Chapter 4
Physiology of Cells
Two Ways Molecules Move
through Cells
• 1) Passive Transport
• 2) Active Transport
Movement of Substances
through Cell Membranes
• Passive transport processes—do not
require any energy expenditure of the cell
membrane
• 4 types of passive transport
• 1) diffusion
• 2) Dialysis
• 3) Osmosis
• 4) Facilitated diffusion
Diffusion
– Diffusion—a passive process
– Molecules spread through the membranes
• Molecules move from an area of high
concentration to an area of low concentration,
down a concentration gradient
• As molecules diffuse, a state of equilibrium will
occur
Simple diffusion
• Simple diffusion
– Molecules cross through the phospholipid
bilayer
– Solutes permeate the membrane; therefore,
we call the membrane permeable
Osmosis
• Osmosis
• Diffusion of water through a selectively
permeable membrane, which limits the
diffusion of at least some of the solute
particles
– Water pressure that develops as a result of
osmosis is called osmotic pressure
Osmosis Cont.
 Potential
osmotic pressure is the maximum
pressure that could develop in a solution
when it is separated from pure water by a
selectively permeable membrane; knowledge
of potential osmotic pressure allows the
prediction of the direction of osmosis and the
resulting change of pressure
Osmosis
• Isotonic—two fluids
that have the same
potential osmotic
pressure
Osmosis
• Hypertonic—“higher
pressure”; cells placed in
solutions that are
hypertonic to intracellular
fluid always shrivel as
water flows out of cell;
this has great medical
importance: if medical
treatment causes the
extracellular fluid to
become hypertonic to the
cells of the body, serious
damage may occur
Osmosis
• Hypotonic—“lower
pressure”; cells
placed in a hypotonic
solution may swell as
water flows into them;
water always
osmoses from the
hypotonic solution to
the hypertonic
solution
Facilitated diffusion
• Facilitated diffusion (mediated passive
transport)
– A special kind of diffusion whereby movement
of molecules is made more efficient by the
action of transporters embedded in a cell
membrane
– Transports substances down a concentration
gradient
– Energy required comes from the collision
energy of the solute
Facilitated diffusion
– Channel-mediated passive transport
– Channels are specific—allow only one type of solute
to pass through
• Gated channels may be open or closed (or inactive)—may be
triggered by any of a variety of stimuli
• Channels allow membranes to be selectively permeable
• Carriers attract and bind to the solute, change shape, and
release the solute out the other side of the carrier
• Carriers are usually reversible, depending on the direction of
the concentration gradient
Active Transport
• Active transport is the movement of solute
particles from an area of low concentration
to an area of high concentration (up the
concentration gradient) by means of a
carrier molecule.
• There are three types:
• 1) Phagocytosis (Endocytosis)
• 2) Pinocytosis (Endocytosis)
• 3) Exocytosis
Endocytosis
– Endocytosis—the plasma membrane “traps”
some extracellular material and brings it into
the cell in a vesicle
• Two basic types of endocytosis:
– Phagocytosis—“condition of cell-eating”; large
particles are engulfed by the plasma membrane
and enter the cell in vesicles; vesicles fuse with
lysosomes, where the particles are digested
– Pinocytosis—“condition of cell-drinking”; fluid
and the substances dissolved in it enter the cell
Phagocytosis
Pinocytosis
Exocytosis
– Exocytosis
• Process by which large molecules, notably
proteins, can leave the cell even though they are
too large to move out through the plasma
membrane
• Large molecules are enclosed in membranous
vesicles that are then pulled by the cytoskeleton to
the plasma membrane, where the contents are
released
• Exocytosis also provides a way for new material to
be added to the plasma membrane
Exocytosis
Cell Metabolism
• Metabolism is the set of chemical
reactions
in a cell
– Catabolism—breaks large molecules into
smaller ones; usually releases energy
– Anabolism—builds large molecules from
smaller ones; usually consumes energy
Enzymes
• Role of enzymes
– Enzymes are chemical catalysts, reducing
activation energy needed for a reaction
– Enzymes regulate cell metabolism
– Chemical structure of enzymes
• Proteins of a complex shape
• The active site is where the enzyme molecule fits
the substrate molecule—the lock-and-key model
The structure of proteins
• Enzymes are important proteins found in living
things. An enzyme is a protein that changes
the rate of a chemical reaction.
• They speed the
reactions in digestion
of food.
• YouTube - Enzyme
Classification and naming of
enzymes
• Enzymes usually have an -ase ending,
with the first part of the word signifying the
substrate or the type of reaction catalyzed
– Oxidation-reduction enzymes—known as
oxidases, hydrogenases, and
dehydrogenases; energy release depends on
these enzymes
– Hydrolyzing enzymes—hydrolases; digestive
enzymes belong to this group
Classification and naming of
enzymes (cont.)
– Phosphorylating enzymes—phosphorylases
or phosphatases; add or remove phosphate
groups
– Enzymes that add or remove carbon
dioxide— carboxylases or decarboxylases
– Enzymes that rearrange atoms within a
molecule—mutases or isomerases
– Hydrases add water to a molecule without
splitting it
General functions of enzymes
– Various chemical and physical agents known
as allosteric effectors affect enzyme action by
changing the shape of the enzyme molecule;
examples of allosteric effectors include the
following:
• Temperature
• Hydrogen ion (H+) concentration (pH)
• Ionizing radiation
• Cofactors
• End products of certain metabolic pathways
General functions of enzymes
(cont.)
– Most enzymes catalyze a chemical reaction in
both directions
– Enzymes are continually being destroyed and
are continually being replaced
– Many enzymes are first synthesized as
inactive proenzymes
Catabolism
– Cellular respiration, the pathway in which
glucose is broken down to yield its stored
energy, is an important example of cell
catabolism
– Cellular respiration has three pathways that
are chemically linked:
1) Glycolysis
2) Citric acid cycle
3) Electron transport system (ETS)
Glycolysis (net yeild of 2 ATP)
• Pathway in which glucose is broken apart into two
pyruvic acid molecules to yield a small amount of
energy (which is transferred to ATP and NADH)
• Includes many chemical steps (reactions that
follow one another), each regulated by specific
enzymes
• Is anaerobic (requires no oxygen)
• Occurs within cytosol (outside the mitochondria)
Citric acid cycle (Krebs cycle)
(net yield of 2 ATP
• Pyruvic acid (from glycolysis) is converted into
acetyl CoA and enters the citric acid cycle after
losing CO2 and transferring some energy to NADH
• Citric acid cycle is a repeating (cyclic) sequence of
reactions that occurs inside the inner chamber of a
mitochondrion. Acetyl splits from CoA and is
broken down, yielding waste CO2 and energy (in
the form of energized electrons), which is
transferred to ATP, NADH, and FADH2
Electron transport system (ETS)
(Net yield of 32 ATP)
• Energized electrons are carried by NADH and
FADH2 from glycolysis and the citric acid cycle to
electron acceptors embedded in the cristae of the
mitochondrion
• As electrons are shuttled along a chain of electronaccepting molecules in the cristae, their energy is
used to pump accompanying protons (H+) into the
space between mitochondrial membranes
Electron transport system (cont.)
– Protons flow back into the inner chamber
through pump molecules in the cristae, and
their energy of movement is transferred to
ATP
– Low-energy electrons coming off the ETS
bind to oxygen and rejoin their protons,
forming water (H2O)
Cell Metabolism
• Anabolism
– Protein synthesis is a central anabolic
pathway in cells
Anabolism
– Deoxyribonucleic acid (DNA)
• A double-helix polymer (composed of nucleotides)
that functions to transfer information, encoded in
genes, that directs the synthesis of proteins
• Gene—a segment of a DNA molecule that consists
of approximately 1000 pairs of nucleotides and
contains the code for synthesizing one polypeptide
Anabolism
– Transcription—mRNA forms along a segment
of one strand of DNA
– Editing
• Noncoding introns are removed, and remaining
exons are spliced together to form the final, edited
version of the mRNA copy of the DNA segment
• Spliceosomes are ribosome-sized structures in the
nucleus that splice mRNA transcripts
Anabolism (cont.)
– Translation
• After leaving the nucleus and being edited, mRNA
associates with a ribosome in the cytoplasm
• tRNA molecules bring specific amino acids to the
mRNA at the ribosome; the type of amino acid is
determined by the fit of a specific tRNA’s anticodon
with mRNA’s codon
• As amino acids are brought into place, peptide bonds
join them—eventually producing an entire polypeptide
chain
Growth and Reproduction of
Cells
• Cell growth and reproduction of cells are
the most fundamental of all living functions
and together constitute the cell life cycle
– Cell growth—depends on using genetic
information in DNA to make the structural and
functional proteins needed for cell survival
– Cell reproduction—ensures that genetic
information is passed from one generation
to the next
Growth and Reproduction of
Cells
• Cell growth—a newly formed cell produces
a variety of molecules and other structures
necessary for growth using the information
contained in the genes of DNA molecules;
this stage is known as interphase
– Production of cytoplasm—more cell material
is made, including growth and/or replication of
organelles and plasma membrane; a largely
anabolic process
Growth and Reproduction of
Cells
• Cell growth (cont.)
– DNA replication (Table 4-4)
• Replication of the genome prepares the cell for
reproduction; mechanics are similar to RNA synthesis
• DNA replication (Figure 4-29)
– DNA strand uncoils and strands come apart
– Along each separate strand, a complementary strand
forms
– The two new strands are called chromatids, instead
of chromosomes
– Chromatids are attached in pairs, and the
centromere is the name of their point of attachment
Growth and Reproduction of
Cells
• Cell growth (cont.)
– Growth phase of the cell life cycle can be
subdivided into the first growth phase (G1),
the [DNA] synthesis phase (S), and the
second growth phase (G2)
Growth and Reproduction of
Cells
• Cell reproduction—cells reproduce by
splitting themselves into two smaller
daughter cells (Table 4-5)
– Mitotic cell division—the process of organizing
and distributing nuclear DNA during cell
division has four distinct phases (Figure 4-31)
Growth and Reproduction of
Cells
• Mitotic cell division (cont.)
– Prophase—“before-phase”
• After the cell has prepared for reproduction during
interphase, the nuclear envelope falls apart as the
chromatids coil up to form chromosomes, which
are joined at the centromere (Figure 4-30)
• As chromosomes are forming, the centriole pairs
are seen to move toward the poles of the parent
cell and spindle fibers are constructed between
them
Growth and Reproduction of
Cells
• Mitotic cell division (cont.)
– Metaphase—“position-changing phase”
• Chromosomes move so that one chromatid of
each chromosome faces its respective pole
• Each chromatid attaches to a spindle fiber
Growth and Reproduction of
Cells
• Mitotic cell division (cont.)
– Anaphase—“apart phase”
• Centromere of each chromosome has split to form
two chromosomes, each consisting of a single
DNA molecule
• Each chromosome is pulled toward the nearest
pole, forming two separate but identical pools of
genetic information
Growth and Reproduction of
Cells
• Mitotic cell division (cont.)
– Telophase—“end phase”
• DNA returns to its original form and location within
the cell
• After completion of telophase, each daughter cell
begins interphase to develop into a mature cell
Growth and Reproduction of
Cells
• Meiosis (Figure 4-31; see also Figure 331)
• Regulating the cell life cycle
– Cyclin-dependent kinases (CDKs) are
activating enzymes that drive the cell through
the phases of its life cycle
– Cyclins are regulatory proteins that control the
CDKs and “shift” them to start the next phase
Cycle of Life: Cells
• Different types of cells have different life
cycles
• Advancing age creates changes in cell
numbers and in their ability to function
effectively
– Examples of decreased functional ability
include muscle atrophy, loss of elasticity of
the skin, and changes in the cardiovascular,
respiratory, and skeletal systems