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Chapter 1
What is a Cell?
By
Benjamin Lewin
1.1 Introduction
• Cells arise only from preexisting cells.
• Every cell has genetic information whose
expression enables it to produce all its
components.
• The plasma membrane consists of a lipid
bilayer that separates the cell from its
environment.
1.2 Life began as a self-replicating
structure
• The first living cell was a self-replicating entity
surrounded by a membrane.
1.3 A prokaryotic cell consists of a single
compartment
• The plasma membrane of a prokaryote surrounds a
single compartment.
• The entire compartment has the same aqueous
environment.
• Genetic material occupies a compact area within the
cell.
• Bacteria and archaea are both prokaryotes but differ
in some structural features.
1.4 Prokaryotes are adapted for growth under
many diverse conditions
• Prokaryotes adapted to many extreme
environmental conditions
• This highlights the variations that are possible
in constructing living cells.
1.5 A eukaryotic cell contains many
membrane-delimited compartments
• The plasma membrane of a eukaryotic cell
surrounds the cytoplasm.
1.5 A eukaryotic cell contains many membrane-delimited
compartments
• Within the cytoplasm there are individual
compartments, each surrounded by a
membrane.
• The nucleus is often the largest compartment
within the cytoplasm
– It contains the genetic material.
1.6 Membranes allow the cytoplasm to
maintain compartments with distinct
environments
• Organelles that are surrounded by
membranes can maintain internal milieus that
are different from the surrounding cytosol.
1.7 The nucleus contains the genetic
material and is surrounded by an envelope
• The nucleus is the largest organelle in the
cell.
• It is bounded by an envelope consisting of a
double membrane.
1.7 The nucleus contains the genetic material and is surrounded by an envelope
• Genetic material is concentrated in one part of the
nucleus.
• Nuclear pores provide the means for transport across the
envelope for large molecules to enter or leave the
nucleus.
1.8 The plasma membrane allows a cell to
maintain homeostasis
• Hydrophilic molecules cannot pass across a lipid
bilayer.
• The plasma membrane is more permeable to
water than to ions.
1.8 The plasma membrane allows a cell to maintain homeostasis
• Osmotic pressure is created by ionic
differences between the two sides of a
membrane.
• The plasma membrane has specific systems
for transporting ions and other solutes into or
out of the cell.
1.8 The plasma membrane allows a cell to maintain homeostasis
• The transport systems allow the cell to
maintain a constant internal environment that
is different from the external milieu.
• Ion channels are proteinaceous structures
embedded in membranes.
– They allow ions to cross the membrane while
remaining in an aqueous environment.
1.9 Cells within cells
• Organelles bounded by envelopes probably
originated by endosymbiosis of prokaryotic
cells.
1.10 DNA is the cellular hereditary material, but
there are other forms of hereditary information
• DNA carries the genetic information that
codes for the sequences of all the proteins of
the cell.
• Information can also be carried in cellular
structures that are inherited.
1.11 Cells require mechanisms
to repair damage to DNA
• The genetic material is continually damaged
by:
– environmental forces
– errors made by cellular systems
• Repair systems to minimize damage to DNA
are essential for the survival of all living cells.
1.12 Mitochondria are energy factories
• All living cells have a means of converting
energy supplied by the environment into the
common intermediate of ATP.
1.13 Chloroplasts power plant cells
• Plastids are membrane-bounded organelles
in plant cells.
• They can develop into chloroplasts and other
specialized forms.
1.14 Organelles require mechanisms for
specific localization of proteins
• All organelles import proteins from the cytosol.
1.15 Proteins are transported to and
through membranes
• Proteins are transported into organelles
through receptor complexes embedded in the
organelle’s membrane.
1.15 Proteins are transported to and through membranes
• Proteins are released into the cytosol after
synthesis.
• For the endoplasmic reticulum, proteins are
transferred into the receptor complex on the
ER membrane during synthesis.
• Proteins then associate with the nucleus, or
an organelle, such as:
– Mitochondria
– Chloroplasts
1.16 Protein trafficking moves proteins
through the ER and Golgi apparatus
• All proteins that are localized in the
– ER
– Golgi apparatus
– plasma membrane
initially associate with the ER during
synthesis.
• Proteins are transported from one
compartment to another by membranous
vesicles.
1.16 Protein trafficking moves proteins through the ER and Golgi apparatus
• The vesicles bud from one membrane
surface and fuse with the next.
• Proteins are transported into the cell from the
exterior by vesicular transport in the reverse
direction.
1.17 Protein folding and unfolding is an
essential feature of all cells
• Protein conformation is a consequence of
primary sequence.
• But often it cannot be achieved by
spontaneous folding.
– It requires assistance from chaperones.
1.18 The shape of a eukaryotic cell is determined
by its cytoskeleton
• The eukaryotic cell cytoskeleton is an internal
framework of filaments, including:
– Microtubules
– Actin filaments
– Intermediate filaments
• It provides an organizing template for many
activities, including anchoring organelles in
place.
1.19 Localization of cell structures is
important
• Localization of certain structures at specific positions
in a cell may be part of its hereditary information.
• Positional effects are important in early development.
1.20 Signal transduction pathways execute
predefined responses
• Events on the outside of the cell can trigger
actions inside the cell by using receptor
proteins embedded in the membrane.
• A receptor spans the membrane and has
domains on both the exterior and interior.
1.20 Signal transduction pathways execute predefined responses
• The receptor is activated when a ligand binds to
the exterior domain.
• Ligand binding causes a change in the structure
or function of the interior domain.
1.21 All organisms have cells that can grow
and divide
• The simplest form of division is shown by some organelles
where the membrane is pinched inward.
1.21 All organisms have cells that can grow and divide
• Bacteria often divide by growing a rigid
septum across the cell as an extension of the
cell wall.
• During mitosis, eukaryotic cells are
extensively reorganized.
• They form the specialized structure of the
spindle.
– It partitions the chromosomes to daughter cells.
1.22 Differentiation creates specialized cell
types, including terminally differentiated
cells
• A multicellular organism consists of many
different cell types that are specialized for
specific functions.
1.22 Differentiation creates specialized cell types, including terminally differentiated cells
• Many differentiated cells have lost the ability
to divide and/or to give rise to cells of different
types.
• Stem cells have the potential to divide to
generate the many different types of cells
required to make:
– an organism
– or a tissue of an organism