Tissues, Organs & Systems
Life is organised in increasing levels of complexity: Cell → Tissue → Organ → Organ System → Organism. Each level performs more complex functions than the one below it.
Organisation Hierarchy
Cell → Tissue → Organ → Organ System → Organism
Tissue = similar cells. Organ = 2+ tissues. System = 2+ organs working together.
A. Animal Tissues
Four Types of Animal Tissue
| Tissue Type | Function | Examples |
|---|---|---|
| Epithelial | Covers body surfaces; lines organs and cavities; acts as barrier and absorbs substances | Skin (epidermis), intestinal lining, lung alveoli, kidney tubules |
| Connective | Supports, binds, protects, and connects other tissues; most diverse tissue type | Bone, Cartilage, Blood (liquid connective tissue), Adipose (fat), Tendons, Ligaments |
| Muscle | Enables movement through contraction — voluntary and involuntary | Skeletal muscle, Smooth muscle, Cardiac muscle |
| Nervous | Transmits electrical impulses (signals) for rapid communication and coordination | Brain, Spinal cord, Peripheral nerves, Sense organs |
⚡ MCQ Tip Blood is a connective tissue (liquid matrix). Cardiac muscle is involuntary and never fatigues. Skeletal muscle is the only voluntary muscle type.
Types of Muscle Tissue
| Type | Control | Location | Key Feature |
|---|---|---|---|
| Skeletal (Striated) | Voluntary (conscious) | Attached to bones | Striped (striated) appearance; fatigues with use; moves limbs |
| Smooth (Non-striated) | Involuntary (unconscious) | Walls of hollow organs (gut, blood vessels, bladder) | No striations; slow sustained contraction; spindle-shaped cells |
| Cardiac | Involuntary (self-stimulating) | Heart wall only | Striated AND branched cells; NEVER fatigues; has intercalated discs |
Skeletal (Striated)
ControlVoluntary (conscious)
LocationAttached to bones
Key FeatureStriped (striated) appearance; fatigues with use; moves limbs
Smooth (Non-striated)
ControlInvoluntary (unconscious)
LocationWalls of hollow organs (gut, blood vessels, bladder)
Key FeatureNo striations; slow sustained contraction; spindle-shaped cells
Cardiac
ControlInvoluntary (self-stimulating)
LocationHeart wall only
Key FeatureStriated AND branched cells; NEVER fatigues; has intercalated discs
B. Plant Tissues
Plant Tissue Types
| Tissue | Type | Function |
|---|---|---|
| Meristematic | Dividing (growth) | Contains actively dividing cells; responsible for plant growth at shoot tips (apical) and root tips |
| Xylem | Vascular (permanent) | Transports water and dissolved minerals from roots → stem → leaves (upward). Dead cells with thick lignified walls. |
| Phloem | Vascular (permanent) | Transports food (sugars/glucose) from leaves to all parts of the plant (both up and down — translocation) |
| Epidermis | Protective (permanent) | Single outer layer covering and protecting plant surfaces; has stomata for gas exchange |
| Parenchyma | Ground (permanent) | Thin-walled, loosely packed; carries out photosynthesis (in leaves) and stores starch |
| Collenchyma | Ground (permanent) | Provides flexible mechanical support in young growing stems and leaves |
| Sclerenchyma | Ground (permanent) | Dead cells with very thick walls; provides rigid support and strength (e.g. in seed coats, nut shells) |
⚡ MCQ Tip Xylem = water & minerals upward (WXMU). Phloem = food (glucose) in all directions. Xylem cells are dead; phloem cells are living. Meristematic = only dividing tissue.
C. Major Human Organ Systems
10 Organ Systems
Digestive System
Mouth, oesophagus, stomach, small intestine, large intestine, liver, pancreas
Breaks down food; absorbs nutrients into blood
Circulatory System
Heart, arteries, veins, capillaries, blood
Transports O₂, nutrients, hormones, and waste products
Respiratory System
Nose, trachea, bronchi, lungs, diaphragm
Gas exchange — O₂ in, CO₂ out
Nervous System
Brain, spinal cord, peripheral nerves, sense organs
Controls and coordinates all body functions; processes information
Endocrine System
Pituitary, thyroid, adrenal glands, pancreas (islets), gonads
Produces hormones for regulation of growth, metabolism, reproduction
Excretory System
Kidneys, lungs, skin, liver
Removes metabolic waste products (urea, CO₂, salts)
Skeletal System
206 bones, cartilage, ligaments, joints
Support, protection of organs, movement, blood cell production
Muscular System
Skeletal, smooth, and cardiac muscles
Movement, locomotion, posture, heat production
Reproductive System
Ovaries/testes, uterus, fallopian tubes, prostate, vas deferens
Production of gametes; sexual reproduction
Immune/Lymphatic System
Lymph nodes, spleen, thymus, white blood cells, lymph vessels
Defence against pathogens; removes toxins; fluid balance
Quick MCQ Revision
| Fact | Answer |
|---|---|
| Organisation hierarchy | Cell → Tissue → Organ → Organ System → Organism |
| Blood tissue type | Connective tissue (liquid matrix = plasma) |
| Cardiac muscle | Involuntary, never fatigues, found only in heart, striated and branched |
| Xylem function | Transports water & minerals upward from roots to leaves |
| Phloem function | Transports food (glucose) from leaves to all parts of plant |
| Meristematic tissue | Only dividing tissue in plants — at shoot and root tips |
| Organ definition | Two or more tissue types working together for a specific function |
| Voluntary muscle | Skeletal (striated) muscle only — under conscious control |
| Smooth muscle location | Walls of hollow organs: gut, blood vessels, bladder, uterus |
| Nervous tissue function | Transmits electrical impulses for communication and coordination |
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