image

OLevel

Biology - 5090

Development of Organisms and Continuity of Life

Paper-2

Development of Organisms and Continuity of Life

Asexual Reproduction


It is a process resulting in the production of genetically identical offspring from one parent, for example, reproduction of potatoes using stem tubers.

Advantages:

It is more energy-efficient as it requires less energy compared to sexual reproduction.

Only one parent is required.

A rapid increase in population.

The good characteristics will always be passed on.

It takes less time than sexual reproduction.

It is more environmentally friendly.

Disadvantages:

Reduced genetic diversity as they are identical offspring.

Negative traits will be passed on, like any disease.

There are fewer chances of survival in varying environments.

Increased competition for local resources.

One disease might affect the whole population.

Results in less evolution.

Example:

Potatoes reproduce using stem tubers. Some of the plant’s stems grow naturally, above ground, producing leaves that photosynthesize. Other stems grow under the soil. Swellings called ubers form on them. Sucrose is transported from the leaves into these underground stem tubers, through the phloem, where it is converted into starch and then stored. The tubers grow larger. Each plant can produce many stem tubers.

Sexual Reproduction


It is the process involving the bonding of the nuclei of sex cells to form a zygote and the production of offspring genetically varies from each other. The nuclei of gametes are haploid, and the nuclei of the zygote are diploid.

Advantages:

Introduces variation, thus increasing biodiversity.

Species can adapt to varying environments.

The disease is less likely to affect individuals.

Chances of survival of the species are increased.

Allows evolution.

It reduces competition.

Disadvantages:

Time-consuming and less energy efficient.

It requires two parents (partners).

There is no rapid increase in population.

Many eggs are wasted.

Offspring may not be as well adapted to the environment on self-pollination.

Differences Between Sexual and

Asexual Reproduction

Sexual Reproduction

Asexual Reproduction

Requires sex cells
Does not require sex cells
Forms genetically non-identical offsprings
Forms genetically identical offsprings
Slower than asexual reproduction
Quick process
Requires two organisms of same species
Requires only one parent
Cells divide by meiosis
Cells divide by mitosis

Mitosis

A type of nuclear division, occurring in all somatic cells, where 2n number of chromosomes remain 2n after cell division, producing genetically identical gametes.

Meiosis

A type of nuclear division, occurring in sex cells, where 2n number of chromosomes become n after cell division. They produce genetically non-identical cells.

Sexual Reproduction in Plants


Plants can reproduce sexually or asexually. The process of asexual reproduction in plants is also known as vegetative reproduction.

Figure (i) Structure of a Flower, Credit: Brainly.in

Structure

All flowers are present on a flower stalk, known as the pedicle. The swollen part on the base of a flower where four layers are present is a receptacle. 

  • 1st layer: Outermost green layer, known as sepal, protects the flower during bad conditions. It also protects the flower when it is in bud form.

  • 2nd layer: Petals are the most conspicuous part. They are usually scented, bright-colored and have a large surface area. They attract insects and provide a platform for insect landing. 

  • 3rd layer: This inner layer of the flower is known as the stamen. It is the male part, consisting of two parts. The long part is known as the filament, and the shorter one is anther. The anther is the site of meiosis. It produces pollen grain which has n number of chromosomes.

  • 4th layer: The inner layer is known as the carpel. It is further divided into three parts: stigma, style, and ovary. Ovules are formed in the ovary. Stigma is feathery, and a platform for the landing of pollen grains. Style is a stalk that holds the stigma in the best position to receive pollen grains.

Pollination

Transfer of pollen grains from the male reproductive part, anther, to the female reproductive part, stigma, is known as pollination.

A bi-sexual flower has both male and female parts, while unisexual flowers only have female parts. 

There are 2 agents: insects and wind.

When pollen matures, the anther bursts, releasing the pollen grains. The pollen either goes to the same flower or another flow of the same plant, or another flower of a different plant of the same species.

Self-Pollination

It is the transfer of pollen grains from the anther of a flower to the stigma of the same flower or different flower on the same plant. 

Advantages:

Ensures pollination.

Less wastage of pollen.

Less energy consumed.

No need for a pollinating agent.

Maintains the unique characteristics of the representatives.

Disadvantages:

Less variation thus does not help in evolution.

Less adaptability to varying environments.

Fewer chances of survival as prone to diseases and other environmental hazards.

Seeds/Crops are usually small.

Less vigorous crops.

Cross-Pollination

It is the transfer of pollen grains from the anther of a flower to the stigma of a flower on a different plant of the same species.

Advantages: 

More variation so aids evolutionary processes.

Higher adaptability to the environment.

Fewer chances of diseases, thus increased chances of survival.

Larger crops/seeds.

Crops are usually more vigorous.

Disadvantages: 

Maximum wastage of pollen.

Less energy-efficient.

More dependency on pollinating agents.

Unique characters do not get preserved.

Large amounts of pollen grains have to be produced to ensure pollination.

Difference Between Insect-Pollinated Flower

and Wind-Pollinated Flower


Insect-Pollinated Flower

Wind-Pollinated Flower

Non-feathering stigma
Feathery stigma to attach max. number of pollen grains
Stigma has a small surface area, making pollination harder
Stigma has a large surface area to make the pollination easier
Stamen is inside the flower, making it hidden and non-pendulous
Stamen is outside the flower
Bright, colorful scented flowers to attract insects
Small, dull, unscented flower
Nectaries present on the base of petals
Sepal absent
Less number of pollen, so less wastage
More number of pollen so more wastage, but higher chances of pollination
Sticky pollen grains to attach to insect’s body hair
Non-sticky pollen grains
Stiff and firmly-attached anthers present outside
Anthers are loosely present on the inside

Figure (ii) Insect-pollinated flower & Wind-pollinated flower, Credit: Slideplayer.com

Fertilization

It occurs when a pollen nucleus fuses with an ovule nucleus. 

Process

On the bursting of the anther, pollen will spread and fall on nearby stigma. If it has landed on the right kind of stigma, pollen grains begin to grow a tube. the pollen tube grows down through the style and the ovary, towards the ovule. It secretes enzymes to digest a track through the style. The ovule is surrounded by several layers of cells, called the integuments. At one end, a small hole is an integument, known as a micropyle. The pollen tube goes into the ovule, through the micropyle. The pollen nucleus travels along the pollen tube to the ovule, and it fuses with the nucleus of the ovule.

The haploid number of chromosomes become diploid, making a zygote. The zygote divides through mitosis, and it changed into a tiny mass of cells, known as an embryo. Embryo changes to mature seed. On putting it in soil, it will germinate and become a fruit.

Seed Germination

It is the sprouting of seeds over a period of time. The stored foods in the seed are starch (corn, wheat), lipids/oil (almonds, peanuts, pistachios), and proteins (beans/lentils).

There are several factors required for the process of germination. 

Water
Oxygen
Temperature

Water enters the seed, breaking the outer-coat of the seed, and it makes the cotyledon swollen. It acts as a solvent. It helps in the breakdown of large, complex molecules to smaller molecules, like starch to glucose, lipids to fatty acids and glycogen, and proteins to amino acids. The enzymes, lipase, amylase, and protease, present in seeds breakdown stored food substances into soluble end products. The seeds need to respire. Oxygen will burn with glucose, producing carbon dioxide, water and a large amount of energy. The energy is needed for the growth of cells, the formation of organelles, cell division, and cell enlargement. A seedling is a tiny shoot/root coming out of the seed. The seeds germinate at a reasonable temperature. A suitable temperature is also required for metabolism and enzymatic activity. Plumule and radicle are the two tissues of embryo, which develop into shoot and roots respectively.

Sexual Reproduction in Humans

Gonads are the sex organs (testes and ovaries) that produce the gametes (sperms and ova).

Male Reproductive System

Testes: produce testosterone, which is the male reproductive hormone, and the sperms, which are the male gametes. They are an outside body as the sperm production needs a temperature lower than the body temperature.

Scrotum: A loose mass of skin in which testes are present. They control the temperature to protect the sperms and guarantee to mature sperm before its release. 

Epididymis: temporarily store sperm.

Sperm ducts: carry sperm to the urethra. Both of the ducts open into the penis. 

Prostate glands: secrete fluids, which along with sperms, are known as semen. Semen provides nutrients, lubricates the track, makes the sperms more mobile and clears the passage for urination. 

Urethra: travels through the penis, carrying the semen and urine. A ring of muscle ensures that the semen and the urine do not mix together.

Penis: ejaculates sperm into the female genital, and it helps in urination.

Female Reproductive System

Ovaries: gonads that produce estrogen and progesterone, which are the female reproductive hormones, and the ova/egg. 

Oviducts: transport the egg. The ovaries are connected to the uterus through an oviduct. The oviducts have ciliated epithelium to help the egg move to the uterus. The eggs are immobile, so they move with the ciliated current.

Uterus: muscular bag with a thick, soft lining, known as the endometrium. It is where the fetus develops.

Cervix: a ring of muscle present at the lower end of the uterus. It dilates during childbirth and keeps the baby in place during pregnancy.

Vagina: It is the birth canal. During sexual intercourse, the penis goes into the vagina. The baby passes through the vagina during childbirth. It also serves as a conduit for menstrual flow. 

Gametes: Sperm and Ova


Adaptations


Egg cell:

They have a large cytoplasm that stores energy and provides nourishment to the zygote. 

Jelly-like coats present acting like a protective layer, and it contains receptors for the sperm to bind to.

It contains few mitochondria so it requires less energy.

No flagellum is present, so ova is swept along the oviduct by peristaltic contraction of oviduct muscles.

Sperm cell:

A smaller cytoplasm present enables faster movement.

The middle piece contains mitochondria, which produces ATP, for the movement of sperm.

Acrosome present, which is a vesicle containing digestive and hydrolytic enzymes to dissolve away through the jelly surrounding the egg cell.

The flagellum is a cytoplasmic extension present in the sperm which makes it mobile by producing swimming movements to enter the vagina.

Difference between Ovum and Sperm

Ovum

Sperm

100 microns, as they have a large cytoplasm
40-60 microns, as they have a small cytoplasm
spherical to reduce pressure by increasing surface area 
streamlined so less resistance in moving to ovum
immboile, and move due to cilia
mobile, due to the presence of flagellum
one egg released per month
millions of sperms released to ensure fertilization

Figure (iii)Ovum and Sperm, Credit: Scienceaid.net

Fertilization and Zygote Development


Fertilization is the fusion of the nuclei from sperm and that of an egg cell. It occurs in the oviduct, and it forms a zygote. 

Millions of sperm are released in semen in one ejaculation. Ejaculation is when the muscles in the walls of the tubes containing sperm contract rhythmically and push sperms out of the penis into the vagina. The wave of contraction begins in the testes, travelling to the penis by passing through the sperm ducts.

The semen is ejaculated in the vagina below the cervix. The sperms use sugars from the semen to release energy which swims through the cervix and uterus, reaching the egg. The eggs produce chemicals that attract sperm to it. Only one of the sperm can fertilize the egg. After fertilization, they become a single cell, called a zygote. As soon as the successful sperm enters the egg cell, the ova’s membrane becomes impenetrable, preventing any other sperm from entering it. The rest of the sperm die. 

The zygote keeps dividing further by mitosis and produces more cells until a tiny mass of cells is formed, known as the embryo. The embryo implants into the endometrium. It receives nutrients from the cytoplasm of the egg. The uterus has a thin, spongy lining in which the embryo sinks itself. This process is known as implantation. 

Pregnancy

After the implantation, the embryo starts further development. Some of its cells grow into embryonic villi which are fixed firmly in the endometrium. The uterus of the mother also grows sets of projections/villi. The two sets of villi grow closely together to form a structure, known as the placenta.

The placenta is the site where nutrients are exchanged between the embryonic blood and the maternal blood. It is the embryo’s life support system.

The embryo also starts to form a protective layer around the baby, known as the amnion. Amnion is a membrane that covers the embryo on its early formation. It is filled with the amniotic fluid which allows free movements of the fetus during later stages of pregnancy. It also protects it by diminishing the risk of any physical injury by acting as a shock absorber. It prevents the fetus from drying out and continuously provides nutrients to it.

After 3-4 months, the majority of the embryonic cells start dividing rapidly producing a fetus. The placenta has two sets of villi: the fetus’ set consisting of the fetus’ blood capillaries and the mother’s set that comprises the mother’s blood capillaries. The blood flows from the mother to the fetus’ blood vessels in the placenta through the umbilical arteries and veins. 

The placenta is joined to the fetus through the umbilical cord. There are veins and arteries present inside it. The capillaries filled with the fetus’s blood are present in the placenta. The fetus’ and the mother’s blood do not mix as they are separated by the wall of the placenta.

Oxygen and food materials in the mother’s blood diffuse across the placenta into the blood of the fetus. Then, they are carried along the umbilical cord to the fetus. Carbon dioxide and waste materials diffuse the other way into the mother’s blood.

The placenta also secretes estrogen and progesterone, which are essential to keep the uterus in good condition. They keep the uterine lining thick and stimulate milk-producing tissues in the mother.

Special Dietary Needs for Pregnant Women


A pregnant woman needs to take extra care of her health, both for herself and her baby’s benefit. She should ensure that her diet contains plenty of calcium for the formation of teeth and bones. If the mother does not take in enough calcium, the fetus starts absorbing it from her bones, making her weak. An intake of extra iron is necessary for growth. It produces hemoglobin in RBCs for the mother and fetus. A lot of blood is required to transport nutrients across the placenta. A small increase in carbohydrates is essential for the production of extra energy, which will help in the movement of her heavier body. Pregnant women should strictly avoid consumption of alcohol as it can cause serious harm to the fetus. 

Difference Between Breastfeeding

and Bottle-feeding

Breastfeeding

Bottle-feeding

Contains unique antibodies and colostrum which provides immunity against diseases
Contains high content of fats and sugar, which provide no immunity
Develops a close relationship between the mother and the baby
No bond formed between the baby and the mother
Cost-effective
Expensive
Low levels of bacteria and pathogens
Easy for bacteria to enter it
Baby gets milk at a suitable temperature
Right temperature is not achieved on boiling the powdered milk with water
Task cannot be shared with another parent
Father can feed, forming a bond between them

Menstrual Cycle


In women, a single egg is released into the oviduct each month. Before the release of this ova, the endometrium becomes thick and spongy to prepare itself for a fertilized egg. It becomes full of blood vessels, ready to supply the embryo with food and oxygen. The egg cell dies if fertilization does not take place. Therefore, the thick endometrium breaks down, and excretes through the vagina, along with the dead egg, over a period of 4-7 days. This results in blood loss and cramps due to the contraction of uterine and abdominal muscles. This is known as menstruation. 

Role of Hormones in the Menstrual Cycle 

The menstrual cycle is controlled by four hormones.

The pituitary gland, in the brain, secretes FSH (Follicle-stimulating hormone) and LH (Luteinizing hormone).

Ovaries secrete oestrogen and progesterone.

At the beginning of the cycle, LH and FSH are secreted. First, a follicle develops in an ovary. The development of the follicle is stimulated by the FSH. This developing follicle secretes estrogen whose concentration increases steadily in the body. Oestrogen builds up the endometrium, making it thick and spongy. Throughout this time, the FSH and LH are secreted to stimulate the follicle for the continuous release of estrogen. On the full development of the follicle, there is a surge in the LH production, which causes ovulation to take place.

Now, the empty follicle stops secreting estrogen. It becomes a corpus luteum. Corpus luteum starts secreting progesterone, which causes a decrease in the FSH and LH. Progesterone maintains the uterine lining, keeping it thick, spongy and well supplied with blood, in case the egg is fertilized. 

If the egg is fertilized, the corpus luteum does not degenerate so quickly. It carries on secreting progesterone until the embryo sinks into the endometrium, and the placenta develops. Then, the placenta secretes progesterone throughout pregnancy. It keeps the endometrium maintained, so the menstruation stops during pregnancy. 

If the egg is not fertilized, the corpus luteum disappears gradually. Progesterone will not be secreted anymore, and the endometrium breaks down, causing menstruation to occur. The cycle repeats. 

Methods of Birth Control


There are five methods of birth control:

1. Natural 
2. Chemical (spermicides)
3. Mechanical 
4. Hormonal 
5. Surgical

Natural Methods:

Abstinence: the couple avoids sexual intercourse completely, or ensure that they don’t have sex during the woman’s fertile period. This method works for women with a regular menstrual cycle.

Body temperature: Around the time of ovulation, body temperature rises slightly, so keep monitoring the body temperature.

Cervical mucus: Around the time of ovulation, the mucus produced in the vagina becomes thicker and slippery. 

Chemical Methods:

Spermicides: These are chemicals, in the form of foam, tablets or jelly, which kill the sperm.

Mechanical Methods:

Condoms: A sheath placed over the penis to trap the semen as it is released, preventing it from entering the vagina. It also prevents the exchange of infection.

Femidom: It acts like a sac that prevents semen’s entry into the vagina.

Diaphragm: a small, circular piece of rubber that fits over the vagina and covers the cervix, stopping sperms from entering into the uterus.

Hormonal Methods:

Intrauterine Device (IUD): device placed inside the uterus by the doctor. It contains hormones that prevent implantation. A plastic string tied to the end of it hangs down the cervix to the vagina.

Intrauterine System (IUS): It interferes with the ability of sperm to find and fertilize an egg, and it also prevents the implantation and development of any fertilized egg. 

Contraceptive pill: This pill stops the production of an egg in the ovaries. It contains estrogen and progesterone. This is usually taken every day and is an effective method. 

Implant: It is a rod inserted in the arm to prevent pregnancy. It has progesterone which prevents the release of eggs from the ovaries. It also thickens the mucus to prevent the entry of sperm.

Injection: High amounts of progesterone entered into the body.

Surgical Methods

Vasectomy: Sperm ducts are cut and ligated, so sperm won’t leave the body.

Sterilization: Oviducts are cut and tied, to ensure egg won’t go into the uterus.

Sexually Transmitted Infections (STIs)


It is the result of the transmission of the infections, through bacteria or viruses, via body fluids, like semen, vaginal fluid and blood, through sexual contact.

Syphilis

It is a bacterial infection transmitted by sexual contact.

Symptoms

Chancres (sores)
Hair loss
Sore throat
Swollen lymph nodes
Muscle pains
Headache
Weight loss

Signs

Rashes on the palm of the hands
Rashes on the soles of feet
Hair loss
Weight loss

Long-term Effects

Organ damage
Brain damage
Blindness
Dementia
Damage to the bones and joints

Treatment

If diagnosed in the early stages, syphilis is easily curable. Penicillin is an antibiotic that is used to treat syphilis at all stages. An alternative to that is doxycycline.

Human Immunodeficiency Virus (HIV)


Methods of transmission:

Through sexual intercourse: HIV can live in vaginal fluids, rectum, and urethra. During intercourse, fluids from both the partners come in contact so the virus is transmitted.

Through blood transfusion: All blood should be screened to check the presence of HIV. Avoid sharing hypodermic needles. 

HIV Leading to AIDS

HIV affects the white blood cells, specifically the lymphocytes. If you have few WBCs in your body, small viruses, like flu, can kill you. Over a long period of time, HIV slowly destroys the lymphocytes. Due to this, your body will be unable to fight against pathogens.

HIV is not the cause of death. Opportunistic diseases, like tuberculosis and diarrhea, kill. A person is likely to develop symptoms of a syndrome. Many different diseases attack one person as they don’t have immunity against the pathogens.






© 2020 O’Level Academy. All Rights Reserved