IB Biology: Cell Biology

Cell biology forms the cornerstone of biological understanding, providing the essential framework from which all other life processes emerge. For IB Biology, mastering this unit is non-negotiable; it underpins topics from genetics to physiology and is heavily assessed in both Paper 1 multiple-choice questions and Paper 2 data-based and structured questions.

Cell Theory and Basic Architecture

All living organisms are composed of cells, the basic structural and functional units of life. This principle is the heart of cell theory, which also states that all cells arise from pre-existing cells and that the chemical reactions of life occur within cells. In IB, you must be able to contrast the two fundamental cell types: prokaryotic and eukaryotic.

Prokaryotic cells, such as bacteria, are simpler and smaller. They lack a membrane-bound nucleus and organelles. Their genetic material is a single, circular DNA molecule found in a region called the nucleoid. They possess a cell wall, a plasma membrane, ribosomes (70S), and may have structures like plasmids, pili, and flagella. In contrast, eukaryotic cells, found in plants, animals, fungi, and protists, are defined by compartmentalization. They contain a true nucleus housing linear DNA chromosomes and a suite of membrane-bound organelles, each with specialized functions. Key organelles include mitochondria for aerobic respiration, chloroplasts (in plants) for photosynthesis, the rough endoplasmic reticulum for protein synthesis, the Golgi apparatus for modification and packaging, and lysosomes for digestion. A classic IB exam task is to interpret electron micrographs, identifying organelles and justifying your conclusions based on structural features.

IB Insight: When drawing comparisons, use a table. A common trap is stating prokaryotes have "no DNA"—they do, it's just not enclosed in a nuclear membrane. Always specify the type of ribosome (70S vs. 80S) as a distinguishing feature.

The Dynamic Cell Membrane

The plasma membrane is not just a static barrier; it is a dynamic, selectively permeable structure that controls exchange with the environment. The fluid mosaic model accurately describes its structure: a bilayer of phospholipids in which various proteins are embedded, like pieces in a mosaic, with the entire layer having fluidity. Cholesterol modulates this fluidity in animal cells. Integral proteins span the membrane, while peripheral proteins are attached to the surface.

Transport across this membrane is categorized. Passive transport requires no cellular energy (ATP) and includes simple diffusion of small, non-polar molecules and facilitated diffusion of larger or charged particles via channel or carrier proteins. Osmosis is the passive diffusion of water across a selectively permeable membrane from a region of lower solute concentration to higher solute concentration. Active transport, however, uses ATP to pump substances against their concentration gradient via protein pumps, such as the sodium-potassium pump. Vesicle-mediated transport involves bulk movement via endocytosis (taking in) and exocytosis (expelling out).

IB Insight: You must be able to explain the consequences of osmosis in animal and plant cells. An animal cell in a hypotonic solution will lyse, while in a hypertonic solution it will crenate. A plant cell in a hypotonic solution becomes turgid (ideal), while in a hypertonic solution it undergoes plasmolysis, a favorite exam diagram.

The Cell Cycle and Mitosis

Growth and asexual reproduction are driven by the cell cycle, an ordered sequence of events leading to cell division. It consists of interphase (G1, S, G2) and mitotic phase (mitosis and cytokinesis). DNA replication occurs during the S phase, so by mitosis, chromosomes are duplicated, consisting of two identical sister chromatids joined at the centromere.

Mitosis is nuclear division resulting in two genetically identical daughter nuclei. It is a continuous process, but for study, it's divided into four phases: prophase (chromosomes condense, nuclear envelope breaks down), metaphase (chromosomes align at the equator), anaphase (sister chromatids are pulled apart to opposite poles), and telophase (chromosomes de-condense, nuclear envelopes reform). Cytokinesis then divides the cytoplasm: in animal cells via a cleavage furrow, and in plant cells via a cell plate formation.

IB Application: Be prepared to calculate mitotic indices from micrograph data or describe the role of mitosis in processes like tissue repair, embryonic development, and asexual reproduction in species like fungi.

Meiosis and Genetic Variation

While mitosis produces clones, meiosis is a reduction division that produces four genetically unique haploid gametes (sperm and egg cells) from one diploid parent cell. This is essential for sexual reproduction. Meiosis involves two consecutive divisions: Meiosis I and Meiosis II.

The key events that generate genetic variation occur in Meiosis I. During prophase I, homologous chromosomes pair up in a process called synapsis, forming bivalents. They exchange segments of DNA through crossing over, creating new combinations of alleles. In metaphase I, the independent assortment of these homologous pairs occurs—how one pair lines up is independent of any other pair. This random orientation leads to a vast number of possible gamete combinations. Anaphase I separates homologous chromosomes, not sister chromatids, reducing the chromosome number from diploid to haploid. Meiosis II then separates the sister chromatids, similar to mitosis.

Common Pitfall: The most frequent error is confusing the products of meiosis I and II. Remember, meiosis I results in two haploid cells, but each chromosome is still duplicated (two chromatids). Only after meiosis II are four haploid cells with unduplicated chromosomes produced.

Stem Cells and Differentiation

Stem cells are unspecialized cells that retain the capacity for both self-renewal (dividing to produce more stem cells) and differentiation (developing into specialized cell types). They are categorized by potency: totipotent (can form any cell, including placental, e.g., early embryo), pluripotent (can form any body cell type, e.g., embryonic stem cells), and multipotent (limited to a specific range, e.g., adult bone marrow stem cells forming blood cells).

Therapeutic uses of stem cells, such as in treating Stargardt's disease (a retinal degeneration) or leukemia via bone marrow transplants, are core IB syllabus examples. You must weigh the ethical considerations, particularly concerning the use of embryonic stem cells, which involves the destruction of embryos, against the use of adult or induced pluripotent stem cells (iPSCs).

IB Assessment Tip: Questions often ask you to "discuss" or "evaluate" the use of stem cells. A balanced argument should include clear biological explanations of their therapeutic potential alongside specific, named ethical arguments from different perspectives.

Common Pitfalls

1. Misidentifying Organelles in Micrographs: Confusing chloroplasts with mitochondria, or rough ER with Golgi. Remember: chloroplasts have thylakoid stacks (grana); mitochondria have cristae; Rough ER is studded with ribosomes and is often sheet-like; Golgi appears as a stack of flattened cisternae.

Correction: Always link structure to function. If the question mentions "protein export," look for extensive rough ER and Golgi. If it mentions "ATP production," look for many mitochondria.

1. Confusing Mitosis and Meiosis Outcomes: Stating that meiosis produces two identical diploid cells.

Correction: Use a mnemonic for meiosis: "It's all about reduction and variation." It produces four unique haploid cells. Mitosis is for growth and repair, producing two identical diploid cells.

1. Osmosis Terminology Errors: Describing water moving from a "high water concentration" to a "low water concentration." While not entirely wrong, it is less precise.

Correction: Use solute concentration terminology as the IB markschemes prefer. State water moves from a region of lower solute concentration (hypotonic) to higher solute concentration (hypertonic).

1. Overlooking the Importance of Cell Size: Forgetting why cells are small. The key concept is the surface area to volume ratio ($SA:V$).

Correction: As a cell grows, its volume increases faster than its surface area. A low $SA:V$ ratio makes diffusion of materials inefficient, limiting cell size. This is why large organisms are multicellular rather than having gigantic single cells.

Summary

• Cell Theory is Fundamental: All living things are made of cells, the smallest unit of life, which arise from pre-existing cells. The distinction between simple prokaryotic cells and compartmentalized eukaryotic cells is critical.
• The Membrane is a Selective, Dynamic Interface: Understand the fluid mosaic model and be able to compare and contrast passive transport (diffusion, osmosis, facilitated diffusion) with active transport and vesicle-mediated processes.
• Mitosis Ensures Genetic Continuity: It is a precisely controlled process for growth and repair, producing two genetically identical diploid daughter cells from one parent cell.
• Meiosis Generates Genetic Variation: Through crossing over and independent assortment in Meiosis I, it produces four genetically unique haploid gametes, which is the basis for sexual reproduction and genetic diversity.
• Stem Cells Hold Therapeutic Potential: Their capacity for self-renewal and differentiation offers treatments for diseases, but their use, particularly embryonic, requires careful ethical evaluation.