Lesson 1: Cells and Organization

Every living thing in Singapore — from the towering dipterocarp trees of Bukit Timah Nature Reserve to the bacteria in a drop of water from Sungei Buloh — is built from cells. A cell is the smallest structural and functional unit of life: the simplest unit that can carry out the life processes of nutrition, movement, growth, excretion, response, respiration and reproduction on its own. Understanding cells and how they are organised into tissues, organs and organ systems is the foundation of all biology at O-Level.

The cell theory and levels of organisation

The cell theory states three principles: all living organisms are made of one or more cells; the cell is the basic unit of life; and all cells arise from pre-existing cells by cell division. Living things are organised in a hierarchy of increasing complexity:

• Cell — the basic unit, e.g. a muscle cell, a leaf mesophyll cell.
• Tissue — a group of similar cells working together to perform one function, e.g. muscle tissue, xylem tissue.
• Organ — a structure made of several tissues performing a specific function, e.g. the heart, a leaf.
• Organ system — several organs working together, e.g. the circulatory system, the root system.
• Organism — the complete living thing.

Animal and plant cells compared

Both animal and plant cells have a cell surface membrane, cytoplasm and a nucleus. The membrane is partially permeable: it controls which substances enter and leave. The cytoplasm is the jelly-like material where most chemical reactions occur. The nucleus contains DNA, the genetic material that controls all cell activities. Mitochondria (singular: mitochondrion) carry out aerobic respiration and release energy; cells needing much energy, such as muscle cells, contain many. Ribosomes are the sites of protein synthesis.

Specialised cells carry special jobs. A red blood cell has no nucleus and a biconcave shape, giving a large surface area for oxygen uptake. A xylem vessel is a hollow, dead tube with lignified walls that transports water. A root hair cell has a long extension that increases surface area for absorbing water and mineral salts from the soil — a structure–function link examiners love to test.

Worked example: magnification and real size

A student draws a plant cell 30 mm long. Under the microscope the cell's actual length is 0.03 mm. The magnification of the drawing is:

magnification = size of image / size of object
              = 30 mm / 0.03 mm
              = ×1000

Rearranged, actual size = image size / magnification. Measure with a ruler in millimetres first, convert to micrometres (1 mm = 1000 µm) when the answer needs it: 0.03 mm = 30 µm, about the width of a large plant cell.

Why cells stay small: surface area to volume

As a cube-shaped cell grows, its volume increases faster than its surface area:

A large cell has too little surface, relative to its volume, to absorb enough nutrients or remove waste fast enough. That is why big organisms are made of many small cells, not a few giant ones.

Practical focus: preparing a slide

To view onion epidermis: peel a thin layer, mount it in a drop of water on a slide, lower a coverslip at 45° to avoid trapping air bubbles, and add a drop of iodine solution to stain the nuclei so they show up brown. View on low power first, focus, then switch to high power. A common practical error is flooding the slide — excess stain runs onto the stage and the cells appear in a dark, unreadable background.

Singapore focus: microscopy in our labs

Singapore's research institutes, such as A*STAR laboratories, use electron microscopes that magnify up to ×500 000 to study viruses — including the dengue virus spread by Aedes mosquitoes in our estates. The same cell principles from this lesson underpin that work: membranes, genetic material and protein synthesis in action.

Common mistakes

• Saying the cell wall controls what enters the cell. The cell wall is fully permeable; the partially permeable membrane does the controlling.
• Claiming plant cells have no vacuole, or animal cells always have one. Plant cells have a large central vacuole; animal cells have small vacuoles or none.
• Stating mitochondria "make" or "create" energy. Mitochondria release energy from glucose during respiration; energy is never created.
• Confusing tissue and organ. The heart is an organ (several tissues); cardiac muscle is a tissue.

Exam Experience

Question 1 (structured, data-based). A student observed a plant cell under a microscope at ×400 total magnification. The cell appeared to be 24 mm long in the field of view drawing. (a) Calculate the actual length of the cell in micrometres. [2] (b) State two structures you would expect to see in this plant cell that are absent from a human cheek cell. [2] (c) The student then looked at a red blood cell. Explain how the red blood cell is adapted to its function. [3]

Question 2. Which row correctly identifies the site of protein synthesis and the site of energy release in a cell? [1] A. protein synthesis — nucleus; energy release — cytoplasm B. protein synthesis — ribosome; energy release — mitochondrion C. protein synthesis — mitochondrion; energy release — ribosome D. protein synthesis — cell membrane; energy release — vacuole

Question 3. A cube-shaped microorganism has sides of 0.02 mm. Explain, using surface area to volume reasoning, why microorganisms rarely exceed a few millimetres in size, and suggest how a large flatworm overcomes this problem. [3]

Marking scheme. Q1(a) Actual length = 24 / 400 = 0.06 mm [1]; = 60 µm [1]. (Award both marks only if the unit conversion is shown; answer must be in µm.) Q1(b) Any two of: cell wall, chloroplasts, large central vacuole [1 each]. Q1(c) Biconcave disc shape increases surface area to volume ratio for faster oxygen diffusion [1]; no nucleus, so more space for haemoglobin / more haemoglobin carried [1]; small and flexible to pass through capillaries [1]. Q2 B [1]. Q3 As size increases, surface area increases more slowly than volume, so SA:V ratio falls [1]; diffusion of nutrients/wastes becomes too slow to supply the cell's interior [1]; flatworm is flat/thin (or has a branched gut), keeping diffusion distances short / increasing surface area relative to volume [1].

Going deeper: from cells to microscopy limits

A school light microscope magnifies up to about ×1500 and can resolve structures about 0.2 µm apart — enough for nuclei and chloroplasts, but not for ribosomes or viruses. Electron microscopes use beams of electrons instead of light and reach magnifications around ×500 000, revealing viruses and cell membranes in detail. The unit ladder matters in questions: 1 mm = 1000 µm, and 1 µm = 1000 nm. A red blood cell is about 7 µm across; a human hair is about 70 µm wide — ten red blood cells side by side would barely span one hair.

Diffusion sets the ultimate size limit. Substances diffuse quickly over micrometres but hopelessly slowly over centimetres, because diffusion time rises with the SQUARE of the distance — ten times the distance means a hundred times the waiting time. That is why no cell relies on diffusion over more than a fraction of a millimetre, and why large organisms evolved transport systems (Lesson 2) to do the long-distance carrying.

Specialisation in context. A nerve cell (neurone) may be a metre long yet microscopically thin — specialised for transmitting impulses, not for diffusion across its length. A sperm cell swims with a tail (flagellum) and packs mitochondria into its midpiece. Whenever an exam presents an unfamiliar cell, ask three questions: What is its job? Which structures are enlarged, multiplied or missing? How does each feature serve the job? That routine answers almost any adaptation question, familiar or not.

Exam-style extra

Question. A student views a cell at ×600. The image of the cell is 18 mm long. (a) Calculate the actual length in µm. [2] (b) State whether the cell is more likely from a leaf or from a human cheek, giving one structural reason that could confirm it. [2]

Marking points. (a) Actual length = 18/600 = 0.03 mm [1] = 30 µm [1]. (b) Either cell could be this size, so the reason must be structural: leaf cell — would show chloroplasts / a regular shape with a cell wall / large vacuole; cheek cell — irregular shape, no wall or chloroplasts [1 for the identification attempt tied to structure, 1 for the correct confirming feature].

Workshop: arguing from structure to function

Examiners pay for a three-link chain: structure → property → function. "The root hair cell has a long thin extension (structure), giving a large surface area in contact with soil water (property), so it absorbs water and mineral ions quickly (function)." One-link answers ("root hairs absorb water") state the function but earn nothing for explaining it.

Scale anchors worth memorising: a human body contains roughly 37 trillion cells of about 200 different specialised types; a typical animal cell is 10–30 µm; the largest common cell visible unaided is an egg cell (about 100 µm in humans; a bird's egg yolk is a single cell far larger). Keep the hierarchy straight under pressure: cell → tissue → organ → organ system → organism. A "tissue" answer for the heart (an organ of muscle, nerve and blood tissues) is a classic lost mark.