Download cell structure and function

CELL STRUCTURE AND
FUNCTION
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What is life?
The living world
Early cells
Prokaryotes
Prokaryote cell features
Eukaryote cells
Functions of Eukaryote
organelles
1. WHAT IS LIFE?
• Life is complex and dynamic – composed of carbon-based
molecules
• Life is organised and self-sustaining – composed of biomolecules
• Life is cellular
• Life is information-based – genes
• Life adapts and evolves – mutations
All living things share four processes:
Growth: an increase in size
Reproduction: an increase in number
Responsiveness: an ability to react to environmental stimuli
Metabolism: controlled chemical reactions
In addition, all living organisms share a cellular structure
2. The Living World
• A protocell could have contained only RNA to function
as both genetic material and enzymes.
• First protocells were heterotrophs using ATP as
energy and carrying out a form of fermentation.
Domains of Life on Earth: 3 domains
1. ARCHAEA: Halophiles and Thermophiles
2. BACTERIA: Cyanobacteria and Heterotrophic bacteria
3. EUKARYA: Flagellates, Fungi, Plants and Animals
3. Early cells
• Bacteria and Archaea are termed as
PROKARYOTES –organisms whose DNA
is not enclosed in a nucleus of the cell.
• EUKARYOTIC cells are aerobic and arose
2.1 billion years ago. They contain nuclei
and organelles.
• PLANTS appeared on land (mud flats)
during the ‘Paleozoic’ period, about 440
million years ago. They provided food for
higher animals to evolve
3.1 Cells
Cells are fundamental units of biology, and
building block of all organisms
Organisms range from:
Unicellular: 1 cell = 1 organism e.g.
Paramecium & Amoeba
Multicellular: 1036 cells = 1 organism,
different cells for different functions, exhibit
division of labor e.g. muscle, skeletal, immune,
lungs, epithelium, etc...)
4. Prokaryotes
Prokaryotes are single-celled microorganisms
characterized by:
• the lack of a membrane-bound nucleus and
• membrane bound organelles.
There are two domains of prokaryote:
1. Eubacteria / Bacteria
2. Archaebacteria/Archaea
4.1 Differences between bacteria and archae
Eubacteria have cell walls composed of peptidoglycan,
Archaebacteria have cell walls composed of various
different substances.
Eubacteria have ester-linked straight-chain membrane
lipids (fatty acids). Archaebacteria have ether-linked
branched-chain member lipids.
Eubacteria and Archaebacteria have differences in their
DNA replication and transcription systems that suggest
independent elaboration in these two groups
Bacteria translation apparatus inhibited by antibiotics (e.g.
streptomycin, tetracycline etc.). Archaea not affected by
antibiotics.
5. Prokaryote cell features
• Has five essential structural components:
1.genome (DNA)
2.ribosomes
3.cell membrane
4.cell wall
5.surface layer
• Structurally, a prokaryotic cell has three architectural
regions:
1.appendages (flagella and pili)
2.cell envelope (capsule, cell wall , plasma
membrane))
3.cytoplasm region (cell genome (DNA) and ribosomes.
5.1 Prokaryote cell
5.2 External structures of prokaryotes (p.60-65,
Bauman)
The external structures of prokaryotic cells include
glycocalyces, flagella, fimbriae, and pili
Glycocalyces (Ch 3, Bauman)
• A glycocalyx is a gelatinous, sticky substance that
surrounds the outside of the cell.
• When the glycocalyx of a prokaryote is firmly attached to
the cell surface, it is called a capsule.
• When loose and water-soluble, it is called a slime layer.
• Both types protect the cell from dessication, and both
increase the cell’s ability to cause disease.
5.2 contd. (Flagella)
• The structures responsible for cell motility include flagella, long
extensions from the cell surface and glycocalyx that propel a cell
through its environment.
• Bacterial flagella are composed of a filament, a hook, and a basal
body. Flagella covering the cell are termed peritrichous flagella and
those found at the ends of a cell are termed polar flagella. (refer
p.62, Bauman)
• Endoflagella are the special flagella of spirochetes that spiral tightly
around the cell instead of protruding into the environment.
• Flagella enable bacterial cells to move clockwise or
counterclockwise, in a series of runs and tumbles.
• Via taxis, flagella move the cell toward or away from stimuli such as
chemicals (chemotaxis) or light (phototaxis).
5.2 contd. (Fimbriae and Pili)
Fimbriae are short, sticky, proteinaceous,
nonmotile extensions of some bacteria that help
cells adhere to one another and to substances in
the environment. They serve an important
function in biofilms, slimy masses of bacteria
adhering to a surface.
Pili (also called conjugation pili) are hollow,
nonmotile tubes of a protein called pilin that
connect some prokaryotic cells. Typically, only
one or two are present per cell. They join two
bacterial cells and mediate the movement of
DNA from one cell to another, a process called
conjugation.
5.3 Cell wall of prokaryotes (p. 65-69)
• Most prokaryotic cells are surrounded by a cell wall (not
found in eukaryotes) that provides structure and
protection from osmotic forces. Cell walls are composed
of polysaccharide chains.
• Walls composed of peptidoglycan, a complex
polysaccharide composed of two alternating sugars
called acetylglucosamine (NAG) and
N-acetylmuranic acid (NAM)
• Gram-positive cells have thick layers of peptidoglycan;
stained cells appear purple under magnification.
• Gram-negative cells have only a thin layer of
peptidoglycan, outside of which is a membrane
containing lipopolysaccharide (LPS); Gram-negative
cells appear pink.
5.4 Prokaryotic Cytoplasmic Membranes (pp. B. 69–73)
• The cytoplasmic membrane is a double-layered structure, called a
phospholipid bilayer, composed of molecules with hydrophobic
lipid tails and hydrophilic phosphate heads.
• The selectively permeable cytoplasmic membrane, not
only,separates the contents of the cell from the outside environment,
but also controls the contents of the cell allowing some substances
to cross it while preventing the crossing of others.
• Although impermeable to most substances, its proteins act as pores,
channels, or carriers to allow or facilitate the transport of substances
the cell needs.
• The relative concentration of chemicals (concentration gradients)
inside and outside the cell and of the corresponding electrical
charges, or voltage (electrical gradients), create an overall
electrochemical gradient across the membrane.
• A cytoplasmic membrane uses the energy inherent in its
electrochemical gradient to transport substances into or out of the
cell using passive process (diffusion and osmosis) and active
process (ATP utilization and translocation)
5.5 Cytoplasm of Prokaryotes
• Cytoplasm is the gelatinous, elastic material inside a
cell. It is composed of cytosol, inclusions, ribosomes,
and in many cells a cytoskeleton. Some bacterial cells
produce internal, resistant, dormant forms called
endospores. Endospores can survive under harsh
conditions, making them a concern to food processors
and health care professionals.
• The liquid portion of the cytoplasm is called cytosol. It is
mostly water, plus dissolved and suspended substances
such as ions, carbohydrates, proteins, lipids, and
wastes. The cytosol of prokaryotes also contains the
cell’s DNA in a region called the nucleoid.
5.5 Cytoplasm contd.
• Deposits called inclusions may be found within the
cytosol of prokaryotes. These may be reserve deposits
of lipids, starch, or other chemicals
• Nonmembranous Organelles: ribosomes and the
cytoskeleton.
a) Ribosomes are the sites of protein synthesis in cells.
They are composed of protein and ribosomal RNA
(rRNA).
b) The cytoskeleton is an internal network of fibers that
play a role forming a cell’s basic shape. Spherical
prokaryotes appear to lack cytoskeletons.
6. EUKARYOTIC CELLS
• Endosymbiotic theory: mitochondria and chloroplast
have 70S ribosomes, circular DNA, and 2 membranes
suggesting that the ancestors of these organelles were
prokaryotic cells that were internalized by other
prokaryotes and then lost the ability to exist outside their
host – thus forming early eukaryotes
• Eukaryotic cells are larger than prokaryotes.
• They have a variety of internal membranes and
structures, they are:
1. Organelles
2. cytoskeleton composed of microtubules,
microfilaments and intermediate filaments
• Eukaryotic DNA is composed of several linear bundles
called chromosomes
6.1 Eukaryotic cell
6.1.1 Animal cell
6.1.2 Plant cell
6.3 External Structures of Eukaryotic Cells (p.77 B.)
• Eukaryotic cells have many external structures similar to those of
prokaryotes, as well as some unique features (flagella, cilia, and
glycocalyces for attachment [in animals and protozoan cells lacking
cell walls])
• Eukaryotic flagella are within the cytoplasmic membrane.
• The shaft of a eukaryotic flagellum is composed of molecules of a
globular protein called tubulin arranged in chains to form hollow
microtubules arranged in nine pairs around a central two.
• The basal body also has microtubules, but in triplets with no central
pair. Eukaryotic flagella have no hook, and do not extend outside the
cell. Rather than rotating, eukaryotic flagella undulate rhythmically to
push or pull the cell through the medium. They do exhibit taxis.
• Some eukaryotic cells are covered with cilia, which have the same
structure as eukaryotic flagella but are much shorter and more
numerous. Their rhythmic beating propels single-celled eukaryotes
through their environment.
Image of flagellum
6.4 Eukaryotic Cell Walls and Cytoplasmic
Membranes (B.pp. 77–80)
• The eukaryotic cells of fungi, algae, plants, and some
protozoa lack glycocalyces;
• Cell wall composed of polysaccharides provides
protection from the environment. It also provides shape
and support against osmotic pressure.
• The cell walls of plants are composed of cellulose,
whereas fungal cell walls are composed of chitin or other
polysaccharides.
• Algal cell walls contain agar, carrageenan, algin, or other
chemicals.
• All eukaryotic cells have cytoplasmic membranes. Like
bacterial membranes, they are a fluid mosaic of
phospholipids and proteins.
6.4.1 Transportation (endocytosis)
• Some eukaryotic cells transport substances into
the cytoplasm via endocytosis, which is an
active process requiring the expenditure of
energy by the cell.
• In endocytosis, pseudopodia—moveable
extensions of the cytoplasm and membrane of
the cell—surround a substance and move it into
the cell.
• When solids are brought into the cell,
endocytosis is called phagocytosis. The
incorporation of liquids is called pinocytosis
6.5 Cytoplasm of Eukaryotes
• The cytoplasm of eukaryotes is more complex than that
of prokaryotes, containing a few nonmembranous and
numerous membranous organelles.
• Three nonmembranous organelles are found in
eukaryotes: ribosomes, a cytoskeleton, and centrioles
• Eukaryotic ribosomes are 80S and are found within the
cytosol (liquid portion of cytoplasm)
• The cytoskeleton is extensive, and composed of both
fibers and tubules. It acts to anchor organelles and
functions in cytoplasmic streaming and in movement of
organelles within the cytosol.
• Centrioles are composed of nine triplets of tubulin
microtubules. Centrosomes play a role in mitosis,
cytokinesis, and in the formation of flagella and cilia.
6.5.1 Membranous Organelles
• Nucleus: - is enclosed in a double membrane and
communicates with the surrounding cytosol via
numerous nuclear pores. Within the nucleus is the DNA
providing the cell with its unique characteristics.
• Nucleolus: The nucleolus produces ribosomes
• Cytosol: The cytosol is the "soup" within which all the
other cell organelles reside and where most of the
cellular metabolism occurs.
-The cytosol is full of proteins that control cell
metabolism including signal transduction pathways,
glycolysis, intracellular receptors, and transcription
factors.
• Cytoplasm: This is a collective term for the cytosol plus
the organelles suspended within the cytosol.
Membranous Organelles
• Mitochondria: Mitochondria provide the energy to a cell.
They are the power centers of the cell. Mitochondria are
membrane-bound organelles (double membrane).
• Vacuole: A vacuole is a membrane-bound sac that plays
roles in intracellular digestion and the release of cellular
waste products.
• Lysosome: -contain hydrolytic enzymes necessary for
intracellular digestion. They are common in animal cells,
but rare in plant cells. Hydrolytic enzymes of plant cells
are more often found in the vacuole
Diagram of mitochondrion
SECTIONS OF MITOCHONDRION
• The different sections are: outer membrane; intermembrane
space; inner membrane and matrix
• The Outer Membrane: This membrane contains a great number of
large transport proteins, which allows for large molecules to enter
with ease. This membrane includes proteins that can convert lipid
substrates into forms that can be used by the matrix.
• The Intermembrane Space: This space contains enzymes that use
ATP to phosphorylate other nucleotides.
• The Inner Membrane: This membrane is highly convoluted, forming
many folds called cristae. This serves to greatly increase the
surface area, allowing more work to be done is a smaller space.
• It contains three major proteins –
(1). the proteins that carry out the oxidation reactions of the
respiratory chain,
(2). an enzyme complex called ATP synthetase which makes ATP,
and
(3). transport proteins which regulate the transfer of molecules into
and out of the matrix. This is where the oxidation phosphorylation
takes place.
The Matrix of Mitochondrion
• The Matrix: The Kreb Cycle takes place
here.
• It also contains:
(1) several copies of the mitochondrial
DNA genome,
(2) special mitochondrial ribosomes,
(3) tRNAs,
(4) and various enzymes required for the
expression of the mitochondrial gene.
CHLOROPLAST IN PLANTS
• Chloroplast: This organelle contains the plant cell's
chlorophyll responsible for the plant's green color.
Structurally it is very similar to the mitochondrion except
it is larger than the mitochondrion, not folded into cristae,
and not used for electron transport
It contains:
• 1. A permeable outer membrane;
• 2. A less permeable inner membrane;
• 3. Inter membrane space;
• 4. A third membrane containing the light-absorbing
system, the electron transport chain, and ATP
synthetase, that forms a series of flattened discs, called
the thylakoids
Image of chloroplast
ORGANELLES (GOLGI AND CELL MEMBRANE)
• Golgi:
-The Golgi apparatus is a single membranebound structure.
-It is actually a stack of membrane-bound
vesicles that are important in packaging
macromolecules for transport elsewhere in the
cell.
• Cell Membrane:
– Every cell is enclosed in a membrane, a
double layer of phospholipids (lipid bilayer)
CYTOSKELETON
• Cytoskeleton:
– Helps to maintain cell shape.
– The primary importance of the cytoskeleton is in cell
motility.
– Provides a supporting structure for the internal
movement of cell organelles, as well as cell locomotion
and muscle fiber contraction could not take place
without the cytoskeleton.
– It is composed of proteinaceous fibers:
1. Microtubules (diameter – 25 nm)
2. Actin filaments (microfilaments), diameter, 5-7nm
3. Intermediate fibers (diameter – 10 nm)
7. SIMILARITIES BETWEEN
EUKARYOTES AND PROKARYOTES
• They both have DNA as their genetic
material.
• They are both membrane bound.
• They both have ribosomes .
• They have similar basic metabolism .
• They are both amazingly diverse in
forms
7.1 Differentiating Prokaryotes and
Eukaryotes
NUCLEUS
Eukaryotes have a nucleus and membrane-bound
organelles , while prokaryotes do not.
DNA
• The DNA of prokaryotes floats freely around the cell; DNA of eukaryotes is
held within its nucleus.
• Prokaryotes also differ from eukaryotes in that they contain only a single
loop of DNA stored in an area named the nucleoid.
• Prokaryotic DNA also lacks the proteins found in eukaryotic DNA .
SIZE
Prokaryotes are usually much smaller than eukaryotic cells. Eukaryotic cells
are, on average, ten times the size
of prokaryotic cells.
CELL WALL
Prokaryotes have cell wall composed of peptidoglycan (a single large polymer
of amino acid and sugar). Cell wall of eukaryotes is not made up of this
polymer.
Differentiating Prokaryotes and
Eukaryotes (contd.)
SUPPORT
In Eukaryotes provided by cytoskeleton;
none in Prokaryotes
PROTEIN SYNTHESIS
In Eukaryotes (animals) Rough Endoplasmic
Reticulum (Rough ER) is involved
In Prokaryotes ribosomes are involved
FAT SYNTHESIS
In Eukaryotes – Smooth ER involved
No fat synthesis in Prokaryotes
Differentiating Prokaryotes and
Eukaryotes (contd.)
ENERGY PRODUCTION
In Eukaryotes – chloroplasts (plants);
mitochondrion (Kreb’s cycle)
In Prokaryotes – chlorophyll (if present) but has no
covering or chloroplast; no mitochondrion and
Kreb’s cycle replaced by fermentation
ENERGY DIGESTION
Lysosomes involved in aging process of cell in
Eukaryotes
No lysosomes in Prokaryotes
Differentiating Prokaryotes and
Eukaryotes (contd.)
MOVEMENT
In Eukaryotes – cilia, flagella and pseudopod
movement
In Prokaryotes – flagella of different structure
involved in locomotion
REPRODUCTION - DNA control
In Eukaryotes – DNA in chromosomes inside
nucleus
In Prokaryotes – DNA in single strand and floating
freely without a nucleus
8. Review Questions
1.
Differentiate among:
i. pili,
ii. fimbriae,
iii. prokaryotic flagella,
iv. eukaryotic flagella,
v. and cilia
using sketches and descriptive labels
2.
Contrast prokaryotic and eukaryotic cells with reference to:
i. Size
ii. Presence of membrane-bound organelles
iii. Structure of flagella
iv. Chemicals in cell walls
v. Type of ribosomes
3.
Compare and contrast 3 types of passive transport across a cell membrane
4.
What factors may prevent a molecule from moving across a cell membrane
5.
Describe the ‘endosymbiotic theory’