Organisms exchange substances with their environment

Describe the relationship between the size and structure of an organism and its surface area to volume ratio (SA:V)

● As size increases, SA:V tends to decrease

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

How is SA:V calculated? Use an example

Divide surface area (size length x side width x number of sides) by volume (length x width x depth)

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 ie. replace lost heat

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

1. Changes to body shape (eg. 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 eg. by ventilation / good blood supply

Exam insight: common mistakes

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

● Thin, flat shape and large surface area to volume ratio

● Short diffusion distance to all parts of cell → rapid diffusion eg. 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 with cells

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

● Tracheoles have thin walls

○ So short diffusion distance to cells

●Many highly branched tracheoles

○ So short diffusion distance to cells

○ So large surface area

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

○ Maintains concentration gradient for diffusion

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

● Thick waxy cuticle → Increases diffusion distance so less evaporation

● Spiracles can open to allow gas exchange AND close to reduce evaporation

● Hairs around spiracles → trap moist air, reducing ψ gradient so less evaporation

Explain how the gills of fish are adapted for gas exchange

● Gills covered with many lamellae

○ Increase surface area for diffusion

● Thin lamellae wall / epithelium

○ So short diffusion distance between water / blood

● Lamellae have a large number of capillaries

○ maintains concentration gradient

Describe Counter Current Flow:

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

2. oxygen concentration higher in water (than blood near)

3. maintains concentration gradient of O2 between water and blood

4. For diffusion along whole length of lamellae

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

● Many stomata (high density) → large surface area for gas exchange

● Spongy mesophyll contains air spaces → large surface area for gases to diffuse through

● Thin → short diffusion distance

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

Xerophyte = plant adapted to live in very dry conditions eg. Cacti and marram grass

● Thicker waxy cuticle

○ 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 surface area to volume ratio

Draw 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 surface area

● Permeable → allows diffusion of O2 / CO2

● Moist → gases can dissolve for diffusion

● Good blood supply from large network of capillaries → maintains concentration gradient

Describe how gas exchange occurs in the lungs

● Oxygen diffuses from alveolar air space into blood down its concentration gradient

● Across alveolar epithelium then across capillary endothelium

Explain the importance of ventilation

● Brings in air containing higher conc. of oxygen & removes air with lower conc. of oxygen

● Maintaining concentration gradients

Explain how humans breathe in and out (ventilation)

Suggest how different lung diseases reduce the rate of gas exchange

● Thickened alveolar tissue (eg. fibrosis) → increases diffusion distance

● Alveolar wall breakdown → reduces surface area

● Reduce lung elasticity → lungs expand / recoil less → reduces concentration gradients of O2 / CO2

Suggest how different lung diseases affect ventilation

● Reduce lung elasticity (eg. 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 & out of lungs (eg. 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

Explain the difference between correlations and causal relationships

● Correlation = change in one variable reflected by a change in another - identified on a scatter diagram

● Causation = change in one variable causes a change in another variable

● Correlation does not mean causation → may be other factors involved

Exam insight: common mistakes

Explain what happens in digestion

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

● That are small enough 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 maltase (attached to cells lining ileum) hydrolyses maltose to glucose

● Hydrolysis of glycosidic bond

Describe the digestion of disaccharides in mammals

● Membrane-bound disaccharidases 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 surface area of lipids for increased / faster lipase activity

● Lipase (made in pancreas) hydrolyses lipids (eg. 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 / surface area 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

● maintain concentration 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 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 concentration 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

Exam insight: common mistakes

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

● Red blood cells contain lots of haemoglobin (Hb) - no nucleus, biconcave, high SA:V, short diffusion path

● Hb associates / binds / loads O2 when partial pressure of O2 (pO2) is high

● forms oxyhaemoglobin which transports O2

● Hb dissociates / unloads O2 near cells / tissues where pO2 is low

Describe the structure of haemoglobin

● Protein with a quaternary structure

● Made of 4 polypeptide chains

● Each chain contains a Haem group containing an iron ion (Fe2+)

Describe the loading, transport and unloading of oxygen in relation to the oxyhaemoglobin dissociation curve

Explain how the cooperative nature of oxygen binding results in an S-shaped oxyhaemoglobin dissociation curve

1. Binding of oxygen changes tertiary / quaternary structure of haemoglobin

2. uncovers Haem group binding sites, making further binding of oxygens easier

Describe evidence for the cooperative nature of oxygen binding

● A low pO2 as oxygen increases there is little / slow increase in % saturation of Hb with oxygen

○ When first oxygen is binding

● At higher pO2, as oxygen increases there is a big / rapid increase in % saturation of Hb with oxygen

○ Showing it has got easier for oxygens to bind

What is the Bohr effect?

• Effect of CO2 concentration on dissociation of oxyhaemoglobin → curve shifts to right

Explain effect of CO2 concentration on the dissociation of oxyhaemoglobin

1. Increasing blood CO2 eg. due to increased rate of respiration

2. Lowers blood pH (more acidic)

3. Reducing Hb’s affinity for oxygen as

tertiary / quaternary

structure changes slightly

4. faster unloading of oxygen

to respiring cells at a given pO2

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

● Different types of Hb are made of polypeptide chains with slightly different amino acid sequences

● Resulting in different tertiary / quaternary structures / shape → different affinities for oxygen

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

Curve shift left → Hb has higher affinity for O2

● More O2 associates with Hb more readily

● At gas exchange surfaces where pO2 is lower

● Eg. organisms in low O2 environments - high altitudes, underground, or foetuses

Curve shift right → Hb has lower affinity for O2

● More O2 dissociates from Hb more readily

● At respiring tissues where more O2 is needed

● Eg. organisms with high rates of respiration /

metabolic rate (may be small or active)

Describe the general pattern of blood circulation in a mammal

Closed double circulatory system - blood passes through heart twice for every circuit around body:

1. Deoxygenated blood in right side of heart pumped to lungs; oxygenated returns to left side

2. Oxygenated blood in left side of heart pumped to rest of body; deoxygenated returns to right

Suggest the importance of a double circulatory system

● Prevents mixing of oxygenated / deoxygenated blood

○ So blood pumped to body is fully saturated with oxygen for aerobic respiration

● Blood can be pumped to body at a higher pressure (after being lower from lungs) ○ Substances taken to / removed from body cells quicker / more efficiently

Draw a diagram to show the general pattern of blood circulation in a mammal, including the names of key blood vessels

Name the blood vessels entering and leaving the heart and lungs

Name the blood vessels entering and leaving the kidneys

● Renal arteries – oxygenated blood → kidneys

● Renal veins – deoxygenated blood to vena cava from kidneys

Name the the blood vessels that carry oxygenated blood to the heart muscle

Coronary arteries - located on surface of the heart, branching from aorta

Label a diagram to show the gross structure of the human heart (inside)

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

● Thicker muscle to contract with greater force

● To generate higher pressure to pump blood around entire body

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

Explain how graphs showing pressure or volume changes during the cardiac cycle can be interpreted, eg. to identify when valves are open / closed

Describe the equation for cardiac output

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

How can heart rate be calculated from cardiac cycle data?

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

Explain how the structure of arteries relates to their function

Function – carry blood away from heart at high pressure

● Thick smooth muscle tissue → can contract and control / maintain blood flow / pressure

● Thick elastic tissue → can stretch as ventricles contract and recoil as ventricles relax, to reduce pressure surges / even out blood pressure / maintain high pressure

● Thick wall → withstand high pressure / stop bursting

● Smooth / folded endothelium → reduces friction / can stretch

● Narrow lumen → increases / maintains high pressure

Explain how the structure of veins relates to their function

Function – carry blood back to heart at lower pressure

● Wider lumen than arteries → less resistance to blood flow

● Very little elastic and muscle tissue → blood pressure lower

● Valves → prevent backflow of bloo

Explain how the structure of capillaries relates to their function

Function - allow efficient exchange of substances between blood and tissue fluid (exchange surface)

● Wall is a thin (one cell) layer of endothelial cells → reduces diffusion distance

● Capillary bed is a large network of branched capillaries → increases surface area for diffusion

● Small diameter / narrow lumen → reduces blood flow rate so more time for diffusion

● Pores in walls between cells → allow larger substances through

Explain the formation of tissue fluid

At the arteriole end of capillaries:

1. Higher blood / hydrostatic pressure inside capillaries (due to contraction of ventricles) than tissue fluid (so net outward force)

2. Forcing water (and dissolved substances) out of capillaries

3. Large plasma proteins remain in capillary

Explain the return of tissue fluid to the circulatory system

At the venule end of capillaries:

1. Hydrostatic pressure reduces as fluid leaves capillary (also due to friction)

2. (Due to water loss) an increasing concentration of plasma proteins lowers water potential in capillary below that of tissue fluid

3. Water enters capillaries from tissue fluid by osmosis down a water potential gradient

4. Excess water taken up by lymph capillaries and returned to circulatory system through veins

Suggest and explain causes of excess tissue fluid accumulation

● Low concentration of protein in blood plasma

○ Water potential in capillary not as low → water potential gradient is reduced

○ So more tissue fluid formed at arteriole end / less water absorbed at venule end by osmosis

● High blood pressure (eg. caused by high salt concentration) → high hydrostatic pressure

○ Increases outward pressure from arterial end AND reduces inward pressure at venule end

○ So more tissue fluid formed at arteriole end / less water absorbed at venule end by osmosis

○ Lymph system may not be able to drain excess fast enough

What is a risk factor? Give examples for cardiovascular disease

● An aspect of a person’s lifestyle or substances in a person’s body / environment

● That have been shown to be linked to an increased rate of disease

● Examples - age, diet high in salt or saturated fat, smoking, lack of exercise, genes

Exam insight: common mistakes