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General Biology: Cell Structure, Membranes, and Transport

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  • Cell Theory (3 tenets)

    1. All living organisms are composed of one or more cells.
    2. The cell is the basic unit of structure and organization in organisms.
    3. Cells arise only from pre-existing cells (omnis cellula e cellula).

  • Four basic features shared by all cells

    Plasma membrane, cytosol, chromosomes, and ribosomes.

  • Differences between prokaryotic and eukaryotic cells

    Prokaryotes: no nucleus, no membrane-bound organelles, size ~1–5 µm.
    Eukaryotes: DNA in nucleus, membrane-bound organelles, size ~10–100 µm.

  • Function of Rough Endoplasmic Reticulum (Rough ER)

    Synthesizes and folds secretory and membrane proteins.

  • Function of Smooth Endoplasmic Reticulum (Smooth ER)

    Lipid synthesis, detoxification, and calcium ion (Ca²⁺) storage.

  • Role of Golgi apparatus

    Modifies, sorts, and packages proteins as they move from the cis to the trans face.

  • Endosymbiont theory evidence

    Mitochondria and chloroplasts have their own DNA, ribosomes, double membranes, and divide independently.

  • Functions of the cytoskeleton components

    Microtubules: organelle movement, chromosome separation.
    Microfilaments: cell shape, muscle contraction, motility.
    Intermediate filaments: structural anchoring and shape.

  • Types of cell junctions and their functions

    Tight junctions: seal cells to prevent fluid leakage.
    Desmosomes: rivet cells together.
    Gap junctions: cytoplasmic channels for communication.
    Plasmodesmata: plant cell cytoplasm connections.

  • Fluid mosaic model of plasma membrane

    Membrane is a mosaic of proteins bobbing in a fluid bilayer of phospholipids, with hydrophilic heads facing water and hydrophobic tails inward.

  • Role of cholesterol in the plasma membrane

    Moderates membrane fluidity by restraining excess fluidity at warm temperatures and preventing tight packing at cold temperatures.

  • Difference between integral and peripheral membrane proteins

    Integral proteins penetrate the hydrophobic core; peripheral proteins attach only to the membrane surface.

  • Why do ions and polar molecules require transport proteins?

    They are impeded by the hydrophobic interior of the membrane and cannot cross efficiently without a hydrophilic path.

  • Two basic types of transport proteins

    Channel proteins provide hydrophilic corridors; carrier proteins bind and shuttle molecules by changing shape.

  • Definition of diffusion

    Movement of particles down their concentration gradient without energy input.

  • Definition of osmosis

    Diffusion of free water across a selectively permeable membrane from low to high solute concentration.

  • Tonicity effects on animal and plant cells

    Animal cells: lysed in hypotonic, normal in isotonic, shriveled in hypertonic.
    Plant cells: turgid in hypotonic, flaccid in isotonic, plasmolyzed in hypertonic.

  • Facilitated diffusion

    Passive transport of specific molecules down their concentration gradient via channel or carrier proteins, requiring no energy.

  • Active transport

    Energy-requiring process (usually ATP) that moves solutes against their concentration gradient using carrier proteins.

  • Electrogenic pumps and membrane potential

    Transport proteins that generate voltage across membranes, e.g., sodium-potassium pump in animals and proton pump in plants.

  • Bulk transport mechanisms

    Exocytosis releases large molecules outside the cell; endocytosis takes in large molecules via phagocytosis, pinocytosis, or receptor-mediated endocytosis.