Gas Exchange in Alveoli and Fish

• Surface area to volume ratio is calculated by dividing the surface area by the volume.

Inspiration

• External intercostal muscles contract, lifting the ribcage up and out.
• The diaphragm contracts, increasing the volume of the thoracic cavity.
• Volume increase lowers pressure, drawing air into the lungs down the pressure gradient.

Expiration

• Internal intercostal muscles contract, moving the ribcage inwards, reducing thoracic cavity volume.
• Volume decrease raises pressure, forcing air out of the lungs down the pressure gradient.

Gas Exchange in Alveoli

• Short diffusion distance due to the thin, single-layered epithelium.
• High surface area due to the many alveoli.
• Dense capillary network maintains a steep concentration gradient with a good blood supply.

Gas Exchange in Fish

• Gills are the site of gas exchange.
• Gill filaments stack, and each filament contains many lamellae which increases surface area.
• Thin gill lamellae with a capillary network close to the surface ensure a short diffusion distance.
• Countercurrent system maintains a concentration gradient by flowing water and blood in opposite directions, preventing equilibrium.
• A diffusion gradient is maintained across gill lamellae's length due to the countercurrent system.

Insect Adaptations to Prevent Water Loss

• Small surface area to volume ratio minimizes water evaporation.
• Waxy exoskeleton is waterproof.
• Spiracles along the abdomen can open or close to regulate gas exchange.
• Trachea attached to spiracles branch into tracheoles, delivering oxygen to tissues.

Gas Exchange in the Tracheal System

• Simple diffusion facilitates gas exchange.
• Cells respire, releasing carbon dioxide and maintaining the concentration gradient.
• Abdominal muscles contract and relax to move gases.
• During flight, lactic acid from aerobic respiration lowers water potential, drawing water from tracheoles into cells via osmosis and air is drawn in.
• Large surface area due to numerous tracheoles.
• Short diffusion distance due to thin tracheole walls and short spiracle-tracheole distance.
• Respiring cells consume oxygen and release carbon dioxide, maintaining the concentration gradient.

Gas Exchange in Plants

• Carbon dioxide diffuses into stomata as guard cells alter shape, becoming less bent and more parallel.
• Carbon dioxide diffuses into the spongy mesophyll, where ample space helps in maintaining concentration gradients.
• Carbon dioxide diffuses from spongy mesophyll into palisade mesophyll for photosynthesis.
• Numerous chloroplasts close to the surface maximize sunlight absorption.

Xerophytic Plant Adaptations

• Xerophytic plants are adapted to survive in water-limited environments.
• Hairs on the plant trap moisture, increasing humidity and reducing evaporation.
• Curled leaves trap water vapour, increasing humidity and reducing evaporation.
• Sunken stomata trap water vapour, increasing humidity and reducing evaporation.

Digestion

• Digestion involves the hydrolysis of large, insoluble molecules into smaller, soluble ones for transport and absorption.

Carbohydrate Digestion

• In salivary glands, amylase hydrolyses polysaccharides into disaccharides by breaking glycosidic bonds.
• Membrane-bound disaccharidases in the duodenum and ileum hydrolyse disaccharides into monosaccharides.
• Sucrase breaks down sucrose into fructose and glucose.
• Lactase breaks down lactose into galactose and glucose.
• Maltase breaks down maltose into glucose.

Protein Digestion

• Endopeptidases hydrolyse peptide bonds within polypeptide chains.
• Exopeptidases hydrolyse peptide bonds at the ends of polypeptide chains.
• Dipeptidases hydrolyse peptide bonds between two amino acids.
• Digestion happens in the duodenum and finishes in the ileum.

Lipid Digestion

• Bile salts (produced in the liver) and lipase (produced in the pancreas) digest lipids.
• Bile salts emulsify lipids into tiny droplets, increasing the surface area for lipase action.
• Lipase hydrolyses ester bonds in triglycerides to form fatty acids and monoglycerides.
• Micelles (fatty acids, monoglycerides, and bile salts) make fatty acids and monoglycerides water-soluble.
• Micelles transport fatty acids and monoglycerides to the plasma cell membrane, releasing them into epithelial cells of the ileum via simple diffusion.
• Fatty acids and monoglycerides are modified into chylomicrons by the Golgi apparatus or form triglycerides in the smooth endoplasmic reticulum.
• They are then packaged into vesicles, released by exocytosis into the lacteal.

Haemoglobin

• Haemoglobin is a protein with a quaternary structure containing four polypeptide chains.
• Contains haem groups with iron where oxygen binds.
• Saturated haemoglobin carries the maximum amount of oxygen.
• Loading of haemoglobin refers to oxygen binding.

Oxygen Dissociation Curve

• The curve is slightly S-shaped.
• High partial pressure of oxygen leads to higher saturation of haemoglobin, indicating a higher affinity.
• Low partial pressure results in easier oxygen unloading, which is common in respiring tissues.

Cooperative Nature of Oxygen Binding

• Initial oxygen binding is difficult, but it alters haemoglobin's quaternary structure, facilitating subsequent oxygen binding.

Bohr Effect

• Carbon dioxide dissolved in blood increases its acidity, which lowers haemoglobin's affinity for oxygen, promoting unloading.
• Carbon dioxide changes the shape of haemoglobin.
• Left shift indicates higher haemoglobin affinity for oxygen.
• Right shift indicates lower haemoglobin affinity for oxygen.

Foetal Haemoglobin

• Foetal haemoglobin has a higher affinity for oxygen, enabling oxygen absorption from adult haemoglobin.

Haemoglobin Adaptations

• Llama haemoglobin has higher oxygen affinity due to high altitudes and lower partial pressure of oxygen.
• Dove haemoglobin has higher oxygen affinity due to high metabolic rate and muscle contraction needs.
• Earthworm haemoglobin has higher oxygen affinity at low partial pressures underground, leading to readier saturation and loading.