1. Chapter at a glance
- All living organisms are composed of cells and products of cells; all cells arise from pre-existing cells (cell theory).
- Cell is the fundamental structural and functional unit of life; anything less than a complete cell does not ensure independent living.
- Prokaryotic cells lack membrane-bound nucleus and organelles (except ribosomes); eukaryotic cells possess membrane-bound nucleus and organelles.
- Plasma membrane is selectively permeable; transport occurs by passive (simple diffusion, osmosis) or active (energy-dependent, e.g., Na+/K+ pump) mechanisms.
- Endomembrane system (ER, Golgi complex, lysosomes, vacuoles) coordinates packaging, transport, digestion and storage.
- Mitochondria and chloroplasts are double-membrane organelles containing their own 70S ribosomes, circular DNA and are sites of ATP synthesis and photosynthesis respectively.
- Ribosomes (70S in prokaryotes, 80S in eukaryotes) are sites of protein synthesis; 9+2 axoneme arrangement is present in cilia and flagella.
- Nucleus contains chromatin, nucleolus and is enclosed by nuclear envelope with pores; chromosomes are classified as metacentric, submetacentric, acrocentric or telocentric based on centromere position.
2. Definitions and laws
Cell theory (as understood today):
(i) all living organisms are composed of cells and products of cells.
(ii) all cells arise from pre-existing cells.
3. Important diagrams and activities
- Figure 8.1: Different shapes of cells – demonstrates variation in cell shape related to function.
- Figure 8.2: Comparison of sizes of prokaryotic, eukaryotic cells and viruses – shows relative sizes and that viruses are smaller than cells.
- Figure 8.3: Plant cell and animal cell – shows presence/absence of cell wall, plastids, large vacuole and centrioles.
- Figure 8.4: Fluid mosaic model of plasma membrane – demonstrates arrangement of phospholipids, proteins and fluidity.
- Figure 8.5: Endoplasmic reticulum (rough and smooth) – shows ribosomes on RER and continuity with nuclear envelope.
- Figure 8.6: Golgi apparatus – shows cis and trans faces and stacked cisternae.
- Figure 8.7: Structure of mitochondrion – shows outer membrane, inner membrane with cristae and matrix.
- Figure 8.8: Sectional view of chloroplast – shows grana, stroma lamellae, thylakoids and stroma.
- Figure 8.9: Ribosome – shows large and small subunits.
- Figure 8.10: Section of cilia/flagella – shows 9+2 axoneme arrangement.
- Figure 8.11: Structure of nucleus – shows nuclear envelope, pores, nucleolus and chromatin.
- Figure 8.12 & 8.13: Chromosome with kinetochore and types of chromosomes based on centromere position – demonstrates metacentric, submetacentric, acrocentric and telocentric chromosomes.
4. Common misconceptions and exam pitfalls
- Do not state Robert Brown discovered the cell (he discovered the nucleus); cell was first observed by Leeuwenhoek.
- Viruses are acellular and do not satisfy cell theory criteria for independent living.
- Mycoplasma lack cell wall; Gram-positive and Gram-negative classification is based on cell envelope staining, not shape.
- Ribosome sedimentation units: prokaryotic = 70S (50S + 30S), eukaryotic = 80S (60S + 40S); mitochondrial and chloroplast ribosomes are 70S.
- Middle lamella is calcium pectate; primary wall is capable of growth, secondary wall is laid inner to it.
- Centrioles are absent in most plant cells but present in animal cells; both cilia/flagella arise from basal bodies.
- Nuclear pores allow bidirectional movement of RNA and proteins; mature RBCs and sieve tube cells lack nucleus yet are considered living in context of the chapter.
5. Formula sheet
No mathematical formulas or equations are present in the chapter.