Nutrition (Edexcel IGCSE Biology)

Water is a reactant in photosynthesis. It reacts with carbon dioxide to produce glucose and oxygen.

False.

Light is required for photosynthesis, but it is not a chemical substance and so is not considered to be a reactant in photosynthesis.

The chemical formula for glucose is C₆H₁₂O₆.

The products of photosynthesis are glucose and oxygen.

False.

Chlorophyll is not a reactant, and so is not used up during the photosynthesis reaction. The role of chlorophyll is to absorb light energy.

6 molecules of water and 6 molecules of carbon dioxide are needed to produce a single glucose molecule.

True.

The photosynthesis equation is the reverse of the equation for aerobic respiration.

The balanced chemical symbol equation for photosynthesis is:

• 6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂

A limiting factor is a factor that limits the rate of photosynthesis.

The three main limiting factors for photosynthesis are:

• Temperature

• Light intensity

• Carbon dioxide concentration

False.

Water is not considered a limiting factor in photosynthesis; this is because plants need relatively little water for photosynthesis, and a lack of water also influences a plant's ability to absorb light and carbon dioxide.

Photosynthesis occurs slowly at low temperatures because molecules (such as enzymes involved in photosynthesis) have little kinetic energy. This means that fewer successful collisions take place.

False.

Rate of photosynthesis only increases with temperature up to a certain point, after which higher temperatures lead to a decrease in the rate of photosynthesis.

At very high temperatures the enzymes that control photosynthesis denature, meaning that the reaction can no longer take place.

False.

In a graph of light intensity against rate of photosynthesis, the level part of the graph shows a point at which a factor other than light intensity is limiting, e.g. temperature or carbon dioxide concentration.

The higher the light intensity, the faster the rate of photosynthesis. This continues until another factor becomes limiting.

The higher the carbon dioxide concentration, the faster the rate of photosynthesis. This continues until another factor becomes limiting.

True.

In a graph of carbon dioxide concentration against rate of photosynthesis, the part of the graph where the line is horizontal is a point at which a factor other than carbon dioxide concentration is limiting. This factor could be temperature or light intensity.

Gas exchange in plants occurs via the stomata. Carbon dioxide diffuses into leaves while oxygen diffuses out.

True.

The waxy cuticle is a waterproof layer that reduces water loss by evaporation from the surface of leaves.

The palisade mesophyll layer contains tall, thin cells that pack together closely. These cells contain many chloroplasts to maximise light absorption.

True.

The air spaces allow the outer surfaces of many cells in the spongy mesophyll to come into direct contact with the air. This maximises the available surface area for gas exchange.

Guard cells can change shape to open or close the stomata. This allows plants to control the rate of water loss from leaves.

Vascular bundles in plants contain xylem and phloem for transport of useful substances, such as water, around the plant.

False.

Phloem vessels carry sucrose and amino acids around inside plants. It is the xylem vessels that carry water and dissolved minerals.

It is beneficial for leaves to be broad and thin because:

• Being broad maximises surface area for light absorption

• Being thin reduces diffusion distance for gas exchange

True.

The palisade mesophyll layer is located just beneath the waxy cuticle and upper epidermis. This means that it is ideally located for light absorption.

False.

The cells of the spongy mesophyll are not especially tall and narrow; this description applies to the cells of the palisade mesophyll.

False.

Plants make carbohydrates during photosynthesis. They can convert carbohydrates into other substances, but may need additional mineral ions from the soil to produce certain molecules.

Nitrogen is present in proteins but not carbohydrates.

Two essential mineral ions required by plants are:

• Nitrates

• Magnesium ions

True.

Nitrates are needed for the production of amino acids, which are joined together to build proteins.

Plants obtain these mineral ions by active transport from the soil into root hair cells.

Magnesium ions are needed for the production of chlorophyll, which is essential for light absorption in photosynthesis.

Yellowing leaves and stunted growth are symptoms of nitrate deficiency.

A symptom of magnesium deficiency in plants is yellowing leaves, particularly in between the leaf veins.

Evolution of oxygen gas can be tested by placing a water plant under water and collecting bubbles of oxygen gas using an upturned funnel and a boiling tube. The presence of oxygen can be tested for by relighting a glowing splint in the gas collected.

The purpose of de-starching the plant by placing it in the dark for 24 hours is to ensure any pre-existing starch in the leaves is used up so it does not affect the results.

True.

Removing chlorophyll by boiling in ethanol is an important step in testing leaves for starch. Removing the green colour allows the results of the starch test to be seen more clearly.

Iodine solution is used to test for starch; it turns blue-black in the presence of starch.

False.

A leaf that has been de-starched and then covered with foil will not turn blue-black when iodine is added but will remain yellow-brown. This is because it will not have had light for photosynthesis, so will not have produced any new starch.

Potassium hydroxide absorbs carbon dioxide from the air. Enclosing a leaf with potassium hydroxide removes carbon dioxide from the air and so prevents photosynthesis from occurring; this demonstrates the need for carbon dioxide in photosynthesis.

A variable that should be kept the same (control variable) when testing the requirement for carbon dioxide in photosynthesis could be:

• Light intensity / whether or not the leaf was exposed to light

• The presence of chlorophyll

• The temperature of the room

This is part of the CORMMS criteria for planning an investigation.

Flames should not be present when boiling leaves in ethanol because ethanol is highly flammable. A water bath should be used for this step.

A variegated leaf has regions of green and white, where the green regions do contain chlorophyll but the white regions do not. Demonstrating that, after exposure to light, the green regions do contain starch while the white regions do not, shows that chlorophyll is needed for photosynthesis.

The independent variable when testing for the importance of chlorophyll in photosynthesis is whether the area of leaf tested is green or white (i.e. whether or not the leaf contains chlorophyll).

This is part of the CORMMS criteria for planning an investigation.

False.

A balanced diet consists of all nutrients in their correct proportions, not necessarily equal quantities.

A balanced diet consists of all essential nutrients in their correct proportions.

The key components of a balanced diet are:

• Carbohydrates

• Proteins

• Lipids

• Dietary fibre

• Vitamins

• Minerals

• Water

True.

Malnutrition can occur when diet is not balanced. This can result from deficiency and/or excess of a particular type of nutrient.

Minerals are chemical elements essential for many processes in the body. Examples include iron and calcium ions.

True.

Calcium is an important mineral required in the formation of teeth and bones.

True.

Carbohydrates, such as sugars and starch, are a source of energy for the body.

Proteins are essential for repair and growth.

Lipids, e.g. fats and oils, act as an energy store, protect vital organs, and help reduce heat loss.

Carbohydrates can be obtained from:

• Pasta

• Rice

• Potatoes

• Cereals

• Bread

• Sugary foods, e.g. fruit juice, chocolate, biscuits

True.

Vitamins A is needed for the production of pigments in the retina. Sources of vitamin A include meat, dairy, eggs and leafy greens.

True.

Water is the main component of the blood, where it is essential for carrying nutrients and waste products, and regulating body temperature. Water is also the main component of cell cytoplasm.

Dietary fibre provides bulk in the intestines, aiding the movement of solid waste and preventing constipation.

Iron is essential for production of haemoglobin, which carries oxygen in the blood.

True.

Nuts, pulses, eggs, meat and fish are all good sources of protein.

False.

Calcium is needed for strong bones and teeth. Connective tissue problems are likely to result from vitamin C deficiency, as vitamin C is needed for production of collagen in, e.g. skin and hair.

Vitamin D aids absorption of calcium, so is essential for strong bones and teeth.

True.

Muscle cells have a higher metabolic rate than other cell types so will need more glucose to fuel respiration.

Examples of factors that affect an individual's energy requirements include:

• Age

• Height

• Biological sex

• Activity levels

• Pregnancy

• Breastfeeding

False.

An individual's energy requirements vary throughout their lifetime based on changing factors such as age and activity levels.

Breastfeeding increases an individual's energy requirements. A breastfeeding mother will need to increase energy intake to provide energy for the child.

Higher activity levels increase energy requirements because of the need to provide energy for contracting muscles.

Energy requirements may change with age as follows:

• Children need more energy for growth

• Older individuals requiring fewer calories due to decreased activity levels

False.

Energy requirements are influenced by various factors beyond height and weight, including activity levels, age, biological sex and pregnancy.

Energy requirements change during pregnancy because pregnant women need to supply energy to support the developing fetus.

The function of the digestive system is to break down large, insoluble food molecules into small, soluble molecules that can be absorbed.

The alimentary canal is the passage through which food travels in the body, starting at the mouth and ending at the anus.

False.

The digestive system includes both the alimentary canal and accessory organs that aid in digestion.

The organs of the alimentary canal through which food passes are:

• Mouth

• Oesophagus

• Stomach

• Small intestine (duodenum and ileum)

• Large intestine

• Rectum

Other organs, e.g. the pancreas, are not in this list because food does not directly pass through them.

True.

Teeth grind food in the mouth; this is mechanical digestion.

Saliva contains amylase enzymes that begin the chemical breakdown of starch into sugars. Saliva also moistens food to aid in swallowing.

The oesophagus is the tube through which food passes between the mouth and the stomach.

True.

Muscular contractions of the stomach wall play a role in mechanical digestion, while protease enzymes start to chemically break down the proteins in food.

False.

The duodenum is the first part of the small intestine, followed by the ileum.

The role of the large intestine is to allow absorption of water from solid waste before it is egested; the remaining solid waste makes up the faeces.

Solid waste is stored in the rectum before egestion.

False.

Accessory organs, such as the pancreas, are not part of the tube through which food passes. Their role is to produce substances, such as enzymes, needed for digestion.

Peristalsis is a mechanism involving wave-like muscle contractions that propel food along the alimentary canal.

False.

Peristalsis occurs throughout the length of the alimentary canal, e.g. also in the oesophagus and large intestine.

Circular muscles in the walls of the alimentary canal contract to reduce the diameter of the tube. When this occurs behind a mass of food it pushes food along the digestive tract.

False.

Longitudinal muscles contract to shorten the length of the alimentary canal during peristalsis. Contraction of circular muscles reduce its diameter.

Mucus in the alimentary canal lubricates the food, reducing friction and aiding the movement of food during peristalsis.

Dietary fibre provides roughage against which the muscles can push during peristalsis.

Enzymes are biological catalysts that speed up chemical reactions without being consumed or changed in the process.

The three main types of digestive enzymes are:

• Carbohydrase

• Protease

• Lipase

Carbohydrase enzymes break down carbohydrates into simple sugars, e.g. glucose.

True.

Amylase breaks down starch into maltose, and then maltase breaks down maltose into glucose.

False.

Maltase breaks down maltose into glucose. This is the second part of starch digestion and occurs after amylase has first broken down starch into maltose.

Carbohydrase enzymes are produced in the:

• Salivary glands

• Pancreas

• Small intestine

False.

Protease enzymes break down proteins into amino acids. Fatty acids are a product of lipid digestion.

Lipase enzymes carry out lipid digestion.

Lipids (fats) are broken down into glycerol and fatty acids by lipase enzymes.

The pancreas produces the following digestive enzymes:

• Lipase

• Amylase

• Protease

Bile is produced by cells in the liver.

Bile is stored in the gall bladder before it is released into the small intestine.

False.

Food does not pass directly through the liver and the gall bladder, so they are accessory organs and do not form part of the alimentary canal.

Bile's main roles are neutralising stomach acid and emulsifying fats.

Emulsification is the process of breaking apart large fat droplets into smaller ones, increasing the available surface area for enzyme action.

True.

Bile is alkaline so works to neutralise hydrochloric acid from the stomach. This creates a suitable pH for enzyme activity in the small intestine.

An alkaline pH is needed in the small intestine because this is the optimum pH for the enzymes that function in this part of the digestive system.

Emulsification increases the surface area of fat droplets, increasing the efficiency of fat digestion by lipase enzymes in the small intestine.

False.

The enzymes of the small intestine work best at a slightly alkaline pH, hence the importance of bile neutralising the stomach acid as it enters the small intestine.

False.

Emulsification breaks large fat droplets into smaller ones but does not involve the breaking of any chemical bonds within fat molecules. Chemical breakdown requires the breaking of bonds.

The majority of digested food is absorbed in the small intestine.

Absorption is the process of moving small, digested food molecules from the digestive system into the blood.

Peristalsis helps to move food through the small intestine.

Features of the small intestine that increase its surface area are:

• It is very long

• It has villi

• The cells of the villi have microvilli

Features of villi which aid absorption are:

• Large surface area

• Short diffusion distance

• Well supplied with blood capillaries

True.

A good blood supply aids absorption as it:

• Shortens the diffusion distance between the intestine and the blood stream

• Maintains concentration gradients as the blood flow removes molecules as soon as they are absorbed

Microvilli further increase the surface area of villi, increasing the rate at which nutrients are absorbed into the bloodstream.

False.

Villi are surrounded by a single layer of cells; this keeps the diffusion distance between the intestine and the blood as short as possible.

True.

When burning food samples the chemical energy stored in the food is released during combustion. More stored energy will result in more energy being transferred.

The variable that is changed between tests is the type of food in the sample being burned. This is known as the independent variable.

This is part of the CORMMS criteria for planning an investigation.

Factors that should be kept the same when investigating the energy content of food include:

• The mass of the food samples should be as similar as possible

• The volume of water

• The distance between the burning food and the water

• The food samples must always be allowed to burn until there is nothing left to burn

This is part of the CORMMS criteria for planning an investigation.

It is important to repeat an investigation several times in order to assess the reliability of results. Repeats allow us to identify anomalies and calculate averages.

In an investigation into the energy content of food the measured variable is the change in temperature of the water (°C).

This is part of the CORMMS criteria for planning an investigation.