Principles of organisationOxford AQA IGCSE Biology: Revision notes
Section 1
How are cells specialised for their functions?
Specialised cells have specific structures that enable them to perform particular functions efficiently. Each type of specialised cell has adaptations that relate directly to its role in the organism.
Red blood cells are specialised for oxygen transport:
- Biconcave disc shape increases surface area for oxygen absorption
- Lack a nucleus in mammals, providing more space for haemoglobin
- Haemoglobin protein binds oxygen reversibly
Sperm cells are specialised for fertilisation:
- Long tail (flagellum) enables movement towards the egg
- Mitochondria in midpiece provide energy (ATP) for tail movement
- Acrosome (vesicle) at the head contains enzymes to penetrate the egg membrane
- Streamlined shape reduces resistance during movement
Egg cells are specialised for reproduction:
- Large size contains stored nutrients (yolk) to support early development
- Nucleus contains genetic material from the female
- Cell membrane has receptors to recognise sperm
- Protective layers (zona pellucida) prevent polyspermy
Ciliated epithelial cells are specialised for moving fluids:
- Multiple cilia (hair-like structures) beat in coordinated waves
- Lined with mitochondria to provide ATP for ciliary movement
- Found in respiratory and reproductive tracts to move mucus and gametes
Root hair cells are specialised for water and mineral absorption:
- Long thin extension increases surface area for absorption
- Thin cell wall and cell membrane allow rapid water uptake
- Packed with mitochondria for active transport of minerals against concentration gradient
- Large vacuole maintains turgor pressure
Palisade cells are specialised for photosynthesis:
- Tall columnar shape packed with chloroplasts
- Chloroplasts concentrated near upper surface to capture maximum light
- Large vacuole supports cell structure and maintains shape
- Thin upper cell wall allows light penetration
Nerve cells (neurones) are specialised for transmitting electrical signals:
- Long axon carries nerve impulses over long distances
- Branched dendrites receive signals from other neurones
- Numerous mitochondria provide ATP for active transport of ions
- Synaptic terminals form connections with other cells
Muscle cells are specialised for contraction:
- Packed with contractile proteins (actin and myosin filaments)
- Numerous mitochondria provide ATP for contraction
- Sarcoplasmic reticulum stores calcium ions for contraction
- Striations visible under microscope (in skeletal muscle) due to organised protein arrangement
Examiners expect you to link structure to function. For each specialised cell, identify one adaptation and explain how it helps the cell perform its role. Use phrases like 'This allows...' or 'This enables...' to show the relationship between structure and function.
Students often confuse cilia with flagella. Remember: cilia are short, numerous, and beat in waves (found on epithelial cells); flagella are long, singular structures used for propulsion (like sperm tails).
Section 2
What are the levels of organisation in living organisms?
Living organisms show a hierarchical organisation from the simplest to the most complex level:
- Cells - the basic unit of life; each cell is the smallest unit that can carry out all life processes
- Tissues - groups of similar cells working together to perform a specific function
- Organs - structures made of different tissues working together to perform a particular function
- Organ systems - groups of organs that work together to perform a major life process
- Organism - a complete living individual composed of multiple organ systems
Each level of organisation builds upon the previous level, and each level has emergent properties that cannot be predicted from the level below. For example, a muscle cell alone cannot produce movement, but when many muscle cells are organised into muscle tissue and then into organs controlled by the nervous system, coordinated movement emerges.
Think of organisational hierarchy like a school: individual students (cells) form classes (tissues), classes combine to form departments (organs), departments work together to run the school (organ systems), and the whole school is the organism.
Example in animals: Muscle cells (specialised cells) → Muscle tissue (groups of muscle cells) → Heart (organ made of muscle, connective, and nervous tissue) → Circulatory system (heart, blood vessels, blood). Each level adds new functions not possible at the level below.
Section 3
What are examples of tissues in animals?
Animal tissues are classified into four main types:
Epithelial tissue - covers body surfaces and lines organs:
- Simple epithelium: single layer of cells for absorption or secretion (e.g. lining of small intestine, lung alveoli)
- Stratified epithelium: multiple layers providing protection (e.g. outer layer of skin)
- Ciliated epithelium: cells with cilia to move fluids (e.g. respiratory tract)
- Columnar epithelium: tall cells for absorption and secretion (e.g. intestinal lining)
- Squamous epithelium: flat cells for diffusion (e.g. alveoli, blood capillary walls)
Connective tissue - binds and supports other tissues:
- Blood (transports substances)
- Bone (provides structural support and protection)
- Cartilage (reduces friction in joints, provides flexibility)
- Tendons (connect muscle to bone)
- Ligaments (connect bone to bone)
- Adipose tissue (stores energy as fat)
Nervous tissue - transmits electrical signals:
- Composed of neurones and glial cells
- Found in the brain, spinal cord, and nerves
- Enables coordination and response to stimuli
Muscle tissue - contracts to produce movement:
- Skeletal muscle: voluntary contraction, striated appearance, attached to bones
- Cardiac muscle: involuntary contraction, striated, forms the heart wall
- Smooth muscle: involuntary contraction, non-striated, found in blood vessel walls and digestive tract
When describing tissues, link the structure of cells to their function. For example: 'Squamous epithelial cells are flat, which provides a short diffusion distance for gases in the lungs' shows understanding of structure-function relationships.
Section 4
What are examples of organs and organ systems in animals?
Major organs in animals are composed of multiple tissues:
| Organ | Tissues Present | Function |
|---|---|---|
| Heart | Cardiac muscle, connective tissue, nervous tissue, epithelium | Pumps blood throughout the body |
| Lung | Epithelial tissue (squamous and ciliated), connective tissue, smooth muscle | Gas exchange (oxygen and carbon dioxide) |
| Stomach | Epithelial tissue (secretory), smooth muscle, connective tissue, nervous tissue | Digestion and mixing of food |
| Brain | Nervous tissue (neurones, glial cells), connective tissue, blood vessels | Coordination and control of body functions |
| Kidney | Epithelial tissue, connective tissue, blood capillaries | Filtration of blood and osmoregulation |
| Skin | Stratified epithelium, connective tissue (dermis), adipose tissue | Protection and temperature regulation |
Major organ systems in animals:
- Circulatory system: heart, blood vessels, blood → transports oxygen, nutrients, and waste
- Respiratory system: lungs, trachea, diaphragm → gas exchange with environment
- Digestive system: mouth, oesophagus, stomach, small intestine, large intestine, liver, pancreas → breakdown and absorption of food
- Nervous system: brain, spinal cord, nerves → coordination and rapid responses
- Endocrine system: glands producing hormones (e.g. pancreas, thyroid) → slow chemical coordination
- Urinary system: kidneys, ureters, bladder, urethra → removal of waste and osmoregulation
- Reproductive system: ovaries/testes, fallopian tubes/sperm ducts, uterus/penis → sexual reproduction
- Muscular system: skeletal muscles, tendons → movement
- Skeletal system: bones, cartilage, ligaments → support and protection
- Immune system: lymphocytes, lymph nodes, spleen → defence against pathogens
The heart is an organ made of cardiac muscle tissue, connective tissue, nervous tissue, and epithelial tissue. It is part of the circulatory system, which includes blood vessels and blood. The circulatory system works with the respiratory system to deliver oxygen to cells and remove carbon dioxide.
Section 5
What are examples of tissues, organs, and organ systems in plants?
Plant tissues:
Dermal tissue - protects plant surfaces:
- Epidermis: single layer of cells covering leaves, stems, and roots
- Cuticle: waxy layer on leaf epidermis that reduces water loss
- Root hair cells: extensions of epidermal cells that increase surface area for water absorption
Ground tissue - fills interior of plant and performs various functions:
- Palisade mesophyll: cylindrical cells packed with chloroplasts for photosynthesis (found in upper leaf)
- Spongy mesophyll: loosely packed cells with air spaces for gas exchange (found in lower leaf)
- Parenchyma: thin-walled cells for storage and gas exchange
Vascular tissue - transports water and dissolved substances:
- Xylem: transports water and mineral ions from roots upwards; made of dead hollow cells
- Phloem: transports dissolved sugars from leaves to other organs; made of living sieve tubes with companion cells
Plant organs:
- Leaf: composed of dermal tissue (epidermis), ground tissue (palisade and spongy mesophyll), and vascular tissue (xylem and phloem) → photosynthesis and gas exchange
- Root: composed of dermal tissue (root hair cells), ground tissue (cortex and endodermis), and vascular tissue (xylem and phloem) → water and mineral absorption, and anchorage
- Stem: composed of dermal tissue (epidermis), ground tissue (cortex and pith), and vascular tissue (xylem and phloem) → support, transport, and growth
Plant organ systems:
- Shoot system: above-ground organs (stems and leaves) → photosynthesis and growth
- Root system: below-ground organs (roots) → water and mineral absorption, anchorage, and storage
- Vascular system: xylem and phloem throughout plant → transport of water, minerals, and dissolved organic substances
When describing plant organs, explain how the arrangement of tissues relates to function. For example: 'Palisade cells are tall and packed with chloroplasts in the upper leaf to maximise light absorption for photosynthesis.'
The plant vascular system is like the human circulatory system: xylem is the 'artery' carrying water upwards, and phloem is the 'vein' carrying dissolved sugars throughout the plant.
Must Know
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Specialised cells have structures adapted to their functions: e.g. red blood cells lack nuclei for more haemoglobin space; root hair cells have large surface area for water absorption; nerve cells have long axons for signal transmission over distance
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Hierarchy of organisation: cells → tissues → organs → organ systems → organism, with each level having emergent properties that cannot be predicted from the level below
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Animal tissues include epithelial (protection and absorption), connective (support and binding), nervous (signal transmission), and muscle tissue (contraction) — each with distinct structures and functions
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Plant tissues include dermal (protection), ground (photosynthesis and storage), and vascular tissue (transport); arranged into organs like leaves, roots, and stems
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Organ systems in animals (circulatory, respiratory, digestive, nervous, urinary, reproductive) and plants (shoot, root, vascular) show how organs work together to maintain life functions
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Link structure to function in all answers: describe how a cell's or tissue's features enable it to perform its role in the organism
That's the notes covered.
Carry on to the next subtopic.