Edexcel International GCSE Biology 4BI1: Level of organisation and biological molecules – Revision Notes

For full explanations and worked examples, read the study guide first.

These notes condense sub-topics 2(a) Level of organisation (point 2.1) and 2(c) Biological molecules (points 2.7–2.14B) of the Pearson Edexcel International GCSE Biology (4BI1) specification, Issue 3, for the June and November series examined under Issue 3. The course is untiered. Point 2.14B is bold with a B reference and is examined on Paper 2 only; the rest can appear on Paper 1 or Paper 2.

Test yourself with the practice questions. Shorter topic-level notes that also cover cell structure (2(b)) are in the cell structure revision notes. Course hub: Edexcel IGCSE Biology. Every point is on the printable checklist, and you can find gaps with the free diagnostics.

2.1 Levels of organisation

organelle → cell → tissue → organ → system → organism

Level	Definition to learn	Examples
Organelle	Structure inside a cell with a specific function	Nucleus, mitochondrion, chloroplast, ribosome
Cell	Basic structural and functional unit of life	Red blood cell, root hair cell, palisade cell
Tissue	Group of similar cells working together for one function	Muscle tissue, epithelium, palisade mesophyll
Organ	Group of different tissues working together	Heart, stomach, brain, leaf, root
System	Group of organs working together	Digestive, circulatory, nervous system

Method in steps: classifying a named structure

1. Is it inside a cell? → organelle.
2. Is it one cell? → cell.
3. Is it many cells of one kind? → tissue.
4. Does it contain more than one kind of tissue? → organ.
5. Is it several organs that share a job? → system.

Worked reminder: one organ, many levels

A leaf is an organ. It contains palisade mesophyll, spongy mesophyll, epidermis, xylem and phloem: several different tissues. The palisade mesophyll is made of many similar palisade cells, and each palisade cell contains many chloroplasts (organelles). The leaf works with the stem and other organs above ground, which together form the shoot system. Being able to walk down the levels like this, from organ to organelle, is a quick way to check any classification.

2.7 Elements

Molecule	C	H	O	N
Carbohydrate	✓	✓	✓	✗
Protein	✓	✓	✓	✓ (often S too)
Lipid	✓	✓	✓	✗

• Nitrogen present → protein.
• Lipids have far less oxygen relative to carbon and hydrogen than carbohydrates.

2.8 Building units

Large molecule	Smaller units	Stored or used by
Starch	Glucose (simple sugar)	Plants (storage)
Glycogen	Glucose (simple sugar)	Animals and fungi (storage)
Protein	Amino acids	All organisms (enzymes, structure)
Lipid	Fatty acids + glycerol	Fats (animals), oils (plants)

• Starch and glycogen have the same unit (glucose), different organisms.
• Protein shape depends on the order of amino acids.
• A lipid = glycerol + fatty acids (usually three).

2.9 Food tests

Test for	Reagent	Heat?	Positive result
Glucose	Benedict’s solution	Yes, water bath	Blue → green / yellow / orange / brick-red
Starch	Iodine solution	No	Orange-brown → blue-black
Protein	Biuret (sodium hydroxide + copper sulfate)	No	Blue → purple
Fat	Ethanol, then pour into water	No	Clear → cloudy white emulsion

Method in steps: any food test answer

1. Prepare the sample: grind solid food with water.
2. Name the reagent and quantity (a few drops, or an equal volume).
3. State heating if needed (Benedict’s only, in a water bath).
4. State the starting colour and the positive colour.
5. Include a control (water instead of food) if asked how to make it valid.

Small reminders:

• Benedict’s colour order tracks glucose amount: green (little) → brick-red (lots).
• Sucrose gives a negative Benedict’s result.
• Emulsion test: the ethanol must go into water to show the cloudiness.

Worked reminder: reading a Benedict’s row

Three glucose solutions give green, orange and brick-red results after the same heating time. Order of glucose concentration, lowest first: green → orange → brick-red. The test only ranks the samples; to compare fairly, use the same volume of sample and Benedict’s solution and the same heating time for each.

2.10 Enzymes as catalysts

• Catalyst: speeds up a reaction, not used up.
• Enzyme: a protein that acts as a biological catalyst in metabolic reactions.
• Catalyse breakdown (starch → sugars) and synthesis (glucose → glycogen; amino acids → protein).
• Active site: region with a specific shape; substrate has a complementary shape.
• Substrate binds → enzyme-substrate complex → products released → enzyme reused.
• Specificity: one enzyme, one type of substrate, because of active site shape.

2.11 and 2.13 Temperature and pH

Condition	What happens	Reversible?
Low temperature	Less kinetic energy, fewer collisions, slow rate	Yes
Optimum temperature	Most successful collisions; fastest rate	–
High temperature	Bonds break, active site changes shape, substrate no longer fits: denatured	No
pH away from optimum	Active site altered; fewer enzyme-substrate complexes	Small shifts: often yes. Extreme pH: denatured

Typical optima: many human enzymes about 37 °C; stomach protease about pH 2; salivary amylase about pH 7.

2.12 Temperature practical (and 2.14B pH practical, Paper 2 only)

Method in steps

1. Pre-warm amylase and starch separately in a water bath at the test temperature.
2. Place iodine drops in a spotting tile.
3. Mix, start the timer, and test one drop every 30 s.
4. Record the time when iodine stays orange-brown.
5. Repeat three times; calculate a mean, leaving out anomalous results.
6. Rate = 1 / time.

Variable	Temperature practical (2.12)	pH practical (2.14B)
Independent	Temperature	pH (buffer solutions)
Dependent	Time for starch to disappear	Time for starch to disappear
Control	pH (buffer), volumes, concentrations, sampling interval	Temperature (one water bath), volumes, concentrations, sampling interval

Small worked reminder

The iodine stays orange-brown after 80 s. Rate = 1/80 = 0.0125 s⁻¹. A time twice as long means half the rate.

Method in steps: describing and explaining an enzyme graph

1. Describe first, with data: “rate rises from 10 °C to a peak at 40 °C, then falls to zero by 60 °C.”
2. Explain the rise: more kinetic energy, more frequent successful collisions, more enzyme-substrate complexes.
3. State the optimum as read from the graph, with its unit.
4. Explain the fall: bonds break, active site changes shape, substrate no longer fits, enzyme denatured.
5. For a pH graph, use the same steps but explain both sides of the peak by the active site being altered.

Must-know distinctions

• Tissue vs organ: similar cells vs different tissues.
• Starch vs glycogen: same unit, plant vs animal store.
• Benedict’s vs iodine: glucose (heat, brick-red) vs starch (no heat, blue-black).
• Slowed vs denatured: cold slows an enzyme reversibly; heat above the optimum changes the active site permanently.
• Time vs rate: longer time = slower rate.
• 2.12 vs 2.14B: temperature practical is on both papers; pH practical is Paper 2 only.

Quick self-test

1. Put in order, smallest first: organ, organelle, system, tissue, cell.
2. Is the heart a tissue or an organ? Give a reason.
3. Which element is in proteins but not in carbohydrates?
4. Name the units that make up glycogen.
5. Name the two kinds of unit that make up a lipid.
6. What colour is a positive biuret test?
7. A sucrose solution is tested with Benedict’s. What is the result?
8. Define the term enzyme.
9. Why does an enzyme stop working at very high temperature?
10. The starch disappears after 125 s. Calculate the rate.
11. Three repeat times are 62 s, 58 s and 60 s. Calculate the mean.
12. In the pH practical, how is temperature controlled, and which paper examines this practical?

Answers

1. Organelle, cell, tissue, organ, system.
2. Organ: it contains several different tissues (such as muscle and nervous tissue) working together.
3. Nitrogen.
4. Glucose (simple sugar) units.
5. Fatty acids and glycerol.
6. Purple (lilac).
7. Negative: stays blue. Sucrose is not detected by Benedict’s.
8. A protein that acts as a biological catalyst, speeding up a metabolic reaction without being used up.
9. Bonds holding its shape break, the active site changes shape and the substrate no longer fits; it is denatured.
10. 1/125 = 0.008 s⁻¹.
11. (62 + 58 + 60) ÷ 3 = 60 s.
12. All tubes kept in the same water bath at one temperature; Paper 2 only (2.14B).

Where marks are usually lost

• Writing “organ = a group of cells” instead of “a group of different tissues”.
• Giving a cell type (for example, a sperm cell) when asked for a tissue.
• Putting nitrogen in lipids, or leaving it out of proteins.
• Saying proteins are made of “amino acids and glycerol” – mixing up the units.
• Leaving out the water bath in the Benedict’s test, or heating the biuret test.
• Describing a positive iodine test as “black” or “purple” rather than blue-black.
• “Enzymes are killed by heat” – enzymes are denatured, and you must say the active site changes shape.
• Saying the substrate “no longer fits” without stating that the active site has changed shape first.
• Quoting the time as the rate; rate = 1/time, and the shortest time is the fastest rate.
• Not pre-warming the enzyme and substrate separately, so the reaction starts at the wrong temperature.

Official syllabus

Pearson Edexcel International GCSE in Biology (4BI1) specification, Issue 3, first examination June 2019, Pearson Education Limited – Topic 2, Structures and functions in living organisms: sub-topics (a) Level of organisation and (c) Biological molecules.