Exchange Surfaces

Question 1

What is the surface area to volume ratio like in multi-cellular organisms?

Answer 1

Small surface area:volume ratio

Question 2

Why is a small surface are:volume ratio a problem to multi-cellular organisms?

Answer 2

Diffusion and active transport alone is not adequate for the organism to obtain the sufficient molecules that are required and therefore this is overcome by having specialised gas exchange surfaces e.g. lungs, gills, tracheoles

Question 3

What physical adaptations do some organisms have to increase its surface area to volume ratio?

Answer 3

Organisms may have a flattened shape such as a flatworm have an elongated thin body which increases its SA:vol ratio

Question 4

Give examples of molecules that need to be interchanged and state what mechanism is used to move these molecules

Answer 4

• Respiratory gases e.g. carbon dioxide, oxygen via simple diffusion
• Nutrients e.g. glucose and amino acids are polar so via facilitated diffusion or active transport. Fatty acids, vits and mins are non-polar so via simple diffusion
• Excretory products e.g. urea, carbon dioxide via simple diffusion

Question 5

Describe features of a specialised system and explain their effects on gas exchange surfaces

Answer 5

• Large surface are increases diffusion rate
• Very thin means short diffusion pathway
• Partially permeable means oxygen and carbon dioxide can diffuse freely through phospholipid bilayer
• Ventilation/movement of surrounding median (e.g. water) maintains steep diffusion gradient
• Transport system (e.g. blood) maintains diffusion gradient

Question 6

What is the equation for working out diffusion?

Answer 6

Diffusion = (SA x Conc difference) / Length of diffusion path

Question 7

How do gases move in and out of the tracheal system?

Answer 7

1. Down concentration gradient - oxygen levels are low at tissue but higher in the tracheae, so oxygen diffuses down the tracheole to tissue. Carbon dioxide diffuses in the opposite direction

2 - Mass transport - Insect’s muscles contract when moving, this squeezes the tracheae and so moves air in and out

3 - The ends of tracheoles are filled with water - when insects carry out a lot of activity, they will need to carry out anaerobic respiration which produces lactate. Lactate is soluble and lowers the water potential in the respiring cells. Water from the tracheoles moves into the muscle cells by osmosis. This loss in water volume in the tracheoles draws more volume of air into the tracheoles. Diffusion now occurs through a gas meaning it occurs much quicker

Question 8

How do insects reduce water loss?

Answer 8

1. Small surface area to volume ratio - minimise the surface area over which water is lost.
2. Waterproof covering - insects have a rigid outer covering of chitin which is covered with a waterproof covering
3. Spiracles - openings of the tracheae which can be closed (at rest) to reduce water loss from tracheae, but must open when insects are active since oxygen is required to enter and carbon dioxide to leave

Question 9

Describe the gills in a fish

Answer 9

Gill arch —> gill filament —> gill lamellae

Question 10

How do gases exchange in fish?

Answer 10

Counter current exchange - water moves in opposite direction to the flow of blood. This maintains the concentration gradient across the whole of the lamellae. The pO2 will always be slightly higher in the water than in the blood capillary so oxygen will always diffuse down a concentration gradient into the capillary

Question 11

Describe gas exchange in plants

Answer 11

• All plant cells require oxygen and produce carbon dioxide during respiration
• Plants also carry out photosynthesis which uses up carbon dioxide and releases oxygen
• This means there is a reduced need to exchange gases with the external environment by using the oxygen made in photosynthesis directly in respiration
• However, the rate of photosynthesis is higher than the rate of respiration so carbon dioxide must be gained from the external environment
• The oxygen produced in photosynthesis diffuses out of the leaves
• Respiration can also occur at night when photosynthesis does not occur. At these times exchange of gases with the environment is essential

Question 12

How do plants limit water loss?

Answer 12

Waterproof covering - cuticle on the surface of leaves

Guard cells and stomata - guard cells can close the stomata to reduce loss of water vapour from the leaf

Some plants are specialised to reduce water loss from transpiration (xerophytes

Question 13

Give examples of xerophytic adaptations and describe their function

Answer 13

Thicker cuticle - 10% of water loss is by evaporation through the cuticle - having a thicker cuticle reduces this even further

Rolling of leaves - Stomata on lower epidermis of leaves means rolling the leaf so the lower epidermis is on the inside will reduce the water potential gradient (between inside leaf and outside the leaf), since there will be water vapour trapped on the underside of the rolled leaf, so water potential just outside the stoma will be high

Hairy leaves - hairs trap water vapour so reduces the water vapour potential gradient

Stomata in pits/grooves - pits trap water vapour so reduces the water vapour potential gradient

Reduced SA:vol ratio of leaves (pines needles) - reduced surface area from less loss of water vapour. Still require leaves for photosynthesis. Some plants adapted to carry out photosynthesis in cells in the stem

Question 14

State the direction of oxygen diffusion in the human gas exchange system

Answer 14

Air > nasal passage/mouth cavity > trachea > bronchi > bronchioles > alveoli <> blood capillaries

Question 15

What type of cells are alveolar walls made from?

Answer 15

Squamous epithelial cells (one cell thick)

Question 16

What features do alveoli have to ensure efficient gas exchange?

Answer 16

1 - Large surface area allows more molecules to diffuse quickly
2 - Thin and permeable so gas can diffuse quickly
3 - A surfactant is produced which prevents alveoli walls sticking together
4 - Large pulmonary capillary network helps maintain a steep concentration gradient

Question 17

Why is diffusion of gases between the alveoli and blood be rapid?

Answer 17

• Erythrocytes are slowed as they pass through the capillaries allowing more time for diffusion
• There is a short diffusion distance between the alveolar air and erythrocytes as the erythrocytes are flattened against the capillary walls
• The walls of the alveoli and capillaries are very thin giving a short diffusion pathway
• The alveoli and capillaries have a very large total surface area
• The breathing movements ventilate the lungs and maintain a steep concentration gradient for faster diffusion
• Action of the heart pumping blood through the capillaries maintains a steep concentration gradient for faster diffusion

Question 18

Describe the mechanism to inspiration

Answer 18

• The diaphragm muscle contracts and flattens
• External intercostal muscles contract and raise the rib cage upwards and outwards
• Volume of the chest cavity increases
• Pressure in the chest cavity drops below atmospheric pressure
• Air enters the lungs down a pressure gradient
• Alveoli stretch

Question 19

Describe the mechanism to expiration

Answer 19

• The alveoli recoil due to elastic tissue , making volume smaller and expelling air
• The diaphragm muscle relaxes and curves upwards
• External intercostal muscles relax and the ribs move downwards and inwards
• Volume of the chest cavity decreases
• Pressure in the chest cavity increases above atmospheric pressure
• Air exits the lungs down a pressure gradient

NOTE: During forced expiration the internal intercostal muscles contract and the external intercostal muscles relax pulling the rib cage down and inwards. This is an example of antagonistic muscle action)

Question 20

Describe how a spirometer trace works

Answer 20

• Chamber filled with oxygen (or air) that floats on a tank of water
• The subject breathes from a disposable disinfected mouthpiece attached to a tube connected to the oxygen chamber and wears a nose clip
• Breathing in removes oxygen from the chamber and the lid falls
• Breathing out pushes air into the chamber and the lid rises
• The movements of the chamber lid are recorded as a spirograph trace on the kymograph

Question 21

What is an average tidal volume?

Answer 21

Usually about 0.5dm^-3

Question 22

What is an average breathing rate?

Answer 22

12-20 breaths per minute

Question 23

What is breathing rate?

Answer 23

The number of breaths taken in or out in one minute (number of peaks per minute)

Question 24

What is pulmonary ventilation?

Answer 24

The total volume of air moved into the lungs in one minute

Question 25

What is the equation for pulmonary ventilation rate?

Answer 25

Pulmonary ventilation rate (dm^3min^-1) = tidal volume x breathing rate

Question 26

What is forced vital capacity (FVC)?

Answer 26

The maximum volume of air that can be inhaled (or exhaled) in one breath, It is approximately 5.0dm^3

Question 27

What is residual volume?

Answer 27

The volume of air which remains in the lungs after you have breathed out as deeply as possible

Question 28

What is forced expiratory volume (FEV1)?

Answer 28

The maximum volume of air that can forcibly breathed out of the lungs in 1 second

Question 29

How is it possible to investigate the rate of oxygen consumption?

Answer 29

- For this investigation there must be soda lime present in the spirometer - Each time you breathe in you take air from the chamber and the trace drops - When you breathe out the usused oxygen in the expired air goes back into the chamber but the carbon dioxide is absorbed by the soda lime. The volume of oxygen taken out of the air into the blood from each breath is the as the volume of carbon dioxide replacing it - With each breath, the volume of oxygen in the spirometer gets less and the trace goes down - If you measure how much they go down over a period of time this will tell you the volume of oxygen you used

Question 30

What is the equation for the rate of oxygen comsumption?

Answer 30

Rate of oxygen comsumption = Volume of oxygen consumed / time

Question 31

What can a respirometer be used to measure?

Answer 31

The oxygen uptake by living organisms such as small maggots, insects or germinating seeds

Question 32

Describe how a respirometer is set up

Answer 32

- KOH is added to one test tube to equal the volume of the living organism on the other side - Having 2 test tubes attached eliminates any atmospheric pressure and temperature changes that could affect the volume of the gases and affect the movement of the coloured liquid in the manometer

Question 33

How does a respirometer work?

Answer 33

- As the organism aerobically respires it will remove molecules of oxygen from the air and it will exhale an equivalent volume of carbon dioxide which will be absorbed by the potassium hydroxide solution - This will decrease the volume of gases in the respirometer and cause the coloured liquid to move towards the living organism - The distance the fluid moves in a time period is recorded and then the colume of oxygen uptake can then be calculated using the formula (pi x r^2 x d) will be the equivalent of the volume of oxygen consumed

Question 34

What is a degenerative disease?

Answer 34

Symptoms get worse over time

Question 35

What is an acute disease?

Answer 35

Symptoms last a short time

Question 36

What is a chronic disease?

Answer 36

Symptoms last a long time

Question 37

Describe asthma

Answer 37

- Smooth muscle lining the bronchioles contracts - Large amount of mucus produced - Causes constriction of the airways making it difficult to breathe - Air flow severely reduced so less oxygen enters the alveoli and diffuses into the bloodstream - Forced expiratory volume will be reduced as a result

Question 38

Describe emphysema

Answer 38

- Usually caused by long term smoking - Foreign particles become trapped in the alveoli, inflammation results and attract phagocytes - Causes the break down of elastic tissue as phagocytes produce an elastase enzyme - The reduction of elastin means the alveoli can't recoil as much on expiration - Also results in the break down of alveoli walls and a reduction of surface area of the alveoli - This will lead to an increase in the person's ventilation rate as a compensation for the reduced oxygen entering the lungs - Symptoms include shortness of breath, wheezing - COPD is a combination of chronic bronchitis and emphysema

Question 39

Describe fibrosis

Answer 39

- Can occur to the build up of scar tissue as a result of infections or exposure to chemicals - Scar tissue is thicker and less elastic than normal lung tissue - Lungs cannot expand as much or hold as much air - Tidal volume reduced - FVC reduced - Rate of gas exchange is slower due to diffusion over thicker pathway

Question 40

Describe pulmonary tuberclorosis

Answer 40

- Bacterial infection where tubucles are formed in the lungs which damage the gas exchange surface - Tidal volume decreases - Higher ventilation rate to compensate - Symptoms include persistent cough with blood laden mucus, shortness of breath, fatigue and weakness

Question 41

Where are salivary glands found and what is their function?

Answer 41

- Found in the mouth | - They secrete saliva containing the enzyme amylase which hydrolyses starch into maltose

Question 42

How does the stomach aid digestion?

Answer 42

- Cells lining the stomach produce hydrochloric acid and protease (a peptidase) enzymes that hydrolyse proteins - The hydrochloric acid provides the optimum pH for the protease enzymes in the stomach - The muscle in the stomach wall will contract and churn the contents known as chyme

Question 43

After the stomach where does the food go?

Answer 43

- Passes through the ileum

Question 44

How does digestion occur at the ileum?

Answer 44

- Pancreatic secretions containing bile and pancreatic carbohydrases, lipases and proteases to hydrolyse carbohydrates lipids and proteins respectively - The epithelial walls of the small intestine also have membrane bound carbohydrases - The inner walls of the ileum are folded into villi which has a large surface area and is further increased by millions of tiny projections called microvilli. These are adapted for absorbing the productsof digestion into the bloodstream

Question 45

What happens to any undigested food after it's been through the small intestine?

Answer 45

- Passes into the large intestine, where water is absorbed from all the secretions - Faeces are formed and stored and are periodically removed via the anus in the process of egestion

Question 46

What are the 2 main stages that digestion takes place in humans?

Answer 46

Physical digestion - large food particles are broken down by the action of the teeth. This increases the surface area for chemical digestion. Churning by the muscles in the stomach wall also physically breaks up the food Chemical digestion - is due to specific enzymes carrying out hydrolysis reactions using water to break chemical bonds

Question 47

Describe how carbohydrates are digested

Answer 47

- Saliva enters the mouth from the salivary glands - The food is mixed with the saliva through chewing - Saliva also contains mineral salts that helps maintain the optimum pH for the amylase enzyme - Amylase hydrolyses alternate glycosidic bonds in starch to produce the disaccharide maltose - The food is then swallowed and enters the stomach, where acid is released, denaturing the amylase enzyme and prevents further hydrolysis of starch - Later food enters the small intestine, where it mixes with pancreatic secretions known as pancreatic juice - Pancreatic juice contains pancreatic amylase, this continues the hydrolysis of any remaining starch into maltose - Alkaline salts are present in bile and are released by the walls of the small intestine, they help to neutralise the acidic contents from the stomach and maintain a neutral optimum pH for the amylase enzyme in the small intestine - Muscle contractions of the walls of the small intestine result in peristalsis, which causes the food to be pushed along - Peristalsis is a series of muscle contractions that occur in your digestive tract to move along the contents - As the food moves along the small intestine it encounters 3 different membrane bound disaccharidase enzymes; maltase, sucrase and lactase which each can hydrolyse single glycosidic bonds to produce monosaccharides

Question 48

What monosaccharides are produced when sucrase hydrolyses sucrose?

Answer 48

Glucose + fructose

Question 49

What monsaccharides are produced when maltase hydrolyses maltose

Answer 49

Glucose + glucose

Question 50

What monosaccharides are produced when lactase hydrolyses lactose?

Answer 50

Glucose + galactose

Question 51

How are lipids digested?

Answer 51

- In the duodenum fats and oils are emulsified first by the action of bile salts - Emulsification is when lipids are split into tiny droplets called micelles - this increases the surface area available to the lipase enzymes and speeds up the rate of hydrolysis - Bile salts are found in bile - Bile is produced by the liver and stored in the gall bladder and then secreted through the bile duct into the small intestine - Lipid micelles are hydrolysed by lipase enzymes - Lipases are produced by the pancreas and are present in pancreatic juice which is released into the small intestine - Lipases hydrolyse the ester bond found in triglycerides to form fatty acids and monoglycerides - A monoglyceride is a glycerol molecule and one fatty acid

Question 52

How are proteins digested?

Answer 52

Endopeptidases - hydrolyse the peptide bonds in the central region of the protein molecule Exopeptidases - hydrolyse the peptide bonds at the ends of the protein molecules Dipeptidases - complete proteins digestion by hydrolysing the peptide bonds in a dipeptide to release amino acids. Dipeptidases are membrane bound being part of the cell surface membrane of the epithelial cells lining the ileum

Question 53

How is the ileum adapted for the absorption of the products of digestion?

Answer 53

1 - The ileum walls are folded and contain many villi, which in turn are made from epithelial cells that have many microvilli, increasing the surface area for diffusion to take place 2 - They are very thin walled, reducing the diffusion pathway 3 - They contain muscle which allows them to move which helps maintain a concentration gradient for faster diffusion 4 - They are well supplied by blood vessels to carry absorbed molecules away which helps to maintain the concentration gradient for faster diffusion 5 - The cell surface membrane of the epithelial cells from microvilli which further increase the surface area for absorption

Question 54

How are triglycerides absorbed into the bloodstream?

Answer 54

- Triglycerides together with bile salts form micelles - Micelles then break down releasing the monoglycerides and fatty acids which can easily diffuse through the cell surface membrane - They are then transported to the endoplasmic reticulum where they reform as triglycerides - Passed to the golgi apparatus where they associate with cholesterol and lipoproteins to form chylomicrons - Chylomicrons move out of the epithelial cells by exocytosis and enter the lymphatic capillaries called lacteals (found in the centre of each villus) - From there they will pass through the lymphatic vessels and finally into the bloodstream