TEAS 7 · Science · 6 min read
Cells: Structure, Organelles and Transport
- Differentiate between prokaryotic and eukaryotic cell structures and identify the specific functions of intracellular organelles.
- Describe the architectural components of the plasma membrane and the structural fibers comprising the cytoskeleton.
- Compare mechanisms of passive and active transport and predict cellular behavioral changes across different osmotic environments.
Cell Theory and Structural Classification of Cells
All living organisms are composed of one or more cells, establishing the cell as the fundamental structural and functional unit of life. Cell theory states that new cells arise from pre-existing cells through cell division; this does not address how the first cells originated. Understanding the cell as the core unit of life helps explain how complex living structures are built from cells, although some cells persist for years without dividing.
Living cells fall into two distinct structural categories: prokaryotic cells and eukaryotic cells. Prokaryotic cells lack a membrane-bound nucleus and membrane-bound organelles. In these cells, prokaryotic DNA is typically circular and located in an un-enclosed region called the nucleoid. Bacteria and archaea are prokaryotes, representing resilient and structurally streamlined forms of cellular life.
In contrast, eukaryotic cells typically contain a membrane-bound nucleus and specialized membrane-bound organelles. Animals, plants, fungi, and protists are eukaryotes. The internal membrane-bound compartments of eukaryotic cells allow complex biochemical reactions to occur simultaneously in isolated, specialized microenvironments.
| Structural Feature | Prokaryotic Cells | Eukaryotic Cells |
|---|---|---|
| Membrane-Bound Nucleus | Absent (DNA resides in un-enclosed nucleoid) | Present (enclosed by nuclear envelope) |
| Membrane-Bound Organelles | Absent | Present (e.g., mitochondria, endoplasmic reticulum) |
| DNA Configuration | Typically circular | Housed as chromatin inside the nucleus |
| Organism Examples | Bacteria and archaea | Animals, plants, fungi, and protists |
Common mistakes
- Confusing the functions of the rough and smooth endoplasmic reticulum: the rough ER folds, inspects, and modifies proteins using bound ribosomes, while the smooth ER synthesizes lipids, metabolizes carbohydrates, detoxifies drugs and toxins, and stores calcium ions in muscle cells.
- Mixing up animal and plant responses in hypotonic solutions: animal cells swell and may lyse because they lack a rigid boundary, whereas plant cells become firm and turgid because the rigid cellulose cell wall limits swelling.
- Reversing the ion ratios of the sodium-potassium pump: primary active transport moves three sodium ions out of the cell and two potassium ions into the cell per ATP molecule, not two sodium out and three potassium in.
- Assuming all ribosomes produce proteins for the same destination: free ribosomes make proteins for the cytosol, nucleus, mitochondria, and peroxisomes, whereas bound ribosomes make proteins for secretion, membrane insertion, or lysosomes.
Key points
- The cell is the fundamental unit of life, all living organisms consist of one or more cells, and all new cells arise from pre-existing cells through cellular division.
- Prokaryotic cells (bacteria and archaea) lack a membrane-bound nucleus and membrane-bound organelles and typically house circular DNA in a nucleoid, whereas eukaryotic cells (animals, plants, fungi, and protists) typically contain a membrane-bound nucleus and specialized organelles.
- The plasma membrane is an amphipathic phospholipid bilayer with hydrophilic heads, hydrophobic tails, embedded proteins, and cholesterol in animal cells; it is supported by actin microfilaments, intermediate filaments, and tubulin microtubules.
- The endomembrane system modifies and sorts materials: the rough ER processes proteins from bound ribosomes, the smooth ER manages lipids and detoxification (and stores calcium ions in muscle cells as the sarcoplasmic reticulum), and the Golgi apparatus packages cargo received at its cis face and shipped from its trans face.
- Mitochondria generate ATP via aerobic cellular respiration and possess circular DNA; typical plant cells have a cellulose cell wall and a central vacuole that maintains turgor pressure, and photosynthetic plant cells have chloroplasts.
- Passive transport moves materials down gradients without energy via simple diffusion, facilitated diffusion, or osmosis through aquaporins, causing crenation or plasmolysis in hypertonic solutions and lysis or turgidity in hypotonic solutions.
- Primary active transport uses ATP to move solutes against their gradients; secondary active transport couples a solute moving down its electrochemical gradient to another moving against its gradient; energy-dependent bulk vesicular transport moves materials by endocytosis or exocytosis without necessarily moving them against a concentration gradient.
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