Living World and Cell Biology
Characteristics of Living Organisms
All living organisms share certain key characteristics that distinguish them from non-living matter:
- Cellular organization: All living things are made up of cells
- Nutrition: Taking in food and using it for energy and growth
- Respiration: Breaking down food to release energy (ATP)
- Excretion: Removing waste products of metabolism
- Growth: Increase in size and mass
- Reproduction: Producing offspring of the same kind
- Responsiveness/Irritability: Reacting to stimuli in the environment
- Movement: Locomotion (external) or internal movement (blood, food)
- Adaptation: Adjusting to environmental changes over time
MR. GREN (Movement, Respiration, Growth, Excretion, Nutrition, Responsiveness) — the mnemonic for characteristics of life.
Cell: The Basic Unit of Life
Discovery
- Robert Hooke (1665): First to observe cells — examined a thin slice of cork under a microscope and saw small compartments, calling them “cells” (Latin for small rooms)
- Anton van Leeuwenhoek (1676): First to observe living cells (bacteria, protozoa) — called them “animalcules”
- Matthias Schleiden (1838): German botanist — all plants are made of cells
- Theodor Schwann (1839): German zoologist — all animals are made of cells
- Rudolf Virchow (1855): “Omnis cellula e cellula” — all cells arise from pre-existing cells
Cell Theory
The cell theory has three main postulates:
- All living organisms are composed of cells
- The cell is the basic unit of life
- All cells arise from pre-existing cells (not spontaneous generation)
Types of Cells
Prokaryotic Cells (Pro = before, karyon = nucleus)
- No nuclear membrane — genetic material (DNA) is scattered in the cytoplasm as a nucleoid
- No membrane-bound organelles (no mitochondria, Golgi bodies, etc.)
- Smaller, simpler cells
- Example: Bacteria, Blue-green algae (Cyanobacteria)
Key features:
- DNA is circular, present in the cytoplasm
- 70S ribosomes (smaller)
- Cell wall present (made of peptidoglycan)
- Reproduction is asexual (binary fission)
Eukaryotic Cells (Eu = true, karyon = nucleus)
- Has a definite nucleus with a nuclear membrane
- Membrane-bound organelles (mitochondria, chloroplast, Golgi apparatus, etc.)
- Larger, more complex
- Example: Plants, Animals, Fungi, Protists
Key features:
- DNA is linear, present inside nucleus
- 80S ribosomes
- More complex internal compartmentalization
Comparison: Plant Cell vs Animal Cell
| Feature | Plant Cell | Animal Cell |
|---|---|---|
| Cell Wall | Present (cellulose) | Absent |
| Chloroplast | Present (photosynthesis) | Absent |
| Vacuoles | Large central vacuole | Small, many |
| Centrioles | Mostly absent (except in lower plants) | Present |
| Shape | Rectangular, fixed shape | Irregular, flexible |
| Lysosomes | Rare | Common |
| Mitochondria | Present but fewer | Many |
| Nutrition | Autotrophic (mostly) | Heterotrophic |
| Size | Larger (10–100 µm) | Smaller (10–20 µm) |
Cell Organelles and Their Functions
Plasma Membrane:
- Made of phospholipid bilayer + proteins (Fluid Mosaic Model — Singer and Nicolson, 1972)
- Selectively permeable — controls what enters and exits
- Contains cholesterol (in animal cells — absent in plant cells)
- Transport: Diffusion, osmosis, active transport, facilitated diffusion
Nucleus:
- Contains genetic material (DNA) organized as chromosomes
- Surrounded by double nuclear membrane with pores
- Contains nucleolus (for ribosome assembly)
- Controls all cell activities (the “brain” of the cell)
- DNA carries genetic code — arranged in histone proteins forming nucleosomes
Mitochondria (Powerhouse of the cell):
- Double membrane — outer smooth, inner folded (cristae)
- Site of aerobic respiration — produces ATP (adenosine triphosphate)
- Has its own DNA (semi-autonomous)
- Called the “powerhouse” because most ATP is produced here
Chloroplast (in plant cells only):
- Double membrane organelle
- Contains chlorophyll (green pigment) — absorbs light energy for photosynthesis
- Has thylakoid membranes stacked into grana
- Semi-autonomous (own DNA and ribosomes)
- Site of photosynthesis (light reaction + dark reaction/Calvin cycle)
Endoplasmic Reticulum (ER):
- Rough ER: Has ribosomes on surface — protein synthesis and transport
- Smooth ER: No ribosomes — lipid synthesis and detoxification
Golgi Apparatus (Golgi body/Bodies):
- Stack of flattened membrane sacs (cisternae)
- Modifies, packages, and ships proteins and lipids
- “Post office of the cell”
Ribosome:
- Site of protein synthesis
- Made of rRNA and proteins
- 70S in prokaryotes, 80S in eukaryotes
Lysosome:
- Contains digestive enzymes (hydrolases)
- Digests worn-out organelles (autophagy), foreign particles
- Called the “suicide bag” — if it bursts, it digests the cell itself
Centrosome/Centrioles:
- Present in animal cells
- Involved in cell division (formation of spindle fibers)
- Forms the basal bodies of cilia and flagella
CTET Exam Focus
- Cell Theory: Schleiden, Schwann, Virchow
- Prokaryotic vs Eukaryotic cells: nucleus membrane, organelles
- Plant cell vs Animal cell: Cell wall, chloroplast, vacuole, centrioles
- Fluid Mosaic Model: phospholipid bilayer + proteins
- Mitochondria: ATP production, cristae
- Chloroplast: chlorophyll, grana, thylakoids, photosynthesis site
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