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Answers

Cell Biology

Exercise 1

1.

a) Unicellular is the term used to describe organisms made up of only 1 cell.

b) Multicellular refers to organisms made up of many cells.

2. Both animal and plant cells have a nucleus, cytoplasm and a cell membrane.

3. The 3 additional features/organelles found in plant cells are chloroplast, cell vacuole and cell wall.

4.

a) The cell membrane controls movement of substances into and out of the cell.

b) The cytoplasm is the aqueous part of the cell made up of mainly water. It is where the chemical reactions occur.

c) The nucleus is where genetic material is stored usually in the form of chromosomes made up of DNA. The nucleus therefore controls the activity of the cell.

Exercise 2

1. A prokaryotic cell has no nucleus like a eukaryotic cell which has a true nucleus. The prokaryotic cell is less complex than a eukaryoitic cell so does not have organelles such as mitochondria. Other differences include size of cell smaller for prokaryotic cells 0.1 - 5 μm whereas eukaryotic cells start from 10 μm in diameter.  

2.

a) Plasmids are rings of DNA containing genetic information that is beneficial but not vital. Plasmids can be transferred between bacteria.

b) Ribosomes are sub-cellular structures that are involved in protein synthesis. Prokaryotic cells also have ribosomes but are smaller in comparison to ribosomes found in eukaryotic cells.

c) Flagella are long whip like structure found on bacteria that rotate and allow bacteria to move. Some bacteria have only one flagellum whilst others may have more than one.

Exercise 3

1. Diffusion is the movement of particles from an area of high concentration to an area of low concentration along a concentration gradient.

2.

a) Increasing the concentration of particles increases the difference in concentration between the two areas across which diffusion occurs. This increases the concentration gradient so increases the rate of diffusion.

b) Increasing temperature will increase the kinetic energy of the diffusing particles so they will move faster. This will increase the rate of diffusion.

c) Increasing the surface area allows more particles to move from one area to another for a given time so this increases the rate of diffusion. Body systems that rely on diffusion will usually be adapted to increase surface area resulting in an increased rate of diffusion. Examples are:

  • the alveoli, an important part of the respiratory system, have a sac like shape that provides a large surface area
  • the small intestine, an important part of the digestive system, has a number of functions one of which is the absorption of digested food. Certain products of digestion are absorbed by diffusion so the lumen of the small intestine has folds called villi which greatly increases its surface area.

3. When multicellular organisms reach a certain size they cannot rely on diffusion alone for transporting substances as the rate of diffusion would be too slow to maintain life processes. To efficiently transport substances such as respiratory gases and food molecules animals have evolved mass flow systems such as the circulatory system where a pump called the heart actively pumps blood containing important substances around the body. Unicellular organisms can rely solely on diffusion as the distance across which diffusion occurs is small.  

4. a) Oxygen in the respiratory system. Carbon dioxide during photosynthesis.

b) Oxygen moves by diffusion from the alveolar airspace across the alveolar epithelium and capillary

wall into the blood. Carbon dioxide moves from the atmosphere into the leaf through stomata.

c) In both cases the gases move in the direction stated because of the concentration gradient. They move from high concentration (in the alveoli and atmosphere) to low concentration (in the blood and inside the leaf).

Exercise 4

1. a) 1500x

b) 500,0000x

2. Light microscope: Advantage is that live organisms can be observed, cheap.

                               Disadvantage is that it has limited magnification so cannot show ultrastructure.

 Electron microscope: Advantage is that it can reveal more detail such as show organelles, also provide 3D images using SEM.

                                  Disadvantage is that preparation of sample to be observed is difficult, EM's are more expensive and cannot easily                                        be moved from one location to another.

3. An electron microscope provides better resolution than a light microscope because the wavelength of an electron is much shorter than light.  

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