3- SA:Vol, Gas exchange, Digestion and absorption

Describe the relationship between the size and structure of an organism and its SA:V

• As size increases, SA:V decreases

• More thin/ flat/ folded/ elongated structures increase SA:V

How is SA:V calculated?

Divide SA by volume

Suggest an advantage of calculating SA:mass for organisms instead of SA:V

Easier/ quicker to find/ more accurate because irregular shapes

What is metabolic rate? Suggest how it can be measured

• Metabolic rate= amount of energy used up by an organism within a given period of time

• Often measured by oxygen uptake= as used in aerobic respiration to make ATP for energy release

Explain the relationship between SA:V and metabolic rate

As SA:V increases (smaller organisms), metabolic rate increases because:

• rate of heat loss per unit body mass increases

• so organisms need a higher rate of respiration

• to release enough heat to maintain a constant body temperature i.e. replace lost heat

Explain the adaptions that facilitate exchange as SA:V reduces in larger organisms

1. Changes to body shape (e.g. long/ thin)

   • Increases SA:V and overcomes (reduces) long diffusion distance/ pathway

2. Development of systems, such as a specialised surface/ organ for gaseous exchange e.g. lungs

   • Increases (internal) SA:V and overcomes (reduces) long diffusion distance/ pathway

   • Maintain a concentration gradient for diffusion e.g. by ventilation/ good blood supply

Explain how the body surface of a single-celled organism is adapted for gas exchange

• Thin, flat shape and large SA:V

• Short diffusion distance to all parts of cell= rapid diffusion e.g. of O2/ CO2

Describe the tracheal system of an insect

1. Spiracles= pores on surface that can open/ close to allow diffusion

2. Tracheae= large tubes full of air that allow diffusion

3. Tracheoles= smaller branches from tracheae, permeable to allow gas exchange

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

• Tracheoles have thin walls

  • so short diffusion distance to cells

• High numbers of highly branched tracheoles

  • so short diffusion distance to cells

  • so large surface area

• Tracheae provide tubes of air

  • so fast diffusion

• Contraction of abdominal muscles (abdominal pumping) changes pressure in body, causing air to move in/out

  • maintains concentration gradient for diffusion

• Fluid in end of tracheoles drawn into tissues by osmosis during exercise (lactate produced in anaerobic respiration lowers water potential of cells)

  • Diffusion is faster through air (rather than fluid) to gas exchange surface

Explain structural and functional compromises in terrestrial insects that allow efficient gas exchange while limiting water loss

• Thick waxy cuticle/ exoskeleton= increases diffusion distance so less water loss (evaporation)

• Spiracles can open to allow gas exchange AND close to reduce water loss (evaporation)

• Hairs around spiracles= trap moist air, reducing water potential gradient so less water loss (evaporation)

Explain how the gills of fish are adapted for gas exchange

• Gills made of many filaments covered with many lamellae

  • increases SA for diffusion

• Thin lamellae wall/ epithelium

  • so short diffusion distance between water/ blood

• Lamellae have a large number of capillaries

  • remove O2 and bring CO2 quickly so maintains conc gradient

• Counter current flow

What is counter current flow?

1. Blood and water flow in opposite directions through/ over lamellae

2. So oxygen conc always higher in water (than blood)

3. So maintains a conc gradient of O2 between water and blood

4. For diffusion along whole length of lamellae

If parallel flow, equilibrium would be reached so oxygen wouldn’t diffuse into blood along the whole gill plate.

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

• Many stomata (high density)= large SA for gas exchange (when opened by guard cells)

• Spongy mesophyll contains air spaces= large SA for gases to diffuses through

• Thin= short diffusion distance

What is a xerophyte?

plant adapted to live in very dry conditions e.g. cacti and marram grass

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

• Thicker waxy cuticles

  • increases diffusion distance so less evaporation

• Sunken stomata in pits/ rolled leaves/ hairs

  • ‘trap’ water vapour/ protect stomata from wind

  • so reduced water potential gradient between leaf/ air

  • so less evaporation

• Spines/ needles

  • reduces SA:V

Describe the gross structure of the human gas exchange system

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

• Flattened cells/ 1 cell thick= short diffusion distance

• Folded= large SA

• Permeable= allows diffusion of O2/CO2

• Moist= gases can dissolve for diffusion

• Good blood supply from large network of capillaries= maintains conc gradient

Describe how gas exchange occurs in the lungs

• Oxygen diffuses from alveolar air space into blood down its conc gradient

• across alveolar epithelium then across capillary endothelium

CO2= opposite

Explain the importance of ventilation

• Brings in air containing higher conc of O2 and removes air with lower conc of oxygen

• maintaining conc gradient

Explain how humans breathe in and out (ventilation)

INSPIRATION (breathing in)

1. Diaphragm muscles contract= flattens

2. External intercostal muscles contract, internal intercostal muscles relax (antagonistic)= ribcage pulled up/ out

3. Increasing volumeand decreasing pressure (below atmospheric) in thoracic cavity

4. Air moves into lungs down pressure gradient

Explain how humans breathe in and out (ventilation)

EXPIRATION (breathing out)

1. Diaphragm relaxes= moves upwards

2. External intercostal muscles relax, internal intercostal muscles may contract= ribcage moves down/ in

3. Decreasing volume and increasing pressure (above atmospheric) in thoracic cavity

4. Air moves out of lungs down pressure gradient

Suggest why expiration is normally passive at rest

• Internal intercostal muscles do not normally need to contract

• Expiration aided by elastic recoil in alveoli

Suggest how different lung diseases reduce the rate of gas exchange

• Thickened alveolar tissue (e.g. fibrosis)= increases diffusion distance

• Alveolar wall breakdown= reduces SA

• Reduce lung elasticity= lungs expand/ recoil less= reduces conc gradients of O2 and CO2

Suggest how different lung diseases affect ventilation

• Reduce lung elasticity (e.g. fibrosis= build up of scar tissue)= lungs expand/ recoil less

  • reducing volume of air in each breath(tidal volume)

  • reducing maximum volume of air breathed out in one breath (forced vital capacity)

• Narrow airways/ reduce airflow in and out of lungs (e.g. asthma- inflamed bronchi)

  • reducing maximum volume of air breathed out in 1 second (forced expiratory volume)

• Reduced rate of gas exchange= increased ventilation rate to compensate for reduced oxygen in blood

Suggest why people with lung disease experience fatigue

cells receive less oxygen= rate of aerobic respiration reduced= less ATP made

Suggest how you can analyse and interpret data to the effects of pollution, smoking and other risk factors on the incidence of lung disease

• Describe overall trend= e.g. positive/ negative correlation between risk factor and incidence of disease

• Manipulate data= e.g. calculate percentage change

• Interpret standard deviations= overlap suggests differences in means are likely to be due to chance

• Use statistical tests= identify whether difference/ correlation is significant or due to chance

  • correlation coefficient= examining on association between 2 sets of data

  • student’s t test= comparing means of 2 sets of data

  • chi-squared test= for categorical data

Suggest how you can evaluate the way in which experimental data led to statutory restrictions on the sources of risk factors

• Analyse and interpret data as above and identify what does and doesn’t support statement

• Evaluate method of collecting data

  • sample size= large enough to be representative of population?

  • participant diversity e.g. age, sex, ethnicity and health status= representative of population?

  • control groups= used to enable comparison?

  • control variables e.g. health, previous medications= valid?

  • duration of study= long enough to show long-term effects?

• Evaluate context= has a broad generalisation been made from a specific set of data?

• Other risk factors that could have affected results?

Explain the difference between correlations and causal relationships

• correlation= change in one variable reflected by a change in another- identified on a scatter graph

• causation= change in one variable causes a change in another variable

• correlation does not mean causation= may be other factors involved

Explain what happens in digestion

• Large (insoluble) biological molecules hydrolysed to smaller (soluble) molecules

• That are small enough to be absorbed across cell membranes into blood

Describe the digestion of starch in mammals

• Amylase (produced by salivary glands/ pancreas) hydrolyses starch to maltose

• Membrane- bound maltose (attached to cells lining ileum) hydrolyses maltose to glucose

• Hydrolysis of glycosidic bond

Describe the digestion of disaccharides in mammals

• Membrane-bound disaccharides hydrolyse disaccharides to 2 monosaccharides:

  • Maltase= maltose= glucose + glucose

  • Sucrase= sucrose= fructose + glucose

  • Lactase= lactose= galactose + glucose

• Hydrolysis of glycosidic bond

Describe the digestion of lipids in mammals, including action of bile salts

• Bile salts (produced by liver) emulsify lipids causing them to form smaller lipid droplets

• This increases SA of lipids for increased/ faster lipase activity

• Lipase (made in pancreas) hydrolyses lipids (e.g. triglycerides) = monoglycerides + fatty acids

• Hydrolysis of ester bond

Describe the digestion of proteins by a mammal

• Endopeptidases= hydrolyse internal (peptide) bonds within a polypeptide= smaller peptides

  • so more ends/ SA for exopeptidases

• Exopeptidases= hydrolyse terminal (peptide) bonds at ends of polypeptide= single amino acids

• Membrane- bound dipeptidases= hydrolyse (peptide) bond between a dipeptide= 2 amino acids

• Hydrolysis of peptide bond

Suggest why membrane- bound enzymes are important in digestion

• Membrane-bound enzymes are located on cell membranes of epithelial cells lining ileum

• (By hydrolysing molecules at the site of absorption they) maintain conc gradients for absorption

Describe the pathway for absorption of products of digestion in mammals

Lumen (inside) of ileum= cells lining ileum (part of small intestine)= blood

Describe the absorption of amino acids and monosaccharides in mammals

CO-TRANSPORT:

1. Na+ actively transported from epithelial cells lining ileum to blood (by Na+/ K+ pump), establishing a conc gradient of Na+ (higher in lumen than epithelial cell)

2. Na+ enters epithelial cell down its conc gradient with glucose against its conc gradient via a co-transport protein

3. Glucose moves down a conc gradient into blood via facilitated diffusion

Describe the absorption of lipids by a mammal, including the role of micelles

• Micelles contain bile salts, monoglycerides and fatty acids

  • make monoglycerides and fatty acids (more) soluble in water

  • carry/ release fatty acids and monoglycerides to cell/ lining of ileum

  • maintain high conc of fatty acids to cell/ lining

• Monoglycerides/ fatty acids absorbed (into epithelial cell) by diffusion (lipid soluble)

• Triglycerides reformed in (epithelial) cells and aggregate into globules

• Globules coated with proteins forming chylomicrons which are then packaged into vesicles

• Vesicles move to cell membrane and leave via exocytosis

  • enter lymphatic vessels and eventually return to blood circulation