What you'll learn
This revision guide covers the fundamental unit of life—the cell—as tested in CXC CSEC Human and Social Biology examinations. You will explore the structural differences between plant and animal cells, understand the specific functions of organelles, and learn how cells are organised into tissues, organs, and systems. This topic forms the foundation for understanding human physiology and is frequently examined through diagrams, comparison questions, and application scenarios.
Key terms and definitions
Cell — the smallest unit of life that can function independently and perform all the necessary functions of life.
Organelle — a specialised structure within a cell that performs a specific function, such as the nucleus or mitochondrion.
Cell membrane — a selectively permeable barrier that controls the movement of substances into and out of the cell.
Cytoplasm — the jelly-like substance inside the cell membrane where chemical reactions occur and organelles are suspended.
Tissue — a group of similar cells working together to perform a specific function.
Organ — a structure composed of two or more different tissues working together to perform a particular function.
Diffusion — the movement of molecules from a region of high concentration to a region of low concentration until evenly distributed.
Osmosis — the movement of water molecules from a region of high water concentration to a region of low water concentration through a selectively permeable membrane.
Core concepts
The basic structure of animal and plant cells
All cells share common features that allow them to carry out life processes. Understanding both similarities and differences between animal and plant cells is essential for CSEC examinations.
Common structures in both cell types:
- Cell membrane: Controls what enters and leaves the cell; made of lipids and proteins
- Cytoplasm: Site of many chemical reactions; contains enzymes and dissolved nutrients
- Nucleus: Contains genetic material (DNA) organised into chromosomes; controls cell activities
- Mitochondria: Site of aerobic respiration; releases energy from glucose
- Ribosomes: Site of protein synthesis; found free in cytoplasm or attached to endoplasmic reticulum
Structures unique to plant cells:
- Cell wall: Made of cellulose; provides structural support and prevents the cell from bursting
- Chloroplasts: Contain chlorophyll; site of photosynthesis where light energy is converted to chemical energy
- Large permanent vacuole: Contains cell sap (water, sugars, salts); maintains cell rigidity through turgor pressure
Key differences summary:
| Feature | Animal Cell | Plant Cell |
|---|---|---|
| Cell wall | Absent | Present (cellulose) |
| Chloroplasts | Absent | Present in green parts |
| Vacuole | Small, temporary | Large, permanent |
| Shape | Irregular | Regular, fixed |
| Storage carbohydrate | Glycogen | Starch |
Functions of major organelles
Understanding organelle function is critical for explaining how cells perform life processes.
Nucleus:
- Controls all cell activities through genes
- Contains chromosomes made of DNA
- Nuclear membrane has pores allowing substances to pass between nucleus and cytoplasm
- Nucleolus within produces ribosomes
Mitochondria:
- Oval-shaped organelles with folded inner membrane (cristae)
- Aerobic respiration occurs here: glucose + oxygen → carbon dioxide + water + energy (ATP)
- Cells requiring large amounts of energy (muscle cells, sperm cells) contain many mitochondria
- Caribbean context: Athletes training at regional competitions like CARIFTA Games require efficient mitochondrial function for energy production
Ribosomes:
- Tiny structures found throughout cytoplasm
- Site where amino acids are joined to form proteins
- Essential for growth and repair
- Some attached to endoplasmic reticulum forming "rough ER"
Cell membrane:
- Selectively permeable—allows some substances through while blocking others
- Small molecules (water, oxygen, carbon dioxide) pass through easily
- Controls uptake of glucose, amino acids, and mineral ions
- Prevents loss of important cell contents
Chloroplasts (plant cells only):
- Contain chlorophyll pigment that absorbs light energy
- Photosynthesis occurs here: carbon dioxide + water → glucose + oxygen (using light energy)
- Found in green parts of plants—leaves, young stems
- Caribbean context: Dasheen, cassava, and breadfruit leaves contain numerous chloroplasts for efficient photosynthesis in tropical conditions
Cell organisation: from cells to systems
Living organisms show hierarchical organisation, with each level building on the previous one.
Cell → Tissue → Organ → Organ System → Organism
Tissues: Groups of similar specialised cells performing the same function.
Examples in animals:
- Epithelial tissue: covers surfaces and lines organs (e.g., skin, intestine lining)
- Muscle tissue: contracts to produce movement (e.g., biceps, heart muscle)
- Nervous tissue: transmits electrical impulses (e.g., brain, spinal cord)
- Connective tissue: supports and connects structures (e.g., blood, bone)
Examples in plants:
- Xylem tissue: transports water and minerals upwards
- Phloem tissue: transports dissolved food substances
- Palisade mesophyll: contains many chloroplasts for photosynthesis
Organs: Structures containing two or more tissue types working together.
Animal examples:
- Heart: contains muscle tissue, nervous tissue, connective tissue, epithelial tissue
- Stomach: contains muscle tissue for churning, epithelial tissue for secretion and absorption
Plant examples:
- Leaf: contains epidermis, palisade mesophyll, spongy mesophyll, xylem, phloem
- Root: contains epidermis, cortex, xylem, phloem
Organ systems: Groups of organs working together to perform major body functions.
Examples:
- Digestive system: mouth, oesophagus, stomach, small intestine, large intestine, liver, pancreas
- Respiratory system: nose, trachea, bronchi, lungs
- Circulatory system: heart, blood vessels, blood
Movement of substances across cell membranes
Cells must exchange materials with their environment to survive. Three key processes enable this exchange.
Diffusion:
The passive movement of particles from high to low concentration until equilibrium is reached. No energy from respiration is required.
Important examples in the body:
- Oxygen diffuses from alveoli into blood capillaries
- Carbon dioxide diffuses from blood into alveoli
- Digested food molecules diffuse from small intestine into blood
- Oxygen diffuses from blood into respiring tissues
Factors affecting diffusion rate:
- Concentration gradient: steeper gradient = faster diffusion
- Temperature: higher temperature = faster diffusion
- Surface area: larger area = faster diffusion
- Distance: shorter distance = faster diffusion
Osmosis:
The movement of water molecules from a region of high water concentration (dilute solution) to a region of low water concentration (concentrated solution) through a selectively permeable membrane.
Effects on cells:
- Animal cells in pure water: water enters by osmosis, cell swells and may burst (lysis)
- Animal cells in concentrated solution: water leaves by osmosis, cell shrinks (crenation)
- Plant cells in pure water: water enters, vacuole expands, cell becomes turgid (firm) but cell wall prevents bursting
- Plant cells in concentrated solution: water leaves, vacuole shrinks, cell becomes flaccid; cytoplasm may pull away from cell wall (plasmolysis)
Caribbean context: When preparing green bananas or dasheen for cooking, placing them in salted water causes cells to lose water by osmosis, making them firmer and easier to peel.
Active transport:
The movement of substances from low to high concentration (against the concentration gradient) using energy from respiration.
Examples:
- Root hair cells absorbing mineral ions from soil even when soil concentration is lower
- Small intestine absorbing all glucose even when gut concentration becomes lower than blood concentration
- Kidney tubules reabsorbing glucose from filtrate back into blood
Specialised cells
Cells become adapted through differentiation to perform specific functions efficiently.
Red blood cells:
- Biconcave disc shape increases surface area for oxygen absorption
- No nucleus provides more space for haemoglobin
- Contains haemoglobin to transport oxygen
- Flexible to squeeze through narrow capillaries
White blood cells:
- Irregular shape allows movement through tissues
- Nucleus present for controlling protein production
- Some produce antibodies; others engulf pathogens (phagocytosis)
Nerve cells (neurons):
- Long fibre (axon) carries impulses over long distances
- Branched endings (dendrites) connect to other neurons
- Myelin sheath insulates and speeds up impulse transmission
Sperm cells:
- Tail (flagellum) for swimming to egg
- Many mitochondria provide energy for movement
- Acrosome contains enzymes to penetrate egg membrane
Root hair cells:
- Long projection increases surface area for water and mineral absorption
- Thin cell wall allows easy absorption
- Large vacuole helps water movement into cell
Palisade cells:
- Columnar shape allows many cells to be packed together
- Numerous chloroplasts for maximum photosynthesis
- Located near upper leaf surface where light intensity is highest
Worked examples
Example 1: Comparing plant and animal cells (6 marks)
Question: State three structural differences between a typical plant cell and a typical animal cell. (6 marks)
Mark scheme answer:
Plant cells have a cell wall made of cellulose; animal cells do not have a cell wall. (2 marks: 1 for plant cell wall, 1 for absence in animal)
Plant cells have chloroplasts for photosynthesis; animal cells do not contain chloroplasts. (2 marks: 1 for chloroplasts in plant, 1 for absence in animal)
Plant cells have a large permanent vacuole containing cell sap; animal cells have small temporary vacuoles or none. (2 marks: 1 for large permanent vacuole in plant, 1 for small/temporary in animal)
Examiner tip: Award yourself 1 mark for each correct feature in plant cells AND 1 mark for correctly contrasting with animal cells. Simply listing features without comparison earns fewer marks.
Example 2: Function of organelles (4 marks)
Question: Name the organelle responsible for each of the following functions:
(a) Controls the activities of the cell (1 mark)
(b) Site of protein synthesis (1 mark)
(c) Releases energy from glucose during respiration (1 mark)
(d) Site of photosynthesis in plant cells (1 mark)
Mark scheme answer:
(a) Nucleus
(b) Ribosome
(c) Mitochondrion (accept: mitochondria)
(d) Chloroplast
Examiner tip: Learn the specific function of each organelle. The question asks you to "name" so just give the organelle name—no explanation needed for these marks.
Example 3: Osmosis application (5 marks)
Question: A student placed pieces of potato in three different solutions for 30 minutes. The table shows the results.
| Solution | Change in mass |
|---|---|
| Pure water | Increased |
| 5% sugar solution | No change |
| 15% sugar solution | Decreased |
(a) Explain why the potato piece in pure water increased in mass. (3 marks)
(b) What can be concluded about the 5% sugar solution? (2 marks)
Mark scheme answer:
(a) Water moved into the potato cells (1 mark) by osmosis (1 mark) from a region of high water concentration (pure water) to a region of lower water concentration (inside the potato cells) (1 mark).
(b) The 5% sugar solution has the same water concentration as the potato cells (1 mark) so there is no net movement of water by osmosis/the solutions are isotonic (1 mark).
Examiner tip: For osmosis questions, always mention: (1) what moved (water), (2) the process (osmosis), (3) the direction of movement using concentration terms.
Common mistakes and how to avoid them
Confusing diffusion and osmosis: Remember osmosis is specifically the movement of water through a selectively permeable membrane. Diffusion applies to any particles moving down a concentration gradient. Don't describe oxygen movement as osmosis—that's diffusion.
Stating "cell wall" for animal cells: Animal cells have only a cell membrane. The cell wall is present exclusively in plant cells, fungi, and bacteria. In diagrams, if you see a thick outer layer with a thinner membrane inside, that's a plant cell.
Saying mitochondria "produces" energy: Energy cannot be created or destroyed. Mitochondria release or transfer energy from glucose during respiration. Use the term "releases energy" rather than "produces" or "makes" energy.
Forgetting the selectively permeable membrane in osmosis definitions: A complete definition of osmosis must mention that water moves through a selectively permeable membrane. Without this, the definition is incomplete and loses marks.
Mixing up xylem and phloem: Xylem transports water and minerals upwards from roots; phloem transports dissolved food (sugars) both up and down. Remember: xylem = water, phloem = food.
Describing cells as "alive" or "dead" without qualification: Red blood cells lose their nucleus during development but remain functional. Use precise terms like "metabolically active" or "non-nucleated" rather than vague terms.
Exam technique for "Cell structure and function"
Command word awareness: "State" or "Name" requires short answers (usually one or two words). "Explain" requires a reason using "because" or "this causes." "Describe" requires detailed observations without explanation. "Compare" requires you to mention both items being compared—don't just describe one.
Diagram questions carry easy marks: When labelling diagrams, use a ruler for label lines, ensure lines touch the structure precisely, and write labels horizontally. Common structures to identify include nucleus, mitochondria, cell membrane, cytoplasm, chloroplasts, cell wall, and vacuole.
Structure-function links: Questions often ask "how is this cell adapted for its function?" Always link a structural feature to its advantage. Example: "Root hair cell has a long projection which increases surface area for absorption of water and minerals."
Mark allocation guides answer length: If a question is worth 3 marks, provide three distinct points. Don't write a long paragraph with only one point—you'll waste time and not earn full marks.
Quick revision summary
Cells are the basic units of life containing organelles with specific functions. The nucleus controls activities, mitochondria release energy, and ribosomes synthesise proteins. Plant cells differ from animal cells by having a cellulose cell wall, chloroplasts, and a large permanent vacuole. Cells are organised into tissues, organs, and organ systems. Substances move across membranes by diffusion (high to low concentration), osmosis (water movement through a membrane), and active transport (low to high concentration using energy). Specialised cells have adaptations matching their functions.