2e Nutrition¶

Part of 2 Structure and Functions in Living Organisms.

Nutrition in this course brings plant food production and human feeding into one topic. The link between them is that both depend on molecules being made, broken down and transported where they are needed.

What You Need to Learn¶

Further detail: Pearson Edexcel International GCSE Biology specification.

On this page you'll learn about photosynthesis and leaf adaptations and mineral ions. You'll also cover balanced diet and digestion and absorption. The notes bring these ideas together into one clear overview of nutrition.

Photosynthesis¶

Photosynthesis is the process by which light energy is converted into chemical energy stored in glucose within chloroplasts.

Word equation: carbon dioxide + water → glucose + oxygen

Symbol equation: 6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂

A limiting factor is the factor in least supply that controls the rate of photosynthesis.

Factors that affect the rate of photosynthesis:

• Temperature: increasing temperature increases the rate of photosynthesis (because enzymes work faster). However, above the optimum temperature the enzymes begin to denature and the rate decreases.
• Light intensity: for most plants, a higher light intensity increases the rate of photosynthesis. Moving a lamp further away reduces the light reaching the plant because the light spreads out. A simple way to model this is light intensity ∝ 1/distance², so doubling the distance reduces intensity to one quarter.
• Carbon dioxide concentration: increasing CO₂ concentration increases the rate of photosynthesis, as CO₂ is a raw material for the reaction.

Reading Limiting-Factor Patterns¶

Be precise in your wording: write light intensity and carbon dioxide concentration, not just light or carbon dioxide.

Practical investigation of photosynthesis: Water plants such as Elodea release bubbles of oxygen during photosynthesis. A lamp placed beside the plant can be moved closer or further away to change light intensity, and the bubble count per minute is used as a measure of photosynthesis rate. Sodium hydrogen carbonate dissolved in the water supplies carbon dioxide. An LED bulb is preferred because it does not significantly heat the water, so temperature remains a controlled variable.

Explore the Elodea practical¶

Use the interactive below to move the lamp and measure how many bubbles the Elodea gives off. You can compare the results using lamp distance or a simple relative light-intensity scale. Open full interactive.

What this simulation does not show¶

• Bubble count is only an estimate of oxygen production because bubble sizes can vary.
• The light-intensity values are relative values worked out from lamp distance, not direct light-meter readings.
• A real practical would also control carbon dioxide supply, temperature and the size of the Elodea piece.

Testing a leaf for starch¶

Starch in a leaf shows that photosynthesis has taken place. This is why the starch test is used when investigating the need for light and chlorophyll.

1. Put the leaf into boiling water for a short time to kill the leaf and stop further chemical reactions.
2. Heat the leaf in ethanol in a hot water bath until it loses its green colour. Ethanol is flammable, so it must be heated in a water bath, not over a naked flame. This step removes chlorophyll so the iodine colour change can be seen clearly.
3. Rinse the leaf in warm water to soften it.
4. Place the leaf on a white tile and add iodine solution.
5. A blue-black colour shows starch is present. If the leaf stays brown/orange, starch is absent.

A plant can be destarched first by leaving it in darkness so any starch made earlier is used up before the test.

• If a destarched plant is left in the light with part of a leaf covered by foil, only the uncovered area turns blue-black. This shows that light is needed.
• If a variegated leaf is tested, only the green areas turn blue-black. This shows that chlorophyll is needed.

Leaf Adaptations and Mineral Ions¶

The leaf is an organ made of different tissues working together mainly for photosynthesis, with gas exchange as a closely linked function.

Explore a leaf cross-section¶

Use the interactive below to connect each main leaf tissue to its job in photosynthesis, gas exchange or transport. It works best as a tissue map rather than as a full cell-level drawing. Open full interactive.

What this diagram does not show¶

• This model is designed to show which tissues are present in a leaf, not full cell ultrastructure.
• Some cell shapes and internal structures are simplified, so the diagram should not be treated as a microscope-accurate drawing.
• Palisade cells are shown mainly to highlight chloroplasts for photosynthesis. Real palisade cells would also contain other organelles such as a large vacuole, nucleus and mitochondria.

Leaves are highly adapted to maximise photosynthesis and gas exchange:

• Xylem delivers water to the leaf, which is needed as a raw material for photosynthesis.
• Stomata allow carbon dioxide to diffuse in and oxygen to diffuse out.
• Phloem carries sugar away from the leaf as sucrose.
• Spongy mesophyll provides air spaces that help gases diffuse through the leaf.
• These same adaptations that make gas exchange efficient also increase water loss, so a leaf is balancing carbon dioxide uptake against water conservation.

Mineral ions for plant growth:

• Magnesium is required for chlorophyll production. Deficiency causes leaves to turn yellow (chlorosis).
• Nitrate is required to produce amino acids (the building blocks of proteins). Deficiency causes stunted growth and yellowing of leaves.

Balanced Diet¶

Humans need a balanced diet to maintain health. The components and their roles are:

These examples help show why balanced diet components matter. Deficiency signs are not separate facts floating on their own: they follow from the normal role of the nutrient in the body.

Factors affecting energy requirements:

• Age: energy needs generally increase through childhood into early adulthood, then decline as activity levels decrease in older adults.
• Activity level: more active people require more energy for movement.
• Pregnancy: additional energy is needed to support foetal growth and to carry the extra mass.

Investigating energy content of food (calorimetry): A food sample can be burned and the heat transferred to water in a test tube. The temperature rise of the water is used to calculate energy transferred: Energy transferred (J) = temperature increase (°C) × mass of water (g) × 4.2 J/g°C

Digestion and Absorption¶

The alimentary canal is the pathway food moves through from mouth to anus. Key structures and their roles:

Mouth:

• Mechanical digestion: teeth break food into smaller pieces, increasing surface area but not changing the molecules themselves.
• Chemical digestion: enzymes break large insoluble molecules into small soluble molecules; in the mouth, salivary amylase begins to break starch into maltose.
• Salivary glands produce saliva to lubricate the food bolus.

Oesophagus:

• A muscular tube from the mouth to the stomach.
• Food moves by peristalsis — wave-like contractions of circular and longitudinal muscles that squeeze the bolus along.

Stomach:

• Muscular stomach walls churn the food, so mechanical digestion continues here.
• Produces gastric juice containing pepsin (a protease that begins protein digestion) and hydrochloric acid, which provides the acidic pH needed for pepsin to work and kills ingested bacteria.
• The acid does not chemically digest the food itself; it creates the right conditions for pepsin.
• Peristalsis continues here; the digested mixture is now called chyme.

Pancreas:

• Produces carbohydrase (amylase), protease and lipase enzymes.
• Secretes these into the duodenum.

Small intestine — Duodenum:

• Receives bile (from the liver via the gallbladder) and pancreatic enzymes.
• Bile has two roles: it is alkaline and neutralises the hydrochloric acid arriving from the stomach (enzymes in the small intestine have a higher optimum pH); and it emulsifies large fat droplets into smaller ones, greatly increasing the surface area for lipase to work on.

Small intestine — Ileum:

• Lined with many villi (finger-like projections) that massively increase the surface area for absorption.
• Microvilli on the surface cells increase the surface area even further.
• Each villus has a thin lining (short diffusion distance), a rich network of capillaries that rapidly remove glucose and amino acids and help maintain a concentration gradient, and a lacteal that absorbs fatty acids and glycerol.
• Some nutrients can also be absorbed by active transport when diffusion alone would not move enough into the blood.

Large intestine:

• Water is absorbed, converting indigestible material into faeces.
• Faeces are stored in the rectum and removed through the anus.

Summary of digestive enzymes:

Common Confusions¶

• Bile is not an enzyme: Bile helps digestion by neutralising stomach acid and emulsifying lipids, but it does not catalyse the chemical breakdown of molecules.
• Balanced does not mean equal amounts: A balanced diet means suitable proportions of each nutrient, not identical quantities of everything.
• Vitamin D vs calcium: Vitamin D helps the body absorb calcium, while calcium is the mineral needed to build strong bones and teeth.
• Emulsification vs digestion: Emulsification by bile physically breaks fat into smaller droplets. Enzymatic digestion by lipase then chemically breaks the fat molecules into fatty acids and glycerol.
• Mechanical vs chemical digestion: Mechanical digestion breaks food into smaller pieces. Chemical digestion uses enzymes to break large insoluble molecules into small soluble molecules.
• Starch is not glucose: They are related but distinct molecules. Only glucose gives a positive Benedict's test result.
• Stomach acid vs digestion: Hydrochloric acid does not digest food itself. Its main roles are killing bacteria and providing the acidic conditions needed for pepsin.

Key Terms¶

• Photosynthesis: the process by which plants convert light energy into chemical energy stored in glucose.
• Chlorophyll: the green pigment in chloroplasts that absorbs light energy for photosynthesis.
• Limiting factor: the factor in least supply that controls the rate of photosynthesis.
• Peristalsis: wave-like muscular contractions that move food along the alimentary canal.
• Amylase: a digestive enzyme that breaks starch into maltose.
• Maltase: a digestive enzyme that breaks maltose into glucose.
• Protease: a digestive enzyme that breaks proteins into amino acids.
• Lipase: a digestive enzyme that breaks lipids into fatty acids and glycerol.
• Bile: an alkaline fluid that neutralises stomach acid and emulsifies fats.
• Emulsification: the physical breakdown of large fat droplets into smaller droplets by bile.
• Villus: a finger-like projection in the small intestine wall that increases surface area for absorption.
• Lacteal: a lymph vessel inside a villus that absorbs fatty acids and glycerol.
• Dietary fibre: indigestible plant material that helps food move through the gut.

Connected Pages¶

• 2c Biological Molecules
• 2f Respiration
• 2g Gas Exchange
• Leaf Cross-Section
• Photosynthesis Rate and Light Intensity
• 5a Food Production
• IGCSE Biology Key Terms Glossary
• Practical Investigation Reference