Cell theory

2.1.1 Outline the cell theory.    

The cell theory states that: 
 

• All living organisms are composed of cells. Multicellular organisms (example: humans) are composed of many cells while unicellular organisms (example: bacteria) are composed of only one cell. Cells are the basic unit of structure in all organisms. 

• Cells are the smallest unit of life. They are the smallest structures capable of surviving on their own.

• Cells come from pre-exsisting cells and cannot be created from non-living material. For example, new cells arise from cell division and a zygote (the very first cell formed when an organism is produced) arises from the fusion of an egg cell and a sperm cell. 

   

2.1.2 Discuss the evidence for the cell theory. 

When scientists started to look at the structures of organisms under the microscope they discovered that all living organisms where made up of these small units which they proceeded to call cells. When these cells were taken from tissues they were able to survive for some period of time. Nothing smaller than the cell was able to live independently and so it was concluded that the cell was the smallest unit of life. For some time, scientists thought that cells must arise from non-living material but it was eventually proven that this was not the case, instead they had to arise from pre-exsisting cells. An experiment to prove this can be done as follows: 
 

• Take two containers and put food in both of these

• Sterilize both of the containers so that all living organisms are killed

• Leave one of the containers open and seal the other closed 

What will happen is that in the open container mold will start to grow but in the container that was sealed no mold will be present. The reason for this is because in the open container, cells are able to enter the container from the external environment and start to divide and grow. However, due to the seal on the other container no cells will be able to enter and so no mold will develop, proving that cells cannot arise from non-living material. 

2.1.3 State that unicellular organisms carry out all the functions of life. 

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

Remember:
1 millimeter (mm) = 10^-3 meters
1 micrometer (μm) = 10^-3 millimeters 
1 nanometer (nm) = 10^-3 micrometers

A molecule = 1 nm
Thickness of cell membrane = 10 nm
Viruses = 100 nm
Bacteria = 1μm
Organelles = up to 10 μm
Eukaryotic cells = up to 100 μm

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

• Take a measurement of the drawing (width or length)

• Take this same measurement of the specimen  

• Remember to convert units if needed to

• Place your values into the equation

• Magnification = length of drawing / length of actual specimen

You can also calculate the length of the specimen if this is unknown: length of the drawing / magnification. 

Conversion of units:
1 centimeter = 10^-2 meters
1 millimeter = 10^-3 meters
1 micrometer = 10^-6 meters
1 nanometer = 10^-9 meters

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

Many reactions occur within the cell. Substances need to be taken into the cell to fuel these reactions and the wast products of the reactions need to be removed. When the cell increases in size so does its chemical activity. This means that more substances need to be taken in and more need to be removed. The surface area of the cell is vital for this. Surface area affects the rate at which particles can enter and exit the cell (The amount of substances that it takes up from the environment and excretes into the environment), whereas the volume affects the rate at which material are made or used within the cell, hence the chemical activity per unit of time. 

As the volume of the cell increases so does the surface area however not to the same extent. When the cell gets bigger its surface area to volume ratio gets smaller. To illustrate this we can use three different cubes. The first cube has a side of 1 cm, the second 3 cm and the third 4 cm. If we calculate the surface area to volume ratio we get:

Cube 1
Surface area: 6 sides x 1² = 6 cm²
Volume: 1³ = 1 cm³
Ratio = 6:1

Cube 2
Surface area: 6 sides x 3² = 54 cm²
Volume: 3³ = 27 cm³
Ratio = 2:1

Cube 3
Surface area: 6 sides x 4² = 96 cm²
Volume : 4³ = 64 cm³
Ratio = 1.5:1

As we can see the cube with the largest surface area and volume has the smallest surface area to volume ratio. If the surface area to volume ratio gets too small then substances won’t be able to enter the cell fast enough to fuel the reactions and wast products will start to accumulate within the cell as they will be produced faster than they can be excreted. In addition, cells will not be able to lose heat fast enough and so may overheat. Therefor the surface area to volume ratio is very important for a cell. 

2.1.7 State that multicellular organisms show emergent properties. 

Multicellular organisms show emergent properties. For example: cells form tissues, tissues form organs, organs form organ systems and organ systems form multicellular organisms. The idea is that the whole is greater than the composition of its parts. For example your lungs are made of many cells. However, the cells by themselves aren’t much use. It is the many cells working as a unit that allow the lungs to perform their function.

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

Every cell in a multicellular organisms contains all the genes of that organism. However, the genes that are activated vary from cell to cell. The reason we have different types of cells in our body (the cells in your eyes are not the same as the ones that make up your hair) is because different genes are activated in different cells. For example, the gene that produces keratin will be active in hair and nail cells. Keratin is the protein which makes up hair and nails. Genes encode for proteins and the proteins affect the cell’s structure and function so that the cell can specialize. This means cells develop in different ways. This is called differentiation. Differentiation depends on gene expression which is regulated mostly during transcription. It is an advantage for multicellular organisms as cells can differentiate to be more efficient unlike unicellular organisms who have to carry out all of the functions within that one cell.

2.1.9 State that stem cells retain the capacity to divide and have the ability to differentiate along different pathways. 

Adults have stems cells in the tissues in their bodies that need to be frequently replaced such as the skin. Stem cells have the ability to produce a wide range of cells which means that they are pluripotent. They retain their ability to divide and produce many different cells by cell division and the process of differentiation. For example, one type of stem cells in the bone marrow produce a variety of red and white blood cells.

2.1.10 Outline one therapeutic use of stem cells. 

Bone marrow transplants are one of the many therapeutic uses of stem cells. 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. 

When a patient has cancer and is given high doses of chemotherapy, the chemotherapy kills the cancer cells but also the normal cells in the bone marrow. This means that the patient cannot produce blood cells. So before the patient is treated with chemotherapy, he or she can undergo a bone marrow harvest in which stem cells are removed from the bone marrow by using a needle which is inserted into the pelvis (hip bone). Alternatively, if stem cells cannot be used from the patient then they can be harvested from a matching donor. After the chemotherapy treatment the patient will have a bone marrow transplant in which the stem cells are transplanted back into the patient through a drip, usually via a vein in the chest or the arm. These transplanted stem cells will then find their way back to the bone marrow and start to produce healthy blood cells in the patient. Therefore the therapeutic use of stem cells in bone marrow transplants is very important as it allows some patients with cancer to undergo high chemotherapy treatment. Without this therapeutic use of stem cells, patients would only be able to take low doses of chemotherapy which could lower their chances of curing the disease.