Exchanging Substances

Question 1

What is the relationship between the size of an organism and its surface area to volume ratio?

Answer 1

As size increases, surface area to volume ratio decreases.

Question 2

How is Surface Area to Volume Ratio calculated?

Answer 2

Surface area (for regular shapes: side length x side width x number of sides) divided by volume (length x width x depth)

Question 3

What is metabolic rate and how is it usually measured?

Answer 3

Metabolic rate is the amount of energy used up by an organism within a given period of time. It is often measured by oxygen uptake because oxygen is used in aerobic respiration to make ATP for energy release.

Question 4

What features do Unicellular Organisms have in relation to Exchanging Substances and what are the advantages and disadvantages of these features?

Answer 4

They have a large surface area to volume ratio, so they can absorb any substances required. They have a short diffusion distance between the outside of the organism to the centre of it, so they can quickly absorb substances from the environment. An advantage of this is that they can exchange materials with their environment. A disadvantage is that they lose heat energy and water quickly, so they can not survive extreme temperatures

Question 5

What features do Multicellular Organisms have in relation to Exchanging Substances and what are the advantages and disadvantages of these features?

Answer 5

They have a small surface area to volume ratio so can not absorb enough substances through small outer surface to support large volume. They have a large diffusion distance between the outside and the centre of the organism so diffusion through outer surface is too slow to supply cells efficiently. An advantage of this is that they lose less heat energy, so can survive in cold environments. A disadvantage is that they often need internal mass transport systems in order to supply the body with vital substances.

Question 6

State and explain the relationship between surface area to volume ratio and metabolic rate.

Answer 6

Organisms with a larger surface area to volume ratio (smaller organisms) have a higher metabolic rate as they lose heat more easily. Therefore, more energy and a higher metabolic rate is required to maintain a constant internal temperature. Per unit of body mass, metabolic rate is higher in small organisms.

Question 7

What are the behavioural and physical adaptations that organisms in cold environments have to prevent heat loss?

Answer 7

• Behavioural: Small mammals with a large surface area to volume ratio will lose heat easily so they need to eat high energy foods such as nuts and seeds to help maintain body temperature. They may also hibernate during winter.
• Physical: Adapted animals will have a compact body shape, giving a smaller surface area to volume ratio. Small mammals with larger surface area to volume ratio may have thick layers of fur to insulate and reduce heat loss.

Question 8

What are the behavioural and physical adaptations that organisms in hot environments have to prevent overheating?

Answer 8

• Behavioural: Large organisms such as hippos spend much of the day in water to help lose heat. Some other organisms may be nocturnal so that they are only active in cold temperatures (at night).
• Physical: Large organisms with low surface area to volume ratio often have large ears which increase their surface area allowing them to lose more heat.

Question 9

What are the behavioural and physical adaptations that organisms in dry environments have to prevent water loss?

Answer 9

• Behavioural: Organisms may be nocturnal so that they are most active in cooler temperatures, reducing the need for cooling by evaporative water loss (sweating), therefore conserving water.
• Physical: Small mammals with a high surface area to volume ratio have structural kidney adaptations so that they produce less urine to conserve water.

Question 10

What is the function of the Waxy Cuticle?

Answer 10

It is waterproof to prevent water loss by evaporation, and transparent to allow light to pass through.

Question 11

What is the function of the Upper Epidermis?

Answer 11

It protects the leaf and is 1 cell thick to allow the light to pass through

Question 12

What is the function of the Palisade Mesophyll?

Answer 12

It is a layer of cells containing large amounts of chloroplasts for photosynthesis.

Question 13

What is the function of the Spongy Mesophyll?

Answer 13

It has air spaces which increases the surface area for gas exchange. The cells within the spongy mesophyll also contain lots of chloroplasts.

Question 14

What is the function of the Xylem?

Answer 14

Transports water from the roots up the plant to the leaves.

Question 15

What is the function of the Phloem?

Answer 15

Transports nutrients, sugars and respiratory products up and down the plant

Question 16

What is the function of the Lower epidermis?

Answer 16

Gases enter and exit via the stoma, which opens and closes

Question 17

Describe the role of the Stomata and how they carry out this role.

Answer 17

Stomata control how much water leaves the plant by transpiration. If there is a higher water potential outside than there is inside the cell, water will move in via osmosis, and if there is a lower water potential outside than inside , water will move via osmosis. When plants have enough water, guard cells are turgid which keeps the pores open, and when plants don’t have enough water, guard cells become flaccid causing the pores to close.

Question 18

Draw and label a plant leaf.

Answer 18

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Question 19

Describe the pathway of air through the tracheal system of an insect

Answer 19

Air enters the trachea through pores on the surface of the exoskeleton called spiracles, which can open and close. Carbon Dioxide and Oxygen will diffuse in and out of the spiracles down the concentration gradient. The tracheae divide into smaller tubes called tracheoles which continue to divide until they branch off into individual body cells. The tracheoles are permeable to allow gas exchange

Question 20

Explain how an insect’s tracheal system is adapted for gas exchange

Answer 20

• Tracheoles have thin walls, so short diffusion distance to cells
• High numbers of highly branched tracheoles, so larger surface area for gas exchange
• Rhythmic contraction of abdominal muscles changes pressure in body, causing air to move in / out, maintaining concentration gradient for diffusion
• Fluid in end of tracheoles drawn into tissues by
  osmosis during exercise increases rate of diffusion
• Spiracles can open and close to maintain the concentration gradient

Question 21

Describe ventilation in insects

Answer 21

By contracting muscles between each body segment, the insect can compress the trachea and therefore pump gases in and out of its body.

Question 22

Describe the structure of fish gills

Answer 22

Each gill is made of lots of thin gill filaments which are attached to a bony gill arch. The gill filaments a covered in small, thin folds called lamellae, which have lots of blood capillaries and a thin layer of cells.

Question 23

What is Counter Current flow?

Answer 23

• Blood and water flow over the lamellae in opposite directions
• So blood is always flowing next to water that has a higher oxygen concentration
• So maintains a concentration gradient of oxygen between water and blood
• For diffusion happens along whole length of lamellae

Question 24

How are gills adapted for gas exchange?

Answer 24

• Gills made of many filaments covered with many lamellae, increasing surface area for diffusion
• Thin lamellae wall / epithelium, so short diffusion distance between water and blood
• Lamellae have a large number of capillaries which remove O2 and bring CO2 quickly so maintains concentration gradient
• Counter current flow system to maintain concentration gradient across the full length of the gill lamellae.

Question 25

Explain how the leaves of dicotyledonous plants are adapted for gas
exchange

Answer 25

They have many stomata , resulting in a large surface area for gas exchange. The spongy mesophyll contains air spaces, allowing a large surface area for gases to diffuse through. The leaves are thin, resulting in a short diffusion distance

Question 26

Explain structural and functional compromises in xerophytic plants that
allow efficient gas exchange while limiting water loss

Answer 26

• Leaves are spikes and therefore have a small surface area, reducing evaporation rate
• Sunken stomata trap water to maintain maintain humid air around the stomata to reduce the water potential gradient
• Stomatal hairs trap water to maintain maintain humid air around the stomata to reduce the water potential gradient
• Extensive root systems maximise water uptake. Some xerophytes have wide, shallow roots to collect rainwater, and other have deep roots to collect groundwater
• Reduced amount of stomata, reducing the amount of places water can evaporate from
• Thicker waxy cuticle to waterproof leaves and stem to reduce evaporation

Question 27

Describe the gross structure of the human gas exchange system

Answer 27

The Trachea spits into Bronchi, when then split into smaller Bronchioles. At the end of each Bronchiole is an Alveolus, surrounded by a capillary network

Question 28

Explain the essential features of the alveolar epithelium that make it
adapted as a surface for gas exchange

Answer 28

• 1 cell thick → short diffusion distance
• Folded → large surface area
• Permeable → allows diffusion of gases
• Moist → gases can dissolve for diffusion
• Good blood supply from large network of capillaries → maintains large concentration gradient

Question 29

Describe how gas exchange occurs in the lungs

Answer 29

Oxygen diffuses from the alveoli, across the alveolar epithelium and the capillary endothelium into blood down its concentration gradient. Carbon dioxide diffuses from the blood, across the capillary endothelium and the alveolar epithelium into the alveoli down its concentration gradient.

Question 30

Explain the importance of ventilation

Answer 30

Brings in air containing higher concentration of oxygen and removes air with lower concentration of oxygen, maintaining concentration gradients

Question 31

Explain how humans breathe in

Answer 31

1. Diaphragm muscles contract and the diaphragm therefore flattens
2. External intercostal muscles contract, and the internal intercostal muscles relax, so the ribcage is pulled up and out
3. This increases volume and decreases pressure in thoracic cavity
4. Air moves into lungs down pressure gradient

Question 32

Explain how humans breathe out

Answer 32

1. Diaphragm muscles relax and the diaphragm therefore moves upwards
2. External intercostal muscles relax, and the internal intercostal muscles contract, so the ribcage moves down and in
3. This decreases volume and increases pressure in thoracic cavity
4. Air moves out of lungs down pressure gradient

Question 33

Suggest why expiration is normally passive at rest

Answer 33

Internal intercostal muscles do not normally need to contract, and expiration is aided by elastic recoil in alveoli

Question 34

Suggest how different lung diseases reduce the rate of gas exchange

Answer 34

• Thickened alveolar tissue (eg. fibrosis) → increases diffusion distance
• Alveolar wall breakdown → reduces surface area
• Reduced lung elasticity → lungs expand / recoil less → reduces concentration gradients of O2 / CO2

Question 35

Suggest how different lung diseases affect ventilation

Answer 35

• Some lung diseases reduce lung elasticity (eg. fibrosis which is a build-up of scar tissue) → lungs expand / recoil less. This reduces volume of air in each breath (tidal volume) and also reduces the maximum volume of air breathed out in one breath (forced vital capacity)
• Some lung diseases narrow airways or reduce airflow in & out of lungs (e.g. asthma, which is inflamed bronchi) Therefore, reduces maximum volume of air breathed out in 1 second (forced expiratory volume)
• Some lung diseases reduce rate of gas exchange, resulting in increased ventilation rate to compensate for reduced oxygen in blood

Question 36

Suggest why people with lung disease experience fatigue

Answer 36

Cells receive less oxygen, so the rate of aerobic respiration is reduced and less ATP made

Question 37

Explain what happens in digestion

Answer 37

Large insoluble biological molecules are hydrolysed into smaller soluble molecules that are small enough be absorbed across cell membranes into the blood

Question 38

Describe the digestion of starch in mammals

Answer 38

• Amylase (produced by salivary glands and pancreas) hydrolyses starch into maltose
• Membrane-bound maltase (attached to cells lining ileum) hydrolyses maltose to glucose
• Hydrolysis of glycosidic bond

Question 39

Describe the digestion of lipids in mammals

Answer 39

• Bile salts (produced by liver) emulsify lipids causing them to form smaller lipid droplets
• This increases surface area of lipids for increased hydrolysis
• Lipase (made in pancreas) hydrolyses lipids into monoglycerides and fatty acids by hydrolysing the ester bonds

Question 40

Describe the digestion of proteins by a mammal

Answer 40

• Endopeptidases hydrolyse internal peptide bonds within the polypeptide chain. This results in the polypeptide chains being broken into shorter chains, so there are therefore more terminal ends for exopeptidases
• Exopeptidases hydrolyse peptides bonds at the terminal ends of the polypeptide chains, releasing singular amino acids
• Membrane bound dipeptidases hydrolyse the peptide bond between dipeptides, releasing 2 amino acids

Question 41

Why are membrane-bound enzymes are important in digestion?

Answer 41

Membrane bound enzymes are located on the cell membrane of the epithelial cells lining the ileum. By hydrolysing molecules at the site of absorption, they maintain concentration gradients for higher absorption rates

Question 42

Describe the absorption of amino acids and monosaccharides in mammals

Answer 42

1. Sodium ions are actively transported from the epithelial cells lining ileum to blood, establishing a concentration gradient of Sodium ions, as the concentration is now higher in lumen than the epithelial cell
2. Sodium ions enter the epithelial cell down its concentration gradient with co-transported glucose or amino acids. This occurs via a co-transporter protein. This establishes a concentration gradient of Glucose or Amino Acids, as the concentration is higher in the epithelial cell than in the blood
3. The Glucose or Amino Acids move down the concentration gradient into blood via facilitated diffusion

Question 43

Describe the absorption of lipids by a mammal

Answer 43

• Micelles carry bile salts, monoglycerides and fatty acids to the epithelial cells of the ilium and breakdown, allowing monoglycerides and fatty acids to diffuse across membrane because they are lipid-soluble.
• The monoglycerides and fatty acids are transported to the Endoplasmic Reticulum where they recombine to form triglycerides again.
• Inside the Golgi, the triglycerides bind with cholesterol and proteins to form chylomicrons.
• Chylomicrons travel in a vesicle to the cell membrane and leave the epithelial cell via exocytosis.
• The Chylomicrons enter lymphatic capillaries called lacteals which transport them away from the small intestine to tissues around the body, where the triglycerides can be hydrolysed and the fatty acids are used by the tissues.

Question 44

Describe the role of red blood cells and haemoglobin in oxygen transport

Answer 44

• Red blood cells contain lots of haemoglobin, which binds with Oxygen at gas exchange surfaces where partial pressure of Oxygen is high
• This forms oxyhaemoglobin which transports Oxygen (each haemoglobin protein can carry 4 oxygen molecules as it has 4 haem groups
• The haemoglobin unloads the oxygen near cells and tissues where the partial pressure of oxygen is low, so that the cells and tissues can respire

Question 45

Describe the structure of haemoglobin

Answer 45

Haemoglobin is a protein with a quaternary structure. It is made up of 4 polypeptide chains, and each chain contains a Haem group containing an iron ion

Question 46

Explain how the cooperative nature of oxygen binding results in an
Sigmoid oxyhaemoglobin dissociation curve

Answer 46

When each Oxygen molecule binds, the tertiary structure of the Haemoglobin changes, exposing more Haem group binding sites, meaning it is easier for Oxygen to bind to the Haemoglobin

Question 47

Describe evidence for the cooperative nature of oxygen binding

Answer 47

• At lower partial pressures of oxygen, as oxygen increases there is a slow increase in % saturation of Haemoglobin with oxygen
• At higher partial pressures of oxygen, as oxygen partial pressure increases, there is a faster increase in % oxygen saturation of Haemoglobin, showing it has become easier for oxygen molecules to bind

Question 48

What factors affect haemoglobin’s affinity for oxygen?

Answer 48

• Partial pressure of Oxygen
• Haemoglobin saturation
• Partial pressure of Carbon Dioxide

Question 49

Explain effect of CO2 concentration on the dissociation of oxyhaemoglobin

Answer 49

An increase in blood carbon dioxide levels due to increased rate of respiration results in the formation of Carbonic Acid, lowering the pH of the blood. This changes the tertiary structure of the Haemoglobin, meaning the Oxygen is unloaded more easily at any given partial pressure of oxygen.

Question 50

Explain the advantage of the Bohr effect during exercise

Answer 50

Oxygen is unloaded more quickly, meaning that the respiring tissues have access to more oxygen so they can aerobically respire at a faster rate. Therefore, more ATP is produced.

Question 51

Explain why different types of haemoglobin can have different oxygen transport properties

Answer 51

Different types of Haemoglobin are made up of polypeptide chains with different amino acid sequences, resulting in different tertiary structures, and therefore different affinities for oxygen

Question 52

Explain how organisms can be adapted to their environment by having different types of haemoglobin with different oxygen transport properties

Answer 52

• Left shift - Oxygen with bind with Haemoglobin at a lower partial pressure. For example, organisms in high altitude or underground environments and foetuses
• Right shift - Oxygen will dissociate from haemoglobin more easily. For example, organisms with high metabolic rate such as small mammals with a larger SA:V ratio, so will lose heat more easily)

Question 53

What type of circulatory system do mammals have?

Answer 53

Closed double circulatory system

Question 54

Describe the general pattern of blood circulation in a mammal

Answer 54

• Deoxygenated blood in right side of heart is pumped to the lungs, and the oxygenated blood returns to left side
• Oxygenated blood in left side of heart pumped to rest of body and the deoxygenated blood returns to right side

Question 55

Suggest the importance of a double circulatory system

Answer 55

• Prevents mixing of oxygenated / deoxygenated blood, so blood pumped around the body is fully saturated with oxygen for aerobic respiration
• Blood can be pumped to body at a higher pressure, so oxygen can reach the body cells more efficiently

Question 56

Name the blood vessels entering and leaving the heart and lungs
and state their function

Answer 56

• Vena cava – Transports deoxygenated blood from the respiring body cells to the heart
• Pulmonary artery - Transports deoxygenated blood from the heart to the lungs
• Pulmonary vein – Transports oxygenated blood from the lungs to the heart
• Aorta – Transports oxygenated blood from heart to the respiring body cells

Question 57

Which blood vessels provide the heart with oxygenated blood?

Answer 57

The coronary arteries

Question 58

Label a diagram to show the gross structure of the human heart

Answer 58

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Question 59

Suggest why the wall of the left ventricle is thicker than that of the right

Answer 59

The left side has thicker muscle to contract with greater force, so it can generate higher pressure to pump blood around entire body

Question 60

Explain the pressure & volume changes and associated valve movements
during the cardiac cycle that maintain a unidirectional flow of blood

Answer 60

• Atrial systole - The atria contract, so the volume decreases, increasing the pressure. The atrioventricular valves open when the pressure in the atria exceeds the pressure in the ventricles, so the blood is pushed into the ventricles
• Ventricular systole - The ventricles contract, so the volume decreases, increasing the pressure. The semilunar valves open when the pressure in the ventricles exceed the pressure in the arteries, so blood is pushed into the arteries
• Diastole - The atria and ventricles relax, so the volume increases, so the pressure decreases. The semi-lunar valves shut when the pressure in the arteries exceeds the pressure in the ventricles, so the blood enters the atria.

Question 61

At what points do the cardiac valves open and close?

Answer 61

• Semi-lunar valves close when the pressure in the artery is higher than the pressure in the ventricles
• Semi-lunar valves open when the pressure in the ventricle is higher than the pressure in the artery
• Atrioventricular valves close when the pressure in the ventricle is higher than the pressure in the atrium
• Atrioventricular valves open when the pressure in the atrium is higher than the pressure in the ventricle

Question 62

What is the equation for cardiac output?

Answer 62

Cardiac output (volume of blood pumped out of heart per minute) = stroke volume (volume of blood pumped in each heart beat) x heart rate (number of beats per minute)

Question 63

How can heart rate be calculated from cardiac cycle data?

Answer 63

Heart rate (beats per minute) = 60 (seconds) / length of one cardiac cycle (seconds)

Question 64

Explain how the structure of arteries and arterioles relates to their function

Answer 64

Their function is to carry blood away from the heart at high pressures. Adaptations include:
* Thick muscular walls - can withstand high pressure, muscles can contract and relax to control blood flow
* Narrow lumen - maintains high pressure

Question 65

Explain how the structure of veins relates to their function

Answer 65

Their function is to carry blood towards the heart at lower pressures. Adaptations include:
* Wide lumen - Maintains blood flow
* Valves - To prevent backflow of blood

Question 66

Explain how the structure of capillaries relates to their function

Answer 66

Their function is to allow efficient exchange of substances between blood and tissue fluid . Adaptations include:
* Walls are one cell thick - reduces diffusion distance
* Capillary bed is a large network - increases surface area for diffusion

Question 67

Explain the formation of tissue fluid

Answer 67

At the arteriole end of capillaries, there is a higher hydrostatic pressure inside the capillaries than outside the capillaries. This forces water and dissolved substances out of the capillaries into the surrounding tissues, and large plasma proteins remain in the capillary

Question 68

Explain the return of tissue fluid to the circulatory system

Answer 68

The water potential outside the capillary is higher than the water potential inside the capillary. Therefore, water enters the capillary via osmosis down a water potential gradient. Excess fluid enters the lymphatic system to become lymph, containing lymphocytes

Question 69

Suggest and explain causes of excess tissue fluid accumulation

Answer 69

• Low concentration of protein in blood plasma or high salt concentration will result in higher water potential, so the water potential gradient is reduced. This will mean more tissue fluid will be formed at arteriole end and less water absorbed at venule end by osmosis
• High blood pressure will result in high hydrostatic pressure. Therefore more tissue fluid will form at arteriole end and less water will be absorbed at venule end by osmosis

Question 70

What is a risk factor, and give an example for cardiovascular disease.

Answer 70

A risk factor is an aspect of a person’s lifestyle, body or environment which has been shown to increase their likelihood of getting a certain disease. Examples for cardiovascular disease include age, salt and fat concentration in diet, smoking, lack of exercise

Question 71

What is the function of xylem tissue?

Answer 71

Transports water and mineral ions up the stem to the leaves

Question 72

How is xylem tissue adapted for its function?

Answer 72

• Cells are joined together with no end walls, forming a long continuous tube, so water can flow as a continuous column
• Made up of dead cells which contain no cytoplasm or nucleus, allowing for easier water flow
• Thick walls made of lignin which provides support and allows the xylem to withstand tension
• Pits in side walls → allow lateral flow of water

Question 73

Explain the cohesion-tension theory of water transport in the xylem

Answer 73

lWater evaporates from the mesophyll due to heat from the sun (Transpiration). This lowers the water potential in the mesophyll, creating a water potential gradient. This causes water to be pulled up the xylem to replace the water lost in transpiration, and this creates tension. Water molecules are cohesive due to Hydrogen bonding, so they form a continuos column. The water molecules also show adhesion to the walls of the xylem vessel.

Question 74

Evidence to support cohesion-tension theory

Answer 74

• If a trunk or stem is damaged and a xylem cell is broken, water does not leak out. Once air enters, the tree can no longer draw up water because the continuous column of water has been broken
• The trunks of trees reduce in diameter during the daytime when transpiration is at its greatest rate. This is because adhesion of water molecules to the walls of the xylem results in tension, pulling the xylem walls in.

Question 75

Name and explain the 4 factors affecting transpiration rate

Answer 75

• Light Intensity - As light intensity increases, the rate of photosynthesis increases. This means that the stomata open to allow CO2 for photosynthesis to enter. This also results in more water being lost.
• Temperature - As temperature increases, the kinetic energy of the water molecules increases. This results in the water evaporating at a faster rate
• Wind intensity - As wind intensity increases, wind blows water molecules away from the stomata, increasing the water potential gradient, so water evaporates at a faster rate
• Humidity - At higher humidity levels, there is more water in the air, so there is a lower water potential gradient between the leaf and the air, so water evaporates at a slower rate

Question 76

What is the function of the phloem?

Answer 76

Transports products of photosynthesis in plants

Question 77

How is the phloem tissue adapted to its function

Answer 77

The sieve tubes have no nucleus and very few organelles, to maximise space for easier flow of substances. The end walls between the phloem cells are perforated to allow substances to pass between the individual cells.

The companion cells contain many mitochondria, to allow a high rate of respiration to make ATP for active transport of substances

Question 78

What is translocation?

Answer 78

The movement of solutes such as sucrose from sources to sinks

Question 79

Explain the mass flow hypothesis for translocation in plants

Answer 79

At the source, sucrose is actively transported into the phloem cells by the companion cells. This lowers water potential in the phloem cells, so water enters laterally from the xylem via osmosis. This increases the hydrostatic pressure in phloem, creating a hydrostatic pressure gradient. So mass flow occurs, as the solute moves from the source to the sink. At the sink, sucrose is removed by active transport to be used by respiring cells or stored in storage organs

Question 80

How does the active loading of sucrose into the companion cell occur at the source?

Answer 80

Hydrogen ions are actively transported out of the companion cell into the cells of the source tissue using the hydrolysis of ATP. This creates a hydrogen concentration gradient across the companion cell membrane. This means that the Hydrogen ions diffuse down the gradient through co-transporters, and they bring a co-transported sucrose molecule with them. This increases the concentration of sucrose in the companion cells, so the sucrose diffuses into the phloem cell

Question 81

What evidence is there to support the mass flow hypothesis?

Answer 81

When sieve tubes are cut, sap is released. This demonstrates that the sap is under pressure within the phloem

Question 82

What evidence is there that goes against the mass flow hypothesis?

Answer 82

Sucrose travels to many different sinks and does not always travel to the one with the highest water potential first, which it should according to the mass flow hypothesis

Question 83

Describe fish ventilation

Answer 83

The fish opens it’s mouth, lowering the floor of the buccal cavity, increasing volume and decresing pressure inside the mouth. Water moves into the mouth down pressure gradient. The fish closes it’s moith, causing floor of buccal cavity to raise, decreasing volume and increasing pressure. This causes the operculum to open, and water is forced over the gill filaments.