Biology AS Chapter 6 - Exchange

Question 1

What are 5 examples of things that need to be interchanged between organisms and their environment?

Answer 1

• Respiratory gases
• Nutrients
• Vitamins
• Excretory products
• Heat

Question 2

What are two examples of ways organisms have evolved to allow efficient exchange of gases?

Answer 2

• A flattened shape so no cell is ever far from the surface
• Specialised exchange surfaces with large areas to increase the surface area to volume ratio (eg lungs and gills)

Question 3

What 5 characteristics must specialised exchange surfaces have to allow effective transfer of materials?

Answer 3

• A large surface area relative to the volume of the organism
• Very thin so the diffusion distance is short
• Selectively permeable
• Movement of the environmental medium to maintain a diffusion gradient
• A transport system to ensure the movement of the internal medium to maintain a diffusion gradient

Question 4

Why are specialised exchange surfaces usually located inside an organism?

Answer 4

For protection because they are thin so can be easily damaged and dehydrated

Question 5

Describe the respiratory system of an insect

Answer 5

They have an internal network of tubes called tracheae which are supported by strengthened rings to prevent them from collapsing. These divide into smaller dead-end tubes called tracheoles which extend throughout the tissues of the insect, allowing oxygen to be brought directly to respiring tissues

Question 6

What are the 3 ways respiratory gases move in and out of the tracheal system in insects?

Answer 6

• Along a diffusion gradient
• Mass transport –> muscle contraction can squeeze the trachea, enabling mass movements of air in and out
• The ends of the tracheoles are filled with water –> lactate is produced during respiration, decreasing water potential and so the water moves into the cells by osmosis. This decreases the volume in the tracheoles and so draws air into them

Question 7

How do gases enter and leave the tracheae in insects?

Answer 7

Through tiny pores called spiracles

Question 8

What is a disadvantage of the tracheal system in insects?

Answer 8

It relies mostly on diffusion and so pathways have to be short, thereby limiting the insects size

Question 9

Describe the countercurrent exchange principle in fish

Answer 9

• Blood that is already well loaded with oxygen meets water which has its max concentration of oxygen and so diffusion of oxygen from the water into the blood occurs
• Blood with little oxygen in it meets water which has had most of its oxygen removed and so oxygen enters the blood via diffusion
• This is due to blood and water flowing in opposite directions and so about 80% of the oxygen available in the water is absorbed into the blood, compared to only 50% if they flow in the same direction

Question 10

What 3 adaptations do the leaves have to allow for rapid diffusion?

Answer 10

• Many small pores, called stomata, and so no cell is far from one and the diffusion pathway is short
• Numerous interconnecting air-spaces that occur throughout the mesophyll so gases readily come into contact with mesophyll cells
• Large surface area of mesophyll cells for rapid diffusion

Question 11

How is the rate of gaseous exchange in plants controlled?

Answer 11

Each stoma (singular of stomata) is surrounded by guard cells which can open and close the stomatal pore, controlling the rate

Question 12

What adaptations do insects have to reduce water loss?

Answer 12

• Small surface area to volume ratio –> minimise the area over which water is lost
• Waterproof coverings –> their covering is a rigid outer skeleton of chitin that is covered with a waterproof cuticle
• Spiracles –> able to open and close to control water loss

Question 13

How do plants limit water loss?

Answer 13

• Waterproof coverings
• Ability to close stomata

Question 14

What are xerophytes?

Answer 14

Plants that are adapted to living in areas where water is in short supply

Question 15

How are xerophytes adapted to limit water loss?

Answer 15

• Thick cuticle –> less water can escape
• Rolling up of leaves –> traps a region of air which becomes saturated with water vapour and has a very high water potential. There is no gradient and so no water is lost
• Hairy leaves –> traps moist air and so the water potential gradient is reduced and less is lost
• Stomata in pits or grooves –> trap moist air and reduce the water potential gradient so less is lost
• Reduced surface area to volume ratio of leaves –> slower rate of diffusion so water loss is reduced

Question 16

Why are the lungs located inside the body?

Answer 16

• Air is not dense enough to support and protect them
• The body would otherwise lose a great deal of water and dry out

Question 17

Describe the structure of the lungs, starting at the trachea

Answer 17

The trachea divides into 2 tubes called the bronchi, each leading to one lung. These then divide into bronchioles and then into alveoli which is where gaseous exchange occurs

Question 18

How do bronchioles control the flow of air in and out of the alveoli?

Answer 18

Their walls are made up of muscle lined with epithelial cells and this muscle allows them to constrict, thereby controlling the flow of air

Question 19

What are the two types of intercostal muscles and how do they differ?

Answer 19

• Internal intercostal muscles –> contraction leads to expiration
• External intercostal muscles –> contraction leads to inspiration

Question 20

Describe the process of inspiration (expiration is just the opposite)

Answer 20

1. The external intercostal muscles contract, while the internal intercostal muscles relax
2. The ribs are pulled upwards and outwards, increasing the volume of the thorax
3. The diaphragm contracts and flattens, increasing the volume of the thorax
4. The increased volume of the thorax results in a reduction of pressure in the lungs
5. Atmospheric pressure is now greater than pulmonary pressure and so air is forced into the lungs

Question 21

Why is diffusion between the alveoli and the blood very rapid?

Answer 21

• Red blood cells are slowed as they pass through pulmonary capillaries, allowing more time for diffusion
• The distance between the alveoli and the red blood cells is reduced as the red blood cells are flattened against the capillary walls
• The walls of alveoli are very thin (one cell) so there is a short diffusion distance
• Alveoli and pulmonary capillaries have a large total surface area
• Steep concentration gradient is always maintained

Question 22

What is the structure and role of the stomach?

Answer 22

A muscular sac with an inner layer which produces enzymes. Its role is to store and digest food, especially proteins, and it has glands which produce enzymes to digest these proteins

Question 23

What is the structure and role of the ileum?

Answer 23

It is a long muscular tube. Food is further digested here by enzymes which are produced by its walls and by glands which pour their secretions into it

Question 24

How is the ileum adapted to its function of digesting food?

Answer 24

The inner walls are folded into villi which gives them a large surface area. These then have millions of tiny projections called microvilli on the epithelial cells of each villus, further increasing the surface area

Question 25

What is the role of the large intestine?

Answer 25

To absorb water

Question 26

What is the role of the rectum?

Answer 26

To store faeces before they are removed via the anus

Question 27

What is egestion?

Answer 27

The process by which faeces is removed from the body via the anus

Question 28

What is the role of the salivary glands?

Answer 28

To pass secretions via a duct into the mouth.These secretions contain the enzyme amylase which hydrolyses starch into maltose

Question 29

What is the role of the pancreas?

Answer 29

It produces pancreatic juice which contains proteases to hydrolyse proteins, lipase to hydrolyse lipids, and amylase to hydrolyse starch

Question 30

Describe the process of the digestion of carbohydrates, using starch as an example

Answer 30

1. Saliva enters the mouth from the salivary glands and is mixed with the food while chewing
2. Saliva contains salivary amylase which starts hydrolysing any starch into maltose
3. The food is swallowed and enters the stomach where the conditions are acidic and so the salivary amylase is denatured
4. The food is passed into the small intestine where it mixes with pancreatic juice
5. This contains pancreatic amylase which continues the hydrolysis of any remaining starch into maltose.
6. Muscles in the intestine wall push the food along the ileum. Its epithelial lining produces maltase which is part of the cell-surface membranes of the epithelial cells that line the ileum (membrane bound disaccharide).
7. The maltase hydrolyses the maltose from the starch breakdown into alpha-glucose

Question 31

How does sucrase hydrolyse sucrose?

Answer 31

It hydrolyses the single glycosidic bond in the sucrose molecule, producing glucose and fructose

Question 32

How does lactase hydrolyse lactose?

Answer 32

It hydrolyses the single glycosidic bond in the lactose molecule, producing glucose and galactose

Question 33

How are lipids digested?

Answer 33

1. Lipids are split up into tiny droplets called micelles by bile salts which are produced in the liver. This is called emulsification and increases the surface area so the action of lipases is sped up
2. Lipases hydrolyse the ester bond in triglycerides to form fatty acids and monoglycerides

Question 34

How are proteins digested?

Answer 34

By a group of enzymes called peptidases (proteases) and there are a few of these:
- Endopeptidases: hydrolyse the peptide bonds between amino acids in the central region of the protein
- Exopeptidases: hydrolyse the peptide bonds on the terminal amino acids of the protein
- Dipeptidases: hydrolyses the bond between 2 amino acids of a dipeptide

Question 35

How do villi in the lumen increase the efficiency of absorption?

Answer 35

• Increase the surface area for diffusion
• Very thin walled, thus reducing the diffusion pathway
• Contain muscle and so are able to move. This helps maintain a diffusion gradient as they are able to mix the contents of the ileum
• Well supplied with blood vessels so blood can carry away absorbed molecules and maintain a diffusion gradient
• The epithelial cells lining the villi possess microvilli which increase the surface area

Question 36

Describe the absorption of triglycerides

Answer 36

1. Micelles contain bile salts and fatty acids/monoglycerides
2. Micelles come into contact with the epithelial cells lining the villi of the ileum
3. Micelles break down and the fatty acids and monoglycerides are absorbed by diffusion into the epithelial cells
4. They are transported to the endoplasmic reticulum where they are recombined to form triglycerides
5. These associate with cholesterol and lipoproteins to form chylomicrons
6. These move out of the epithelial cells by exocytosis and enter lymphatic capillaries called lacteals that are found in the centre of each villus
7. From here, the chylomicrons pass into the blood system and are hydrolysed by an enzyme so they can diffuse into cells