# Tissues, Organs and Organ Systems
Multicellular organisms are organised into a hierarchy of structures, from individual cells all the way up to complete organ systems. Understanding these levels of organisation is essential for GCSE Biology and underpins many other topics, including digestion, circulation, and plant biology.
1. Levels of Organisation
The hierarchy of organisation in multicellular organisms is:
Cells
- The basic building blocks of all living things
- Each cell has a specific structure suited to its function
- Examples: red blood cells, nerve cells, epithelial cells
Tissues
- A tissue is a group of similar cells working together to perform a specific function
- Examples:
- Muscular tissue — contracts to cause movement
- Glandular tissue — produces and secretes substances (enzymes, hormones)
- Epithelial tissue — covers and lines surfaces (e.g., inside of the gut, skin surface)
- Nervous tissue — transmits electrical impulses
- Connective tissue — supports and connects structures (e.g., bone, blood, cartilage)
Organs
- An organ is a group of different tissues working together to perform a specific function
- Each organ contains several types of tissue
- Examples:
- Stomach — contains muscular tissue (churning), glandular tissue (enzyme secretion), and epithelial tissue (lining)
- Heart — contains cardiac muscle tissue, connective tissue, and epithelial tissue
- Leaf — a plant organ containing epidermal tissue, palisade mesophyll tissue, spongy mesophyll tissue, and vascular tissue (xylem and phloem)
Organ Systems
- An organ system is a group of organs working together to perform a major bodily function
- Examples are detailed below
2. Animal Organ Systems
Digestive System
- Function: Break down food into small, soluble molecules that can be absorbed into the blood
- Organs: Mouth, oesophagus, stomach, small intestine (duodenum and ileum), large intestine, rectum, anus
- Associated organs: Liver, pancreas, gall bladder, salivary glands
Circulatory System
- Function: Transport oxygen, nutrients, hormones, and waste products around the body
- Organs: Heart, blood vessels (arteries, veins, capillaries)
- Blood components: Red blood cells, white blood cells, platelets, plasma
Respiratory System
- Function: Gas exchange — oxygen into the blood, carbon dioxide out
- Organs: Trachea, bronchi, bronchioles, lungs (alveoli), diaphragm
Nervous System
- Function: Detect and respond to stimuli; coordinate body functions
- Organs: Brain, spinal cord (CNS), nerves, sense organs
Endocrine System
- Function: Produce hormones that regulate body processes
- Organs: Pituitary gland, thyroid, adrenal glands, pancreas, ovaries/testes
Excretory System
- Function: Remove metabolic waste products
- Organs: Kidneys, ureters, bladder, urethra; lungs (CO₂); skin (sweat)
Reproductive System
- Function: Produce gametes and enable reproduction
- Organs: Ovaries, uterus, fallopian tubes (female); testes, vas deferens, penis (male)
Musculoskeletal System
- Function: Support, protection, and movement
- Organs: Bones (skeleton), muscles, tendons, ligaments, joints
3. The Digestive System in Detail
The digestive system is a key example of how organs work together:
| Organ | Tissue Types Present | Function |
|---|---|---|
| Mouth | Epithelial, muscular, glandular | Mechanical digestion (teeth); chemical digestion (salivary amylase) |
| Oesophagus | Muscular, epithelial | Moves food to stomach by peristalsis |
| Stomach | Muscular, glandular, epithelial | Produces hydrochloric acid and protease enzymes; churns food |
| Small intestine | Epithelial, glandular, muscular | Digestion completed; nutrients absorbed through villi |
| Large intestine | Epithelial, muscular | Absorbs water from undigested food |
| Pancreas | Glandular | Produces digestive enzymes (amylase, protease, lipase) |
| Liver | Glandular | Produces bile (stored in gall bladder) to emulsify fats |
4. Plant Organ Systems
Plants also have organs and organ systems:
Plant Organs
- Leaves — photosynthesis and gas exchange
- Stems — support and transport
- Roots — absorption of water and minerals; anchorage
- Flowers — reproduction
Plant Tissues
| Tissue | Function |
|---|---|
| Epidermal tissue | Covers and protects outer surfaces of the plant |
| Palisade mesophyll | Main site of photosynthesis (packed with chloroplasts) |
| Spongy mesophyll | Gas exchange; air spaces allow diffusion of gases |
| Xylem | Transports water and minerals from roots to leaves (dead, hollow cells with lignin walls) |
| Phloem | Transports dissolved sugars from leaves to rest of plant (translocation; living cells with companion cells and sieve plates) |
| Meristem tissue | Found at root and shoot tips; cells that can divide and differentiate into any plant cell type |
The Leaf as an Organ
A leaf contains multiple tissues working together:
- Upper epidermis — protective layer, often with a waxy cuticle to reduce water loss
- Palisade mesophyll — just below the upper surface; cells are tall and packed with chloroplasts for photosynthesis
- Spongy mesophyll — below palisade layer; irregular cells with air spaces for gas exchange
- Lower epidermis — contains stomata (tiny pores) controlled by guard cells for gas exchange and transpiration
- Vascular bundles — contain xylem and phloem for transport
5. Enzymes — An Overview
Enzymes are essential to the functioning of organ systems, particularly digestion:
- Enzymes are biological catalysts — they speed up chemical reactions without being used up
- They are proteins with a specific 3D shape
- The active site has a shape complementary to the substrate (lock-and-key model)
- Factors affecting enzyme activity: temperature, pH, substrate concentration
- If temperature or pH is too extreme, the enzyme denatures (active site changes shape, substrate can no longer fit)
Key Digestive Enzymes
| Enzyme | Substrate | Product | Where Produced |
|---|---|---|---|
| Amylase | Starch | Maltose (sugars) | Salivary glands, pancreas |
| Protease | Proteins | Amino acids | Stomach, pancreas |
| Lipase | Lipids (fats) | Fatty acids + glycerol | Pancreas |
Worked Example: **Question:** Explain how the stomach is an example of an organ. *(3 marks)*
Problem
Solution
An organ is made up of different types of tissue working together to perform a function. The stomach contains:
- Muscular tissue — contracts to churn and mix food with digestive juices
- Glandular tissue — secretes hydrochloric acid and enzymes (protease/pepsin)
- Epithelial tissue — lines the inner and outer surfaces of the stomach
These different tissues work together to break down food mechanically and chemically, making the stomach an organ.
Practice Questions
- Put these in order from smallest to largest: organ, tissue, organ system, cell. (1 mark)
- Give two examples of tissues found in the human body and state the function of each. (4 marks)
- Explain why a leaf is described as an organ. (3 marks)
- Name the organ system responsible for transporting oxygen around the body. (1 mark)
- Describe the roles of xylem and phloem in a plant. (4 marks)
Answers
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Exam Tips
- Learn the order: Cells → Tissues → Organs → Organ Systems → Organism.
- When asked to explain why something is an organ, always name at least two different tissue types and describe how they work together.
- Don't confuse tissues and organs — a tissue is made of similar cells; an organ contains different tissues.
- For plant questions, remember that xylem carries water UP and phloem carries sugar BOTH ways.
Frequently Asked Questions
Is blood a tissue or an organ?
Blood is a tissue (specifically, a type of connective tissue). It is made up of similar components (red blood cells, white blood cells, platelets, and plasma) working together to transport substances.
What's the difference between the small intestine and large intestine?
The small intestine is where most digestion and absorption of nutrients occurs (it has villi to increase surface area). The large intestine mainly absorbs water from undigested food before it is egested.
Do all organisms have organ systems?
No. Only complex multicellular organisms have organ systems. Simple organisms like bacteria (unicellular) carry out all functions within a single cell. Some simple multicellular organisms may have tissues but not fully developed organ systems.
Summary
- Living organisms are organised into: cells → tissues → organs → organ systems → organisms.
- A tissue is a group of similar cells with a specific function; an organ contains different tissues working together.
- Key animal organ systems include the digestive, circulatory, respiratory, nervous, and endocrine systems.
- Plants have organs too: leaves, stems, roots, and flowers, containing tissues like xylem, phloem, and mesophyll.
- The stomach is a classic exam example of an organ containing muscular, glandular, and epithelial tissue.