Key Biology Practicals and Processes

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• Biology
• Upper Secondary (Key Stage 4)
• GCSE

Key Biology Practicals and Processes/AQA GCSE Biology Paper 1H Revision

Learning Objective

At the end of the lesson you will be able to describe and explain core biology practicals and key biological processes including food tests, osmosis, photosynthesis, microscopy, enzymes, and the structure of the heart.

Food Tests: Introduction

Food tests help us identify the main nutrients in food, including starch, reducing sugars, proteins, and lipids.
These tests involve adding specific chemical reagents that react with the nutrient to produce a visible colour change, indicating its presence. This enables accurate analysis of food composition in biological and chemical investigations.

Test for Starch

Add iodine solution to food. A blue-black colour shows starch is present.

Test for Sugars

Use Benedict's solution and heat. A red, yellow, or green colour shows reducing sugars are present.

Test for Proteins

Add Biuret reagent. A purple colour indicates protein is present.

Test for Lipids

Add ethanol and water. A milky white emulsion shows lipids are present.

Osmosis Practical: Overview

Osmosis is the movement of water from a dilute solution to a concentrated one through a semi-permeable membrane.

Method:

• Cut equal-sized potato cylinders (same length/diameter)

• Measure initial mass using a balance

• Place in different sugar/salt solutions

• Leave for a set time

• Remove, dry, and measure final mass

Results:

• Mass increase → water entered (higher water potential outside)

• Mass decrease → water left

Definition of Osmosis:
Movement of water from high water potential to low water potential across a partially permeable membrane

Variables:

• Independent: concentration of solution

• Dependent: change in mass

• Control: size, time, temperature

Improvements:

• Repeat and calculate mean

• Blot potato dry before measuring

Osmosis Experiment Steps

Place potatoes in different concentrations of sugar solution. After time, reweigh and observe mass changes.

Cut equal potato pieces and blot to remove moisture. Weigh each piece.

Investigation

Set-up

Osmosis Results

Potatoes in pure water gain mass. Potatoes in concentrated solution lose mass. This supports osmosis.

Photosynthesis Practical

Investigate the rate of photosynthesis using pondweed and count oxygen bubbles to measure photosynthesis.

Method:

• Place pondweed in water with sodium hydrogencarbonate (CO₂ source)

• Shine a lamp at different distances

• Count oxygen bubbles produced per minute

Variables:

• Independent: light intensity (distance from lamp)

• Dependent: rate of photosynthesis (bubble count)

• Control: temperature, CO₂ concentration

Results:

• Closer lamp → higher light intensity → faster photosynthesis

• More bubbles = higher rate

Limitations:

• Counting bubbles is inaccurate (size varies)

Improvements:

• Use a gas syringe to measure volume of oxygen

Variables in Photosynthesis

Change the light intensity or CO2 level to see how the rate of photosynthesis changes.

Photosynthesis Results

Increasing light intensity or carbon dioxide concentration increases the rate of photosynthesis because they provide more energy or reactants for the reaction. However, this only continues up to a maximum rate, after which the graph plateaus because another factor, such as temperature, becomes the limiting factor, preventing further increase.

Microscopy Practical: Aims

Microscopy helps us observe cells in detail using light microscopes and prepared slides.

Method:

• Prepare a thin sample (onion/cheek cells)

• Add stain (iodine for plant, methylene blue for animal)

• Place coverslip carefully (avoid air bubbles)

• Start on low power, then increase magnification

Magnification Formula:

• Magnification = image size ÷ real size

Key Skills:

• Measure image using ruler

• Convert units (mm → µm)

Important Conversion:

• 1 mm = 1000 µm

Common Questions:

• Calculate magnification

• Calculate real size

Microscopy: Steps

Place a thin sample on a slide, stain if needed, add a cover slip, and view under a microscope.

Cell Structures Seen

You may see cell walls, nuclei, cytoplasm, and chloroplasts, depending on the sample.

Enzymes: Lock and Key Model

Enzymes fit specific substrates like a lock and key. They speed up reactions without being used up.

How Enzymes Work:

• Enzymes are biological catalysts (speed up reactions)

• Each enzyme has a specific active site

• Substrate fits into active site → enzyme-substrate complex forms

• Reaction occurs → product released

• Enzyme remains unchanged and can be reused

Key Term:
“Substrate has a complementary shape to the active site”

Enzymes – Temperature & pH

Effect of Temperature:

• Low temperature → slow reaction (less kinetic energy)

• Optimum (~37°C in humans) → maximum rate

• High temperature → enzyme denatures
  (active site changes shape → substrate no longer fits)

Effect of pH:

• Each enzyme has an optimum pH

• Example:

  • Stomach enzymes → acidic (pH 2)

  • Intestinal enzymes → alkaline (pH 8)

Key Idea:

• Extreme pH → denaturation → loss of function

Factors Affecting Enzymes

pH

Temperature

pH too high or low can denature the enzyme and slow the reaction.

Too hot: enzyme denatures.

Optimum temperature: fastest rate.

Digestive Enzymes

Amylase

Breaks down starch into sugars. Acts in the mouth and small intestine.

Protease and Lipase

Protease breaks proteins into amino acids. Lipase breaks fats into fatty acids and glycerol.

Extra (for higher marks):

• Bile (from liver):

  • Neutralises stomach acid (creates alkaline conditions)

  • Emulsifies fats → increases surface area for lipase

Heart Structure: Overview

The heart has four chambers: left/right atria (top), left/right ventricles (bottom), separated by valves.

The Heart:

• Muscular organ that pumps blood

• Double circulatory system (heart → lungs → heart → body)

4 Chambers:

• Right atrium (receives deoxygenated blood)

• Right ventricle (pumps to lungs)

• Left atrium (receives oxygenated blood)

• Left ventricle (pumps to body)

Blood Flow:
Body → right atrium → right ventricle → lungs → left atrium → left ventricle → body

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Heart Function

Blood flows from body to right side, then to lungs. Oxygenated blood enters the left side, pumped to the body.

Heart Key Features

Left Ventricle:

• Thicker muscular wall

• Generates high pressure to pump blood around body

Valves:

• Prevent backflow of blood

• Ensure one-directional flow

Blood Vessels:

• Arteries: carry blood away, thick walls, high pressure

• Veins: carry blood to heart, have valves

• Capillaries: very thin walls (one cell thick), allow exchange

Reflection

For 6-mark questions:

• Describe method clearly

• Include variables (independent, dependent, control)

• Mention repeat + mean

• Identify anomalies

Key Scientific Vocabulary (for higher marks)

• Optimum → the best condition for maximum rate

• Denature → when an enzyme’s active site changes shape and stops working

• Active site → the part of an enzyme where the substrate binds

• Diffusion/Osmosis → movement of particles/water across a gradient

• Rate of reaction → how fast a reaction occurs

Lesson Summary

You have learned about key practicals and core biological processes, including food tests, osmosis, photosynthesis, microscopy, enzymes, and heart structure.