Topic 2.1 Cell theory

Question 1

2.1.1 Outline the cell theory

Answer 1

• All organisms are composed of one or more cells
• Cells are the smallest units of life
• All cells come from pre-existing cells

Question 2

2.1.2. Discuss the evidence for the cell theory.

Answer 2

• 1st principle: microscopes - all living things, when viewed under a microscope, have been found to be made of cells and cell products
• 2nd principle - we have not been able to find any living entity that is not made of at least one cell
• 3rd principle: experimental evidence: 1860s, Louis Pasteur showed living organisms wouldn’t ‘spontaneously’ reappear in sealed and sterile conditions; he sterilized chicken broth by boiling, and life re-established in broth only after exposure to pre-existing cells

Question 3

2.1.3 Can unicellular organisms carry out all functions of life?

Answer 3

Yes. Functions include metabolism, growth, reproduction, homeostasis, response and nutrition.

Question 4

2.1.4 Compare the relative sizes of molecules, cell membrane thickness, viruses, bacteria, organelles and cells, using the appropriate SI unit.

Answer 4

In decreasing order of size:

• Cells = 100 micrometres
• Organelles = 10 micrometres
• Bacteria = 1 micrometre
• Viruses = 100 nanometres
• Membranes (thickness) = 10 nanometres
• Molecules = 1 nanometre

Question 5

2.1.5 Calculate the linear magnification of drawings and the actual size of specimens in images of known magnification.

Answer 5

Linear magnification:

Magnification = image/drawing size ÷ real size of specimen

Actual size:

Actual size = size of image ÷ magnification

or

Actual size = field diameter of microscope ÷ fraction of field covered by specimen

Question 6

2.1.6 Explain the importance of surface area to volume ratio as a factor limiting cell size.

Answer 6

Rate of heat/waste production & resource consumption are functions of volume in cell.

Cell size affects rate of chemical reactions inside because: SA (membrane) controls movement of materials in & out of cell; volume (cell interior) carries out chemical reactions. As cell’s volume increases, SA increases at slower rate.

Therefore, SA:V ratio decreases as cell grows. Large cells not as efficient at moving materials in & out as small cells. Therefore, cells are limited to a size they can attain and efficiently carry out functions of life.

Note:

• Large animals have more cells, not larger cells
• Larger cells have modifications that allow them to function efficiently (e.g. long & thin instead of spherical, infoldings/outfoldings to increase SA)

Question 7

2.1.7 What kind of organisms show emergent properties?

Answer 7

Multicellular organisms show emergent properties.

Emergent properties: whole is greater than the composition of its parts i.e. cells form tissues, which form organs, which form organ systems, which form multicellular organisms. Individual cells aren’t of much use – cells as a unit allow lungs to perform their function.

Question 8

2.1.8 Explain that cells in multicellular organisms differentiate to carry out specialized functions by expressing some of their genes but not others.

Answer 8

Multicellular organisms start off as a single cell that has the ability to reproduce at a rapid rate. Differentiation is the process during development whereby newly formed cells become more specialized and distinct from one another as they mature to produce all the necessary cell types that form the organism.

• All cells of an individual organism share an identical genome
• Expression of specific genes within a given cell by chemical signals will cause it to differentiate from other cells like it

BONUS:

• Differentiation is advantageous for multicellular organisms since cells can differentiate to be more efficient vs. unicellular organisms who carry out all functions within one cell
• Active genes usually packaged in expanded and accessible form (euchromatin); inactive genes are mainly packaged in condensed form (heterochromatin); differentiated cells will have different regions of DNA packaged as heterochromatin and euchromatin depending on their function
• Some cells greatly/completely diminish ability to reproduce once specialized (e.g. nerve and muscle cells); others reproduce throughout their life (e.g. epithelial cells like skin)

Question 9

2.1.9 What are stem cells able to do?

Answer 9

Stem cells retain the capacity to divide and have the ability to differentiate along different pathways. Stem cells are unspecialized and have 2 key qualities:

• *Self renewal**: can continuously divide and replicate
• *Pluripotency**: have capacity to differentiate into specialized cells

Question 10

2.1.10 Outline one therapeutic use of stem cells.

Answer 10

Bone marrow transplants — blood stem cells found in the bone marrow give rise to the red blood cells, white blood cells and platelets in the body. These stem cells can be used in bone marrow transplants to treat people who have certain types of cancer (e.g. leukaemia) and are immuno-compromised as a result of chemotherapy.

Other:

Retinal cells — Replace dead cells in retina to cure diseases like glaucoma and macular degeneration
Skin cells — Graft new skin cells to replace damaged cells in severe burn victims
Nerve cells — Repair damage caused by spinal injuries to enable paralysed victims to regain movement

Stem cells can be used to replace damaged or diseased cells with healthy, functioning ones. They can only be distinguished from other cells on the basis of their behaviour. There are embryonic, pluripotent and tissue-specific stem cells. Stem cells can be found in different places (e.g. in bone marrow, peripheral blood and umbilical cord blood).