# Cell Biology Core Concepts Review
While the ACT Science section focuses on data interpretation rather than content knowledge, having a solid understanding of cell biology helps you work through biology passages faster and with greater confidence. This guide reviews the key cell biology concepts most likely to appear.
1. Cell Structure
Prokaryotic vs. Eukaryotic Cells
| Feature | Prokaryotic | Eukaryotic |
|---|---|---|
| Nucleus | No (nucleoid region) | Yes (membrane-bound) |
| Organelles | Few, no membrane-bound | Many membrane-bound |
| Size | Small (1–10 μm) | Larger (10–100 μm) |
| Examples | Bacteria, Archaea | Animals, Plants, Fungi |
Key Organelles
- Nucleus: Contains DNA; controls cell activities
- Mitochondria: Cellular respiration — produces ATP
- \(C_6H_{12}O_6 + 6O_2 \rightarrow 6CO_2 + 6H_2O + \text{ATP}\)
- Chloroplasts (plants): Photosynthesis
- \(6CO_2 + 6H_2O \xrightarrow{\text{light}} C_6H_{12}O_6 + 6O_2\)
- Ribosomes: Protein synthesis
- Cell membrane: Phospholipid bilayer; selectively permeable
- Endoplasmic reticulum (ER): Rough ER (protein processing), Smooth ER (lipid synthesis)
- Golgi apparatus: Packages and ships proteins
2. Cell Transport
Passive Transport (no energy required)
- Diffusion: Movement from high to low concentration
- Osmosis: Diffusion of water across a selectively permeable membrane
- Facilitated diffusion: Uses channel or carrier proteins
Active Transport (requires ATP)
- Pump proteins: Move substances against the concentration gradient
- Endocytosis: Cell engulfs materials (phagocytosis, pinocytosis)
- Exocytosis: Cell expels materials via vesicles
Osmosis and Tonicity
- Hypertonic solution: Cell loses water (shrivels/crenation)
- Hypotonic solution: Cell gains water (swells/may lyse)
- Isotonic solution: No net water movement
3. Cell Division
Mitosis (somatic cells)
- Produces 2 genetically identical daughter cells
- Phases: Prophase → Metaphase → Anaphase → Telophase
- Purpose: Growth, repair, asexual reproduction
Meiosis (gametes)
- Produces 4 genetically unique haploid cells
- Two divisions: Meiosis I (homologous pairs separate) and Meiosis II (sister chromatids separate)
- Crossing over in Prophase I creates genetic variation
4. Worked Example
Q: A passage describes an experiment where red blood cells are placed in solutions of different salt concentrations. In Solution A (0.9% NaCl), cells remain normal. In Solution B (0.2% NaCl), cells swell and burst. In Solution C (5% NaCl), cells shrink.
Explain these results using osmosis.
A:
- Solution A (0.9%) is isotonic — water moves in and out equally, so cells maintain their shape
- Solution B (0.2%) is hypotonic — water moves into the cell by osmosis (from low solute to high solute concentration), causing swelling and lysis
- Solution C (5%) is hypertonic — water moves out of the cell, causing it to shrink (crenation)
5. Practice Questions
Q1. Which organelle would be most abundant in muscle cells? Why?
A1. Mitochondria. Muscle cells require large amounts of ATP for contraction, so they contain many mitochondria to carry out cellular respiration.
Q2. A cell has 46 chromosomes. After mitosis, how many chromosomes does each daughter cell have? After meiosis?
A2. After mitosis: 46 chromosomes (diploid, identical to parent). After meiosis: 23 chromosomes (haploid, genetically unique due to crossing over and independent assortment).
Want to check your answers and get step-by-step solutions?
Summary
- Know the difference between prokaryotic and eukaryotic cells
- Understand key organelle functions, especially mitochondria and chloroplasts
- Distinguish passive vs. active transport and predict osmosis outcomes
- Know the outcomes of mitosis (2n → 2n) vs. meiosis (2n → n)