Understand Cell Structure | Varsity Tutors

SECTION 1

Historical Context & Motivation

For most of human history, people had no idea that living things were made of microscopic units. The naked eye can only resolve objects about 0.1 mm across, and most cells are far smaller than that. It was the invention of the microscope in the seventeenth century that opened a hidden world and launched one of the most important ideas in biology: the cell theory. Understanding cell structure matters because every process in your body — from breathing to thinking — happens inside or between cells.

1665

Robert Hooke Coins 'Cell'

Using a compound microscope, Robert Hooke examined thin slices of cork and observed tiny box-like compartments. He named them cells because they reminded him of monks' rooms (cellae) in a monastery.

1674

Leeuwenhoek Sees Living Cells

Antonie van Leeuwenhoek used single-lens microscopes he crafted himself to observe living organisms in pond water, which he called animalcules. He was the first person to see bacteria and protists.

1838–1839

Schleiden & Schwann Propose Cell Theory

Botanist Matthias Schleiden concluded all plants are made of cells; zoologist Theodor Schwann extended this to animals. Together they established that the cell is the basic unit of life.

1855

Virchow: Cells from Cells

Rudolf Virchow declared "Omnis cellula e cellula" — all cells arise from pre-existing cells. This completed the classical cell theory and discredited the idea of spontaneous generation.

1950s–present

Electron Microscopy Reveals Ultrastructure

The development of transmission and scanning electron microscopes allowed biologists to see organelles like ribosomes, endoplasmic reticulum, and the double membrane of the nucleus in stunning detail.

These discoveries raised a central question that the IB Biology course addresses: if every living organism is built from cells, what structures do cells share, and how do differences in cell structure relate to differences in function? This lesson explores the answers by examining prokaryotic cells, eukaryotic cells, and the key organelles inside them.

SECTION 2

Core Principles of Cell Structure

Before diving into specific organelles, it helps to grasp a few foundational ideas that unify all of cell biology. These principles underpin everything from single-celled bacteria to the trillions of cells in your body.

Cell Theory

All living organisms are composed of one or more cells; the cell is the smallest unit of life; all cells arise from pre-existing cells. This is the unifying framework for biology.

Compartmentalisation

Eukaryotic cells use membrane-bound organelles to separate incompatible chemical reactions, increasing efficiency and allowing specialised functions.

Surface Area to Volume Ratio

As a cell grows, its volume increases faster than its surface area. This limits cell size because the plasma membrane must supply enough nutrients and remove enough waste for the entire volume.

Prokaryote vs. Eukaryote

Prokaryotic cells lack a true nucleus and membrane-bound organelles. Eukaryotic cells have a nucleus enclosed by a nuclear envelope, plus organelles such as mitochondria and the endoplasmic reticulum.

Structure–Function Relationship

Every organelle has a shape and composition that matches its job. For instance, mitochondria have highly folded inner membranes (cristae) to maximise surface area for ATP production.

✦ KEY TAKEAWAY

Think of a cell like a school building. The whole building is the cell, the main office (nucleus) stores important records (DNA), the cafeteria (mitochondria) produces the energy everyone needs, the hallways (endoplasmic reticulum) transport materials, and the custodial staff (lysosomes) clean up waste. Prokaryotic cells are like a small one-room schoolhouse — everything happens in one open space — while eukaryotic cells are like a large campus with specialised rooms for every function.

SECTION 3

Visual Overview — Animal Cell Structure

A generalised animal cell showing major organelles. The nucleus houses DNA, mitochondria generate ATP, the Golgi apparatus processes and packages proteins, and the rough ER synthesises proteins with attached ribosomes.

The diagram above shows a typical animal cell, which is a eukaryotic cell — a cell that possesses a true, membrane-bound nucleus. Notice how the cell is not a random soup of molecules. Each organelle occupies its own space and carries out a specific function, much like rooms in a building. The plasma membrane surrounds the entire cell and controls what enters and leaves. Inside, the cytoplasm (the jelly-like fluid filling the cell) provides a medium in which organelles float and chemical reactions occur. The nucleus contains the cell's genetic instructions in the form of DNA, and the nucleolus inside it is where ribosomal RNA is assembled.

The rough endoplasmic reticulum (RER) is studded with ribosomes and folds proteins destined for export or insertion into membranes. The smooth endoplasmic reticulum (SER) lacks ribosomes and is involved in lipid synthesis and detoxification. After proteins are made, they travel to the Golgi apparatus, which modifies, sorts, and packages them into vesicles. Lysosomes are membrane-bound sacs filled with digestive enzymes that break down worn-out organelles and engulfed pathogens.

SECTION 4

How Cell Size Is Constrained — The SA:V Ratio

You might wonder why cells are so small — typically between 1 and 100 micrometres (µm). The answer lies in a simple mathematical relationship: as a cell grows, its volume increases faster than its surface area. Since the plasma membrane (surface) is responsible for exchanging materials with the environment and the volume is what needs to be supplied, a cell that gets too large cannot sustain itself. This relationship is captured by the surface-area-to-volume ratio (SA:V).

SURFACE AREA OF A CUBE

SA = 6s²

Where s is the side length of the cube.

VOLUME OF A CUBE

V = s³

Volume grows as the cube of the side length, which is why it outpaces surface area as the cell enlarges.

SA:V RATIO

SA:V = 6s² ÷ s³ = 6/s

As s increases, the ratio decreases. A smaller cell therefore has a more favourable SA:V ratio for efficient exchange of materials.

How increasing cell size reduces the SA:V ratio

Side length (µm)	Surface area (µm²)	Volume (µm³)	SA:V ratio
1	6	1	6:1
2	24	8	3:1
3	54	27	2:1
6	216	216	1:1

✦ KEY TAKEAWAY

Imagine you are wrapping gifts. A small box is easy to cover with wrapping paper, but a huge box needs vastly more paper relative to what fits inside. Cells face the same problem: a larger cell has proportionally less membrane surface to service its growing interior. That is why most cells stay tiny, and why larger organisms add more cells rather than making bigger ones.

SECTION 5

Prokaryotic vs. Eukaryotic Cells — A Detailed Comparison

All cells fall into one of two broad categories. Prokaryotic cells (from the Greek pro = before, karyon = kernel/nucleus) evolved first and include all bacteria and archaea. Eukaryotic cells (eu = true) arose later and include protists, fungi, plants, and animals. The table below summarises their differences.

Side-by-side comparison of a prokaryotic bacterium (left) and a eukaryotic animal cell (right). Notice the absence of a membrane-bound nucleus in the prokaryote — its DNA floats freely in the nucleoid region.

Key differences between prokaryotic and eukaryotic cells

Feature	Prokaryotic Cell	Eukaryotic Cell
Size	0.1–5 µm	10–100 µm
Nucleus	No true nucleus; DNA in nucleoid region	True nucleus enclosed by nuclear envelope
DNA shape	Circular, naked (not associated with histones)	Linear chromosomes associated with histone proteins
Ribosomes	70S (smaller)	80S (larger); 70S in mitochondria and chloroplasts
Membrane-bound organelles	Absent	Present (mitochondria, ER, Golgi, lysosomes, etc.)
Cell wall	Usually present (peptidoglycan in bacteria)	Present in plants (cellulose) and fungi (chitin); absent in animals
Reproduction	Binary fission	Mitosis and meiosis
Examples	Escherichia coli, Staphylococcus	Human cells, plant cells, yeast

💡 IB Exam Tip

IB Biology exams often ask you to draw and label a prokaryotic cell. Be sure you can sketch the cell wall, plasma membrane, cytoplasm, ribosomes (70S), nucleoid with circular DNA, and optionally a plasmid and flagellum. For eukaryotic cells, you should be able to identify at least eight organelles and state a function for each.

SECTION 6

Worked Example — Calculating SA:V and Identifying Organelles

Cube-Shaped Cell Model — SA:V Calculation

Step 1 — Read the ProblemA model cell is shaped like a cube with a side length of 4 µm. Calculate its surface area, volume, and SA:V ratio. Then predict what would happen to the cell's efficiency if the side length doubled to 8 µm.

Step 2 — Calculate Surface Area (s = 4 µm)SA = 6s² = 6 × (4)² = 6 × 16 = 96 µm²

SA = 96 µm²

Step 3 — Calculate Volume (s = 4 µm)V = s³ = (4)³ = 64 µm³

V = 64 µm³

Step 4 — Calculate SA:V RatioSA:V = 96 ÷ 64 = 1.5:1. Alternatively, using the formula 6/s = 6/4 = 1.5.

SA:V = 1.5:1

Step 5 — Predict Effect of Doubling Side LengthIf s = 8 µm: SA = 6 × 64 = 384 µm²; V = 512 µm³; SA:V = 384 ÷ 512 = 0.75:1. The ratio halved. This means the larger cell has proportionally less membrane surface to exchange materials with its environment, reducing its metabolic efficiency. In real organisms, this limitation is why cells tend to remain small or develop internal membranes (like cristae in mitochondria) to increase internal surface area.

Doubling side length halves the SA:V ratio from 1.5:1 to 0.75:1

Identifying Organelles from an Electron Micrograph Description

Step 1 — Read the DescriptionAn electron micrograph shows a cell with a double membrane surrounding a dense, darkly staining region containing linear structures. Scattered throughout the cytoplasm are oval structures with highly folded inner membranes. Near one side, stacked flattened sacs are visible with small vesicles budding off. Identify these three structures.

Step 2 — Identify the Double-Membrane StructureA double membrane (envelope) surrounding dense material with linear structures (chromosomes) is the nucleus. The double membrane is the nuclear envelope, and the darkly staining region is chromatin.

Nucleus

Step 3 — Identify the Oval StructuresOval organelles with highly folded inner membranes (cristae) are mitochondria. The folds increase surface area for aerobic respiration and ATP production.

Mitochondria

Step 4 — Identify the Stacked Flattened SacsStacked flattened membrane sacs (cisternae) with vesicles budding off describe the Golgi apparatus. It modifies proteins and lipids received from the ER and packages them into vesicles for transport.

Golgi apparatus

SECTION 7

Plant Cells vs. Animal Cells — Similarities and Differences

Both plant and animal cells are eukaryotic, so they share most organelles — nucleus, mitochondria, ribosomes, ER, and Golgi apparatus. However, plant cells possess several additional structures that reflect the demands of a photosynthetic, sessile lifestyle. The table below highlights the key contrasts.

Comparison of animal and plant eukaryotic cells

Feature	Animal Cell	Plant Cell
Cell wall	Absent	Present (cellulose)
Chloroplasts	Absent	Present — site of photosynthesis
Large central vacuole	Small vacuoles (if any)	Large, permanent vacuole for storage and turgor
Centrioles	Present — organise the mitotic spindle	Generally absent in higher plants
Lysosomes	Present	Rare; vacuole performs similar roles
Shape	Irregular / rounded	Fixed, rectangular shape due to rigid cell wall
Energy organelle	Mitochondria only	Mitochondria and chloroplasts

✦ KEY TAKEAWAY

Think of a plant cell as an animal cell wearing armour and carrying a solar panel. The cellulose cell wall is the armour — rigid structural support that animal cells lack. The chloroplasts are the solar panels, capturing light energy to make glucose. Both cell types still need mitochondria (the power plant) to convert that glucose into ATP, which is why plant cells have both chloroplasts and mitochondria.

SECTION 8

Connecting to Advanced Concepts — Endosymbiotic Theory and Cell Specialisation

Understanding basic cell structure prepares you for more advanced ideas in IB Biology. One of the most elegant explanations for the origin of eukaryotic cells is the endosymbiotic theory, proposed by Lynn Margulis in 1967. This theory states that mitochondria and chloroplasts were once free-living prokaryotes that were engulfed by a larger ancestral cell. Instead of being digested, they formed a mutualistic relationship, eventually becoming permanent organelles.

Evidence supporting the endosymbiotic theory

Evidence	Explanation
Double membrane	The inner membrane may be the original prokaryote's membrane; the outer membrane formed from the host's engulfing vesicle.
Own DNA	Mitochondria and chloroplasts have their own circular DNA, similar to prokaryotic genomes.
70S ribosomes	These organelles contain 70S ribosomes (the prokaryotic type), not the 80S ribosomes found in the eukaryotic cytoplasm.
Binary fission	Both mitochondria and chloroplasts replicate by dividing in two, similar to bacterial reproduction.
Size	Mitochondria and chloroplasts are roughly the same size as prokaryotic cells (1–10 µm).

Another advanced concept you will encounter is cell specialisation (differentiation). While all cells in a multicellular organism contain the same DNA, they express different genes, resulting in cells with different structures suited to different functions. For example, red blood cells lose their nucleus to maximise space for haemoglobin; neurons develop long axons to transmit electrical signals over distance; and root hair cells in plants extend thin projections to increase surface area for water absorption. This structure–function relationship is a recurring theme throughout IB Biology and builds directly on your understanding of organelles.

🔭 Looking Ahead

In later IB Biology topics, you will study how cells communicate (cell signalling), how they divide (mitosis and meiosis), and how disruptions to normal cell structure and division can lead to diseases such as cancer. A solid grasp of cell structure is the foundation for all of these topics.

SECTION 9

Practice Problems

PROBLEM 1 — CONCEPTUAL

State three features that are found in prokaryotic cells but not typically found in eukaryotic cells, and explain why one of these features supports the idea that prokaryotes are simpler organisms.

PROBLEM 2 — BASIC CALCULATION

A spherical cell has a radius of 5 µm. Calculate its surface area (SA = 4πr²) and volume (V = 4/3 × πr³), then determine its SA:V ratio. Use π ≈ 3.14.

PROBLEM 3 — INTERMEDIATE

A student examines two cells under an electron microscope. Cell A has a cell wall made of cellulose, chloroplasts, a large central vacuole, and a nucleus. Cell B has a cell wall made of peptidoglycan, 70S ribosomes, and a circular chromosome in the cytoplasm but no membrane-bound nucleus. Classify each cell as prokaryotic or eukaryotic, and identify which kingdom each is most likely from.

PROBLEM 4 — APPLIED

Antibiotics such as tetracycline target 70S ribosomes to inhibit bacterial protein synthesis but do not affect 80S ribosomes. Using your knowledge of cell structure, explain why these antibiotics kill bacteria without harming human cells. Then explain why such antibiotics might cause side effects in mitochondria.

PROBLEM 5 — CRITICAL THINKING

Some scientists have proposed that viruses should be considered a form of life, while others argue they should not. Using the three tenets of cell theory and your understanding of cell structure, construct an argument for why viruses do not satisfy the classical definition of life. Then suggest what this implies about whether cell theory is a complete description of all biological entities.

SUMMARY

Lesson Summary

This lesson explored the foundational concept that all living organisms are made of cells, as established by cell theory. We distinguished between prokaryotic cells (no nucleus, 70S ribosomes, circular DNA) and eukaryotic cells (membrane-bound nucleus, 80S ribosomes, compartmentalised organelles). Key eukaryotic organelles include the nucleus (stores DNA), mitochondria (ATP production via aerobic respiration), rough and smooth ER (protein and lipid synthesis), Golgi apparatus (modification and packaging), and lysosomes (intracellular digestion).

We explored the surface-area-to-volume ratio (SA:V = 6/s for a cube), which explains why cells remain small: as size increases, the ratio decreases, limiting efficient exchange of materials. We compared plant and animal cells, noting that plant cells additionally possess a cellulose cell wall, chloroplasts for photosynthesis, and a large central vacuole. Finally, the endosymbiotic theory connects cell structure to evolution, explaining how mitochondria and chloroplasts likely originated as engulfed prokaryotes — supported by their double membranes, own DNA, 70S ribosomes, and binary fission.