Overview of the cell structure
Readings and Objectives
• Reading
– Russell: 2.5a, 21.2, 22
– Cooper: Chapter 1
• Objectives
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Cell Theory
Basic properties of cells
Cell size
Cell diversity
Prokaryotic vs eukaryotic cells
• Diversity
• Structure
Properties of Cells
Basic properties of cells
• Order and complexity
– surrounded by a semi-permeable membrane, similar in
composition and function
– Similar chemical composition: similarity of structural patterns
of macromolecules and their functions
– Similar biochemical and regulatory processes
– hierarchal complexity from molecules to supramolecular
structures
• Self replication by division: all cells come from existing cells by
division, process of division share similarities among cells
Properties of Cells
Basic properties of cells
• Cells are small (see module 3)
1200
S=6(X)^2
1000
V=(X)^3
800
S or V
– Varied between ~0.5 to a few
100 um (eg E.coli ~0.6 um, RBC
8 um, skin epithelial cells ~30
nm and ameoba ~500 um)
– Why cells are generally small?
– Limiting factor: Surface to
volume ratio (S/V)
– Surface is the portal of supply
for the volume of cell
600
400
200
0
1
2
3
4
5
6
X (side of cube)
7
8
9
10
Properties of Cells
Basic properties of cells
• Cells are small (see module 3)
– S/V vs X in nonlinear
– S/V exponentially reduced as
X increases
• Surface (size of cell) reaches a
limit that cannot support the
cell’s need (volume)
• Eukaryotes: have increased
the S/V by extending internal
membrane surfaces
7.00
6.00
S/V
5.00
4.00
3.00
2.00
1.00
0.00
1
2
3
4
5
6
X (cube side)
7
8
9
10
Properties of Cells
Properties…
• Interaction with the environment
• All have transmittable genetic program
– DNA: similar structure and function, being inherited
– Genes: units of genetic function, basic similarity
– Evolution: plasticity of genetic information, functional divergence in
response to environmental cues
• Energy biogenesis
• Similarity of metabolic pathways, eg. glycolytic pathway, Krebs
cycle, adaptive variations on a general theme
• Use of ATP as the universal cellular energy currency
 Chemical and mechanical activity
• Similar enzymes carry out metabolic reactions → energy
• Use of energy to do mechanical work
Cell theory
Early observations
• Robert Hooke (1665): first to observe unit structures in cork,
called them “cells”
• Antone Van Leeuwenhoek: discovers microbial single “cells”
• Matthias Schleiden and Theodor Schwann (1839):
Independently concurred that all living organisms,
– All living organisms consist of unit structures called Cells
(The Cell theory)
• Rudolf Virchow (1855): expanded the cell theory,
– Each cell is the result of division of previous cells
• Modern cell biology addresses the question “how cells work”
• To understand the molecular basis of cellular processes
Cell diversity
• Domains of life: cells are organized in three domains
• Procaryotes- Eubacteria and Arhaebacteria
• Eucaryotes- Protista, Fungi, Plants and Animals
• Similarities: Procaryotes and eucaryotes
– Cell membrane
– energy metabolism
– Genetic code
• Differences:
– Procaryotes are unicellular
– Except for many members of protista the rest of eukaryotes are
multicellular
– Size: procaryotes 0.5-5 um, eucaryotes 5-500 um
– Membrane bound organelles present in eucaryotes but not procaryotes
– Reproduction: cell fission in procaryotes vs mitosis in eukarotes
Procaryotes: Structure
Bacterial cytoplasm is surrounded by a cell
membrane, a cell wall, and for some with a
polysaccharide capsule
Cell wall: used for protection, shape, rigidity
• Composed of peptidoglycan, a polysaccharide
of alternating acetlylated muramic acid and
glucoseamine
• β14 glycosidic bond btw sugars
• cross-linked by a short oligopeptide
Two types of bacteria based on cell wall
structure
– Gram positive: multiple layers of peptidoglycan
– Gram negative: few layers
of peoptidoglycan
G+ bacteria
G- bacteria
Procaryotes: Structure
• Plasma membrane
– Beneath the cell
wall is the plasma
membrane, a
phospholipid
bilayer with
associated proteins
– Steroid like
molecules instead
of cholesterol
Procaryotes: Structure
Cytoplasm: lacks membrane
bound organelles
• Ribosomes
–70S; 3 rRNA + 55 proteins
(in 2 subunits 50S and
30S)
–Protein synthesis
• Chromosome: single
supercoiled chromosome that
resides in a region called
nucleoid (not membrane bound)
Procaryotes: Structure
• Bacteria have appendages
with specific functions
– Flagella, composed of flagellin
helps bacteria move
– Fimbriae are small bristlelike
fibers that allow bacteria to
attach themselves to surfaces
– Sex pili (F pilus) used for
conjugation to transfer DNA
from one bacterium to another
• Genes coding for F-pilus are on F
plasmid
Procaryotes: reproduction
Reproduction
• Fission instead of mitosis
• Chromosomal DNA
replication
Cytokinesis
• formation of divisome (several
proteins that tag the centre of cell)
• Cell wall peptidoglycan
deposited
• Chromosomes separated
Eucaryotic cells
Diversity
• Protista: free-living marine
unicellular, some photosynthetic
• e.g. Giardia lamblia,
dinoflagellates, Paramecium, Plasmodium
• Multicellular organisms
– Fungi, Plants, Animals
– Differentiation
– Model organisms
(a) Saccharomyces cerevisiae (yeast)
(b) Arabidopsis thaliana
(c) Caenorhabditis elegans
(d) Drosophila melanogaster
(e) Mus musculus
Eucaryotic cells Structure
Characteristics:
• Typically 10-30 um
• Separation of DNA and cytoplasm by nuclear envelope
• Presence of membrane-bound compartments with specialized
functions: Mitochondria, chloroplasts, ER, Golgi complex
• Highly specialized motor proteins
• Nuclear envelope and
internal membranes
– Originated from cell
membrane
– differentiated and
acquired special functions
Eucaryotic cells
• organelles can be
divided into four
categories
– The nucleus and
ribosomes
– endomembrane
system
– energy-related
organelles
– cytoskeleton
Can you meet these objectives?
• Describe the cell theory
• Describe the basic properties of cells
• Outline the major advances leading to modern
cell biology
• Describe the properties of procaryotic cells
• Contrast and compare prokaryotes and
eukaryotes
• Discuss the cellular structure of prokaryotes