Chapter 4 The Basic Structure of Cells 4

Chapter 4 The Basic Structure of Cells 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 1

4. 1 The Role of Cells in Organisms (p. 78) Role of cells in organisms Cell: the basic unit of life They control the basic functions of life: • Growth • Reproduction • Movement • Detection of environmental changes • Response 2

4. 2 The Microscope (p. 78) How to magnify an object? 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 times 3

4. 2 The Microscope (p. 79) What are the different parts of a light microscope and their functions? Coarse adjustment knob Fine adjustment knob Stage 4 Eyepiece Body tube Nosepiece Objective Clip Condenser Diaphragm Mirror

4. 2 The Microscope (p. 82 -83) Practical 4. 1 Using the light microscope for low-power magnification 5

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 3. How can the specimen be brought into focus? 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 6

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 7 Low power Smaller Less Large Brighter High power Greater More Smaller Dimmer

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 Complete the table below: 8 Eyepiece Objective Magnification of the object 5 x 5 x 25 x 5 x 10 x 50 x 10 x 10 x 400 x 15 x 40 x 600 x

4. 3 The Basic Structure of a Cell (p. 86) What common features are found in animal cells? Drawings of some human cells 9

4. 3 The Basic Structure of a Cell (p. 87) Diagram showing a typical animal cell (not drawn to scale) 10

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) 11

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 - food granules (glycogen) - vacuoles - organelles (structures in a cell in which certain functions and processes are carried out) Mitochondria (singular: mitochondrion) • Rod-like structure • Function: 12 - release energy from the breakdown of food (respiration)

Mitochondria 13

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 - control the normal activities of the cell 14

chromosome nucleus cell Deoxyribo. Nucleic Acid. DNA 15

4. 3 The Basic Structure of a Cell (p. 88) What common features are found in plant cells? Drawings of some plant cells Root hair cell 16 Guard cells Palisade mesophyll cells Epidermal cells

4. 3 The Basic Structure of a Cell (p. 89) 17 Diagram showing a typical plant cell (not drawn to scale)

4. 3 The Basic Structure of a Cell (p. 89) Characteristics of plant cells: • 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 18

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 19

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. ) 20

4. 3 The Basic Structure of a Cell (p. 92) What are the similarities and differences between animal cells and plant cells? Common structures: • Both possess cytoplasm surrounded by a cell membrane • Both possess mitochondria • Both possess a nucleus surrounded by a nuclear membrane 21

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 Plant cells Have a cellulose cell wall 2. Usually irregular in shape Usually regular in shape 3. Have small or no vacuoles Usually have a large central vacuole 4. Do not have chloroplasts Some have chloroplasts 5. Usually have glycogen Usually have starch grains granules 6. Nucleus usually at the centre of the cell edge of the cell 22

4. 4 Levels of Organization in Organisms (p. 95) How cells are organized to form organism? 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 23

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 • 24 Strong and rigid cells supporting the plant body

4. 4 Levels of Organization in Organisms (p. 96) How are cells organized into higher levels? 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 25

26 bone muscle

Vascular tissue 27

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 28

4. 4 Levels of Organization in Organisms (p. 97) Other examples: Some external organs of animals: • Eyes • Ears • Nose • Skin 29

4. 4 Levels of Organization in Organisms (p. 97) Other examples: Some internal organs of Small animals: intestine • • Lungs Liver Stomach Kidneys Liver Stomach Pancreas Uterus Caecum 30 Heart Lung

4. 4 Levels of Organization in Organisms (p. 97) Fruit Other examples: For plants: • Roots • Stems • Leaves • Flowers • Fruits 31 Flower Leaf

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 32

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 33

4. 5 Discovery of Cells (p. 99) Discovery of cells 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” 34

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 35

4. 5 Discovery of Cells (p. 100) Electron microscope: • Allows scientists to observe even smaller structures inside a cell 36

4. 5 Discovery of Cells (p. 100) Scanning electron microscope: • Provides a threedimensional surface view of structures • With very high magnification 37

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 found in animal cells. 38 Ans: Chloroplasts are found in plant cells, but mitochondria are found in both plant and animal cells