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RELEVANT LEARNING OUTCOME:
(a) Outline the cell theory with the understanding that cells are the smallest unit of life, all cells come from pre-existing cells and living organisms are composed of cells.
(f) Discuss how viruses challenge the cell theory and concepts of what is considered living.
💡Concepts of the Cell Theory:
💡Concepts of the Cell Theory:
The cell is the basic unit of structure and organization in organisms.
All organisms are made up of one or more cells.
All cells come from pre-existing cells.
👁️🗨️Watch the video to understand how the cell theory was developed!
What do you notice about how it all began, and what the process was like?
🖊️Check Your Understanding: Lecture Notes Checkpoint
🖊️Check Your Understanding: Lecture Notes Checkpoint
Which of the following statements is not supported by the cell theory?
a) The cell contains hereditary information that is passed from the parent cell to daughter cell during cell division.
b) Cells form by free-cell formation, like the formation of crystals in solution, in the presence of other cells.
c) The function of an organism depends on the total activity of individual cells.
d) Some living things, known as unicellular organisms, are made up of only one cell and the other living things are made up of many cells are known as multicellular organisms.
Answer: (c) - This statement is not supported by cell theory because it suggests that cells are generated spontaneously. All our cells come from pre-existing cells.
💡Viruses challenge the cell theory!
💡Viruses challenge the cell theory!
It has been debated extensively whether viruses are living or non-living. To determine that, we must first understand the characteristics of life.
👁️🗨️Watch the video that describes some of the characteristics of life.
In light of those characteristics, would you consider the virus to be living or non-living?
🖊️Check Your Understanding: Lecture Notes Checkpoint
🖊️Check Your Understanding: Lecture Notes Checkpoint
1) List, and explain, two characteristics that would qualify the virus to be a living thing.
Any two from this list:
They can adapt to unfavourable environments -> viruses exhibit high mutation rates which increase genetic variation, conferring a greater chance of survival in the unfavourable environment.
They are able to reproduce --> viruses reproduce only by infecting a host cell, after which they can take over the genetic machinery of the host cells.
They are able to respond to external stimuli --> viruses can switch their reproductive cycle (more of that in the topic of 'Genetics of Viruses') when conditions are suitable.
They contain the hereditary material --> the genetic characteristics of a virus can be passed on from one generation to the next through replication of its nucleic acid (DNA/RNA).
2) In what ways are viruses non-living?
They are acellular, having no cytoplasm or cellular organelles.
They do not grow, divide or increase in size. New viral components are synthesized and assembled within the infected host cell.
They do not move on their own but can only be carried by host cells or an extracellular medium.
They have no ability or regulating their own internal environment.
They lack enzymes for any metabolic activity, depending on the host cell's metabolic machinery to replicate.
💡Viruses are obligate intracellular parasites.
💡Viruses are obligate intracellular parasites.
We can't call viruses 'cells' because they do not have organelles, and so, we regard them as 'particles'. However, unlike other particles, such as dust or sand that are inanimate (i.e., lifeless), the viral particle has some living characteristics as well. For example, viruses can reproduce, even though only in a host cell.
So, what exactly is a virus?
Viruses are infectious particles that cannot survive without its living host cell. They are obligate intracellular parasites that are dependent on their living host cells for molecular resources essential of life.
👁️🗨️ The unnarrated video below explains the term with a little more details.
Virus - Obligate intracellular parasite.mp4🖊️Check Your Understanding
🖊️Check Your Understanding
Attempt Qns 1 to 3 of the learning goals.
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