Single celled organisms

Single Celled Organisms

• Some organisms, unlike plants and animals, consist of only one cell.

• These organisms are said to be “unicellular”

• An amoeba is another name given to an organism that consists of only 1 cell.

• These unicellular organisms are so small that they must be seen with a microscope.

• Single celled organisms can be classified into 3 main categories:

– Protists

– Bacteria

– Viruses

• Protists contain a cell membrane, nucleus and various organelles.

• Some protists contain chloroplasts.

• These organisms play an important role in the worlds oxygen production.

• They are also an important part of many aquatic food chains.

Protists

• Some protists cause disease.

• These “bad” protists are known as pathogens.

• Amoebic dysentery and malaria are two diseases caused by such pathogens.

Bacteria

• These are the smallest and simplest forms of life on earth.

• Bacteria lack most of the organelles that other cells have.

• They do not have a nucleus or nuclear membrane.

• Bacterial shapes:

• 1. Bacilli

– Rod shaped

• 2. Spirilla

– Spiral shaped

• 3. Cocci

– Sphere shaped

• Bacteria contain a single strand of DNA that controls all of their functioning.

• In addition, they may contain one or more plasmids.

• Plasmids are circular DNA molecules that contain non-essential genes.

• Beneficial bacteria:

– In our digestive system

– Make yogurt, cosmetics, swiss cheese

– Recycle organic matter

– Photosynthesis producing oxygen

– Food source for other organisms

• Harmful bacteria:

– Produce toxins (poisons) that affect cell

functioning

• Examples: E. coli and Salmonella

– Bacteria that grow on food and cause nausea,

cramps, vomiting, diarrhea

Bacteria and Disease

• The best way to prevent bacterial disease is through sanitation.

• Washing your hands, food, and plates before eating prevents large amounts of bacteria from entering our bodies.

•

Treating Bacterial Disease

• Antibiotics are used to treat disease.

• Antibiotics kill or slow the spread of disease.

How Antibiotics Work

• Most pathogens have antigens on their outer surface. Like a key.

• Specific antibodies are able to attach themselves to these antigens.

• In some cases the antibody kills the disease.

• In other cases it slows the spread of it down enough so that other parts of the bodies defense system can kill it.

Viruses

• Are NOT cells (non-living)

• Smaller than bacteria (3000 could fit across a period at the end of a sentence!)

• MUST reproduce inside living cells

• Have 2 parts:

– Protein cover

– Genetic material

• To be able to reproduce, a virus needs a host cell.

• Once inside, viruses “hijack” the host’s DNA and enzymes to reproduce itself.

• Some modify the genetic info in the DNA

• When viral particles are produced, they are released from the cell (lysis)

• Examples: influenza (flu), smallpox, measles, HIV

Vaccines

• Weakened or dead forms of a virus

• Stimulate immune system to develop and launch a defense

• If you are later infected by the virus, your body can destroy it quickly

• Doesn’t work for all viruses!

– Eg: no vaccine for HIV

ReproductionReproduction

Reproduction of Viruses

Two types:

1. lytic cycle – destroys the host cell very

quickly

2. lysogenic cycle – a.k.a. ‘slow’ or latent

virus. Can remain in a cell for years before becoming active destroying the cell

The Lytic cycle

1. Attachment: virus attaches to

the cell using it’s spikes and

receptor sites on the c.m.

2. Insertion of viral genes into

the cell. Viral DNA becomes

part of the cells DNA and

takes over the cell’s activities

3. Manufacture of viral parts

4. Assembly of viruses

5. Lysis: the cell bursts releasing

the new viruses

1 Adsorption 2 Penetration 3 Early Replication

4 Late Replication 5 Maturation 6 Release

Examples of lytic viruses

• common cold

• bacteriophage (aka phage)

• influenza

• Ebola

• cold sore (herpes I)

• Genital herpes II

The Lysogenic cycle

1. Attachment

2. virus injects genetic material into cell

3. genetic material is inserted into the host cell’s DNA. It is now a prophage

4. viral genes remain dormant for a length of time, but are copied with the host cell’s DNA when cell division occurs

5. without warning, the cell will switch into the lyticcycle, producing and releasing many viruses

6. In some types, the viral DNA will cause the cell to become a cancer cell! No viruses are produced in this case.

When the cell divides,

the prophage becomes

part of any new cells!

This will produce many

new cells which will

now produce thousands

of new viruses!

LysogenicLysogenic CycleCycle…

Lysogenic viruses

• Herpes

• Hepatitis

• HIV

• Some phages

Life cycle summary

Living qualities of viruses

1. Capsid or protein coat

2. genetic material either DNA or RNA

3. Some have enzymes (proteins)

4. Some have a lipid coat or capsule

Non-living qualities of viruses

1. Acellular (Non-cellular)

2. Do not grow

3. Do not make or use energy

4. Do not respond to stimuli

5. Cannot reproduce on their own

6. Can be crystallized

Are viruses alive?

• viruses require cells to reproduce, and are not cellular

• they don’t fit the Cell Theory: all living things are composed of cells

No, they are not considered to be alive!