Reproduction

 

Reproduction

Reproduction is the process of making more of the same kind of organism. 

There are two types of reproduction that you need to be aware of: 

• Asexual reproduction
• Sexual reproduction

 

Asexual reproduction

Asexual reproduction is the process resulting in the production of genetically identical offspring from a single parent i.e. cloning 

Advantages of asexual reproduction are: 

• Quick 
• Only single parent required 
• Good genetic characteristics always passed on 
• No dispersal so offspring will grow in the same favorable environment 

Disadvantages of asexual reproduction are:

• Little variation i.e. less adaptability to a changing environment 
• Unlikely to withstand disease if parent not resistant 
• Lack of dispersal lead to increased competition for nutrients 

 

Sexual reproduction

Sexual reproduction is the process involving the fusion of two gametes (one from each parent) to form a zygote. This results in the production of offspring that is genetically different from the parents. 

Gametes are sex cells that have half of the normal chromosome number (haploid).

Therefore, when the male and female gamete fuse together they form a new cell with the normal chromosome number (diploid). 

Advantages of sexual reproduction are:

• Variation among offspring, and therefore more adaptable to a changing environment 
• More likely to withstand disease 
• In plants, seed dispersal reduces competition for nutrients as offspring will grow in a different environment

Disadvantages of sexual reproduction are:

• Requires the fusion of two gametes 
• Slower process 

 

Plant reproduction

Plants mostly reproduce sexually, and therefore that will be the focus of this section. 

The sexual reproduction in plants involves the fusion of the male gamete (pollen) and the female gamete (ovule). 

A plant flower has both a ‘male part’ containing the pollen and the ‘female part’ which contains the ovule. 

• Male part = Stamen 
• Female part = Pistil/carpel 

Sexual reproduction occurs when the pollen from the stamen of one flower successfully reaches the ovule of either the same flower, or a different flower. 

This transfer of pollen is called pollination, and we will look into this in a bit more detail down below. 

Important concepts to understand at this stage are: 

• Transfer of pollen is pollination
• Successful pollination results in fertilization 
• A flower can self pollinate i.e. pollen transfer within the same flower or a different flower of same plant
• A flower can cross pollinate i.e. pollen transfer to a different flower of a different plant 

Structure of a flower 

The term carpel & pistil has the same meaning, but I would suggest using carpel in your examination as this is the official term for IGCSE. 

The carpel is the female reproductive part of the plant. It is made of three important structures:

• Stigma = The sticky surface that catches pollen 
• Style = Links stigma to ovary 
• Ovary = Contains ovules which develop into seeds after fertilization 

The stamen is the male reproductive part of the plant. It is made of two important structures:

• Anther = Contains pollen 
• Filament = Supports the anther 

 

Pollination

Pollination is the transfer of the pollen (male sex cell) from the anther to the stigma. 

Once the pollen lands on the stigma, it is transferred to the ovule (female sex cell). 

The fusion of the pollen and ovule leads to what we call fertilization. 

There are main two methods by which plants facilitate pollination

• Insect pollination 
• Wind pollination

Insect pollination

Insect pollination uses insects that land on the flower to carry pollen. 

As insects move around within the flower, some pollen become caught onto the insect’s body. The insect therefore physically carries pollen and successful pollination occurs when it rubs its body against a stigma of the same flower (self pollination) or a different flower (cross pollination). 

Wind pollination

Wind pollination uses the wind to carry pollen. 

Pollen that gets carried by the wind may end up on the stigma of the same flower (self pollination) or a different flower (cross pollination). 

Insect pollinated vs wind pollinated flower

There are some key differences between an insect pollinated flower and a wind pollinated flower. 

Cross pollination vs self pollination

Self pollination is the transfer of pollen from the another of one flower to the stigma of the same flower, or a different flower of the same plant. 

• Advantages
  • Only one parent required so less reliance on pollinators
  • Less competition among offspring
• Disadvantages
  • Less variation (since genes are all from the same plant) 
  • Less adaptable to changing environment and resistance to disease 

Cross pollination is the transfer of pollen from the anther of one flower to the stigma of another flower on a different plant (of same species)

• Advantages
  • Increased variation
  • Greater adaptability to changing environment and more resistant to disease 
• Disadvantages
  • More reliance on pollinators 

 

In the above diagram, A and B represent self pollination. C represents cross pollination. 

 

Fertilization

When a pollen grain lands on the stigma of the correct species, a pollen tube will begin to grow. 

It grows through the style, enters through a small gap in the ovary called the micropyle, and eventually reaches the ovule. 

The nucleus of the pollen then passes along the pollen tube and fuses with the neuclus of the ovule, resulting in successful fertilization. 

The zygote eventually develops into a seed. The seed remains dormant until the conditions are right, and germination occurs. 

 

Germination

Germination is the development of a plant from a seed or spore after a period of dormancy.

The conditions that must be met for germination to occur are as follows:

• Water – For the activation of enzymes 
• Oxygen – For respiration for growth 
• Temperature – Optimum temperature for enzymes 

Investigation of germination

• Set up boiling tubes each containing 10 cress seeds on cotton wool

• Leave tubes in set environmental conditions for a period of time 
• A, B and C placed in an incubator of 20°C 
• D is placed in a fridge of 4°C
• Compare the growth/germination of each of the test tubes 
• Results are as follows: 
  • A(no water) = No germination
  • B (control) = Germination 
  • C (no oxygen) = No germination
  • D (cold temperature) = No germination 

 

Human reproduction

Human reproduction is focused on the male gamete (sperm) fertilizing the female gamete (egg).

 

Male reproductive system

• Penis
  • Can become firm and erect for insertion into the vagina 
• Testis
  • Sperm production
• Scrotal sac 
  • Holds testes outside of body to keep it colder than body temperature (ideal for sperm production)
• Epididymis 
  • Sperm storage 
• Vas deferns/sperm duct
  • Tube connecting testis to urethra  
• Prostate gland 
  • Add nutrients & fluid to sperm to make it semen 
• Urethra 
  • Passes semen or urine out through the penis (but never at the same time)

 

Female reproductive system

• Vagina 
  • Entry point for penis 
• Cervix 
  • Ring of muscle separating vagina from uterus 
• Uterus 
  • Location of fetal development 
• Ovary 
  • Production of egg cells
• Oviduct/fallopian tubes 
  • Site of fertilization
  • Movement of egg cell to uterus via cilia in the wall

 

Human pregnancy overview

1. Sperm enters vagina 
2. Sperm passes through cervix 
3. Sperm enters oviduct 
4. If an egg is present, it becomes fertilized
5. Fertilization results in zygote formation
6. Zygote divides to make an embryo (ball of cells) 
7. Embryo implants into the uterus walls
8. Embryo develops further to form a fetus 
9. Fetus develops within uterus/womb 
10. Birth of a baby 

 

Fetal development & birth

Once an egg is fertilized by sperm, it forms a zygote. The zygote undergoes cell division to form a ball of cells called the embryo.  

The emrbyo implants itself into the uterus lining, where it begins to develop into a fetus 

Fetal development

 

The placenta brings fetal blood supply close to the mother’s blood without mixing. Nutrients diffuse from the mother’s blood into the fetus’ blood. Oppositely, waste diffuses from the fetus’ blood into the mother’s blood. 

The umbilical cord carries fetal blood to and from the placenta. This cord is therefore essential for the nutrient/waste exchange between the fetal blood and the mother’s blood. 

The amniotic fluid protects the fetus from physical harm, and is held inside the amniotic sac. 

 

Umbilical cord

 

• Umbilical artery
  • Carries blood from the fetus to the placenta 
• Placenta 
  • Nutrient/waste exchange 
  • Nutrients pass from mother’s blood to fetal blood 
  • Waste products poss from fetal blood to mother’s blood
• Umbilical vein
  • Carries blood from the placenta to the fetus 
  • Some toxins such i.e. nicotine or pathogens i.e. rubella, can cross the placenta from the mother’s blood and affect the fetus 

Ante-natal care of pregnant woman

Before birth, the baby obtains all dietary requirements from the mother via the placenta. 

It is therefore very important that the mother’s diet is well balanced with: 

• Amino acids/proteins for growth 
• Calcium for bone development 
• Iron for RBC formation

Process of childbirth

The process of childbirth is as follows:

• Labour is triggered by the oxytocin hormone
• Muscular walls of the uterus contract
• Pressure from contraction breaks the amniotic sac and releases amniotic fluid 
• Contractions become more violent and push the baby down the cervix 
• The cervix becomes dilated for the baby to pass through 
• The vagina stretches in order to allow the baby to be born 
• The Baby is still attached to the placenta via umbilical cord post birtth 
• The umbilical cord is therefore cut and tied 
• The placenta breaks away from the uterus wall and passes out

 

Breast-milk vs formula

It is important to understand the ongoing debate about the advantages and disadvantages of breast feeding compared to formula. 

Advantages: 

• No risk of allergic reaction
• Natural antibodies in breast milk (passive immunity) 
• Breast milk is at ideal temperature 
• No additives 
• Bonding between mother & baby 

Disadvantages 

• Difficult to do in public 
• Painful 
• Mother must be present 

 

Puberty

Puberty  is the name for the time when the body begins to change as a child transitions into adulthood. 

• Testosterone triggers the development of secondary sexual characteristics in males
• Estrogen triggers the development of secondary sexual characteristics in females 

 

Menstrual cycle

There are four main hormones involved in the menstrual cycle: 

1. Estrogen – Builds the uterus lining 
2. Progesterone – Maintains the uterus lining 
3. FSH – Causes a single follicle in the ovary to mature (a mature follicle contains the egg, along with other surrounding cells) 
4. LH – Secreted from pituitary glands which stimulate ovulation

 

The stages of the cycle can be summarized as follows: 

1. Uterus lining breaks down due to the lack of progesterone 
2. Estrogen levels rise to build the uterus lining 
3. FSH causes a single follicle to mature 
4. On day 14, estrogen levels drop and LH levels spike to cause ovulation 
5. The ovum gets released into the oviduct, leaving behind the remains of the mature follicle.
6. The cells that get left behind after ovulation become the corpus luteum 
7. The corpus luteum secretes progesterone which maintains the thick uterus lining in preparation for implantation

 

Human birth control

There are various methods of birth control that you must be familiar with.

Common methods of birth prevention include: 

• Natural 
  • Abstinence 
  • Withdrawal 
  • Rhythm 
• Chemical  
  • Contraception
  • Spermicide
• Mechanical 
  • Condoms 
  • Femidom
  • Diaphragm
• Surgical 
  • Vasectomy 
  • Laparotomy 

Common methods of birth promotion include: 

• Artificial insemination
• Fertility drugs 
• In-vitro fertilization 

 

Sexually transmitted disease

Sexually transmitted diseases (STIs) are infections that can be transmitted via body fluids during sexual contact. 

HIV (human immunodeficiency virus) is an example of an STI. An HIV infection can lead to AIDS (acquired immunodeficiency syndrome). 

As the name suggests, the HIV virus destroys the body’s immune system. The virus attacks lymphocytes in the blood stream, and a decreased lymphocyte count leads to reduced antibodies, and therefore compromised immunity. 

Methods of transmission include: 

• Unprotected sex 
• Sharing needles 
• Mother to fetus via the placenta 

Methods to prevent the spread of HIV include: 

• Condom usage 
• Abstinence of sexual intercourse 
• Using sterilized needles instead of sharing 
• Bottled milk