expLoreon your own
suMMary
sections 3.1, 3.2 A plasma membrane
surrounds the inner region of cytoplasm of
a living cell. In a eukaryotic cell, including
human cells, membranes divide the cell
into organelles, compartments that separate
metabolic reactions in the cytoplasm.
section 3.4 Cell membranes consist
mainly of phospholipids and proteins. The
phospholipids form a lipid bilayer. Various
kinds of proteins in or attached to the
membrane perform most of its functions.
Some membrane proteins are transporter proteins. Others
are receptors. Still others have carbohydrate chains that serve
as a cell’s identity tags.
section 3.6 The largest organelle is the
nucleus, which contains the genetic material
DNA. The nucleus is surrounded by a double
membrane, the nuclear envelope. Pores in
the envelope help control the movement of
substances into and out of the nucleus.
A cell’s DNA and proteins associated with it are called chro-
matin. Each chromosome in the nucleus is one DNA molecule
with its associated proteins.
section 3.7 The endomembrane system
includes the endoplasmic reticulum (ER),
Golgi bodies, and various vesicles. In this
system new proteins are modified into final
form and lipids are assembled. Unwanted
materials may be broken down in lysosomes
and peroxisomes.sections 3.8, 3.9 Mitochondria carry
out the oxygen-requiring reactions that make
ATP, a nucleic acid that is the cell’s energy
currency. These reactions occur in the inner
compartment of mitochondria.
The cytoskeleton gives a cell its shape
and internal structure. It consists mainly
of microtubules and microfilaments; some types of cells also
have intermediate filaments. Microtubules are the structural
framework for cilia or flagella, which develop from centrioles
and are used in movement.section 3.10 A cell’s plasma mem-
brane is selectively permeable—only certain
substances may cross it, by way of transport
mechanisms.
In diffusion, substances move down their
concentration gradient. Osmosis is the name
for the diffusion of water across a selectively
permeable membrane in response to a concentration gradi-
ent, a pressure gradient, or both. In passive transport, a solute
moves down its concentration gradient through a membrane
transporter protein.section 3.11 In active transport, a
solute is pumped through a membrane
protein against its concentration gradient.
Active transport requires an energy boost, as
from ATP.
Cells use vesicles to take in or expel large molecules or
particles. In exocytosis, a vesicle moves to the cell surface and
fuses with the plasma membrane. In endocytosis, a vesicle
forms at the surface and moves inward. In phagocytosis, an
endocytic vesicle brings organic matter into a cell.In this chapter you learned that an enzyme can act only on certain
substrates. Because your saliva contains enzymes that can use some substances
as substrates but not others, you can easily gain some insight into practical impacts of
this concept (Figure 3.30). Start by holding a bite of plain cracker in your mouth for
30 seconds, without chewing it. What happens to the cracker, which is mostly starch
(carbohydrate)? Repeat the test with a dab of butter or margarine (lipid), then with a
piece of meat, fish, or even scrambled egg (protein). Based on your results, what type
of biological molecules do your salivary enzymes act upon?Figure 3.30 Enzymes digest the different
kinds of biological molecules in foods.© Cengage Learning/Gary Head64 Chapter 3Copyright 2016 Cengage Learning. All Rights Reserved. May not be copied, scanned, or duplicated, in whole or in part. Due to electronic rights, some third party content may be suppressed from the eBook and/or eChapter(s).