Nutrition — Edexcel Linear - OL Revision Notes & Topical Questions

Plants use glucose as a source of energy in respiration.

A limiting factor is something present in the environment in such short supply that it restricts a life process; the rate of photosynthesis is limited by whichever factor is lowest.

Where the line is rising, the limiting factor is the one on the x-axis.

Photosynthesis therefore takes carbon dioxide out of the atmosphere and adds oxygen, whereas respiration does the reverse, and the two processes keep the atmosphere balanced.

Leaves have complex structures with layers of different tissues containing specially adapted cells.

Leaves are adapted for photosynthesis by having a large surface area, a palisade layer near the top that is hit directly by light, and an air-space-filled spongy layer.

Plants must make proteins, lipids and nucleic acids themselves because they do not eat. Carbohydrates contain C, H and O, but proteins also contain nitrogen. Two fundamental mineral ions are nitrate (for amino acids and proteins) and magnesium (for chlorophyll). A lack of nitrate leads to stunted growth; a lack of magnesium causes yellow leaves (chlorosis) and poor photosynthesis.

To show that oxygen is released, use a water plant such as Elodea in a beaker of water. Oxygen produced as bubbles leaves the cut end of the stem; counting the bubbles per minute estimates the rate. To show the need for light:

Glucose is not tested because it is quickly converted to other substances or stored as starch; starch is stored in chloroplasts and is a reliable indicator of photosynthesis.

To compare deficiencies

• a plant lacking nitrate has stunted growth and yellow older leaves, because nitrate is moved to new growth;
  • a plant lacking magnesium has yellow leaves because it cannot make chlorophyll.

Remember to name the correct tissue when explaining transport: xylem delivers the water and mineral ions the leaf needs, and phloem carries away the sucrose made, so photosynthesis in the mesophyll depends on both.

The leaf from the potassium hydroxide flask stays orange-brown because it could not photosynthesise without carbon dioxide; the control leaf turns blue-black because it had everything needed.

Safety

• ethanol is extremely flammable, so turn the Bunsen burner off at that stage and heat the ethanol in an electric water bath, not a beaker over a flame.
• Wear eye protection;
  • potassium hydroxide is corrosive.

CORMS questions will probably ask about unfamiliar experiments, so practise applying CORMS to many different practical scenarios.

A balanced diet contains all the food groups in the correct proportions. An unbalanced diet can lead to malnutrition, which causes many health problems.

The amount of energy needed depends on age, activity and sex; young people and active people need more. For example, teenagers and pregnant women need more energy, and a manual worker needs more than an office worker.

In the carbon dioxide experiment the conclusion depends on comparing the two leaves: if both leaves turned blue-black, carbon dioxide may still have entered the flask, so the seal must be airtight.

Hint for examswhen asked to name a variable that was controlled, give a specific one such as the same plant, the same light intensity and the same time left in the light, rather than just saying “the conditions”.

The digestive system is an example of an organ system. It includes the organs of the alimentary canal (the tube food passes through) and the accessory organs (liver, gall bladder and pancreas) that work together to break large insoluble molecules into small soluble ones that can be absorbed.

Peristalsis is a mechanism that moves food along the alimentary canal. It is controlled by circular and longitudinal muscles in the gut wall.

The purpose of digestion is to break large, insoluble molecules into smaller, soluble molecules that can be absorbed into the bloodstream. Food is partly digested mechanically (chewing, churning, emulsification), which increases surface area, but digestion mainly takes place chemically: bonds holding the large molecules together are broken by enzymes made in different parts of the system.

Pepsin is made in the stomach and breaks proteins into smaller polypeptide chains; proteases made in the pancreas and small intestine break these chains into amino acids. The pancreas is an accessory organ: food does not pass through it, but it releases digestive enzymes into the small intestine and makes the hormone insulin.

Bile is an alkaline substance produced by cells in the liver and stored in the gall bladder before being released into the small intestine. It neutralises the acid from the stomach and its bile salts emulsify lipids, as described in the next lesson.

The wall of the alimentary canal has layers of muscle, glands and a lining that suits each region.

Bile has two roles.

Emulsification is like tearing a large piece of paper into smaller pieces: it is mechanical digestion, not chemical, because breaking something into smaller pieces does not break bonds or change the chemical structure of the molecules.

Absorption is the movement of small digested molecules from the digestive system into the blood (glucose and amino acids) and lymph (fatty acids and glycerol). Small soluble molecules move by diffusion and sometimes active transport; water is absorbed by osmosis, mainly in the small intestine but also in the large intestine.

The structure of a villus in relation to its function is a frequent exam question, so learn these adaptations and how each affects the rate of absorption.

Burn food samples to compare energy contents using simple calorimetry.

The larger the rise in water temperature, the more energy the food stored.

Sources of error

• some food may not burn completely, so not all its energy is released (solution: relight the sample until it will not light);
  • heat is lost to the surroundings (use a shield and insulation, and keep the tube close to the flame).
• The measured value is lower than the true one.