Mitosis

Mitosis is a type of cell division whose major purpose is growth and to replace worn out cells. It produces two new cells that are identical to each other, and to the parent cell.

We demonstrated the 5 phases of mitosis with the help of frosting on donuts:

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1. Interphase

-The DNA in the cell is copied in preparation for cell division, this results in two identical full sets of chromosomes

-Outside of the nucleus are two centrosomes, each containing a pair of centrioles, these structures are critical for the process of cell division

-During interphase, microtubules extend from these centrosomes

2. Prophase:

-The chromosomes condense into X-shaped structures

-Each chromosome is composed of two sister chromatids, containing identical genetic information

-At the end of prophase the membrane around the nucleus in the cell dissolves away releasing the chromosomes

-The mitotic spindle, consisting of the microtubules and other proteins, extends across the cell between the centrioles as they move to opposite poles of the cell

3. Metaphase:

-The chromosomes line up neatly end-to-end along the centre (equator) of the cell

-The centrioles are now at opposite poles of the cell with the mitotic spindle fibres extending from them

-The mitotic spindle fibres attach to each of the sister chromatids

4. Anaphase:

-The sister chromatids are then pulled apart by the mitotic spindle which pulls one chromatid to one pole and the other chromatid to the opposite pole

5. Telophase:

-At each pole of the cell a full set of chromosomes gather together

-A membrane forms around each set of chromosomes to create two new nuclei

-The single cell then pinches in the middle to form two separate daughter cells each containing a full set of chromosomes within a nucleus. This process is known as cytokinesis

 

 

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Observing mitosis in root cells

Mitosis is a process where a single cell divides into two identical daughter cells. To see mitosis in action one needs to look at living cells. This could be examined by the following experiment.

Preparation

  1. Cut off 1-2 cm of the root tips. Put in a small volume of ethanoic acid on a watch glass
  2. Meanwhile, heat 10-25 cm3 of 1 M hydrochloric acid to 60 °C in a water bath
  3. Wash the root tips in cold water for 4-5 minutes and dry on filter paper
  4. Use a mounted needle to transfer the root tips to the hot hydrochloric acid and leave for 5 minutes.
  5. Wash the root tips again in cold water for 4-5 minutes and dry on filter paper
  6. Use the mounted needle to remove two root tips onto a clean microscope slide
  7. Cut each about 2 mm from the growing root tip. Discard the rest, but keep the tips
  8. Add a small drop of stain and leave for 2 minutes
  9. Break up the tissue with scissors and add a drop of water
  10. Cover with a coverslip and squash

Investigation

 -View the root tips under a microscope and look for the chromosomes within cells which are actively dividing

-Observe the different stages of Mitosis

 

Investigating Osmosis using Potato Strips

Image result for potato strips in ribena and water

Osmosis is defined as the net movement of particles of a solvent (a substance that dissolves another to form a solution) along its concentration gradient, across a partially permeable membrane, until an equilibrium (a state of rest or balance due to the equal action of opposing forces) is established.

The aim of this experiment is to prove the effect Osmosis has on the object in question, namely the potato strips.

Ethical issues

There are no ethical issues associated with this experiment

Health and Safety

Potatoes are a foodstuff and therefore low hazard. However, its raw state and the location of the experiment, which was in a laboratory, prohibited its consumption.

 In order to achieve this, the following materials and apparatus were used:

-3 Potato strips

-Water (hypotonic solution)

-Ribena (hypertonic solution)

-ruler

-3 identical test tubes

-paper towels

– test tube holder

– a potato chipper

The procedure is as follows:

 

  1. We observed each strip individually by feeling its consistency
  2. We weighted and measured them, recording the initial mass and length

  Initial length Initial mass
A 4,5cm 3,85g
B 4,5cm 3,38g
C 4,5cm 3,69g
  1. We put Ribena in the first test tube, a mixture solution in the second, and water in the third
  2. We placed a strip in each test tube and let them set for about 15min
  3. We remove the strips after 15 minutes and dab on tissue
  4. We weighted and measured the potato strips anew and observed their state
  Final length Final mass
A (Ribena) 4,3cm 3,25g
B (mix) 4,3cm 2,96g
C (water) 4,7cm 3,85g
  1. Finally, we performed % difference calculations for the mass and length using the formula:

(final – initial) x 100% / initial

  Length change Mass change
A -0,04% -0,15%
B -0,04% -0,12%
C 0,05% 0,04%

Limitations

There were several limitations to this experiment, which may have hindered or altered its accuracy and end results:

  • We did not measure the exact amount of liquid put into each test tube: we did not exclude the factor of liquid amount having an effect over the potato strip
  • There was no exact mixture between water and Ribena: the imprecise mixture deprives us from knowing which fluid was the dominant one
  • Use of normal water: normal water is not as hypotonic as distilled water and urges a smaller visible reaction
  • Uneven temperature
  • No time recording: there was no specific time span in which the potatoes stayed in the liquid and they were also placed one after the other, so they did not spend the exact same time inside it
  • The mass and percentage of change was rounded and thus the result is not precise

In order to avoid potential flaws, next time the experiment should be measured precisely, be timed and use distilled water.

 

Results and Conclusions:

The following results and conclusions were deduced by this experiment:

Potatoes in water:

An increase in mass of the potato strip due to the movement of water molecules into the plant cells via osmosis. The water is hypotonic. This means it possesses high water solution and is low on sugar. In contrast, the potato is hypertonic: it has low water potential and a high sugar solution. When put into contact, water will diffuse into the potato and hence expand its size, making the final size greater that its initial results.

This is also relevant to the increase in length. Since the intake on water molecules occupies extra space, it also changes the volume of the potato by pushing against the cell membrane and the proximate cellulose cell wall. This results into an expansion in all dimensions, including length. However, the expansion is limited by the stiffness of the cellulose cell.

The push/force acting upon the surface area of the rigid cell produces turgor pressure and hence strips are turgid.

Potatoes in Ribena:

There is a decrease in the mass of the potato strips due to the movement of water molecules outside of the plant cells. Ribena is mainly a sugary solution and hence is hypertonic to the potato cells, which are hypotonic. The water molecules will move along their concentration gradient out of the cells and thus make the potato strip shrink.

The decrease in the number of water molecules results in loss of volume and length.

Since the turgor pressure exerted by the water molecules against the cell membrane/wall is less, its condition remains flaccid.

Potatoes in Mixture of both:

There was a decrease in weight and length present but not as great as the one by Ribena. The solution still contained a higher sugar rate and is therefore hypertonic. However, the contrast between hypertonic and hypotonic is smaller and hence the result is less visible.

In order to generalize and prove the statements mentioned above, the class compared its results and established the following realizations:

 

  Average change in mass Average change in length
Water -0,7g -0,2cm
Mix -0,15g 0cm
Ribena 0,2g 0,1cm

 

 

 

 

 

 

Models of membrane structure

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In 1935 Hugh Davson and James Danielli suggested that the phospholipid bilayer is sandwiched between layers of globular proteins. Thad been evidence of the presence of proteins and since both membrane and proteins could act as barriers, the belief seemed plausible. This became known as the Davson-Danielli model.

 However, the theory had several flaws. It remains unclear how proteins contain the flexible fluid, as they are generally stiff and can’t cope well with outside movement. Another problem was that the amount and/or type of membrane protein varied greatly between different cells.

In 1966, with the help of freeze fracturing: the splitting of cell membranes between the two lipid layers and thus revealing a 3D view of the surface texture, the model was disproved as it displayed that the proteins were actually embedded inside the layer and were amphipathic.

Thus came the creation of the Fluid mosaic model of Singer and Nicolson in 1975. With the help of fluorescent labelling, it became clear that the components of the membrane were able to move within its structure. Two different colored membranes were fused together and instead of maintaining the same colors, they diffused into each other and moved around.

Further details can be found in the following video: 

 

Exceptions to the cell theory

It is a well-known fact that in order to sustain a hypothesis in most work and knowledge fields, one needs to back it up with evidence. In science however, the aim is to disprove it. Only then could humanity move forward and develop successfully.

One of the most significant theories in biology is that cells are the smallest units of life and that all living organisms are made out of cells. However, even in cell theory there are organisms and tissues that are not made of typical cells and hence do not answer the requirements of the belief.

The first example is the Skeletal muscle. It is composed by muscle fibres and are enclosed by a membrane. However, they are much larger than the average cell and contain hundreds of nuclei.

Another example is the Giant algae (Acetabularia). Due to its big proportions, its estimated to consist of numerous small cells. In reality, its composed from one giant cell as it has only one nucleus and t=is therefore not multicellular.

The third and last example is the Aseptate Fungi. It is composed of thread-like structures called hyphae. They are not divided into sub-units, containing a single nucleus. Instead they consist of long sections, full with nuclei.

Despite these exceptions, the overall trend for living organisms being composed of cells is very strong. The cell theory has not been abandoned.

Onion cells

Real Lab Procedure

1.Cut open an onion

2.Use forceps to peel a thin layer of epidermis from the inside

3.Lay the layer of epidermis on a microscope slide

4.Add a drop of iodine/methylene solution to the layer

5.Carefully place a cover slip over the layer

Observations

There are a large number of regularly shaped cells lying side by side and each cell has a distinct cell wall.

A distinct nucleus is present on the periphery of each cell.

A large vacuole is present at the centre of each cell, and is surrounded by the cytoplasm.

Conclusion

As cell walls and large vacuoles are clearly observed in all the cells, the cells placed for observation are plant cells.