UNIT A Chapter 3: Cell Structure and Function
Chapter 3: Cell Structure and Function
In this chapter, you will learn about how cell structures have critical
roles to play in the health of an organism.
What other cellular organelles
have a similar function to the
lysosome?
Why doesn’t the cell “clean
up” the faulty lysosomes?
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UNIT A Chapter 3: Cell Structure and Function
Section 3.5
3.5 The Permeability of the Plasma Membrane
The plasma membrane is selectively permeable, allowing
passage of only certain molecules.
Figure 3.17 How molecules cross
the plasma membrane. Molecules
that can diffuse across the plasma
membrane are shown with long
back-and-forth arrows. Substances
that cannot diffuse across the
membrane are indicated by the
curved arrows.
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UNIT A Chapter 3: Cell Structure and Function
Section 3.5
Passage of Molecules Across the Membrane
• Some substances freely cross the membrane. They move
“down” their concentration gradient (from high
concentration to low concentration).
• Some substances are unable to freely cross and are
transported by proteins or vesicles. They may go “up,” or
against, their concentration gradient.
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UNIT A Chapter 3: Cell Structure and Function
Section 3.5
Diffusion
Diffusion is the movement of molecules down their
concentration gradient. It does not require energy. The rate of
diffusion is affected by factors such as temperature, pressure,
and molecule size.
• A solution contains a solute in a solvent. Diffusion occurs
until there is an equal distribution of solute and solvent.
Figure 3.18
Process of
Diffusion.
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UNIT A Chapter 3: Cell Structure and Function
Section 3.5
Diffusion of Oxygen
Only a few types of molecules
can diffuse across the plasma
membrane.
• Gases can diffuse across the
bilayer
• Oxygen enters cells and
carbon dioxide leaves
• In lungs, oxygen moves
from the alveoli to blood
in the capillaries
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Figure 3.19 Gas exchange in lungs. Oxygen (O2)
diffuses into the capillaries of the lungs because
there is a higher concentration of oxygen in the
alveoli (air sacs) than in the capillaries.
UNIT A Chapter 3: Cell Structure and Function
Section 3.5
Osmosis
Osmosis is the diffusion of water molecules across a
selectively permeable membrane due to a difference in
concentration.
• There is a net movement of water and changes in solute
concentration on both sides of the membrane
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Figure 3.20 Osmosis demonstration.
UNIT A Chapter 3: Cell Structure and Function
Section 3.5
Isotonic, Hypotonic, and Hypertonic Solutions
Isotonic solutions have the same
concentration of solute and solvent as the
solution inside the cell, and water will not
enter or leave the cell.
Hypotonic solutions have a lower
concentration of solute than solution inside
the cell, and water will enter the cell.
Hypertonic solutions have a higher
concentration of solute than solution inside
the cell, and water will leave the cell.
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Prefixes:
iso: the same as
hypo: less than
hyper: more than
_____________
tonicity: refers to
osmotic pressure
UNIT A Chapter 3: Cell Structure and Function
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Figure 3.21 Osmosis in animal and plant cells.
Section 3.5
UNIT A Chapter 3: Cell Structure and Function
Section 3.5
Transport by Carrier Proteins
The plasma membrane stops the passage of most molecules
into and out of the cell. However, biologically important
molecules do pass. They do so because of carrier proteins that
exist in the plasma membrane.
• Carrier proteins are specific and each binds to specific
molecules
• Carrier proteins are required for both facilitated transport
and active transport of substances across the plasma
membrane
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UNIT A Chapter 3: Cell Structure and Function
Facilitated Transport
• Assists in transport of molecules across the membrane
by binding to those molecules
• Occurs down a concentration gradient and does not
require ATP
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Figure 3.22 Facilitated transport.
Section 3.5
UNIT A Chapter 3: Cell Structure and Function
Section 3.5
Active Transport
• Assists transport of substances across the membrane by
binding to them
• Occurs against a concentration gradient and requires
energy, usually in the form of ATP
Proteins involved in active transport are often called pumps
because they use energy to pump substances against their
concentration gradient.
• One important carrier protein pump is the sodiumpotassium pump. It moves sodium ions to the outside of
the cell and potassium ions to the inside of the cell.
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UNIT A Chapter 3: Cell Structure and Function
Figure 3.23 The sodiumpotassium pump. The
same carrier protein
transports sodium ions
(Na+) to the outside of the
cell and potassium ions
(K+) to the inside of the cell
because it undergoes an
ATP-dependent change in
shape. Three sodium ions
are carried outward for
every two potassium ions
carried inward. Therefore,
the inside of the cell is
negatively charged
compared to the outside.
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Section 3.5
UNIT A Chapter 3: Cell Structure and Function
Section 3.5
Bulk Transport
Macromolecules are transported into and out of the cell by
vesicle formation, called membrane-assisted transport in
energy-dependent processes.
• Exocytosis is a way substances can exit a cell
• Endocytosis is way substances can enter a cell
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UNIT A Chapter 3: Cell Structure and Function
Section 3.5
Exocytosis
During exocytosis, a vesicle fuses with the membrane and the
substance it is carrying is secreted outside of the cell.
• Neurotransmitters, hormones, and digestive enzymes are
examples of substances secreted in this way
Figure 3.24 Exocytosis. Exocytosis
deposits substances on the outside
of the cell and allows secretion to
occur.
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UNIT A Chapter 3: Cell Structure and Function
Section 3.5
Endocytosis
During endocytosis, cells take in substances by vesicle
formation.
• The plasma membrane folds in on itself and then pinches
off to form an intracellular vesicle
Endocytosis occurs in one of three ways.
• Phagocytosis
• Pinocytosis
• Receptor-mediated endocytosis
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UNIT A Chapter 3: Cell Structure and Function
Section 3.5
Phagocytosis
During phagocytosis, the material being taken into the cell is
large, such as a food particle or another cell.
• Common in unicellular organisms and occurs in certain
types of human white blood cells
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From Figure 3.25 Three methods of endocytosis. a. Phagocytosis occurs when the
substance to be transported into the cell is large. Amoebas ingest by phagocytosis.
Digestion occurs when the resulting vacuole fuses with a lysosome.
UNIT A Chapter 3: Cell Structure and Function
Section 3.5
Pinocytosis
During pinocytosis, vesicles form around liquid or very
small particles.
• Common in blood cells, intestinal cells, and plant root
cells
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From Figure 3.25 Three methods of endocytosis. b. Pinocytosis occurs when a
macromolecule such as a polypeptide is transported into the cell. The result is a vesicle
(small vacuole).
UNIT A Chapter 3: Cell Structure and Function
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Section 3.5
UNIT A Chapter 3: Cell Structure and Function
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Section 3.5
UNIT A Chapter 3: Cell Structure and Function
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Section 3.5