Biology Human Reproduction PDF

## Organisms and Environment ### 13.1 Human reproductive system We learnt that flowering plants can reproduce sexually and/or asexually. How about humans? Humans carry out sexual reproduction only. Their reproductive systems are specialized for this way of reproduction. #### A Male reproductive s...

## Organisms and Environment ### 13.1 Human reproductive system We learnt that flowering plants can reproduce sexually and/or asexually. How about humans? Humans carry out sexual reproduction only. Their reproductive systems are specialized for this way of reproduction. #### A Male reproductive system Fig 13.1 shows the front and side views of the male reproductive system. **Front view** **(ureter)** **(urinary bladder)** erectile tissue urethra Singular testis penis **(right ureter)** seminal vesicle prostate gland **(rectum)** Cowper's gland epididymis scrotum **Side view** **(sperm duct)** vas deferens seminal vesicle prostate gland Cowper's gland epididymis scrotum holds testis outside the body **(urinary bladder)** vas deferens erectile tissue penis urethra testis **Note:** Structures in brackets do not belong to the reproductive system **Fig 13.1 Male reproductive system in humans** The male reproductive system consists of: 1. **A pair of gamete-producing organs: testes** - They produce sperms (male gametes) and male sex hormones. - They are located in a sac called the **scrotum**, which hangs outside the abdominal cavity. This keeps the testes at 2-3°C lower than the body temperature (37°C). The lower temperature is optimal for sperm development. 2. **A system of ducts:** epididymis, vas deferens and urethra | Duct | Function | |----------------|-------------------------------------------------------------------------------------| | Epididymis | It is a long coiled tube that lies on each testis. It stores sperms temporarily. | | Vas deferens | It carries sperms from the epididymis to the urethra. | | Urethra | It discharges semen* out of the body during ejaculation*. (It also serves to discharge urine.) | 3. **Glands:** seminal vesicles, prostate gland and Cowper’s glands - They secrete **seminal fluid** into the ducts. Seminal fluid: - provides a medium for sperms to swim in - contains sugars that act as an energy source for sperms - contains mucus for lubrication - is alkaline. It helps neutralize any acidic urine in the male's urethra and acidic secretions in the female's vagina. - The seminal fluid and the sperms make up the **semen**. 4. **A copulatory organ: penis** - It contains **erectile tissue** and many blood vessels - It transfers semen into the female's vagina during sexual intercourse. ### 13.2 Human gametes Sexual reproduction in humans involves the fusion of a sperm (male gamete) and an ovum (female gamete). What are the structures of these gametes and how are they formed? #### A Structure of human gametes 1. **Sperms** A sperm looks like a tadpole. It is very small, about 0.05 mm long. It consists of three parts: a head (contains a cap called the **acrosome** and a **nucleus**), a midpiece (contains many mitochondria) and a tail. 2. **Ova** An ovum is spherical in shape. Its diameter is about 0.1 mm. It is much larger than the sperm as it contains a lot of food reserves. It cannot move by itself. It consists of a nucleus, **cytoplasm** and a cell membrane covered by a **jelly coat**. ### 13.3 Secondary sexual characteristics in humans Increased production of sex hormones during puberty does not only stimulate sperm production in males and development of ova in females, but also leads to the development of **secondary sexual characteristics**. Some of these characteristics are shown in Fig 13.14 | | Male | Female | |---------------|---------------------------|------------------------------------| | **Similarity** | Hair grows in armpits and pubic area | Hair grows in armpits and pubic area | | **Difference** | Larynx enlarges and voice deepens. Shoulders broaden . The body becomes more muscular . Beard grows. | Breasts develop. Hips broaden . More fat is deposited under the skin. | **Note:** Primary sexual characteristics refer to the reproductive organs that we are born with. ### 13.4 Menstrual Cycle We learnt that an ovum is released into the oviduct about every 28 days. If the ovum meets a sperm in the oviduct, fertilization may occur. The fertilized ovum will develop into an embryo. To prepare for the implantation of the embryo, the uterine lining thickens and its blood supply increases before ovulation. If fertilization does not occur, the thickened uterine lining will break down. The cycle of building up and breaking down of the uterine lining is called the **menstrual cycle**. Key events in the cycle are shown below. * Days 1 to 5 * If fertilization does not occur, the yellow body degenerates. * The thickened uterine lining breaks down. The uterine lining (including some blood) and the unfertilized ovum are discharged through the vagina. This process is called **menstruation**. It lasts about 5 days. * Some follicles in the ovary start to develop. * Days 6 to 13 * Follicles continue to develop. The uterine lining thickens and the blood supply increases to prepare for the implantation of an embryo. * Day 14 * Ovulation occurs * Days 15 to 28 * The ruptured follicle becomes the yellow body, which releases a hormone to increase and maintain the thickness of the uterine lining. * The uterine lining becomes very thick. It us ready to receive an embryo. If fertilization occurs and the woman gets pregnant, the degeneration of the yellow body is delayed. The uterine lining does not break down. Instead, it continues to thicken. Menstruation does not occur until the baby is born. Figure 13.16 shows the changes in the thickness of the uterine lining in relation to the events occurring in the ovary. **Note:** The first day of menstruation is considered Day 1 of the cycle. The changes in the uterine lining and ovaries are brought about by the interactions of hormones. The first menstruation occurs at puberty. The menstrual cycle repeats until about the age of 50, when a woman reaches **menopause**. At menopause, ovulation and menstruation become less regular and gradually stop. Then the woman can no longer become pregnant. **Fertile Period:** After ovulation, an ovum is capable of being fertilized for 1 to 2 days only. As sperms can live in the female body for up to 5 days, the highest chance for a woman to get pregnant is to have sexual intercourse a few days before and after ovulation. This period is called the **fertile period**. ### 13.5 Fertilization A new life begins when fertilization takes place. How do the sperm and the ovum meet each other and fuse together? #### A Transfer of sperms into female body Sperms are brought into a female body during sexual intercourse (or called copulation or mating). #### B Journey of sperms towards oviducts After entering the vagina, the sperms swim through the cervix and the uterus into the upper parts of the oviducts (site of fertilization). #### C Fusion of sperm and ovum 1. When sperms meet an ovum in the oviduct, they try to penetrate it. 2. The sperms release **enzymes** from their acrosomes to digest a path through the jelly coat of the ovum. 3. The head of one of the sperms successfully passes through the cell membrane and enters the cytoplasm of the ovum. The midpiece and tail are left outside. Once this happens, the jelly coat of the ovum changes to prevent the entry of other sperms. 4. The nucleus of the sperm fuses with the nucleus of the ovum. This process is called **fertilization** and the fertilized ovum is called a **zygote**. The zygote carries the genetic materials of both parents. The diploid number of chromosomes is restored in it. ### 13.6 Pregnancy #### A Implantation After fertilization, the zygote is moved down the oviduct. During the journey, the zygote divides repeatedly by **mitotic cell division** to form a ball of cells called an **embryo**. After reaching the uterus, the embryo embeds itself into the thickened uterine lining. This process is called **implantation**. It occurs about seven days after fertilization. It marks the beginning of pregnancy. #### B Development of embryo 1. **Formation of amnion** About five days after implantation, some cells of the embryo form a membrane called the **amnion**. The amnion encloses the embryo. The cavity between the amnion and the embryo is filled with a clear, slightly yellowish liquid called **amniotic fluid**. | Functions of amniotic fluid | |------------------------| | It acts as a cushion to protect the embryo from mechanical injury. | | It provides a stable environment, e.g. a constant temperature, for the development of the embryo. | | It prevents the embryo from desiccation. | | It allows the embryo to move around easily. | | It lubricates the vagina during childbirth. | 2. **Formation of placenta** Besides the amnion, the embryo also develops finger-like villi that grow into the uterine lining. These villi, together with some maternal uterine tissue, form a disc-shaped organ called the **placenta**. The placenta is connected to the embryo by the **umbilical cord**. The umbilical cord contains two umbilical arteries which carry deoxygenated blood from the embryo to the placenta, and an umbilical vein which carries oxygenated blood from the placenta to the embryo. The embryonic villi in the placenta contain the embryo’s capillaries. The villi are extended into the maternal blood space within the uterine lining. The embryo's blood and the maternal blood are separated. This separation: - avoids breakage of embryo's blood vessels by the high blood pressure of maternal blood. - avoids clumping of blood in case the blood types of the embryo's blood and the maternal blood are incompatible. - prevents the entry of certain pathogens or toxins from the maternal blood into the embryo’s blood. The placenta allows the exchange of materials between the embryo and the mother: - Nutrients (e.g. glucose, amino acids, minerals), oxygen and antibodies diffuse from the maternal blood to the embryo's blood. - Carbon dioxide and other metabolic wastes (e.g. urea) diffuse from the embryo’s blood to the maternal blood. #### C Development of foetus About eight weeks after fertilization, major organs of the embryo are formed. The embryo is now called a **foetus**. The foetus continues to grow until it is ready to be born. It takes about 38 weeks from fertilization to birth. Human pregnancy can be divided into three 3-month periods. The foetal development in each period is shown below. **First 3 months** **Week 4:** - embryo - urinary bladder - The heart beats. - The backbone is formed. **Week 8:** - foetus - Length: 4 cm - Weight: 3 g - All major organs are formed. - Eyes, nose and ears can be seen. **Week 12:** - Length: 9 cm - Weight: 45 g - The foetus can move its arms and legs. **Note:** The period from fertilization to birth is called the gestation period. Bigger-sized animals usually have a longer gestation period. Besides allowing the exchange of materials between the embryo and the mother, the placenta also secretes hormones to maintain the thickness of the uterine lining for the development of the embryo. #### D Prenatal care Prenatal care is vital for the health of the foetus as well as that of the mother. Below are some points to note for a pregnant woman. **Do's:** * Have a balanced diet. * Do gentle exercise regularly * Have prenatal check-ups regularly **Don'ts:** * Do not smoke. * Do not drink alcohol. * Do not take drugs without seeking the doctor's advice. **Middle 3 months:** - Length: 19 cm - Weight: 460 g - Hair starts to grow. - External reproductive organs can be seen. **Last 3 months:** - Length: 27 cm - Weight: 1.3 kg - Eyes begin to open. **Week 38:** - Length: 34 cm - Weight: 2.9 kg - Bones and muscles are well developed. ### 13.7 STSE connections #### Non-invasive prenatal testing - To check whether the foetus carries certain genetic diseases, a pregnant woman can undergo **amniocentesis**. In amniocentesis, a needle is placed through the abdomen to collect some **amniotic fluid**. The fluid contains foetal cells which can be checked for any abnormal chromosomes. - Now a safer test is available. By simply taking a blood sample from the mother, **foetal DNA** can be checked for abnormalities. - In the past, many people held the concept that maternal blood is free of foetal DNA. #### NOS ideas * Scientific knowledge is tentative and subject to change. * Science is affected by social and cultural factors. * Scientists may not arrive at the same conclusions from the same set of data. ### 13.8 Formation of twins There is usually one baby from a single pregnancy. But in some cases, a mother gives birth to two babies. These two babies are called **twins**. There are two types of twins, **identical twins** and **fraternal twins** (or non-identical twins). Their formation is shown below. #### Identical twins After fertilization, a zygote divides into two cells. If each cell develops into an embryo, identical twins will result. #### Fraternal twins If two ova are released at the same time and fertilized by two different sperms, two zygotes will be formed and fraternal twins will result. ### 13.9 Did you know? **Most children delivered at a single birth** - Besides twins, multiple births with three or more babies are also possible. ### 13.10 Key learning 1. What happens to a zygote before implantation takes place? 2. What are the functions of the amniotic fluid? 3. What is the importance of separating embryo’s blood and the maternal blood? 4. What are the functions of the placenta? 5. How do identical twins differ from fraternal twins?