Welcome to Cells and Tissues: The Building Blocks of the Human Body
Welcome to your revision notes for Cells and Tissues! This chapter forms the foundation of Unit AS 7: Understanding the Physiology of Health and Illness in your CCEA AS Level Health and Social Care course.
Have you ever wondered how trillions of microscopic parts work together to keep a person alive, walking, and thinking? Just like a house is built from individual bricks, mortar, walls, and rooms, the human body is organised in a precise biological hierarchy. Understanding how healthy cells and tissues work—and what happens when they get damaged—is essential for understanding illness, medical treatments, and the care needs of service users.
Don't worry if physiology feels overwhelming at first! We will break everything down step by step using everyday analogies, clear definitions, and helpful tips to secure top marks in your 2-hour AS 7 examination.
1. The Hierarchy of Biological Organisation
The human body is organised into six distinct structural levels, moving from the microscopic sub-units up to the complete living person:
1. Organelle: A specialised sub-cellular structure within a cell that performs a specific metabolic job (e.g., a mitochondrion or ribosome).
2. Cell: The basic structural and functional unit of all living organisms (e.g., a muscle cell or red blood cell).
3. Tissue: A group of specialised cells of similar structure and function working together with their extracellular matrix to perform a specific task (e.g., ciliated epithelial tissue).
4. Organ: A structure made up of two or more distinct tissue types working together to perform coordinated physiological functions (e.g., the heart, stomach, or lungs).
5. Organ System: A group of organs that cooperate to carry out major bodily functions (e.g., the cardiovascular system or respiratory system).
6. Organism: The integrated, complete living individual (e.g., a human being).
Analogy: Building a House
Think of organelles as the clay and water, the cell as an individual brick, the tissue as a brick wall, the organ as a complete room (like the kitchen), the organ system as the whole house layout with plumbing and wiring, and the organism as the entire occupied home!
Memory Trick: Remember the Order
Use the mnemonic phrase: Old Chefs Taste Orange Soup Often
Organelle \(\rightarrow\) Cell \(\rightarrow\) Tissue \(\rightarrow\) Organ \(\rightarrow\) System \(\rightarrow\) Organism
Common Examiner Pitfall: Tissues vs Organs
Watch out! In AS 7 exams, students often incorrectly call the heart, stomach, or skin a "tissue". The heart is an organ because it is made of several different tissues working together (cardiac muscle tissue, epithelial lining, nervous tissue, and connective tissue).
Quick Review: Level Check
Sub-cellular worker: Organelle
Basic unit of life: Cell
Team of similar cells: Tissue
Group of tissues: Organ
Team of organs: Organ System
Whole person: Organism
2. Animal Cell Structure and Organelles
Human cells are eukaryotic, meaning their genetic material is enclosed inside a true nucleus. Inside each cell, microscopic structures called organelles carry out life-sustaining processes.
The Factory Analogy of the Cell
Imagine the cell as an active manufacturing factory. Each organelle has a distinct department role:
1. Nucleus (The Main Office & Blueprint Archive):
Enclosed by a double membrane called the nuclear envelope, which contains small openings called nuclear pores. It houses the genetic material (DNA / chromatin) and contains the nucleolus (the site where ribosomes are assembled). The nucleus controls all cellular metabolic activities and directs protein synthesis.
Exam tip: Never write that the nucleus is "the brain of the cell"—examiners will not award marks for this! Always state that it contains DNA / genetic material and controls cellular activities and protein synthesis.
2. Cell / Plasma Membrane (The Security Gate):
A selectively permeable phospholipid bilayer embedded with proteins and cholesterol. It controls the entry and exit of nutrients, ions, and waste products, maintaining a stable internal environment (cellular homeostasis).
3. Cytoplasm and Cytosol (The Factory Floor):
The gel-like aqueous fluid (cytosol) containing dissolved ions, nutrients, enzymes, and organelles. It is the site of vital metabolic chemical reactions, such as the early stages of cellular respiration.
4. Mitochondria (The Power Generators):
Double-membraned, rod-shaped organelles. The inner membrane is folded into finger-like projections called cristae, surrounding an internal fluid called the matrix. Mitochondria are the site of aerobic cellular respiration, which breaks down glucose in the presence of oxygen to release energy in the form of ATP (adenosine triphosphate).
Exam tip: Avoid writing that mitochondria "create" or "make" energy (energy cannot be created). State that they are the site of aerobic respiration to release energy / produce ATP.
5. Ribosomes (The Assembly Workers):
Tiny, non-membrane-bound complexes of ribosomal RNA (rRNA) and proteins (80S ribosomes in human cells). They are the exact site of protein synthesis (translation), assembling amino acids into polypeptide chains.
6. Rough Endoplasmic Reticulum / RER (The Assembly Line):
A network of flattened, membrane-bound sacs studded with ribosomes on the outer surface. The RER folds, modifies, and packages newly synthesised proteins destined for secretion or incorporation into cell membranes.
7. Smooth Endoplasmic Reticulum / SER (The Custom Workshop):
A tubular membrane network with no ribosomes. The SER synthesises lipids, phospholipids, and steroid hormones, stores calcium, and helps detoxify drugs and harmful metabolic by-products.
8. Golgi Apparatus (The Packaging and Dispatch Department):
A series of flattened, curved membrane sacs (cisternae). It receives proteins and lipids from the ER, modifies them (e.g., adding carbohydrate chains to form glycoproteins), sorts them, and packages them into membrane-bound vesicles for transport within the cell or export outside the cell via exocytosis.
9. Lysosomes (The Recycling and Waste Disposal Team):
Spherical membrane-bound sacs containing powerful acidic hydrolytic (digestive) enzymes. They break down cellular waste, damaged organelles (autophagy), and engulf foreign pathogens (such as bacteria during phagocytosis).
Quick Review: Key Organelles and Roles
Nucleus: Holds DNA; controls cell activities.
Plasma Membrane: Selectively regulates entry and exit.
Mitochondria: Site of aerobic respiration; releases ATP.
Ribosomes: Site of protein synthesis.
Rough ER: Folds and modifies proteins (has ribosomes).
Smooth ER: Synthesises lipids and steroids (no ribosomes).
Golgi Apparatus: Modifies, packages, and tags proteins into vesicles.
Lysosomes: Contain digestive enzymes to break down waste.
3. The Four Primary Human Tissue Classifications
A tissue is defined as a group of specialised cells of similar structure working together to perform a specific physiological function. In the human body, all tissues belong to four main classifications:
1. Epithelial tissue (coverings and linings)
2. Connective tissue (support, protection, and binding)
3. Muscle tissue (movement and mechanical force)
4. Nervous tissue (communication and electrical impulses)
Classification 1: Epithelial Tissue
General Characteristics:
Epithelial tissues form continuous, protective sheets that cover the outer surfaces of the body (skin), line internal cavities, and line hollow organs. They are avascular (they lack direct blood vessels and receive oxygen and nutrients by diffusion from the underlying connective tissue) and rest upon an underlying basement membrane.
Epithelial tissues are classified by the shape of the cells (squamous = flat; cuboidal = cube; columnar = tall column) and the number of layers (simple = one layer; stratified = multiple layers).
Key Sub-types to Know for AS 7:
A. Simple Squamous Epithelium:
Structure: A single, ultra-thin layer of flattened, scale-like cells.
Function: Provides a very short diffusion pathway for the rapid exchange of gases and fluids.
Locations: Air sacs (alveoli) of the lungs, lining of blood capillaries (endothelium).
B. Simple Cuboidal Epithelium:
Structure: A single layer of cube-shaped cells with central round nuclei.
Function: Secretion and absorption.
Locations: Kidney tubules, surface of ovaries, ducts of small glands.
C. Simple Columnar Epithelium:
Structure: A single layer of tall, column-shaped cells, often fitted with microvilli to increase surface area, interspersed with mucus-producing goblet cells.
Function: Active absorption of nutrients and secretion of digestive juices and protective mucus.
Locations: Lining of the stomach and small intestine (gastrointestinal tract).
D. Ciliated Epithelium (e.g., Pseudostratified Ciliated Columnar):
Structure: Columnar cells equipped with microscopic, hair-like projections called cilia on their exposed surface, alongside mucus-secreting goblet cells.
Function: Goblet cells produce sticky mucus to trap inhaled dust, dirt, and pathogens; the wave-like beating of the cilia sweeps the mucus upwards away from the lungs toward the throat to be swallowed or coughed out.
Locations: Respiratory tract (trachea, bronchi).
E. Stratified Squamous Epithelium:
Structure: Multiple thick layers of cells, with flattened squamous cells at the outer surface.
Function: Provides heavy-duty physical protection against mechanical friction, wear and tear, chemical damage, and pathogens.
Locations: Outer layer of the skin (epidermis), lining of the mouth, oesophagus, and vagina.
Classification 2: Connective Tissue
General Characteristics:
Connective tissue is the most abundant and widely distributed tissue in the human body. Unlike epithelial tissue, connective tissue is not made of tightly packed cells. Instead, it consists of cells widely dispersed within a non-cellular extracellular matrix made of a gel-like ground substance and protein fibres (such as collagen for strength and elastin for flexibility).
Exam tip: Always mention the extracellular matrix when defining connective tissue in exam questions!
Key Sub-types to Know for AS 7:
A. Areolar (Loose) Connective Tissue:
Structure: A loose, soft web of collagen and elastin fibres surrounded by a semi-fluid ground substance.
Function: Cushions and protects body organs, binds skin to underlying muscles, and holds internal structures in place.
Locations: Beneath epithelial sheets; surrounding blood vessels and nerves.
B. Adipose Tissue (Fat):
Structure: Packed with specialised cells called adipocytes, each storing a large droplet of triglycerides (fat).
Function: Long-term energy storage, thermal insulation to prevent heat loss, and mechanical shock-absorbing cushioning around delicate organs.
Locations: Subcutaneous layer beneath the skin; padding around the kidneys and eyeballs.
C. Dense Fibrous Connective Tissue:
Structure: Dense, closely packed bundles of parallel collagen fibres with high tensile strength.
Function: Resists extreme pulling forces in one direction.
Locations: Tendons (which attach muscle to bone) and ligaments (which connect bone to bone at joints).
D. Cartilage:
Structure: A firm, resilient, and flexible tissue containing specialised cells called chondrocytes housed within tiny spaces called lacunae, surrounded by a smooth collagen matrix. It is avascular.
Function: Reduces friction between moving bones at joints, absorbs shock, and provides flexible structural support.
Locations: Articular ends of long bones at joints (hyaline cartilage), rings of the trachea, intervertebral discs (fibrocartilage), outer ear (elastic cartilage).
E. Bone (Osseous Tissue):
Structure: A hard, mineralised extracellular matrix reinforced with calcium phosphate salts (hydroxyapatite) and strong collagen fibres, containing mature bone cells called osteocytes.
Function: Provides a rigid structural framework, protects delicate internal organs (e.g., skull protecting the brain, ribs protecting the heart/lungs), stores calcium, and acts as levers for movement.
F. Blood and Lymph (Fluid Connective Tissue):
Structure: A liquid extracellular matrix called plasma carrying formed cellular elements: erythrocytes (red blood cells), leukocytes (white blood cells), and thrombocytes (platelets).
Function: Erythrocytes transport oxygen bound to haemoglobin; leukocytes provide immune defence against pathogens; platelets control blood clotting; plasma transports dissolved nutrients, carbon dioxide, urea, and hormones throughout the body.
Classification 3: Muscle Tissue
General Characteristics:
Muscle tissue is highly cellular and rich in blood vessels. It is specialised for contraction (shortening) to generate mechanical tension and movement.
There are three distinct types of muscle tissue:
1. Skeletal Muscle:
Microscopic appearance: Long, cylindrical fibres that are striated (striped pattern of actin and myosin filaments) and multinucleated (many nuclei per cell located at the periphery).
Control: Voluntary (under conscious control via the somatic nervous system).
Location: Attached to bones across the skeleton.
Function: Produces body movement, locomotion, and maintains posture.
2. Cardiac Muscle:
Microscopic appearance: Striated, branching fibres with a single central nucleus per cell (uninucleate). Individual cells are joined end-to-end by specialised junctions called intercalated discs, which allow rapid electrical conduction across the heart.
Control: Involuntary (contracts automatically without conscious effort).
Location: Walls of the heart (the myocardium).
Function: Rhythmic, fatigue-resistant contraction to pump blood through the circulatory system.
3. Smooth (Visceral) Muscle:
Microscopic appearance: Non-striated (smooth appearance), composed of spindle-shaped cells with tapered ends and a single central nucleus (uninucleate).
Control: Involuntary (controlled automatically by the autonomic nervous system).
Location: Walls of hollow internal organs and tubular vessels (e.g., stomach, intestines, urinary bladder, uterus, and blood vessel walls).
Function: Propels substances along internal tracts via rhythmic waves of contraction (peristalsis in the gut) and regulates blood vessel diameter (vasoconstriction and vasodilation).
Summary Table of Muscle Types
Skeletal Muscle: Striated | Multinucleated | Voluntary | Attached to bones
Cardiac Muscle: Striated | Uninucleate (with intercalated discs) | Involuntary | Heart wall
Smooth Muscle: Non-striated (spindle-shaped) | Uninucleate | Involuntary | Hollow organs & vessels
Classification 4: Nervous (Neural) Tissue
General Characteristics:
Nervous tissue forms the brain, spinal cord, and peripheral nerves. It is specialised for the rapid generation, transmission, and processing of electrical nerve impulses (action potentials).
Nervous tissue consists of two major cell populations:
1. Neurons (Nerve Cells):
The highly excitable, conducting cells of the nervous system. A neuron has three main structural parts:
Dendrites: Branch-like extensions that receive chemical signals and electrical impulses from receptors or other neurons and carry them toward the cell body.
Cell Body (Soma): Contains the nucleus, mitochondria, and organelles; integrates incoming signals.
Axon: A long, single extension that conducts electrical nerve impulses away from the cell body toward a synapse or effector organ (muscle or gland). Many axons are wrapped in an insulating myelin sheath to dramatically speed up impulse transmission.
2. Neuroglia (Glial Cells):
Non-conducting supporting cells that outnumber neurons. They protect, nourish, physically support, and insulate neurons (e.g., Schwann cells and oligodendrocytes produce the protective myelin sheath; other glial cells maintain the chemical balance around neurons).
4. Applying Physiology to Health, Illness, and Service Users
In Unit AS 7, you must be able to link cellular and tissue damage directly to chronic diseases and explain how these conditions impact the holistic health and wellbeing of service users.
Always structure the holistic impact on a service user using the PIES model:
P = Physical: Bodily symptoms, mobility, pain, fatigue, medication side-effects.
I = Intellectual: Concentration, memory, ability to attend school/work, learning new care management techniques.
E = Emotional: Anxiety, fear, low self-esteem, depression, loss of identity.
S = Social: Relationships with family/friends, social isolation, leisure participation, financial security and employment.
Clinical Application Example 1: Destruction of Ciliated Epithelium in Chronic Bronchitis
Physiological Breakdown:
Cigarette smoke and environmental pollutants paralyse and destroy the delicate cilia of the respiratory ciliated epithelium, while stimulating goblet cells to produce excessive mucus. Because the cilia can no longer beat rhythmically to sweep mucus upwards, thick mucus accumulates in the bronchi and bronchioles, obstructing airways and trapping pathogens, leading to chronic inflammation and frequent bacterial chest infections.
PIES Holistic Impact on the Service User:
Physical: Persistent severe coughing, chest tightness, chronic breathlessness (dyspnoea), exhaustion, and frequent lung infections.
Intellectual: Reduced oxygen levels can cause brain fog; the individual may need to learn complex inhaler techniques and pulmonary rehabilitation exercises.
Emotional: Feelings of frustration and fear during acute breathless episodes; low mood or depression due to the loss of physical independence.
Social: Difficulty walking or leaving the house leads to social isolation; inability to maintain full-time employment can lead to reduced income and financial stress.
Clinical Application Example 2: Breakdown of Cartilage in Osteoarthritis
Physiological Breakdown:
In osteoarthritis, the smooth, protective articular cartilage covering the ends of bones in synovial joints (e.g., knees, hips) gradually wears away and breaks down. Without this shock-absorbing, low-friction layer, bone ends rub directly against bone, causing joint inflammation, bone spur development, severe pain, and joint stiffness.
PIES Holistic Impact on the Service User:
Physical: Chronic joint pain, morning stiffness, reduced joint mobility, difficulty walking or climbing stairs, and sleep disruption.
Intellectual: Constant chronic pain drains mental energy, making it hard to concentrate at work or during daily tasks.
Emotional: Frustration over losing independence; anxiety regarding potential surgery (e.g., total hip replacement); feeling older than their years.
Social: Giving up active hobbies, sports, or family outings; relying on family or carers for daily living tasks; strain on personal relationships.
5. Chapter Summary and Exam Checklist
To secure maximum marks in your AS 7 examination questions on Cells and Tissues, ensure you can:
1. State the biological hierarchy in order: Organelle \(\rightarrow\) Cell \(\rightarrow\) Tissue \(\rightarrow\) Organ \(\rightarrow\) Organ System \(\rightarrow\) Organism.
2. State the precise functions of all eukaryotic organelles without using forbidden shortcuts (e.g., mitochondria release energy as ATP via aerobic respiration; the nucleus holds DNA and regulates cellular activities).
3. Define a tissue as a group of similar cells working together to perform a specific function.
4. Explain the key features of the four main tissue types: epithelial (sheets, avascular, basement membrane), connective (cells in an extracellular matrix with fibres), muscle (contractile, striated vs non-striated, voluntary vs involuntary), and nervous (neurons and supporting neuroglia).
5. Apply tissue dysfunction to a chronic illness (e.g., damage to ciliated epithelium in bronchitis; breakdown of articular cartilage in osteoarthritis) and evaluate the holistic impact using the PIES framework.