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Tissues, organs and functioning organ systems within an organism
In order to ensure an organism made from many millions of cells survives, the cells’ activities and functions needs to be
organised. The level of organisation within multicellular organisms can be described under three main headings:
Tissues
A collection of cells that are similar
to each other and perform a
common function. These may be
attached to each other but may
not be. Examples include phloem
and xylem in plants, and epithelial
and nervous tissues in animals
Organs
A collection of tissues working
together to perform a particular
function. Examples include the
leaves of plants and the liver in
animals
Organ systems
Made up of several organs
working together to perform an
overall life function. Examples
include the excretory system and
the reproductive system
EPITHELIAL TISSUES IN ANIMALS
Animal tissues in general are grouped under four main categories:
 epithelial tissue – layers and linings
 connective tissue – hold structures together and provide support
 muscle tissue – cells specialised to contract and move certain body parts
 nervous tissue – cells that convert certain stimuli into electrical impulses and conduct those impulses
Epithelial tissues form sheets which cover surfaces. Almost all organs in the body have some kind of epithelial tissue
involved. Simple epithelia are one cell thick. Cells rest on a basement membrane (a network of collagen and
glycoprotein, secreted by the underlying cells and that holds the epithelial cells in position).
Squamous (pavement) epithelia cover many surfaces in the body including the cheeks, blood vessels and alveoli. The
individual cells are smooth, flat and very thin. They fit closely together to provide a low-friction surface. Their thinness
allows for rapid diffusion.
Ciliated epithelia have cilia. Cells can be cubodial, as in the bronchioles, of columnar, as in the oviduct. The cells with
cilia waft rhythmically, moving material over the surface, for example to move the egg along the oviduct. There are also
mucus-secreting goblet cells present. In breathing tubes, the mucus traps dirt and microbes whilst cilia move it
upwards.
TISSUES IN A LEAF
1 – Upper epidermis: secretes a waxy cuticle to protect
against pathogens and prevent dramatic water loss.
Transparent to allow the light to reach palisade layer
2 – Palisade mesophyll: many chloroplasts circulating the
cells for photosynthesis – long and thin cells mean fewer
cell walls for light to pass through
3 – Spongy mesophyll: spread out to provide air spaces
for uptake of CO2 for palisade cells and excretion of O2
4 – Lower epidermis: as upper epidermis, but also has
stomata for gas exchange, controlled by guard cells
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TRANSPORT TISSUES IN PLANTS
Plants need to move water and minerals from the soil through their roots and stems and up into
their leaves. They also need to be able to move the products of photosynthesis around the plant to
be used for growth or stored in other places for later use.
In a plant, meristems are points at which meristem cells are produced. These are the only
undifferentiated cells in a plant which can specialise into any other form of cell needed. Meristem
regions are the root, shoot tips and a ring around the stem or trunk. Meristem cells can become
cells which can become part of the transport tissues needed for the above functions.
Some meristem cells produce small cells which elongate to become xylem cells. The xylem
vessel walls are reinforced and made waterproof by deposits of lignin.
The lignin kills the cell contents and breaks down the ends
of the cells so that the stack of cells becomes one long
hollow tube, with a wide lumen. Xylem tissue is used
to transport water and minerals up the plant.
Phloem
Phloem tissue consists of sieve tubes and companion
cells. The meristem tissue produces cells which elongate and
Xylem
line up end-to-end to form a long tube. Their ends do not
completely break down, but form sieve plates between the cells.
The plates allow the movement of materials up or down the tubes. Next
to each sieve tube is a companion cell. These are very metabolically active. They move the products of photosynthesis up
and down the plant.
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