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What grade would I get on this 2012 paper, and how can I improve?

QUESTION ONE: CULTURING MICROBES
A nutrient agar plate was left open on the bench in a laboratory. It was then sealed up and incubated at 25°C.

The plate shown has colonies of both bacteria and fungi.
Discuss the growth of colonies shown on the plate.
In your answer you should:
• describe the conditions needed for micro-organisms to reproduce
• describe how the micro-organisms arrived on the plate
• explain the relationship between the structures and functions of the micro-organisms, and how
they look on the plate
• explain why bacteria and fungi have survived and reproduced on the agar plate, but viruses
have not.

Bacteria need moisture, nutrients, and warmth to reproduce. Viruses need a host cell to replicate in. The microorganisms arrived on the plate from the air. The fungus arrived in spores, and then germinated on the agar, while the bacteria arrived as single bacterial cells, which reproduced This. Fungi look fuzzy, as they have sporangia above the agar, which is how they reproduce, as the sporangia then burst and release the spores which germinate and form new fungi. Bacteria on the other hand are shiny due to the slime capsule which surrounds the organisms. They reproduce by binary fission, by duplicating their DNA and pinching their cell wall to form two cells.

The bacteria and fungi could survive and reproduce on the agar plates, as all the conditions needed for them to reproduce were supplied, meaning they could carry out extracellular digestion to forge energy required for reproduction. Viruses on the other hand require a host cell to reproduce, as they inject their genetic material into the host cell, instructing it to reproduce. Since there are no living cells on the agar, the virus will not be able to reproduce, meaning it will die.

Unrelated question: Why can’t viruses use fungi or bacterial cells to reproduce?

QUESTION TWO: PRESERVING FOOD
Fruit and vegetables can be canned so that they can be stored for a long time. Analysis of food canned over 45 years ago shows it to be safe to eat.
When food is canned, the can and its contents are heated to very high temperatures and then quickly sealed with a sterilised lid to exclude any air.

(a) Discuss at least TWO ways in which contamination with micro-organisms is reduced in the canning of food. In your answer you should relate each of these ways to the life processes of micro-organisms.

Before food is canned, it is heated to a high temperature, in order to kill any microbes that are already present, as the high temperature denatures the enzymes, so the microbes cannot carry out life processes. This means that the food is initially sterile. When food is canned, it is stored in an airtight container, so airborne microbes cannot enter and multiply.

(b) Explain why canned food must be refrigerated once opened.
In your answer you should refer to the life processes of micro-organisms.
When the can is opened, airborne bacteria can enter the food, and cause it to spoil. If the food gets refrigerated, it will keep the food at a low temperature, so the enzymes in any bacteria that enter the food will become slowed down. This means the microorganisms will not be able to carry out life processes as fast, so they will not reproduce as fast, so the food can be kept longer without spoiling.

QUESTION THREE: SEWAGE TREATMENT
Sewage contains water-borne waste from toilets and household waste water, along with some industrial waste and storm water. The main aim of sewage treatment is to remove or reduce solid waste and organic material. Once this is done, the water can be safely discharged into rivers or the sea.

(a) Explain why sewage treatment is important.
Sewage treatment is important, as human faeces contain large quantities of bacteria. This could get into drinking water and cause infection. Additionally, if there is a lot of waste in waterways, the naturally occurring bacteria will begin to break this down, and use up large amounts of dissolved oxygen. This will make the water anaerobic, so many fish can die.

(b) During secondary treatment, sewage enters an aeration tank. The tank contains bacteria, and has air pumped into it. Discuss the role of bacteria in the aeration tanks during treatment of sewage.

In your answer you should:
• describe the role of the bacteria in the sewage plant and explain how this relates to their life processes
• explain the significance of air being added into the aeration tank
• consider the raw materials that enter the tanks and the products generated from the bacterial action

The bacteria in the sewage plant are what breaks down the sewage. They do this via extracellular digestion, where enzymes are excreted onto the sewage, and breaks down the molecules. The sewage is then reabsorbed by the bacteria as nutrients, giving them energy to carry out life processes such as reproduction and feeding.

The bacteria that should break down the sewage are aerobic, which means they should be constantly supplied with oxygen. This means they can carry out anaerobic respiration, which is much more efficient, and waste will be broken down much faster. If the tank was not supplied with oxygen, all the obligate aerobes would die, and anaerobic bacteria would take over. This would cause methane production, making the tank become smelly