Lecture Notes – Cell Biology

HKDSE Biology 1 By Michael Ho~*


Molecules of Life
1. Water
 Water (H2O) is the most abundant compound on Earth’s surface and it is
essential for all life on Earth.

 A water molecule consists of two hydrogen atoms covalently bonded to one
oxygen atom.

 Attractive forces (hydrogen bonds) form between water molecules. These
hydrogen bonds are sufficiently strong to create many of the properties of water.

Biological Functions

1. Freezing Properties
Ice is less dense than water and hence ice floats on water surface, insulating the
aquatic organisms below. It allows aquatic animals to survive in cold region.

2. High Latent Heat of Fusion
Large amount of energy has to be removed from water to form ice. The aquatic
environment and cell contents are slow to freeze in cold weather.

3. High Latent Heat of Evaporation
Large amount of energy is required for evaporation of water. It increases heat loss
and provides an effective cooling effect to mammals through sweating and plants
through transpiration.

4. High Specific Heat Capacity
Large amount of energy is required to change the temperature. Water helps to keep
the temperature constant and hence provides a stable environment for aquatic life.

5. High Transparency
Light can penetrate through water which allows photosynthesis of underwater plants
and vision for aquatic animals.

6. High Surface Tension and Adhesion
Water molecules are held together by hydrogen bonds which allow formation of a
continuous water column in plants for water transport.



7. High Polarity
Water is a universal solvent due to its high polarity. Water can dissolve many
substances and hence facilitates chemical reactions and serves as transport
medium.

8. Incompressibility
Water is incompressible and provides turgidity for plants and support for animals with
hydro-skeleton.

2. Biomolecules
 A biomolecule is any molecule that is produced by a living organism, including
Carbohydrates, Proteins, Lipids and nucleic acids.

Function

Carbohydrates  as main source of energy - oxidized to release energy

 as an energy reserve
Lipids
 form an insulator under skin to reduce heat loss

Proteins  build up body tissues for growth and repair
(polypeptides)  as enzymes to speed up reactions

Nucleic acids
 carry genetic information
(DNA and RNA)



3. Inorganic Ions
 Inorganic ions are vital for cellular activities in animals and plants.

Biological Functions

1. Sodium ions (Na+)
 take part in the transmission of nerve impulse

 maintain osmotic balance

2. Potassium ions (K+)

 maintain osmotic balance

3. Calcium ions (Ca2+)

 chemical component of bones and teeth

4. Iron ions (Fe2+)
 chemical component of haemoglobin in red blood cells

5. Magnesium ions (Mg2+)
 chemical component of bones and teeth as well as chlorophyll

6. Nitrate ions (NO3-)
 synthesis of amino acids and nucleotides

7. Phosphate ions (PO4-)
 synthesis of phospholipids, ATPs, bones and teeth



Microscope Development and Cell Theory
 Organisms are made up of cells. Cells are very small and cannot be seen with
the naked eye. People did not even know that cells existed until the microscopes
were invented.

Discovery of Cell
 In 1665, An Englishman, Robert Hooke, looked at a thin slice of cork through his
two-lens microscope. He observed some tiny, hollow structures and called them
‘cells’.

Hooke's microscope Cork cells under Hook's microscope

 The ‘cells’ Hooke observed were in fact the cell walls of dead cork cells.
However, no one knew about the existence of cell walls at that time.



Cell Theory
 In 1839, Schwann proposed the 'cell theory'.

 The cell theory states that:

1. All organisms are made up of one or more cells.
2. The cell is the basic unit of life; it is the smallest unit that
shows all the characteristics of life.
3. All cells come from pre-existing cells.

Modern light microscope Electron microscope

Large organelles in cell are discovered More organelles are discovered and
studied in details

maximum resolution ~0.2 m maximum resolution ~200 nm



Light microscope
 The light microscope can produce an image magnified by up to 1000 times.

 The maximum resolution is about 0.2 m.

Procedure in using the Light Microscope
1. Place the microscope in a well-lit area
2. Position the low power objective lens above the stage hole
3. Adjust the mirror and the diaphragm to provide the best illumination
4. Clip the slide on the stage
5. While watching from the side, turn the coarse focus knob to lower the objective
lens until it almost touches the slide
6. Look through the eyepiece. Turn the coarse focus knob counter-clockwise to
bring the low power objective lens until a clear image is seen
7. If necessary, change to a high power objective lens
8. Adjust the fine focus knob to get a sharper image

Preparing Onion Tissue Sample



Transmission electron microscope
 The transmission electron microscope makes use of electron beams that pass
through the specimen to form an image.

 It produces two-dimensional, black and white
images magnified by up to 800,000 times.

Scanning electron microscope
 The scanning electron microscope makes use of an electron beam to scan over
the surface of a specimen.

 It produces three-dimensional, black and white
images magnified by up to 30,000 times.

Advantages of electron microscopy
 electrons have a shorter wavelength than light, providing a higher resolution
 the maximum useful magnification is higher than that of light microscope

Disadvantages of electron microscopy
 Specimens are always dead and so the structure of a living specimen using
electron microscope can NEVER be studied
 The processes involved in preparing specimens might damage / distort their true
structure


Resolution and Magnification
Resolution: The ability to distinguish between structures that are very close together
Magnification: The ratio of the size of an object and its image

Magnification =

Unit of length Symbol Factor
Gigametre Gm
Megametre Mm
Kilometre km
Metre m
Centimetre cm
Millimetre mm
Micrometre m
Nanometre nm
Picometre pm

Concept Check
1. The following is the compound light microscope.

Eyepiece of 50x and objective of 100x was used,
the magnification of the image would be ________________ = ________


2. The following shows the images produced by 2 types of microscope.

_________________ _________________

______________________________


3.

(a) What is the type of microscope used for studying this cell? (1 mark)

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(b) The cell was stained with uranium salts in preparation for a
transmission electron microscope. Explain how this stain caused the
nucleus to show a dark shade compared to the light shade of the
cytoplasm. (4 marks)

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(c) Calculate the actual length of the nucleus. (2 marks)

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(d) Distinguish the terms resolution and magnification. (2 marks)

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(e) Determine the magnification of this micrograph. (2 marks)

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Prokaryotae and Eukaryotae
Prokaryotae Eukaryotae

lack a distinct nucleus bounded by a have a true nucleus that is separated
membrane from cytoplasm by a nuclear envelope

lack membrane-bound organelles such cytoplasm contains membrane-bound
as mitochondria and chloroplasts organelles

infolding of cell membrane forms mitochondria for respiration
mesosomes for respiration

Single circular DNA and some small Genetic material (DNA) is carried on a
circular DNA called plasmids in number of chromosomes
cytoplasm

Prokaryotic cell
Prokaryotae are a kingdom of organisms including the bacteria and cyanobacteria.



Eukaryotic Cell


1. Nucleus
a) Structure

 It is enclosed by a nuclear membrane of double membranes that is
perforated by nuclear pore.

 It contains chromatin.

 It is enclosed by a nuclear membrane of double membranes that is
perforated by nuclear pore.

 It contains chromatin.



b) General functions of nucleus

 It contains genetic materials (DNA) and can be able to synthesize DNA.

 It synthesizes genetic messages (mRNA) to be transported to cytoplasm
through nuclear pore for protein synthesis.

2. Cytoplasm
 It contains water and dissolved substances.

 Function: It is the site of cellular activities.

3. Endoplasmic Reticulum (ER)
a) Structure

 It is the complex network of folded membranes running throughout the
cytoplasm. It consists of two types: rough ER and smooth ER.

 Rough ER has many ribosomes attached on the outer surface while
smooth ER has none.


b) Function

(i) Rough ER
 Rough ER is concerned with the transport of protein synthesized by ribosomes
on its surface.

(ii) Smooth ER
 Smooth ER is concerned with the synthesis and transport of lipids and steroids.


4. Mitochondrion

a) Structure

 rod-shaped, bounded by a double membrane.

 outer membrane is smooth while inner membrane is highly folded.
b) Function

 it carries out aerobic respiration, producing ATP molecules which are the
form of energy to drive cellular activity.


5. Chloroplast (found in some plant cells only)

a) Structure

 It contains chlorophyll to absorb light for photosynthesis.

 It is surrounded by double membranes and contains a system of flattened
membrane-bond sacs to hold chlorophyll.
b) Function

 It is the site of photosynthesis producing sugars from carbon dioxide and
water using light energy trapped by chlorophyll.

 it converts light energy to chemical energy.


6. Vacuole
a) Structure

 It is a fluid-filled sac bounded by a membrane.

 Plant cells usually have a large central vacuole which is surrounded by a
membrane called tonoplast and contains a fluid called cell sap.
b) Functions

 It helps keep plant cells in a turgid state.

 Osmotic uptake of water results in cell expansion during cell growth.

 It is used for the storage of water and food, etc.

 It provides a place to hold harmful substances or metabolic wastes so as
not to interfere the normal metabolism of the cytoplasm.

7. Cell Wall
 It is made up of cellulose and is therefore rigid.

 It is fully permeable.

 Function: gives shape, support and protection to the cell.

Structural Differences between a Plant Cell and an Animal Cell
Plant cell Animal cell

has cell wall has NO cell wall

has large vacuole has NO large vacuole

has chloroplasts has NO chloroplasts


Concept Check



Cell Membrane
1. Chemical composition
 membranes isolated from red blood cells were chemically analyzed and found to
be composed of phospholipids and proteins.


2. Model of membrane structure
 Cell membranes are actually phospholipid bilayers. Such a bilayer could exist as
a stable boundary between 2 aqueous media, because the molecular
arrangement shelters the hydrophobic tails from water, while exposing the
hydrophilic heads to water.

 Fluid Mosaic Model

 The membrane is a fluid phospholipid bilayer with a mosaic of protein
molecules floats in it.

 Both phospholipid and protein molecules are mobile to varying degree within
the membrane which is therefore, regarded as a fluid structure.

 Proteins penetrate into the hydrophobic interior of the membrane - some
penetrate only part of the way into the membrane while others penetrate all
the way through.

 The arrangement of proteins varies according to the type and function of the
membrane.


3. Properties of membranes
 Selective permeability

 Hydrophobic / non-polar / water-insoluble substances and simple, small
molecules can move across the phospholipid bilayer freely.

 Ions / water / water-soluble substances can move across the membrane
through the channel proteins or carrier proteins.

 Fluidity of membranes

 The phospholipid and some protein molecules can move laterally.

 It allows the membrane to change its shape and fuse itself during cell division.

 Specific in function
Different membranes have their unique collections of proteins which determine
most of the membrane’s specific functions.

Functions of membrane proteins:

Membrane protein Function

channel protein to transport certain ions and water-soluble substances

carrier protein needed in active transport

receptor protein to receive chemical signals

enzyme to speed up reactions in the cell

with antigen molecules attached to form glycoprotein for
recognition protein
recognition purposes



Mechanisms of Membrane Transport
1. By diffusion

 Diffusion is the net movement of particles down a concentration gradient / from
a region of higher concentration to a region of lower concentration.

 When the particles have become evenly distributed (i.e. concentration gradient =
0), there will be no net movement of particles.

 No metabolic energy is required, hence no ATP is required.

 The diffusion of a substance will be unaffected by concentration differences of
other substances.

 The diffusion rate is determined by:
1. diffusion distance
2. temperature
3. size
4. shape of the substance
5. electrical charges


2. By active transport

 It is the movement of substances across the cell membrane in the expense of
energy from respiration (ATP is required).

 Substances can be moved against a concentration gradient, i.e. from a region of
lower concentration to a region of higher concentration.

 The substance is moved in one direction only.

 Specific carriers (proteins) are required.

Examples:
- food absorption at the small intestine: By _______________ + ________________
- absorption of some minerals by roots: By _______________ + ________________


3. By phagocytosis
 It is the intake of large solid particles (e.g. cells) by invagination of cell
membrane, forming a food vacuole.

 It is carried out by some single-celled organisms (e.g. Amoeba) and certain
types of white blood cells.

 It requires energy and must involve the cell membrane.

4. By osmosis

 It refers to the net movement of water molecules from a region of higher water
potential to a region of lower water potential (down a potential gradient)
across a selectively permeable membrane.


a) Water potential ()
 It is the tendency of water molecules to move.

 The more solute dissolved in water, the lower is its water potential.

 Since the water potential of pure water is defined as zero, the water
potential of solutions always a negative value.

b) Water relations in animal cells
 In hypotonic solution:
1. water potential of the external solution is higher than that if the cell
2. water enters the cell by osmosis
3. cell becomes swell and then burst

 In hypertonic solution:
1. water potential of the external solution is lower than that of the cell
2. water leaves the cell by osmosis
3. cell becomes shrinks

 In isotonic solution: cell has no change in size and shape


c) Water relations in plant cells
 In hypotonic solution: The cell becomes turgid

∵ The rigid cell wall prevents the cell form bursting

 In hypertonic solution:
The cell becomes plasmolysed (its cytoplasm shrinks and detaches from
the cell wall)

 In isotonic solution: The cell becomes flaccid

Concept Check
1. One end of the middle portion of a spring onion leaf has been cut lengthwise
into 4 sections and then immersed in different concentration of sucrose
solution. Arrange the solutions in ascending concentration.

2. Why is it incorrect to say that 'sea water is a hypertonic solution'. (1 mark)



In hypotonic solution In isotonic solution In hypertonic solution

In the cells of the epidermis of the leaf of certain plants that contain colored pigment,
the liquid inside the vacuoles are colored. They can be seen under the microscope
without staining.

Normal condition Plasmolysed condition

Concept Check
Why can plasmolysis not take place in an animal cell? (2 marks)


Type 1 – Dialysis Tubing
Dialysis tubing:

 differentially permeable

 has many small pores which allow small molecules (e.g. water, glucose, ions) to
pass through but not large ones (e.g. sucrose, proteins)

Before carrying out the experiment, make sure that
1. the knot of the dialysis tubing should be tied tightly
2. the outside of the dialysis tubing should be rinsed with distilled water
3. the dialysis tubing should be examined for any damage

(a) Describe and explain the change in the water level of the capillary tube in set-
up A. (3 marks)

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(b) Describe and explain the change in the water level of the capillary tube in set-
up B. (3 marks)

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Type 2 – Potato Container

(a) Explain the level of solution inside the 'potato bowl' in set-up A after 12 hours.
(4 marks)

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(b) Explain the level of solution inside the 'potato bowl' in set-up B after 12 hours.
(4 marks)

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Type 3 – Potato Strip
A 10cm long potato strip was placed in a sugar solution. Its length was measured at
regular intervals over a period of 3 hours. The results are shown in the table below:

Time (hour) Length of the potato strip (cm)

0 10.0
0.5 9.7
1.0 9.4
1.5 9.1
2.0 8.9
2.5 8.8
3.0 8.8

(a) Explain the change in length of the potato strip in the first hour. (4 marks)

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(b) (i) Which period of time does the length of potato strip remain unchanged?
(1 mark)

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(ii) Explain why there was no change in length during this period?
(2 marks)

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(c) What perimeter can be measured instead of the length of potato strip?
(1 mark)

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