Describe the meaning of the terms: cell, tissue, organ, organ system and organism.

2.1 Cell Structure – Learning Objective

Describe the meaning of the terms: cell, tissue, organ, organ system and organism.

The Cambridge IGCSE Biology (0610) specification also requires you to:

• compare plant, animal and bacterial cells (structures and functions),
• identify cell structures in labelled diagrams,
• state the function of each structure,
• recognise a range of specialised cells,
• understand how new cells are produced,
• use the magnification formula and calculate cell size (including unit conversion), and
• plan and interpret a simple microscope investigation.

Key Definitions (AO 1)

Term	Definition
Cell	The basic structural and functional unit of all living organisms; the smallest unit that can carry out all life processes.
Tissue	A group of similar cells (plus the extracellular material that surrounds them) that work together to perform a specific function.
Organ	A structure made up of two or more different types of tissue that cooperate to carry out a particular task.
Organ system	A set of organs that cooperate to perform a complex function for the whole organism.
Organism	An individual living entity capable of carrying out all life processes; may be unicellular or multicellular.

1 The Cell – Levels of Organisation (AO 1 & 2)

1.1 Comparison of Plant, Animal and Bacterial Cells

Feature	Plant cell (eukaryote)	Animal cell (eukaryote)	Bacterial cell (prokaryote)
Typical size	10–100 µm	10–30 µm	0.5–5 µm
Cell wall	Cellulose – rigid support & protection	Absent	Peptidoglycan – shape & protection
Plasma membrane	Phospholipid bilayer (present)	Phospholipid bilayer (present)	Phospholipid bilayer (present)
Nucleus	True nucleus with nuclear envelope	True nucleus	Absent (DNA free in cytoplasm)
DNA organisation	Linear chromosomes	Linear chromosomes	Circular chromosome(s); often plasmids
Chloroplasts	Present – photosynthesis	Absent	Absent
Vacuole(s)	One large central vacuole (storage, turgor)	Small, numerous (e.g., lysosomes)	Usually none
Mitochondria	Present – cellular respiration	Present	Absent (energy from plasma‑membrane enzymes)
Ribosomes	70 S (free) & 80 S (bound to rough ER)	70 S (free) & 80 S (bound to rough ER)	70 S ribosomes, free in cytoplasm
Other organelles	Golgi, ER, peroxisomes, etc.	Golgi, ER, lysosomes, centrioles	Very simple; no membrane‑bound organelles

1.2 Major Cell Structures and Their Functions (AO 2)

Structure	Location	Function
Cell wall	Outside plasma membrane (plants, bacteria)	Provides rigidity, protection and prevents excessive water uptake.
Plasma membrane	All cells	Regulates entry/exit of substances; maintains internal environment.
Nucleus	Animal & plant cells	Houses DNA; controls cell activities.
Chromosome(s)	Nucleus (eukaryotes) or cytoplasm (prokaryotes)	Carry genetic information.
Plasmid	Cytoplasm of bacteria	Extra‑chromosomal DNA; often carries antibiotic‑resistance genes.
Chloroplast	Plant cells	Site of photosynthesis – converts light energy to chemical energy (glucose).
Mitochondrion	All eukaryotic cells	Site of cellular respiration – produces ATP.
Vacuole	Plant cells (large central) & animal cells (small)	Stores water, nutrients, waste; in plants maintains turgor pressure.
Ribosome	Free in cytoplasm or attached to rough ER	Synthesises proteins.
Endoplasmic reticulum (ER)	Cytoplasm	Rough ER – protein synthesis; Smooth ER – lipid synthesis & detoxification.
Golgi apparatus	Cytoplasm	Modifies, sorts and packages proteins & lipids for transport.
Cilia / Flagella	Surface of some animal & protozoan cells	Movement of cell or movement of fluid over cell surface.

1.3 Specialised Cells (AO 2)

Cell type	Location	Key structural feature	Function (one sentence)
Root‑hair cell	Root epidermis (plant)	Long tubular extension of epidermal cell	Increases surface area for water & mineral absorption.
Palisade mesophyll cell	Leaf (plant)	Tightly packed, many chloroplasts	Major site of photosynthesis.
Neurone	Animal nervous tissue	Long axon & dendrites	Transmits nerve impulses over long distances.
Red blood cell	Blood (animal)	Biconcave disc, no nucleus	Carries oxygen via haemoglobin; maximises surface area.
Ciliated epithelial cell	Respiratory tract (animal)	Numerous motile cilia on apical surface	Moves mucus and trapped particles out of the lungs.
Sperm cell	Testes (animal) / pollen (plant)	Flagellum (animal) or elongated shape (plant)	Motile gamete that delivers male genetic material.
Egg cell (ovum)	Ovary (animal) / ovule (plant)	Large, nutrient‑rich cytoplasm, nucleus at one pole	Female gamete that fuses with sperm to form a zygote.

1.4 How New Cells Are Produced (AO 3)

• Mitosis – division of a somatic cell; produces two genetically identical diploid daughter cells. Purpose: growth, repair and asexual reproduction in some plants.
• Meiosis – specialised division that halves the chromosome number; produces four genetically diverse haploid gametes. Purpose: sexual reproduction.
• Binary fission – simple splitting of a bacterial cell; produces two genetically identical daughter cells. Purpose: asexual reproduction in prokaryotes.

1.5 Magnification & Cell‑Size Calculations (AO 2)

Magnification formula

\[ \text{Magnification} = \frac{\text{Size of image on screen (mm)}}{\text{Actual size of specimen (mm)}} \]

When the image size is measured on a computer screen, the actual size of the cell can be calculated as:

\[ \text{Actual size (µm)} = \frac{\text{Image size (mm)} \times 1000}{\text{Magnification}} \]

Conversely, to convert a real‑world measurement (µm) into the size that will appear on the screen (mm):

\[ \text{Image size (mm)} = \frac{\text{Actual size (µm)} \times \text{Magnification}}{1000} \]

Example

• Image of an onion epidermal cell measures 2.5 mm on the screen.
• Microscope magnification = 400×.
• Actual cell size = (2.5 mm × 1000) ÷ 400 = 6.25 µm.

2 From Cell to Organism (AO 1)

2.1 Tissue

• Group of similar cells plus the extracellular material that work together.
• Four basic animal tissue types:
  1. Epithelial – lines surfaces & cavities.
  2. Connective – supports, binds, stores energy.
  3. Muscle – produces movement.
  4. Nervous – transmits impulses.

2.2 Organ

• Structure composed of at least two different tissue types.
• Example – Stomach (animal)
  • Epithelial lining – protects inner surface.
  • Smooth muscle layers – mix and propel food.
  • Connective tissue – holds layers together.
  • Nervous tissue – regulates secretion.
• Example – Leaf (plant)
  • Epidermis – protective outer layer.
  • Mesophyll (palisade & spongy) – photosynthesis.
  • Vascular tissue (xylem & phloem) – transport of water, minerals and sugars.
  • Cuticle – waxy layer reducing water loss.

2.3 Organ System

• Group of organs that cooperate to perform a complex function.
• Human examples (relevant to the syllabus):
  1. Digestive system – mouth, oesophagus, stomach, small & large intestines, liver, pancreas.
  2. Respiratory system – nose, pharynx, larynx, trachea, bronchi, lungs.
  3. Circulatory system – heart, arteries, veins, capillaries.
  4. Nervous system – brain, spinal cord, peripheral nerves.

2.4 Organism

• The complete living entity, comprising all organ systems (multicellular) or a single cell (unicellular).
• All levels of organisation work together to maintain homeostasis.

3 Visualising the Hierarchy

4 Practical Investigation Ideas (AO 3)

• Microscope observation of different cell types – prepare slides of onion epidermis, cheek cells and a bacterial smear; use a calibrated ocular micrometer to measure cell dimensions.
• Staining techniques – apply iodine to plant cells (stains starch) and methylene blue to animal cells (highlights nuclei) and record the structures that become visible.
• Effect of osmotic pressure on plant cells – place onion strips in distilled water, 0.5 M NaCl and 0.5 M sucrose; observe plasmolysis or turgidity under the microscope.
• Investigating cell division – use onion root tips, stain with acetocarmine, and count the number of cells in each phase of mitosis.
• Measuring cell size (magnification checklist)
  1. State a clear hypothesis (e.g., “Onion epidermal cells are larger than cheek cells”).
  2. Identify variables: independent – cell type; dependent – measured cell diameter; controlled – magnification, temperature, slide preparation.
  3. Calibrate the ocular micrometer using a stage micrometer.
  4. Measure at least 10 cells of each type and record the image size (mm).
  5. Calculate actual size using the formula in 1.5.
  6. Present results in a table and draw a simple conclusion.

5 Supplementary (Extended) Content (Optional)

6 Key Points to Remember (AO 1)

• All living organisms are built from cells.
• Similar cells → tissue; different tissues → organ; organs → organ system; organ systems → organism.
• Plant, animal and bacterial cells share common features (membrane, DNA, ribosomes) but differ in wall composition, organelles and size.
• Each cell structure has a specific function that contributes to the cell’s overall activity.
• Specialised cells are adaptations that allow tissues and organs to perform their specialised roles.
• New cells arise by mitosis (diploid → diploid), meiosis (diploid → haploid) or binary fission (prokaryote).
• Use the magnification formula and remember the conversion 1 mm = 1000 µm.
• When planning a microscope investigation, state a hypothesis, identify variables, take systematic measurements and interpret the data.

7 Check Your Understanding (Practice Questions)

1. Compare a plant cell with an animal cell, listing at least five structural differences and the function of each difference.
2. Identify the labelled parts of the diagram below (cell wall, plasma membrane, nucleus, chloroplast, mitochondrion, vacuole, ribosome, bacterial cell wall, plasmid). (Provide a labelled diagram in your answer.)
3. Give two examples of specialised cells and describe how their structure relates to their function.
4. Explain why the stomach is classified as an organ rather than a tissue.
5. Calculate the actual diameter of a red blood cell if its image measures 1.8 mm on a screen at 1000× magnification.
6. Describe a simple experiment to observe the effect of a hypertonic solution on plant cells.
7. Differentiate between mitosis and meiosis in terms of purpose, chromosome number of daughter cells and number of divisions.
8. State which organ system includes the heart and explain one way this system contributes to homeostasis.