IGCSE Biology Edexcel - Nutrition Notes

2.18 The Process of Photosynthesis

Photosynthesis Theory:
- Photosynthesis is an endothermic reaction where sunlight energy is transferred to chloroplasts in green plants.
- Chlorophyll, a green pigment inside chloroplasts, absorbs sunlight energy.
- Green plants utilize this energy to produce glucose (a carbohydrate) from carbon dioxide and water.
- Oxygen is produced as a waste product during this process.
- Photosynthesis: the process by which plants manufacture carbohydrates from raw materials using light energy.
Plants are:
- Autotrophs: They can create complex molecules like glucose from simple ones (carbon dioxide and water).
- Producers: They produce their own food, making them the first organisms in all food chains.
Products of Photosynthesis:
- Plants use glucose for energy in respiration.
- They also use it to:
  - Produce starch for storage.
  - Synthesize lipids as an energy source in seeds.
  - Form cellulose to construct cell walls.
  - Produce amino acids (proteins) by combining with nitrogen and other mineral ions absorbed by roots.

Fate of Glucose

Glucose produced in photosynthesis is used in various ways:
- Respiration: To release energy for the cell.
- Storage: Converted into insoluble starch for storage, which doesn't affect osmosis in cells.
- Cellulose: Used for strength in cell walls. Many glucose molecules joined together form cellulose.
- Energy Storage: Converted into fats and oils for energy storage in seeds.
- Transport: Converted into sucrose for transport around the plant.
- Amino Acids/Proteins: Combined with nitrates from the soil to form amino acids, which then form proteins.
Glucose Molecule: $C6H{12}O_6$

Raw Materials for Photosynthesis

Carbon dioxide and water are the raw materials.
Light energy is required but is not a substance, so it cannot be a raw material.

2.19 Photosynthesis Equations

Word Equation:
- Carbon Dioxide + Water → Glucose + Oxygen (in the presence of light and chlorophyll)
Reactants & Products:
- Carbon dioxide diffuses into the leaf through stomata.
- Water is taken up by the roots and transported to the leaves through the xylem.
- Glucose is used to make substances needed by the plant and for respiration to release energy.
- Oxygen diffuses out of the leaf through the stomata and is used in repiration.
Balanced Chemical Equation:
- $6CO2 + 6H2O → C6H{12}O6 + 6O2$ (in the presence of light and chlorophyll)
The photosynthesis equation is the reverse of the aerobic respiration equation.

2.20 Factors Affecting the Rate of Photosynthesis

Limiting Factors:
- Plants' photosynthesis rate is limited by the factor in shortest supply.
- A limiting factor restricts life processes due to its short supply in the environment.
- Water is necessary, but not typically a limiting factor, because the amount needed is less compared to the water transpired from the plant.
Three main limiting factors:
- Temperature
- Light Intensity
- Carbon Dioxide Concentration
- Number of Chloroplasts or amount of chlorophyll.
Temperature:
- Affects kinetic energy of particles, influencing the movement of carbon dioxide and water within the plant.
- Lower temperature: less kinetic energy, resulting in fewer successful collisions.
- Increasing temperature: increases kinetic energy, increasing collisions between reactants and enzymes, thus forming more products.
- Higher temperatures: can denature enzymes controlling photosynthesis, reducing the rate.
Light Intensity:
- Affects the amount of energy the plant has for photosynthesis.
- More light leads to a faster rate of photosynthesis until another factor becomes limiting.
Carbon Dioxide Concentration:
- The more carbon dioxide available, the faster the rate of reaction, until another factor becomes limiting.
Chlorophyll:
- The more chloroplasts, the faster the photosynthesis rate.
- Factors affecting chlorophyll:
  - Diseases (e.g., tobacco mosaic virus).
  - Lack of nutrients (e.g., magnesium).
  - Loss of Leaves (fewer chloroplasts).

Interpreting Graphs of Limiting Factors:

Increasing Rate: Limiting factor is whatever is on the x-axis.
Rate Not Increasing: Limiting factor is something other than what is on the x-axis (temperature, light intensity, or carbon dioxide concentration).

2.21 Leaf: Structure & Adaptations

Leaf Structure:
- Leaves have complex structures with different tissues containing specially adapted cells.
Leaf Structures Table:
- Wax Cuticle: Protective layer that prevents water evaporation.
- Upper Epidermis: Thin and transparent to allow light to enter the palisade mesophyll layer.
- Palisade Mesophyll: Column-shaped cells packed with chloroplasts to absorb more light for photosynthesis.
- Spongy Mesophyll: Contains air spaces increasing the surface area to volume ratio for gas diffusion (mainly carbon dioxide).
- Lower Epidermis: Contains guard cells and stomata.
- Guard Cell: Absorbs and loses water to open and close stomata, allowing carbon dioxide to diffuse in and oxygen out.
- Stomata: Where gas exchange takes place; opens during the day and closes at night. Evaporation of water also takes place here. Greater concentration on the underside of the leaf to reduce water loss.
- Vascular Bundle: Contains xylem and phloem to transport substances to and from the leaf.
- Xylem: Transports water into the leaf for mesophyll cells to use in photosynthesis and for transpiration from stomata.
- Phloem: Transports sucrose and amino acids around the plant.

Adaptations of Plant Leaves for Photosynthesis Table

Large Surface Area: Increased surface area for the diffusion of carbon dioxide and absorption of light for photosynthesis.
Thin: Allows carbon dioxide to diffuse to palisade mesophyll cells quickly.
Chlorophyll: Absorbs light energy so that photosynthesis can take place.
Network of Veins: Allows the transport of water to leaf cells and carbohydrates from the leaf for photosynthesis.
Stomata: Allows carbon dioxide to diffuse into the leaf and oxygen to diffuse out.
Epidermis (Thin and Transparent): Allows more light to reach the palisade cells.
Thin Cuticle (Wax): Protects the leaf without blocking sunlight.
Palisade Cell Layer (Top of Leaf): Maximizes light absorption as it directly hits chloroplasts in the cells.
Spongy Layer: Air spaces allow carbon dioxide to diffuse through the leaf, increasing the surface area.
Vascular Bundles: Thick cell walls support stem and leaf.

2.22 Plants & Mineral Ions

Mineral Ions:
- Photosynthesis provides carbohydrates, but plants need other biological molecules like proteins, lipids, and nucleic acids (DNA).
- Plants make these substances themselves as they do not eat.
- Carbohydrates contain carbon, hydrogen, and oxygen, while proteins contain nitrogen.
Two Main Mineral Ions:
- Nitrogen
- Magnesium
- Without these, plants cannot photosynthesize or grow properly, obtaining these elements as mineral ions actively absorbed from the soil by root hair cells.
- 'Mineral' is a term describing any naturally occurring inorganic substance.
Mineral Ion Function and Deficiencies in Plants Table:
- Magnesium:
  - Function: Needed to make chlorophyll.
  - Deficiency: Causes yellowing between the veins of leaves (chlorosis).
- Nitrate:
  - Function: Source of nitrogen needed to make amino acids (to build proteins).
  - Deficiency: Causes stunted growth and yellowing of leaves.

2.23 Practical: Investigating Photosynthesis

Practical: Evolution of Oxygen
- Demonstrate oxygen evolution from photosynthesis using water plants like Elodea or Cabomba (pondweed).
- Oxygen gas produced is released as photosynthesis occurs.
- The oxygen released can be seen as bubbles leaving the cut end of the pondweed.
Apparatus:
- Beaker
- Water Plant
- Funnel
- Boiling Tube
- Splint
- Bunsen burner (for oxygen test)
- Heat proof mat
Method:
- Submerge a bundle of water plant shoots in a beaker of water underneath an upturned funnel.
- Fill a boiling tube with water and place it over the end of the funnel.
- As oxygen is produced, the bubbles of gas collect in the boiling tube and displace the water.
Results and Analysis:
- Show that the gas collected is oxygen by relighting a glowing splint.
Practical: Investigating Light & Photosynthesis
- Leaves cannot be directly tested for glucose as it is quickly used up, converted, transported, or stored as starch.
- Testing a leaf for starch indicates photosynthesis activity.
- Starch is stored in the chloroplasts where photosynthesis occurs.
Apparatus:
- Beakers
- Leaf tissue
- Bunsen burner
- Tripod
- Gauze platform
- Prongs
- Ethanol
- Apron
- Safety goggles
- Gloves
- Iodine solution
- White tile
Investigating Light:
- Method Part 1:
  - Destarch the plant by placing it in a dark cupboard for 24 hours.
  - Partially cover a leaf with aluminum foil and place the plant in sunlight for a day.
  - Remove the covered leaf and test for starch using iodine (Method Part 2).
- Method Part 2: Testing for Starch:
  - Drop the leaf in boiling water to kill tissue and break down cell walls.
  - Transfer the leaf into hot ethanol in a boiling tube for 5-10 minutes to remove chlorophyll.
  - Rinse the leaf in cold water to soften the tissue.
  - Spread the leaf out on a white tile and cover it with iodine solution.
- Results and Analysis:
  - The entire green leaf will turn blue-black if photosynthesizing.
  - The aluminum foil covered area will remain orange-brown (no sunlight, no photosynthesis), while the exposed area turns blue-black.
- This shows that light is needed for photosynthesis and starch production.
Safety:
- Ethanol is flammable; turn off the Bunsen burner when using it.
- Heat ethanol in an electric water bath instead of over an open flame.
Applying CORMS Evaluation:
- C (Change): Presence or absence of light.
- O (Organism): Leaves from the same plant species, age, and size.
- R (Repeat): Repeat the investigation for reliable results.
- M1 (Measurement 1): Observe the color change of the leaf when iodine is applied.
- M2 (Measurement 2): After a day.
- S (Same): Control room temperature.
Practical: Investigating Carbon Dioxide & Photosynthesis
- Use the iodine test for starch to investigate the requirement for carbon dioxide in photosynthesis.
Apparatus:
- Conical flasks
- Potassium hydroxide solution
- Clamps
- Clamp stands
- A plant
- Beakers
- Bunsen burner
- Tripod
- Gauze platform
- Prongs
- Ethanol
- Apron
- Safety goggles
- Gloves
- Iodine solution
- White tile
Method:
- Destarch the plant by placing it in a dark cupboard for 24 hours.
- Enclose one leaf with a conical flask containing potassium hydroxide (absorbs $CO_2$ ).
- Enclose another leaf with a conical flask without potassium hydroxide (control).
- Place the plant in bright light for several hours.
- Test both leaves for starch using iodine solution.
  - Drop the leaf in boiling water.
  - Transfer the leaf into hot ethanol in a boiling tube for 5-10 minutes.
  - Rinse the leaf in cold water.
  - Spread the leaf out on a white tile and cover it with iodine solution.
Results:
- The leaf from the flask with potassium hydroxide remains orange-brown (no $CO_2$ , no photosynthesis).
- The leaf from the flask without potassium hydroxide turns blue-black (all requirements met).
Applying CORMS Evaluation:
- C (Change): Presence or absence of carbon dioxide.
- O (Organism): Leaves from the same plant species, age, and size.
- R (Repeat): Repeat the investigation for reliable results.
- M1 (Measurement 1): Observe the color change of the leaf when iodine is applied.
- M2 (Measurement 2): After a day.
- S (Same): Control room temperature and light intensity.
Practical: Investigating Chlorophyll and Photosynthesis
- Testing a leaf for starch indicates photosynthesis activity.
- Use a variegated leaf (partially green, partially white) to test if chlorophyll is needed for photosynthesis.
Apparatus:
- Beakers
- Leaf tissue (variegated leaves)
- Bunsen burner
- Tripod
- Gauze platform
- Prongs
- Ethanol
- Apron
- Safety goggles
- Gloves
- Iodine solution
- White tile
Method:
- Drop the leaf in boiling water to kill the tissue and break down the cell walls.
- Transfer the leaf into hot ethanol in a boiling tube for 5-10 minutes to remove chlorophyll.
- Rinse the leaf in cold water to soften the leaf tissue.
- Spread the leaf out on a white tile and cover it with iodine solution.
Safety:
- Ethanol is flammable; turn off Bunsen burner when using it.
- Heat ethanol in an electric water bath instead of over an open flame.
Results and Analysis:
- Only green areas of the leaf (containing chlorophyll) stain blue-black.
- White areas (no chlorophyll) remain orange-brown (no photosynthesis, no starch).
Applying CORMS Evaluation:
- C (Change): Presence or absence of chlorophyll.
- O (Organism): Leaves from the same plant species, age, and size.
- R (Repeat): Repeat the investigation for reliable results.
- M1 (Measurement 1): Observe the color change of the leaf when iodine is applied.
- M2 (Measurement 2): After a day.
- S (Same): Control room temperature and light intensity.

2.24 Balanced Diet

Importance of a Balanced Diet:
- A balanced diet contains all the food groups in the correct proportions:
  - Carbohydrates
  - Proteins
  - Lipids
  - Dietary Fibre
  - Vitamins
  - Minerals (mineral ions)
  - Water
Malnutrition:
- An unbalanced diet can lead to malnutrition, causing various health problems.
Causes & Effects of Malnutrition Table:
- Starvation:
  - Cause: Taking in less energy than is used over a long period.
  - Effect: Body breaks down energy stores (fat and muscle), leading to weight loss and damage to the heart and immune system.
- Coronary Heart Disease:
  - Cause: Diet too high in saturated fat and cholesterol.
  - Effect: Fat deposits build up in arteries, reducing blood flow to the heart, potentially leading to heart attacks.
- Constipation:
  - Cause: Lack of fiber in the diet.
  - Effect: Food lacks bulk for muscles to push through the alimentary canal, increasing the risk of bowel cancer.
- Obesity:
  - Cause: Taking in more energy than is used.
  - Effect: Extra energy is stored as fat, contributing to heart disease and type 2 diabetes.

2.25 Sources & Functions of Dietary Elements

Sources & Functions of Dietary Elements:
- Carbohydrate:
  - Function: Source of energy.
  - Sources: Bread, cereals, pasta, rice, potatoes.
- Protein:
  - Function: Growth and repair.
  - Sources: Meat, fish, eggs, pulses, nuts.
- Lipid:
  - Function: Insulation and energy storage.
  - Sources: Butter, oil, nuts.
- Dietary Fibre:
  - Function: Provides bulk (roughage) to push food through the intestine.
  - Sources: Vegetables, whole grains.
- Vitamins:
  - Function: Needed in small quantities to maintain health.
  - Sources: Fruits and vegetables.
- Minerals:
  - Function: Needed in small quantities to maintain health.
  - Sources: Fruits, vegetables, meats, dairy.
- Water:
  - Function: Needed for chemical reactions in cells.
  - Sources: Water, juice, milk, fruits, vegetables.
Vitamins and Minerals Functions and Sources:
- Calcium:
  - Function: Needed for strong teeth and bones; involved in blood clotting. Deficiency can lead to osteoporosis.
  - Sources: Milk, cheese, eggs.
- Vitamin D:
  - Function: Helps the body absorb calcium for strong bones and teeth.
  - Sources: Oily fish, dairy products, also made naturally by the body in sunlight.
- Vitamin C:
  - Function: Essential part of collagen protein, which makes up skin, hair, gums, and bones. Deficiency causes scurvy.
  - Sources: Citrus fruit, strawberries, green vegetables.
- Vitamin A:
  - Function: Needed to make the pigment in the retina for vision.
  - Sources: Meat, liver, dairy, leafy green vegetables like spinach,eggs.
- Iron:
  - Function: Needed to make hemoglobin, the pigment in red blood cells that transports oxygen.
  - Sources: Red meat, liver, leafy green vegetables like spinach.

2.26 Variation in Energy Requirements

Dietary Needs of Individuals:
- Nutritional requirements vary throughout an individual’s lifetime.
- Same food groups are required, but in different quantities, depending on factors such as:
  - Age
  - Height
  - Sex
  - Activity levels
  - Pregnancy
  - Breastfeeding
Variations in Dietary Requirements Table:
- Age:
  - Energy needs increase towards adulthood for growth. Children need a higher proportion of protein for growth. Energy needs decrease as adults age.
- Activity Levels:
  - The more active, the more energy is required for muscle contraction and respiration.
- Pregnancy:
  - Energy requirements increase to support fetal growth and the mother's larger mass. Extra calcium and iron are needed for fetal bone, teeth, and blood development.
- Breastfeeding:
  - Energy requirements increase, and extra calcium is still needed to make high-quality breast milk.
- Sex:
  - Males tend to have higher average energy requirements due to a larger proportion of muscle compared to fat.

2.27 Human Alimentary Canal: Structure & Function

Structure & Function of the Alimentary Canal:
- The digestive system is an organ system where several organs work together to digest and absorb food.
- Digestion is when large, insoluble molecules (e.g., starch, proteins) are broken down into smaller, soluble molecules that can be absorbed into the bloodstream.
- These smaller molecules (e.g., glucose, amino acids) are used for energy (via respiration) or building other molecules for growth, repair, and function.
- The human digestive system comprises the alimentary canal and accessory organs.
- The alimentary canal is the channel through which food passes, starting at the mouth and ending at the anus.
- Digestion occurs within the alimentary canal.
- Accessory organs produce substances needed for digestion (e.g., enzymes and bile), but food does not pass directly through them.
Alimentary Canal and Accessory Structures Table:
- Mouth/Salivary Glands:
  - Mechanical digestion occurs here; teeth break food into smaller pieces.
  - Amylase enzymes in saliva start digesting starch into maltose.
  - The tongue shapes food into a bolus (ball) and lubricates it with saliva for easy swallowing.
- Oesophagus:
  - Tube connecting the mouth to the stomach, where the food bolus goes after being swallowed.
  - Wave-like contractions (peristalsis) push the food bolus down without relying on gravity.
- Stomach:
  - Food is mechanically digested by churning actions while protease enzymes start chemically digesting proteins.
  - Hydrochloric acid kills bacteria in food and provides the optimum pH for protease enzymes.
- Small Intestine:
  - The first section (duodenum) is where food from the stomach finishes being digested by enzymes produced here and secreted from the pancreas.
  - The pH of the small intestine is slightly alkaline (around pH 8-9).
  - The second section (ileum) is where absorption of digested food molecules takes place.
  - The ileum is long and lined with villi to increase the surface area over which absorption can take place.
- Large Intestine:
  - Water is absorbed from the remaining material in the colon to produce feces.
  - Feces are stored in the rectum and removed through the anus.
- Pancreas:
  - Produces all three types of digestive enzymes: amylase, protease, and lipase.
  - Secretes enzymes in an alkaline fluid into the duodenum to raise the pH of fluid coming from the stomach.
- Liver:
  - Produces bile to emulsify fats (break large droplets into smaller droplets)—an example of mechanical digestion.
  - Amino acids not used to make proteins are broken down here (deamination), producing urea.
- Gall Bladder:
  - Stores bile to release into the duodenum as required.
Stages of Food Breakdown:
- Ingestion
- Mechanical Digestion
- Chemical Digestion
- Absorption
- Assimilation
- Egestion

2.28 Peristalsis

Peristalsis:
- Mechanism moving food along the alimentary canal.
- Muscles in the walls of the esophagus create waves of contractions, forcing the bolus along.
- Once in the stomach, the bolus is churned into chyme, continuing to the small intestine.
Controlled by:
- Circular Muscles: Contract to reduce the diameter of the lumen of the esophagus or small intestine.
- Longitudinal Muscles: Contract to reduce the length of that section of the esophagus or small intestine.
Mucus:
- Produced to lubricate the food mass and reduce friction.
Dietary Fiber:
- Provides the roughage required for the muscles to push against during peristalsis.

2.29 Role of Digestive Enzymes

Role of Digestive Enzymes:
- The purpose of digestion is to break down large, insoluble molecules into smaller, soluble molecules that can be absorbed into the bloodstream.
- Food is partially digested mechanically (by chewing, churning, and emulsification) in order to break large pieces of food into smaller pieces of food which increases the surface area for enzymes to work on.
- Chemical digestion is controlled by enzymes produced in different areas of the digestive system.
- Enzymes are biological catalysts - speed up chemical reactions without being used up.
Three main types of digestive enzymes:
- Carbohydrases
- Proteases
- Lipases
Carbohydrases:
- Break down carbohydrates to simple sugars such as glucose.
- Amylase breaks down starch into maltose; produced in the salivary glands, pancreas, and small intestine.
- Maltase then breaks down maltose into glucose.
Proteases:
- A group of enzymes that break down proteins into amino acids.
- Pepsin breaks down proteins into smaller polypeptide chains in the stomach.
- Proteases from the pancreas and small intestine break the peptides into amino acids.
Lipases:
- Enzymes that break down lipids (fats) to glycerol and fatty acids.
- Lipase enzymes are produced in the pancreas and secreted into the small intestine.
Pancreas:
- Accessory organ producing digestive enzymes and hormones regulating blood sugar (insulin and glucagon).

2.30 Bile: Production & Storage

Bile Production & Storage:
- Bile is an alkaline substance produced by cells in the liver.
- It is stored in the gallbladder before being released into the small intestine.

2.31 Bile: Function

The Role of Bile:
- Two main roles:
  1. Neutralizing the hydrochloric acid from the stomach due to its alkaline properties.
    - Essential as enzymes in the small intestine have a higher (more alkaline) optimum pH than those in the stomach.
  2. Breaking apart large drops of fat into smaller ones (emulsification), increasing their surface area.
    - The alkalinity and larger surface area allows lipase to chemically break down fat (lipids) molecules into glycerol and fatty acids faster.
Emulsification
- This is mechanical digestion, not chemical digestion - tearing a large piece of paper into smaller pieces.

2.32 Small Intestine: Structure & Adaptations

Absorption of Food & Water
- Stages of Food Breakdown:
  - Ingestion
  - Mechanical digestion
  - Chemical digestion
  - Absorption
  - Assimilation
  - Egestion
- Absorption is the movement of small digested food molecules from the digestive system into the blood (glucose and amino acids) and lymph (fatty acids and glycerol).
- Absorption occurs through diffusion and sometimes active transport.
- Water is absorbed (by osmosis) primarily in the small intestine, but also in the colon.
- Assimilation is the movement of digested food molecules into the body cells where they are used.
- Egestion is the passing out of undigested food (as feces) through the anus.
Adaptations of the Small Intestine
- Adapted for absorption: very long and has highly folded surface with millions of villi (tiny, finger-like projections).
- These adaptations massively increase the surface area of the small intestine, allowing absorption to take place faster and more efficiently
- Peristalsis helps by mixing together food and enzymes and by keeping things moving along the alimentary canal
Villi of the small intestine
- Villi have several specific adaptations which allow for the rapid absorption of substances:
- A large surface area to volume ratio
- Microvilli on the surface of the villus further increase the surface available for absorption
- A short diffusion distance
  - The wall of a villus is only one cell thick
- A steep concentration gradient
  - The villi are well supplied with a network of blood capillaries that transport glucose and amino acids away from the small intestine in the blood
  - A lacteal (lymph vessel) runs through the centre of the villus to transport fatty acids and glycerol away from the small intestine in the lymph
- Enzymes produced in the walls of the villi assist with chemical digestion
- The movement of villi helps to move food along and mix it with the enzymes present

2.33B Practical: Energy Content of a Food Sample

Practical: Energy Content of a Food Sample:
- Investigate the energy content of food in a simple calorimetry experiment.
Apparatus:
- Boiling tube
- Boiling tube holder
- Bunsen burner
- Mounted needle
- Measuring cylinder
- Balance/scales
- Thermometer
- Water
- Food samples
Method:
- Measure 25 $cm^3$ of water and pour it into the boiling tube.
- Record the starting temperature of the water.
- Weigh the initial mass of the food sample.
- Set fire to the sample of food using the Bunsen burner and hold the sample 2cm from the boiling tube until it has completely burned.
- Record the final temperature of the water.
- (Once cooled) weigh the mass of any remaining food and record
- Repeat the process with different food samples (popcorn, nuts, crisps).
Results:
- A larger increase in water temperature indicates a larger amount of energy contained by the sample.
- We can calculate the energy in each food sample using the following equation:
  - Energy transferred (J) = (mass of water (g) x 4.2 x temperature increase (°C)) ÷ (mass of food (g))
Limitations:
- Incomplete burning of the food sample
  - Solution: Relight the food sample until it no longer lights up
- Heat energy is lost to the surroundings
  - Solution: Whilst heat lost means that the energy calculation is not very accurate, so long as the procedure is carried out in exactly the same way each time (with the same distance between food sample and boiling tube), we can still compare the results
Applying CORMS Evaluation:
- Change - We are changing the type of food in the sample
- Organisms - This is not relevant to this investigation as we aren't using an organism
- Repeat - We will repeat the investigation several times for each food sample
- Measurement 1 - We will measure the change in temperature of the water
- Measurement 2 - The mass of the food will be measured after the food sample has burned out
- Same - We will control the volume of water used, the distance between the food sample and the boiling tube during burning, the food will also be relit every time it goes out until it no longer lights