section 3: organisms exchange substances with their environment

Question 1

physical digestion

Answer 1

broken down into smaller pieces by chewing with the teeth and churning by the muscles in the stomach walls
makes surface area larger for chemical digestion

Question 2

chemical digestion

Answer 2

hydrolyses large, insoluble molecules into smaller, soluble ones
uses enzymes

Question 3

digestion of starch

Answer 3

1. saliva enters the mouth from the salivary gland and is mixed with food during chewing
2. salivary amylase starts to hydrolyse any starch to maltose, saliva also contains mineral salts that help to maintain the optimum almost neutral pH
3. food enters the stomach, conditions are acidic so amylase becomes denatured and hydrolysis of starch stops
4. food then moves into the small intestine and mixes with pancreatic juices
5. the pancreatic juice contains pancreatic amylase that continues the hydrolysis of starch to maltose, alkaline salts are produced to maintain neutral pH
6. the epithelial lining of the ileum has a membrane bound disaccharidase - maltase that breaks down the maltose into alpha glucose

Question 4

what enzyme breaks down sucrose

Answer 4

sucrase

Question 5

what enzyme breaks down lactose

Answer 5

lactase

Question 6

what bonds do the disaccharidases break down

Answer 6

glycosidic bonds

Question 7

what is a membrane-bound disaccharidase

Answer 7

a disaccharidase that does not get released into the lumen of the ileum but is a part of the cell-surface membrane of the epithelial cells that line the ileum

Question 8

what enzymes hydrolyse lipids

Answer 8

lipases

Question 9

explain lipid digestion

Answer 9

firstly lipids are split up into tiny droplets called micelles by bile salts (produced in the liver) - this is called emulsification and increases surface area so lipases can act faster
lipases (produced in the pancreas) hydrolyse the ester bond found in triglycerides into fatty acids and monoglycerides

Question 10

what is a monoglyceride

Answer 10

a monoglyceride is a glycerol molecule with a single fatty acid attached

Question 11

what are the 3 main peptidases that hydrolyse proteins

Answer 11

1. endopeptidases
2. exopeptidases
3. dipeptidases

Question 12

what do endopeptidases do

Answer 12

hydrolyse the peptide bonds between amino acids in the central region of a protein molecule forming a series of peptide molecules

Question 13

what do exopeptidases do

Answer 13

hydrolyse the peptide bonds on the terminal amino acids of the peptide molecules formed by the endopeptidases,
they progressively release dipeptides and single amino acids

Question 14

what do dipeptidases do

Answer 14

hydrolyse the bond between two amino acids of a dipeptide,

they are membrane-bound so are part of the cell-surface membrane of the epithelial cells lining the ileum

Question 15

what is the ileum

Answer 15

a section of the small intestine

Question 16

how is the ileum adapted for its function of absorbing the products of digestion

Answer 16

• the wall is folded and has projections called villi which increase the surface area so there is more space for diffusion
• very thin walled so diffusion pathway is shorter
• villi contain muscle so can move, this movement means they can mix the contents of the ileum to maintain a diffusion gradient
• well supplied with blood vessels so that blood can carry away absorbed molecules to also maintain diffusion gradient
• villi have microvilli that further increase surface area for absorption

Question 17

how are amino acids and glucose absorbed

Answer 17

1. the sodium-potassium pump means there is constantly a higher concentration of sodium ions in the lumen than in the epithelial cells
2. so the sodium ions diffuse into the epithelial cells down their concentration gradient through a co-transport protein, and take either amino acid or glucose molecules with them
3. the glucose or amino acids then move into the bloodstream using facilitated diffusion

Question 18

absorption of triglycerides

Answer 18

1. the broken down monoglycerides and fatty acids remain with the bile salts and form small droplets called micelles
2. the micelles come into contact with the epithelial cells lining the villi of the ileum and break down into monoglycerides and fatty acids
3. these can then diffuse into the epithelial cells
4. inside the epithelial cells the monoglycerides and fatty acids are transported to the endoplasmic reticulum where they are recombined to form triglycerides
5. they then move to the golgi apparatus and are associated with cholesterol and lipoproteins to form chylomicrons
6. the chylomicrons move out of the epithelial cells by exocytosis and into the lymphatic capillaries called lacteals
7. they move through the lymphatic vessels and into the blood system
8. the triglycerides in the chylomicrons are then hydrolysed by an enzyme on the endothelial cells of blood capillaries where they can then diffuse into cells

Question 19

how does the loading and unloading of oxygen occur in the body

Answer 19

• at the gas-exchange surface carbon dioxide is constantly being removed
• the pH is slightly raised due to the low concentration of carbon dioxide
• this shape also increases the affinity of haemoglobin for oxygen, so it is not released while being transported in the blood to the tissues
• in the tissues, carbon dioxide is produced by respiring cells
• carbon dioxide is acidic in solution, so the pH of the blood within the tissues is lowered
• the lower pH changes the shape of haemoglobin into one with a lower affinity for oxygen
• so haemoglobin releases its oxygen into the respiring tissues

Question 20

what 3 ways do respiratory gases move in and out of the tracheal system?

Answer 20

• alone a diffusion gradient - when cells are respiring oxygen is used to and so it’s concentration towards the end of the tracheoles falls, this creates a diffusion gradient, this means oxygen diffuses from the atmosphere along the tracheae and tracheoles to the cells, but this also works in the opposite way for carbon dioxide to diffuse out of the tracheae
• mass transport - contraction of muscles in insects can squeeze the trachea enabling mass movement of air in and out, which further speeds up the exchange of respiratory gases
• the ends of tracheoles are filled with water - during periods of major activity, the muscle cells around the tracheoles carry out some anaerobic respiration, this produces lactate which is soluble and lowers water potential of the muscle cells, so water moves into them, the water in the ends of the tracheoles decreases in volume and draws air further into them, also means final diffusion pathway is gas not liquid so diffusion is more rapid

Question 21

phloem

Answer 21

• where translocation occurs - the process by which organic molecules and some mineral ions are transported from one part of the plant to another
  • made up of sieve tube elements - long, thin structures arranged end to end
  • their end walls (where they meet) are perforated to form sieve plates
  • associated with each sieve tube element are companion cells

Question 22

mass flow theory

Answer 22

1. transfer of sucrose into sieve tube elements from photosynthesising tissue:
   - sucrose is manufactured from the products of photosynthesis in cells with chloroplasts
   - sucrose diffuses down a concentration gradient by facilitated diffusion from the photosynthesising cells into companion cells
   - meanwhile, hydrogen ions diffuse down a concentration gradient through carrier proteins into the sieve tube elements
   - sucrose molecules are transported with the hydrogen ions via co-transport through co-transport carrier proteins
2. mass flow of sucrose through sieve tube elements:
   - step 1 causes the sieve tubes to have lower water potential
   - as the xylem has a much higher water potential, water moves from the xylem into the sieve tubes by osmosis, creating high hydrostatic pressure in them
   - at the respiring cells (sink), sucrose is used up or stored as starch so sucrose levels are low there
   - so sucrose actively transports into the respiring cells from the sieve tubes, lowering their water potential
   - due to the lowered water potential, water also moves into these respiring cells via osmosis
   - therefore the hydrostatic pressure of the sieve tubes is now lowered
   - the sucrose solution has moved down a hydrostatic pressure gradient through the sieve tubes = mass flow
3. transfer of sucrose from the sieve tube elements into storage or other sinks:
   - the sucrose is actively transported by companion cells, out of the sieve tubes and into sink cells

Question 23

positive cooperativity

Answer 23

binding of the first oxygen molecule makes binding of the second easier

Question 24

Bohr effect

Answer 24

higher concentration of carbon dioxide = lower haemoglobin affinity for oxygen

Question 25

how do insects reduce water loss?

Answer 25

• small surface area to volume ratio to minimise the area over water can be lost
• they have a rigid exoskeleton made of chitin that is covered with a waterproof cuticle to ensure it is water tight
• spiracles that can be closed to reduce water loss

Question 26

how are insects adapted for efficient gas exchange?

Answer 26

• have an internal network of tubes called tracheae which are supported by strengthened rings to prevent them from collapsing
• the tracheae divide into smaller dead-end tubes called tracheoles that extend throughout the full body so air is brought directly to the respiring tissues so there is a short diffusion pathway

Question 27

process of inspiration

Answer 27

1. the external intercostal muscles contract, while the internal internal intercostal muscles relax
2. the ribs are pulled upwards and outwards, increasing the volume of the thorax
3. the diaphragm muscles contract, causing it to flatten, which also increases the volume of the thorax
4. the increased volume of the thorax means reduction of pressure in the lungs
5. atmosphere pressure is now greater than pulmonary pressure so air is forced into the lungs

Question 28

process of expiration

Answer 28

1. the internal intercostal muscles contract, and external intercostal muscles relax
2. the ribs more downwards and inwards, decreasing the volume of the thorax
3. diaphragm muscles relex and so it is pushed again and the volume of the thorax is further decreased
4. the decreased volume of the thorax increases the pressure in the lungs
5. pulmonary pressure is now greater than that of the atmosphere, air is forced out of the lungs

Question 29

how to calculate pulmonary ventilation rate ?

Answer 29

pulmonary ventilation rate = tidal volume x breathing rate

Question 30

how are alveoli adapted for efficient gas exchange?

Answer 30

• red blood cells are slowed as they pass through pulmonary capillaries, allowing more time for diffusion
• walls of alveoli and capillaries are very thin which also decreases diffusion distance
• alveoli and pulmonary capillaries have very large surface areas
• blood flow through the pulmonary capillaries maintains a concentration gradient
• breathing constantly ventilates the lungs and the heart constantly circulates blood around the alveoli to ensure a steep concentration gradient is kept
• red blood cells are flattened against the capillary walls so diffusion distance is shorter

Question 31

what does a shift to the left mean?

Answer 31

the further to the left the curve is, the greater it’s affinity of haemoglobin for oxygen (so loads oxygen readily but unloads less easily)

Question 32

what does a shift to the right mean in an oxygen dissociation curve?

Answer 32

the further to the right, the lower the haemoglobins affinity for oxygen is (so it unloads oxygen easily but loads oxygen less readily)

Question 33

cardiac output calculation

Answer 33

cardiac output = heart rate x stroke volume