Class notes c Chi Lam Memorial College A.L. Bio. Notes

Buddhist Chi Hong Chi Lam Memorial College A.L. Bio. Notes (by Denise Wong)
The Cell ...... Page 28

Organelles of cells:
Introduction :
- The cell is the fundamental unit of life.
- The modern ‘Cell theory’ states :
i) All living organisms are composed of cells.
ii) All new cells are derived from other cells.
iii) Cells contain the hereditary material of an organism which is passed from parent to
daughter cells.
iv) All metabolic process take place within cells.

Microscopy :
1. Light microscope :
- It is the most common type of microscopes.
- The degree of detail which can be seen with a microscope is called resolution or resolving
power. This measures its ability to distinguish two objects which are close together.
- The resolving power is inversely proportional to the wavelength of light being used. This
means that the resolving power of any light microscope is limited because the wavelength
of light has a fixed range.
- At best it can distinguish two points which are 0.2µm apart and magnify around 1500 times.
2. Electron microscope :
- It works on the same principal as the light microscope except that instead of light rays, a
beam of electrons is used.
- In practice it magnifies just over 500,000 times.
- The image produced by the electron microscope cannot be detected directly by the naked
eye. Instead, the electron beam is directed onto a screen from which black and white
photographs, called photoelectromicrographs, can be taken.
3. A comparison of the light and electron microscope :
Light microscope Electron microscope
Advantage Disadvantage
0

Cheap to purchase Expensive to purchase
Cheap to operate – uses a little electricity for a light bulb. Expensive to operate – requires up to 100,000 volts to
produce the electron beam.
Small and portable – can be used almost anywhere. Very large and must be operated in special rooms.
Unaffected by magnetic fields. Affected by magnetic fields.
Preparation of material is relatively quick and simple, Preparation of material is lengthy and requires
requiring only a little expertise. considerable expertise and sometimes complex
equipment.
Material rarely distorted by preparation Preparation of material may distort it.
Both living or dead material may be viewed. A high vacuum is required and living material cannot be
observed.
Natural colour of the material can be observed. All images are in black and white.
Disadvantage Advantage
Magnifies objects up to 1500x. Magnifies objecs over 5000,000x.
Can resolve objects up to 200nm apart. Has a resolving power for biological specimens of around
1nm.
The depth of field is restricted. It is possible to investigate a greater depth of field.

, Buddhist Chi Hong Chi Lam Memorial College A.L. Bio. Notes (by Denise Wong)
The Cell ...... Page 29

The Types of Cells :

- Prokaryotic cells were probably
the first forms of life on earth.
There are no true nucleus and no ****
membrane- bounded organelles Insert U.B. p39 fig 4.4
within a prokaryotic cell. This ****
occurs only in bacteria and the
blue- green algae




Fig. 31 Structure of prokaryotic cell, e.g. a
generalised bacterial cell.

- The development of eukaryotic cells from prokaryotic ones involved considerable changes.
The essential changes was the development of membrane- bounded organelles within the
outer plasma membrane of the cell.
- The presence of membrane- bound organelles confers four advantages :
i) increase surface area for the metabolic process to take place; (enzymes are embedded
in the membrane)
ii) contain enzymes for a particular metabolic pathway;
iii) control the rate of any metabolic reaction in an organelle as membrane of the organelle
control the passage of reactants.
iv) harmful substance can be isolated inside an organelle.
- There are two main kinds of eukaryotic sells, they are plant and animal cells. Belows are the
differences between them.
Plant Cell Animal Cell
Cellulose cell wall surrounds the cell membrane No cell wall (only a membrane surrounds the cell)
Pits and plasmodesmata present in the cell wall Absent
Middle lamella join cell walls of adjacent cells Cells are joined by intercellular cement
Present of plastids e.g. chloroplast Absent
Mature cells have a large single, central vacuole Vacuoles, e.g. contractile vacuoles, if present, are
filled with cell sap small and scattered throughout the cell
Tonoplast present around vacuole Absent
Nucleus at edge of the cell Nucleus always in the central
Lysosome not normally present Lysosomes always present
Centrioles absent in higher plant Centrioles present
Cilia and flagella absent in higher plant Present
Starch grains used for storage Glycogen granules used for storage
Only meristematic cells can divide Almost all cells are capable of division
Few secretion are produced Wide variety of secretions are produced

, Buddhist Chi Hong Chi Lam Memorial College A.L. Bio. Notes (by Denise Wong)
The Cell ...... Page 30



Exercise : (95 I 1)
Tabulate the major differences in cellular organization between prokaryotic and eukaryotic
organisms. [4 marks]


Ultra-structure of the cell :
****
Insert BSI 3 rd Ed. P135 fig 5.10, 5.11
****




Fig. 32 Ultrastructure of a generalised animal cell as seen with the electron microscope.




Fig. 33 Ultrastructure of a generalised plant cell as seen with the electron microscope.


`

, Buddhist Chi Hong Chi Lam Memorial College A.L. Bio. Notes (by Denise Wong)
The Cell ...... Page 31

1. Cell membranes :
- They are described as selectively permeable, since apart from small molecules, such as
water, larger molecule e.g. glucose, amino acids, glycerol and ions can diffuse slowly
through them. And they also exert a measure of active control over what substances they
allow through.
- As organic solvent (alcohol) penetrate membranes even more rapidly than water, this
suggested that membranes have non- polar portions; in other words they contain lipids.
- After careful chemical analysis, it is found that membranes are comprised almost entirely
of proteins and lipids (phospholipids, glycolipids and sterols).
- The fluid- mosaic model can be used to describe the detailed structure of plasma
membrane.
The fluid-mosaic model (Singer-Nicholson model ) :
Ø It was put forward in the early 1970s by S.J. Singer and G.L. Nicholson.
Ø The protein molecules vary in size and have a much less regular arrangement.
Ø Some proteins occur on the surface of the phospholipid layer, while others extend
into it, some even extend completely across it.
Ø Viewed from the surface, the proteins are dotted throughout the phospholipid bilayer
in a mosaic arrangement.
Ø The hydrophilic phosphate heads of the phospholipids face outwards into the
aqueous environment inside and outside the cell.
Ø The hydrocarbon tails face inwards and create a hydrophobic interior.
Ø Hydrophilic molecules will be repelled by the hydrophobic tails of the phospholipid,
they can pass the membrane only through the pores formed by proteins that spans the
membrane or by protein carriers.
Ø The phospholipids are fluid and move about rapidly by diffusion in their own layers.
Ø Membranes also contain cholesterol which disturbs the close packing of
phospholipids and keeps them more fluid. This can be important for organisms living
at low temperatures when membranes can solidify. Cholesterol also increase
flexibility and stability of membranes. Without it membranes break up.
Ø Glycoproteins and glycolipids are important recognition features of the cells.


Fig. 34 The fluid-mosaic
model of the plasma
membrane.
A Functional approach P27, fig2.22a




- Function :
• separate contents of cells from their external environment;
• controlling exchange of substances between two cells;
• form separate compartments inside cells in which specialised metabolic pathways can
take place, i.e. not interfering each other;
• as receptor sites for recognizing external stimuli;
• the glycoproteins on the surface act as cell identity markers, i.e. antigens;
• site for reaction to take place, e.g. protein on the membrane of chloroplast and
mitochondria take part in the energy transfer system.