In multicellular organisms, cells are not like Stentor: they are specialised to carry out specific functions and are often clustered together or in layers. In animals, a layer of ciliated cells forms a ciliated epithelium, which is a tissue.
A tissue is a group of similar, specialised cells in a multicellular organism that carries out a specific function, or several related functions.
Examples of specialised cells:Sperm cells, Red blood cells, Neutrophils, Palisade cells, Root hair cells, Guard cells.
The thin, flat cells that line the alveoli in the lungs form a tissue called squamous epithelium. Other examples of animal tissue are; Cartilage and bone (for support), muscle (for movement), and blood (for transport)
Tissues are grouped into organs. Animals have many organs, such as; lungs, hearts and kidneys. Examples or plant organs are; roots, stems and leaves.
An organ is a group of different tissues that form a distinct structure and function together.
Xylem and phloem are the transport tissues in plants. Xylem transports water and ions. Phloem transports sugars (mainly sucrose) and other compounds made by plants.
Xylem and phloem come from dividing meristem cells such as cambium. Meristem cells undergo differentiation to form the different kinds of cells in the transport tissues.
Cross section of root Cross section of stem
Xylem and Phloem structure and function
Xylem - meristem cells produce small cells that elongate. Their walls are reinforced & waterproofed by lignin. This kills cell contents & ends of cells break down & become continuous, long tubes with a wide lumen.
- Xylem tissue is well suited for transporting water and minerals up the plant. It also helps support the plant.
Phloem - consists of sieve tubes and companion cells. Meristem tissue produces cells that elongate and line up end-to-end to form a long tube Their ends do not break down completely, but form sieve plates between cells. The sieve plates allow the movement of materials up or down the tubes.
- Next to each sieve tube is a companion cell. Companion cells are very metabolically active. Their activities play an important role in moving the products of photosyntheses up and down the plant in the sieve tubes.
Organs work together to perform one major body function e.g. digestion, coordination.
Organs including trachea, lungs, diaphragm and rib cage work together to bring about ventilation of the gas exchange surface in the lungs. the heart and blood vessels work together to move blood around the body.
In multicellular organisms, cells are specialised to carry out specific tasks. There are some functions that all cells carry out - they all respire to provide themselves with energy; they all make their own proteins, membranes and organelles. But in an animal, cells deep in the body cannot obtain their own food and oxygen directly from the environment - they depend on cells in the gut and lungs to do this. They rely on the heart to keep a constant supply of blood providing the substances they need and removing their wastes. They also rely on the lungs and kidneys to excrete this waste to the outside world.