HUMAN BIOLOGY

Cells and how they work 65

© Cengage Learning

section 3.13 The chemical reactions in
a cell are collectively called its metabolism. A
metabolic pathway is a stepwise sequence of
chemical reactions catalyzed by enzymes—
catalytic molecules that speed up the rate of
metabolic reactions. Each enzyme interacts only with a specific
substrate, linking with it at one or more active sites.
Anabolism builds large, energy-rich organic compounds
from smaller molecules. Catabolism breaks down molecules
to smaller ones. Most anabolic reactions run on energy from
ATP, which is replenished by way of the ATP/ADP cycle.

sections 3.14, 3.15 In human cells, cellular
respiration produces ATP molecules. This pathway
releases chemical energy from glucose and other
organic compounds. Cellular respiration begins
with glycolysis (in the cytoplasm), which makes a
little ATP. Then in mitochondria the Krebs cycle and electron
transport generate a large amount of ATP in steps that
require oxygen. These steps are the aerobic pathway of cellular
respiration, in which oxygen is the final acceptor of electrons
removed from glucose. The typical net energy yield of cellular
respiration is thirty-six ATP.

section 3.16 In cells, complex carbo-

hydrates are broken down to the simple

sugar glucose, the body’s main metabolic

fuel. Alternatives to glucose include fatty

acids and glycerol from triglycerides and

sometimes amino acids from proteins.

reView Questions

1. Describe the general functions of the following in a
   eukaryotic cell: the plasma membrane, cytoplasm, DNA,
   ribosomes, organelles, and cytoskeleton.


2. Which organelles are in the endomembrane system?


3. Distinguish between the following pairs of terms:
   a. diffusion; osmosis
   b. passive transport; active transport
   c. endocytosis; exocytosis


4. What do enzymes do in metabolic reactions?


5. In aerobic cellular respiration, which reactions occur
   only in the cytoplasm? Which ones occur only in a cell’s
   mitochondria?


6. For the diagram of the aerobic pathway shown in the next
   column, fill in the number of molecules of substances
   formed at each stage.

NADH, FADH 2

NADH pyruvate

1 glucose

Glycolysis ATP (net)

Electron Transport

Chain

Krebs

Cycle CO 2

ATP

ATP

seLF-Quiz Answers in Appendix VI

1. The plasma membrane.
   a. surrounds the cytoplasm
   b. separates the nucleus from the cytoplasm
   c. separates the cell interior from the environment
   d. both a and c


2. The is responsible for a eukaryotic cell’s shape,
   internal organization, and cell movement.


3. Cell membranes consist mainly of a.
   a. carbohydrate bilayer and proteins
   b. protein bilayer and phospholipids
   c. phospholipid bilayer and proteins


4. carry out most membrane functions.
   a. Proteins c. Nucleic acids
   b. Phospholipids d. Hormones


5. The passive movement of a solute through a membrane
   protein down its concentration gradient
   is an example of.
   a. osmosis c. endocytosis
   b. active transport d. diffusion

ATP

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