1SS_Cells, Tissues, Organs and Systems_Notes B2

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1SS_Cells, Tissues, Organs & Systems_Notes B2 © Just Education Holdings Pte Ltd

Name: ____________________________________ Class Code: ______________ Date: __________ 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

JUST SCIENCE TUITION CENTRE Secondary 1 Notes B2

Topic B2 Cells, Tissues, Organs & Systems

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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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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 17th 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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• 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.

IntriguingIntriguingIntriguingIntriguing 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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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

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1SS_Cells, Tissues, Organs and Systems_Notes B2