Biology RPs and Food Tests

What is the test for starch?

• Add iodine solution to food.

• Positive = blue-black

• Negative = orange-brown

What is the test for glucose?

• Add Benedict’s solution, then heat in water bath (~75°C).

• Positive = green/yellow/orange/brick red (depends on amount)

• Negative = stays blue

What is the test for lipids?

• Add Sudan III stain and shake gently.

• Positive = red layer forms on top

• Negative = no separate red layer

🛑 Note: Sudan III is flammable and harmful — mention goggles and lab coat!

What is the test for proteins?

• Add Biuret solution (sodium hydroxide + copper sulfate).

• Positive = lilac/purple

• Negative = blue

RP1 - Microscopy

Use a microscope to view cells (e.g., onion or cheek cells).

• Prepare a slide (stain → cover slip → focus)

• Use low → high magnification

• Measure size using scale bar

RP2 - Culturing microorganisms

pre-inoculation:

• sterilise Petri dish and agar

• pass inoculating loop through flame

• to kill unwanted bacteria

inoculation:

• use swab to spread bacterium into agar

• open the lid of dish as little as possible

• to prevent microbes from the air entering

post-inoculation:

• seal dish with tape

• to prevent microbes from entering

• incubate at 25°C

• to allow growth of bacteria

RP3 - Osmosis in potatoes

• Measure and record the mass of potato pieces

• Place potato pieces into different concentrations of salt solution

  • Use at least 3 different concentrations of salt solution

• Leave the potato pieces in the salt solutions for 30 mins

• Remove potato pieces from salt solutions + blot dry

• Calculate the change in mass (record new mass)

• Repeat each concentration 2 more times

• Calculate the mean change in mass

• Plot percentage change

Control variables:

• Use the same size + mass of potato pieces

• Use same type of potato

• Keep potato pieces in solution for same amount of time

• Volume is solution

RP5 - Enzymes (amylase and pH)

Investigate how pH affects amylase breaking down starch.

• Mix amylase + starch solution together with a buffer solution

• At regular intervals,

• Use iodine in spotting tile

• Time how long until no starch remains (iodine stays orange)

• Vary pH buffer

• Control: temp, volume, concentration

Amylase and starch solutions are mixed together with a buffer solution to control the pH. The mixture is kept at 37°C using a water bath.

At regular intervals, drops of the mixture are added to iodine solution on a spotting tile. Iodine turns blue-black if starch is present.

The time taken for the iodine to stop turning blue-black is recorded. This shows how quickly starch has been broken down. The experiment is repeated at different pH values. The fastest reaction shows the optimum pH for amylase.

RP6 - Photosynthesis (light intensity + pondweed)

Measure rate of photosynthesis by counting bubbles/volume of oxygen.

• A piece of pondweed is placed in water with sodium hydrogencarbonate to provide carbon dioxide.

• Change light intensity by moving lamp

• Count the number of oxygen bubbles released in a fixed time

• (The closer the lamp the higher the LI)

• Control: CO₂, temp, same pondweed

• Use ruler for distance, stopwatch for timing

• Optional: Use gas syringe for accuracy

As light intensity increases, the rate of photosynthesis increases until it reaches a maximum where another factor becomes limiting.

RP7 - plan and carry out an investigation into the effect of a factor on human reaction time

Reaction time is measured using a ruler drop test.

• A ruler is held vertically and dropped without warning. The distance fallen before being caught is recorded and converted into reaction time.

• Repeated several times

• Calculate a mean

• The investigation can be repeated while changing a factor such as caffeine intake, tiredness, distractions, or dominant vs non-dominant hand.

• Using repeats improves reliability and helps identify anomalies.

RP8 - Investigate the effect of light or gravity on the growth of newly germinated seedlings.

Seeds are germinated and placed in a box.

For light experiments, light is allowed from one direction only. For gravity experiments, the box is rotated.

• After several days, the direction of shoot and root growth is observed.

• Shoots grow towards light (positive phototropism) and against gravity (negative gravitropism).

• Roots grow downwards due to gravity.

• This happens because auxin redistributes in the plant, causing uneven growth.

RP9 - Measure the population size of a common species in a habitat.

Use sampling techniques to investigate the effect of a factor on the distribution of this species.

• Use a quadrat of 1m by 1m

• Place the quadrats randomly

• Using a random number generator to find the coordinates of 10 quadrats

• Count the number of plants in each

• Calculate the mean per quadrat

• Find the area of the field

• Find the population by multiplying the mean x area of field

RP10 - Investigate the effect of temperature on the rate of decay of fresh milk by measuring pH change.

Milk samples are placed at different temperatures.

• The pH of each sample is measured at regular intervals using a pH probe or indicator.

• As bacteria grow, they produce acids which lower the pH.

• A faster decrease in pH shows a higher rate of decay.

• Higher temperatures increase enzyme activity in bacteria, causing milk to spoil faster.