Cell Biology (SL) — IB Biology SL SL Study Guide

For: IB Biology SL candidates sitting IB Biology SL.

Covers: Core Cell Biology concepts including cell theory, ultrastructure of prokaryotic and eukaryotic cells, the fluid mosaic model of membrane structure, and active/passive membrane transport mechanisms.

You should already know: IGCSE Biology, basic chemistry.

A note on the practice questions: All worked questions in the "Practice Questions" section below are original problems written by us in the IB Biology SL style for educational use. They are not reproductions of past IBO papers and may differ in wording, numerical values, or context. Use them to practise the technique; cross-check with official IBO mark schemes for grading conventions.

1. What Is Cell Biology?

Cell biology is the study of the structure, function, and behavior of cells, the smallest fundamental unit of life. As the first core topic in the IB Biology SL syllabus, it underpins every subsequent unit from molecular biology to physiology and ecology, and accounts for 15-20% of your total exam assessment. Questions on this topic appear in both Paper 1 (multiple choice) and Paper 2 (structured response and short essay), making it a high-yield area to master.

2. Cell theory

Cell theory has three universally accepted core tenets, developed over 200 years of microscopic and experimental research:

1. All living organisms are composed of one or more cells
2. The cell is the basic unit of structure and function in all living organisms
3. All cells arise from pre-existing cells, with no spontaneous generation of life from non-living material. The third tenet was experimentally validated by Louis Pasteur’s 1859 swan-neck flask experiment, which showed sterile broth remained free of microbes unless exposed to pre-existing microbial cells in the environment.

Examiners frequently ask for limitations of cell theory, so you must memorize three well-documented exceptions:

• Striated muscle fibers: Large (up to 30mm long) multi-nucleated cells that violate the assumption that all cells are small, single-nucleated units
• Acetabularia (giant algae): Single-celled organisms up to 10cm long with a complex, differentiated structure, contradicting the idea that large organisms must be multicellular
• Aseptate fungal hyphae: Thread-like fungal structures with no dividing cell walls, containing many nuclei in a single continuous cytoplasm, violating the tenet that all living things are made of discrete, individual cells.

Worked example (2-mark exam question): Outline one exception to cell theory.

Full-mark answer: Acetabularia (1 mark) is a single-celled giant algae that grows up to 10cm long, with a distinct root, stalk, and cap structure, contradicting the cell theory assumption that all large, complex organisms are made of many small cells (1 mark).

3. Ultrastructure of eukaryotic and prokaryotic cells

All cells are classified as either prokaryotic or eukaryotic, with distinct ultrastructural differences visible under electron microscopy.

Prokaryotic cells

Prokaryotes (domains Bacteria and Archaea) are the oldest, simplest cell type, with no membrane-bound nucleus or membrane-bound organelles. Key structures include:

• Cell wall (made of peptidoglycan in bacteria, for structure and protection)
• Plasma membrane (selectively permeable barrier)
• Cytoplasm (gel-like matrix where metabolic reactions occur)
• 70S ribosomes (site of protein synthesis, smaller than eukaryotic ribosomes)
• Nucleoid region (contains circular, naked DNA with no histone proteins)
• Optional structures: Plasmids (small extra loops of DNA), pili (for attachment and DNA exchange), flagella (for movement), capsule (protective outer layer)

Eukaryotic cells

Eukaryotes (animals, plants, fungi, protists) have a membrane-bound nucleus containing linear DNA wrapped around histone proteins, plus specialized membrane-bound organelles that carry out specific functions. Key organelles to identify from electron micrographs:

• Nucleus (stores DNA, controls cell activities, has a dark nucleolus for ribosome production)
• Rough endoplasmic reticulum (rER: covered in 80S ribosomes, synthesizes proteins for secretion)
• Smooth endoplasmic reticulum (sER: no ribosomes, synthesizes lipids and detoxifies substances)
• Golgi apparatus (modifies, sorts, and packages proteins into vesicles for secretion)
• Mitochondria (site of aerobic respiration, produces ATP, has folded inner cristae membranes)
• Chloroplasts (plant cells only: site of photosynthesis, contains stacks of thylakoids called grana)
• Large central vacuole (plant cells only: stores water and solutes, maintains turgor pressure)
• Lysosomes (animal cells only: contain digestive enzymes to break down waste material)
• Cell wall (plant/fungi only: cellulose for plants, chitin for fungi, provides structural support)

Worked example (3-mark exam question): Compare the ultrastructure of prokaryotic and eukaryotic cells.

Full-mark answer: Prokaryotes have 70S ribosomes, while eukaryotes have larger 80S ribosomes (1 mark); prokaryotic DNA is circular and naked in a nucleoid region, while eukaryotic DNA is linear, wrapped around histones, and contained in a membrane-bound nucleus (1 mark); prokaryotes have no membrane-bound organelles, while eukaryotes have specialized membrane-bound organelles including mitochondria, endoplasmic reticulum, and Golgi apparatus (1 mark).

4. Membrane structure — fluid mosaic

The currently accepted model of cell membrane structure is the fluid mosaic model, named for two key properties: fluid (phospholipids and embedded proteins can move laterally within the membrane layer) and mosaic (proteins are scattered throughout the phospholipid bilayer like tiles in a mosaic).

Core structure: Phospholipid bilayer

Each phospholipid molecule has a hydrophilic (polar, water-attracted) phosphate head and two hydrophobic (non-polar, water-repelled) fatty acid tails. When placed in an aqueous environment, phospholipids spontaneously arrange into a bilayer, with heads facing outward toward the cytoplasm and extracellular fluid, and tails facing inward, forming a stable, selectively permeable barrier that blocks large or polar molecules from passing through directly.

Additional membrane components

• Integral proteins: Span the entire width of the bilayer, acting as channel proteins, carrier proteins, or active transport pumps
• Peripheral proteins: Attached to the inner or outer surface of the bilayer, acting as enzymes, cell adhesion molecules, or parts of cell signaling pathways
• Cholesterol: Found only in animal cell membranes, regulates fluidity: at high temperatures it restricts phospholipid movement to reduce fluidity, at low temperatures it prevents phospholipids from packing tightly to maintain fluidity and avoid membrane rupture
• Glycoproteins/glycolipids: Carbohydrate chains attached to proteins or lipids on the outer membrane surface, responsible for cell recognition, immune response, and cell-to-cell signaling.

Worked example (2-mark exam question): Explain why phospholipids form a bilayer in aqueous environments.

Full-mark answer: The hydrophilic phosphate heads of phospholipids are attracted to the aqueous cytoplasm and extracellular fluid (1 mark), while the hydrophobic fatty acid tails are repelled by water, so they aggregate in the center of the bilayer away from aqueous environments (1 mark).

5. Membrane transport mechanisms

Cells move materials across their membrane via two broad categories of transport: passive (no ATP required, moves down a concentration gradient) and active (requires ATP, moves against a concentration gradient).

Passive transport

1. Simple diffusion: Small non-polar molecules (O₂, CO₂, steroid hormones) move directly through the phospholipid bilayer from an area of high concentration to low concentration, no energy or proteins required.
2. Facilitated diffusion: Large polar molecules (glucose, amino acids) or charged ions (Na⁺, K⁺) cannot pass through the hydrophobic bilayer core, so they move down their concentration gradient via integral channel proteins or carrier proteins, no ATP required.
3. Osmosis: The net movement of water across a selectively permeable membrane, from an area of high water potential (low solute concentration) to an area of low water potential (high solute concentration). In animal cells, a hypertonic solution (higher solute concentration than cytoplasm) causes water loss and cell shrinkage (crenation), while a hypotonic solution causes water gain and cell bursting (lysis). In plant cells, the rigid cell wall prevents lysis, so hypotonic solutions create turgor pressure, the healthy state for most plants.

Active transport

1. Protein pumps: Integral membrane proteins use ATP to move molecules against their concentration gradient. For example, the sodium-potassium pump moves 3 Na⁺ ions out of the cell and 2 K⁺ ions into the cell for every ATP molecule used, critical for nerve impulse transmission.
2. Bulk transport: Moves very large molecules (proteins, whole bacteria) across the membrane using vesicles, requires ATP:

• Endocytosis: The membrane folds inward to form a vesicle, bringing material into the cell (phagocytosis = "cell eating" of solid material, pinocytosis = "cell drinking" of liquid material)
• Exocytosis: A vesicle containing waste or secreted material fuses with the plasma membrane, releasing its contents outside the cell, used for secretion of hormones, neurotransmitters, and digestive enzymes.

Worked example (2-mark exam question): Explain why facilitated diffusion is classified as passive transport even though it uses integral proteins.

Full-mark answer: Facilitated diffusion moves molecules down their concentration gradient (1 mark), so it does not require ATP input, meeting the definition of passive transport regardless of protein use (1 mark).

6. Common Pitfalls (and how to avoid them)

• Pitfall 1: Stating that all cells have a membrane-bound nucleus, or forgetting exceptions to cell theory. Why students do it: They only memorize the core tenets of cell theory and ignore the required limitations. Correct move: Always list the three standard exceptions (striated muscle, Acetabularia, aseptate fungal hyphae) if asked about cell theory limitations, and explain each clearly for full marks.
• Pitfall 2: Mixing up 70S and 80S ribosomes, or stating prokaryotes have no ribosomes. Why students do it: They confuse prokaryotic and eukaryotic organelle sizes, and assume "no membrane-bound organelles" means no organelles at all. Correct move: Memorize the rule: Prokaryotes = 70S ribosomes, Eukaryotes = 80S ribosomes. Mentioning this difference in compare questions will always earn you an easy mark.
• Pitfall 3: Describing the fluid mosaic model as static, or claiming all proteins are only on the membrane surface. Why students do it: They misinterpret the "mosaic" label, forgetting proteins are embedded in the bilayer. Correct move: Explicitly state that proteins can be integral (spanning the bilayer) or peripheral (surface-attached), and that all components can move laterally, not fixed in place.
• Pitfall 4: Defining osmosis as the movement of solute across a membrane, or claiming water moves from high to low solute concentration. Why students do it: They mix up solute and water potential gradients. Correct move: Always define osmosis as the movement of water down a water potential gradient, or from low to high solute concentration, never the reverse.
• Pitfall 5: Classifying bulk transport as passive because it does not use protein pumps. Why students do it: They only associate active transport with protein pumps. Correct move: Remember any transport that requires ATP (including endocytosis and exocytosis) is active, regardless of whether it uses pumps or vesicles.

7. Practice Questions (IB Biology SL Style)

Question 1 (Paper 1, 1 mark)

Which of the following observations provides evidence for the third tenet of cell theory? A) Striated muscle fibers have multiple nuclei B) Sterile broth in a sealed swan-neck flask shows no microbial growth C) Acetabularia is a single-celled organism up to 10cm long D) Eukaryotic cells have 80S ribosomes while prokaryotic cells have 70S ribosomes

Solution: Correct answer B. The third tenet of cell theory states all cells come from pre-existing cells. Pasteur’s swan-neck flask experiment (option B) disproved spontaneous generation, showing no new cells form in sterile material without exposure to pre-existing cells. Options A and C are exceptions to cell theory, option D is a structural difference between cell types.

Question 2 (Paper 2, 3 marks)

Outline three functions of components of the fluid mosaic membrane.

Full-mark solution: Any three of the following, 1 mark each: 1) Phospholipid bilayer forms a selectively permeable barrier that blocks large polar molecules from entering the cell. 2) Integral channel proteins allow large polar molecules or ions to cross the membrane via facilitated diffusion. 3) Cholesterol regulates membrane fluidity in animal cells across changing temperatures. 4) Glycoproteins on the outer membrane surface enable cell recognition and immune system function.

Question 3 (Paper 2, 4 marks)

A freshwater amoeba (single-celled eukaryote) is moved from its natural habitat to a beaker of seawater, which has a much higher solute concentration than the amoeba’s cytoplasm. Explain the changes that will occur to the amoeba, with reference to membrane transport.

Full-mark solution: 1 mark per valid point, max 4: Seawater is hypertonic compared to the amoeba’s cytoplasm, meaning it has a lower water potential (1 mark). Water will move out of the amoeba across its plasma membrane by osmosis, down the water potential gradient (1 mark). As the amoeba loses water, its cytoplasm will shrink, and the cell will shrivel (1 mark). The amoeba may use active transport mechanisms to pump solutes into its cytoplasm to equalize water potential and reduce water loss, if it can survive the initial osmotic shock (1 mark).

8. Quick Reference Cheatsheet

9. What's Next

Mastering cell biology is non-negotiable for success in the rest of the IB Biology SL syllabus. Your knowledge of membrane transport will directly support your study of physiology topics including gas exchange in the respiratory system, nutrient absorption in the digestive system, and nerve impulse transmission in neurobiology. Your understanding of prokaryotic and eukaryotic structure will underpin units on molecular biology (DNA replication, gene expression) and ecology (microbial roles in nutrient cycles and ecosystem function). Even topics like evolution and genetics rely on a baseline understanding of cell structure and function.

If you struggle with any of the concepts in this guide, or want more personalized practice questions tailored to your weak areas, you can ask Ollie for 1:1 support at any time on the homepage. Once you feel confident with cell biology, you can move on to our dedicated study guide for Molecular Biology (SL), the next core unit in the IB Biology SL syllabus, to build on the foundational knowledge you have gained here.