Patterns in Nature. Cell theory. Cells are the smallest living units of organisms All cells come from pre-existing cells. Each organism is made of one or more cells. Evidence to support cell theory. Cell theory was formulated over a period of
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Cell theory was formulated over a period of
about 300 years in parallel with the development
of the microscope. Microscopes enabled scientists
to make observations of tissues from
organisms. As microscopes
improved scientists began to study
the internal structure of cells and
identify cell organelles.
The light compound microscope uses light to view
specimens. It has two lenses.
The electron microscope was developed in the
1930s and uses a beam of electrons instead of
light as a source of energy. There are two types
of electron microscope—the transmission
microscope, transmits electrons through the
specimen, and the scanning electron
microscope in which bounces electrons off the
An organelle is a small structure within a cell that
performs a specific function. Each organelle has a
structure that suits its function. Organelles are
situated in the cytoplasm of cells and include:
- Nucleus & nucleolus - Vacuoles
- Mitochondria - Ribosomes
- Endoplasmic reticulum - Golgi bodies
Chemical compounds in cells can be divided into
Volume is the amount of space occupied by an
object, expressed in cubic units.
Volume = 2 x 2 x 2
Surface area is the area of the outer surface of
an object expressed in square units.
SA = 6 x 2 x 2
Surface are to volume ratio is the amount of
surface area of an object compared with its
SA : V = 24 : 8
= 3 : 1
Substances move into and out of cells across the
cell membrane. The cell membrane covers the
surface of the cell and therefore has a surface area.
The contents of the cell occupy a space or volume.
The ratio of SA : V influences the rate at which
substances can move into and out of the cell.
As objects become larger their SA : V ratio
decreases. Cells are small because this keeps their
SA : V ratio high. This in turn ensures that transport
of substances across the surface area of the cell is
fast enough to service the metabollic activities in
the volume of the cell.
Diffusion is the movement of a substance from where it is more concentrated to where it is less concentrated.
Osmosis is diffusion
of water across a
membrane from an
area where it is at a
concentration to an
area where it is
Plants are autotrophs which means they are able to
make organic materials from inorganic materials.
They do this by the process of photosynthesis. Other
organisms, such as animals and fungi, that depend
directly or indirectly on the organic compounds
produced by producers are called heterotrophs.
Photosynthesis can be
described using a
water + carbon oxygen + glucose
What structures ensure that plants have a ready
supply of the materials needed for photosynthesis?
Leaves, stems and roots therefore combine to
provide the sunlight energy, carbon dioxide and
water that plants need for photosynthesis.
In heterotrophic organisms the digestive system
provides the means by which nutrients are taken in
and broken down. Large insoluble food molecules
are converted into small soluble ones that can be
absorbed and made available to the body cells.
The function of teeth is to physically break down food
into smaller pieces and so increase the surface area
for enzyme activity
Humans have four
types of teeth: incisors,
Incisors are flat sharp teeth for cutting and biting.
On either side of the four incisors are two canines,
which are adapted for ripping and tearing food.
Premolars and molars are on either side of the
canines and are used for grinding and crushing food.
Multicellular organisms need specialised systems to:
Gases exchange across the surface of cells all the
time. Oxygen moves into cells and carbon dioxide
moves out. Surfaces where gases enter and leave
the body are called respiratory surfaces. Their
Different organisms have different strategies for
Mammals have lungs.
The respiratory surface
of the alveoli, where
most gas exchange
occurs are protected
within the body.
Most amphibians have soft moist skin with an
extensive blood capillary network just below the
surface. As a result gas exchange can occur
directly through the skin. Most carbon dioxide is lost
through the skin.
A frog’s lungs consist of a pair of hollow sacs. Frogs
first take air into a space behind the mouth called
a buccal cavity.
This is separated from the lung by a glottis which
opens and closes to control the movement of air into
and out of the lung.
Fish need to obtain their oxygen from water. Gills
are the respiratory surfaces fish use for exchanging
dissolved gases with the water around them.
Gills are protected by bony plates and are
constructed of many thin stacked layers that provide
a large surface area for gas exchange. In addition,
blood capillaries are just inside the surface of the gill
tissue so that diffusion can occur efficiently.
Insects have a tracheal system where gases are
transported to and from cells through a network of
fine tubes (tracheae and tracheoles). These tubes
are open to the air at the body surface. The
openings are called spiracles.