HESI A2 · Biology · 6 min read
Cells, Organelles and Membrane Transport
- Distinguish between prokaryotic and eukaryotic cellular structures, genetic organization, and surface boundaries.
- Identify the distinct functions of eukaryotic organelles involved in synthesis, energy transformation, structural support, and waste processing.
- Compare passive and active membrane transport mechanisms, detailing tonicity effects and vesicle-mediated processes.
Prokaryotic and Eukaryotic Cell Architecture
All living organisms are organized into cellular units, which fall into two primary structural categories: prokaryotic cells and eukaryotic cells. Bacteria and Archaea represent the prokaryotic organisms. In contrast, animals, plants, fungi, and protists are composed of eukaryotic cells. The hallmark distinction between these groups centers on internal organization. Prokaryotic cells lack a membrane-bound nucleus and membrane-bound organelles, whereas eukaryotic cells possess a true, membrane-bound nucleus that houses their genetic material.
The physical arrangement of genetic material differs markedly between these two cellular designs. In prokaryotes, DNA is typically circular and rests in an unbound region termed the nucleoid. Eukaryotes, however, maintain their nuclear DNA as multiple linear chromosomes that are organized and tightly associated with histone proteins. Despite these structural distinctions in genetic storage, both prokaryotic and eukaryotic cells possess ribosomes to execute protein synthesis. Even so, their translational machinery is not identical: prokaryotic ribosomes are 70S and smaller than eukaryotic cytosolic ribosomes, which are 80S.
Both cell types are bounded by lipid-based plasma membranes, though archaeal membrane lipids differ from those found in bacteria and eukaryotes. Outside this lipid boundary, many cells construct a protective cell wall, yet the chemical composition varies according to taxonomic group. Most bacterial cell walls are composed of peptidoglycan. Plant cell walls, on the other hand, are primarily composed of cellulose, while fungal cell walls contain chitin and glucans. These distinct extracellular layers provide reinforcement while preserving internal integrity.
| Feature | Prokaryotic Cells | Eukaryotic Cells |
|---|---|---|
| Nucleus | Lacks a membrane-bound nucleus (DNA in nucleoid) | Possesses a true, membrane-bound nucleus |
| Membrane-Bound Organelles | Absent | Present |
| DNA Structure | Typically circular and unbound | Multiple linear chromosomes with histone proteins |
| Ribosome Size | 70S ribosomes (smaller) | 80S cytosolic ribosomes |
| Representative Organisms | Bacteria and Archaea | Animals, plants, fungi, and protists |
Common mistakes
- Assuming plant cells burst in hypotonic solutions: animal cells swell and may lyse in sufficiently hypotonic conditions, whereas plant cells generally become turgid because their cell walls resist expansion.
- Confusing the specific roles of the rough and smooth endoplasmic reticulum; the rough ER aids in protein folding and transport, while the smooth ER synthesizes lipids, metabolizes carbohydrates, and detoxifies drugs and toxins.
- Believing that facilitated diffusion requires cellular energy because it involves transport proteins; facilitated diffusion moves solutes down their gradient and does not consume ATP.
- Mixing up the directions and ion counts of the sodium-potassium pump; it exports three sodium ions and imports two potassium ions per hydrolyzed ATP molecule.
Key points
- Prokaryotes (Bacteria and Archaea) lack a membrane-bound nucleus and membrane-bound organelles, holding typically circular DNA in a nucleoid, whereas eukaryotes possess a true nucleus with linear chromosomes bound to histones.
- Prokaryotes have smaller 70S ribosomes, while eukaryotic cytosol contains larger 80S ribosomes; both cell types have lipid-based plasma membranes, but not all cells have walls, animal cells lack them, while bacterial, plant, and fungal cell walls have different compositions.
- Eukaryotic synthesis and energy rely on the nucleus, nucleolus, ribosomes, rough ER, smooth ER, Golgi apparatus, mitochondria with cristae, and in plants, chloroplasts with thylakoids and a large central vacuole.
- Cell maintenance and architecture are sustained by hydrolytic lysosomes, peroxisomes neutralizing hydrogen peroxide, a three-part cytoskeleton, centriole-containing centrosomes, and microtubule-based cilia and flagella.
- The plasma membrane is a fluid mosaic of phospholipids, cholesterol, integral proteins, peripheral proteins, glycoproteins, and glycolipids that displays selective permeability.
- Passive transport (simple diffusion, facilitated diffusion, osmosis via aquaporins) requires no cellular energy, while active transport (sodium-potassium pump, phagocytosis, pinocytosis, exocytosis) consumes energy, typically from ATP hydrolysis.
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