Chapter 15
The Basic Unit of
Life- The Cell
Principles of Science II

This lecture will help you understand:
1. Characteristics of Life
2. Macromolecules Needed for Life
3. Cell Types: Prokaryotic and Eukaryotic
4. The Microscope
5. Tour of a Eukaryotic Cell
6. The Cell Membrane
7. Transport into and out of Cells
8. Cell Communication
9. How Cells Reproduce
10.How Cells Use Energy
11. ATP and Chemical Reactions in Cells
12.Photosynthesis
13.Cellular Respiration and Fermentation

Characteristics of Life
• All living things
– use energy.
– develop and grow.
– maintain themselves.
– have the capacity to reproduce.
– are parts of populations that evolve.

Characteristics of Life
• All living things use energy.
– Plants take electromagnetic energy from sunlight and
convert it to chemical energy.
– Animals convert energy from food into chemical
energy.
– Living things use this chemical energy to build
structures and fuel their activities.
– How living things use energy is consistent with the
laws of physics.

Characteristics of Life
• All living things develop and grow, changing
over time.
• Living things maintain themselves.
– They build structures (bones, stems).
– They repair damage (heal wounds).
– They maintain their internal environments (body
temperature, water balance).

Characteristics of Life
• All living things have the capacity to reproduce.
– Asexual reproduction occurs when an organism
reproduces by itself.
– Sexual reproduction occurs when organisms produce
sperm and eggs that join to develop into new individuals.

Characteristics of Life
• All living things are parts of populations that
evolve.
– Populations do not remain constant.
– Populations change over time, across generations.
– Populations may evolve in response to their
environments.

Macromolecules Needed for Life
• Proteins (amino acid Leucine)
• Carbohydrates (Glucose)
• Lipids (Palmitric Acid)
• Nucleic acids (Guanine DNA)

Cell Types: Prokaryotic and Eukaryotic
• Prokaryotic cells have no nucleus.
• Prokaryotes are almost always
single-celled, microscopic
organisms. Their DNA is
found in a single circular
structure. They usually
have an outer cell wall.

Cell Types: Prokaryotic and Eukaryotic
• Eukaryotic cells have a nucleus and
may be single-celled or multicellular.
They
– have their DNA inside the nucleus.
– have linear chromosomes.
– have various organelles.
– are larger than prokaryotic cells.

The Microscope
• Light microscopes allow people to view cells and
make out the larger features within them.
• Electron microscopes allow people to view even
smaller structures within cells.

The Microscope
Light microscope

The Microscope
Electron microscope

Tour of a Eukaryotic Cell
• Eukaryotic cells contain many parts, including:
– Cell membrane
– Nucleus
– Cytoplasm
– Cytoskeleton
– Organelles
• All the parts of the
cell have specific
functions.

Tour of a Eukaryotic Cell
• Plant cells also contain (animal cells do NOT)
– a cell wall to make the cell rigid.
– chloroplasts that perform photosynthesis.

The Cell Membrane
• The cell membrane
– defines a cell's boundary.
– controls what moves into and out of the cell.
– consists of a phospholipid bilayer, membrane
proteins, and short carbohydrates.

The Cell Membrane
• The fluid mosaic model describes the structure
of the cell membrane, a mosaic of proteins and
phospholipids, almost all of which can move
fluidly around the membrane.

Transport into and out of cells
• Cells take in many resources, including water,
oxygen, and organic molecules.
• Cells also discard wastes.
• Transport occurs through:
– Diffusion
– Facilitated diffusion
– Active transport
– Endocytosis and exocytosis

• Diffusion is the tendency of molecules to move from an
area of high concentration to an area of low
concentration.
– Molecules diffuse
across the phospholipid
bilayer of the cell
membrane.
– Diffusion requires no
energy from the cell.
It is a form of passive
transport.
Transport into and out of cells

Transport into and out of cells
• Facilitated diffusion occurs when a transport protein
binds to a molecule and moves it down a concentration
gradient.
– Facilitated diffusion requires no energy from the cell.
It is a form of passive transport.

Transport into and out of cells
• Active transport occurs when a transport protein moves
a molecule against its concentration gradient.
– Active transport requires energy from the cell.

Transport into and out of cells
• Larger amounts of material can be transported into and
out of cells through endocytosis and exocytosis.
– In endocytosis, a vesicle pinches off from the cell
membrane.
– In exocytosis, a vesicle fuses with the cell membrane.

Cell Communication
• Cells communicate with one another using special
molecules.
• Communication between adjacent cells occurs when
molecules move from one cell to another through special
"doorways."
– Animal cells have gap junctions.
– Plant cells have plasmodesmata.

Cell Communication
• Long-distance communication relies on
message molecules traveling through the
bloodstream.
– Receptors are membrane proteins.
– A receptor binds a message molecule with a lock-
and-key fit.
– The binding of the receptor to a message molecule
initiates a series of chemical reactions that results in
the target cell's response to the message.

Cell Communication

How Cells Reproduce
• In mitosis, one parent cell divides
into two daughter cells that have
the same genetic information as
the parent cell.

How Cells Reproduce
• A dividing cell goes
through the stages
of the cell cycle:
1. Gap 1
2. Synthesis
3. Gap 2
4. Mitosis/cytokinesis

How Cells Reproduce
• The phases of mitosis:
1. Prophase: Chromosomes condense, and nuclear
membranes break down.
2. Metaphase: Chromosomes line up along the
equatorial plane.
3. Anaphase: Sister chromatids are pulled apart and
move to opposite poles of the cell.
4. Telophase: New nuclear membranes form around
each set of chromosomes.
• After mitosis, cytokinesis occurs: The cytoplasm
divides, completing cell division.

How Cells Reproduce

How Cells Use Energy
• Two things are necessary for a chemical
reaction to occur:
– Must be consistent with the law of conservation of
energy.
• Exothermic (energy-releasing) reactions occur
spontaneously.
• For all other reactions, cells rely on usable energy
stored in molecules of ATP.
– Activation energy needed for initial breaking of bonds.

How Cells Use Energy
• ATP provides energy for chemical reactions in cells.
• Cells obtain energy from ATP when one phosphate
group is removed, leaving ADP.

How Cells Use Energy
• Cells eventually turn ADP back into ATP by adding a
phosphate group during cellular respiration.

How Cells Use Energy
• The sodium-potassium pump uses active transport to
control the levels of sodium ions (Na+
) and potassium ions
(K+
) in cells. This process uses one molecule of ATP.

How Cells Use Energy
• Reacting molecules must collide with an
activation energy that is needed for the initial
breaking of bonds.
• The activation energy for many essential
chemical reactions is very high.
• A catalyst is a substance that lowers the
activation energy, allowing a reaction to happen
more quickly.
• The catalysts in cells are enzymes—large,
complex proteins.

How Cells Use Energy
• An enzyme binds the reactants at its active site and
releases the products.
• In the process, the enzyme is not altered or destroyed; it
can be used again and again.

How Cells Use Energy
• Cells regulate enzymes
– Cells control the synthesis and degradation of
enzymes.
– Enzyme function depends on temperature, pH, and
other features of the environment.
– Inhibitors can block the function of enzymes.

How Cells Use Energy
• Two types of enzyme inhibition
– In competitive
inhibition, the
inhibitor binds
to the active site
of an enzyme so
that the enzyme
cannot bind its
substrate.
– Noncompetitive inhibition occurs when an inhibitor
binds to a different part of the enzyme, changing the
active site so that the enzyme can no longer bind to
its substrate.

Photosynthesis
• The process organisms use to convert light energy from
the Sun into chemical energy.
• Conducted in the chloroplasts of plants.
• Occurs in two stages: the light-dependent reactions and
the light-independent reactions.
• Photosynthesis is the ultimate source of (practically) all
organic molecules on Earth.

Photosynthesis
Light-dependent reactions

Photosynthesis
• Light-independent reactions
– The light-independent reactions make use of stored
energy from the light-dependent reactions.
– Carbon is fixed, moved from atmospheric CO2 to the
sugar glucose.
– The molecules produced during photosynthesis are
used as a starting point for building all of the
macromolecules of life.

Cellular Respiration and Fermentation
• Cells break down glucose to produce ATP.
• Cellular respiration is aerobic (uses oxygen).
• Cellular respiration yields 38 molecules of ATP
from every molecule of glucose.

Cellular Respiration and Fermentation
• Cellular respiration occurs in three steps:
– Glycolysis
– The Krebs cycle
– Electron transport

Cellular Respiration and Fermentation
• Glycolysis takes place in the cytoplasm of cells.
– The glucose molecule is split into two
molecules of pyruvic acid, releasing two
molecules of ATP.

Cellular Respiration and Fermentation
• The Krebs cycle occurs in the cell's mitochondria.
– In the Krebs cycle, pyruvic acid is converted to acetic
acid and bound to a molecule of coenzyme A. The
result—acetyl-CoA—is broken down into CO2.
– Two molecules of ATP are harvested. Additional energy
is stored in the molecules NADH and FADH2.

Cellular Respiration and Fermentation
• In electron transport, electrons carried by NADH
and FADH2 are sent down electron transport
chains.
– In the process, the electrons lose energy, which is
used to pump hydrogen ions across a membrane
inside the mitochondria.
– At the end of the chain, the electrons combine with O2
to make water.
– The concentration gradient generated by pumping
hydrogen ions is used to make ATP.

Cellular Respiration and Fermentation
• In certain cells, under certain conditions, glycolysis is
followed by fermentation.
– Fermentation uses no oxygen and generates no ATP.
– But, fermentation regenerates the molecules
necessary to keep glycolysis going, so cells can
continue to obtain energy through glycolysis.
• Alcoholic fermentation by yeast is used to bake bread
and make beer and wine.
• Lactic acid fermentation occurs in muscle cells when
there is not enough oxygen for cellular respiration to
continue. It is also used by red blood cells. Lactic acid
fermentation by bacteria and yeast is used to make
yogurt and cheese.