Cell is the base of life
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The Endomembrane system
A membranous system of interconnected tubules and
cisternae
Membranes of the endomembrane system vary in structure,
composition, thickness and behavior
The endomembrane system includes:
Nuclear envelope
Endoplasmatic reticulum
Golgi apparatus
Lysosomes
Vacuoles
Plasma membrane (related to endomembrane)
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ER manufactures membranes
Endoplasmatic reticulum (ER) – network within the
cytoplasm – extensive membranous network of tubules
and sacs (cisternae) which sequesters its internal lumen
(cisternal space) from the cytosol.
Consist of smooth and rough ER.
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Smooth ER
Participates in the synthesis of lipids, phospholipids and
steroids
Participates in carbohydrate metabolism
Detoxifies drugs and poisons
Stores calcium ion necessary for muscle contraction
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Rough ER
Manufactures secretory proteins and membranes
Proteins to be secreted are synthesized by ribosomes
attached to rough ER
Polypeptide chain is threaded through ER membrane
into the lumen or cisternal space
Protein folds into its native conformation
Undergo modification: oligosaccharide are added to the
proteins in order to make glycoprotein
Proteins departs in a transport vesicle pinched off from
transitional ER adjacent to the rough ER site production
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Rough ER
Glycoproteins – protein covalently bonded to
carbohydrate
Oligosaccharide – small polymer of sugar units
Transport vesicle – membrane vesicle in transit from
one part of the cell to another
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Rough ER and membrane production
Membrane proteins are produced by ribosomes.
Growing polypeptide anchors by hydrophobic regions
into the ER membrane
Enzymes within the ER membrane synthesize
phospholipids from raw materials in the cytosol
Newly expanded ER membrane can be transported as a
vesicle to other parts of the cell
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Apparatus Golgi
Golgi apparatus – organelle made of stacked, flattened
membranous sacs (cisternae), that modifies, stores and
routes products of the ER
Has a distinct polarity. Membranes of cisternae at opposite
ends differ in thickness and composition.
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Apparatus Golgi
Two poles are called the cis face (forming face) and the
trans face (maturing face)
Cis face, which is closely associated with transitional ER,
receives products by accepting transport vesicles from the
ER.
Trans face pinches off vesicles from the Golgi and transports
molecules to other sites
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Golgi products in transit
from one cisternae to the
next, are carried in transport
vesicles.
The Golgi:
alters some membrane
phospholipids
modifies the oligosaccharide
portion of glycoproteins
target products for various
parts of the cell
sorts products for secretion
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Apparatus Golgi
Apparatus Golgi
Many polysaccharides
including hyaluronic acid
are Golgi products
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Lysosomes
Lysosomes are relatively large vesicles formed by the Golgi:
- organelles which are membrane-enclosed bag of
hydrolytic enzymes that digest all major classes of
macromolecules.
Enzymes include lipases, carbohydrases, proteases, and
nucleases
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Lysosomes
Lysosomal membrane performs two important functions:
Sequesters potentially destructive hydrolytic enzymes
from the cytosol
Maintains the optimal acidic environment for enzyme
activity by pumping H+s inward from the cytosol to the
lumen
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Function of the lysosomes
a. Intracellular digestion
Phagocytosis – cellular process of ingestion, in which
the plasma membrane engulfs substances and pinches
off to form a particle-containing vacuole
Lysosomes may fuse with food-filled vacuoles, and
their hydrolytic enzymes digest the food:
• Amoeba and other protists
• Human macrophages
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Function of the lysosomes
b. Recycle cell’s own organic material
Lysosomes may engulf other cellular organelles or
part of the cytosol and digest them (autophagy)
Resulting monomers are released into the cytosol
where they can be recycled into new
macromolecules
c. Programmed cell destruction
This process is important during metamorphosis and
development
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The formation and functions of lysosomes
The cell encloses
food in a vacuole.
The food vacuole
fuses with a
lysosome, and
hydrolytic enzymes
digest the food.
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The formation and functions of lysosomes
After hydrolysis, simple sugars, amino acids, and other
monomers pass across the lysosomal membrane into
the cytosol as nutrients for the cell.
Lysosomes recycle
the molecular
ingredients of
organelles
(autophagy).
The cell continually
renews itself
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Lysosomes and human disease
Symptoms of inherited storage diseases result from
impaired lysosomal function.
Lack of a specific lysosomal enzymes causes substrate
accumulation which interferes with lysosomal metabolism
and other cellular functions
Pompe’s disease – the missing enzyme is a carbohydrase
that breaks down glycogen – glycogen accumulation
damages the liver
Tay-Sachs disease – brain impairment by accumulation of
lipids
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The formation and functions of lysosomes
Transformation of a tadpole into a frog
and
Disappearance of tissue between the hands
fingers of human embryos
are done by digestion with lysosomes
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Diverse function of vacuoles
Food vacuoles – vacuole
formed by phagocytosis
Contractile vacuoles – pump
water excess out of the cell
(in protozoa)
Central vacuole enclosed by
a membrane (tonoplast)
exist in mature plants.
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Diverse function of vacuoles
Central vacuole
-is the major food
storage (protein storage
in seeds);
-stores inorganic ions
(K+ and Cl-);
-sequesters dangerous
metabolic by-products
from the cytoplasm
-contains soluble
pigments in some cells;
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Diverse function of vacuoles
Central vacuole
-helps against predators
by containing poisonous
compounds;
- plays a role in plant
growth by absorbing
water and elongating the
cell;
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Relationships between endomembranes
Membrane and secretory proteins produced by the ER flows
in the form of transport vesicles to the Golgi.
Golgi pinches off vesicles:
Vesicles give rise to
lysosomes and
vacuoles and
fuse with and add to
plasma membrane.
The membrane
expends and releases
secretory proteins
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Other membranous organelles
Mitochondria and chloroplasts
– the main energy transformers of cells
Mitochondria and chloroplasts are organelles that
transduce energy acquired from the surroundings into
forms useable for cellular work
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Other membranous organelles
Mitochondria are the sites of cellular respiration:
catabolic process that generates ATP by extracting
energy from sugars, fats and other molecules
Chloroplasts the sites of photosynthesis:
they convert solar energy to chemical energy by
absorbing sunlight and using it to drive the
synthesis of organic compounds from CO2 and H2O
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Mitochondria
Enclosed by double membranes that are not part of
endomembrane system (the membrane proteins are
synthesized by free ribosomes)
Contain ribosomes and some DNA that programs a small
portion of their own protein synthesis
Are semiautonomous organelles that grow and reproduce
within the cell
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Mitochondria
outer membrane
inner membrane
Cristae
Matrix
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Mitochondria
Found in nearly all eukaryotes cells
Number of mitochondria depends on the cell’s metabolic
activity
Are about 1 μm in diameter and 1-10 μm in length
Are dynamic structures that move, change their shape
and divide
Mitochondria contain their own DNA (termed mDNA) and
are thought to represent bacteria-like organisms
incorporated into eukaryotic cells over 700 million years
ago (perhaps even as far back as 1.5 billion years ago).
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Mitochondria
They function as the sites of energy release (following
glycolysis in the cytoplasm) and ATP formation (by
chemiosmosis).
Smooth outer membrane is highly permeable to small
solutes, but it blocks passage of proteins and other
macromolecules
Convoluted inner membrane contains embedded
enzymes that are involved in cellular respiration. It folds
into a series of cristae, which are the surfaces on which
ATP is generated.
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Mitochondria
Intermembrane space – a narrow region between the
inner and outer mitochondrial membranes
Reflects the solute composition of the cytoplasm, because
the outer membrane is permeable
Mitochondrial matrix – compartment enclosed by the inner
membrane, contains enzymes that catalyze many
metabolic steps of cellular respiration.
Some enzymes of respiration and ATP production are
actually embedded in the inner membrane.
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Muscle Cell Mitochondria
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Plastids
Plastids are also membrane-bound organelles that only
occur in plants and photosynthetic eukaryotes.
They include amyloplasts, chromoplasts and chloroplasts.
Amyloplasts – colorless plastids that store starch in roots
and tubers
Chromoplasts – plastids containing pigments other than
chlorophyll; responsible for fruits and flowers color.
Chloroplasts – chlorophyll-containing plastids which are
the sites of photosynthesis in eukaryotes.
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Plastids
Chloroplasts are found in eukaryotic algae, leaves and
other green plant organs
Are lens-shaped and measure about 2-5m
Are dynamic structures that change shape, move and
divide.
Functional compartments:
Intermembrane space – separates the two membranes
Inside the chloroplast is another membranous system –
thylakoids – segregates the interior of the chloroplast into
two compartments: thylakoid space and stroma.
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Plastids
Thylakoids function in the steps of photosynthesis that
initially convert light energy to chemical energy
Collectively a stack of thylakoids are a granum [plural =
grana]) floating in a fluid termed the stroma.
Photosynthetic reactions that use chemical energy to
convert carbon dioxide to sugar occur in the stroma
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Chloroplasts
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Chloroplasts
Like mitochondria, chloroplasts have their own DNA,
termed cpDNA.
Chloroplasts of Green Algae (Protista) and Plants
(descendants of some Green Algae) are thought to have
originated by endosymbiosis of a prokaryotic alga similar
to living Prochloron (Prochlorobacteria).
Chloroplasts of Red Algae (Protista) are very similar
biochemically to cyanobacteria (also known as blue-green
bacteria.
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Peroxisomes
Peroxisomes are roughly spherical and often have a
granular or crystalline core that is probably a dense
collection of enzymes. This peroxisome is in a leaf cell.
Notice its proximity to two
chloroplasts and a
mitochondrion.
These organelles cooperate
with peroxisomes in certain
metabolic functions (TEM).
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Peroxisomes
Peroxisomes do not bud from the endomembrane system.
They grow by incorporating proteins and lipids made in the
cytosol.
They increase in number by
splitting in two when they reach
a certain size.
Peroxisomes in liver detoxify alcohol
by transferring H to O and producing
H2O2
Peroxidase destroys toxic H2O2 by
converting it to H2O
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Peroxisomes
Peroxisomes convert fatty acids to smaller molecules that
can be used by mitochondria in the process of cellular
respiration.
In plant seeds glyoxysomes, special peroxisomes, convert
fatty acids to sugar.
This provides growing seedlings with energy and carbon
source.
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Cytoplasm
The cytoplasm was defined earlier as the material between
the plasma membrane (cell membrane) and the nuclear
envelope.
Fibrous proteins that occur in the cytoplasm, referred to as
the cytoskeleton maintain the shape of the cell.
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Cytoplasm
Microtubules function in cell division and serve as a
"temporary scaffolding" for other organelles.
Actin filaments are thin threads that function in cell division
and cell motility.
Intermediate filaments are between the size of the
microtubules and the actin filaments.
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Reading
Ch. 6 pp. 108-111
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