1SS_Cells, Tissues, Organs and Systems_Notes B2

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1SS_Cells, Tissues, Organs & Systems_Notes B2
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Cells - Introduction

The cell is the basic building block of life. All living things are made up of cells. Cells are alive
and they grow and multiply. Cells are so tiny that they can be seen only by using a microscope.

A unicellular organism is made up of only one cell. All functions take place within the cell to
keep itself alive. Examples are the paramecium, euglena, amoeba and desmid.

A multicellular organism is made up of more than one cell. An example is a human being.

Cells come in different shapes, sizes and structures to perform different tasks. In
multicellular organisms, each cell does a different job. This is possible due to cell
differentiation, in which a cell changes its structure to take on a new function. An example
is the red blood cell, which is adapted to transport oxygen around the body.

Animal Cells and Plant Cells

• Animal Cells

An animal cell is usually smaller and irregularly shaped than a plant cell. It consists of
the following structures:


Parts of Cell

Functions


Cell
membrane

• Single layer
• Thin and partially permeable
• Allows only some materials to pass through but not others
(e.g. allows gases to pass through freely but stopping ions)


Nucleus

• Bound by two layers of membrane
• Controls all the chemical reactions that take place in the
cell
• Contains chromosomes, which contain instructions for
protein production

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Secondary 1 Notes B2
Topic B2 Cells, Tissues,
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1SS_Cells, Tissues, Organs & Systems_Notes B2
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Chromosomes

• Thread-like structures which store chemical instructions
and inherited characteristics of an organism
• Chromosomes contains genes, which are passed down
from parents to children, and are responsible for the
different characteristics of an organism
• Normally seen only when the cell is in the dividing stage. In
non-dividing stage, the genetic material or DNA exists as
finer threads called chromatin.


Cytoplasm

• Jelly-like substance which contains many organelles (tiny
structures within the cell performing specific functions)
• Chemical reactions take place in it.
• Exists in 2 forms depending on the condition of the cell:
- Sol state (liquid state)
- Gel state (Semi-solid state)


Vacuoles

• Tiny, numerous spaces containing air, liquid or food
particles
• Not permanent structures























Fig 1: An animal cell
Cell membrane
Cytoplasm
Vacuole
Nucleus
Chromosome (seen only
when the cell is in the
dividing stage)
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1SS_Cells, Tissues, Organs & Systems_Notes B2
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The World of the Minute

The invention of the microscope is one of the biggest breakthroughs of all times. It allows the
human eye to observe very tiny objects. Cells and microscopic organisms were unknown of until
as late as the 17
th
century. Anton van Leeuwenhoek (1632-1723), the father of microscopy,
was the first to see bacteria, yeast, the rich life in a drop of water, and the circulation of blood
cells in blood capillaries, with the help of the first microscope he built. Robert Hooke (1635-
1703) later confirmed Leeuwenhoek's discovery of the presence of tiny living organisms in a
drop of water and improved on Leeuwenhoek’s microscope design.



The light microscope uses a combination of lenses to concentrate light. However, even with
perfect lenses, magnifications can only be made up to 1000 times. Objects which are smaller
than half the wavelength of light will show up as a blur. This is why even tinier objects need a
stronger microscope to be seen.

Electron microscopes were developed due to this limitation of light microscopes. The scanning
electron microscope (SEM) and transmission electron microscope (TEM) use electron
beams instead of light, and electromagnets instead of lenses to concentrate the electrons. It is
possible for an electron microscope to magnify the original specimen up to 100,000 times!

The TEM is similar to the light microscope. It allows us to see the size, shape and arrangement
of finer particles within the cell. The SEM, on the other hand, allows us to see the surface
features of a cell and its texture.

There are many advantages of using an electron microscope. The specimen can be viewed at a
much higher magnification. The area of focus can be larger and resolution greater. With higher
magnification, minute features can be seen. Equally important is a greater resolution, in which
closely spaced features can be distinguished. However, only the light microscope allows for the
observation of living cells in action. This is because the harsh chemical treatment of cells during
preparation for viewing under an electron microscope often kills the cells.






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1SS_Cells, Tissues, Organs & Systems_Notes B2
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• Plant Cells

A plant cell is usually bigger than an animal cell and takes on a more regular shape. It
consists of the following structures:


Parts of Cell


Functions

Cell wall

• Fully permeable layer
• Made up of a thick wall of cellulose, which is fibrous
in nature.
• Provides support to the cell, giving the cell its
regular shape.


Cell
membrane

• Thin and partially permeable layer
• Controls the movement of materials in and out of the
cell


Cytoplasm

• Thin lining in plant cell that contains many
substances in it


Nucleus

• Controls all the chemical reactions that take place in
the cell
• Stores genetic material just as in the animal cell


Starch
grains

• Disc-like structures which store food
• Can be found in cytoplasm


Vacuole

• Big in size and few in number
• Contains the cell sap
• Responsible for water regulation and helps the cell
keeps its shape
• Enclosed by a membrane known as the tonoplast


Cell sap

• Liquid containing dissolved substances such as
sugar and salt and found in vacuole.
• Helps to keep cell firm by absorbing water


Chloroplasts

• Tiny disc-like structures containing a green pigment
called chlorophyll
• Chlorophyll allows plant cells to absorb energy from
sunlight and make food (sugars) from carbon
dioxide and water. The process is known as
photosynthesis

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Fig 2: A plant cell

• Comparing Animal and Plant Cells

Similarities between the plant cell and the animal cell:

• Both have nuclei
• Both have chromosomes in the nucleus
• Both have cell membranes

Differences between an animal cell and a plant cell:

• Animal cells do not have no chloroplasts whereas plant cells do.

• Animal cells do not have no cellulose cell wall whereas plant cells do.
(This lack of a rigid cell wall allows animals to develop a greater diversity of
cell types)

• Animal cells have vacuoles which are small and numerous in numbers
while plant cells have vacuoles which are big in size and few in number.

• In animal cells, cytoplasm fills up the whole cell whereas in plant cells, the
cytoplasm is reduced to only a thin lining.

• Plant cells store food in the form of starch grains which are absent in
animal cells. Animal cells store their food in the form of glycogen instead.


Chloroplast containing
chlorophyll
Cell wall
Cell membrane
Tonoplast
Vacuole containing
cell sap
Nucleus
Chromosome (seen only
when the cell is in the
dividing stage)
Cytoplasm
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A Cell So Unusual…

Living cells can be divided into two main types - prokaryotic (cell without a distinct nucleus)
and eukaryotic (cell with a distinct nucleus). Plant and animal cells contain nuclei and hence
are known as eukaryotic cells. A bacterial cell, on the other hand, has no nucleus to store its
genetic material. Instead, its genes can be found floating within the cytoplasm as a single
circular DNA, known as nucleoid.

Plant and animal cells are more highly organised with many subcellular structures called
organelles as compared to a bacterial cell. They also tend to be larger and more complex.

Bacterial cells, on the other hand, are smaller and simpler in structure, without any organelles.
They have outer cell walls that give them shape. This is exactly what a plant cell wall does,
except that these special bacterial cell walls are made up of polysaccharide and proteins, but
not cellulose.





Intriguing Intriguing Intriguing Intriguing Website Website Website Website

Recommended websites for interested students

- An interesting and interactive site on cell structure and function
http://www.cellsalive.com/


A bacterial cell: Note that unlike plant and animal cells, a bacterial cell has
no well-defined nucleus.
Nucleoid
Cytoplasm
Cell wall Cell membrane
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1SS_Cells, Tissues, Organs & Systems_Notes B2
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Unicellular and Multicellular Organisms

Living organisms are grouped into unicellular and multicellular organisms. Unicellular organisms
are composed of only one cell while multicellular organisms are composed of many cells,
sometimes even millions of cells. In a muliticellular organism, the cells are differentiated into
tissues, organs and systems.

The cell of a unicellular organism performs all the necessary life processes within the
cell.

The cells in a multicellular organism develop from a single cell (fertilized egg cell). This
cell divides to give rise to more cells and these new cells will undergo changes so as to
better perform a specific task. This is known as cell differentiation.

The cells are specialized to perform different functions. This is called division of labour.

The advantages of division of labour among cells are:

- by working as a team, the cells are able to perform different functions to increase
efficiency
- cells can also remain relatively small, thus maintaining a large surface area to
volume ratio, while forming an organism that is large.

Specialised cells come in different shapes, sizes and structures to perform different
tasks. As such, each type of cell does a different job. Hence, the structure of a cell is closely
related to its function. This is a form of structural adaptation of the cell.

The following are examples of some of the specialized cells in plants and animals.


Cell Structure


Adaptation to Function

Root hair cell (plant)






• The root hair cell has a long and narrow structure
which helps to increase the surface area to volume
ratio, which is important in maximising the
efficiency of absorption of water and nutrients from
the soil




Red blood cell (animal)



• The red blood cell has no nucleus so as to pack
more oxygen-carrying pigment (haemoglobin) into
the cell.
• The cell is circular and biconcave (due to the lack of
nucleus in the centre of the cell) to increase the
surface area to volume ratio for oxygen to diffuse
into and out of the cell faster
• Its small size also allows it to squeeze through the
tiny blood capillaries easily