1. Discovery of Cells

  • In 1665, Robert Hooke observed the structure of cork using a self-designed microscope. He saw small compartments that resembled honeycombs and called them “cells,” meaning “little rooms” in Latin.
  • This was the first observation of the existence of separate units in living organisms, which was crucial for the development of biology.
  • The concept of “cells” is still used today in biology to refer to the basic units of life.

2. Living Organisms and Cells

  • Unicellular organisms: Organisms made of a single cell (e.g., Chlamydomonas, Paramecium, bacteria).
  • Multicellular organisms: Organisms made up of many cells that group together to form different parts of the body (e.g., plants, animals).
  • Every multicellular organism starts from a single cell, and cells divide to form more cells.

3. Cell Types and Differences

  • Cells can look different based on their function and the part of the organism they come from.
    • Example: Cells from the human body can include blood cells, nerve cells, muscle cells, and more.
    • Onion peel cells, regardless of the onion’s size, appear similar under a microscope.

4. Key Cell Discoveries

  • Robert Hooke (1665): Discovered cells in cork.
  • Leeuwenhoek (1674): Discovered free-living cells in pond water.
  • Robert Brown (1831): Discovered the nucleus in cells.
  • Purkinje (1839): Coined the term “protoplasm” for the cell fluid.
  • Schleiden & Schwann (1839): Developed the cell theory, stating all living organisms are made of cells.
  • Virchow (1855): Proposed that all cells arise from pre-existing cells.
  • Electron Microscope (1940s): Allowed detailed observation of cell structures and organelles.

5. Basic Functions of Cells

  • Each cell has specific components called cell organelles that help the cell perform its functions. These organelles include the nucleus, mitochondria, ribosomes, etc.
  • Even though cells have different functions, they all have similar basic structures.
  • The shape and size of a cell are related to its function.
    • Example: Nerve cells have a fixed, long shape, while an amoeba can change shape.

6. Plasma Membrane (Cell Membrane)

  • The plasma membrane is the outer covering of the cell that separates it from the environment.
  • It controls the movement of materials in and out of the cell. The membrane is selectively permeable, meaning it allows some substances to pass through while blocking others.

7. Transport Mechanisms in Cells

  • Diffusion: Movement of substances from an area of high concentration to low concentration.
    • Example: Oxygen enters cells via diffusion when its concentration inside the cell is low, and carbon dioxide exits the cell when its concentration is high.
  • Osmosis: A type of diffusion where water molecules move across a selectively permeable membrane.
    • Hypotonic Solution: Water moves into the cell causing it to swell.
    • Isotonic Solution: No net movement of water, cell stays the same size.
    • Hypertonic Solution: Water moves out of the cell causing it to shrink.

8. Examples of Osmosis

  • In the egg experiment: If an egg is placed in pure water, it swells as water enters (osmosis). In a concentrated salt solution, the egg shrinks as water leaves.
  • Plant roots absorb water through osmosis.
  • Raisins or apricots: When placed in plain water, they swell due to water intake, and they shrink when placed in a concentrated sugar or salt solution.

9. Endocytosis

  • The plasma membrane is flexible and can engulf food or other materials from the external environment. This process is called endocytosis.
    • Example: An amoeba engulfs food through endocytosis.

Extra Knowledge to Help with Competitive Exams:

  • Cell Theory: Understanding the basic principles of the cell theory is crucial for exams. The key is to know that cells are the fundamental unit of life, all organisms are made up of cells, and cells arise from pre-existing cells.
  • Diffusion and Osmosis: These are important processes for moving substances in and out of cells. You should be familiar with examples and how they relate to different solutions (hypotonic, isotonic, hypertonic).
  • Microscope Use: Understanding how to use a microscope and prepare slides for observation is a common practical question in exams.

Cell Wall

  • Plant Cells: Besides the plasma membrane, plant cells have an outer, rigid cell wall, primarily made of cellulose. This structure gives strength and rigidity to plants.
  • Osmosis and Plasmolysis: When a plant cell loses water due to osmosis, the cell membrane shrinks away from the cell wall. This process is called plasmolysis.
  • Experiment:
    • Observing living and dead plant cells (like from a Rhoeo leaf) under a microscope helps us see the effects of plasmolysis. Only living cells undergo plasmolysis because they can absorb water by osmosis.
  • Role of Cell Wall in Osmosis: In plant cells, fungi, and bacteria, the cell wall helps prevent bursting when the cell takes in water (osmosis), especially in hypotonic solutions.
  • Turgor Pressure: The cell wall withstands the internal pressure from water intake and prevents the cell from bursting.

Nucleus

  • Nucleus Structure: The nucleus is a dark, spherical structure in the center of the cell. It has a nuclear membrane with pores for material transfer between the nucleus and cytoplasm.
  • Chromosomes and DNA: Inside the nucleus, chromosomes carry DNA (genetic information) in the form of genes. DNA directs cell functions and inheritance.
  • Chromatin: In non-dividing cells, DNA is in a chromatin form, which becomes organized into chromosomes when the cell divides.
  • Nucleoid in Prokaryotes: In prokaryotes (like bacteria), the nucleus isn’t membrane-bound; it’s called a nucleoid.
  • Prokaryotic vs. Eukaryotic Cells:
    • Prokaryotes: Simple cells without a nucleus or membrane-bound organelles.
    • Eukaryotes: Complex cells with a nucleus and membrane-bound organelles.

Cytoplasm

  • Cytoplasm Structure: The cytoplasm is the jelly-like substance inside the cell, enclosed by the plasma membrane. It contains organelles that perform specific functions for the cell.
  • Prokaryotic vs. Eukaryotic Cytoplasm: In prokaryotic cells, the cytoplasm is less organized, whereas eukaryotic cells have more specialized organelles within the cytoplasm.
  • Importance of Membranes: Membranes in cells separate different functions and protect the cell from the outside environment. Viruses, lacking membranes, are not considered living until they enter a host cell.

Cell Organelles

  • Organelles: Specialized structures inside the cell, each performing unique tasks. In eukaryotes, organelles are membrane-bound, while prokaryotes lack membrane-bound organelles.
  • Examples of Organelles:
    • Endoplasmic Reticulum (ER): A network of tubes and sacs involved in transporting proteins (rough ER with ribosomes) and making lipids (smooth ER).
    • Golgi Apparatus: Modifies, stores, and packages proteins and lipids, and is involved in forming lysosomes.
    • Lysosomes: Contain digestive enzymes to break down waste and foreign substances; often referred to as the “suicide bags” of the cell because they can destroy the cell by digesting it.
    • Mitochondria: Known as the powerhouses of the cell, they produce ATP, the energy currency of the cell.
    • Plastids (in plants): Store food or pigments. Chloroplasts are a type of plastid that perform photosynthesis in plants.
    • Vacuoles: Storage sacs; large in plants for storing nutrients and maintaining cell pressure.

Cell Division

  • Purpose of Cell Division: Cells divide for growth, to replace dead cells, and for reproduction (formation of gametes).
  • Mitosis: A type of division where a single cell divides into two identical cells. This process is important for growth and tissue repair.
  • Meiosis: A special type of division that produces gametes (sperm and egg cells) with half the number of chromosomes of the parent cell, ensuring genetic diversity during reproduction.

Comparing Prokaryotic and Eukaryotic Cells:

  • Prokaryotic Cells: Small, no nucleus or membrane-bound organelles, single chromosome, and simple structure.
  • Eukaryotic Cells: Larger, with a nucleus, multiple chromosomes, and membrane-bound organelles like the ER, Golgi apparatus, mitochondria, etc.

Additional Notes for Competitive Exams:

  • Prokaryotes vs. Eukaryotes: Key difference lies in the presence of membrane-bound organelles and a defined nucleus. Prokaryotes are simpler (e.g., bacteria), while eukaryotes (e.g., plant, animal cells) are more complex.
  • Importance of the Nucleus: The nucleus is the control center of the cell, directing growth, development, and reproduction. The chromosomes within carry DNA that guides all cell functions.
  • Plastids: Only found in plant cells, essential for storing food (leucoplasts) and performing photosynthesis (chloroplasts).

THESE ALL ARE THE NOTES OF CHAPTER 5 SCIENCE. AND AFTER SOME TIME YOU GET IMPORTANT QUESTIONS HERE. *#THANKS FOR VISITING, VISIT AGAIN#* 😊