Cells
Created by Miss Clarissa Ng | www.clartutors.com
1 The Cell — Basic Unit of Life
Cell: The basic structural and functional unit of all living organisms. All cells arise from pre-existing cells.
🔬 Unicellular vs Multicellular Organisms
| Type | Description | Examples |
|---|---|---|
| Unicellular | Made of ONE cell that carries out all life functions on its own | Amoeba, Euglena, bacteria |
| Multicellular | Made of MANY cells; cells are the basic building blocks organised into tissues → organs → systems | Plants, animals (e.g. humans) |
Key Point: In multicellular organisms, cells are the basic building blocks that are organised into tissues → organs → systems. Typical plant and animal cell diagrams are models — they represent the various real forms of cells.
2 Parts of a Typical Cell & Their Functions
Organelles are specialised subunits or compartments within a cell that carry out specific functions.
| Part | Function |
|---|---|
| Nucleus | Membrane-bound; contains chromosomes with genetic material (DNA) that can be passed down to the next generation. Controls activities in the cell; responsible for reproduction and growth, repair of worn-out parts, and heredity. |
| Cytoplasm | Gel-like substance containing organelles and various substances; site where many chemical reactions take place. |
| Cell membrane | Controls the movement of substances into and out of the cell (partially permeable). |
| Vacuole | Filled space surrounded by a membrane; contains water and dissolved substances such as sugars and mineral salts (cell sap). In plant cells, keeps the plant turgid due to turgor pressure. |
| Mitochondrion | Site of respiration — releases energy from food for the cell's activities. |
| Chloroplast | Contains chlorophyll; traps light to produce sugar during photosynthesis. Found in plant cells only. |
| Cell wall | Rigid outer layer made of cellulose; supports the plant and gives it a regular shape. Found in plant cells only. |
⚠️ Exam Tip
The syllabus requires you to know that the nucleus contains genetic material (DNA) passed to the next generation. The abbreviation "DNA" is sufficient — spelling out the full name or its structure is NOT required.3 Plant Cells vs Animal Cells
| Feature | Plant Cell | Animal Cell |
|---|---|---|
| Cell wall | ✅ Present (gives fixed shape) | ❌ Absent |
| Chloroplasts | ✅ Present (photosynthesis) | ❌ Absent |
| Vacuoles | One large central vacuole | Small but numerous vacuoles |
| Shape | Fixed, regular (rectangular) | Irregular / round |
🦠 Special Case: Euglena
Euglena has chloroplasts (like a plant cell) but does NOT have a cell wall. It can move to places with sunlight so that it can perform photosynthesis.
⚠️ Exam Tip
You may be asked to infer whether an organism is an animal or a plant based on its cell structures: if the cell has a cell wall and/or chloroplasts → plant; if it lacks both → animal. Euglena (chloroplasts, no cell wall) is the classic trap — it is a unicellular organism, not a plant.4 The Light Microscope
| Part | Function |
|---|---|
| Eyepiece (ocular lens) | Magnifies the image; you look through it (usually 10×). |
| Objective lens | Magnifies the object by a particular factor (e.g. 4×, 10×, 40×). |
| Coarse adjustment knob | Moves the stage up and down for initial focusing. |
| Fine adjustment knob | Makes small adjustments to sharpen the focus. |
| Stage | Holds the slide with the specimen. |
| Diaphragm / aperture | Controls the amount of light passing through the specimen. |
| Light source (mirror) | Provides light to illuminate the specimen. |
🧮 Total Magnification
Total magnification = eyepiece × objective lens, e.g. 10× × 40× = 400×
🧪 Using the Microscope Safely & Correctly
- Carry with TWO hands — one hand holds the arm, the other supports the base laterally and horizontally.
- The image seen is inverted (upside-down and reversed).
- To make cell structures clearer, stain the sample before viewing — e.g. cells can be stained using methylene blue.
Technology & knowledge: The development of the microscope is an example of how advances in technology lead to new knowledge — it allowed scientists to discover cells and study their internal structure.
5 Specialised Cells & Division of Labour
Cell specialisation: The process by which a cell becomes structurally different from other cells so that it can perform a specific function more efficiently. A specialised cell is adapted in structure to carry out its particular role.
Division of labour: The assignment of different functions to specialised structural units, leading to efficiency in the organism.
Significance:
- Different types of cells perform specific functions entirely and efficiently
- Allows different functions to be performed at the same time in a multicellular organism
- Cell specialisation makes division of labour possible
🐭 Examples: Animal Cells
| Specialised Cell | Adaptation | Function / Advantage |
|---|---|---|
| Red blood cell | No nucleus or other organelles; biconcave shape; no mitochondria | Packs more haemoglobin per cell → transports more oxygen to respiring tissues. High surface area : volume ratio lets oxygen diffuse in and out faster. Does not use up the oxygen it carries. |
| Nerve cell (neuron) | Long cytoplasmic extensions | Increases the rate of transmission of impulses to and from the brain. |
| Intestinal epithelial cell | Many small projections called microvilli, forming a brush border | Increases the surface area : volume ratio for faster absorption of nutrients in the small intestine. |
| Sperm cell | Motile flagellum (tail); head contains enzymes; neck packed with mitochondria | Swims to the egg for fertilisation. Enzymes break down the outer layers of the egg so it can fuse with the egg cell. Mitochondria supply energy for movement. |
🌱 Examples: Plant Cells
| Specialised Cell | Adaptation | Function / Advantage |
|---|---|---|
| Root hair cell | Long narrow projection; no waxy cuticle and concentrated cell sap; numerous mitochondria | Increases surface area : volume ratio for efficient absorption of water and mineral salts. Steep concentration gradient lets water enter quickly by osmosis. Energy supplied for active transport of mineral ions. |
| Xylem vessel | Long hollow tubes with no cross walls, cytoplasm or protoplasm; lignin deposited on walls | Lignin strengthens the tube; hollow centre allows water and dissolved minerals to be transported up the plant. |
| Guard cell | Uneven cellulose in walls — thicker layer on the side facing the stoma; contains chloroplasts | Regulates the opening and closing of stomata for gas exchange and to control transpiration. Glucose formed during photosynthesis changes the shape of the guard cells. |
6 Levels of Organisation in an Organism
| Level of Organisation | Definition | Example |
|---|---|---|
| Cell | The basic structural and functional unit of all living organisms | Muscle cell, nerve cell, root hair cell |
| Tissue | A group of similar cells working together to perform a specific function | Muscular tissue; photosynthetic (palisade) tissue in leaves |
| Organ | A group of different tissues working together for a specific function | The heart — consists of different tissues (e.g. muscle tissue, nerve tissue) |
| Organ system | Different organs working together to carry out related functions | Nervous system — brain, spinal cord and sensory organs |
| Organism | A single living thing made up of organ systems working together | Human; flowering plant |
🧬 Tissues — Animal vs Plant
Tissues are groups of similar cells working together to perform a specific function.
| Animal tissues | Plant tissues |
|---|---|
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❤️ Organs — Animal vs Plant
Organs are groups of different tissues working together for a specific function.
| Animals | Plants |
|---|---|
| Liver, kidneys, heart, stomach, brain | Leaves, stem, roots |
🫀 Human Organ Systems
An organ system consists of different organs working together for a specific function.
| System | Organs | Function(s) |
|---|---|---|
| Digestive system | Mouth, oesophagus, stomach, small intestine, large intestine, rectum, anus | To break down food into small, soluble nutrients to be absorbed into the bloodstream |
| Skeletal system | Skull, vertebrae, ribcage, pelvic girdle, thigh bone | To support our body and give us shape, and to protect our internal organs; enables movement together with the skeletal muscles |
| Respiratory system | Nose, trachea, bronchi, bronchioles, lungs | For intake of air into the body and exchange of oxygen and carbon dioxide in the lungs |
| Excretory system | Kidneys, ureter, bladder, urethra | To remove metabolic waste products like urea |
| Nervous system | Brain, spinal cord, sensory organs | To coordinate and regulate bodily functions in response to changes in the environment |
| Circulatory system | Heart, arteries, veins, capillaries, blood | To transport soluble nutrients, oxygen, waste products, carbon dioxide and hormones to different parts of the body |
| Reproductive system | Male — testes, sperm duct, sex glands, penis; Female — ovaries, oviducts, uterus, vagina | For reproduction and continuation of human species |
🌿 Plant Organ Systems
The transport system in plants consists of the root system and the shoot system. The transport system ensures delivery of water and nutrients to the various plant organs.
| System | Organs | Function(s) |
|---|---|---|
| Root system | Roots | Water and mineral salts enter plants through the roots |
| Shoot system | Leaves, buds, flowers, stem | Transports water to the leaves for photosynthesis |
📊 Summary of the Levels of Organisation in an Organism
Animals:
| Cell | Tissue | Organ | Organ system |
|---|---|---|---|
| Nerve cell | Nervous tissue | Spinal cord | Nervous system |
| Muscle cell | Muscular tissue | Muscle | Muscular system |
| Red blood cells | Blood | — | Circulatory system |
Plants:
| Cell | Tissue | Organ | Organ system |
|---|---|---|---|
| Palisade cells, spongy cells, epidermal cells, guard cells, xylem, phloem | Photosynthetic tissue; vascular tissue; epidermis tissue | Leaf | Shoot system |
| Xylem, phloem | Vascular tissue; supporting tissue | Stem | Shoot system |
| Root hair cell, xylem | Xylem tissue | Root | Root system |
Hierarchy: Cell → Tissue → Organ → Organ system → Organism. Each level is built from the one below it, and each performs a specific role in the organism.