Download VERTEBRATES Vertebrates are members of the larger phylum

VERTEBRATES
Vertebrates are members of the larger phylum Chordata, and show all of the major chordate
features at some point in their life cycles: notochord, dorsal hollow nerve cord, pharyngeal
slits, and a post-anal tail. But what makes a vertebrate a vertebrate, is the presence of the
vertebral column, or backbone, which surrounds and protects the main nerve cord. Vertebrates
belong to the phylum chordate. Vertebrates are the most complex and successful life forms on
earth.
During the embryonic development of all chordates, they pass through a stage called the
pharyngula, having these features:
1. A dorsal, tubular nerve cord, running from anterior to posterior. At its anterior end, it becomes
enlarged to form the brain.
2. A flexible, rodlike notochord that runs dorsal to the digestive tract and provides internal
support. In vertebrate chordates, it is replaced by a vertebral column or backbone long before
maturity.
3. Pairs of gill pouches. These lateral outpocketings of the pharynx are matched on the exterior
by paired grooves. In aquatic chordates, one or more pairs of gill pouches break through to the
exterior grooves, forming gill slits. These provide an exit for water taken in through the mouth
and passed over the gills.
4. A tail that extends behind the anus.
Seven classes of vertebrates are recognized:
Class Agnatha: These are jawless, fish-like animals with poorly developed fins. Lampreys and
hagfishes are the only jawless vertebrates to survive today. They both have a round mouth and
for this reason are often referred to as cyclostomes. They are the most primitive of the
vertebrates. Besides lacking jaws, their notochord persists for life, never being completely
replaced by a backbone even in the lampreys. They have no scales.
Class Chondrichthyes: This comprises 800 living species of sharks, rays, skates and rat fishes,
all of which have a cartilaginous skeleton, true jaws, and a number of other distinctive
characters. They got their names from the fact that their skeletons are made of cartilage, not
bone. With their gills exposed to sea water, all marine fishes are faced with the problem of
conserving body water in a strongly hypertonic environment. Sea water is about 3.5% salt, over 3
times that of vertebrate blood. The cartilaginous fishes solve the problem by maintaining such a
high concentration of urea in their blood (2.5% — far higher than the 0.02% of other
vertebrates).
Class Osteichthyes: includes some 20,000 species of true fishes, with a bony skeleton, a sutured
skull, teeth fused to the jaws, lobed or rayed fins, and a number of other distinguishing features.
Bony Fishes (Osteichthyes). They have paired fins. As their name indicates, the skeleton in this
group is made of bone. The group is subdivided into the ray-finned fishes (Actinopterygii) and
lobe-finned fishes (Sarcopterygii).
Class Amphibia: This includes some 3,500 living species of frogs, toads, salamanders, newts,
and caecilians, all of which have four limbs (making them tetrapods), a moist glandular skin,
external fertilization, and a complex life cycle. Some have poison glands. Amphibians are
vertebrates that spend part of their lives under water (breathing with gills) and the remainder on
land (breathing with lungs). Amphibians are cold-blooded; their body temperature depends on
the temperature of their environment. All amphibians have moist skin (their skin must be moist
to absorb oxygen, even though they breathe with their lungs). Most amphibians lay shell-less,
jelly-covered eggs, usually in the water. There are 3 groups (orders) of living amphibians: frogs
and toads (Anurans); newts and salamanders (Urodeles); caecilians (the worm-like
Gymnophiones ).
Class Reptilia: These are four-legged, tailed animals, with dermal scales, internal fertilization
and direct development. Living reptiles include about 6,200 species of crocodiles, turtles,
lizards, snakes, and tuataras. Important extinct groups of reptiles include the dinosaurs,
hadrosaurs, ichthyosaurs, pterosaurs and plesiosaurs. Reptiles (meaning "to creep") are a group
of animals that have (or modified scales), they breathe through lungs and usually lay eggs. They
are cold-blooded vertebrates. They all have scales (or plates, in the case of turtles) that prevent
them from losing moisture through their skin and protects against injury. Scales are made up of
keratin, the same material found in fingernails. Most lay eggs. There are four main groups of
reptiles: turtles and tortoises; lizards and snakes; crocodiles and alligators; and lastly tuatara.
Class Aves: the birds: Birds are animals that have wings, feathers, a beak, strong hollow bones
and powerful flight muscles. They are warm-blooded (or homoiothermic) tetrapods whose
forelimbs are specialized for flight (although some species are secondarily flightless or even
flightless). Birds have a characteristic covering of feathers and reproduce by laying eggs. Not all
flying animals are birds; and not all birds can fly. The ability to fly has developed independently
many times throughout the history of the Earth. Bats (flying mammals), pterosaurs (flying
reptiles from the time of the dinosaurs), and flying insects are not birds. The fastest running bird
is the ostrich, but it cannot fly. The ostrich is also the fastest two-legged runner of all the animals
on Earth. The ostrich is the largest bird. Bird is quite varied; most can fly, some can run very
well, some swim, and some do combinations of these. They all have feathers. All birds hatch
from eggs, and most birds build nests for the eggs. Some birds deposit their eggs in other birds’
nests (like the cowbird). Bird bones are mostly hollow, thus very light (yet strong). Flightless
birds include kiwi, ostrich and emu. The only bird that can fly backwards is the humming bird.
Class Mammalia: This includes more than 4,000 species of homoiothermic tetrapods, with
epidermal hair and female mammary glands for suckling the young. All give birth to young,
although a very few, primitive species reproduce by laying eggs.
Characteristics
They possess mammary glands – produce milk to feed young
They have sweat glands – only mammals perspire (in order to maintain body temperature)
They are warm-blooded/homoiothermic (use food energy to maintain certain temperature)
They have two pairs of limbs
All have hair at some time in their life, and most have an outer hair (or fur) covering, though
some are practically hairless (whale, elephant).
Breathing is by lungs, diaphragm assists with this (mammals are the only animal that have
diaphragm)
They have well developed brains.
As with most animal groups, there are more herbivores (plant-eaters) than there are carnivores
(meat-eaters).
Types of Mammals
Monotremes: These are egg-laying mammals. Monotremes are very rare and unusual!
Marsupials: They give birth to their young in a very immature state; most female marsupials
have pouches. Their babies live in the pouches until they are big enough to survive on their own.
Placental Mammals: Their young are born at a more advanced stage than the young of other
mammals. Before birth, the young are fed through a placenta. Most mammals are placental
mammals.
Vertebrate Skeletal System
Fig I: The Human Skeleton
The skeletal system consists of the bones, ligaments and cartilages joined together to form a
strong, flexible framework for the body. The vertebrate skeleton is divided into 2 namely, axial
skeleton and appendicular skeleton. It is composed of 206 bones (Axial skeleton: longitudinal
axis, 80 bones; Appendicular skeleton: limbs, 126 bones).
Axial Skeleton
The axial skeleton consists of the skull, vertebral column, and rib cage and the sternum.
The Skull
The skull encloses and protects the brain and its related structures. It has a number of individual
bones tightly fitted together by immovable joints. At birth many of these joints are not
completely sutured together as bone, leading to a number of "soft spots" or fontanels, which do
not completely join until the age of 14-18 months. The cranium is made up of 8 bones and facial
skeleton of 14 bones. With the exception of the mandible, all are immovable and joined by
sutures.
Vertebral Column
The vertebral column supports the head and encloses the spinal cord. There are 33 bones or
vertebrae in the spine, i.e., 7 cervical vertebrae in the neck, 12 thoracic vertebrae in the upper
back corresponding to each pair of ribs, 5 lumbar vertebrae in the lower back, 5 sacral vertebrae
are fused together to form 1 bone called the sacrum and 4 coccygeal vertebrae that are also fused
to form the coccyx or tailbone. We refer to the vertebrae by their names and numbers so that the
cervical vertebrae are C1, C2, C3………..C7 where “C” stands for “cervical” and the number is
the position of the vertebrae counting down from the head. Therefore the thoracic vertebrae
become T1 –T12. The lumbar are L1 – L5, and the sacrum and coccyx do not have numbers and
each is thought of as one bone. Its functions include:
1) Support weight – transmits weight to pelvis and lower limbs
2) Houses and protects spinal cord - spinal nerves leave cord between vertebrae
3) Permits movements 4) Provides for muscle attachments - muscles of back; also muscles of head, neck, upper
extremity, thorax
The Rib
This is a curved, slightly twisted strip of bone, making up part of the skeleton of the thorax. In
man there are 12 pairs of ribs. The head of each rib fits into one of the 12 thoracic vertebrae of
the backbone. The first seven pairs are termed true ribs, connected directly to the sternum by
their costal cartilages. The next 3 pairs are called false ribs, because each is connected to the rib
above it, while the last 2 pairs are called floating ribs because they are not connected in front to
the sternum. The rib (ribcage) houses and protects the heart and lungs.
Sternum
This is also called the breastbone. It is usually 15- 20 cm long, extending from the base of the
neck to just below the diaphragm and forming the front part of the skeleton of the thorax.
The Appendicular Skeleton
The appendicular skeleton consists of the limbs and their girdles. The upper limbs are attached to
the pectoral girdle while the lower limbs are attached to the pelvic girdle.
The Pectoral Girdle
The Pectoral Girdle is composed of two scapulae and two clavicles, one on each side of the body
in order to form two shoulders. The purpose of the pectoral girdle is to provide a point where the
upper extremity, the arms, attach to the axial skeleton, and it also provides attachment points for
many muscles that move these limbs. The clavicle helps to hold the scapula (and arm) in
position, but also allows the scapula to move, resulting in increased mobility of the shoulder and
arm. The scapula is a triangular-shaped bone that is mobile, serves as an attachment site for the
arm bone (humerus) to the trunk, muscles moving the arm and muscles holding or moving the
scapula. The Acromion process is the part of the scapula joining the clavicle. The Glenoid
cavity (fossa) is the part of the scapula joining to the arm bone (humerus). This ball-and-socket
joint allows a wide range of arm movements.
The Pelvic Girdle
The pelvic girdle consists of the right and left coxae (hip bones) and the sacrum. Its functions
include:
1.It supports the weight of the head, upper limbs and trunk.
2. The hip joint between the pelvic girdle and lower limbs allows movement of the lower
limbs.
3. Muscles that move the lower limbs, trunk, and arms attach to the pelvic girdle.
4. The pelvic girdle supports and protects abdominopelvic organs.
The pelvis consists of the two coxae, the sacrum and the coccyx. Coxa (pl. coxae) consists of the
fused ilium, ischium, and pubis bones. The sacroiliac joint connects the sacrum and ilium. The
pubic symphysis (symphysis pubis) is the anterior connection of the two coxae (at the pubis
bones).
The Hip joint is the connection between the head of the femur and the acetabulum of the coxa; it
transfers weight from the coxa to the femur while allowing a wide range of movement. The
acetabulum is the depression in the coxa that holds the head of the femur, but allows it to move.
The obturator foramen decreases the weight of the
coxa and is mostly closed by a connective tissue membrane. The superior entrance to the pelvic
girdle is the pelvic inlet and the inferior exit is the pelvic outlet. It is a passageway for the fetus
during delivery.
Female vs. Male Pelvis
The female pelvis has larger pelvic inlet and outlet, the pelvic inlet is oval, and subpubic angle is
greater than 90 degrees. Contrariwise, the male pelvis has a smaller pelvic inlet and outlet, the
pelvic inlet is heart-shaped, and subpubic angle is less than 90 degrees.
Appendages
Human limbs are built on a pentadactyl pattern, i.e., they end in five digits. The biggest bone in
the fore limb is the humerus. The head of the humerus fits into the glenoid cavity and articulates
interiorly with the scapula and posteriorly with the other smaller bones, i.e, the ulna and the
radius, preceded by the metacarpals and the phalanges (fingers) in that order.
A similar arrangement is seen in the hind limb. The biggest bone in the hind limb is the femur.
Following the femur posteriorly are 2 smaller bones called tibia and fibula, which are followed
by the tarsals (ankle bones), which are then preceded by the metatarsals and the phalanges (the
toes).
The Joints
A joint is a place where two or more bones meet. Most joints are movable. The opposing
surfaces of the two bones are lined with cartilaginous, fibrous or soft (synovial) tissue, this
permits the joints to move smoothly without damaging the bones. There are 3 main classes of
joints, namely: diarthrosis (freely moveable) examples include, gliding joint, hinge joint
(ginglymus) and pivot joint (trochoid joint) and ball-and-socket joint; amphiarthrosis (slightly
moveable), e.g., the joints between the ribs and the sternum and synarthrosis (immovable) e.g.,
fixed joint.
• Hinge Joints - A hinge joint can only move in one direction – up or down. The knee and
elbow are hinge joints
• Pivot Joints - The pivot joint allows rotation. Your wrist and the attachment of the head is
called the spinal chord are pivot joints. This is why you can turn your head.
• Ball-and-Socket Joints - These joints allow you to turn your arms and legs in a complete
circle. There is a ball or knob at the end of one bone. At the end of another is a hole or socket. A
joint is formed where the ball fits into the socket. Shoulder joints and hip joints are ball-andsocket joints.
• Gliding Joints – A gliding joint allows two bones to move one over another. These occur
between the vertebrae in the backbone and also in the carpus and tarsus. The amount of
movement is small.
• Fixed joints – These are joints where the bones are tightly joined together and cannot move.
The skull bones are fixed joints.
Fig II: Joints of the Human Skeleton
Muscles
A muscle is a special kind of tissue that brings about movement. The kind of movement that
muscles carry out depends on where the muscles are located. Bones and joints will be useless
without muscles. Moving, breathing and swallowing are all functions of the muscles. When
muscles contract, they pull the body parts to which they are attached. This action usually causes
some movement. Sometimes, muscular contraction resists motion, as when they help the body
parts in postural positions. Muscles are also responsible for the movement of the body fluids
such as blood and urine, and they function in heat production that helps maintain body
temperature. There are 600 muscles in the human body. Most muscles are attached to bones and
extended from one bone to another. Each end of the muscles is attached to a bone by a strong
connective tissue, a tendon.
Bones of the muscles move only when the muscles attached to them pull. Muscles do not push;
they only pull. This is accounts for the fact why most muscles come in pairs. The muscles of the
arm are good examples of how muscles work as partners. The two muscles of the arms are called
biceps and triceps. The biceps is a flexor muscle. It produces a flexing or bending action by
pulling the lower arm bone at the joint towards the body. The triceps is an extensor muscle. It
produces an extending or straightening action by returning the bone to the relaxed position. Not
all muscles are connected to the bones. For example, heart muscles work by squeezing. Some
muscles are connected to the skin. This is how you are able to smile by moving 14 muscles. It
takes 75 muscles to frown.
Fig III: Muscles of the Upper Arms
Types of Muscles
There are three kinds of muscles:
(1) Skeletal muscle, also known as striated or voluntary muscles. It makes up about half
of an individual’s body weight.
(2) Smooth or involuntary muscles, which forms the muscle layers of the digestive
tract and other internal organs; and
(3) cardiac or heart muscles.
The skeletal muscle consists of a mass of muscle fibers grouped together and are attached to
bones. This muscle has a striation when seen under a microscope. Striped or striated muscles are
used to make fast, precise movement. The movement of striped muscles of striped muscles can
be controlled. A muscle that can be controlled is called a voluntary muscle.
Fig IV: A Skeletal Muscle
The smooth muscles are made of long, thin cells that form layers. Their movements cannot be
controlled. The smooth muscle is a type of involuntary muscle. Smooth muscles control many of
the internal organs of the body. Examples of smooth muscles are the muscles of the stomach,
arteries and uterus.
Fig V: A Smooth Muscle
Cardiac muscle is found only in the heart. It works like a smooth involuntary muscle. The
cardiac muscle looks like bundles of fibers twisted into a rope. Fibers from one bundle branch
into the next bundle. Thus, the bundles of cardiac muscles are all connected and can work
together to keep the heart pumping.
Fig VI: A Cardiac Muscle.
References
Czura, A.W. (2004). Skeletal System: Axial and Appendicular Skeleton Lecture Materials.
SCCC BIO130 Chapters Lecture Notes, pp 7- 8.
Daniel, L. (1994). Life science. Westerville, OH: Merill Publishing Co., Mcmillan/McGrawHill.
Grabowski, T. (2003). Principles of Anatomy and Physiology. N.Y.: John Wiley and Sons,
Inc.
Holo, W. (1984). Human Anatomy and Physiology. 3rd ed. Iowa: W. C. Brown
Publishers. Dubuque.
Hopson, J.L. and Wessells, N.K. (1990). Essentials of Biology. USA: McGraw-Hill Publishing
Company.
Kaskel, A., Hummer, P.J. & Daniel, L. (1981). Biology on Everyday Experience. USA:
Merill Publishing Co.
Marieb, E. N. (2004). Human Anatomy & Physiology 6th ed. San Francisco: Pearson Benjamin
Cummings,
Mariele, E.N. (1998). Essentials of Human Anatomy and Physiology. 3rd ed. New York,
USA: Addison-Wesley Longman, Inc.
Martini, F. H. (2004). Fundamentals of Anatomy & Physiology 6th ed. San Francisco: Pearson
Benjamin Cummings,
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Reproduction
Reproduction is the process by which a living organism is able to produce more of its own kind.
The continuity of life on earth, from its origin to the present day, has been possible only because
of reproduction. Living organisms reproduce in two ways—asexual and sexual reproduction.
In consequence, the scope of this work encompasses the modes of reproduction in plants and
animals especially humans, population growth and its control and sexually transmitted diseases.
Asexual Reproduction: This is the reproduction which requires only one parent. Simpler forms
of life such as bacteria, amoeba and yeasts, reproduce asexually. In all types of asexual
reproduction the offspring are exactly like the parent because they have the same DNA as the
parent. Asexual reproduction involves the production of an offspring from body parts other than
reproductive organs. It is a common process of reproduction in lower plants and animals.
Basic features of asexual reproduction
i) It involves only one organism i.e. different sexes are not involved.
ii) New individuals produced are genetically identical to the single parent.
iii) It is a fast mode of multiplication.
Sexual reproduction
Sexual reproduction is the formation of a new individual following the union of two gametes. In
humans and the majority of other eukaryotes, plants as well as animals, the two gametes differ in
structure and are contributed by different parents
Basic features of sexual reproduction
i) It is the production of offspring by the fusion of egg and sperm, which are the sex cells
or gametes.
ii) Upon fertilization, the male and female gametes unite to form a zygote, which develops
into a mature organism.
Parthenogenesis
This is a type of reproduction in which an organism develops from an unfertilized ovum. It is
common in plants and occurs in some lower animals e.g., sap-sucking insects such as aphids.
Female aphids are able to produce young ones without having their eggs fertilized by a male.
This permits them to increase in numbers very rapidly when environmental conditions are
beneficial. Moreover, few animals that are capable of parthenogenesis also have a sexual stage in
their life cycles, which produces genetic variety in their young.
Asexual Reproduction in Organisms
1. Fission
Fission is of two types: Binary fission and multiple fission.
Binary fission: In binary fission, two individuals are formed from a single parent. This type of
reproduction is found in organisms like bacteria, yeast and Amoeba
FIG1: Binary Fission in Amoeba
Multiple fission: In multiple fission, many individuals are formed from a single parent. This
type of reproduction by multiple fission occurs during unfavourable conditions. In this type of
reproduction, the unicellular organism develops a protective covering called cyst over the cell.
The nucleus of the cell divides repeatedly producing many nuclei. Many daughter cells are
produced within the cyst. The cyst breaks and small offsprings are liberated. This type of
reproduction is seen in many algae and in some protozoans, such as the malaria parasite
(Plasmodium).
Fig 2: Multiple Fission in Plasmodium
Budding: In this type of reproduction, a bulb-like projection or outgrowth arises from the parent
body known as bud, which detaches and forms a new organism. For example, Hydra reproduces
by budding (Fig.3). A small protruberance arises from one side of its body, which grows,
develops tentacles and gets detached to lead an independent life. Yeasts also reproduce by this
method.
Fig 3: Budding in Hydra
Regeneration or Fragmentation
In this type of reproduction, the body of an individual breaks up into two or more parts and each
part develops into a complete individual. Example: Planaria.
4. Spore formation
In lower forms of life like the alga, Chlamydomonas and even fungi the protoplast of the cell
divides to form 4–8 spores. These being motile are termed as zoospores. When spores are
released in the surrounding medium they develop into new plants. In fungi motile and non motile
spores are formed which are designed for aerial dispersion.
REPRODUCTION IN PLANTS
Like animals, plants also reproduce both asexually and sexually. Asexual reproduction in plants
is either by vegetative propagation or vegetative reproduction of plant parts. Sexual reproduction
is by fusion of male and female gametes and it occurs in flowering plants.
Vegetative propagation or vegetative reproduction in plants
Vegetative reproduction (or vegetative propagation) is a form of asexual reproduction in plants
in which a bud grows and develops into a new plant. In this type of reproduction, any vegetative
part of the plant body like leaf, stem or root develops into a complete new plant. Vegetative
reproduction can take place by two methods—natural and artificial.
Vegetative reproduction by natural methods
This type of vegetative reproduction can involve roots, stem or leaves. Some common modes of
vegetative reproduction are given below:
i) By roots
The roots of sweet potato produce new plants when planted in the soil.
ii) By stem
When the stem of some plants are cut off, they will form roots and grow into new plants when
placed in the soil e.g. geranium. Underground horizontal stems, or rhizomes, are also important
reproductive structures, particularly in grasses and sedges. Rhizomes invade areas near the parent
plant, and each node can give rise to a new flowering shoot.
The noxious character of many weeds results from this type of growth pattern. Corms, bulbs, and
tubers are also stems specialized for storage and reproduction. In many plants the stem develops
buds on it.
The part of the stem that bears buds serves as an organ for vegetative multiplication, e.g. the
modified parts of stem, such as runners of grass, suckers of mint and Chrysanthemum, bulbs of
onion and tulip, rhizomes of ginger, corms of Colocasia (cocyam) and tubers of potato, etc.
iii) By leaves
In some plants, e.g. in Bryophyllum and Bigonia, adventitious buds are developed in the margins
of their leaves. When the leaf falls on moist soil, these buds develop into small plantlets, which
can be separated and grown into independent plants
iv) By Runners.
Some plants reproduce by means of runners— long, slender stems that grow along the surface of
the soil. In the cultivated strawberry, for example, leaves, flowers, and roots are produced at
every other node on the runner. Just beyond each second node, the tip of the runner turns up and
becomes thickened. This thickened portion first produces adventitious roots and then a new shoot
that continues the runner.
v) By Suckers
The roots of some plants—for example, cherry, apple, raspberry, and blackberry—produce
“suckers,” or sprouts, which give rise to new plants. Commercial varieties of banana do not
produce seeds and are propagated by suckers that develop from buds on underground stems.
B) Vegetative propagation by artificial methods
Some plants can be propagated artificially. The methods of artificial propagation include
grafting, layering, cutting and tissue culture.
i) Grafting: It is the method of obtaining a superior quality plant from two different plants,
taking the root system of one plant and the shoot system of another plant. The plant whose root
system is taken is called stock. The plant whose shoot system is taken is called scion. The ends to
be grafted, of the stock and the scion, are cut obliquely and placed face to face and are bound
firmly with tape (Fig. 4). The stock supplies all the desired nutrients to the scion. This technique
has been used in raising superior quality plants of mango, apples, roses, rubber and citrus.
Fig 4: Vegetative propagation by grafting
ii) Cutting: In some plants like rose and sugarcane, this method is used quite frequently. Stem
cuttings with nodes and internodes are placed in moist soil which give rise to adventitious roots,
and grow into new plants.
iii) Layering: Layering is the development of roots on a stem while it is still attached to the
parent plant. The stem or the branch that develops adventitious roots while still attached to the
parent plant is called a layer. It is a means of reproduction in black raspberries, jasmine
(Jasminum), blackberries, etc.
iv) Tissue culture: This is a modern technique of vegetative propagation. In this technique, a
small piece of tissue is cut from a plant and is transferred to a container with nutrient medium
under aseptic conditions. The tissue utilizes nutrients from the medium, divides and re-divides,
and forms a callus. Small portions of this callus are transferred to another medium which induces
differentiation and plantlets are produced. These plantlets are transplanted in soil to form an
adult plant. Orchids, Chrysanthemum, Asparagus and many other plants are now being grown by
using plant tissue culture technique.
Sexual reproduction in plants
In flowering plants, flower is the reproductive part of a plant. Most flowers have both male and
female reproductive organs. A typical flower has four whorls, namely: calyx (sepals), corolla
(petals), androecium (stamens) and gynoecium (carpels) (Fig. 5). The androecium and
gynoecium are directly concerned with sexual reproduction. The androecium is the male part of
the flower. It consists of stamens. Each stamen has anther and a filament. Each anther
possesses many pollen grains, which are the male gametes in pollen sacs. Gynoecium is the
female reproductive part of a flower. The female part contained in this whorl is called pistil.
Each pistil consists of three parts—an upper flat stigma, a medial, long, cylindrical style, and a
lower, swollen ovary.
• The stigma receives pollen grains during pollination.
• The style bears the stigma at a suitable position to receive the pollen grains.
• The ovary contains ovules that are found attached to the placenta. Ovules are the structures in
which embryo sacs develop, and mature into seeds after fertilization. The arrangement of ovules
in the ovary is called placentation.
Fig 5: Parts of a Flower.
Pollination
Pollination is the process of transfer of pollen grains from the anther to the stigma of a flower. It
is of two types:
i) Self-pollination: If the pollen grains from the anther of a flower are transferred to the stigma
of the same flower, it is termed as self-pollination or autogamy (auto: self; gamy: marriage) e.g.
pea and china rose.
ii) Cross pollination: If the pollen grains from anther of one plant reach the stigma of a flower
on another plant of the same species, then this is called as cross pollination or allogamy (allos:
other; gamy: marriage). Cross pollination has the advantage of increasing the chances of
variations.
Fertilization
• After pollination, the pollen grains germinate on the stigma to produce a pollen tube.
• This tube grows down through the style and finally reaches the ovule.
• The ovule contains the egg cell inside the embryo sac.
• The tip of the pollen tube ruptures in the ovule and discharges two male gametes into it.
• One of the male gametes fuses with the egg to form the zygote. This fusion is called
fertilization.
• The other male gamete fuses with the diploid secondary nucleus and forms the endosperm
nucleus.
• The zygote that is formed as a result of fertilization divides several times and gives rise to an
embryo. The endosperm nucleus grows to form the endosperm of the seed. Following
fertilization, the sepals, petals, style and stigma degenerate and usually fall off. The ovary wall
ripens and forms the pericarp of the fruit. Each ovule develops into a seed. The seed contains a
potential plant or embryo. The whole ovary after fertilization changes into a fruit
Fig 6: Zygote Formation
REPRODUCTION IN HUMANS
Humans reproduce sexually. Reproductive organs in humans are described below.
Male reproductive system
The male reproductive system in humans consists of the following organs—a pair of testes, a
pair of epididymis, a pair of vasa deferentia, an ejaculatory duct, a urethra, penis and accessory
glands (Fig. 7). The testes produce, sperms, the male gametes. The process of formation of
sperms in the testes of an organism is called spermatogenesis. Each testis contains certain
coiled tubes called seminiferous tubules that are actually responsible for the production of
sperms. These sperms are released from the testes and stored in the epididymis until mating. At
the time of mating, the sperms are passed from the epididymis through the vas deferens to the
ejaculatory duct. The ejaculatory duct opens into the urethra. In human males, the urethra is a
common passage for sperms and urine. The urethra passes through an organ called penis, which
is the copulatory organ (organ for transfer of sperms during mating or copulation) in humans.
During their passage from the epididymis to the urethra, the sperms are mixed with certain
secretions from the accessory glands. The sperms along with the secretions form the semen.
During copulation, semen is discharged. The process of discharging of semen is called
ejaculation. In one ejaculation about 200,000,000 (2 × 108) sperms are discharged.
Fig 7: The Male Reproductive System in Humans
Female reproductive system
The female reproductive system consists of a pair of ovaries, a pair of oviducts (or fallopian
tubes), uterus and vagina (Fig. 8). A pair of ovaries lies in the lower part of the abdominal cavity,
one on each side of the body. Ovaries produce ova and secrete female sex hormones, oestrogen
and progesterone. The process of formation of egg in the ovary is known as
oogenesis. There is a pair of oviducts or fallopian tubes in the human female reproductive
system. One end of each oviduct is funnel-shapped. It collects the eggs released by the ovary.
Both fallopian tubes open into the uterus. The uterus is a pear-shaped, muscular, thick-walled
organ.
The lower end of the uterus opens into the vagina that opens to the outside by a genital opening.
Vagina is the organ where the penis is inserted during coitus for the discharge of semen. It serves
as the birth canal during childbirth. In a human female, the urethra and the genital duct have
separate openings.
Fig 8: The Female Reproductive System in Humans
Puberty
The reproductive organs in human beings become functional at an age of 13–14 years in males
and 12–13 years in case of females. This age is known as puberty. During sexual maturity,
hormonal changes take place in males and females, and under the influence of some hormones
secondary sexual characteristics are developed.
• Development of secondary sexual characteristics in males include: deepening of voice,
widening of shoulders, appearance of beard and moustaches, and growth of axillary and pubic
hair.
• Development of secondary sexual characteristics in females include: growth of axillary and
pubic hair, widening of pelvis and hip, enlargement of breasts and initiation of the menstrual
cycle.
SEXUAL CYCLE IN HUMAN FEMALES (MENSTRUAL CYCLE)
The period of life during which a female has the capacity to produce young ones is called the
fertility period. In human females, it extends from about 12–13 years (puberty) up to 45–50
years (menopause). Between puberty and menopause, the female reproductive system passes
through a regular monthly sequence of events called the menstrual cycle. The events of
menstrual cycle are given here.
• During each menstrual cycle, an ovum matures and is released once every 28 days.
• The menstrual cycle starts with menstrual flow, during which cellular lining of the uterus is
shed off along with blood flow. This process continues for 3–5 days.
• From the 5th up to the 13th day of the onset of menstrual cycle, growth and maturation of the
graafian follicle takes place. It consists of an ovum and a mass of cells surrounding it.
• The graafian follicle produces the female hormone, oestrogen.
• The cells lining the uterus grow rapidly and develop a dense network of blood vessels.
• The release of the ovum from the ovary is called ovulation. Ovulation takes place 12–13 days
after the onset of menstruation. The graafian follicle ruptures to release the ovum.
• The cells of the ruptured follicle form the corpus luteum, which secretes the hormone,
progesterone.
• The ovum reaches the uterus via the fallopian tube on the 13th or 14th day and remains there up
to the 16th day (for 48–72 hours).
• If the ovum does not receive any sperm during this period it starts degenerating. At the end of
the 28th day this ovum is rejected along with the uterine lining.
• This marks the start of a slow disintegration of the thickened lining of the uterus and the next
menstrual cycle.
However, if the ovum receives sperm, it is in the fallopian tube, the two unite to form a zygote.
This is called fertilization. Fertilization occurs in the fallopian tube. The zygote immediately
begins to divide and forms a mass of cells called morula, which passes down to the uterus and
fixes itself to the wall of the uterus (known as implantation). Menstruation does not occur and
the female is said to be pregnant. The developing young one or the foetus is attached to the
uterus by a tissue called placenta. Placenta supplies oxygen and nourishment from the maternal
blood to the foetus. It also transports carbon dioxide and excretory waste from the foetal blood to
the maternal blood. Placenta also produces two hormones— progesterone and oestrogen. Under
the influence of these hormones neither ovulation nor menstruation takes place till pregnancy
continues. The Umbilical cord is a tough structure that serves as the blood vascular connection
between the foetus and uterine wall. From the first few weeks of development, the embryo is
enclosed in a sac called amnion, which is filled with amniotic fluid. This fluid acts as a shockabsorber and helps to protect the embryo from damage.
Test tube babies
In some women, the fallopian tube gets blocked, which prevents the ova from being fertilized.
This problem can be overcome by the test tube baby technique. In this technique, one or more
mature ova are sucked from a woman’s ovaries using a special syringe. The sperms are taken
from her husband. These sperms and ova are kept together in a container for a few hours for
fertilization to take place. When a sperm fertilizes an ovum a zygote is formed, which divides
repeatedly to form an embryo.
This embryo is them inserted into the woman’s uterus where it gets implanted and develops into
a baby. Fertilization of an egg by a sperm outside the body of the female is called in-vitro
fertilization (IVF).
Twins
In every reproductive cycle, usually, an ovary releases only one ovum. But, sometimes more than
one egg may be released and fertilized by more than one sperm or an ovum may divide into two
or more cells after fertilization which separate and develop as different individuals. This is how
twins, triplets and quadruplets etc. are produced.
a) Identical twins
When a fertilized egg divides into two independent sets of cells, both of which continue to
divide, two identical embryos are produced from the same egg. The twins thus produced are
identical twins.
b) Non-identical or fraternal twins
When two eggs are produced at the same time and a different sperm fertilizes each egg, nonidentical or fraternal twins are produced.
POPULATION CONTROL
Reproduction serves to replace the older generation. It is also a phenomenon that leads to
increase in the number of individuals of a species to ensure that at least some will survive in the
struggle for existence.
In humans, due to a variety of reasons more and more children have been surviving. This has
currently led to what we call the population explosion. The increase in population has created
many problems like problems of food, shelter, clothing, etc. So it has become very essential to
limit the human population. There are various ways to prevent fertilization and hence to check
the increase in population. Let us study about few such methods.
Education
Imparting education to the people about various ways of fertility control is the most effective
method of population control. Education helps to make people aware of the advantages of a
small family and the disadvantages of a large family.
Preventive Methods
These methods prevent the fusion of the egg and the sperm. Two important preventive methods
are discussed below.
(i) Vasectomy
This is a method of sterilization in males. In this method, each vas deferens is cut and tied at both
cut ends by a thread (ligature).
(ii) Tubectomy
This is a method of sterilization in females. In this method, the fallopian tube is cut and the two
ends are tied to prevent passage of ova down the fallopian tubes. Thus, in this method, the eggs
continue to be released but do not reach the fallopian tube and no fertilization takes place.
Contraceptive Methods
These methods involve prevention of fertilization and conception. The main methods of
contraception are given below:
i) Natural Methods of Contraception
• Rhythm method of contraception: In this method copulation is avoided for those days when
the ovum is available for fertilisation. Thus, fertilization can be avoided.
• Coitus interruptus: In this method, the penis is withdrawn from the vagina prior to
ejaculation.
ii) Mechanical Methods of Contraception
In this approach, various mechanical methods are used to prevent the passage of semen to the
fallopian tube or to prevent implantation.
• Condoms: It is a thin rubber tube worn over the penis before sexual intercourse. The ejaculate
gets collected in this tube and is not discharged into the vagina.
• Diaphragm: It is fitted over the cervix in a woman’s body by a doctor to prevent the entrance
of sperms into the cervical canal.
• Intra uterine device (IUD): IUD or loop is made of plastic or stainless steel. It is inserted in
the uterus, which releases certain secretions that prevent the implantation of embryo in the
uterine wall.
iii) Chemical Methods of Contraception
•Spermicides: Strong spermicidal (sperm-killing) creams, jellies, etc. are applied in the vagina
before copulation, which kill the sperms and prevent fertilization.
•Oral Contraceptives or Pills: The oral contraceptives or pills are taken daily, which prevent
ovulation in females. These pills prevent ovulation but allow monthly shedding of the uterine
lining through menstrual bleeding.
iv) By Medical Termination of Pregnancy (MTP)
MTP methods are also known as contraceptive methods. They are applied if conception has
taken place. Abortion and aspiration are two corrective methods in which pregnancy can be
terminated by either mechanical method or by using hormones.
SEXUALLY TRANSMITTED DISEASES
Diseases which spread through sexual contact are called sexually transmitted diseases (STD).
Sometimes microorganisms may infect areas around reproductive parts. During the act of sexual
intercourse, these microorganisms may be easily transmitted from one person to another. There
are so many STIs and most are of viral origin, moreover, a good number of them are incurable.
Three important sexually transmitted diseases are:
i) Syphilis
ii) Gonorrhoea
iii) Acquired Immuno Deficiency Syndrome (AIDS)
Syphilis and Gonorrhoea
Causative organism
Both these diseases are caused by bacteria. Syphilis: Treponema pallidum and Gonorrhoea:
Neisseria gonorrhoeae
Modes of spread
Sexual contact with the infected person
Incubation period
Symptoms of gonorrhoea disease occur in about 2-5 days and that of syphilis are seen in 10-90
days.
Symptoms
The common symptoms of these bacterial diseases are given below:
i) Fever, followed by sores appearing on the skin, in the throat and urinogenital areas
especially vagina or penis, anus, rectum and mouth.
ii) Break out of rashes on hands, feet and palms.
iii) White patches in the mouth.
iv) Acne-like warts in the groin area.
v) Hairfall occurs in patches from infected areas.
Prevention and cure
Abstinence is the best method to avoid them, but any of the underlisted can be helpful:
i) Having sexual intimacy with only one person
ii) Avoiding prostitution and homosexuality
iii) Taking appropriate medical treatment
Acquired Immuno Deficiency Syndrome (AIDS)
AIDS is caused by the Human immunodeficiency virus (HIV). Upon entry into the body, the
virus lives and grows in the body fluids and blood cells of the infected person.
Mode of transmission
HIV may be transmitted in the following ways:
(i) Sexual contact with the affected person i.e. through semen or vaginal fluid.
(ii) Exposure to infected blood and blood products. This may occur by using the same sharp
object, e.g., needle and syringe, clippers etc with an infected person, and by use of infected
blood during blood transfusion.
(iii) Organ transplant from an affected person to a healthy person.
(iv) During pregnancy, from an infected mother’s blood to her baby’s blood. Although
Prevention of Mother to Child Transmission Therapy (PMTCT), is now possible.
Incubation period
The average incubation period of HIV virus is about 28 months (range 15–57 months).
Symptoms
i) The person feels fatigued or tired, suffers from loss of weight and fever, and sweats profusely.
ii) Persistent dry cough, oral rash and shortness of breath may be observed.
iii) Headache, visual disturbance and vomiting are also witnessed.
iv) Gastro-intestinal problems like mild diarrhoea may occur.
v) Skin blotches, eczema, fungal infection and sometimes skin cancer may be observed.
vi) Nervous system may be affected, the brain may be badly damaged leading to a loss of
memory, and ability to speak and to think.
vii) A completely infected AIDS patient may die within three years of infection.
Prevention and control
Although there is no cure for AIDS, the HIV infection can be prevented by
i) Avoiding multiple sex partners
ii) Using a condom or other method of contraception
iii) Avoiding prostitution and homosexuality
iv) Screening of blood before transfusion
v) Treatment of all blood and other products used in transfusion to destroy the HIV
vi) Avoiding sharing of injection needles
vii) Avoiding pregnancy if the mother is HIV positive
viii) Educating people.
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