Revision:Living Organisms consist of cells

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Advanced Subsidiary’s AS Biology Revision Notes

Unit 1: Cells, Exchange and Transport – Module 1 Cells

AS Biology – Revision Notes – Living Organisms consist of cells

Disclaimer: As chunky as these notes look, this was the notes I made during class while I was in college. You can make them shorter if you want, but if you want to complete Biology experience then you’ve found the right place.

By the end of this lesson, I should be able to;

• State the resolution and magnification that can be achieved by a light microscope.
• Explain the difference between magnification and resolution.
• Explain the need for staining samples in light microscopy.

The Cell Theory

In the 1660’s, Robert Hooke developed a compound microscope using several lenses. He used it to examine slices of cork taken from under the bark of an oak tree. Hooke noted that the slices where made up of chambers that resembled the cells or rooms where monks lived so he called the tiny chambers in the cork, ‘cells’. With better microscopes, other scientists studied the biological material and saw that all plant and animal material was made up by cells.

The Cell Theory states;

• All living things consist of cells.
• New cells are formed only by the division of pre-existing cells.
• The cells contain information that act as instructions for growth. This information can be passed onto new cells.

Investigating Cell make-up

In order to investigate cells, we need to be able to produce images that are both enlarged and more detailed. In the past 60 years light microscopes have improved and electron microscopes developed. This has enabled scientists to study cells in more detail. Other scientists have studied the chemical reactions going on in different parts of the cell. The results of these investigations have enabled us to understand how the structure of the cell parts allows them to carry out their functions.

By the 1840’s the cell theory – discovered by two scientists, Schleiden and Schwann – was accepted. The cell theory, as extended by the work of Virchow in 1855 and Weisman in 1880 stated the above.

Microscope and Resolution

In the dark, a car’s headlights at some distance away appear as on light source – if you took a photograph it will only show one light source. You could enlarge the photograph many times but it will still only show as one light source. It would be increasingly blurred as the magnification increased. This is because magnification on its own does not increase the level of detail.

The term resolution refers to the ability to see two distinct points separately. In the example above, as the car moves closer, the one light source you saw in the distance ‘resolves’ into two when the car is close enough for your eye to see the two headlights as separate points.

In order to investigate cells and their component parts, you need both high resolution and high magnification.

Key Definitions

Magnification > Is the degree to which the size of the image is larger than the object itself.

Resolution > Is the degree to which it’s possible to distinguish between two objects that are very close together. The higher the resolution, the greater level of detail you can see.

The Light Microscope

• Light microscope was a number of lenses to produce to produce an image that can be viewed directly at the eyepieces.
• Light passes from a bulb under the stage, through a condenser lens, then through the specimen.
• This beam of light is focused through the objective lens, then through the eyepiece lens.
• To view specimens at different magnifications, light microscopes have a number of objective lenses that can be rotated into position.
• Usually four objective lenses are present; x4, x10, x40 and x100. The x100 objective is an oil immersion lens.
• The eyepiece lens then magnifies the image again. This is usually x10.

The total magnification of any specimen is given by multiplying the objective magnification by the eyepiece magnification.

Advantages and Limitations of the Light Microscope

Magnification > Most light microscopes are capable of magnification up to x1500.

Resolution > The maximum resolving power using light is 200 nanometres. This means that if the two objects are closer than 200 nanometres, they will be seen as one object. This limit is due to the magnification of the wavelength of light. Two objects can be distinguished only if light waves can be passed through them.

Specimen > A wide range of specimens can be viewed using light microscope. These include living organisms such as Euglina and Daphina. You can also view thin sections of larger plants and animals, and smear preparations of blood or cheek cells.

The light microscope is widely used in education, laboratory analysis and research. It doesn’t have high resolution so it can’t give detailed information about internal cell structures.

Preparation of Specimens for the Light Microscope

A lot of biological material is not coloured, so you can’t see the details. Also, some material distorts when you try to cut it into thin sections.

Preparation of slides to avoid these problems involves the following steps.

Staining > Coloured stains are chemicals that bind to chemicals in the specimen. This allows specimen to be seen. Some stains bind to specific cell structures. Acetic Orcein stains DNA dark red. Gentian violet stains bacterial cell walls.

Sectioned > Specimens are embedded in wax. Thin sections are then cut without distorting the structure of the specimen. This is particularly useful for making sections of soft tissue, such as brain.

Advanced Subsidiary's AS Biology Questions

Unit 1: Cells, Exchange and Transport - Module 1 Cells

AS Biology – Questions – Living Organisms consist of cells

Disclaimer: Please note that some of these questions contain relevant scientific knowledge which may be required in the exam.

A lot of the questions asked in exams specify the candidate to use scientific knowledge, to help build the confidence to use this knowledge I have added questions specifically for this reason.

One last thing, most of these questions are in correlation with each other just to make things a little bit easier. Enjoy!

Introductory Questions

1. State what Eukaryotic Cells consist of.
2. Which organelle or cell structure is common to all cells?
3. List the 7 characteristics of living things. (Mrs Gren Anyone?)

Test Yourself

1. How many different types of cell do you know?
2. What are the limitations of the light microscope and the electron microscope in observing cells?
3. How big are cells?
4. Why can we observe chromosomes in a dividing cell but not in a non-dividing cell?
5. What happens when cell division gets out of control?
6. What does ‘partially permeable’ mean?
7. What is found inside cells?

The development of the Cell Theory

1. In what year(s) did Robert Hooke develop a compound microscope that used several lenses?
2. Who was the name of the scientist that developed the compound microscope using several lenses?
3. What did Robert Hooke use the compound microscope for?
4. Why did Hooke use the term ‘cell’?
5. What does the cell theory state?

Investigating Cell Make-Up

1. Explain how cells are investigated.
2. Light microscopes and electron microscopes have been developed and improved, describe what this has allowed scientists to do.
3. Outline the two features that are needed for microscopes to investigate cells and their component parts, collectively.

The Light Microscope

1. What does the light microscope have that produces an image which can be viewed directly at the eyepieces?
2. Outline briefly, the sequence in which light passes through the bulb to the eye pieces.
3. State the part(s) that light microscopes have which enables them to view specimens at different magnifications.
4. State how many objective lenses are present in light microscopes and the sizes in which they magnify images.
5. Describe the function of the eyepiece lens.
6. Explain the difference between magnification and resolution.

Advantages and Limitations of the Light Microscope

1. What are the main advantages and limitations of the light microscope? E.g. (Magnification, Resolution, Specimen)

Preparation of Specimens for the Light Microscope

1. Outline briefly, the two problems that occur when viewing specimens with the light microscope.
2. Describe the two methods used to overcome these problems and explain how they overcome the problem.
3. Explain what coloured stains are and outline their function.
4. State the names of the chemicals which stain DNA and bacterial cell walls.
5. Outline briefly, the specific cell structures Acetic Orcein and Gentian Violet stain.
6. Name and label each part of the light microscope and light path diagram. (Should be able to draw and identify each part on the light path diagram)
7. Why do sections of tissue need to be cut into thin slices for examination under a microscope?
8. Suggest why light microscopes are so useful in biology.

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• Advanced Subidiary
• AS Biology OCR
• F211
• Module 1 - Cells, Exchange and Transport