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General Biology (Bio107)
Chapter 4 – Cells
Cells are the smallest living
units of life
• Two types of cells exist on earth:
1. Prokaryotic cells
2. Eukaryotic cells
• All cells are surrounded by a phospholipid bilayermade barrier called the plasma membrane.
• The semifluid substance within the membrane is
the cytosol, containing the organelles.
• All cells contain chromosomes which have
genes in the form of DNA.
• All cells also have ribosomes, tiny organelles that
make proteins using the instructions contained in
genes.
• Besides showing a difference in
size (prokaryotic cells are small),
a major difference between
prokaryotic and eukaryotic cells
is the location of chromosomes.
• In eukaryotic cells, chromosomes
are contained in a membraneenclosed organelle, the nucleus.
• In much smaller prokaryotic cells,
the DNA is concentrated in the
nucleoid without a membrane
separating it from the rest of the
cell.
The prokaryotic cell is much simpler in structure, lacking a
nucleus and the other membrane-enclosed organelles of the
eukaryotic cell.
• Eukaryotic cells are generally much bigger
than prokaryotic cells.
• The logistics of carrying out metabolism set
limits on cell size.
– At the lower limit, the smallest bacteria,
mycoplasmas, are between 0.1 to 1.0 micron.
– Most bacteria are 1-10 microns in diameter.
– Eukaryotic cells are typically 10-100 microns in
diameter.
• Metabolic requirements also set an upper
limit to the size of a single cell.
• As a cell increases in size its volume
increases faster than its surface area.
– Smaller objects have a greater
ratio of surface area to volume.
Fig. 7.5
Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings
• The plasma membrane functions as a selective
barrier that allows passage of oxygen, nutrients,
and wastes for the whole volume of the cell.
Fig.
7.6
Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings
• The volume of cytoplasm determines the
need for this exchange.
• Rates of chemical exchange may be
inadequate to maintain a cell with a very
large cytoplasm.
• The need for a surface sufficiently large to
accommodate the volume explains the
microscopic size of most cells.
• Larger organisms do not generally have
larger cells than smaller organisms - simply
more cells.
Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings
Eukaryotic cell
• A eukaryotic cell has extensive and elaborate
internal membranes, which partition the cell
into compartments and membraneous
organelles.
• These membranes also participate in
metabolism as many enzymes are built into
membranes.
• The barriers created by membranes provide
different local environments that facilitate
specific metabolic functions.
Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings
• Different membranous organelles can be found
in eukaryotic cells:
1. Rough endoplasmic reticulum (rER)
2. Smooth endoplasmic reticulum
3. Golgi apparatus
4. Mitochondrion
5. Lysosomes
6. Peroxisomes
• Each type of membranous organelle has a
unique combination of lipids, proteins and
enzymes for its specific functions.
– For example, those in the membranes of
mitochondria function in cellular respiration.
Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings
Fig. 7.7
Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings
• Cells of eukaryotic photosynthesizing life
forms, e.g. algae and plants, contain
unique sets of organelles, most namely:
1. Tonoplast (central vacuoles)
and
2. Chloroplasts
• Their plasma membrane is further
surrounded by a thick, protective cell wall.
Fig. 7.8
Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings
Plasma membrane
• Phospholipid bilayer-made barrier between inside
and outside of cell; also contains cholesterol
• Semi-permeable; only water, gases and lipophilic
molecules can freely cross
• Controls entry of materials with the help of
selective transport proteins, e.g. carrier, porins,
channels
• Receives chemical and mechanical signals with
the help of receptor proteins
• Transmits signals between intra- and extracellular spaces
Facilitated Diffusion
Active Transport
Solutes are transported across plasma membranes with the
use of ATP-derived energy; capable to transport from an
area of lower concentration to an area of higher
concentration
Example: Sodium-potassium pump
Na+
gradient
Extracellular fluid
Na+/K+ ATPase
Cytosol
K+
gradient
Cytosol
K+
gradient
3 Na+ expelled
2K+
3 Na+
1
P
3 Na+
1
ATP
2
ADP
3
P
+
4 2K
imported
17
Nucleus
• The nucleus contains chromosomal DNA and
most of the genes in a eukaryotic cell.
– The nucleus of each human cell contains 46
chromosomes.
– Some genes are located in mitochondrial and
chloroplast DNA.
• The nucleus averages about 5 microns in diameter.
• The nucleus is separated from the cytoplasm by a
double membrane.
– These are separated by 20-40 nm.
• Where the double membranes are fused, a nuclear
pore allows gene-regulating proteins, large
macromolecules and particles to pass through.
Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings
• The nuclear side
of the envelope is
lined by the
nuclear lamina,
a network of
intermediate
filaments that
maintain the
shape of the
nucleus.
Fig. 7.9
Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings
• Within the nucleus, the DNA and associated
proteins (histones) are organized into fibrous
material, called chromatin.
• In a normal cell they appear as diffuse mass.
• However when the cell prepares to divide, the
chromatin fibers coil up to be seen as separate
structures, chromosomes.
• Each eukaryotic species has a characteristic
number of chromosomes.
– A typical human cell has 46 chromosomes, but
sex cells (eggs and sperm) have only 23
chromosomes.
Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings
• In the nucleus is a region of densely stained fibers
and granules adjoining chromatin, the nucleolus.
– There, ribosomal RNA (rRNA) is synthesized
and assembled with proteins from the cytoplasm
to form ribosomal subunits.
– The subunits pass from the nuclear pores to the
cytoplasm where they combine to form
ribosomes.
• The nucleus directs protein synthesis by
synthesizing messenger RNA (mRNA).
– mRNA travels to the cytoplasm and combines
with ribosomes to translate its genetic message
into the primary structure of a specific
polypeptide.
Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings
Ribosomes
• Ribosomes which contain rRNA and protein are
responsible for synthesis of new proteins in cells.
• A ribosome is composed of two subunits that
combine to carry out protein synthesis.
Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings
• Cell types that synthesize large quantities of
proteins (e.g., pancreas) have large numbers of
ribosomes and prominent nuclei.
• Some ribosomes, free ribosomes, are suspended
in the cytosol and synthesize proteins that function
within the cytosol.
• Other ribosomes, bound ribosomes, are attached
to the outside of the rough endoplasmic reticulum.
– These synthesize proteins that are either
included into membranes or for export from the
cell.
• Ribosomes can shift between roles depending on
the polypeptides they are synthesizing.
Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings
Endoplasmic Reticulum (ER)
• Structure: network of folded membranes
• Functions: synthesis, intracellular transport
• Types of E.R.
– Rough E.R.: studded with ribosomes (sites of
protein synthesis & protein folding)
– Smooth E.R. lacks ribosomes. Functions:
•
•
•
•
lipid synthesis
release of glucose in liver cells into bloodstream
drug detoxification (especially in liver cells)
storage and release of Ca2+ in muscle cells (where
smooth E.R. is known as sarcoplasmic reticulum or
SR)
Endoplasmic Reticulum (E.R.)
Copyright 2010, John Wiley &
Sons, Inc.
Golgi Complex
• Structure:
– Flattened membranes (cisterns) with bulging
edges (like stacks of pita bread)
• Functions:
– Receive protein from rER, modify and sort
proteins  glycoproteins and lipoproteins
that:
• Become parts of plasma membranes
• Are stored in lysosomes, or
• Are exported by exocytosis
Golgi Complex
Copyright 2010, John Wiley &
Sons, Inc.
Small cell organelles
• Lysosomes: contain digestive enzymes
– Help in final processes of digestion within cells
– Carry out autophagy (destruction of worn out parts of
cell) and death of old cells (autolysis)
– Important for phagocytotic cells (e.g. macrophages)
– Tay-Sachs: hereditary disorder; one missing lysosomal
enzyme leads to nerve destruction
• Peroxisomes: special forms of metabolism,
detoxify; abundant in liver; produce hydrogen
peroxide
• Proteasomes: digest unneeded or faulty proteins
– Faulty proteins accumulate in brain cells in persons with
Parkinson or Alzheimer disease.
Lysosomes
• Membrane-bounded sacs which contain
many hydrolytic enzymes that recycle
macromolecules, e.g. DNA, lipids, proteins,
back into their monomers.
• Lysosomal enzymes work best at acidic pH of 5.
– Proteins in the lysosomal membrane pump
hydrogen ions from the cytosol to the lumen
of the lysosomes.
• While rupturing one or a few lysosomes has little
impact on a cell, but massive leakage from
lysosomes can destroy an cell by autodigestion.
• Lysosomes fuse with phagosomes which space
allows the cell to digest macromolecules safely.
• Lysosomal enzymes and membranes are
synthesized by rough ER and then transferred to
the Golgi where they bud off.
• Lysosomes play crucial
role in following cell
processes:
1. Food digestion
2. Autophagy
(“organelle
recycling”)
3. Phagocytosis
+ bacterial kill
• Lysosomes also play a critical role in the
programmed destruction of cells (“apoptosis”)
in multicellular organisms.
– This process allows reconstruction during the
developmental process.
• Several inherited diseases affect lysosomal
metabolism (“lysosomal storage disorders”).
– These individuals lack a functioning version of
a normal hydrolytic enzyme.
– Lysosomes are engorged with indigestable
substrates.
– These diseases include Pompe’s disease in
the liver and Tay-Sachs disease in the brain.
Mitochondria
• Structure:
– Sausage-shaped with many folded membranes
(cristae) and liquid matrix containing enzymes
– Have some DNA, ribosomes (can make
proteins)
• Function:
– Nutrient energy is released and trapped in ATP;
so known as “power houses of cell”
– Chemical reactions require oxygen
• Abundant in muscle, liver, and kidney cells
– These cells require much ATP
Mitochondria
Chloroplast
• Structure:
– Oval-shaped with many stacked phospholipid
sacs (= thylacoids) and liquid stroma containing
enzymes
– Have large ring-formed DNA, ribosomes (can
make proteins)
• Function:
– Convert solar energy into ATP and nutrient
energy (glucose) by a process called
“photosynthesis”
– Chemical reactions require carbon dioxide
• Abundant in cells of green algae and plants
Chloroplast
Inner Chloroplast
Membrane
Thylacoid
Stroma
Centrosome
• Structure:
– Two centrioles arranged perpendicular to each
other
• Composed of microtubules: 9 clusters of 3 (triplets)
– Pericentriolar material
• Composed of tubulin that grows the mitotic spindle
• Function: important for microtubule
formation and assembly; important for
movement of chromosomes to ends of cell
during cell division (mitosis) and for vesicular
transport, e.g. in neurons
Centrosome
Copyright 2010, John Wiley &
Sons, Inc.
Cytoskeleton
• Maintains shape of
cell
• Positions organelles
• Changes cell shape
• Includes:
microfilments,
intermediate
filaments,
microtubules
Copyright 2010, John Wiley &
Sons, Inc.
Microtubules
• Build 9+2 protein core of cilia and flagella.
• Associated with ATP-consuming motor proteins.
• Important for movement of:
1. Cilia
2. Flagella (sperm)
3. Vesicles (axons)
4. Chromosomes
(mitosis & meiosis)
• Build from polymerized
tubulin monomers.
• Monomer: Tubulin
• Microtubules play a major role in cell motility.
– This involves limited movements of parts of the
cell.
• The microtubules interacts with ATP-consuming
motor proteins, e.g. dynein.
– In cilia and flagella motor proteins pull
components of the cytoskeleton past each other.
– This is also true
in muscle cells.
• A flagellum of a sperm has an undulatory
movement.
• Cilia move more like oars with alternating power
and recovery strokes.
– They generate force perpendicular to the cilia’s
axis.
Microfilaments
• Build protein core of microvilli of epithelial cells.
• Builds cortical network underneath cell membrane.
• Important for pseudopodia
formation and movement
of cells, e.g. white blood
cells.
• Build from polymerized
protein monomers.
• Monomer: Actin
Intermediate Filaments
• Are intermediate in size with 8 - 12 nm diameter.
• They are specialized for bearing tension.
– Intermediate filaments are
built from a diverse class of
subunits from a family
of proteins called keratins.
• Intermediate filaments are
more permanent fixtures of the
cytoskeleton than are the
other two classes.
• They reinforce cell shape
and also fix organelle location.