Download Eukaryotic cell structure (Lecture 3-4)

Eukaryotic cell structure (Lecture 3-4)
How can one define life?
The simplest definition is that any living thing must have three general properties:
• metabolism
• growth
• reproduction
Life on Earth is incredibly extensive and, to make it easier to study, biologists have broken
living systems up into generalized hierarchical levels:
molecules;
organells;
cells;
tissues;
organs;
organisms;
populations
The cell theory
1. Cells are the fundamental units of life, because a cell is the simplest unit capable of
independent existence.
2. All living things are made of cells.
Mattias Schleiden and Theodor Schwann (1839) called cells “units of life” – cell theory.
Cell elemental composition
Cells are 90% water.
The remaining molecules, the dry weight is approximately:
• 50% protein
• 15% carbohydrate
• 15% nucleic acid
• 10% lipid
• 10% miscellaneous
Cell element composition
Total approximate composition by element:
• 60% H
• 25% O
• 12% C
•
5% N
•
8% all the others including P, S and traces of Na, Mg, Cl, K, Ca, Fe, and even
less of certain other metals.
Similarities between eu- and prokaryotes:
1. They both have DNA as their genetic material.
2. They are both membrane bound.
3. They both have ribosomes.
4. They have similar metabolism.
Major differences
1. Eukaryotes have a nucleus and membrane-bound organelles, while prokaryotes do
not.
2. Prokaryotic DNA floats freely around the cell; eukaryotic is held within its nucleus.
3. Eukaryotic/prokaryotic size is 10/1.
4. The DNA of eukaryotes is much more complex.
5. Prokaryotes have a cell wall composed of peptidoglycan, a polymer of amino acids
and sugar. Some eukaryotic cells also have cell walls, but none made of
peptidoglycan.
Plant cell (Fig.6.9)
Is surrounded by membrane and cell wall
Contains: a nucleus, ribosomes, ER, Golgi apparatus, mitochondria, peroxisomes,
microfilaments and microtubules, plastids, chloroplasts and large central vacuole.
The vacuole (called tonoplast) stores chemicals, breaks down macromolecules, and, by
enlarging, plays a major role in plant growth.
Cell wall helps maintain the cell's shape and protects the cell from mechanical damage.
The cytosol of adjacent cells connects through trans-wall channels called plasmodesmata.
No lysosomes, no centrioles, no flagella
Animal cells (Fig.6.9)
No central vacuole, no chloroplasts, no cell wall, no plasmodesmata
Ribosomes are the sites of protein synthesis. (Fig.6.11)
They are not membrane-bound and thus occur in both prokaryotes and eukaryotes.
Eukaryotic ribosomes are slightly larger than prokaryotic ones.
They consist of a small and larger subunits.
Biochemically the ribosome consists of ribosomal RNA (rRNA) and some 50 structural
proteins.
Often ribosomes cluster on the endoplasmic reticulum, in which case they resemble a series
of factories adjoining a railroad line.
Bound ribosomes, those presently attached to the endoplasmic reticulum (ER), produce
secretory proteins.
Free ribosomes mainly make proteins that will remain dissolved in the cytosol.
Bound and free ribosomes are identical and can alternate between these two roles.
The Endomembrane system
A membranous system of interconnected tubules and cisternae
Membranes of the endomembrane system vary in structure changing in composition,
thickness and behaviour
The endomembrane system includes (Find them all on Fig.6.9):
Nuclear envelope
Endoplasmatic reticulum
Golgi apparatus
Lysosomes
Vacuoles
Plasma membrane (related to endomembrane)
ER manufactures membranes (Fig.6.12)
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.
Consists of smooth and rough ER.
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
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
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
Rough ER and membrane production
Membranes 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
Apparatus Golgi (Fig.6.13)
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.
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
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 og glycoproteins
target products for various parts of the cell
sorts products for secretion
Lysosomes (Fig.6.14)
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
Lysosomal membrane performs two important functions:
Sequesters potentially destructive hydrolytic enzymes from the cytosol
Maintains the optimal acidic environment for enzymeactivity by pumping H+s
inward from the cytosol to the lumen
The functions of lysosomes (Fig.6.14)
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
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
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 impairement by accumulation of lipids
Diverse function of vacuoles
Food vacuoles – phagocytosis
Contractile vacuoles – pump water excess out of the cell
Central vacuole enclosed by a membrane (tonoplast) exist in mature plants.
It develops by the coalescence of smaller vacuoles derived from ER and Golgi apparatus.
- 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;
- help against predators by containing poisonous compounds;
- plays a role in plant growth by absorbing water and elongating the cell;
- contributes to the large ratio of membrane surface area to cytoplasmic
volume
Relationships between endomembranes (Fig.6.16)
Membrane produced by the ER flows in the form of transport vesicles to the Golgi.
Golgi produces lysosomes and vacuoles
The membrane expends and releases secretory proteins
Molecule composition and metabolic function is modified upon the transition
Other membranous organelles (Figs 6.17-18)
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
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
They both:
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
Mitochondria
Found in nearly all eukaryotes cells
Number of mitochondria per cell varies and directly correlates with the cell’s
metabolic activity
Are about 1 mm in diameter and 1-10mm 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).
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.
Intermembrane space – a narrow region between the inner and outer mitochondrial
membranes
Reflects the solute composition of the cytosol, 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.
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.
Chloroplasts are found in eukaryotic algae, leaves and other green plant organs
Are lens-shaped and measure about 2-5mm
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.
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
Chloroplasts (Fig.6.18)
Chloroplasts contain many different types of accessory pigments:
Leukoplasts store starch, sometimes protein or oils.
Chromoplasts store pigments associated with the bright colors of flowers and/or fruits.
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.
Peroxisomes (Fig.6.19)
Peroxisomes are roughly spherical and often have a granular or crystalline core that is
probably a dense collection of enzymes.
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.
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.
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.
Reading
Campbell et al. Biology. Ch. 6. A tour of the cell, 102-111