Class 9th Biology Chapter 4 Cells and Tissues Notes
The cell theory, formulated in the 19th century, is a fundamental principle in biology that states:
- All organisms are composed of one or more cells.
- Cells are the smallest living units and the basic unit of organization in all organisms.
- Cells arise only from pre-existing cells through cell division.
- Cellular structures and functions include:
Cell Wall: A non-living and strong component located outside the plasma membrane in plant cells and some other organisms. It provides shape, strength, protection, and support to the cell.
Cell Membrane: A thin and elastic barrier that covers the cytoplasm of all prokaryotic and eukaryotic cells. It functions as a semi-permeable barrier, allowing some molecules to pass through while keeping others inside the cell. The cell membrane also plays a role in sensing chemical messages and identifying other cells.
Cytoplasm: The semi-viscous and semi-transparent substance between the plasma membrane and the nuclear envelope. It contains water and various organic and inorganic molecules. The cytoplasm provides space for organelles and serves as the site for many metabolic reactions.
Cytoskeleton: A network of microfilaments and microtubules that helps maintain the cell’s shape and allows it to change shape. Microtubules are made of tubulin protein and are also the major component of cilia and flagella, while microfilaments are made of actin protein.
Cell Organelles: These are small structures within eukaryotic cells that perform specialized functions. Some important organelles include:
Nucleus: Contains genetic material (DNA) and controls cell activities.
Endoplasmic Reticulum: Involved in protein synthesis and lipid metabolism.
Golgi Apparatus: Processes, modifies, and packages proteins for transport.
Mitochondria: Produce energy (ATP) through cellular respiration.
Chloroplasts: Found in plant cells and are the site of photosynthesis, where sunlight is converted into energy.
Vacuoles: Store water, nutrients, and waste materials.
Lysosomes: Contain enzymes for digestion and recycling of cellular materials.
Peroxisomes: Break down toxic substances in the cell.
Long Questions Answer
Short Answer Questions
Exercise and MCQ’s
The history of cell theory dates back to the 17th century when Robert Hooke first described cells in cork, and later, Antonie van Leeuwenhoek observed living cells in pond water. In the 19th century, Matthias Schleiden and Theodor Schwann proposed cell theory, and Rudolf Virchow added the idea that all cells arise from pre-existing cells.
Cell theory revolutionized the study of biology and has since been expanded and refined, providing a fundamental understanding of life’s basic unit and its essential components. Electron microscopes have further advanced cell research by allowing scientists to observe cellular structures at much higher magnifications and resolutions.
Important Conceptual Question Answers
Who presented the idea that all animals and plants are somehow related, leading to the question of a fundamental unit of structure shared by all organisms?
Aristotle
When were cells first described, and who was the scientist responsible for this discovery?
Cells were first described in 1665.
The scientist who made this discovery was Robert Hooke.
How did Robert Hooke observe cells for the first time, and what did he call the compartments in cork?
Robert Hooke observed cells by using his self-made light microscope.
He called the compartments in cork “cellulae.”
What did Antonie van Leeuwenhoek observe a few years later under his microscope, and what did he call them?
Antonie van Leeuwenhoek observed tiny organisms from pond water under his microscope.
He called them “animalcules.”
Who proposed that all living cells arise from pre-existing cells, and what Latin phrase did he use to describe this idea?
Rudolf Virchow proposed that all living cells arise from pre-existing cells.
He used the Latin phrase “Omnis cellula e celula.”
What are the three fundamental principles of cell theory in its modern form?
All organisms are composed of one or more cells.
Cells are the smallest living things and the basic unit of organization of all organisms.
Cells arise only by divisions in previously existing cells.
What did Jean Baptist de-Lamarck propose in 1809 regarding cellular tissues?
He proposed that no body can have life if its parts are not cellular tissues or are not formed by cellular tissues.
Who discovered the nucleus in the cell, and in which year did this discovery take place?
Robert Brown discovered the nucleus in the cell in 1831.
Which two scientists proposed the cell theory in its initial form, and what did they state in their propositions?
Matthias Schleiden and Theodor Schwann proposed the cell theory.
Matthias Schleiden stated that all plants are aggregates of individual cells, while Theodor Schwann reported that all animal tissues are also composed of individual cells.
What important extension of the cell theory was proposed by Rudolf Virchow?
Rudolf Virchow proposed that all living cells arise from pre-existing cells.
What experimental proof did Louis Pasteur provide in 1862 regarding cell theory?
Louis Pasteur provided experimental proof that all living cells arise from pre-existing cells.
What are sub-cellular or acellular particles, and give examples of such particles?
Sub-cellular or acellular particles are entities that are not composed of cells but show some characteristics of living organisms.
Examples include viruses, prions, and viroids.
What is the function of the cell wall in plant cells and some other organisms?
The cell wall provides shape, strength, protection, and support to the inner living matter (protoplasm) of the cell.
What is the primary chemical component of the outer layer of the plant cell wall?
The primary chemical component of the outer layer of the plant cell wall is cellulose.
Which German scientist studied plant tissues and made the first statement of cell theory?
Matthias Schleiden studied plant tissues and made the first statement of cell theory.
What is the fluid-mosaic model of the cell membrane, and what are its main components?
The fluid-mosaic model describes the cell membrane’s structure as a lipid bilayer with embedded protein molecules. The main components of the cell membrane are proteins and lipids, with small quantities of carbohydrates.
What is the role of the cytoplasm in a cell?
The cytoplasm is the semi-viscous and semi-transparent substance that provides space for the proper functioning of organelles and acts as the site for various biochemical (metabolic) reactions.
What is the function of microtubules in the cytoskeleton, and what protein are they made of?
Microtubules in the cytoskeleton help cells hold their shape and are also a major component of cilia and flagella.
They are made of tubulin protein.
Name two organelles found in eukaryotic cells and their respective functions.
Nucleus: Contains genetic material (DNA) and controls cell activities.
Mitochondria: Produce energy (ATP) through cellular respiration.
What are the three fundamental principles of cell theory in its modern form?
All organisms are composed of one or more cells.
Cells are the smallest living things and the basic unit of organization of all organisms.
Cells arise only by divisions in previously existing cells.