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.
Learning Objectives¶
| ID | Official specification wording | Main teaching sections |
|---|---|---|
2e-lo-1 |
2.18 understand the process of photosynthesis and its importance in the conversion of light energy to chemical energy 2.19 know the word equation and the balanced chemical symbol equation for photosynthesis 2.20 understand how varying carbon dioxide concentration, light intensity and temperature affect the rate of photosynthesis |
Photosynthesis |
2e-lo-2 |
2.21 describe the structure of the leaf and explain how it is adapted for photosynthesis 2.22 understand that plants require mineral ions for growth, and that magnesium ions are needed for chlorophyll and nitrate ions are needed for amino acids 2.23 practical: investigate photosynthesis, showing the evolution of oxygen from a water plant, the production of starch and the requirements of light, carbon dioxide and chlorophyll Humans |
Leaf Adaptations and Mineral Ions |
2e-lo-3 |
2.24 understand that a balanced diet should include appropriate proportions of carbohydrate, protein, lipid, vitamins, minerals, water and dietary fibre 2.25 identify the sources and describe the functions of carbohydrate, protein, lipid (fats and oils), vitamins A, C and D, the mineral ions calcium and iron, water and dietary fibre as components of the diet 2.26 understand how energy requirements vary with activity levels, age and pregnancy |
Balanced Diet |
2e-lo-4 |
2.27 describe the structure and function of the human alimentary canal, including the mouth, oesophagus, stomach, small intestine (duodenum and ileum), large intestine (colon and rectum) and pancreas 2.28 understand how food is moved through the gut by peristalsis 2.29 understand the role of digestive enzymes, including the digestion of starch to glucose by amylase and maltase, the digestion of proteins to amino acids by proteases and the digestion of lipids to fatty acids and glycerol by lipases 2.30 understand that bile is produced by the liver and stored in the gall bladder 2.31 understand the role of bile in neutralising stomach acid and emulsifying lipids 2.32 understand how the small intestine is adapted for absorption, including the structure of a villus 2.33B practical: investigate the energy content in a food sample |
Digestion and Absorption |
Photosynthesis¶
Photosynthesis is an endothermic reaction in 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
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. Light intensity is inversely proportional to the square of the distance from the light source (light intensity ∝ 1/distance²). For example, doubling the distance from the light 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.
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.
Investigating the need for light and chlorophyll: - A leaf can be half-covered with foil and left in light. After removing chlorophyll with hot ethanol and treating with iodine, only the exposed half turns blue-black (starch present), showing that light is needed. - A variegated leaf (with green and white areas) will only show starch in the green areas after the same treatment, demonstrating that chlorophyll is required.
Leaf Adaptations and Mineral Ions¶
Leaves are highly adapted to maximise photosynthesis and gas exchange:
| Structure | Function |
|---|---|
| Waxy cuticle | Reduces water loss by evaporation; protective layer at the top of the leaf |
| Upper epidermis | Very thin and transparent, allowing light to reach the palisade cells below |
| Palisade mesophyll | Packed with chloroplasts for rapid photosynthesis |
| Spongy mesophyll | Air spaces increase the surface area to volume ratio, speeding diffusion of gases |
| Lower epidermis | Contains guard cells and stomata |
| Guard cells | Kidney-shaped cells that open and close the stomata; they absorb water and become turgid to open stomata when water is plentiful |
| Stomata | Pores where gas exchange (CO₂ in, O₂ out) and water vapour loss by evaporation occur; open during the day and close at night |
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:
| Component | Food sources | Function |
|---|---|---|
| Carbohydrates | Bread, cereals, pasta, rice, potatoes | Main energy source |
| Proteins | Meat, fish, eggs, pulses | Growth and repair |
| Lipids | Butter, oil, nuts | Energy storage and insulation |
| Dietary fibre | Vegetables, bran | Roughage to keep food moving through the gut; deficiency causes constipation |
| Vitamin A | Carrots, green vegetables | Needed for vision (especially in dim light) and growth |
| Vitamin C | Citrus fruits | Helps the body absorb iron |
| Vitamin D | Margarine, oily fish | Helps the body absorb calcium |
| Calcium | Milk, dairy products | Bone and tooth strength; deficiency can cause rickets (bowing of bones) |
| Iron | Red meat | Needed for haemoglobin production; deficiency causes anaemia |
| Water | Water, juice, milk | Needed for all cell reactions |
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. - Chemical digestion: 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: - Produces gastric juice containing pepsin (a protease that begins protein digestion) and hydrochloric acid, which provides the acid pH needed for pepsin to work and kills ingested bacteria. - 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 villi (finger-like projections) that massively increase the surface area for absorption. - Each villus has a thin lining (short diffusion distance), a rich network of capillaries (rapid removal of absorbed substances) and large surface area for maximum absorption of glucose, amino acids, fatty acids and glycerol.
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:
| Substrate | Enzyme | Product |
|---|---|---|
| Starch | Amylase (mouth, pancreas) | Maltose |
| Maltose | Maltase (small intestine) | Glucose |
| Proteins | Proteases (stomach, pancreas, small intestine) | Amino acids |
| Lipids | Lipase (pancreas, small intestine) | Fatty acids + glycerol |
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.
- 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.
- Starch is not glucose: They are related but distinct molecules. Only glucose gives a positive Benedict's test result.
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: any factor that slows down the rate of photosynthesis when in short supply.
- 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 made by the liver, stored in the gallbladder, 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.
- Dietary fibre: indigestible plant material that helps food move through the gut.