Lecture 5
Biology
Cytoplasm:
Cytoplasm represents everything enclosed by the plasma membrane,
with the exclusion of the nucleus. It is present in all cells where metabolic
reactions occur. It consists mainly of a viscous fluid medium that includes
salts, sugars, lipids, vitamins, nucleotides, amino acids, RNA, and
proteins which contain the protein filaments, actin microfilaments,
microtubules, and intermediate filaments. These filaments function in
animal and plant cells to provide structural stability and contribute to cell
movement.
Many of the functions for cell growth, metabolism, and replication are
carried out within the cytoplasm. The cytoplasm performs the functions
of energy production through metabolic reactions, biosynthetic processes,
and photosynthesis in plants. The cytoplasm is also the storage place of
energy within the cell.
Cytosol:
a subset of cytoplasm, refers only to the protein-rich fluid
environment, excluding the organelles.
Cytoskeleton:
The cytoskeleton structure, located just under the
membrane, is a network of fibers composed of proteins, called protein
filaments. This structure is connected to other organelles. In animal cells,
it is often organized from an area near the nucleus. These arrays of
protein filaments perform a variety of functions:
• Establish the cell shape
• Provide mechanical strength to the cell
• Perform muscle contraction
Lecture 5
Biology
• Control changes in cell shape and thus produce locomotion
• Provide chromosome separation in mitosis and meiosis .
• Facilitate intracellular transport of organelles
Nucleus:
The nucleus is often called the control center of the cell. It is the largest
organelle in the cell, usually spherical with a diameter of 4–10µm, and is
separated from the cytoplasm by an envelope consisting of an inner and
an outer membrane. All eukaryotic cells have a nucleus. The nucleus
contains DNA distributed among structures called chromosomes, which
determine the genetic makeup of the organism. The chromosomal DNA is
packaged into chromatin fibers by association with an equal mass of
histone proteins. The nucleus contains openings (100 nm across) in its
envelope called nuclear pores, which allow the nuclear contents to
communicate with the cytosol.
Figure 2. shows a schematic of a nucleus. The inside of the nucleus also
contains another organelle called a nucleolus, which is a crescent-shaped
structure that produces ribosomes by forming RNA and packaging it with
ribosomal protein. The nucleus is the site of replication of DNA and
transcription into RNA. In a eukaryotic cell, the nucleus and the
ribosomes work together to synthesize proteins.
Lecture 5
Biology
Figure 2. Schematics of the structure of the nucleus.
Mitochondria:
Mitochondria are large organelles, globular in shape (almost like fat
sausages), which are 0.5–1.5µm wide and 3–10µm long. They occupy
about 20% of the cytoplasmic volume. They contain an outer and an inner
membrane, which differ in lipid composition and in enzymatic activity.
The inner membrane, which surrounds the matrix base, has many in
foldings, called cristae, which provide a large surface area for attachment
of enzymes involved in respiration. The matrix space enclosed by the
inner membrane is rich in enzymes and contains the mitochondrial DNA.
Mitochondria serve as the engine of a cell. They are self-replicating
energy factories that harness energy found in chemical bonds through a
process known as respiration, where oxygen is consumed in the
production of this energy. This energy is then stored in phosphate bonds.
In plants, the counterpart of mitochondria is the chloroplast, which
utilizes a different mechanism, photosynthesis, to harness energy for the
synthesis of high-energy phosphate bonds.
Lecture 5
Biology
Endoplasmic Reticulum:
The endoplasmic reticulum consists of flattened sheets, sacs, and tubes
of membranes that extend throughout the cytoplasm of eukaryotic cells
and enclose a large intracellular space called lumen. There is a continuum
of the lumen between membranes of the nuclear envelope. The rough
endoplastic reticulum (rough ER) is close to the nucleus, and is the site of
attachment of the ribosomes. Ribosomes are small and dense structures,
20nm in diameter, that are present in great numbers in the cell, mostly
attached to the surface of rough ER, but can float free in the cytoplasm.
They are manufactured in the nucleolus of the nucleus on a DNA
template and are then transported to the cytoplasm. They consist of two
subunits of RNA (a large, 50S, and a small, 30S) that are complexed with
a set of proteins. Ribosomes are the sites of protein synthesis. The process
of protein synthesis using a messenger RNA template is described below.
The rough ER transitions into a smooth endoplastic reticulum (smooth
ER), which is generally more tubular and lacks attached ribosomes. The
smooth ER is the primary site of synthesis of lipids and sugars and
contains degradative enzymes, which detoxify many organic molecules.
Golgi Apparatus:
This organelle is named after Camillo Golgi, who described it. It
consists of stacked, flattened membrane sacs or vesicles, which are like
shipping and receiving departments because they are involved in
modifying, sorting, and packaging proteins for secretion or delivery to
other organelles or for secretion outside of the cell. There are numerous
membrane-bound vesicles (<50nm) around the Golgi apparatus, which
are thought to carry materials between the Golgi apparatus and different
compartments of the cell.
Lecture 5
Lysosomes:
Biology
These are bags (technical term: vesicles) of hydrolytic
enzymes that are 0.2–0.5µm in diameter and are single-membrane bound.
They have an acidic interior and contain about 40 hydrolytic enzymes
involved in intracellular digestions.
Peroxisomes:
These are membrane-bound vesicles containing
oxidative enzymes that generate and destroy hydrogen peroxide. They are
0.2–0.5µm in diameter.
Chloroplast:
This cell organelle exists only in plants. It contains
pigments, called chlorophylls, which harvest light energy from the sun.
The chloroplast is the site of photosynthesis, where light energy from the
sun is converted into chemical energy to be utilized by the plant cell
(synthesis of ATP).