Cell Biology — IB Biology HL HL Study Guide

For: IB Biology HL candidates sitting IB Biology HL.

Covers: Core Cell Biology subtopics including cell theory and origins, eukaryotic and prokaryotic cell ultrastructure, fluid mosaic cell membrane model, membrane transport mechanisms, and cell cycle/mitosis.

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 HL 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 the basic unit of life: the cell. It forms the foundational core of the IB Biology HL syllabus, accounting for ~15% of total HL exam marks across Papers 1, 2, and 3, with direct links to every subsequent topic from genetics to ecology. Common synonyms include cellular biology and cytology; exam questions frequently integrate this topic with biochemistry (Topic 2) and genetics (Topic 3) to test cross-topic application skills.

2. Cell theory and origins

The modern cell theory has three core tenets, first formalized in the 19th century by Schleiden, Schwann, and Virchow:

1. All living organisms are composed of one or more cells
2. The cell is the smallest unit of independent life, capable of carrying out all seven functions of life: nutrition, metabolism, growth, response, excretion, homeostasis, and reproduction
3. All cells arise from pre-existing cells via division, with no spontaneous generation of new cells from non-living material

Examiners frequently ask you to evaluate cell theory, so memorize the three common syllabus-listed exceptions: striated muscle fibers (multi-nucleated, up to 30cm long, far larger than typical animal cells), aseptate fungal hyphae (no cross-walls, continuous multi-nucleated cytoplasm), and giant Acetabularia algae (single-celled, up to 10cm long, no multicellular structure).

For eukaryote origins, you are required to understand the endosymbiotic theory: an ancestral anaerobic prokaryote engulfed an aerobic proteobacterium that evolved into the mitochondrion, while a later engulfment of a photosynthetic cyanobacterium evolved into the chloroplast in photosynthetic eukaryotes. Key evidence for this theory includes: mitochondria/chloroplasts have 70S ribosomes (matching prokaryotes), circular naked DNA, a double membrane, and replicate independently via binary fission.

Worked example (2 marks): Question: List two pieces of evidence supporting the endosymbiotic theory of eukaryote origin. Answer: 1. Mitochondria possess circular, naked DNA identical in structure to prokaryotic genomes (1 mark). 2. Mitochondria replicate independently of the host cell via binary fission, the same division mechanism used by prokaryotes (1 mark). Exam note: Vague answers like "they are the same size as prokaryotes" do not receive marks unless you specify the 1-10μm size range matching typical prokaryotes.

3. Ultrastructure of eukaryotic and prokaryotic cells

Prokaryotes (bacteria and archaea) are simple, unicellular organisms with no membrane-bound nucleus or organelles. Key structural features include: 70S ribosomes, circular naked DNA stored in a nucleoid region (no nuclear envelope), peptidoglycan cell walls (archaea have pseudopeptidoglycan), optional pili (for adhesion or DNA transfer) and flagella (for locomotion), and small extrachromosomal DNA molecules called plasmids. They divide via binary fission, not mitosis.

Eukaryotes (animals, plants, fungi, protists) have a membrane-bound nucleus containing linear DNA wrapped around histone proteins, 80S cytoplasmic ribosomes, and specialized membrane-bound organelles: mitochondria for aerobic respiration, rough endoplasmic reticulum for protein synthesis, Golgi apparatus for protein modification and sorting, lysosomes for intracellular digestion in animal cells, and chloroplasts for photosynthesis in plant cells. Plant cells also have a cellulose cell wall and large central vacuole, while animal cells have centrioles for spindle formation during mitosis.

Worked example (2 marks): Question: Identify two structural differences between a prokaryotic and eukaryotic cell visible on a transmission electron micrograph. Answer: 1. Prokaryotic cells lack a membrane-bound nucleus surrounding their DNA, while eukaryotic cells have a distinct nuclear envelope enclosing linear chromosomes (1 mark). 2. Prokaryotic cells have smaller 70S ribosomes, while eukaryotic cells have larger 80S ribosomes visible in the cytoplasm (1 mark). Exam tip: Do not reference cell wall chemical composition, as this is not visible on an electron micrograph.

4. Cell membranes — fluid mosaic model

The fluid mosaic model, first proposed by Singer and Nicolson in 1972, describes the structure of all cell membranes. It is named for two key properties: fluid because phospholipids and embedded proteins can move laterally within the bilayer, and mosaic because scattered proteins, glycoproteins, glycolipids, and cholesterol form a mosaic pattern when viewed from the extracellular surface.

Core structural components:

1. Phospholipid bilayer: The foundational structure, with hydrophilic phosphate heads facing outward to the aqueous cytoplasm and extracellular fluid, and hydrophobic fatty acid tails facing inward to form a non-polar core. This creates a selectively permeable barrier that only allows small non-polar molecules (O₂, CO₂, steroid hormones) to pass freely.
2. Integral proteins: Span the entire bilayer, functioning as channel/carrier proteins for transport, cell surface receptors, or membrane-bound enzymes.
3. Peripheral proteins: Attached to the inner or outer surface of the bilayer, used for cytoskeleton anchoring, cell adhesion, or temporary enzyme binding.
4. Cholesterol: Only present in animal cell membranes, acting as a fluidity buffer: it reduces fluidity at high temperatures by restricting phospholipid movement, and prevents stiffening at low temperatures by disrupting tight packing of fatty acid tails.
5. Glycoproteins/glycolipids: Carbohydrate chains attached to proteins or lipids on the extracellular surface, functioning in cell recognition, immune response, and cell adhesion.

Worked example (3 marks): Question: Explain how the structure of phospholipids contributes to the stability of the cell membrane. Answer: 1. Phospholipids have a hydrophilic phosphate head and hydrophobic fatty acid tails (1 mark). 2. When placed in aqueous solution, phospholipids spontaneously arrange into a bilayer with hydrophobic tails facing inward away from water, and hydrophilic heads facing outward toward the cytoplasm and extracellular fluid (1 mark). 3. This arrangement is stabilized by hydrophobic interactions between the fatty acid tails and hydrogen bonding between the phosphate heads and surrounding water molecules, creating a stable, flexible barrier (1 mark).

5. Membrane transport mechanisms

Membrane transport is split into two broad categories, based on energy requirements:

Passive transport (no ATP input, moves down a concentration/electrochemical gradient)

1. Simple diffusion: Movement of small non-polar molecules directly through the phospholipid bilayer, e.g. O₂ diffusing from alveoli into blood capillaries.
2. Facilitated diffusion: Movement of large, polar, or charged molecules via integral channel or carrier proteins, e.g. glucose entering red blood cells via GLUT transporters, water moving through aquaporin channels.
3. Osmosis: Net movement of water across a selectively permeable membrane from a region of high water potential (low solute concentration) to low water potential (high solute concentration). HL note: Examiners expect you to use the term water potential, not just solute concentration, in answers.

Active transport (requires ATP input, moves against a concentration/electrochemical gradient)

1. Primary active transport: Uses ATP directly to power protein pumps that move molecules against their gradient, e.g. the Na⁺/K⁺ ATPase pump in nerve cells, which pumps 3 Na⁺ out of the cell and 2 K⁺ into the cell per ATP molecule hydrolyzed.
2. Bulk transport: Movement of large molecules or particles via vesicle formation: endocytosis brings materials into the cell (phagocytosis = "cell eating" of solid particles, pinocytosis = "cell drinking" of dissolved solutes), while exocytosis releases materials out of the cell (e.g. neurotransmitters from nerve cells, insulin from pancreatic beta cells).

Worked example (2 marks): Question: A cell has an internal sodium concentration of 10 mmol dm⁻³, and the extracellular sodium concentration is 140 mmol dm⁻³. Identify the type of transport required to move sodium out of the cell, and explain your answer. Answer: 1. Active transport (1 mark). 2. Sodium is being moved against its concentration gradient from a region of low (10 mmol dm⁻³) to high (140 mmol dm⁻³) concentration, which requires energy input from ATP (1 mark).

6. Cell cycle and mitosis

The cell cycle is the sequence of events between one cell division and the next, split into two broad phases: interphase and M phase (mitosis + cytokinesis).

1. Interphase: The longest phase (~90% of the cell cycle), split into three sub-phases: G₁ (cell grows, synthesizes proteins, carries out normal metabolic functions), S phase (DNA replication occurs, chromosomes duplicate into two identical sister chromatids attached at a centromere), G₂ (cell duplicates organelles, checks for DNA damage, and prepares for mitosis).
2. Mitosis: Nuclear division that produces two genetically identical diploid daughter nuclei, used for growth, tissue repair, and asexual reproduction. It has four distinct stages:

• Prophase: Chromosomes condense, nuclear envelope breaks down, spindle microtubules form from centrioles (animal cells only) and attach to chromosome centromeres.
• Metaphase: Chromosomes line up along the equator (metaphase plate) of the cell.
• Anaphase: Sister chromatids separate at the centromere, and spindle microtubules pull identical chromatids to opposite poles of the cell.
• Telophase: Chromosomes decondense, nuclear envelopes reform around each set of chromosomes, and the spindle breaks down.

3. Cytokinesis: Division of the cytoplasm to form two separate daughter cells: animal cells form a cleavage furrow via actin filaments pinching the membrane inward, while plant cells form a cell plate from Golgi vesicles carrying cell wall material, which grows outward to form a new cell wall and membrane.

A key HL calculation is the mitotic index, the proportion of cells in a sample that are undergoing mitosis, used to assess cancer growth (cancer cells have a higher mitotic index than normal cells): $$\text{Mitotic Index}=\frac{\text{Number of cells in mitosis}}{\text{Total number of cells observed}}$$

Worked example (2 marks): Question: A student observes 200 root tip cells, 26 of which are in mitosis. Calculate the mitotic index of the sample, and state what it indicates. Answer: Mitotic index = $\frac{26}{200}=0.13$ (or 13%) (1 mark). A mitotic index of 13% indicates a rapidly dividing tissue, consistent with a growing root tip (1 mark).

7. Common Pitfalls (and how to avoid them)

• Wrong move: Stating that all cells follow cell theory with no exceptions when asked to evaluate the theory. Why students do it: They memorize the tenets but forget IBO explicitly lists exceptions. Correct move: Always reference 1-2 exceptions (striated muscle, aseptate fungi, giant algae) and explain how they deviate from standard tenets to gain full marks.
• Wrong move: Mixing up 70S and 80S ribosomes, claiming prokaryotes have 80S ribosomes. Why students do it: They forget that mitochondria and chloroplasts in eukaryotes have 70S ribosomes as endosymbiotic evidence. Correct move: Memorize: prokaryotes = 70S, eukaryote cytoplasm = 80S, eukaryote organelles (mitochondria/chloroplasts) = 70S.
• Wrong move: Stating cholesterol only makes cell membranes more rigid. Why students do it: They only remember the high temperature effect of cholesterol. Correct move: Describe cholesterol as a fluidity buffer: it reduces fluidity at high temperatures and increases fluidity at low temperatures by preventing tight packing of fatty acid tails.
• Wrong move: Confusing crenation and plasmolysis, stating animal cells plasmolyse in hypertonic solutions. Why students do it: They use the terms interchangeably for water loss. Correct move: Plasmolysis only occurs in plant cells with a cell wall, where the cell membrane pulls away from the cell wall; animal cells crenate (shrivel) in hypertonic solutions.
• Wrong move: Defining mitosis as division of the entire cell. Why students do it: They mix up mitosis and cytokinesis. Correct move: Define mitosis as nuclear division (division of the nucleus into two identical daughter nuclei), and cytokinesis as the subsequent division of the cytoplasm.

8. Practice Questions (IB Biology HL Style)

Question 1 (3 marks)

a) List two pieces of evidence for the endosymbiotic origin of chloroplasts (2 marks) b) State one structural feature of chloroplasts that is not evidence for endosymbiosis (1 mark)

Worked Solution

a) Award 1 mark each for any two of the following:

• Chloroplasts have circular, naked DNA identical in structure to prokaryotic genomes
• Chloroplasts have 70S ribosomes matching the ribosome size of prokaryotes
• Chloroplasts replicate independently of the host cell via binary fission
• Chloroplasts have a double membrane, consistent with being engulfed by a host cell b) Award 1 mark for any of the following:
• Presence of thylakoid membranes/grana for photosynthesis
• Presence of chlorophyll pigments for light absorption
• Large central starch granule for storage of photosynthetic products Exam note: Structural features shared with prokaryotes do not receive marks for part b.

Question 2 (4 marks)

A red blood cell with an internal solute concentration of 0.9% NaCl is placed in a solution of 5% NaCl. a) Identify the tonicity of the 5% NaCl solution relative to the red blood cell (1 mark) b) Describe the net movement of water that will occur, and name the transport mechanism (2 marks) c) State what will happen to the red blood cell (1 mark)

Worked Solution

a) The 5% NaCl solution is hypertonic (1 mark) b) Water moves out of the red blood cell from a region of higher water potential (inside the cell, 0.9% NaCl) to a region of lower water potential (outside the cell, 5% NaCl) down the water potential gradient (1 mark). The mechanism is osmosis, a form of passive transport (1 mark). c) The red blood cell will crenate (shrivel) as it loses water (1 mark) Exam note: Do not reference plasmolysis, as this only occurs in plant cells with a rigid cell wall.

Question 3 (3 marks)

A student observes 120 cells in an onion root tip sample. 18 cells are in prophase, 7 in metaphase, 5 in anaphase, 10 in telophase, and the rest are in interphase. a) Calculate the mitotic index of the sample, show your working (2 marks) b) State one use of the mitotic index in medical diagnosis (1 mark)

Worked Solution

a) First calculate total number of cells in mitosis: $18+7+5+10=40$ cells (1 mark). Mitotic index = $\frac{\text{Number of cells in mitosis}}{\text{Total number of cells observed}}=\frac{40}{120}=0.33$ (or 33%) (1 mark) b) The mitotic index is used to identify cancerous tissue, as cancer cells divide more rapidly and have a higher mitotic index than normal healthy tissue (1 mark)

9. Quick Reference Cheatsheet

10. What's Next

Cell biology is the foundational core of the entire IB Biology HL syllabus, so mastering these concepts will directly support your learning of all subsequent topics. For example, membrane transport is critical for understanding plant water relations (Topic 9), nerve impulse transmission (Topic 6), and kidney function (Topic 11), while mitosis is a prerequisite for learning meiosis and genetic inheritance (Topic 3) and cancer biology (Topic 6). Endosymbiosis also links to evolution (Topic 5), as it explains the origin of complex eukaryotic life that evolved into all multicellular organisms on Earth.

If you have questions about any of the concepts in this guide, or want to practice more exam-style questions tailored to your weak spots, you can ask Ollie, our AI tutor, at any time for personalized explanations and feedback. You can also find more study guides for IB Biology HL topics on the homepage, including practice paper sets and mark scheme breakdowns to help you prepare for your exams.