Levels of organisation:

Adapted Cells:

Red blood celltransport of oxygen– biconcave shape
– no nucleus
-has haemoglobin
Muscle cell contracts to get structures
closer together
-many protein fibres in
cytoplasm to shorten cell
when energy is available
Ciliated cellmove and push mucus– tiny hairs called cilia
Root hair cellabsorb minerals and water-elongated shape for more
surface area
xylem vesseltransport water, support
-no cytoplasm so water
passes freely
-no end walls so all cells connect to form a tube
-lignin makes in waterproof
palisade cellcarries out photosynthesis– regular shape so many
can fit in small space
-many chloroplasts

Specialisation of cells

Most cells, when they have finished dividing and growing, become specialised. When cells are specialised:

  • they do one particular job
  • they develop a distinct shape
  • special kinds of chemical change take place in their cytoplasm

(a) cilated cells

These cells form the lining of the nose and windpipe, and the tiny cytoplasmic ‘hairs’, called cila, are in a continual flicking movement which creates a stream of fluid (mucus) that carries dust and bacteria through the bronchi and trachea, away from the lungs.

cilated cells

(b) root hair cell

These cells absorb water and mineral salts from the soil. The hair-like projection on each cell penetrates between the soil particles and offers a large absorbing surface. The cell membrane is able to control which dissolved substances enter the cell.

root hair cell

(c) xylem vessels

These cells transport mineral ions from the roots to the leaves. A substance called lignin impregnates and thickens the cell walls making the cells strong and impermeable. This gives the stems strength. The lignin forms distinctive patterns in the vessels – spirals, ladder shapes, reticulate (net-like) and pitted. Xylem vessels are made up of a series of long xylem cells joined end-to-end. Once a region of the plant has stopped growing, the end walls of the cells are digested away to form a continuous, fine tube. The lignin thickening prevents the free passage of water and nutrients, so the cytoplasm in the cells dies. Effectively, the cells form long, strong straws.

xylem vessels

(d)) palisade mesophyll cells

These are found underneath the upper epidermis of plant leaves. They are columnar (quite long) and packed with chloroplasts to trap light energy. Their function is to make food for the plant by photosynthesis using carbon dioxide, water and light energy.

(e) nerve cells

These cell are specialised for conducting electrical impulses along the fibre, to and from the brain and spinal cord. The fibres are often very long and connect distant parts of the body tot the CNS, e.g. the foot and the spinal column. Chemical reactions cause the impulses to travel along the fibre.

(f) red blood cells

These cells are distinctive because they have no nucleus when mature. They are tiny disc-like cells which contain a red pigment called haemoglobin. This readily combines with oxygen and their function is the transport of oxygen around the body.

red blood cells

(g) sperm cell

Sperm cells are male sex cells. The front of the cell is oval shaped and contains a nucleus which carries genetic information. There is a tip, called an acrosome, which secretes enzymes to digest the cells around an egg and the egg membrane. Behind this is a mid-piece which is packed with mitochondria to provide energy for movement. The tail moves with a whip-like action enabling the sperm to swim. Their function is reproduction, achieved by fertilising an egg cell.

sperm cell

(h) egg cell

Egg cells (ova, singular: ovum) are larger than sperm cells and are spherical. They have a larger amount of cytoplasm, containing yolk droplets made up of protein and fat. The nucleus carries genetic information. The function of the egg cell is reproduction.

egg cell

Examiners tips

  1. You need to be able to give examples of tissues, organs and organ systems in both plants and animals. A leaf is an organ made up of a number of tissues, e.g. upper epidermis, palisade, mesophyll.
  2. If you draw a diagram to support an exam answer, make sure you refer to it in your written answer. Annotation is more likely to help you gain extra mark.

Example of annotation

Action of phagocyte

cytoplasm forms pseudopodia to surround and engulf bacteria – enzymes are released to digest and kill bacteria

Animal cells

Cell typeAppearanceFunctions and adaptations
Red blood cellTransports oxygen from the lungs to the tissues where aerobic
respiration occurs. The cytoplasm is filled with the pigment
haemoglobin, which carries oxygen. The cells have no nucleus, leaving more space for haemoglobin, and they are very flexible (they can be forced through even the narrowest of blood vessels).
Muscle cellContracts so that structures can be brought closer together. Muscle cells are long, and have many protein fibres in the cytoplasm. These fibres can shorten the cell when energy is available.
Ciliated cellHas a layer of tiny hairs(cilia) which can move and push mucus from one place to another. The mucus can transport trapped dust and microbes when it is pushed by the cilia.
Motor nerve cellConducts nerve impulses. The cell has a long fibre called an axon along which impulses travel, a fatty sheath which gives electrical insulation and a many-branched ending which can connect with many other cells

Plant cells

Cell typeAppearance Functions and adaptations
Root hair cellAbsorbs minerals and water from the soil water. The cell has a
long extension (a root hair) which increases the surface area for the absorption of materials.
Xylem vesselTransports water and supports the plant. The cell has no cytoplasm (so water can pass freely), no end wall (so that many cells can form a continuous tube) and walls strengthened with a waterproof substance called lignin.

Cell differentiation and specialisation

Cells don’t all look the same.They have different structures to suit their different functions.

Cells differentiate to become specialised

  1. Differentiation is the process by which a cell changes to become specialised for the job.
  2. As cell change, they develop different subcellular structures and turn into different types of cells. This allows them to carrry out specific functions.
  3. Most differentiation occurs as an organism develops. In most animal cells, the ability to differentiate is then lost at an early stage, after they become specialised. However lots of plant cells don’t ever lose this ability.
  4. The cells that differentiate in mature animals are mainly used for repairing and replacing cells, such as skin or blood cells.
  5. Some cells are undifferentiated cells – they’re called stem cells.

You need to know these examples of specialised cells

SPERM CELLS are specialised for REPRODUCTION

The function of a sperm is basically to get the male DNA to the female DNA. It has a long tail and a streamlined head to help it swim to the egg. There are lot of mitochondria in the cell to provide the energy needed. It also carries enzymes in its head to digest through the egg cell membrane.


The function of nerve cells is to carry electrical signals from one part of the body to another. These cells are long (to cover more distance) and have branched connections at their ends to connect to other nerve cells and form a network throughout the body.

MUSCLE CELLS are specialised for CONTRACTION

The function of a muscle cell is to contract quickly. These cells are long (so that they have space to contract) and contain lots of mitochondria to generate the energy needed for contraction.

ROOT HAIR are specialised for absorbing WATER and MINERALS

Root hair cells are cells on the surface of plant roots, which grow into long “hairs” that stick out into the soil. This gives the plant a big surface area for absorbing water and mineral ions from the soil.


Phloem and xylem cells from phloem and xylem tubes, which transport substances such as food and water around plants. To form the tubes, the cells are long and joined end to end. Xylem cells are hollow in the centre and phloem cells have very few subcellular structures, so that stuff can flow through them.

Organelle: a specialised part of a cell that has its own function, a structure within a cell (e.g. nucleus, vacuole, cytoplasm and chloroplast are all organelles of a plant cell).

Cell: the smallest part of a living structure that can operate as an independent unit e.g. the red blood cell

Tissue: a group of cells with similar structures, working together to perform a shared function e.g. muscle tissue

Organ: a structure made up of a group of tissues, working together to perform specific functions e.g. the heart

Organ system: a group of organs with related functions, working together to perform body functions e.g. respiratory system

Organism: an individual made of organ systems which work to keep that organism alive e.g. a cat

How cell form tissues

  1. Cells forming an epithelium: A thin layer of tissue, e.g. the lining of the mouth cavity. Different types of epithelium form the internal lining of the windpipe, air passages food canal, etc., and protect these organs from physical or chemical damage.
  2. Cells forming a small tube: e.g. a kidney tubule. Tubules such as this carry liquids from one part of an organ to another.
  3. One kind of muscle: Forms a sheet of muscle tissue. Blood vessels, nerve fibres and connective tissues will also be present. Contractions of this kind of muscle help to move food along the food canal or close down small blood vessels.
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How cells form tissues
  1. Cells forming part of a gland: The cells make chemicals which are released into the central space and carried away by a tubule. Hundreds of cell groups like this would form a gland like the salivary gland.

Levels of organisation

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Specialised cells combine to form tissues ….

Cells with similar structures and functions are massed together in tissues. Some plant and animal tissues are shown in the tables below.

Animal tissueMain Function
EpitheliumLines tubes such as the gut and covers
surfaces such as the skin
Connective tissueBinds and strengthens other tissues, such
as tendons
BloodTransports substances around the body,
and defends against disease
Skeletal tissueSupports and protects softer tissues, and
allows movement
Nervous tissueSets up nerve impulses and transmits them
around the body
Muscle tissueContracts to support and move the body
Plant tissueMain functions
EpidermisProtects against water loss, and may
be involved in absorption of water and ions
ParenchymaFills spaces between other plant
tissues and may be involved in
storage, as in the potato tuber
Vascular tissueTransports materials through the
plant body
Strenghtening tissueSupports the plant

…. tissues combine to form organs

Several tissues may be combined to form an organ, a complex structure with a particular function, such as the small intestine shown right.

…. organs combine to form organ systems

In complex organisms, several organs work together to perform a particular task. These organs form an organ system.

Each cell, tissue and organ in an organism has a specialised part to play (there is division of labour) but their activities must be coordinated.

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