4.5 Chapter 4 Summary
Christelle Sabatier
Relevant Course Learning Objectives
- Describe the composition of a cell membrane.
- Describe what types of molecules, based on polarity, need membrane proteins to cross cell membranes
- Describe the different forms of membrane transport
- Contrast channels using facilitated diffusion with pumps using active transport
- Interpret models of membrane transport presented as cell diagrams.
- Model the type of transport needed for a molecule to cross the membrane based on its molecular structure.
4.1 Cell Types and Compartments
Key Distinctions Between Prokaryotic and Eukaryotic Cells
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Eukaryotes possess a true nucleus (membrane-bound) and multiple organelles (e.g., mitochondria, endoplasmic reticulum, Golgi apparatus, lysosomes, chloroplasts in plants).
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Cytoplasm is the space between the plasma membrane and nuclear envelope, filled with cytosol, cytoskeleton, and dissolved molecules, and it is where many metabolic reactions occur.
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Organelle compartmentalization refers to when organelles create specialized internal environments (e.g., acidic lumens in lysosomes) to optimize biochemical functions.
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Plasma membranes are phospholipid bilayers embedded with proteins and cholesterol that regulate molecular traffic and maintain cellular integrity.
4.2 Plasma Membrane Structure and Components
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Explained by the fluid-mosaic model: A dynamic bilayer of phospholipids where proteins, cholesterol, and carbohydrates are interspersed and laterally mobile.
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Selectivity: Amphipathic nature allows small nonpolar molecules (O₂, CO₂, fat-soluble vitamins) to diffuse directly, whereas ions and polar molecules require transport proteins.
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Membrane asymmetry: Inner and outer leaflets differ in lipid and protein composition; glycoproteins and glycolipids on the exterior aid cell recognition and adhesion.
4.3 Passive Membrane Transport
Passive transport involves movement down a concentration gradient without using ATP.
Main types:
- Simple diffusion: nonpolar molecules pass directly through the bilayer (e.g., O2, CO2).
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Facilitated diffusion:
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Channel proteins form hydrophilic pores (e.g., aquaporins for water, ion channels for Na⁺, K⁺, Cl⁻).
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Carrier proteins (e.g., GLUT transporters) bind specific substrates and undergo conformational changes to shuttle molecules.
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Osmosis and Tonicity
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Osmosis: Water flows through a semipermeable membrane toward higher solute concentration, often via aquaporins.
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Tonicity: Describes solution effects on cell volume:
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Hypotonic: water enters → cells swell/lyse
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Isotonic: balanced, no net change
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Hypertonic: water exits → cells shrink/crenate
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Biological responses: For example, plant cell turgor or animal osmoregulation via vacuoles and kidneys.
4.4 Active Membrane Transport
Active transport moves substances against gradients and requires energy (typically ATP).
Types:
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Primary active transport:
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Directly uses ATP.
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For example, Na⁺/K⁺‑ATPase pumps Na⁺ out and K⁺ in, critical for membrane potential and secondary transport mechanics.
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Secondary active transport:
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Uses energy from existing gradients.
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Symporters (e.g., Na⁺/glucose co-transport) move two substances simultaneously in the same direction.
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Antiporters (e.g., Na⁺/H⁺ exchanger) move in opposite directions.
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Additional Mechanisms
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ATP-binding cassette (ABC) transporters, P-type pumps, V-type pumps transport ions, toxins, and drugs.
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Endocytosis and exocytosis are bulk transport processes (e.g., phagocytosis, pinocytosis, receptor-mediated endocytosis, vesicle fusion for secretion).
Summary Table
| Process | Energy | Direction | Examples/Components |
|---|---|---|---|
| Simple diffusion/osmosis | None | High to low | O₂, CO₂, H2O, nonpolar molecules |
| Facilitated diffusion | None | High to low | Ion channels, aquaporins, carrier proteins |
| Primary active transport | ATP | Low to high | Na⁺/K⁺‑ATPase, Ca²⁺ pumps |
| Secondary active transport | Ion Gradient | Low to high | Na⁺/glucose symporter, Na⁺/H⁺ antiporter |
| Bulk transport (endo/exocytosis) | ATP | Vesicular | Macromolecules, receptor-mediated uptake |
Practice Questions
Licenses and Attributions
“4.5 Chapter 4 Summary” was initially generated by ChatGPT4.0 and then modified by Christelle Sabatier. “4.5 Chapter 4 Summary” is licensed under CC-BY-NC 4.0.
