Distance Education Centre

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DISTANCE EDUCATION CENTRE

BIOLOGY UNIT 3 2014

BIOLOGY REVISION BOOKLET

This booklet is specifically prepared for students who have not covered these topics in Biology Unit 1 or Biology Unit 2 in Year 11. It will also be useful for new students who have been out of school for a while and are returning to study; it will help to prepare you for the study of Biology Unit 3 in year 12.

There are countless millions of individual living things, or organisms, on Earth. Some kinds are familiar to us, such as the various animals and plants which surround us; others such as fungi and bacteria are less familiar and may even be invisible to the naked eye.

Biology - the scientific study of life – is concerned with the ways in which biologists investigate the living world, with the knowledge that has come from those investigations, and with the ways that knowledge can be applied to help solve human problems.

How is a city similar to a cell?

We all live in a civilised society - we either live in or have visited a large town or city. Did you know that the organisation within a city can be compared to the organisation within a single plant or animal cell?

Cities have a clearly defined edge, whether it is a strong, defensive wall as in Medieval times, or a more modern ring-road. Cells, too, have an outer edge called the plasma membrane, which controls what enters and leaves the cell. Many energy-rich activities occur within the city and hence the city needs a constant supply of energy to be distributed to where it is needed. In the cell the mitochondria perform this function. During this process, however, waste is produced, and like the waste-disposal service available to a city, the cell has to recycle, break down or remove any waste that is generated.

A city is organised so that different activities occur in different buildings or areas - you would not want a hospital in the same building as an abattoir! So, too, a cell, which is capable of carrying out thousands of chemical reactions at once, has different membrane-bound compartments where these reactions occur - they are the organelles. These compartments represent the 'factories" of the cell. New goods and products which are needed by the cell, such as proteins and glucose are continually being made from raw materials. This production usually takes place on a cellular production line until the final product is achieved.

The sites of various activities are not randomly distributed within either a city or a cell. In a city there are industrial zones, residential zones and central business districts. These are all linked by an effective communication system. It is the same in a cell; a cell has a complex set of structures that define the centre, distinguish one end from the other and provide routes for transportation.

Like cities, cells are highly organised, so at the city centre lies the city council who are in charge of the city, and at the centre of the cell lies the nucleus. Inside the nucleus are the chromosomes, which carry in chemical code on the DNA all the instructions on how the cell is to develop and function.

In this booklet we will investigate the structure and functioning of cells and how they are organised to achieve the amazing array of events that occur within them.

Life at the molecular level

Living things share a suite of characteristics: movement, respiration, sensitivity, growth, reproduction, excretion and nutrition.

Living organisms are amazingly diverse in appearance - from tiny insects to huge trees, from worms to elephants, from bacteria to mushrooms. But the closer you look at all of them, the more similar they become.

Just like a city has buildings (and streets and drains), a tree has leaves (and branches and a trunk); just like the buildings are made of smaller units (bricks), so the tree is made of smaller units (cells).

Like the tree, all of these organisms are composed of cells.

So, what is a cell?

Cell [sel] (plural cells) / noun

basic unit of living thing:the smallest independently functioning unit in the structure of an organism, usually consisting of one or more nuclei surrounded by cytoplasm and enclosed by a membrane.Cells also contain organelles such as mitochondria, lysosomes, and ribosomes.

(Source: Encarta http://encarta.msn.com/dictionary_/cell )

The cell is the site of life; it is the functioning unit structure from which living organisms are made. This is one of the fundamental principles of biology known as The Cell Theory. While cells share many common features, there are also differences between cells that are related to their particular roles in organisms.

If we are to understand life we need to understand how cells work. In the 17th Century, microscopes capable of viewing cells were made for the first time. The English scientist Robert Hooke (1635 – 1703) was the first person to observe cells - he viewed thin pieces of cork through a microscope. What he saw had no name then; he called them “cells” because they reminded him of little boxes, like the cells of a honeycomb. Since then, studies of cells from all types of organisms by various scientists have led to the formulation of the cell theory which states that:

• All organisms are composed of cells (and the products of cells).

• All cells come from pre-existing cells.

• The cell is the smallest living organisational unit.

When Robert Hooke viewed the cork cells under his microscope he was only looking at a very thin slice and so was not looking at whole cells. Cells are three-dimensional. However, like Hooke, when looking under the microscope we are only seeing a two dimensional slice through a cell or one side of a whole cell.

Go to the following website to see cells in 3-D:

·  http://www.ibiblio.org/virtualcell

Now try these questions:

Question 1
(a) / Why were cells unknown until the 17th century?
(b) / Biologists often cut several sections of a cell to find out about its structure.
Give two reasons why having several sections from different areas of the same cell is important.

Please note: Answers to all of the questions in this booklet are provided at the end of the booklet, so you don’t have to do the questions - you can refer to the response sheet if you like.

Exceptions to the cell theory

Viruses present some problems to the cell theory.

What are viruses? Are they living or non-living organisms?

Viruses are the smallest living things known – they can be over a million times smaller than bacteria. Viruses are so small that they can only be seen under a special type of microscope called an electron microscope. Electron microscopes are used to study fine details and very small things – they can magnify things by up to one million times. (You will learn more about microscopes later in this chapter).

Some viruses are harmless, but many cause illnesses, ranging from the common cold (Influenza virus) to AIDS (HIV).

Viruses are not regarded as cells because they do not have the basic structures of cells (i.e. the nucleus, cytoplasm and cell membrane). They not only lack the complex structures found in cells but only show a few characteristics of living things. They cannot become active outside a living host cell, which means that they cannot live on their own. Instead they force their way into the cells of living creatures and use these cells to make more viruses. After a time these infected cells die and the viruses set off to find new cells to attack. They simply exist as inert virus particles called virions.

The main parts of a typical virus are shown below. After you have read about cells, you will see how different viruses really are.

Figure 1: A typical Virus

Looking at Cells

There are many different types of cells. The shell of an emu egg has a single cell inside it (which is made up of the egg yolk and white). On the other hand, about a hundred thousand bacterial cells could fit on an area the size of a full stop! So, you see, cells have a large range of sizes.

Most living things start life as just one cell. In fact, you started life as only one cell. So how did you become so big?

How does a single cell become millions of cells?

A single cell can divide into two, these two then grow to full size and divide into four, and so on. They do this by a process known as Mitosis (you will study this process later in the course).

Not only can cells replicate, they can also differentiate (meaning: to become different) to become many different types of cells.

Your body now has a variety of tissues - these are groups of the same types of cells. Examples of tissues are muscle tissue and connective tissue. Groups of tissues in turn make up organs such as your heart and lungs and groups of organs form organ systems, such as the respiratory system.

As various organisms have adapted to their environment, the degree of cellular organisation has altered. All cells need a constant input of substances like oxygen, glucose and a removal of wastes such as carbon dioxide and nitrogenous wastes.

There is really no such thing as a typical cell. Cells are specialised for many different purposes and their structures reflect those purposes. However, there are some features that are shared by all cells. Examination of cells using various microscopes reveals much about their internal organisation. Each living cell is a small compartment with an outer boundary known as the plasma membrane (also referred to as the cell membrane or plasmalemma). Inside each living cell is a fluid, known as the cytosol which consists mainly of water containing many dissolved substances.

Another feature of all living cells is that they contain genetic material in the form of DNA, which carries hereditary information, directs the cell’s activities, and is passed accurately from generation to generation. Living things can be classified into two different kinds on the basis of their internal structure - prokaryotes and eukaryotes.

* Prokaryotic cells

Prokaryotic cells are relatively simple and very little internal structures can be seen, even with an electron microscope. They lack membrane-bound organelles and in particular, they lack a clearly defined structure to house their DNA - the nucleus. They contain a single circular DNA chromosome in the middle of the cell. They are small cells which range in size from 0.5 to 1.0 micrometre (or micron).

Organisms that are made of prokaryotic cells are called Prokaryotes and include bacteria and cyanobacteria. They are

unicellular organisms which means that they are made up of just a single cell (uni: meaning “one”) and are so small that they are invisible to the naked eye.

Figure 2: Prokaryotic Cell Structure

Courtesy: http://micro.magnet.fsu.edu/cells/procaryotes/images/procaryote.jpg

* Eukaryotic cells

Eukaryotic cells have a much more complex structure than prokaryotic cells. They contain many different kinds of membrane-bound structures called organelles suspended in the cytosol. These organelles carry out specific functions within the cell. One of these organelles is a nucleus with a clearly defined membrane called a nuclear membrane or nuclear envelope. The DNA of a eukaryotic cell is located inside the nucleus.

Eukaryotic cells are relatively large cells which range in size from 30 to 150 micrometres.

Organisms that are made up of eukaryotic cells are called Eukaryotes. They are multicellular organisms (multi: made up of

many cells) and include all animals, plants, fungi and protists. So the majority of organisms typically contain eukaryotic cells.

Figure 3: Anatomy of a eukaryotic cell

Courtesy: http://micro.magnet.fsu.edu/cells/bacteriacell.html

Key properties of Prokaryotes and Eukaryotes

Characteristics / Prokaryotes / Eukaryotes
Size of cell / Small cell size (0.2 – 2 µm.) / Larger cell size(10 -200 µm.)
Nucleus / No nuclear membrane or nucleoli / True nucleus consisting of nuclear membrane and nucleoli
Membrane enclosed organelles / Absent / Present
Cell wall / Usually present, chemically complex.
Rigid cell walls / When present, chemically simple.
Flexible cell walls
Chromosomes(DNA) / Single circular chromosome / Multiple linear chromosomes, enclosed in nucleus
Cell division / Binary fission (cell splitting) / Mitosis

Note: The pictures you see of cells here, in the textbook and other sources are only flat two-dimensional images - just like photographs of you are flat two-dimensional images. Cells, also like you, are three dimensional in reality.

Eukaryote organelles

Just as a human body contains many specialised organs essential for survival, such as the heart, kidneys, liver, lungs and brain, a cell contains many organelles.

Organelles are subcellular structures involved in specific functions of the cell. In other words, each organelle has a specific job to carry out within the cell to keep the cell working. Many organelles are found in most cells. Each organelle is surrounded by a membrane and they are found inside the cytosol of the cell.

Organelles within a cell do not act in isolation, but interact with each other. The normal functioning of each kind of cell depends on the combined actions of its various organelles. All types of cells perform similar basic processes and many also carry out highly specialised functions. The activities of cells require considerable energy, and require the production of a variety of biological molecules that are assembled into new organelles, used for repair or exported from the cell. All these processes are catalysed by enzymes and are precisely regulated. Some biochemical processes involve hundreds of enzymes operating sequentially along a complex integrated chemical pathway; each step is tightly controlled.