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Sexual Reproduction in Biology

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Sex: Biology Explained


This article explores the variety and similarities of sexual reproduction in plants, frogs, chickens, and people in a way suitable for pre-teens as their first "sex talk."


Almost every living thing both plant and animal- produces offspring through a process known as sex.

During the process of sex, two individuals of the same species each contribute a tiny portion of their own bodies a special living cell containing half of the material needed to create an offspring. The two special cells combine to form a complete new cell capable of growing.

The two individuals working together to create an offspring are called adults. With just a few exceptions in some species, one of the adults has to be a male and one has to be a female. The reason is that the special cells that come together to form a complete cell are special in different ways. One of these types of special sex cell is called a sperm, which is produced by the male. The other type is called an ovum, which is produced by the female.

When a sperm and an ovum combine together, the new cell begins dividing, first into two cells, then into four. Increasing to eight cells, to sixteen cells and so on, the potential offspring, the embryo, continues growing. As it becomes larger, the embryo's cells start to become specialized and develop into the different parts that make the embryo begin resembling the original two individuals who created it.

Eventually the embryo has all of the working parts that make it an identifiable member of its species. No longer called an embryo, it may be called depending on the species a seedling, a plantlet, a larva, a tadpole, a chick, a pup, a calf, or a baby. It continues growing until it becomes an adult, and then is able to produce offspring of its own.


In plants, we see the sex process happening when the wind or insects carry pollen grains from one flower to another. The pollen contains sperm. When it lands on a special part of the flower called a pistil, the sperm inside the pollen burrows in to find the ovum. When the sperm and the ovum combine, the new cell begins growing, eventually becoming a seed. The plant embryo is inside the seed.

In plants, the embryo will typically stay dormant inside the seed until conditions are right for it to continue growing such as when it is exposed to warm temperatures and water. Then it will sprout and grow into a new plant.


Frogs go about sex differently. The female discharges a mass of eggs (ova) from her body in a sticky jelly that attaches to leaves, grasses, or twigs under water. The male then discharges a huge amount of sperm from his body that settle on the eggs. When an individual sperm finds an egg and combines with it, the resulting embryo grows until it hatches into a tadpole.


For chickens to produce offspring, the male uses a small organ it can extend from between its legs. Because the organ, called a penis, is small, the male has to get very close to the female from behind so that the penis can reach into the opening under her tail, called a vagina. As male and female are touching, and the penis enters the vagina, the male releases a large amount of sperm. The female stores the sperm inside her vagina for as long as 10 to 15 days.

Every day or two, the female releases an egg. As the egg passes through the vagina before being laid, it is exposed to the sperm. When one of the sperm combines with the egg, it starts the growth of the embryo. The egg is laid in a nest with a hard shell around it, and the embryo inside grows until it is able to break out of the eggshell and begin living in the outside world.

Chickens will lay eggs even if they have no sperm inside their vagina. An ovum that has not yet combined with a sperm is called an unfertilized egg. When a sperm and ovum join, it is called a fertilized egg. When chickens lay eggs that have not been fertilized by sperm, those unfertilized eggs will never start dividing and growing. Most of the eggs we buy in the store are unfertilized. They will never grow into chickens.


In human beings, the male also has a penis between his legs, and the female also has a vagina. Since people don't have tails, the female's vagina is located between her legs.

To produce a baby, the male puts his penis into the female's vagina. The sperm does not come out automatically, though. To get the sperm to come out, the male has to push his penis in and out of the vagina to create sexual stimulation. With enough stimulation, the penis ejects sperm into the vagina. The ejection of sperm is called ejaculation or an orgasm, and the process of pushing the penis in out of the vagina is called sex, or having sex.

As the male pushes his penis in and out of the female's vagina, it also creates sexual stimulation for the female. With enough stimulation, the female can also have an orgasm. The female orgasm causes muscle contractions inside the vagina, which help pull the sperm deeper inside her where they can reach an ovum. The sperm can still reach an ovum, however, even if the female doesn't have an orgasm

Males and females usually consider orgasm to be highly pleasurable, and will often have sex simply for the enjoyment of the orgasm. The pleasure associated with an orgasm is nature's way of making sure that humans have sex and produce offspring.

Offspring are not produced every time a male and female have sex. The female produces only one egg about once each month. The egg stays inside the female for about a week, waiting to be joined by a sperm. If the egg and a sperm don't connect, the egg is flushed out through the vagina accompanied by blood that was being built up in advance to help the potential embryo grow. Unlike a chicken, though, the unfertilized egg is very tiny and cannot be seen.

When an unfertilized egg and the extra blood are flushed out, it is called menstruation, and the flushing of blood lasts for several days. Until an egg becomes fertilized, the female will menstruate about once a month in what is called the menstruation cycle.

In the menstruation cycle, an egg and the extra storage of blood are produced, and the egg waits for a sperm. If the sperm doesn't arrive within about a week, the egg and blood are flushed out through the vagina by menstruation. After about two weeks, another egg is produced and the cycle starts over.

Also unlike chickens, sperm does not live very long inside a human female's vagina. After a day or so, if the sperm haven't found an egg to join with, the sperm cells die.

When a sperm and an egg join and the egg becomes fertilized, it is called impregnation. The female is considered pregnant. Since a female's egg is only in her body for about a week, the female can only become pregnant if she has sex during that week. Many people try to plan sex so they have it during that week if they want to have a baby, or during the other three weeks if they don't want to have a baby.

The exact time when an egg is ready to be fertilized is not perfectly predictable, however. Sometimes people will have sex and the female will become pregnant when they were not intending to have a baby.

When a sperm and an egg join, and the egg begins forming an embryo, the embryo attaches itself to the lining of a pouch-like organ inside the female called a uterus. When it attaches, it develops blood vessels that connect to the female's blood vessels inside the uterus. The blood flowing through the female's body then also flows through the embryo, which is how the embryo gets nourishment to grow.

The embryo grows inside the uterus for about nine months. As it grows and its cells divide and start to differentiate into a head and arms and legs, it begins to more and more resemble a human being. When it has fully developed all its limbs and organs, and is ready to exist outside of the female, the process of birth begins.

During birth, the female begins having muscle contractions that start pushing the new baby out of the uterus. The uterus is connected to the vagina, so that is where the baby comes out.

In the uterus, the baby's blood vessels are connected from its belly button to a detachable part of the uterus called a placenta. The blood vessels are connected through a tube called the umbilical cord. During birth, the baby and the placenta both come out still connected together by the umbilical cord.

Since it is no longer needed, the umbilical cord is cut and tied into a knot so it doesn't bleed. This doesn't hurt the baby because the umbilical cord can't feel pain. Eventually, the umbilical cord dries up and falls off, leaving a belly button behind.

Then the baby grows, becomes an adult, and the sex process starts all over again.

More about this author: Steve Holder

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