Shapes Galore
Varied cell shapes: RBC, nerve cell, epithelial
Shape mirrors duty
Cells adopt specific forms that match their roles.
Biconcave red blood cells bend easily and expose more surface for gas exchange.
A nerve cell’s long, branched axon carries impulses swiftly over distance.
Columnar or flat epithelial cells create tight sheets for protection or absorption.
Key Points:
- RBC: biconcave disc → high surface area, easy capillary passage.
- Nerve cell: long & branched → rapid signal conduction.
- Epithelial cell: columnar/flat sheet → cover, protect, absorb.
Sizing Things Up
Relative Scale: Virus → PPLO → Bacterium → Human Cell
A virus measures about 20–300 nm; even the tiniest PPLO (mycoplasma) is roughly 300 nm.
Typical bacteria are 1–5 µm, whereas a eukaryotic cell often spans 20 µm or more.
Key Points:
- Virus: 0.02–0.3 µm
- PPLO (Mycoplasma): ~0.3 µm
- Bacterium: 1–5 µm
- Eukaryotic cell: 20–100 µm
Plant & Animal Cells
Plant vs Animal cell diagrams
Spot the unique parts
Plant and animal cells share the same core organelles for life activities.
Focus on the structures that make each cell type distinct.
Key Points:
- Plant cell: rigid cellulose cell wall outside the membrane.
- Plant cell: single large central vacuole for storage & turgor.
- Animal cell: lacks cell wall, has small vacuoles and contains centrioles for cell division.
- All other organelles are common to both cell types.
Fluid Mosaic Membrane
Fluid mosaic model (simplified)
A Lipid-Protein Sea
Phospholipids form a double layer; hydrophobic tails meet inside, heads face water, letting the sheet stay fluid.
Proteins float in this layer, moving laterally to relay signals and transport molecules.
Key Points:
- Cholesterol slots between phospholipid tails, keeping fluidity steady across temperatures.
- Membrane proteins act as channels, carriers, enzymes and receptors.
- Lateral movement of lipids and proteins lets the plasma membrane flex and self-heal.
Endoplasmic Reticulum
RER (ribosome dotted) and SER network inside a cell.
Rough ER vs Smooth ER
Endoplasmic reticulum (ER) forms a continuous membrane network from the nucleus into the cytoplasm.
Its rough and smooth domains look similar but perform distinct cellular jobs.
Key Points:
- Rough ER: Ribosome-studded cisternae; synthesises secretory & membrane proteins, then sends them to the Golgi.
- Smooth ER: Ribosome-free tubules; produces lipids, phospholipids & steroids, detoxifies drugs, stores Ca²⁺.
Golgi Packaging Hub
Stacked cisternae of the Golgi apparatus
Cis-to-Trans Assembly Line
Cis-face, pressed against the ER, receives fresh proteins and lipids.
Moving cisternae add or trim sugars, creating the final molecular labels.
Trans-face packs the labeled cargo into vesicles that head to membranes or outside.
Key Points:
- Cis-face = receiving dock
- Trans-face = shipping desk
- Vesicles = delivery trucks
Chloroplast Factory
Grana stacks & stroma in a chloroplast
Grana & Stroma Roles
Light reaction happens on grana—flattened thylakoid stacks.
Chlorophyll here captures sunlight to form \( \text{ATP} \) and \( \text{NADPH} \).
Dark reaction (Calvin cycle) unfolds in the surrounding stroma.
Stroma enzymes use CO₂ plus the ATP & NADPH to build glucose.
Key Points:
- Grana = site of light reaction, energy capture.
- Stroma = site of dark reaction, carbon fixation.
- Structure keeps the two phases organised and efficient.