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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
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
Evidence to support cell theory

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.

development of cell theory
Development of Cell Theory
  • 1590 The first compound (two lens) microscope was made by Hans and Zacharias Janssen.
  • 1665 Robert Hooke studied thin slices of cork under the microscope and described what he saw as small boxes or cells.
  • 1676 Anton van Leeuwenhoek, saw microorganisms under the microscope when he viewed a few drops of pond water.
development of cell theory5
Development of Cell Theory
  • 1824 Frenchman Henri Dutrochet suggested that all organisms are composed of cells.
  • 1827 Scottish botanist, Robert Brown discovered and described the nucleus in plant cells.
  • 1838 German biologists Schleiden and Schwann advanced the idea that all organisms are made of cells.
  • 1859 German biologist Rudolph Virchow stated that all cells divide
the light compound microscope
The light compound microscope

The light compound microscope uses light to view

specimens. It has two lenses.

  • Objective lens: is placed over the microscope slide containing the specimen. It collects light passing through the object and forms a magnified image of it.
  • Ocular lens (eyepiece): is placed at the top of the barrel. It collects the magnified image from the objective lens and further magnifies this image.
electron microscope
Electron microscope

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


cell organelles
Cell Organelles

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

- Lysosomes

chemicals in cells
Chemicals in Cells

Chemical compounds in cells can be divided into

two groups:

  • Organic substances: always contain carbon atoms. Examples include carbohydrates, lipids (fats), proteins and nucleic acids
  • Inorganic substances: may or may not contain carbon and are found in living and non-living things. Examples include water, salts, phosphates, carbon dioxide and metals such as potassium.

Volume is the amount of space occupied by an

object, expressed in cubic units.

Volume = 2 x 2 x 2

= 8cm3




surface area
Surface area

Surface area is the area of the outer surface of

an object expressed in square units.

SA = 6 x 2 x 2

= 24cm2




surface area to volume ratio
Surface area to volume ratio

Surface are to volume ratio is the amount of

surface area of an object compared with its


SA : V = 24 : 8

= 3 : 1




surface area to volume ratio13
Surface area to volume ratio

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.

surface are to volume ratio
Surface are to volume ratio

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


autotrophs and heterotrophs
Autotrophs and heterotrophs

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

word equation:


water + carbon oxygen + glucose


plant structures photosynthesis
Plant structures & photosynthesis

What structures ensure that plants have a ready

supply of the materials needed for photosynthesis?


  • Flat shape provides a large surface area exposed to sunlight
  • Stomata provide access into the leaf for CO2
  • Chloroplasts trap the energy of sunlight
  • Xylem and phloem vessels transport materials to and from the leaves
plant structures photosynthesis21
Plant structures & photosynthesis


  • Xylem vessels give rigidity to stem and transport water and minerals from the roots to the rest of the plant
  • Branching of stems allows layers of leaves at different levels thereby increasing total area available for sunlight
  • Phloem transports products of photosynthesis via the stem to the rest of the plant
plant structures photosynthesis22
Plant structures & photosynthesis


  • Tap water and minerals salts
  • Root hairs give a large surface area to volume ratio and this increases the area available for absorption of water and mineral salts.

Leaves, stems and roots therefore combine to

provide the sunlight energy, carbon dioxide and

water that plants need for photosynthesis.

mammalian digestion
Mammalian Digestion

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.

mammalian teeth
Mammalian teeth

The function of teeth is to physically break down food

into smaller pieces and so increase the surface area

for enzyme activity

(chemical breakdown).

Humans have four

types of teeth: incisors,

canines, premolars

and molars.

mammalian teeth25
Mammalian teeth

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.

exchange and transport in multicellular organisms
Exchange and transport in multicellular organisms

Multicellular organisms need specialised systems to:

  • Obtain nutrients (digestive system)
  • Exchange gases with the external environment (respiratory system)
  • Distribute gases and nutrients to cells (circulatory system)
  • Remove cellular wastes (excretory system)
gas exchange
Gas exchange

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

characteristics include:

  • All living cells must be moist for gas exchange to occur. Oxygen and carbon dioxide are dissolved in water before they diffuse across respiratory surfaces
gas exchange29
Gas exchange
  • Respiratory surfaces have a large surface area to maximise diffusion
  • Respiratory surfaces have a good blood supply to carry oxygen to body cells
  • Respiratory surfaces are thin so that gases can quickly move into the blood

Different organisms have different strategies for

exchanging gases.

gas exchange in mammals
Gas exchange in mammals

Mammals have lungs.

The respiratory surface

of the alveoli, where

most gas exchange

occurs are protected

within the body.

gas exchange in frogs
Gas exchange in frogs

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.

gas exchange in frogs33
Gas exchange in frogs

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.

gas exchange in fish
Gas exchange in fish

Fish need to obtain their oxygen from water. Gills

are the respiratory surfaces fish use for exchanging

dissolved gases with the water around them.

gas exchange in fish35
Gas exchange in fish

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.

gas exchange in an insect
Gas exchange in an insect

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.