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Chapter 2 : Cell Structure
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The cell theory states: All living things or organisms are made
of cells and their products. New cells are created by old cells
dividing into two. Cells are the basic building units of life. All
metabolic processess take places within cells.
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The generally accepted parts of modern cell theory include:All
known living things are made up of one or more cells.All living
cells arise from pre-existing cells by division.The cell is the
fundamental unit of structure and function in all living
organisms.The activity of an organism depends on the total activity
of independent cells.Energy flow (metabolism and biochemistry)
occurs within cells.Cells contain hereditary information (DNA)
which is passed from cell to cell during cell division.All cells
are basically the same in chemical composition in organisms of
similar species.Modern interpretation :
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2.1 Prokaryotic Cells
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The feature that makes a prokaryotic cell very different from a
eukaryotic cell is its genetic material. The bacterial DNA is in
direct contact with the cytoplasm. There is no enclosing membrane
and therefore no recognisable nucleus. Prokaryotes possess a
single, continuous, circular thread of DNA. The DNA is located in a
region of the cytoplasm called a nucleoid.Small structures called
plasmids may also be present. These carry DNA with only a few genes
responsible for special metabolic pathways and resistance to
antibiotics. Plasmids can transfer between bacteria and it is this
property that has made them important in genetic engineering.
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The ribosomes found in prokaryotic cells are smaller than those
of eukaryotes and are involved in protein synthesis.The rate at
which bacteria divide requires a high level of protein synthesis
and thus many ribosomes are needed. Thus ribosomes may constitute
as much as 40% of the cell mass. Prokaryotic cells possess 70S
ribosomes whereas eukaryotic cells possess 80S ribosomes.(S stands
for Svedberg units and is a measure of how rapidly the ribosomes
sediment in a centrifuge. 80S ribosomes sink quickest because they
are heaviest)
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2.1 Prokaryotic CellsIncludes bacteria and cyanobacteriaHave DNA
not enclosed by nuclear membraneLack organelles bounded by a double
membrane
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2.2 The EukaryotesIncludes the protoctists, fungi, animals, and
plantsHave chromosomes surrounded by well-defined nuclear
membranes
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Differences between Prokaryotes and Eukaryotes
ProkaryotesEukaryotesE.g. bacteria, cyanobacteriaE.g.
protoctists, fungi, animals and plantsAverage diameter : 0.5 5
mAverage diameter of cell : 10 100 m
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Differences between Prokaryotes and Eukaryotes
ProkaryotesEukaryotesCircular double strands of DNA, not
surrounded by double-membraned envelopeMost DNA are associated with
histone proteins to form chromosomesSurrounded by double-membraned
nuclearCircular DNA are presented in mitochondria and
chloroplastsSome bacteria have small circular palsmidsPlasmids are
absent
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Differences between Prokaryotes and Eukaryotes
ProkaryotesEukaryotesFew organellesNo double-membraned
organellesMany organellesHave double-membraned organelles such as
nucleus, mitochondria and chloroplasts No mitosis or meiosisNo
spindle formationMitosis, meiosis, or both can occurThere is
spindle formation
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Differences between Prokaryotes and Eukaryotes
ProkaryotesEukaryotesRibosomes are smaller, 70S ribosomes occur
as free particles in cytoplasmRibosomes are larger, 80S ribosomes
occur as free particles in cytoplasm or are bound to ERRigid cell
Walls containing murein (peptidoglycan)Cell walls of plants and
algae contain cellulose, fungi contain chitin and animal cells have
no cell walls
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Differences between Prokaryotes and Eukaryotes
ProkaryotesEukaryotesMesosomes in bacteria and plasma membrane
of cyanobacteria contain respiratory enzymes.No mitochondriaNo
mesosomes.Mitochondria function as sites for cellular respiration
Some are photoautotrophsPhotosynthetic membranes not stacked into
granaNo chloroplastsChloroplasts contain grana
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Differences between Prokaryotes and Eukaryotes
ProkaryotesEukaryotesFlagella, if present, contain flagellin,
and lack microtubulesFlagella, if present, have a `9 + 2
arrangement of microtubules Some prokaryotic cells have enzymes
that can fix atmospheric nitrogen for use in amino acid
synthesisEukaryotic cells do not contain enzymes that can fix
atmospheric nitrogen
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2.2.1 Animal Cell
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Cell Structure Overview
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2.2.2 Plant Cell
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Differences between Plant Cell and Animal Cell
Animal CellPlant CellNo cell wall, only plasma membraneHave
rigid cell wall and plasma membraneNo plasmodesmata and
pitsPalsmodesmata and pits presents in cell wall
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Differences between Plant Cell and Animal Cell
Animal CellPlant CellNo chloroplastsChloroplasts present in
photosynthetic cellsSmall, temporary vacuolesLarge, permanent
central vacuole filed with cell sap
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Differences between Plant Cell and Animal Cell
Animal CellPlant CellNo tonoplastTonoplast around vacuoleNucleus
often central. Ctoplasm throughtout the cellNucleus usually
peripheral
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Differences between Plant Cell and Animal Cell
Animal CellPlant CellCentrioles presentNo centriolesContain
glycogen granules Contain starch granules
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Differences between Plant Cell and Animal Cell
Animal CellPlant CellSome cells have cilia or flagella, e.g.
ciliated cells of trachea, oviducts, flagellum of spermatozoonNo
cilia or flagellaLysosomes presentLsosomes usually absent except
insectivorous plants e.g. nepenthes utricularia
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Differences between Plant Cell and Animal Cell
Animal CellPlant CellSmaller than plant cellsOften larger than
animal cells although sizes varies greatly
