4.1
The Role of Cells in Organisms
4.2
The Microscope
4.3
The Basic Structure of a Cell
4.4
Levels of Organization in Organisms
4.5
Discovery of Cells

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4.1 The Role of Cells in Organisms (p. 78)
Cell: the basic unit of life
They control the basic functions of life:
• Growth
• Reproduction
• Movement
• Detection of environmental changes
• Response

4.2 The Microscope (p. 78)
Hand lens / dissection microscope
• With magnification of 10 ~ 20 times
• For examining the external features of relatively large specimens
Microscopes :
• Light microscope : with magnification up to
1,500 times
• Electron microscope : with magnification up to 1,000,000 times
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4.2 The Microscope (p. 79)
Coarse adjustment knob
Fine adjustment knob
Stage
Eyepiece
Body tube
Nosepiece
Objective
Clip
Condenser
Diaphragm
Mirror
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4.2 The Microscope (p. 82-83)
Practical 4.1
Using the light microscope for low-power magnification

4.2 The Microscope (p. 82-83)
Analysis
1. How should the slide be placed and fixed in the correct position for observation?
Ans: Place the slide on the stage so that the specimen is located at the centre of the hole. Hold the slide in position with the clips
2. Which parts of the microscope is concerned with the direction, brightness and uniformity of light respectively?
Ans: Mirror, diaphragm and condenser
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Ans:
(a) Watch the objective from the side.
(b) Turn the coarse adjustment knob to lower the body tube until the objective is close but not touching the slide.
(c) Look down through the eyepiece and turn the coarse adjustment knob to raise the body tube slowly until the specimen comes into focus.
(d) Turn the fine adjustment knob for fine adjustment of focus

4.2 The Microscope (p. 81)
Table showing the comparison between low power and high power magnification:
Magnification power
Details of the specimen
Field of view
Image brightness
Low power
Smaller
Less
Large
Brighter
High power
Greater
More
Smaller
Dimmer
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4.2 The Microscope (p. 81)
Critical Thinking 4.1
How can the magnification of an object be worked out?
The total magnification: magnifying power of the eyepiece  magnifying power of the objective
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Complete the table below:
Eyepiece
5x
5x
10x
10x
15x
Objective
5x
10x
10x
40x
40x
Magnification of the object
25x
50x
100x
400x
600x

4.3 The Basic Structure of a Cell (p. 86)
Drawings of some human cells
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4.3 The Basic Structure of a Cell (p. 87)
Diagram showing a typical animal cell (not drawn to scale)
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4.3 The Basic Structure of a Cell (p. 87)
Cell membrane
• Located on the outer surface of the cell
• A thin, elastic layer
• Composed of fats and proteins
• Function:
- control the movement of substances in and out of the cell (selectively permeable)
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4.3 The Basic Structure of a Cell (p. 87)
Cytoplasm
• Bounded by the cell membrane
• A flexible, jelly-like substance
• The site for cellular activities
• Water is the main constituent
• Contains: chemical substances which
food granules (glycogen)
vacuoles
organelles (structures in a cell in certain functions and processes are
• Rod-like structure
• Function:
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- release energy from the breakdown of food (respiration)

Mitochondria
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4.3 The Basic Structure of a Cell (p. 87)
Nucleus (pleural: nuclei)
• A dense and spherical structure in the cytoplasm
• Surrounded by a nuclear membrane
 controls the movement of substances in and out of the nucleus
• Contains chromosomes
- composed of DNA and proteins
- carry genetic materials
• Functions:
- determine the organism ’ s characteristics
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- control the normal activities of the cell

15 chromosome nucleus cell
DeoxyriboNucleic Acid.
DNA

4.3 The Basic Structure of a Cell (p. 88)
Drawings of some plant cells
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Root hair cell Guard cells
Epidermal cells
Palisade mesophyll cells

4.3 The Basic Structure of a Cell (p. 89)
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Diagram showing a typical plant cell (not drawn to scale)

4.3 The Basic Structure of a Cell (p. 89)
• Comparatively more regular in shape
• Have all the structures found in an animal cell
(e.g. cell membrane, cytoplasm, nucleus and mitochondria)
• Have a cell wall
• Possess a large central vacuole
• Have chloroplasts for photosynthesis
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4.3 The Basic Structure of a Cell (p. 89)
Cell wall
• A thick and rigid layer surrounding the cell membrane
• Mainly composed of cellulose
• Freely permeable
• Functions:
gives a definite cell shape
protects the delicate internal structures
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4.3 The Basic Structure of a Cell (p. 89)
Chloroplasts
• A kind of organelle
• Contain chlorophyll and enzymes
• Function:
- carry out photosynthesis
Vacuole
• Surrounded by a membrane
• Filled with cell sap
- which contains a solution of sugars, proteins, minerals and other substances
• Function:
- acts as a store of various substances
(e.g. water, food, pigments, enzymes, wastes, etc.)
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4.3 The Basic Structure of a Cell (p. 92)
Common structures:
• Both possess cytoplasm surrounded by a cell membrane
• Both possess mitochondria
• Both possess a nucleus surrounded by a nuclear membrane
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4.3 The Basic Structure of a Cell (p. 92)
Table showing the differences between animal cells and plant cells:
Animal cells
1. Do not have a cell wall
2. Usually irregular in shape
Plant cells
Have a cellulose cell wall
Usually regular in shape
3. Have small or no vacuoles
4. Do not have chloroplasts
Usually have a large central vacuole
Some have chloroplasts
5. Usually have glycogen granules
6. Nucleus usually at the centre of the cell
Usually have starch grains
Nucleus usually at the edge of the cell
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4.4 Levels of Organization in Organisms (p. 95)
Cellular level
Unicellular organism:
• Made up of a single cell
• Carries out all the life activities necessary for survival on its own
• Examples: Amoeba, bacteria
Multicellular organism:
• Composed of a large number of cells
• Example: human
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4.4 Levels of Organization in Organisms (p. 95)
Multicellular organism:
All the body cells carry out the basic life processes on their own
(e.g. respiration, process of getting rid of waste products)
Different types of cells work together to carry out particular functions
In human body:
• Muscle cells  movement
• Nerve cells  transmitting signals
• Red blood cells  carrying oxygen
• White blood cells  killing germs
In plant body:
• Cells with chloroplasts  photosynthesis
• Strong and rigid cells  supporting the plant body

4.4 Levels of Organization in Organisms (p. 96)
Tissue level
A tissue is formed when the same type of cells are associated to carry out a particular function
For examples:
In animals: bone, muscle, nerves etc.
In plants: vascular tissue
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4.4 Levels of Organization in Organisms (p. 97)
Organ level
An organ is formed when different types of tissues are grouped together to perform a more complicated function
Example: The heart is an organ composed of various tissues, including
- cardiac muscles
- blood vessels
- nervous tissues
- connective tissues
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4.4 Levels of Organization in Organisms (p. 97)
Other examples:
Some external organs of animals:
• Eyes
• Ears
• Nose
• Skin
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4.4 Levels of Organization in Organisms (p. 97)
Other examples:
Some internal organs of animals:
Small intestine
• Lungs
• Liver
• Stomach
• Kidneys
Caecum
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Heart
Lung
Liver
Stomach
Pancreas
Uterus

4.4 Levels of Organization in Organisms (p. 97)
Fruit Flower Leaf
Other examples:
For plants:
• Roots
• Stems
• Leaves
• Flowers
• Fruits
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4.4 Levels of Organization in Organisms (p. 98)
System level
A system is formed when different organs and tissues work together to carry out a particular process
Examples:
In human body: digestive system, breathing system, excretory system, circulatory system, skeletal system, muscular system, nervous system, endocrine system, reproductive system
Examples:
In plants: transporting system & supporting system
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4.4 Levels of Organization in Organisms (p. 98)
The different levels of organization in a multicellular organism can be summarized as: cells  tissues  organs  systems  organism
The digestive system, breathing system and excretory system
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4.5 Discovery of Cells (p. 99)
Robert Hooke:
• A British scientist in the seventeenth century
• He observed a thin piece of cork under the microscope that he made on his own
• He saw the structures that look like “little rooms”
& gave the name “cells”
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4.5 Discovery of Cells (p. 99)
After further observation and studies by scientists, the cell theory is formulated
Cell theory: The cell is the basic unit of structure and function of living things. There are many different types of cells but they are similar in their basic structure
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4.5 Discovery of Cells (p. 100)
Electron microscope:
• Allows scientists to observe even smaller structures inside a cell
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4.5 Discovery of Cells (p. 100)
Scanning electron microscope:
• Provides a threedimensional surface view of structures
• With very high magnification
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Chapter 4 (p. 100)
The following statements contain some wrong concepts commonly held by students. Point them out and give correct statements.
1. The outermost layer of a cell is the cell membrane.
Ans: It is true only of animal cells since plant cell has a cell wall outside the cell membrane
2. Vacuoles are found only in plant cells.
Ans: Sometimes, small vacuoles are also found in animal cells
3. A large number of cells form a tissue.
Ans: Only large number of similar cells form a tissue
4. All living animal and plant cells contain cytoplasm, cell membrane and nucleus.
Ans: The statement is true in most ceases but mature red blood cells have no nucleus
5. Chloroplasts are found in plant cells and mitochondria are
38 found in animal cells.
Ans: Chloroplasts are found in plant cells, but mitochondria are found in both plant and animal cells