Chapter 5: The Fundamental Unit of Life

 

Discovery of the Cell

Robert Hooke’s Observation (1665)

• While examining a thin slice of cork, Robert Hooke observed a structure resembling a honeycomb with many small compartments.
• He used a self-designed microscope for this observation.
• The cork was obtained from the bark of a tree.
• 
  
  
• Hooke named these compartments "cells", derived from the Latin word "cellula", meaning "a little room".

Significance of the Discovery

• This was the first recorded observation of cells in 1665.
• It led to the realization that living things are made up of separate units (cells).
• The term "cell" is still used in biology today.

This discovery laid the foundation for cell theory, a fundamental concept in biology.

What Are Living Organisms Made Up Of?

Discovery of Cells

• Robert Hooke first observed cells in 1665 in a thin slice of cork.
• Cells are the basic structural and functional units of all living organisms.
• The invention of microscopes led to the discovery of the microscopic world.

Activity 5.1: Observing Onion Peel Cells

1. Peel off the epidermis from the inner side of an onion bulb.
2. Place it in water to prevent drying.
3. Transfer it to a glass slide with water and add safranin stain.
4. Cover it with a cover slip (avoid air bubbles).
5. Observe under a compound microscope (low and high power).

Observations

• Cells appear box-like and similar in structure.
• Regardless of onion size, cells look the same.
• Cells are building blocks of organisms.

Unicellular and Multicellular Organisms

• Unicellular organisms: A single cell forms an entire organism (e.g., Amoeba, Chlamydomonas, Paramecium, Bacteria).

• Multicellular organisms: Many cells work together to form a body (e.g., fungi, plants, animals).

Activity 5.2: Observing Different Plant Cells

• Temporary mounts of leaf peels, root tips, and onion peels can be prepared.

Key Questions

1. Do all cells look alike in shape and size? (No)
2. Do all cells have the same structure? (No)
3. Are there differences between cells from different plant parts? (Yes)
4. What similarities do we find? (All have a basic cellular structure)

Cell Diversity

• Cells have different shapes and sizes based on their function.
• Amoeba has a changing shape, while nerve cells have a fixed, elongated shape.

Cell Organelles and Functions

• Division of Labour exists in both multicellular organisms and individual cells.
• Each cell has organelles that perform specific functions:
  • Synthesis of new materials
  • Waste removal
  • Energy production
• All cells have similar organelles, regardless of their function or organism type.

Conclusion

• Cells are the fundamental units of life.
• Unicellular organisms function as a single cell, while multicellular organisms have specialized cells.
• Cell organelles play a crucial role in keeping the cell alive and functional.

Structural Organization of a Cell

1. Components of a Cell

The cell has three main parts:

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

2. Plasma Membrane (Cell Membrane)

• Outermost covering of the cell.
• Separates the cell’s internal contents from its external environment.
• Selectively permeable membrane – allows certain materials to enter or exit the cell.

3. Transport Across the Plasma Membrane

(i) Diffusion

• Movement of molecules from higher concentration to lower concentration.
• Example: Exchange of gases (O₂ & CO₂) in and out of the cell.

(ii) Osmosis

• Special case of diffusion where water molecules move across a selectively permeable membrane.
• Moves from higher water concentration to lower water concentration until equilibrium is reached.

4. Effects of Osmosis on Cells

1. Hypotonic Solution (Low solute concentration, high water)

   • Water enters the cell, causing it to swell.

2. Isotonic Solution (Equal solute & water concentration)

   • No net movement of water, cell remains same size.

3. Hypertonic Solution (High solute concentration, low water)

   • Water exits the cell, causing it to shrink.

5. Experiments Demonstrating Osmosis

Egg Experiment:

• Egg swells in pure water (osmosis causes water intake).
• Egg shrinks in salt solution (osmosis causes water loss).

Dried Raisins/Apricots Experiment:

• Swell in water.
• Shrink in concentrated sugar/salt solution.

6. Importance of Diffusion & Osmosis

• Gas exchange (O₂ & CO₂) in cells.
• Water absorption by plant roots.
• Nutrient and waste transport within cells.

7. Endocytosis

• Process of engulfing food/material from the environment.
• Example: Amoeba takes in food through endocytosis.

8. Plasma Membrane Structure

• Made of lipids and proteins.
• Flexible, can bend and fold.
• Can be observed under an electron microscope.

9. Electron Microscope

• Advanced microscope that provides high-resolution images of cell structures.
• Used to study detailed cell membranes and organelles.

CELL WALL

• Plant Cell Wall:

  • Plant cells, in addition to the plasma membrane, have a rigid outer covering called the cell wall, which is located outside the plasma membrane.
  • The cell wall is primarily made of cellulose, a complex substance that provides structural strength and rigidity to the plant cell.

• Plasmolysis:

  • Plasmolysis is the phenomenon that occurs when a living plant cell loses water through osmosis, causing the contents of the cell to shrink and pull away from the cell wall.
  • This can be observed when the plant cell is placed in a hypertonic solution (such as a strong sugar or salt solution).

• Activity to Observe Plasmolysis:

  1. Mount a peel of Rhoeo leaf in water on a slide and examine it under the high power of a microscope. You will see small green granules (chloroplasts) containing chlorophyll.
  2. Then add a strong solution of sugar or salt to the mounted leaf and observe under the microscope. You will notice the cell contents shrink away from the cell wall. This is plasmolysis.
  3. Boil the Rhoeo leaves to kill the cells, mount one leaf on a slide, and observe it under a microscope. Add the strong sugar or salt solution again. No plasmolysis will occur in dead cells.

• Role of the Cell Wall:

• The cell wall allows plant cells, fungi, and bacteria to withstand very dilute (hypotonic) external media without bursting.
• When cells are in such media, they absorb water by osmosis, causing the cell to swell. The cell wall exerts pressure against the swollen cell, preventing it from bursting and helping the cell maintain structural integrity.
• Due to the strength of the cell wall, plant cells can withstand much greater changes in the surrounding environment compared to animal cells, which lack a cell wall.

Nucleus and Cell Structure

1. Observation of Cells

• When observing onion peel cells, iodine solution is used to stain the cells, allowing differentiation of cell parts.
• Different regions of the cell absorb the stain differently, appearing lighter or darker.
• Other staining solutions like safranin or methylene blue can also be used for better visibility of cell parts.

2. Cheek Cell Observation

• Scrape the inside of your cheek with an ice cream spoon and transfer the material onto a glass slide.
• Add methylene blue solution to color the cells, then place a cover slip.
• Under the microscope, nucleus appears as a dark, spherical or oval structure in the center of each cell.

3. Structure of the Nucleus

• The nucleus has a double-layered membrane with pores that control the movement of materials between the nucleus and the cytoplasm.
• Chromosomes inside the nucleus contain DNA (Deoxyribonucleic Acid), which carries genetic information and is responsible for inheritance.
  • Chromosomes are only visible during cell division; otherwise, DNA is in the form of chromatin (a thread-like structure).
  • Genes are functional segments of DNA that help in constructing and organizing cells.

4. Role of the Nucleus

• Cellular Reproduction: The nucleus controls cell division, allowing one cell to divide into two new cells.
• Cell Development: It directs chemical activities that determine the cell's development and final form.

5. Prokaryotes vs. Eukaryotes

• In prokaryotic cells (e.g., bacteria), the nucleoid is a poorly defined region containing nucleic acids but lacks a nuclear membrane.
• Prokaryotes have no membrane-bound organelles, and their cytoplasmic functions are carried out by simpler structures.
• Eukaryotic cells have a defined nucleus with a nuclear membrane and membrane-bound organelles (e.g., mitochondria, plastids, ribosomes).

Prokaryotic Cell Diagram:

• Components:
  • Ribosomes
  • Plasma membrane
  • Cell wall
  • Nucleoid

6. Key Takeaways

• The nucleus is a central organelle involved in genetic material storage, cell division, and controlling cellular activities.
• Prokaryotes and eukaryotes differ in their nuclear structure and organelles.
• Observing cells under a microscope using staining techniques helps in identifying their structure and function.

Nucleus and Cell Structure

1. Observation of Cells

• When observing onion peel cells, iodine solution is used to stain the cells, allowing differentiation of cell parts.
• Different regions of the cell absorb the stain differently, appearing lighter or darker.
• Other staining solutions like safranin or methylene blue can also be used for better visibility of cell parts.

2. Cheek Cell Observation

• Scrape the inside of your cheek with an ice cream spoon and transfer the material onto a glass slide.
• Add methylene blue solution to color the cells, then place a cover slip.
• Under the microscope, nucleus appears as a dark, spherical or oval structure in the center of each cell.

3. Structure of the Nucleus

• The nucleus has a double-layered membrane with pores that control the movement of materials between the nucleus and the cytoplasm.
• Chromosomes inside the nucleus contain DNA (Deoxyribonucleic Acid), which carries genetic information and is responsible for inheritance.
  • Chromosomes are only visible during cell division; otherwise, DNA is in the form of chromatin (a thread-like structure).
  • Genes are functional segments of DNA that help in constructing and organizing cells.

4. Role of the Nucleus

• Cellular Reproduction: The nucleus controls cell division, allowing one cell to divide into two new cells.
• Cell Development: It directs chemical activities that determine the cell's development and final form.

5. Prokaryotes vs. Eukaryotes

• In prokaryotic cells (e.g., bacteria), the nucleoid is a poorly defined region containing nucleic acids but lacks a nuclear membrane.
• Prokaryotes have no membrane-bound organelles, and their cytoplasmic functions are carried out by simpler structures.
• Eukaryotic cells have a defined nucleus with a nuclear membrane and membrane-bound organelles (e.g., mitochondria, plastids, ribosomes).

Prokaryotic Cell Diagram:

• Components:
  • Ribosomes
  • Plasma membrane
  • Cell wall
  • Nucleoid

6. Key Takeaways

• The nucleus is a central organelle involved in genetic material storage, cell division, and controlling cellular activities.
• Prokaryotes and eukaryotes differ in their nuclear structure and organelles.
• Observing cells under a microscope using staining techniques helps in identifying their structure and function.

Cytoplasm and Cell Organelles

1. Cytoplasm Definition

• The cytoplasm is the fluid-filled region inside the cell membrane that takes up little to no stain.
• It is the area where various cellular processes occur and contains numerous cell organelles.

2. Cell Organelles

• Organelles are specialized structures within the cytoplasm, each performing specific functions essential for the cell's activities.
• Organelles are typically enclosed by membranes, maintaining a compartmentalized environment.

3. Membranes in Prokaryotes vs. Eukaryotes

• Prokaryotic Cells: Do not have membrane-bound organelles or a defined nuclear region.
• Eukaryotic Cells: Have a defined nuclear membrane and membrane-enclosed organelles (e.g., mitochondria, plastids, endoplasmic reticulum).

4. Viruses and Membranes

• Viruses lack membranes and do not exhibit life characteristics until they invade a living host cell.
• Once inside a host, viruses utilize the host's cell machinery to reproduce and multiply.

5. Key Takeaways

• The cytoplasm is essential for housing organelles and supporting cellular functions.
• Membrane-bound organelles are a hallmark of eukaryotic cells and help compartmentalize various functions, while prokaryotes lack these features.
• Viruses, which lack membranes, cannot carry out life processes independently and require a host to multiply.

Cell Organelles

1. Membrane-bound Organelles

• Membrane-bound organelles are specialized structures inside the cell, each performing specific functions.
• These structures help compartmentalize chemical activities, keeping different functions separate to maintain cell efficiency.
• These organelles are mainly found in eukaryotic cells, which distinguish them from prokaryotic cells that lack membrane-bound organelles.

2. Importance of Cell Organelles

• In large, complex cells, such as those in multicellular organisms, a variety of chemical activities are needed to maintain the cell's structure and function.
• Organelles help manage these activities by isolating them within distinct areas.

3. Examples of Key Organelles

Some key organelles essential for cell function include:

• Endoplasmic Reticulum (ER): A network of membranes involved in protein and lipid synthesis.
• Golgi Apparatus: Responsible for modifying, sorting, and packaging proteins and lipids for transport.
• Lysosomes: Contain enzymes for digesting waste materials and cellular debris.
• Mitochondria: The powerhouse of the cell, producing energy (ATP) through cellular respiration.
• Plastids: Found in plant cells, they are involved in photosynthesis, storage, and pigment production.

4. Visualization of Organelles

• Many of these organelles can only be observed under an electron microscope due to their small size.

5. Significance

• Organelles play a crucial role in maintaining the specialized functions of cells, contributing to their overall survival and efficiency.

Endoplasmic Reticulum (ER)

1. Structure of Endoplasmic Reticulum (ER)

• Endoplasmic Reticulum (ER) is a large network of membrane-bound tubes and sheets.
• It appears as long tubules or round/oblong bags (vesicles) under a microscope.
• The ER membrane is structurally similar to the plasma membrane.

2. Types of Endoplasmic Reticulum

• There are two types of ER:
  • Rough Endoplasmic Reticulum (RER):
    • Appears rough due to ribosomes attached to its surface.
    • Ribosomes are sites for protein synthesis.
  • Smooth Endoplasmic Reticulum (SER):
    • Lacks ribosomes and appears smooth.
    • Involved in the synthesis of lipids (fat molecules), important for cell function.

3. Functions of Endoplasmic Reticulum

• Protein Synthesis:

  • Ribosomes on the RER manufacture proteins, which are then transported via the ER to various locations in the cell as needed.

• Lipid Synthesis:

  • SER aids in the manufacture of lipids, which are critical for the cell's structure and function.

• Membrane Biogenesis:

  • Some proteins and lipids synthesized in the ER help in building the cell membrane.

• Transport of Materials:

  • The ER serves as channels for transporting materials, particularly proteins, between the cytoplasm and the nucleus, or within different regions of the cytoplasm.

• Cytoplasmic Framework:

  • Provides a surface for biochemical activities in the cell.

• Detoxification (in liver cells):

  • In vertebrates, SER plays an important role in detoxifying poisons and drugs, particularly in liver cells.

4. Variation in Appearance

• The ER's appearance can vary significantly across different cell types but always forms a network system.

Golgi Apparatus

1. Structure of Golgi Apparatus

• The Golgi apparatus consists of a system of membrane-bound vesicles (flattened sacs) arranged in parallel stacks called cisterns.
• These membranes are often connected to the membranes of the ER, making the Golgi apparatus a key component of the cellular membrane system.

2. Functions of Golgi Apparatus

• Packaging and Transport:

  • The Golgi apparatus packages materials synthesized near the ER and sends them to various destinations inside or outside the cell.

• Storage:

  • It stores products that are synthesized by the cell until they are required.

• Modification:

  • The Golgi apparatus modifies products, such as complex sugars, which are synthesized from simpler sugars.

• Formation of Lysosomes:

• The Golgi apparatus is involved in the formation of lysosomes, which are essential for digestion and waste removal within the cell.

Lysosomes

1. Structure of Lysosomes

• Lysosomes are membrane-bound sacs filled with digestive enzymes.
• These enzymes are synthesized by the rough endoplasmic reticulum (RER).

2. Functions of Lysosomes

• Waste Disposal System:
  • Lysosomes act as the cell's waste disposal system, breaking down foreign materials and worn-out organelles.
• Digestion of Foreign Materials:
  • They digest any foreign materials that enter the cell, such as bacteria or food particles.
• Breaking Down Old Organelles:
  • Lysosomes also help in breaking down old or damaged cell organelles to maintain cellular health.
• Digesting Organic Materials:
  • The enzymes in lysosomes are powerful and capable of breaking down all types of organic material.

3. Self-Digestion (Suicide Bags)

• Self-Digestion:
• In case of cellular damage or disturbance in metabolism, lysosomes may burst and release their enzymes to digest the cell’s own components.
• This process is referred to as the lysosome's role in self-digestion, earning them the nickname "suicide bags" of the cell.

Mitochondria

1. Structure of Mitochondria

• Mitochondria are double-membraned organelles:
  • Outer membrane: Porous.
  • Inner membrane: Deeply folded to increase surface area for chemical reactions.

2. Function of Mitochondria

• Known as the powerhouses of the cell because they are responsible for energy production.
• ATP Production:
  • Mitochondria generate energy in the form of ATP (Adenosine triphosphate) through chemical reactions.
  • ATP is the energy currency of the cell, used for various chemical activities and mechanical work within the cell.

3. Unique Features

• Own DNA and Ribosomes:
• Mitochondria have their own DNA and ribosomes, which allow them to produce some of their own proteins.

Plastids

1. Types of Plastids

• Chromoplasts (colored plastids):
  • Contain pigments like chlorophyll, which give plants their green color.
  • Involved in photosynthesis.
  • Also contain yellow or orange pigments.
• Leucoplasts (colorless plastids):
  • Function mainly as storage sites for materials such as starch, oils, and protein granules.

2. Structure of Chloroplasts

• Internal Organisation:
  • Composed of numerous membrane layers embedded in a substance called stroma.
  • Similar to mitochondria in their external structure.

3. Unique Features of Plastids

• Like mitochondria, plastids have their own DNA and ribosomes, allowing them to produce some of their own proteins.

Vacuoles

1. Structure and Size

• Small vacuoles in animal cells.
• Large vacuoles in plant cells, with the central vacuole sometimes occupying 50-90% of the cell's volume.

2. Function in Plant Cells

• Storage of substances like amino acids, sugars, organic acids, and proteins.
• Provide turgidity and rigidity to the cell, helping maintain its shape.

3. Function in Unicellular Organisms

• Food vacuoles in organisms like Amoeba store ingested food.
• Specialized vacuoles in unicellular organisms help in expelling excess water and wastes from the cell.

Cell Division

1. Purpose of Cell Division

• New cells are formed for:
  • Growth of the organism.
  • Replacing old, dead, or injured cells.
  • Formation of gametes for reproduction.

2. Types of Cell Division

Mitosis:

• Process: A single cell (mother cell) divides to form two identical daughter cells.
• Chromosome number: Daughter cells have the same number of chromosomes as the mother cell.
• Function: Involved in growth and repair of tissues.

Meiosis:

• Process: Specific reproductive cells divide to form gametes (eggs and sperm).
• Chromosome number: The resulting daughter cells have half the number of chromosomes as the mother cell.
• Result: Four new cells are produced, as opposed to two in mitosis.
• Function: Involved in sexual reproduction, leading to offspring after fertilization.

Why Reduced Chromosome Number in Meiosis?

• The chromosome number is halved in meiosis to maintain the stability of chromosome number across generations when two gametes (each with half the chromosomes) fuse during fertilization.

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