Biology II: The Cell
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The cell theory states that all living things are made of cells, the cell is the basic unit of structure and function, and all cells arise from pre-existing cells.
Microscopy
- Magnification = image size ÷ actual size; resolution is the ability to distinguish two close points. The light microscope resolves about 0.2 µm; the transmission electron microscope (TEM) shows internal structure at high resolution; the scanning electron microscope (SEM) shows surfaces in 3D.
- Use stains (methylene blue, iodine, Gram stain) and measure with an eyepiece graticule calibrated against a stage micrometer.
- Convert units: 1 mm = 1000 µm; 1 µm = 1000 nm.
Prokaryotic and Eukaryotic Cells
| Feature | Prokaryote (bacteria, archaea) | Eukaryote (plants, animals, fungi, protists) |
|---|---|---|
| Nucleus | None; DNA in a nucleoid region | Membrane-bound nucleus |
| Size | About 1 to 10 µm | About 10 to 100 µm |
| Ribosomes | 70S | 80S (cytoplasm); 70S in mitochondria and chloroplasts |
| Organelles | No membrane-bound organelles | Many |
| DNA | Usually a circular chromosome; plasmids | Linear chromosomes with histones |
| Cell wall | Peptidoglycan (bacteria) | Cellulose (plants), chitin (fungi); none in animals |
Organelles and Their Functions
Nucleus (DNA, nucleolus); rough ER (protein synthesis) and smooth ER (lipids, detoxification); Golgi apparatus (modification, packaging, secretion); lysosomes (digestion); mitochondria (aerobic respiration; double membrane, cristae, matrix); chloroplasts (photosynthesis; thylakoids, grana, stroma); vacuole (storage, turgor); ribosomes; cytoskeleton (support, movement); centrioles. The endosymbiotic theory proposes that mitochondria and chloroplasts descend from engulfed prokaryotes (Margulis, 1970). Plant cells also have cell walls and plasmodesmata; animal cells have cell junctions (tight junctions, desmosomes, gap junctions).
The Plasma Membrane
The fluid mosaic model (Singer & Nicolson, 1972) describes a phospholipid bilayer with embedded proteins, cholesterol, and glycoproteins. Proteins act as channels, carriers, receptors, enzymes, and recognition sites.
Movement Across Membranes
| Process | Description | Energy |
|---|---|---|
| Simple diffusion | Movement down a concentration gradient (O₂, CO₂) | None |
| Facilitated diffusion | Through channel or carrier proteins | None |
| Osmosis | Diffusion of water across a selectively permeable membrane | None |
| Active transport | Against the gradient using carrier proteins, for example the sodium-potassium pump (3 Na⁺ out, 2 K⁺ in) | ATP |
| Endocytosis and exocytosis | Bulk transport using vesicles | ATP |
Water potential (Ψ) predicts the movement of water: Ψ = Ψs + Ψp (solute potential, which is negative, plus pressure potential). Water moves from higher (less negative) to lower (more negative) water potential. Pure water at atmospheric pressure has Ψ = 0. A plant cell in a hypotonic solution becomes turgid; in a hypertonic solution it undergoes plasmolysis; animal cells burst (lyse) or shrink (crenate).
The Cell Cycle and Cell Division
- Interphase (G₁, S, G₂) and M phase. DNA replicates in S phase. Checkpoints (controlled by cyclins and cyclin-dependent kinases) regulate progression; failure of control can lead to cancer.
- Mitosis: prophase, metaphase, anaphase, telophase, followed by cytokinesis; produces two genetically identical diploid cells; used for growth, repair, and asexual reproduction.
- Meiosis: two divisions (meiosis I and II) producing four genetically different haploid cells. Variation arises from crossing over in prophase I and independent assortment of homologous chromosomes. Non-disjunction causes aneuploidy, such as Down syndrome (trisomy 21).
| Feature | Mitosis | Meiosis |
|---|---|---|
| Number of divisions | One | Two |
| Daughter cells | Two, diploid, identical | Four, haploid, different |
| Crossing over | No | Yes |
| Function | Growth, repair | Gamete formation |
Common Mistakes
- Confusing diffusion with osmosis.
- Drawing organelles to the wrong relative size.
- Mixing up the stages of mitosis and meiosis, particularly anaphase I and II.
CHAPTER 5