Levels of Organisation

Multicellular organisms are built up in a hierarchy of organisation, each level more complex than the one below:

1. Cell — the smallest unit of life. Each cell is specialised (red blood cell, palisade cell, sperm cell).
2. Tissue — a group of similar cells that work together to perform the SAME function. Examples: muscle tissue, epithelial tissue, xylem tissue.
3. Organ — a group of DIFFERENT tissues working together to perform a particular function. Examples: heart, small intestine, leaf, kidney.
4. Organ system — a group of organs working together to perform a major body function. Examples: digestive system, circulatory system.
5. Organism — the complete individual, made up of all its organ systems.

Edexcel mark-scheme keywords:

• Tissue = SIMILAR cells, SAME function.
• Organ = DIFFERENT TISSUES, particular function.
• Organ SYSTEM = several ORGANS, major body function.

Writing 'made of cells' for an organ — a frequent examiner-report complaint — loses the mark every time. Use the keyword 'different tissues'.

Let's trace one tissue from a single cell all the way up to the whole human organism using the digestive system.

• Cell: one epithelial cell lining the inner wall of the small intestine. It has microvilli on its upper surface (a feature that increases surface area for absorption).
• Tissue: many epithelial cells together form epithelial tissue lining the lumen of the small intestine.
• Organ: epithelial tissue + muscle tissue (circular and longitudinal) + connective tissue + glandular tissue together form the small intestine — a single organ.
• Organ system: the small intestine joins forces with the stomach, liver, pancreas, oesophagus, large intestine and other organs to form the digestive system — a single organ system.
• Organism: the digestive system, plus the circulatory, respiratory, nervous, endocrine, excretory and other systems, all working together, form a complete human (the organism).

Why this matters. When Edexcel ask 'describe the hierarchy of organisation', they expect named tissues and organs at each step, not just labels. ECF (error carried forward) applies — if you wrongly name a tissue early on but use it consistently going up, you still earn marks for the higher levels.

Plants follow the same five levels. Don't assume the hierarchy is only for animals — Edexcel often uses plant examples on Paper 2.

• Cell: a palisade mesophyll cell packed with chloroplasts.
• Tissue: many palisade cells form palisade mesophyll tissue, the main photosynthetic tissue.
• Organ: palisade tissue + spongy mesophyll tissue + upper and lower epidermis + xylem + phloem all combine to form the leaf — an organ.
• Organ system: all the leaves + stem + branches + petals + roots make up the shoot system (above ground) and the root system (below ground).
• Organism: the whole plant (e.g. a pea plant, an oak tree).

Tissue types unique to plants:

Tissue	Role
Epidermis	Single layer of transparent cells; protects and allows light through.
Palisade mesophyll	Column-shaped cells packed with chloroplasts; main site of photosynthesis.
Spongy mesophyll	Loosely packed cells with air spaces; gas exchange between cells and stomata.
Xylem	Hollow dead tubes; transport WATER and dissolved minerals up from roots.
Phloem	Living sieve tubes + companion cells; transport SUCROSE and amino acids (translocation).

Edexcel pitfall. Don't write 'xylem' inside an animal organ — xylem only exists in plants. Likewise, animal tissues like 'cardiac muscle' don't exist in plants.

Cells in a multicellular organism don't all look alike — they are specialised for different jobs. The process by which an unspecialised cell becomes specialised is called differentiation.

How it works:

• During development, a cell switches some genes on and others off.
• The cell makes different proteins as a result, and its structure changes (e.g. develops a long axon, a flagellum, no nucleus, many chloroplasts).
• Once fully differentiated, most cells stay specialised for the rest of their life. Some cells (e.g. stem cells in bone marrow, meristem cells in plant growing tips) keep the ability to divide and differentiate later.

Differentiation matters because it allows division of labour — each cell type is more efficient at its task than a single 'jack of all trades' cell could ever be. It is what makes the build-up of tissues, organs and organ systems possible.

Important distinction. Mitosis produces two genetically identical daughter cells. Differentiation is what happens AFTERWARDS to make those daughter cells become different from one another by switching different genes on or off. They are NOT the same process.

Edexcel routinely asks 'describe a feature of cell X and explain how it suits its function'. Memorise these spec-named examples — each is worth 1-2 marks per feature with the structure-to-function link.

Red blood cell (oxygen transport):

• Biconcave disc shape → large surface-area-to-volume ratio for fast O₂ diffusion.
• No nucleus → more room inside for haemoglobin.
• Packed with haemoglobin → binds large quantities of O₂.
• Flexible cell membrane → squeezes through narrow capillaries.

Sperm cell (fertilisation):

• Flagellum (tail) → propels the sperm towards the egg.
• Acrosome at the tip → contains digestive enzymes to break down the egg's outer layers.
• Many mitochondria in the midpiece → release ATP to power the tail's beating.
• Streamlined head → reduces drag while swimming.

Nerve cell (impulse transmission):

• Long axon → carries electrical impulses over long distances (e.g. foot to spinal cord).
• Many dendrites → form connections (synapses) with many other neurones.
• Myelin sheath → insulates the axon and speeds up impulse transmission.

Root hair cell (water + mineral absorption):

• Long, hair-like extension → large surface area in contact with soil water.
• Thin cell wall + cell-surface membrane → short diffusion path for water (osmosis).
• Many mitochondria → release ATP for active transport of mineral ions against the concentration gradient.
• No chloroplasts (below ground, no light).

Ciliated epithelial cell (mucus clearance):

• Many cilia on the upper surface → beat to sweep mucus + trapped microbes up the trachea, away from the lungs.

Edexcel template for the A-mark. Always write 'feature X, so that Y'. Stating 'biconcave' without saying 'so that diffusion is faster' loses half the available marks.