Biology Notes: Introduction, Lecture notes for Biology
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Biology Notes: Introduction, Lecture notes for Biology

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Introduction to General Biology. Cell Biology.
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Introduction to

cell biology

Caril Alvin kradril SHS general biology 1 Department of biology College of science Polytechnic university of the Philippines

OBJECTIVES:

 Explain the postulates of the cell theory;

 Describe the structure and function of major and subcellular organelles;

 Distinguish prokaryotic from eukaryotic cells according to their distinctive features;

 Classify different cell types and specify the function(s) of each; and

 Describe some cell modifications that lead to adaptation to carry out specialized functions.

I. THE CELL: BASIC PRINCIPLES AND THEORY

THE CELL THEORY

I. All living organisms are composed of one or more cells.

II. The cell is the basic structure and organization in organism.

III. All cells come from pre-existing cells.

SOME HISTORICALLY IMPORTANT EVENTS IN CELL BIOLOGY

 1590 – Zacharias JansenA invented the compound microscope.

 1665 – Robert HookeB, using an improved compound microscope, examined cork and used the term “cell” to describe its basic units.

 1650 to 1700 – Anton Van LeeuwenhoekC, using a good quality simple lens (mag. x200), observed nuclei and unicellular organisms, including bacteria. In 1676, bacteria were described for the first time as “animalcules”.

A.

B.

C.

 1831 to 1833 – Robert Browna described the nucleus as a characteristic spherical body in plant cells.

 1838 to 1839 – Matthias Schleidenb (a botanist) and Theodore Schwannc (a zoologist) produced the “cell theory” which unified the ideas of the time by stating that the basic unit of structure and function in living organisms is the cell.

 1840 – Jonnanes Purkinjed gave the name protoplasm to the contents of cells, realising that the latter were the living material, not the cell walls.

o Later the term cytoplasm was introduced (cytoplasm + nucleus = protoplasm).

a

b

c

d

 1855 – Rudolf Virchow showed that all cells arise from pre- existing cells by cell division.

 1866 – Ernst Haeckel established that the nucleus was responsible for storing and transmitting heredity characters.

 1866 to 1888 – Cell division studied in detail and chromosomes described.

 1880 to 1883 – plastids, e.g. chloroplasts, discovered.

 1890 –mitochondria discovered.

 1898 –Golgi apparatus discovered.

 1887 to 1900 – improvements in microscopes, fixatives, stains and sectioning.

o Cytology† started to become experimental.

o Embryology was studied to established how cells interact during growth of a multicellular organism.

o Cytogenetics‡, with its emphasis on the functioning of the nucleus in heredity, became a branch of cytology.

 1900 – Mendel’s work, forgotten since 1865, was rediscovered giving an impetus to cytogenetics.

o Light microscope had almost reached the theoretical limits of resolution, thus slowing down the rate of progress.

 1930s – Electron microscope developed, enabling much improved resolution.

 1946 to present – Electron microscope became widely used in biology, revealing much more detailed structure in cells.

o This “fine” structure is called ultrastructure.

Who gave this statement???

“I saw thousands of tiny empty chambers in cork and called it cells” (1665).

“I observed tiny living organisms” (1676).

“As I observed in my studies, all plants are made up of cells” (1838).

“All animals are made up of cells” (1839).

“Please mate, all cells came from pre- existing cells” (1858).

II. CELLULAR ULTRASTRUCTURE AND FUNCTIONS

THREE (3) MAJOR COMPONENTS

CELL MEMBRANE

Structure:

 Two layers of lipid (bilayer) sandwiched between two (2) protein layers.

Functions:

 A partially permeable barrier controlling exchange between the cell and its environment.

 Different types of membranes differ in thickness but most fall within the range 5-10 nm, for example cell surface membranes are 7.5 nm wide.

 Membranes are lipoprotein structures (lipid + protein), with carbohydrates (sugar) portions attached to the external surfaces of some lipid and protein molecules. Typically, 2–10% of the membranes is carbohydrate.

Summary of the Features of Biological Membranes

 The lipids spontaneously form a bilayer owing to their polar heads and non-polar tails.

 The proteins are variable in function.

 The sugar are involved in recognition mechanisms.

 The two (2) sides of a membrane may differ in composition and properties.

 Both lipids and proteins show rapid lateral diffusion in the plane of the membrane unless anchored or restricted in some way.

CELL WALL

Structure:

 A rigid cell wall surrounding the cell, consisting of cellulose microfibrils running through a matrix of other complex polysaccharides, namely hemicellulose and pectic substances.

 May be secondarily thickened in some cells.

Functions:

 Provides mechanical support and protection.

 It allows a pressure potential to be developed which aids in support.

 It prevents osmotic bursting of the cell.

 It is a pathway for movement of water and mineral salts.

 Various modifications, such as lignification, for specialized functions.

NUCLEUS

Structure:

 Largest cell organelle, enclosed by an envelope of two (2) membranes that is perforated by nuclear pores.

 It contains chromatin which is the extended form taken by chromosomes during interphase.

 It also contains a nucleolus.

o Nuclear envelope – composed of a lipid bilayer that separates the nuclear content from the cytoplasm.

• The double membrane is separated by approximately 50 nm.

• The outer membrane is continuous with the endoplasmic reticulum.

o Nuclear pores – selective channels that facilitates the inward and outward movement of molecules.

o Nucleoplasm – the fluid portion of the nucleus where the genetic material is suspended.

o Nucleolus – a suborganelle of nucleus, the site where the subunits of the ribosome are assembled and include the synthesis and maturation of ribosomal RNA for release in the cytoplasm where protein synthesis occurs.

• Around 10, 000 ribosomes per minutes are needed by the cells.

o DNA molecule – a long strand present in the nucleus, which wounds around histone proteins to form a helical structure termed as chromatin strands.

o Chromosomes – formed during cell division by the condensation of chromatin strands.

• In prokaryotes, the chromosomes are circular and no membrane enclosing the chromosomes.

Functions:

 Chromosome contain DNA, the molecule of inheritance.

 DNA is organized into genes which control all the activities of the cell.

 Nuclear division is the basis of cell replication, and hence reproduction.

 The nucleolus manufactures ribosomes.

RIBOSOMES

Structure:

 Very small organelles consisting of a large and a small subunit.

 They are made of roughly equal parts of protein and RNA.

 Slightly smaller ribosomes are found in mitochondria and chloroplasts in plants.

Functions:

 Sites of protein synthesis, holding in place the various interacting molecules involved.

 They are either bound to the ER or lie free in the cytoplasm.

 They may form polysomes (polyribosomes), collections of ribosomes strung along messenger RNA.

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