StudyNotesHub - The Fundamental Unit of Life

1. Introduction to Cells

Discovery: Robert Hooke (1665) first observed cells in cork using a self-designed microscope, naming the box-like structures "cells" (Latin for "little room").
Significance: Hooke’s observation revealed living organisms consist of separate units, a foundational concept in biology.
Later Discoveries:
- Leeuwenhoek (1674): Discovered free-living cells in pond water using an improved microscope.
- Robert Brown (1831): Identified the nucleus in cells.
- Purkinje (1839): Coined "protoplasm" for the cell’s fluid content.
- Schleiden (1838) & Schwann (1839): Proposed the cell theory: all plants and animals are made of cells, and cells are the basic unit of life.
- Virchow (1855): Expanded cell theory, stating all cells arise from pre-existing cells.
- Electron Microscope (1940): Enabled detailed observation of cell organelles.
Cell Definition: Cells are the basic structural and functional units of all living organisms.
Types of Organisms:
- Unicellular: Single-celled organisms (e.g., Amoeba, Chlamydomonas, Paramecium, bacteria) where one cell performs all functions.
- Multicellular: Organisms with many cells (e.g., fungi, plants, animals) showing division of labour for different functions.
Cell Uniformity: All cells in an organism (e.g., onion bulb) have similar structures under a microscope, regardless of the organism’s size.
Activity 5.1 (Onion Peel):
- Peel onion epidermis, mount in water with safranin, and observe under a microscope.
- Cells appear as rectangular, tightly packed structures (like Fig. 5.2 in the textbook).
- Observation: Cells from different-sized onions look similar, confirming cells as the basic building units.
Activity 5.2 (Plant Parts):
- Mount leaf peels, root tips, or onion peels and observe under a microscope.
- Questions:
  - Shape/Size: Cells vary (e.g., Amoeba changes shape, nerve cells have fixed shapes).
  - Structure: Different cells have unique structures based on function.
  - Plant Parts: Cells from roots, stems, or leaves differ in shape and function.
  - Similarities: All cells have a plasma membrane, nucleus, and cytoplasm.

Activity: Observe onion peel cells under a microscope!

2. Cell Structure

Basic Components: All cells have three main parts: plasma membrane, nucleus, and cytoplasm.
Plasma Membrane (Cell Membrane):
- Outermost layer separating cell contents from the environment.
- Composition: Made of lipids and proteins, flexible, and visible only under an electron microscope.
- Selectively Permeable: Allows entry/exit of specific materials, regulating cell interactions.
- Diffusion:
  - Process: Spontaneous movement of substances (e.g., CO₂, O₂) from high to low concentration.
  - Example: CO₂ exits cells when its concentration is higher inside; O₂ enters when lower inside.
  - Role: Facilitates gaseous exchange between cells and their environment.
- Osmosis:
  - Definition: Diffusion of water across a selectively permeable membrane toward higher solute concentration.
  - Types of Solutions:
    - Hypotonic: Higher water concentration outside; cell gains water, swells (e.g., cell in dilute solution).
    - Isotonic: Equal water concentration; no net water movement; cell size unchanged.
    - Hypertonic: Lower water concentration outside; cell loses water, shrinks (e.g., cell in concentrated salt solution).
- Endocytosis: Process where the flexible membrane engulfs food/material (e.g., Amoeba takes in food).
- Activity 5.3 (Egg Osmosis):
  - Dissolve egg shell in dilute HCl, place in water (swells due to osmosis) or salt solution (shrinks).
  - Observation: Water moves into egg in hypotonic solution, out in hypertonic solution.
- Activity 5.4 (Raisins):
  - Place raisins in water (swell) or concentrated sugar/salt solution (shrink).
  - Observation: Confirms osmosis in unicellular organisms and plant cells.
Cell Wall (Plants Only):
- Location: Rigid outer layer outside the plasma membrane in plants, fungi, and bacteria.
- Composition: Mainly cellulose, providing structural strength.
- Plasmolysis:
  - Definition: Shrinkage of cell contents away from the cell wall in hypertonic solutions (e.g., salt/sugar).
  - Occurs only in living cells, not dead ones.
  - Activity 5.6 (Rhoeo Leaf):
    - Mount Rhoeo leaf in water, observe chloroplasts; add sugar/salt solution to see plasmolysis.
    - Boil leaf (kills cells), repeat; no plasmolysis in dead cells.
- Role: Allows cells to withstand hypotonic environments without bursting by exerting pressure against swelling.
Nucleus:
- Structure: Double-layered nuclear membrane with pores for material exchange between nucleus and cytoplasm.
- Components:
  - Chromosomes: Rod-shaped, visible during cell division, made of DNA and proteins.
  - DNA: Contains genes (functional DNA segments) for cell organization and inheritance.
  - Chromatin: Entangled, thread-like DNA in non-dividing cells; organizes into chromosomes during division.
- Role: Controls cell reproduction, development, and chemical activities.
- Prokaryotes vs. Eukaryotes:
  - Prokaryotes: Lack nuclear membrane; nuclear region (nucleoid) contains only nucleic acids (e.g., bacteria).
  - Eukaryotes: Have nuclear membrane and membrane-bound organelles (e.g., plants, animals).
- Activity 5.7 (Cheek Cells):
  - Scrape cheek, mount with methylene blue, observe under microscope.
  - Observation: Cells are flat, with a darkly stained nucleus near the center.
Cytoplasm:
- Definition: Fluid content inside the plasma membrane, containing organelles.
- Role: Site of many cell activities; contains membrane-bound organelles in eukaryotes.
- Prokaryotes: Lack membrane-bound organelles; functions performed by less-organized cytoplasmic parts.
- Example: Viruses lack membranes and show no life characteristics until they use a host cell’s machinery.

Activity: Try the egg osmosis experiment!

3. Cell Organelles

Overview: Eukaryotic cells have membrane-bound organelles for specific functions; prokaryotic cells lack these.
Endoplasmic Reticulum (ER):
- Structure: Network of membrane-bound tubules and sheets, similar to plasma membrane.
- Types:
  - Rough ER (RER): Has ribosomes, appears rough, synthesizes proteins.
  - Smooth ER (SER): No ribosomes, synthesizes lipids, detoxifies poisons (e.g., in liver cells).
- Functions:
  - Transports proteins (via RER) and lipids within the cell or to the nucleus.
  - Membrane Biogenesis: Proteins and lipids from ER build cell membranes.
  - Provides a surface for biochemical activities.
Golgi Apparatus:
- Discovery: Camillo Golgi (1898), Nobel Prize winner (1906) for nervous system studies.
- Structure: Stacks of membrane-bound vesicles (cisterns), connected to ER.
- Functions:
  - Stores, modifies, and packages ER-synthesized materials (e.g., proteins, lipids).
  - Converts simple sugars to complex sugars.
  - Forms lysosomes.
Lysosomes:
- Structure: Membrane-bound sacs with digestive enzymes from RER.
- Functions:
  - Digests foreign materials (e.g., bacteria) and worn-out organelles.
  - Breaks complex substances into simpler ones.
  - Suicide Bags: May burst during cell damage, digesting the cell itself.
Mitochondria:
- Structure: Double-membraned; outer membrane porous, inner membrane folded (increases surface area).
- Functions:
  - Powerhouse of the Cell: Produces ATP (energy currency) for chemical and mechanical work.
  - Contains own DNA and ribosomes, can synthesize some proteins.
Plastids (Plants Only):
- Types:
  - Chromoplasts: Colored; chloroplasts (with chlorophyll) perform photosynthesis.
  - Leucoplasts: Colorless; store starch, oils, or proteins.
- Structure: Double-membraned with stroma (like mitochondria); contain own DNA and ribosomes.
Vacuoles:
- Structure: Storage sacs; small in animal cells, large (50-90% cell volume) in plant cells.
- Functions:
  - Plants: Store cell sap (amino acids, sugars, organic acids, proteins), provide turgidity/rigidity.
  - Unicellular Organisms: Food vacuoles (e.g., Amoeba) store food; specialized vacuoles expel excess water/wastes.

Fun Fact: Mitochondria have their own DNA, like tiny cells within cells!

4. Cell Division

Purpose: Cells divide for growth, repair, replacement of dead/injured cells, and reproduction (gamete formation).
Mitosis:
- Process: Mother cell divides into two identical daughter cells with the same chromosome number.
- Role: Drives growth and tissue repair in organisms.
Meiosis:
- Process: Occurs in reproductive cells; involves two divisions, producing four daughter cells with half the chromosome number of the mother cell.
- Role: Forms gametes for reproduction; reduced chromosome number ensures proper chromosome count after fertilization.

Activity: Compare mitosis and meiosis!

Next Chapter

Ready to dive deeper into science? Explore the next chapter to learn more exciting concepts!

Go to Chapter 6