Energy and Metabolism
Chapter 4
Part 2
4.5 Movement of Ions and Molecules
 For metabolism to work, a cell must keep its
internal composition stable – even when
conditions outside are greatly different
 Selective permeability
• Membrane property that allows some substances,
but not others, to cross
Selective Permeability of Cell Membranes
Diffusion
 Molecules or ions tend to follow their own
concentration gradient and diffuse into an
adjoining region of fluid in which they are less
concentrated
 Diffusion
• Net movement of molecules or ions from a region
of higher concentration to a region of lower
concentration
Osmosis and Tonicity
 Water molecules tend to diffuse in response to
their own concentration gradient
 Osmosis
• Net diffusion of water molecules across a
selectively permeable membrane between two
fluids with different water concentrations
Osmosis
Osmosis and Tonicity
 Tonicity describes relative concentrations of
solutes in fluids separated by a selectively
permeable membrane
• Hypotonic: Low solute concentration relative to
another fluid
• Hypertonic: High solute concentration relative to
another fluid
• Isotonic: Same solute concentration relative to
another fluid
Fig. 4-12 (a), p. 71
B Red blood cells
immersed in an isotonic
solution do not change in
volume. The fluid portion
of blood is typically
isotonic with cytoplasm.
C Red blood cells
immersed in a hypertonic
solution shrivel up
because more water
diffuses out of the cells
than into them.
D Red blood cells
immersed in a hypotonic
solution swell up
because more water
diffuses into the cells
than out of them.
Fig. 4-12 (b-d), p. 71
4.6 Membrane Crossing Mechanisms
 Gases, water, and small nonpolar molecules can
diffuse across a lipid bilayer
 Most other molecules and ions cross only with
the help of transport proteins, which gives a cell
or membrane-enclosed organelle control over
which substances enter and exit
Transport Proteins
 Each type of transport protein moves a specific
ion or molecule across a membrane
 The types of transport proteins in a membrane
determine which substances cross it
• Examples: glucose transporters, calcium pumps
Passive and Active Transport
 Passive transport
• Concentration gradient drives a solute across a
cell membrane through a transport protein
• Requires no energy input
 Active transport
• A transport protein use energy, usually from ATP,
to pump a solute across a cell membrane against
its concentration gradient
Passive Transport
Extracellular
Fluid
glucose
1
Cytoplasm
lipid bilayer
Fig. 4-13a, p. 72
2
Fig. 4-13b, p. 72
3
Fig. 4-13c, p. 72
Active Transport
Sarcoplasmic
Reticulum
Cytoplasm
calcium
A
B
C
Fig. 4-14, p. 73
Cotransport
 Cotransporter
• Active transport protein that moves two
substances across a membrane in opposite
directions at the same time
 Example: sodium-potassium pump
• ATP powers an active transport protein that
pumps Na+ out of and K+ into a cell
Cotransport: Sodium-Potassium Pump
Endocytosis and Exocytosis
 Endocytosis
• Process by which a cell takes in a small amount
of extracellular fluid by a ballooning inward of its
cellular membrane
 Exocytosis
• Process by which a cell expels a vesicle’s
contents to extracellular fluid by merging the
vesicle with the plasma membrane
Phagocytosis
 Phagocytosis (“cell eating”)
• Endocytic pathway by which cells such as
macrophages and other white blood cells engulf
particles such as microbes or cellular debris
• Amoebas also are phagocytic cells
Phagocytosis