What is a Cell?

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Chapter 1

• What is a Cell?
• By
• Benjamin Lewin

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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.

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1.2 Life began as a self-replicating structure

• The first living cell was a self-replicating
  entity surrounded by a membrane.

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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.

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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.

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1.5 A eukaryotic cell contains many
membrane-delimited compartments

• The plasma membrane of a eukaryotic cell
  surrounds the cytoplasm.

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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.

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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.

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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.

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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.

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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.

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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.

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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.

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1.9 Cells within cells

• Organelles bounded by envelopes probably
  originated by endosymbiosis of prokaryotic cells.

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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.

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1.11 Cells require mechanismsto 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.

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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.

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1.13 Chloroplasts power plant cells

• Plastids are membrane-bounded organelles in plant
  cells.
• They can develop into chloroplasts and other
  specialized forms.

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1.14 Organelles require mechanisms for specific
localization of proteins

• All organelles import proteins from the cytosol.

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1.15 Proteins are transported to and through
membranes

• Proteins are transported into organelles through
  receptor complexes embedded in the organelles
  membrane.

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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

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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.

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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.

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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.

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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.

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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.

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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.

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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.

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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.

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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.

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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.

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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