Biology 12

Chapter 3 Cellular Energy • MHR 79

Figure 3.15, on page 80, shows the two possible
fermentation pathways. In each case, NADH
molecules in the cytosol (formed by glycolysis II)
are oxidized to form NAD+, allowing glycolysis
to continue.

Figure 3.14Following the formation of pyruvate from
glucose, pyruvate may enter either aerobic cellular
respiration or anerobic cellular respiration.

glucose

pyruvate

oxygen present

aerobic cellular respiration

(reactions inside

mitochondria)

anerobic cellular respiration

(reactions outside

mitochondria)

oxygen absent

Manipulating Nutrient Intake

People who want to lose weight or reduce body fat

sometimes turn to restrictive low-fat or low-carbohydrate

diets. These types of diets may lead to weight loss in the

short term, but often have detrimental effects on the

body’s metabolic function.

The Effects of Extreme Low-Fat Diets

For example, some people try to control their weight by

severely restricting their fat intake, instead ingesting large

amounts of pasta, bread, and other carbohydrates

Someone who severely restricts their fat intake will not

necessarily prevent further accumulation of fat on their

body. This is particularly true when someone who is

not eating enough fat compensates by overeating

carbohydrates. Glucose from carbohydrates can be

converted to fatty acids via acetyl Co-A during the

transition reaction. Glucose can also be converted to

glycerol through glyceraldegyde-3-phosphate, as shown

below. In short, if you ingest food that contains more

energy than you can use, your body will store it as

fat, regardless of whether the food is carbohydrate,

protein, or fat.

Excess glucose is used to synthesize fats.

The Effects of Extreme

Low-Carbohydrate Diets

On the opposite end of the scale, extreme low-

carbohydrate diets, which tend also to be low in nutrients

overall, can also cause metabolic problems. People on

these diets are forced to derive their energy primarily from

stored fats. Breaking down lipids to form acetyl-CoA for

the Krebs cycle, however, produces byproducts that can

be harmful in large amounts.

These byproducts are compounds with ketone groups.

Some of these compounds are acids. The condition

caused by the build-up of ketones is called ketosis, and

is characterized by the smell of acetone on the breath. If

the condition progresses, the acidic ketones can lower

the blood pH to dangerous levels. Brain damage or even

death can result. This extreme condition is called

ketoacidosis.

Most experts agree that the best way to stay healthy and

maintain a healthy weight is to eat a broad variety of

foods that provide the right balance and quantity of

carbohydrates, protein, and fats.

Follow-Up

1.What percentage of your daily food intake comes

from carbohydrates, lipids, and proteins? Keep a

food log for several days to monitor your intake of

carbohydrates, proteins, and fats. Should you make

any adjustments to your diet to provide a healthier

mix of dietary energy sources?

2.To help people to follow a healthy, balanced diet,

organizations and governments issue dietary

guidelines. Do some research on the Internet to

find out about several different dietary guidelines

produced for the general public. For example, you

might choose to compare the Canada Food Guide

and the Federal Dietary Guidelines (U.S.).

(a)How do the guidelines compare with respect

to recommended nutrient sources and the

percentage of each type of nutrient?

(b)Which set of guidelines is easier to understand

and follow?

(c)Make recommendations on how you would

improve or expand upon each set of guidelines.

glucose

fats

glyceraldehyde-3-phosphate

pyruvate

glycerol

fatty acids

acetyl-CoA