Understanding Cells and tissues: Edexcel IGCSE Human Biology Explained

Cells and tissues form the structural foundation of the entire edexcel igcse human biology course. Every organ system you study later, from the heart to the kidneys, is built from the cell types and tissue arrangements covered here. Mastering edexcel igcse human biology cells and tissues gives you the strongest possible start.

This section of the specification spans three interconnected areas: cell structure and DNA, genetic engineering and cell division, and tissues and specialised cells. Grasping the logic that connects them, how DNA encodes proteins, how cells divide to grow and repair, and how cells become specialised for particular functions, gives you a framework that supports almost every other topic in the edexcel igcse human biology course. These edexcel igcse human biology revision notes for the 4HB1 specification break down each area with the precision the exam demands.

Cell structure: what you need to know for the exam

The specification requires you to recognise cell structures as seen under both a light microscope and an electron microscope (TEM images only). The key structures and their functions are:

Structure	Function	Visible under
Nucleus	Contains chromosomes made of DNA; controls cell activities	Light microscope and TEM
Chromosomes	Carry genes (lengths of DNA) that code for specific proteins	Light microscope (during division) and TEM
Cell membrane	Controls what enters and leaves the cell; partially permeable	TEM (too thin for light microscope)
Mitochondria	Site of aerobic respiration; release energy from glucose	TEM
Endoplasmic reticulum	Transports proteins and other molecules through the cell	TEM
Ribosomes	Site of protein synthesis; where amino acids are assembled into polypeptides	TEM

Exam tip: A common mistake is stating that mitochondria "make energy." Energy cannot be created or destroyed. Mitochondria release energy from glucose through aerobic respiration. The edexcel mark scheme specifically rewards the word "release" rather than "make" or "produce energy."

DNA structure and replication

The specification describes DNA as two strands coiled to form a double helix. Each strand is made of nucleotides, and the strands are linked by complementary base pairs joined by hydrogen bonds. The base pairing rules are:

Adenine (A) pairs with Thymine (T)
Cytosine (C) pairs with Guanine (G)

DNA replication occurs when a cell prepares to divide. The process follows a logical sequence:

The two strands of the DNA molecule separate (the hydrogen bonds between complementary bases break).
Free nucleotides from the cytoplasm line up alongside each exposed strand.
Complementary base pairing ensures that A pairs with T and C pairs with G on each new strand.
The enzyme DNA polymerase joins the new nucleotides together, forming the sugar-phosphate backbone of each new strand.
Two identical DNA molecules result, each containing one original strand and one new strand.

Worked example: base pairing

If one strand of DNA has the base sequence A-T-G-C-C-A, what is the complementary strand?

Answer: Apply the base pairing rules. A pairs with T, T pairs with A, G pairs with C, C pairs with G, C pairs with G, A pairs with T. The complementary strand is T-A-C-G-G-T.

Protein synthesis: from gene to polypeptide

A gene is a length of DNA containing a sequence of bases that codes for a specific protein. The journey from gene to protein involves two stages: transcription and translation.

Transcription occurs in the nucleus:

The relevant section of DNA unwinds and the two strands separate.
One strand acts as a template for building a molecule of messenger RNA (mRNA).
RNA nucleotides line up alongside the template strand using complementary base pairing (except that RNA uses uracil (U) instead of thymine (T)).
The completed mRNA molecule detaches and moves out of the nucleus through a nuclear pore.

Translation occurs at the ribosomes in the cytoplasm:

The mRNA attaches to a ribosome.
Transfer RNA (tRNA) molecules carry specific amino acids to the ribosome.
Each tRNA has an anticodon that matches a codon (a sequence of three bases) on the mRNA.
The amino acids are joined together in the correct sequence to form a polypeptide chain.

Key distinction: DNA is double-stranded, contains deoxyribose sugar, and uses thymine. RNA is single-stranded, contains ribose sugar, and uses uracil. The exam regularly tests whether you can state these differences clearly.

DNA mutations

A DNA mutation involves a change in the sequence of bases in a gene. This could lead to a change in the amino acid sequence of the protein coded for by that gene, which could then alter the shape and function of the protein and affect the phenotype of the individual. Not all mutations cause a visible change: some base changes still code for the same amino acid, and some altered proteins function normally.

Genetic engineering

The specification requires you to outline the principles of genetic engineering. The process involves:

Identifying and cutting out the desired gene from the DNA of one organism using enzymes.
Inserting the gene into the DNA of another organism (the host).
The host organism then produces the protein coded for by the inserted gene.

Two applications you must know:

Genetically modified bacteria to produce human insulin. The human insulin gene is inserted into bacterial DNA. The bacteria multiply rapidly and produce human insulin, which is harvested and used to treat diabetes.
Genetically modified plants to produce vaccines and to improve health. Plants can be engineered to carry genes that produce vaccine antigens, making vaccines cheaper and easier to distribute.

Cell division: mitosis

Mitosis occurs during growth, repair, cloning and asexual reproduction. It produces two genetically identical diploid daughter cells from one parent cell. The four main stages are:

Stage	What happens
Prophase	Chromosomes condense and become visible. The nuclear membrane begins to break down.
Metaphase	Chromosomes line up along the middle (equator) of the cell. Spindle fibres attach to each chromosome.
Anaphase	The spindle fibres shorten, pulling the two copies of each chromosome to opposite poles of the cell.
Telophase	Nuclear membranes reform around each set of chromosomes. The cytoplasm divides, producing two genetically identical daughter cells.

Worked example: why mitosis matters

A student cuts their finger. Explain how mitosis helps the wound heal.

Answer: Cells around the wound undergo mitosis, producing genetically identical copies of the damaged cells. These new cells replace the damaged tissue, closing the wound. Because the daughter cells are genetically identical to the originals, the repaired tissue functions normally.

Stem cells

Stem cells have the ability to develop into different types of body cells. The specification distinguishes two types:

Embryonic stem cells: Found in early-stage embryos. Can differentiate into any cell type in the body (they are "pluripotent").
Adult stem cells: Found in certain tissues such as bone marrow. More limited in the range of cell types they can become.

The advantages, disadvantages and ethics of stem cell research are commonly tested:

Consideration	Embryonic stem cells	Adult stem cells
Advantage	Can become any cell type, making them useful for treating a wide range of conditions	No ethical concerns about embryo destruction; lower risk of immune rejection if taken from the patient
Disadvantage	Requires destruction of an embryo; risk of immune rejection; potential to form tumours	Limited range of cell types; harder to grow in the laboratory
Ethical issue	Some people believe the embryo is a human life from conception and should not be destroyed for research	Generally fewer ethical objections

Tissues and specialised cells

Cells are grouped into tissues, and tissues are organised into organs. The specification requires you to describe the structure of five tissue types:

Bone: Hard, mineralised tissue that supports and protects the body. Contains living cells within a matrix of calcium phosphate.
Muscle: Three types must be known. Voluntary (skeletal) muscle is striated and under conscious control, used for movement. Involuntary (smooth) muscle is found in the walls of organs such as the gut and blood vessels, not under conscious control. Cardiac muscle is found only in the heart, is striated, and contracts rhythmically without tiring.
Blood: A fluid tissue containing red blood cells, white blood cells, platelets and plasma.
Nervous tissue: Made up of neurones (nerve cells) that transmit electrical impulses.
Epithelium: Covers surfaces and lines cavities. Squamous epithelium is flat and thin (lining the cheek). Ciliated epithelium has hair-like projections (lining the trachea) that waft mucus and trapped particles upwards.

Specialised cells for reproduction

The egg cell (ovum) and sperm cell are both adapted for their roles:

Feature	Egg cell	Sperm cell
Size	Large, to carry a food store (cytoplasm) for the early embryo	Very small and streamlined for swimming
Cytoplasm	Abundant, containing nutrient reserves	Minimal, to reduce weight
Cell membrane	Changes after fertilisation to prevent entry of additional sperm	Contains enzymes in the acrosome to penetrate the egg
Mitochondria	Some present	Many, packed in the middle section to provide energy for the tail to swim
Tail	Absent	Long tail (flagellum) for propulsion towards the egg

Self-check questions

Test yourself on the key concepts from this section. Write your answer before checking.

Name the cell structure where aerobic respiration takes place.
State the complementary base sequence for the DNA strand: G-C-A-T-T-A.
Describe the role of DNA polymerase in DNA replication.
Explain why RNA uses uracil instead of thymine.
During which stage of mitosis do chromosomes line up along the equator of the cell?
Give one advantage and one disadvantage of using embryonic stem cells.
Name two differences between voluntary muscle and cardiac muscle as seen under a light microscope.
Explain how the structure of a sperm cell is adapted for its function.

Answers: (1) Mitochondria. (2) C-G-T-A-A-T. (3) DNA polymerase joins the new nucleotides together to form the new DNA strand during replication. (4) This is simply a structural difference between DNA and RNA; the specification requires you to state it, not explain why it evolved. (5) Metaphase. (6) Advantage: can become any cell type. Disadvantage: requires destruction of an embryo, or risk of immune rejection, or potential to form tumours. (7) Both are striated, but cardiac muscle contracts rhythmically without fatigue and is not under conscious control, while voluntary muscle is under conscious control and tires. (8) The sperm has a long tail (flagellum) to swim towards the egg, many mitochondria to provide energy for movement, a streamlined shape to reduce resistance, and an acrosome containing enzymes to penetrate the egg cell membrane.

These edexcel igcse human biology notes on cells and tissues provide the foundation you need. On the Green Bridge CBT platform, you can work through igcse 4hb1 cells and tissues practice questions that mirror the format and difficulty of the real exam. Every cells and tissues edexcel igcse question you practise builds the precision that earns marks. For the full picture of how this topic connects to the rest of the course, see the edexcel igcse human biology explained overview article, and use the edexcel igcse human biology practice questions on the platform to test yourself under timed conditions.