Investigating Cells

• Typical plant cell may be about 0.1mm in diameter
• Animal cell may be 0.02mm in diameter
• Prokaryotic cells smaller- often 0.002mm long
• Scientists use units that have different prefixes

• Not possible to see cells as seperate objects using naked eye
• Ability to see two or more objects as seperate- called resolution
• Light microscope- developed in late 16th century and gave greater resolution than human eye
• Allowed scientists to see plant, animal and bacterial cells
• Some sub-cellular structures even smaller than resolution achieved by light microscope- cannot be seen using this method
• 1933- Scientists used electron microscope
• Electron microscope passes electrons rather than light through specimen and can give much better resolution
• Cells can be seen in finer detail
• Structures inside mitochondria and chloroplasts can be studied- helps scientists find out how they work
• Ribosomes- can be seen and role in making proteins ca be studied

• When microscope used to look at cells- scientists often take photographs or produce drawings
• Images are many times larger than real cell or structure
• Magnification- how many times larger the image is than real object
• Magnification not some as resolution
• Light microscopes can be built to magnify more, but after a certain point images do not get any clearer

Equation for magnification-

Magnification = size of real image / size of real object

• Binary Fission- used by bacteria as a type of simpole cell division to multiply
• Example of asexual reproduction
• Can multiply as frequently as one every 20 mins- with enough nutrients and suitable temp
• Bacteria can be grown in nutrient broth solutons or as colonies on type of jelly called agar- This is called a culture
• Agar is usually in bottom of flat dish called Petri dish
• Cultures of microorganisms need to be uncontaminated- important so speicific strains can be used to test effects of antibiotics or disinfectants
• Uncontaminated cultures can be produced by the following sterile technique-
• 1- Petri dishes and agar must be sterilised before use to kill unwanted organisms
• 2- Inoculating loop is sterelised by passing through a flame
• 3- Cooled inoculating loop used to transfer bacteria to the agar
• 4- Lid of Petri dish must be quickly removed and replaced when transferring bacteria- secured with tape to stop it coming off otherwise microorganisms from air may contaminate the culture
• 5-Dish stored upside down to stop condensation dripping onto agar surface
• Procedure called aseptic technique and enses that only required microroganisms are grown
• Cultures should be incubated at max temp of 25*C in schools- reduces likelihood of growth of harmful bacteria.

Uncontaminated cultures can be produced by the following sterile technique-

• 1- Petri dishes and agar must be sterilised before use to kill unwanted organisms
• 2- Inoculating loop is sterelised by passing through a flame
• 3- Cooled inoculating loop used to transfer bacteria to the agar
• 4- Lid of Petri dish must be quickly removed and replaced when transferring bacteria- secured with tape to stop it coming off otherwise microorganisms from air may contaminate the culture
• 5-Dish stored upside down to stop condensation dripping onto agar surface
• Procedure called aseptic technique and enses that only required microroganisms are grown
• Cultures should be incubated at max temp of 25*C in schools- reduces likelihood of growth of harmful bacteria.
• In Industries- higher temps can be used for rapid growth
• Fungi can also be grown on agar in Petri dishes- Viruses cannot be grown on agar as they need living cells to reproduce- Have to be grown inside suitable host cells
• In optimum conditions- bacteria can divide approx every 20 mins- Mean average division time
• Area of colony of bacteria calculated by: area of circle = πR2

1. Arrange these structures in order of size with the largest first:

Bacterium               Liver Cell              Nucleus       Ribosome

2. A nucleus is measured as 0.005mm in diameter. How many micrometres is this?

3. A student draws a cheek cell. The cell in their drawing is 50mm wide. In real life the cell is 0.025mm. What is the magnification of their drawing?

4. Write down two precautions that should be taken to stop a bacterial culture getting contaminated.