CHAPTER 4 Highlights 87
Key Concepts
Study CHAPTER HIGHLIGHTS
ZONE
Key Terms
Section 1
passive transport (74)
concentration gradient (74)
equilibrium (74)
diffusion (75)
osmosis (76)
hypertonic solution (77)
hypotonic solution (77)
isotonic solution (77)
ion channel (78)
carrier protein (80)
facilitated diffusion (80)Section 2
active transport (81)
sodium-potassium pump (81)
endocytosis (83)
exocytosis (83)
receptor protein (84)
second messenger (85)BIOLOGYBIOLOGY
Unit 1—Cell Transport and Homeostasis
Use Topics 1– 6 in this unit to review the key
concepts and terms in this chapter.Passive Transport
Passive transport is the movement of substances across
the cell membrane without the use of energy by the cell.
Diffusion is the movement of a substance from an area of
high concentration to an area of lower concentration.
Osmosis is the diffusion of free water molecules across a
selectively permeable membrane.
Ion channels are proteins that have a pore through which
ions can cross the cell membrane.
In facilitated diffusion, a carrier protein transports a
substance across the cell membrane down the concentra-
tion gradient of the substance.Active Transport
Active transport is the movement of a substance against
the concentration gradient of the substance. Active trans-
port requires cells to use energy.
In animal cells, the sodium-potassium pump uses energy
supplied by ATP to transport sodium ions out of the cell
and potassium ions into the cell.
During endocytosis, substances are moved into a cell by a
vesicle that pinches off from the cell membrane.
During exocytosis, substances inside a vesicle are released
from a cell as the vesicle fuses with the cell membrane.
Communication between cells often involves signal mol-
ecules that are bound by receptor proteins on cells.
A signal molecule that is bound by a receptor protein on
a cell can change the activity of the cell in three ways:
by enabling specific ions to cross the cell membrane, by
causing the formation of a second messenger, or
by speeding up chemical reactions inside the cell.