides, and they are easily visible under a microscope because of their substantial size. They contain distinct pigments that give plants specific colors. Plastids can be categorized into three types—chloroplasts, chromoplasts, and leucoplasts—based on the specific pigments they contain.
Types of Plastids
(a) Chloroplasts
Chloroplasts play a vital role in capturing light energy for the process of photosynthesis, utilizing pigments like chlorophyll and carotenoids. They are organelles enclosed by a double membrane. Notably, the inner membrane of chloroplasts has relatively low permeability.
Chloroplast
(b) Chromoplasts: Chromoplasts contain fat-soluble carotenoid pigments such as carotene, xanthophylls, and others. These pigments give the plant parts a yellow, orange or red colour.
(c) Leucoplasts: Leucoplasts are colourless plastids of varied shapes and sizes that store nutrients. There are three types of leucoplasts:
(i) Amyloplasts store carbohydrates (starch), e.g., potato
(ii) Elaioplasts store oils and fats
(iii) Aleuroplasts store proteins
Ribosomes were initially observed as dense particles under the electron microscope by George Palade in 1953. They consist of ribonucleic acid (RNA) and proteins and are not enclosed by any membrane. Ribosomes are tiny structures made up of ribonucleic acid (RNA) and proteins.
Ribosomes
Eukaryotic ribosomes have a sedimentation coefficient of 80S, while prokaryotic ribosomes have a sedimentation coefficient of 70S. Each ribosome is composed of two subunits: a larger subunit and a smaller subunit.
The cytoskeleton is a complex system of proteinaceous filamentous structures, including microtubules, microfilaments, and intermediate filaments, found within the cytoplasm.
Cytoskeleton
Collectively, these components are known as the cytoskeleton, and they serve various essential roles within the cell. These functions encompass providing mechanical support, enabling cell motility, and maintaining the cell's shape.
Cilia (singular: cilium) and flagella (singular: flagellum) are slender, hair-like projections extending from the cell membrane. Cilia are relatively small structures that function like oars, generating movement in either the cell or the surrounding fluid. Flagella, on the other hand, are longer and primarily responsible for propelling the cell. It's worth noting that prokaryotic bacteria also possess flagella, but their structure differs from that of eukaryotic flagella.
Cilia and Flagella
Under the electron microscopic study of cilia and flagella show that:
[Question: 891009]
The centrosome is an organelle typically composed of two cylindrical structures known as centrioles, surrounded by a less structured pericentriolar material. Within the centrosome, both centrioles are oriented perpendicular to each other and exhibit an organization resembling a cartwheel.
Centrosome and Centrioles
The concept of the nucleus as a cellular organelle was first introduced by Robert Brown back in 1831. Later on, Flemming coined the term "chromatin" to describe the material within the nucleus that could be stained with basic dyes.
Nucleus
Chromosomes are thread-like structures found in the nucleus of eukaryotic cells, such as those in humans and many other organisms. They are made up of DNA, which contains the genetic information or genes that carry instructions for various cellular functions and traits.
Types of Chromosome
Microbodies are small, membrane-bound vesicles found in both plant and animal cells. These microbodies are notable for containing a variety of enzymes within their structure. These enzymes are involved in diverse cellular processes and metabolic reactions. Microbodies are essential for compartmentalizing specific enzymatic activities, allowing cells to carry out various biochemical reactions efficiently and independently within these specialized vesicles.
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Chapter Notes: Cell: The Unit of Life
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Plant cells are fundamental to the structure and function of plants. They possess unique organelles like chloroplasts and a central vacuole that are not found in animal cells.
Plant Cell
An animal cell is the fundamental structural and functional unit of animal organisms. It is a microscopic, membrane-bound structure that serves as the basic building block of animal tissues and organs.
Animal Cell
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1. What is a cell and why is it considered the basic unit of life? |
2. What are the key components of a prokaryotic cell? |
3. How do eukaryotic cells differ from prokaryotic cells? |
4. What is cell theory and what are its main tenets? |
5. Why are cells important in the study of biology? |
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