Bile is not an enzyme it emulsifies fat to increase surface area for lipase.
Part 2 of 8Learning objectives
1
Describe photosynthesis with word and balanced symbol equations.
2
Explain how light intensity, CO2 concentration and temperature affect the rate of photosynthesis.
3
Relate the structure of a leaf and the role of mineral ions to plant growth.
4
Describe the components of a balanced diet and the functions of the alimentary canal.
5
Explain how peristalsis moves food through the gut.
Part 3 of 8Essential knowledge
1
Process of photosynthesis
Process of photosynthesis chlorophyll in chloroplasts absorbs light energy, which is used to join carbon dioxide and water into glucose; oxygen is given off.
Photosynthesis: light, carbon dioxide and water used to produce glucose and oxygen
Glucose may be respired, stored as insoluble starch, used to make cellulose for cell walls, converted to sucrose for transport in the phloem,
or combined with nitrate ions to make amino acids.
2
Factors affecting the rate
Factors affecting the rate increasing light intensity or CO2 concentration raises the rate until another factor becomes limiting, when the graph levels off. Temperature affects the enzymes that control photosynthesis: the rate rises to an optimum, then falls as enzymes denature.
3
Leaf structure and adaptations
Leaf structure and adaptations
leaves are broad and flat (large surface area for light) and thin (short diffusion distance for gases).
A waxy cuticle reduces water loss
the transparent upper epidermis lets light through
the palisade mesophyll is packed with chloroplasts near the top surface
the spongy mesophyll has air spaces for gas diffusion
stomata controlled by guard cells allow gas exchange
vascular bundles bring water (xylem) and remove sugars (phloem).
Leaf cross-section and adaptations for photosynthesis
4
Plants and mineral ions
Plants and mineral ions nitrate ions are needed to make amino acids and proteins for growth; magnesium ions are needed to make chlorophyll. Shortage of nitrate causes stunted growth and yellow leaves; shortage of magnesium causes yellowing between the veins.
Healthy plant compared with nitrate and magnesium deficiency
5
Practical — investigating photosynthesis
Practical — investigating photosynthesis
destarch a plant by leaving it in the dark for 48 hours,
then test leaves for starch after exposure to different conditions: cover part of a leaf with foil (light needed), use a variegated leaf,
or enclose a leaf with sodium hydroxide / soda lime to absorb CO2 (CO2 needed).
(chlorophyll needed)
The rate can be measured by counting oxygen bubbles from pondweed (e.g. Elodea) at different lamp distances, keeping temperature constant with a water bath.
6
Balanced diet
Balanced diet carbohydrates and lipids provide energy; proteins are for growth and repair; vitamin A (vision), vitamin C (prevents scurvy), vitamin D (calcium absorption, strong bones); calcium (bones and teeth) and iron (haemoglobin, prevents anaemia); fibre helps peristalsis and prevents constipation; water is the solvent for reactions.
Energy requirements depend on activity level, age and pregnancy.
7
Alimentary canal structure and function
Alimentary canal structure and function the processes are ingestion, digestion, absorption, assimilation and egestion.
Human digestive system and alimentary canal
Food passes mouth → oesophagus → stomach → small intestine → large intestine (colon) → rectum → anus.
(duodenum and ileum)
The salivary glands, pancreas and liver add secretions.
Villi in the ileum have microvilli, thin walls one cell thick and a rich capillary network plus a lacteal, giving fast absorption.
8
Peristalsis
Peristalsis the gut wall has circular and longitudinal muscles. Circular muscles contract behind the food bolus (narrowing the gut) while longitudinal muscles relax, and in front the reverse happens, so a wave pushes food forward. Dietary fibre gives the muscles something to push against.
9
Practical — energy in food
Burn a weighed food sample beneath a boiling tube holding a measured volume of water and record the temperature rise. Energy per gram (J/g) = mass of water (g) × temperature rise (°C) × 4.2 ÷ mass of food (g). Example: 20 g of water warmed by 15 °C by 2.0 g of food gives 630 J/g.
10
Why calorimetry values are too low
Much of the heat escapes to the air and the glass instead of the water, and the food may not burn completely. Improve accuracy by holding the burning food close to the tube, shielding or insulating the tube, relighting the sample until it no longer burns, or using a calorimeter.
11
Testing the gas from pondweed
Collect the bubbles from pondweed in an upturned water-filled test tube over a funnel. The gas relights a glowing splint, showing it is oxygen made in photosynthesis. The volume collected per minute can be used as a measure of rate.
12
Deficiency of vitamin D and calcium
Too little vitamin D in children causes rickets, with soft, poorly formed bones, because calcium is not absorbed well. A long-term shortage of calcium weakens bones and teeth and can lead to fragile bones (osteoporosis) later in life.
13
Changing nutritional needs
Growing children need relatively more protein for growth. During pregnancy extra energy, calcium and iron are needed to build the fetus's bones, teeth and blood; breastfeeding mothers need extra energy and calcium to make milk. Energy needs fall as adults get older and less active.
14
Where digestive enzymes are made
Amylase
salivary glands and pancreas.
Protease: stomach (pepsin), pancreas and small intestine.
Lipase: pancreas.
Maltase: small intestine lining.
The pancreas releases its enzymes into the duodenum in an alkaline juice, so the small intestine works at about pH 8–9.
15
Mechanical vs chemical digestion
Mechanical digestion (chewing, stomach churning, emulsification by bile) breaks food into smaller pieces without breaking chemical bonds, increasing the surface area for enzymes. Chemical digestion uses enzymes to break the bonds in large molecules, producing small soluble molecules.
16
Absorption routes
Glucose and amino acids pass into the blood capillaries of the villi; fatty acids and glycerol enter the lacteal (lymph). Most water is absorbed by osmosis in the small intestine, and the colon absorbs much of the rest.
17
Reading limiting-factor graphs
Where the curve is rising, the factor on the x-axis is limiting the rate. Where the curve has levelled off, a different factor (light, CO2 concentration or temperature) is now limiting. Water is rarely limiting because plants need very little of it for photosynthesis.
Part 4 of 8Process & sequence
Light energy is absorbed by chlorophyll in chloroplasts of palisade cells.
Carbon dioxide diffuses into the leaf through open stomata and air spaces.
Water is absorbed by root hairs and carried up the xylem to the leaf.
Light energy is converted to chemical energy, building glucose from CO2 and H2O.
Oxygen is released as a by-product and diffuses out through stomata.
Glucose is used for respiration or converted to starch, cellulose, sucrose, proteins and lipids.
From leaf to glucosewrite these steps in this order; each step is a marking point.
Light absorbed by chlorophyll
CO2 enters through stomata
Water arrives through xylem
Glucose made in chloroplast
Oxygen released
Glucose stored as starch
Part 5 of 8Worked examples
A student moves a lamp from 10 cm to 20 cm away from pondweed. Using the inverse square law, how does light intensity change, and what happens to the bubble rate if light is limiting?
Light intensity is proportional to 1/d2.
Doubling the distance gives (1/2)2 = 1/4 of the original intensity.
If light is the limiting factor, fewer oxygen bubbles are produced per minute because the rate of photosynthesis falls.
Explain why the rate of photosynthesis levels off at high light intensity.
At high light intensity light is no longer limiting.
Another factor, such as carbon dioxide concentration or temperature, is now in short supply and limits the rate,
so increasing light further has no effect.
Part 6 of 8Summary
Photosynthesis turns light energy into chemical energy stored in glucose.
Rate is controlled by the limiting factor: light, CO2 or temperature.
Leaves are adapted for gas exchange and light absorption.
A balanced diet supplies all nutrients in the right amounts.
Enzymes digest food; villi absorb the products in the small intestine.
Part 7 of 8Keep going
Nutrition Edexcel IGCSE Biology 4BI1: what to do next
These Nutrition Edexcel IGCSE Biology 4BI1 notes covered the key ideas, worked examples and exam technique for this topic. Check your Nutrition Edexcel IGCSE Biology 4BI1 understanding with the practice questions below, then reread any Nutrition Edexcel IGCSE Biology 4BI1 point you were unsure about before moving on.
