12. Living Organisms
• A living thing is composed of one or more
cells. These units, generally too small to be
seen with the unaided eye, are organized into
tissues. A tissue is a series of cells that
accomplish a shared function. Tissues, in
turn, form organs, such as the stomach and
kidney. A number of organs working together
compose an organ system. An organism is a
complex series of various organ systems.
20. Many-celled (Multi-cell) Organisms
• Higher animals, including man, are made up of
millions of living cells which vary widely in
structure and function but have different
features in common.
23. Cell Structure
• All cells are similar in that they contain a gelatinous substance
called protoplasm. Protoplasm is the
viscid, translucent, polyphasic colloid with water as the
continuous phase that makes up the essential material of all
cells. It is composed mainly of nucleic
acids, proteins, lipids, carbohydrates, and inorganic salts. The
protoplasm surrounding the nucleus is known as the cytoplasm
and that composing the nucleus is the nucleoplasm (also called
karyoplasm).
• The cell contains an outer membrane, the plasma (cell)
membrane, a nucleus (a spherical or oval organelle often near
the center) and cytoplasm, in which are cell organelles (little
organs) suspended in a fluid, the cytosol, and inclusion bodies
containing secretion and storage substances.
25. Plasma Membrane (1)
• The plasma membrane is formed from a double
layer of lipids and proteins and composes the cell's
outer membrane. It surrounds the protoplasm of the
cell and helps maintain its shape and structure. The
plasma membrane separates each cell from its
environment. The function of the plasma membrane is
to regulate active transport. This process selectively
allows certain substances to enter and leave the cell
while barring others. It also sends and receives
chemical and electrical messages, including signals for
the cell to manufacture proteins or to divide.
27. Plasma Membrane (2)
• The moving of substances in and out of cells is either a passive
process or an active process. Passive transport requires no energy
and can occur by simple diffusion (the spreading or scattering of
particles), which is the passage of molecules of one substance from
a region where it is present in high concentration to the molecules
of another substance to form a uniform mixture of the two.
Oxygen, nitrogen, and other small molecules that can dissolve
easily in lipids move readily back and forth across the bilayer. Some
molecules such as glucose, or sodium and potassium ions, cannot
cross the membrane on their own by simple diffusion. They need
selective transport proteins or special channels to allow them in
and out of a cell. In active transport the cell works to bring
molecules in and push them out. The energy needed for active
transport is derived from molecules of a higher energy phosphate
known as ATP (adenosine triphosphate).
29. Nucleus (1)
• The nucleus is a small sphere in the central
portion of a cell. It is the biggest, densest and
most obvious structure within the cell and is the
control center of the cell. The nucleus is enclosed
in a double layer of nuclear membrane. At
numerous points these membranes are
joined, forming the rims of circular openings, the
water filled nuclear pores, through which large
molecules e.g. ribonucleic acid (RNA) can pass in
and out of the nucleus. Within the nuclear
membrane is nucleoplasm. It contains one or two
small spherical bodies called nucleoli.
30. Nucleus (2)
• The nucleolus contains the DNA coding. The
function of DNA (Deoxyribonucleic acid) is to
pass on the characteristics of one generation
of cells to the next. The DNA is contained by a
number of chromosomes, which consist of
long strands of DNA tightly wound into coils
with proteins called histones. The combination
of DNA and histone proteins is known as
chromation. The nucleolus also controls the
synthesis of some of the cell's RNA.
32. Nucleus (3)
• DNA molecules are too large to pass out of the
nucleus. Hence part of the DNA molecule assembles
(by a process called transcription) a nucleic acid which
is smaller than DNA, called messenger ribonucleic acid
(mRNA) which can pass into the cytoplasm. mRNA
carries the code for polypeptide and protein assembly
to the ribosomes. Amino acids are also carried to the
ribosomes attached to other, smaller RNA
molecules, called transfer RNA (tRNA). Polypeptides
and proteins can then be assembled on the ribosomes
from the amino acids according to the mRNA code (a
process of translation).
36. Chromosomes (1)
• A chromosome is a rod-shaped structure containing
genes that is found in the cell nucleus. It is composed
of long strands of DNA with many proteins attached. In
general, all chromosomes have similar structures with
slight variations. The DNA and protein is found in the
arms of the chromosome and genes are located at
specific sites. Genes are small segments of the DNA
molecule. Each gene provides information required to
determine a protein's amino acid sequence. The two
arms of the chromosome can usually be identified as
the short arm and the long arm.
38. Chromosomes (2)
• There is a narrow area on each chromosome
called centromere. The centromere is the point
that appears to attach the spindle fibers during
mitosis. Chromosome movement occurs about
centromere. Chromosomes normally occur in
pairs. There are 23 pairs of chromosomes in
human cells, for a total of 46. One of each pair is
contributed by the mother and one by the father.
Chromosomes are also responsible for the
determination of sex. Normally, females have two
identical X-chromosomes, whereas males have
one X and Y chromosome.
41. Cytoplasm
• Cytoplasm is a jelly-like substance of protoplasm which
occupies the region between the plasma membrane
and the nucleus. It is composed of all the structures
inside a cell, excluding the nucleus. The cytoplasm is
the site of most of the intermediary metabolism of the
cell: food is converted into forms that can be used to
build cell parts; chemical energy is released from food
and transferred to an area where energy is needed in
chemical reactions; specific compounds, such as
proteins, are synthesized to be used within the cell
itself or to be exported to other parts of the organism.
In general, it carries on the work of the cell. The
cytoplasm contains:
42. Endoplasmic Reticulum (1)
• The endoplasmic reticulum is a network of interconnected
tubular and sac-like channels. The space between their
walls is continuous with the space of the nuclear
membrane and can thus transport substances from one
part of the cell to another. One form of ER, rough or
granular endoplasmic reticulum, has ribosomes attached to
its outer surface and the other form, smooth or
agranular, has no ribosomes. The spaces between both
types are connected. Ribosomes on rough ER synthesize
proteins while smooth ER is involved in carbohydrate
metabolism. Specialized types of ER are present in some
cells e.g. in skeletal muscle cells smooth ER stores calcium
ions which are liberated to initiate contraction of muscle
cells.
44. Endoplasmic Reticulum (2)
• The endoplasmic reticulum appears to serve several functions. Its
membranes provide an increase in surface area where chemical
reactions can occur. The channels of the reticulum provide both
storage space for products synthesized by the cell and
transportation routes through which material can travel to other
parts of the cell. The endoplasmic reticulum is also a cell's
membrane factory. Phospholipids and cholesterol, the main
components of membranes throughout the cell, are synthesized in
the smooth portion of ER. These compounds form the coating of
protein filled sacs, called vesicles, that break off from ER, migrate to
another organelle, fuse with it, and then deposit the protein cargo.
Most of the proteins leaving ER are still not mature. They must
undergo further processing in another organelle, the Golgi
apparatus, before they are ready to perform their functions within
or outside the cell.
45. Ribosomes (1)
• Granules, called ribosomes, shaped somewhat like
balls, are sites of protein synthesis. Ribosomes are
extremely tiny. A single cell may contain thousands of
ribosomes. Each ribosome is made of two unequally
sized subunits, which are composed of at least 40
different proteins and a form of RNA called ribosomal
RNA. Within these ribosomes, various chemicals called
amino acids, guided by signals from the nucleus are
assembled in precisely the right arrangement to form
proteins, the major part of the organic matter in living
cells. Proteins perform most of the significant chemical
reactions that occur in cells. They are also important in
maintaining its structure.
47. Ribosomes (2)
• Proteins are long strings of amino acids attached to one
another like beads in a necklace. Different proteins have
different sequences of amino acids, which are
determined, or coded, by the DNA. In protein synthesis, an
RNA copy of the DNA of a gene is transported to the
cytoplasm, where ribosomes, other RNAs and enzymes
come together to translate the RNA structure into a specific
amino acid sequence, or protein. Protein synthesis occurs
through the interaction of three kinds of RNA molecules.
During translation, a strand of messenger RNA moves
between the two parts of a ribosome. It provides the coded
message for the amino acid sequence. The ribosome
"reads" the message of the mRNA in groups of three, rather
than one nucleotide at a time. These groups are called
codons.
49. Ribosomes (3)
• Each codon specifies one of twenty different amino
acids or is a signal to start or stop making protein. The
amino acids called for by the mRNA are brought from
the cytoplasm to the ribosome by tRNA. This small
molecule is a connector: one end carries three
nucleotides, known as anticodon, which will join to a
codon in the mRNA according to the rules of base
pairing. The molecule's other end carries an amino
acid. As the mRNA passes through the ribosome, tRNA
brings the correct amino acids in and they are linked
together by peptide bonds to form a polypeptide
chain. When all amino acids for a protein chain are
formed, the chain is released.
50. Ribosomes (4)
• Some ribosomes move freely in the cytoplasm and
some are attached to the surface of the endoplasmic
reticulum. The two kinds of ribosomes play similar
roles in the manufacture of proteins. But while free
ribosomes leave the proteins free to float in the
cytoplasm, the bound ribosomes transfer their finished
proteins into the endoplasmic reticulum. Proteins
synthesized by endoplasmic reticulum ribosomes pass
into the ER lumen then to the Golgi apparatus where
they are processed. Proteins manufactured by free
ribosomes perform their functions in the cytosol.
51. The Golgi Apparatus
• The Golgi apparatus consists of a collection of
membrane-enclosed sacs. Proteins from the
endoplasmic reticulum have their structures
altered here. This alteration is a kind of label
which determines whether the protein will be (a)
passed into lysosomes, (b) stored in secretory
granules or (c) inserted into plasma membrane.
Once final processing of the protein is
complete, the proteins are removed from the
Golgi apparatus and are moved to their final
destinations in vesicles.
53. Lysosomes
• Lysosomes are large single-membrane structures with no
dividing membrane inside. Lysosomes are manufactured by
the endoplasmic reticulum and Golgi apparatus. They vary
in shape and size because they fuse with other vesicles to
carry out their functions. Lysosomes contain digestive
enzymes that break down large molecules, such as
proteins, fats, and nucleic acids, into smaller constituents
that can be oxidized by the mitochondria. Lysosomes also
digest bacteria. When a bacterium enters the
cell, lysosomes fuse with the vesicle of engulfed material
and release digestive enzymes to break up the material.
Lysosomes are known to contain over 40 different enzymes
that can digest almost anything in the cell, including
proteins, RNA, DNA, and carbohydrates.
54.
55. Peroxisomes
• Peroxisomes are also single-membrane
organelles. They contain (a) enzymes which
combine oxygen and hydrogen to form
hydrogen peroxide (H2O2) and (b) an enzyme
(catalase) which converts H2O2 to water.
58. Mitochondria (1)
• Mitochondria are the largest organelles in an animal cell, after the
nucleus. They are sausage or oval shaped structures surrounded by
a double-layered membrane. The inner and outer membranes are
separated by a fluid-filled gap. Mitochondria can change shape
quite readily. They swell or contract in response to various
hormones and drugs and during ATP (adenosine triphosphate)
manufacture.
• Mitochondria are now sometimes referred to as the
powerhouse of cells because these organelles release the majority
of the energy obtained from food and make it available to the
energy-consuming process of the cell. Energy is generated from
sugars and fatty acids. Specialized enzymes that trap energy from
the breakdown of sugar are imbedded in the inner layer. Besides
supplying energy, mitochondria also help the concentration of
water, calcium, and other charged particles (ions) in the cytoplasm.
59.
60. Mitochondria (2)
• Mitochondria use oxygen to release the chemical energy
stored in food. This process is called cellular respiration or
catabolism. The biochemical reactions of cellular
respiration fall into two groups: the carbon pathway, in
which sugar is broken down into carbon dioxide and
hydrogen; and the hydrogen pathway, which transfers
hydrogen to oxygen in stages, forming water and releasing
energy. In the hydrogen pathway, the hydrogen's electrons
pass through an "electron transport chain" made up of
enzymes. The electrons give up part of their energy as they
move from enzyme to enzyme. This energy is then stored in
molecules of ATP (adenosine triphosphate). In the end, 38
molecules of ATP are formed for every ever molecule of
sugar that is used up in respiration.
61. Mitochondria (3)
• Mitochondria have some of their own DNA
molecules and ribosomes and are self-replicating.
They "reproduce" by splitting in half.
• An interesting characteristic of human
mitochondria is the fact that all of a person's
mitochondria are descendants of those of his or
her mother; no paternal mitochondria are
present. This is unlike nuclear DNA which is
equally derived from both parents.
62. Centrosome
• The centrosome consists of two rod-like
structures called centrioles arranged at right
angles to one another. It is concerned with the
synthesis of microtubules, e.g. the spindle and
aster microtubules present during cell
division.
64. Secretory Vesicles
• All secretory substances are formed by the
endoplasmic reticulum - Golgi apparatus system.
They are then released from Golgi apparatus into
the cytoplasm inside storage vesicles called
secretory vesicles or secretory granules.
• In addition to the above-mentioned organelles
the cytoplasm may contain any of a variety of
rod-like filaments, microfilaments and
microtubular structures, depending on the
function of the cell.
