Section 2 of the Pearson Edexcel IGCSE Biology specification is the largest and most heavily examined part of the course. This article addresses the first five topics: levels of organisation, cell structure, biological molecules, movement of substances, and nutrition.
The structure and functions in living organisms section spans ten topics in the 4BI1 syllabus. The first half, covered here, lays the groundwork for everything from respiration to coordination. These edexcel igcse biology revision notes walk through each topic with the precision the specification demands, providing worked examples and flagging the areas where candidates most frequently lose marks.
Levels of organisation
The Edexcel IGCSE Biology specification requires you to describe the levels of organisation in organisms. The hierarchy is as follows, from smallest to largest functional unit:
- Organelles - structures within cells that perform specific functions (e.g. mitochondria, ribosomes)
- Cells - the basic structural and functional units of life
- Tissues - groups of similar cells working together to perform a function (e.g. muscle tissue)
- Organs - structures made of different tissues working together (e.g. the heart, the leaf)
- Organ systems - groups of organs working together to perform a major body function (e.g. the digestive system)
This hierarchy applies to multicellular organisms. Unicellular organisms such as bacteria carry out all life processes within a single cell and do not have tissues or organs.
Cell structure
Cell structure is fundamental to the igcse biology course. You must know the structures present in plant and animal cells, their functions, and the key differences between the two cell types.
| Structure | Function | Plant cells | Animal cells |
|---|---|---|---|
| Nucleus | Contains genetic material (DNA); controls cell activities | Present | Present |
| Cytoplasm | Jelly-like substance where most chemical reactions occur | Present | Present |
| Cell membrane | Controls the movement of substances into and out of the cell | Present | Present |
| Mitochondria | Site of aerobic respiration; releases energy (ATP) | Present | Present |
| Ribosomes | Site of protein synthesis | Present | Present |
| Cell wall | Made of cellulose; provides structural support and prevents the cell from bursting | Present | Absent |
| Chloroplasts | Contain chlorophyll; site of photosynthesis | Present (in green parts) | Absent |
| Permanent vacuole | Contains cell sap; helps maintain turgor pressure | Present (large, central) | Absent or small and temporary |
Cell differentiation and stem cells
Cell differentiation is the process by which a cell becomes specialised for a particular function. As cells differentiate, they develop specific structures. A root hair cell, for instance, has a long extension to increase surface area for water absorption. A red blood cell has no nucleus to maximise space for haemoglobin, and its biconcave shape increases the surface area for oxygen absorption.
Stem cells are undifferentiated cells that can divide and develop into different cell types. The edexcel specification requires you to understand both the advantages and disadvantages of using stem cells in medicine. Advantages include the potential to treat conditions such as paralysis or diabetes by replacing damaged cells. Disadvantages include ethical concerns about the use of embryonic stem cells and the risk that stem cells may divide uncontrollably, potentially leading to tumour formation.
Biological molecules
The three main groups of biological molecules are carbohydrates, proteins and lipids (fats and oils). Each is a large molecule built from smaller basic units:
| Molecule | Elements present | Basic units | Example |
|---|---|---|---|
| Carbohydrates | Carbon, hydrogen, oxygen | Simple sugars (e.g. glucose) | Starch, glycogen, cellulose |
| Proteins | Carbon, hydrogen, oxygen, nitrogen (sometimes sulfur) | Amino acids | Enzymes, haemoglobin, antibodies |
| Lipids | Carbon, hydrogen, oxygen | Fatty acids and glycerol | Fats (solid at room temperature), oils (liquid) |
Food tests
The specification requires you to be able to investigate food samples for the presence of glucose, starch, protein and fat. These are the four standard food tests:
- Glucose: Benedict's reagent. Heat the food solution with Benedict's reagent. A colour change from blue to green, yellow, orange or brick-red (depending on glucose concentration) indicates glucose is present.
- Starch: Iodine solution. Add iodine to the food sample. A colour change from brown/orange to blue-black indicates starch.
- Protein: Biuret test. Add Biuret reagent (sodium hydroxide then copper sulfate) to the food sample. A colour change from blue to purple/lilac indicates protein.
- Fat/lipid: Ethanol emulsion test. Dissolve the food in ethanol, then add water. A cloudy white emulsion indicates fat is present.
Enzymes
Enzymes are biological catalysts. They speed up metabolic reactions without being used up themselves. Each enzyme has an active site with a specific shape that is complementary to its substrate. This is the lock-and-key model: the substrate fits into the active site the way a key fits into a lock.
Two factors critically affect enzyme function:
Temperature: As temperature increases, enzyme activity increases because molecules have more kinetic energy, leading to more frequent collisions between enzyme and substrate. Above the optimum temperature, the enzyme begins to denature: the active site changes shape permanently, so the substrate can no longer fit. The reaction rate falls sharply. For most human enzymes, the optimum temperature is around 37 degrees Celsius.
pH: Each enzyme has an optimum pH at which it works fastest. Deviations from this pH alter the shape of the active site, reducing enzyme activity. Extreme pH values cause denaturation. Pepsin, for example, works best at pH 2 (acidic conditions in the stomach), while pancreatic lipase works best at around pH 8 (alkaline conditions in the duodenum).
Movement of substances into and out of cells
Three processes move substances across cell membranes: diffusion, osmosis and active transport. Understanding how and why they differ is essential for the edexcel igcse biology explained content in this section.
| Process | Definition | Energy required? | Example |
|---|---|---|---|
| Diffusion | Net movement of particles from an area of higher concentration to an area of lower concentration | No (passive) | Oxygen moving from alveoli into blood capillaries |
| Osmosis | Net movement of water molecules across a partially permeable membrane from a dilute solution to a more concentrated solution | No (passive) | Water entering root hair cells from the soil |
| Active transport | Movement of substances against the concentration gradient (from low to high concentration) | Yes (requires ATP from respiration) | Mineral ions absorbed by root hair cells from the soil |
Factors affecting the rate of movement include surface area to volume ratio (a larger ratio increases the rate), distance (shorter distance means faster movement), temperature (higher temperature increases kinetic energy) and concentration gradient (a steeper gradient increases the rate of diffusion and osmosis).
Explain why.
Answer: The sugar solution has a lower water concentration than the potato cells. Water moves out of the potato cells by osmosis, through the partially permeable cell membrane, from the dilute solution inside the cells to the more concentrated sugar solution outside. The cells lose turgor pressure and become flaccid (soft and floppy). If enough water is lost, the cell membrane pulls away from the cell wall, a process called plasmolysis.
Nutrition
Nutrition is among the most commonly examined topics in the igcse 4BI1 structure and functions in living organisms: level of organisation to nutrition section. It covers both plant nutrition (photosynthesis) and human nutrition (diet and digestion).
Photosynthesis
Photosynthesis is the process by which plants convert light energy into chemical energy stored in glucose. The word equation is:
carbon dioxide + water → glucose + oxygen
The balanced chemical symbol equation is:
6CO2 + 6H2O → C6H12O6 + 6O2
Three factors affect the rate of photosynthesis: light intensity, carbon dioxide concentration and temperature. Each can act as a limiting factor. Increasing light intensity increases the rate up to a point; beyond that, another factor (such as CO2 concentration) becomes limiting.
The leaf is adapted for photosynthesis in several ways: a large surface area to absorb light, a thin shape to keep diffusion distances short, chloroplasts concentrated in the palisade mesophyll layer, and stomata on the lower surface for gas exchange.
Plants also require mineral ions. Magnesium ions are needed to make chlorophyll (without them, leaves turn yellow). Nitrate ions are needed to make amino acids, which are then assembled into proteins.
Human nutrition: a balanced diet
A balanced diet includes carbohydrates, proteins, lipids, vitamins (A, C and D), mineral ions (calcium and iron), water and dietary fibre in appropriate proportions. Energy requirements vary with activity level, age and pregnancy.
The alimentary canal and digestion
The human alimentary canal runs from the mouth to the rectum. Its key structures and their roles are:
- Mouth: Mechanical digestion (teeth) and chemical digestion (salivary amylase begins starch breakdown)
- Oesophagus: Moves food to the stomach by peristalsis (waves of muscular contraction)
- Stomach: Produces hydrochloric acid (kills bacteria, provides acidic pH for pepsin) and protease enzymes
- Duodenum (first part of small intestine): Receives bile from the liver (via the gall bladder) and pancreatic enzymes
- Ileum (second part of small intestine): Main site of absorption; lined with villi
- Colon (large intestine): Absorbs water from undigested food
- Rectum: Stores faeces before egestion
- Pancreas: Produces amylase, protease and lipase enzymes
The three main classes of digestive enzyme are:
- Amylase and maltase: Digest starch into glucose
- Proteases: Digest proteins into amino acids
- Lipases: Digest lipids into fatty acids and glycerol
Bile plays two roles. It neutralises the acidic chyme from the stomach, creating the alkaline conditions that pancreatic enzymes require. It also emulsifies lipids, breaking large fat droplets into smaller ones, which increases the surface area available for lipase to act on.
The small intestine is adapted for absorption through its villi. Each villus has a thin wall (one cell thick) for a short diffusion distance, a dense network of blood capillaries to maintain a concentration gradient and carry absorbed nutrients away, and a lacteal to absorb fatty acids and glycerol. The villi also give the small intestine a very large surface area.
Worked example: enzyme graph interpretation
Model answer:
1. Between 10 and 40 degrees, increasing temperature gives molecules more kinetic energy.
2. This leads to more frequent successful collisions between enzyme and substrate, increasing the rate of reaction.
3. At 40 degrees (the optimum temperature), the rate is at its maximum.
4. Above 40 degrees, the enzyme begins to denature: the active site changes shape so the substrate can no longer fit, and the rate of reaction decreases rapidly.
Self-check practice questions
These edexcel igcse biology practice questions test your understanding of the edexcel igcse biology structure and functions in living organisms: level of organisation to nutrition material. Write your answers before checking them against the notes above.
- State the five levels of organisation in a multicellular organism, from smallest to largest.
- Name three structures found in plant cells but not in animal cells.
- Describe the Biuret test for protein and state the expected colour change.
- Explain why an enzyme stops working above its optimum temperature. Use the term "active site" in your answer.
- Distinguish between diffusion, osmosis and active transport in terms of energy requirement and direction of movement.
- Write the balanced symbol equation for photosynthesis.
- Explain two ways in which villi are adapted for absorption in the small intestine.
- State the roles of bile in digestion.
These edexcel igcse biology notes cover the densest portion of the specification. Mastery of cell structure, enzymes, membrane transport and nutrition provides the scaffolding for every subsequent topic in Section 2. Revisit any areas of uncertainty before progressing to respiration, gas exchange and transport, where the same principles of diffusion, active transport and enzyme action reappear in new contexts.
The structure and functions in living organisms: level of organisation to nutrition edexcel igcse content forms the backbone of Paper 1 (2 hours, 110 marks) and reappears in Paper 2 (1 hour 15 minutes, 70 marks). Invest the time here, and it pays dividends across both papers.
Edexcel IGCSE Biology notes on cells, biological molecules, enzymes, diffusion, osmosis and nutrition: the core of Section 2 explained with worked examples.
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