BIOL #17: Nutrition

Question 1

Nutrition

Answer 1

Nutrition refers to the general process by which food is taken into the body, taken apart, and taken up by the body’s cells.

Question 2

Heterotrophs vs. Autotrophs

Answer 2

Animals are heterotrophs—they obtain the energy and nutrients they need from other organisms rather than making their own food as plants (autotrophs) do.

Question 3

Diets

Answer 3

Animals have diverse diets and a variety of means for obtaining food.

• Herbivores eat mainly plants or algae (e.g. cows, termites)
• Carnivores eat mainly other animals (e.g. sharks, spiders)
• Omnivores eat both plants and animals (e.g. crows, humans)

However, most animals are opportunistic feeders – eating what is available even if outside their normal dietary range.

Question 4

Opportunistic Feeders

Answer 4

However, most animals are opportunistic feeders – eating what is available even if outside their normal dietary range.

Question 5

Nutritional Requirements

Answer 5

To meet the continuous requirements for ATP in the body, animals ingest and digest nutrients, including carbohydrates, proteins, and lipids, for use in cellular respiration and energy storage.

An animal’s diet must also supply the raw materials needed for biosynthesis in order for the organism to grow, maintain itself, and reproduce.

Animals must obtain two types of organic precursors from food with which they can construct a large variety of organic molecules:

1) Source of organic carbon (e.g. sugar)
2) Source of organic nitrogen (e.g. protein)

Question 6

Essential Nutrients

Answer 6

The materials that an animal’s cells require but cannot synthesize are called essential nutrients

• These materials are obtained from dietary sources and include certain minerals and preassembled organic molecules
• Some nutrients are essential for all organisms, whereas others are needed only by certain species

There are four classes of essential nutrients:

Essential amino acids cannot be synthesized and must be obtained from food.
- Most animals, including adult humans, require 8 amino acids in their diet

Essential fatty acids are the ones that animals do not have the enzymes to synthesize – these are fatty acids that contain one or more double bonds (i.e. unsaturated)
- Example: humans require linoleic acid (found in seeds, grains, vegetables) to make some cell membrane phospholipids

Vitamins are organic compounds that are vital for health but are required only in specific small amounts; several function as coenzymes in critical reactions.

Minerals are inorganic nutrients (e.g. iron, sulfur, sodium and chloride) that are usually required in small amounts; certain minerals serve as coenzymes in critical reactions or are required for the functioning of nerve cells and maintaining osmotic pressure (e.g. electrolytes).

Question 7

Human Vitamin Requirements

Answer 7

Vitamins are classified as water-soluble and fat-soluble

Moderate overdoses of water-soluble vitamins (e.g. vitamin C) are probably harmless because excesses of these vitamins are excreted in urine

Excesses of fat-soluble vitamins (e.g. vitamin A) are deposited in body fat, so overconsumption may result in accumulating toxic levels of these compounds

Question 8

Ingestion

Answer 8

Ingestion is the act of eating or feeding

Animals utilize four main feeding mechanisms:

• Suspension/ Filter feeding
• Substrate feeding
• Fluid feeding
• Bulk feeding

Question 9

Digestion

Answer 9

Digestion is the breakdown of food into small enough pieces to allow for absorption—the uptake of nutrients.

• Mechanical digestion (the physical breakdown of food) typically proceeds chemical digestion (enzymatic hydrolysis of food) because it increases the surface area available for chemical digestion to occur.
• Carbohydrates, proteins, and lipids must be chemically broken down because these molecules are too large to pass through membranes to enter the cells of the body.

Question 10

Absorption

Answer 10

Absorption is the process of taking up small molecules, such as simple sugars and amino acids, after food digestion.

Question 11

Elimination

Answer 11

Elimination is the process of passing undigested material out of the digestive system.

Question 12

Intracellular Digestion

Answer 12

Intracellular digestion involves the simplest digestive compartments – enzymatic hydrolysis in food vacuoles within cells
- This type of digestion occurs in organisms with very simple body plans, e.g. single-celled organisms and sponges

Cells take in solid food via phagocytosis or liquid via pinocytosis and the newly formed food vacuoles fuse with lysosomes
- Lysosomes contain hydrolytic enzymes and compartmentalize digestion within cells.

Question 13

Extracellular Digestion

Answer 13

Extracellular digestion involves the breakdown of food in compartments that are continuous with the outside of an animal’s body.

Extracellular digestion come in two general designs:

1) Incomplete digestive tracts.
2) Complete digestive tracts.

Question 14

Incomplete Digestive Tracts

Answer 14

Incomplete digestive tracts have a single opening through which food is ingested and waste is eliminated (e.g. hydra).

The mouth opens into a chamber, called a gastrovascular cavity, where digestion takes place.

Question 15

Complete Digestive Tracts

Answer 15

Complete digestive tracts

• Also called the alimentary canal or gastrointestinal (GI) tract
• have two openings – one at the mouth and the other at the anus
• The interior of this tube communicates directly with the external environment via these openings.

Complete digestive tracts have three advantages:

1) Animals can feed on large pieces of food.
2) Since the tract is unidirectional, chemical and physical processes can be separated within the canal, so that they occur independently of each other and in a specific sequence.
3) Material can be ingested and digested continuously.

Question 16

Digestive Processes: Mammals

Answer 16

Three major types of biomolecules must be broke down during digestion: carbohydrates, lipids, and proteins

• This chemical processing (digestion), starting in the mouth, continues in the stomach and finishes in the small intestine.
• Accessory glands (salivary glands, pancreas, liver and gallbladder) are not a direct part of the complete digestive tract but secrete digestive juices through ducts into the gastrointestinal tract.

The small molecules that result from this digestion are absorbed in the small intestine, along with water, vitamins, and minerals (ions).

More water is absorbed in the large intestine, producing feces (undigested waste) that eventually exit the body at the anus.

Question 17

The Oral Cavity

Answer 17

Digestion begins in the oral cavity (mouth), starting with the tearing and crushing activity of teeth during chewing (i.e. mechanical digestion).

During chewing, enzymes start to break down some components of the food:

• Salivary glands produce amylase which begins the breakdown of carbohydrates.
• Cells in the tongue secrete lipase, which begins the breakdown of lipids.

Salivary glands in the mouth also release water and glycoproteins called mucins. When mucins contact water, they form the slimy substance called mucus.

The combination of water and mucus makes food soft and slippery enough to be swallowed.
- The formed ball of food and saliva is called a bolus.

Question 18

The Pharynx

Answer 18

The pharynx (throat region) opens to the esophagus and the trachea (windpipe).

During the act of swallowing, a flap of cartilage, called the epiglottis, covers the glottis – which is the opening to the trachea.
- This directs the bolus of food to the esophagus.

Question 19

The Esophagus

Answer 19

The bolus of food then enters the esophagus – a muscular tube connecting the mouth and stomach.

The act of swallowing stimulates a reflex (automatic reaction to a stimulus):
- A wave of muscle contractions mixes the bolus of food (segmentation) and propels it to the stomach (peristalsis).

Question 20

The Stomach

Answer 20

The stomach is a tough, muscular pouch bracketed on both ends by valves called sphincters.

After eating, muscular contractions in the stomach result in churning that mixes and breaks down the food mechanically.

The stomach secretes a digestive fluid called gastric juice which helps break down food chemically. The mixture of ingested food and gastric juice is called chyme.

The lumen of the stomach is highly acidic; the predominant acid in the stomach is hydrochloric acid (HCl).

Question 21

Protein Digestion in the Stomach

Answer 21

Two components of gastric juice carry out chemical digestion of protein: hydrochloric acid (HCl) and the enzyme pepsin
- HCl lowers pH and denatures (unfolds) proteins, exposing their peptide bonds. Pepsin can easily break these bonds when proteins are in this denatures state – thus a low pH is an optimal pH environment for pepsin activity.

Question 22

Secretory Cells

Answer 22

The stomach epithelium contains several types of secretory cells, each of which is specialized for a particular function:

Chief cells contain pepsinogen which is a precursor of pepsin.

• Pepsinogen is converted to active pepsin when it contacts the acidic environment of the stomach lumen – HCl activates this enzyme by interacting with the molecule and exposing its active site (active pepsin can also activate pepsinogen)
• Secretion of pepsin in an inactive form is important: It prevents destruction of proteins in the cells where the enzyme is synthesized.

Parietal cells secrete hydrogen and chloride ions, which form HCl – decreasing the pH of the gastric juice to as low as 1.5.

Mucous cells secrete mucus which lines the gastric epithelium and protects the stomach tissue from damage by HCl.

Question 23

Production of Gastric Juice

Answer 23

The production of gastic juice:

1) Pepsinogen and HCl are secreted into the lumen of the stomach
2) HCl converts pepsinogen to pepsin
3) Pepsin then activates more pepsinogen, starting a chain reaction so that protein digestion can begin

Question 24

Ulcer

Answer 24

An ulcer is a hole in the epithelium that damages the underlying basement membrane and tissues.

Ulcers in the stomach lining or the first part of the small intestine can result in intense abdominal pain.

Ulcers were initially thought to result from excess acid in the stomach and psychological stress, however, researchers have determined that the majority of ulcers are caused by an acid-tolerant bacterium called Helicobacter pylori.
- Now most stomach ulcers are treated using antibiotic treatments.

Question 25

The Small Intestine

Answer 25

Partially digested food passes from the stomach into the small intestine

• a 6 m (20 ft) long tube – the longest part of the alimentary canal
• The small intestine includes 3 parts: the duodenum, jejunum, and ileum

In the first part of the small intestines (duodenum), chyme from the stomach mixes with secretions from the pancreas and liver to continue digestion and begins to move through the tube.

At the end of the small intestine, digestion is complete and most nutrients—along with much water—has been absorbed.

The small intestine has an enormous surface area (in humans, it’s roughly the size of a tennis court) for absorption of nutrients due to projections called villi, which in turn have projections called microvilli.

Because each villus contains blood vessels and a lymphatic vessel called a lacteal, nutrients pass quickly from epithelial cells into the body’s transport systems.

Question 26

Liver

Answer 26

The liver (an accessory gland) aids chemical digestion by producing bile, a mixture of substances that contain bile salts.
- Bile salts act as detergents and emulsify (break down) lipids.

After bile is produced by the liver, it is stored and concentrated in the gallbladder.
- Bile is released into the duodenum of the small intestine to begin the breakdown of lipids.

Question 27

Pancreas

Answer 27

The pancreas (an accessory gland) aids chemical digestion by producing several substances that are secreted into the duodenum:
- An alkaline (basic) solution rich in bicarbonate that neutralizes the acidity of the chyme that enters the duodenum (acts as a buffer)
- Pancreatic lipase, which breaks down lipids into fatty acids
- Pancreatic amylase, which breaks down carbohydrates
- Pancreatic proteases which break down protein
\+ These proteases (trypsin and chymotrypsin) are secreted into the duodenum in inactive forms and are triggered to activate once safely in the duodenum – much like the activation of pepsin in the stomach.

Question 28

Hormones in Digestive System

Answer 28

Hormones are involved in stomach and intestinal functioning.

• After being stimulated by nerves or the arrival of food, certain stomach cells produce the hormone gastrin. In response, parietal cells begin secreting HCl.
• A hormone produced in the small intestine, cholecystokinin (CCK), stimulates the secretion of digestive enzymes from the pancreas and the secretion of bile stored in the gallbladder to begin the digestion of lipids, etc.

Question 29

Hormones in Digestive System

Answer 29

Hormones are involved in stomach and intestinal functioning.

• After being stimulated by nerves or the arrival of food, certain stomach cells produce the hormone gastrin. In response, parietal cells begin secreting HCl.
• A hormone produced in the small intestine, cholecystokinin (CCK), stimulates the secretion of digestive enzymes from the pancreas and the secretion of bile stored in the gallbladder to begin the digestion of lipids, etc.

Question 30

Absorption in the Small Intestine

Answer 30

Depending on the nutrient, transport across the epithelial cells from the lumen to the blood vessels can be passive or active.

Nutrients may pass down their concentration gradient via facilitated diffusion (e.g. fructose)

Nutrients may pass against their concentration gradient via active transport with cotransporters (e.g. amino acids, small peptides, vitamins, and glucose)

Question 31

Liver in Nutrients Absorption

Answer 31

The capillaries and veins that carry nutrient-rich blood away from the villi of the small intestine all converge into the hepatic portal vein, a blood vessel that leads directly to the liver.

The liver serves two main functions in the digestive process:
- The liver can regulate the distribution of nutrients to the rest of the body
- The liver can remove toxic substances before the blood circulates throughout the body
+ The liver is the primary site for the detoxification of many organic molecules, including drugs, that are foreign to the body.

Question 32

Water Absorption in the Small Intestine

Answer 32

Some water absorption occurs in the small intestines.

When solutes from digested material are brought into the epithelium of the small intestine via active transport, water follows passively by osmosis.

Question 33

The Large Intestine

Answer 33

By the time digested material reaches the end of the small intestine, a large amount of water and virtually all of the available nutrients have been absorbed.

The primary function of the last part of the alimentary canal, the large intestine, is to compact wastes that remain and absorb enough water to form feces.

These processes occur in the colon, the main section of the large intestine.

Feces are held in the rectum, the final part of the large intestine, until they can be excreted via the anus.

Question 34

The Cecum and Appendix

Answer 34

The cecum is an outpocketing of the digestive tract located at the start of the large intestine.

The cecum is greatly enlarged in herbivorous species such as rabbits, koalas, and leaf-eating primates that use it as a fermentation chamber (harboring symbiotic bacteria) for processing cellulose.

In humans (and many carnivores), the cecum is dramatically reduced in size and has a minor function in defense against invading bacteria and viruses (immunity). 
- Because its size and function differ from those of a cecum, it is called the appendix in humans.

Question 35

Water Reabsorption in the Large Intestine

Answer 35

Large amounts of water accompany digestion because it acts as a solvent for the digestive juices

The transmembrane water channels aquaporins play a major role in water movement from the large intestine back into the body.
- The presence of aquaporins allows more water molecules to move across the epithelia quickly

Water is reabsorbed by osmosis when Na+ and other ions are pumped from the lumen of the large intestines into the epithelial cells (water moves towards high solute concentration).

Diarrhea results when viral or bacterial infections cause the colon lining to absorb less water than usual

Constipation results when feces move along the colon too slowly.
- Cellulose fiber helps move food through the small intestine and feces through the colon

Question 36

Nutritional Homeostasis

Answer 36

When digestion is complete, amino acids, fatty acids, ions, and sugars enter the bloodstream and are delivered to the cells that need them.

Digesting too much of a nutrient, or too little, can be problematic or even fatal, however.

Question 37

diabetes mellitus (DM)

Answer 37

People with the disease diabetes mellitus (DM) experience abnormally high levels of glucose in their blood.

Over the course of a lifetime, the chronic glucose imbalance associated with diabetes can lead to reduction of circulation in the legs, blindness, and even heart failure.

The disease is caused by problems with a hormone called insulin.

Question 38

Glucose Homeostasis

Answer 38

Excess glucose in the body is stored as glycogen in the liver and muscle tissue

The pancreatic hormones, insulin and glucagon maintain glucose homeostasis by tightly regulating the synthesis and breakdown of glycogen

Insulin levels rise after a carbohydrate-rich meal and stimulate the liver to convert and store glucose, entering from the hepatic portal vein, as glycogen

Between meals, when there is a much lower glucose concentration in the blood coming from the small intestine, glucagon stimulates the liver to break down glycogen stores, releasing glucose into the bloodstream

The combined action of insulin and glucagon results in blood exiting the liver at concentrations between 70-110 mg/100 ml at nearly all times – i.e. glucose homeostasis.

Question 39

Diabetes Can Take Several Forms

Answer 39

Diabetes is caused by either a deficiency of insulin or a decreased response to insulin:
- Type I Diabetes mellitus: caused by an inability to synthesize insulin
+ Pancreas cannot make insulin
- Type II Diabetes mellitus: caused by defective versions of the insulin receptor
+ Liver cannot respond to increase the glycogen production

In both cases diabetes leads to abnormally high (sometimes potentially fatal) blood glucose levels.

Currently, Type I DM is treated with insulin injections and careful attention to diet.

Type II DM is managed through prescribed diets, monitoring blood glucose levels, and drugs that increase cellular responsiveness to insulin.

The challenge is to achieve blood glucose homeostasis in the absence of the body’s normal regulatory mechanisms.

There is currently an epidemic of type II DM in human populations.

Although some individuals have a genetic predisposition for Type II DM, there is strong evidence that environmental conditions, such as increased fat intake and decreased physical activity, significantly decrease the responsiveness of insulin receptors.
- The incidence of Type II DM is correlated with obesity.