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Chapter 2
The Cell: the Basic Unit of
Life
Contents
1
2
3
Cells
Cell Organization
Two Fundamental Cell
Types
Cells
 Basic unit of living
organism
 First named by Robert
Hook in the 17th century
 Different types but the
same essential properties
 Same building blocks:
proteins, carbohydrates,
fats, and nucleic acids
Liver
Neutrophil
Goldfish skin
Neuron
Essential Functions of Living Cells 1
 Growth
 Metabolism
• Catabolism: breaking down large molecules to generate
building blocks and energy
• Anabolism: Generation of large molecules using building
blocks and energy
Essential Functions of Living Cells 2
 Reproduction
 Cell cycle
• Cyclical process of cell growth and division
• Daughter cell must receive a correct copy of genetic material
DNA replication before cell division
Cell cycle
Essential Functions of Living Cells 3
 Maintenance of internal
environments
 Use energy to maintain the
internal environments
• Unique molecules
– Specific proteins, DNA etc.
• Molecules also exit outside but with
different concentrations
– Water, salts, sugar etc.
Amoeba
Essential Functions of Living Cells 4
 Response to external
environments
 Sense a change in their
environment
 Respond
• Maintaining osmotic homeostasis
• Bacterial chemotaxis
• Release of digestive enzymes
from stomach cells
• …..
Essential Functions of Living Cells 5
 Communication with each other
 Between cells in an organism
• e.g. Nerve cell and muscle
 Between single cell organisms
• e.g. Mating of yeast cells, quorum sensing of bacteria
Essential Functions of Living Cells 6
 Differentiation in multicellular organisms
 Cells differentiate to cells with specific functions
 Specific cells organize into different tissues and
organs
Common Cellular Processes
 Constant supply of energy
 Need energy for all the cellular activities
 Energy source
• Sun: photosynthetic plant or bacteria
• Food and Chemicals
 Chemical reactions
 Enzymes: protein catalyst accelerating chemical
reactions
Common Cellular Processes
 Cell processes occur in a series of small steps
 Pathway: a process consisting of a series of steps
 Regulation of processes
 Regulation of various processes by regulation of
protein-protein and protein-DNA interactions
 Cell cycle, blood sugar levels, blood pressure, body
water balance etc.
Cell Organization
 Interaction of molecules with water
 Important factor for determining the molecular organization within
a cell
 Hydrophilic (water soluble) or Hydrophobic (water insoluble)
• Congregation of hydrophilic parts with other hydrophilic parts
• Congregation of hydrophobic parts with other hydrophobic parts
 Binding of molecules
 Specificity of molecular
bindings determines cellular
processes
 Binding: fitting between
molecules
• Depends on shape and
chemical properties (charge)
Cellular Membranes I
 Provide structural organization
 Lipid bilayer with hydrophobic core and hydrophilic face
 Plasma (cell) membrane: Hydrophobic barrier between inside
(cytoplasm) and outside of the cell
 Internal membranes
•
•
•
•
Nucleus
Endoplasmic reticulum, Golgi apparatus
Mitochondria
Chloroplast
Cellular Membranes II
 Control molecular transport across the
membrane
 Free diffusion
• Small, electrically neutral or slightly charged
molecules (CO2, O2, water)
 Transport through membrane-bound
channels and transporters
Two Fundamental Cell Types
 Prokaryotic cells
 Prokaryote (pro; before, karyon: kernel or nucleus)
 No nuclear membrane
 Small (0.2-2 m), mostly single-celled organisms
• Eubacteria : common bacteria, e.g. E.coli, blue-green algae
• Archaea (Archaebacteria)
 Eukaryotic cells
 Eukaryote (well-formed nucleus)
 Nuclear and internal membranes  organelles
 Larger than prokaryotes (10-100 m)
• Single-celled: yeast, green algae, amoebae
• Multicellular: fungi, plant, animal
The Scale of Life
The phylogenetic tree of life as defined by
comparative ribosomal RNA sequencing
Two Fundamental Cell Types
 Procaryote & Eucaryote
 Primary Difference: presence or absence of nucleus
(more details in Table 2.1 and 2.2)
Procaryotic Cells
 Procaryote
 (pro; before, karyon: kernel or nucleus)
 No nuclear membrane
 Small (0.5-3 m), mostly single-celled organisms
 Eubacteria : common bacteria
(e.g. E. coli, blue-green algae )
 Archaea (Archaebacteria): methanogens (methane-producing),
thermoacidophiles,
and halobacteria (high salt)
• Live in extreme environments.
• Possess unusual metabolism.
Eubacteria
 Divided into two groups by gram stain.
 Gram-negative cell (e.g. E. coli)
•
•
•
•
Outer membrane
Peptidoglycan
Cytoplasmic (inner) membrane
Periplasm
– Between inner and outer membranes
 Gram-positive cell (e.g. Bacillus subtilis)
•
•
•
•
No outer membrane
Peptidoglycan
Teichoic acid
Cytoplasmic membrane
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23
Teichoic acids
Teichoic acids (cf. Greek τειχος, teichos,
"wall") are bacterial polysaccharides[1] of
glycerol phosphate or ribitol phosphate
linked via phosphodiester bonds
They can be covalently linked to Nacetylmuramic acid of the
peptidoglycan layer, to the lipids of
the cytoplasmic membrane, or to a
terminal D-alanine in the
tetrapeptide crosslinkage between
N-acetylmuramic acid units.
Cellular Membranes
 Provide structural organization
 Lipid bilayer with hydrophobic core and hydrophilic face
 Plasma (cell) membrane: Hydrophobic barrier between inside
(cytoplasm) and outside of the cell
 Internal membranes for eucaryote
•
•
•
•
Nucleus
Endoplasmic reticulum, Golgi apparatus
Mitochondria
Chloroplast
Cytoskeleton and Centrioles
Viruses
 No independent
reproduction
 Genetic material (DNA or
RNA)
 Proteins (Capsid)
 Host specificity
 Bacteria (bacteriophages),
human etc
 Tissue specificity
Microbial Diversity
 psychrophile: optimum temp < 20oC
 mesophile: 20oC < optimum temp < 50oC
 thermophile: 50oC < optimum temp
 aerobic: growth in the presence of oxygen
 anaerobic: growth without oxygen
 facultative: growth under either circumstances
 coccus: spherical or elliptical
 bacillus: cylindrical or rod
 spirillum: spiral
Naming Cells
 Escherichia coli (E. coli)




written in italic
Escherichia: genus
coli: species
various strains of E. coli --- (ex) E. coli K12
Viruses
 Not cells
 No independent reproduction (not alive by itself)
• Genetic material (DNA or RNA)
• Proteins (Capsid)
 “Viruses are in the semantic fog between life and non-life.”
(Campbell and Reece, Biology, 6e, p 339.)
 Are viruses living beings?
“The answer to that question is ‘no’, inasmuch as viruses are incapable of
independent life.” (de Duve, Life Evolving, p.313)
 Conclusion:
Viruses do not fit the basic definition of cellular life.
• Require host for all cellular activities
• No metabolic capability of their own
Bacteriophage
 Bacteriophage: virus infecting bacteria
 lytic cycle: reproduction of virus (Fig. 2.1)
 lysogenic cycle: Phage DNA is incorporated
into the host DNA
Eucaryote
 Eucaryotic cells
 Eukaryote (well-formed nucleus)
 Larger than procaryotes (10-100 m)
• Single-celled: yeast, green algae, amoebae
• Multicellular: fungi, plant, animal
 Nuclear and internal membranes  organelles
•
•
•
•
Nucleus --- contains chromosomes as nuclear material
Mitochondria --- powerhouse of a cell
Golgi body --- responsible for the secretion of certain proteins
Vacuole --- responsible for food digestion, osmotic regulation,
and waste-product storage
• Chloroplast --- responsible for photosynthesis
• endoplasmic reticulum, lysosome, glyoxysome ….
Nucleus and Mitochondria
Endoplasmic Reticulum
Cell Cycle in a Eucaryote
 Cell Cycle (Fig. 2.4)
 S phase:
--- DNA synthesis
 M phase:
--- mitosis
 G 1, G 2 :
--- G strands for gap between
S and M phase
--- Cell growth
Mitosis
 DNA replication
 Mitosis
 Prophase
• Condensation of chromosomes and disappearance of nuclear
membrane
 Metaphase
• Alignment of chromosome in the center
• Pulling by spindle fibers attached to the kinetochore
 Anaphase
• Splitting of chromatids and pulling to the opposite ends of the cell
 Telophase
• Decondensation of chromosome
• Formation of new nuclear membrane
 Cytokinesis
 Cell division after mitosis
 Interphase
 The time between cell division and the next mitosis (G1, S, G2)
Mitosis and Cytokinesis
Chromosome
 Tightly packed complex of DNA and histone proteins
Eucaryotic Cells
 Fungi
 Yeasts --- single small cells of 5- to 10-m size
 Molds --- filamentous fungi, have a mycelial structure
 Algae
 Unicelluar algae (microalgae) --- 10 to 30 m
 Plantlike multicelluar algae
 Protozoa
 Unicellular, motile, relatively large (1 - 50 mm) --- amoeba
 Plant cells
 Animal cells
 http://multimedia.mcb.harvard.edu/anim_innerlif
e_hi.html