(a) What are intestinal glands? Describe the roles of intestinal juices in digestion and absorption of food. 20 (b) Define…

(a) Intestinal glands and the roles of intestinal juice

Intestinal glands (crypts of Lieberkühn) are simple tubular glands in the mucosa of the small intestine, opening between the villi. Their epithelium has enterocytes, mucus-secreting goblet cells, Paneth cells at the base (antimicrobial lysozyme and defensins), enteroendocrine cells (secretin, cholecystokinin) and stem cells that renew the epithelium every few days. They secrete intestinal juice (succus entericus), about 1.8 L a day, watery and alkaline (pH 7.5–8.0), with mucus, bicarbonate and electrolytes. Brunner's glands in the duodenal submucosa add alkaline mucus. The juice itself holds few enzymes; most digestive enzymes sit on the brush border of the enterocytes.

Role in digestion:

• enterokinase (enteropeptidase) converts trypsinogen to trypsin, which activates the other pancreatic zymogens
• carbohydrates: maltase, sucrase, lactase and isomaltase (α-dextrinase) give glucose, fructose and galactose
• proteins: aminopeptidases and dipeptidases split peptides into amino acids
• nucleotidases and nucleosidases split nucleotides into sugar, phosphate and bases; a little intestinal lipase finishes glyceride digestion
• bicarbonate neutralises acid chyme and sets the optimum pH; mucus lubricates and protects the mucosa

Role in absorption:

• the watery juice is a vehicle that dissolves the chyme and brings digested food to the villi, whose folds, villi and microvilli greatly increase the surface area
• terminal digestion on the brush border is coupled to uptake: glucose and galactose enter by Na⁺-linked SGLT1, fructose by GLUT5, and all leave through GLUT2 into the blood capillaries; amino acids and small peptides enter by Na⁺- or H⁺-coupled carriers
• fatty acids and monoglycerides enter from bile-salt micelles, are rebuilt into triglycerides, packed into chylomicrons and pass into the lacteals
• water, electrolytes and vitamins are absorbed along with the nutrients

(b) Thermoregulation in homeotherms

Thermoregulation is the maintenance of a nearly constant core body temperature (about 37 °C in humans) by balancing heat production against heat loss, whatever the environmental temperature. Homeotherms (birds and mammals) do this by negative feedback: thermoreceptors in the skin and hypothalamus signal the hypothalamic thermostat (preoptic area of the anterior hypothalamus), which controls the effectors.

In hot climates (heat loss increased, heat gain reduced):

• cutaneous vasodilation raises skin blood flow, so more heat is lost by radiation and convection
• sweating (humans, horses) and panting (dogs, cats, pigs) cool by evaporation; birds use gular fluttering
• metabolic rate and muscle tone fall, because sympathetic and thyroid activity decrease
• behaviour: seeking shade or water, nocturnal activity, burrowing; large ears (elephant) act as radiators

In cold climates (heat conserved, heat production increased):

• cutaneous vasoconstriction, through the sympathetic system, reduces heat loss; counter-current heat exchange between arteries and veins of the limbs keeps the extremities cool and the core warm
• piloerection, dense fur, feathers and subcutaneous fat or blubber insulate
• shivering thermogenesis: rhythmic muscle contraction releases heat
• non-shivering thermogenesis: thyroxine, adrenaline and noradrenaline raise the metabolic rate, and brown adipose tissue (infants, hibernators) oxidises fat with uncoupling protein UCP1, releasing heat instead of ATP
• behaviour: huddling, burrowing, migration; some mammals hibernate

(c) Structure of the retina and mechanism of vision in the mammalian eye

The labelled diagram of the retina should show its layers from the choroid side to the vitreous side:

• pigment epithelium
• photoreceptor layer: rods and cones with outer and inner segments
• outer nuclear layer (photoreceptor nuclei) and outer plexiform layer
• inner nuclear layer (bipolar, horizontal and amacrine cells) and inner plexiform layer
• ganglion cell layer, and the nerve fibre layer running to the optic nerve
• the fovea (cones only) and the optic disc (blind spot), with light entering from the vitreous side

The retina is inverted: light crosses all the layers before it reaches the rods and cones.

Mechanism of vision:

1. The cornea and lens focus an inverted image on the retina.
2. In the dark, cGMP keeps Na⁺ channels open in the rods, so the cell is depolarised and releases glutamate continuously.
3. In light, 11-cis-retinal of rhodopsin (opsin plus a vitamin A derivative) changes to all-trans-retinal and activates opsin.
4. Activated opsin activates the G protein transducin, which activates phosphodiesterase; this breaks down cGMP.
5. Falling cGMP closes the Na⁺ channels, the rod hyperpolarises and glutamate release falls.
6. Bipolar cells pass the change to ganglion cells, whose axons form the optic nerve; impulses reach the visual cortex of the occipital lobe through the lateral geniculate nucleus.
7. Retinal is converted back to the 11-cis form in the pigment epithelium, regenerating rhodopsin.

Rods work in dim light; cones, with three opsins (blue, green, red), give colour vision and acuity.