reproduction

Question 1

Asexual Reproduction

Answer 1

Asexual reproduction does not involve sex cells or fertilisation

Only one parent is required so there is no fusion of gametes and no mixing of genetic information

As a result, the offspring are genetically identical to the parent and to each other (clones)

Asexual reproduction is defined as a process resulting in genetically identical offspring from one parent

Question 2

examples of asexual reproduction

Answer 2

Question 3

Advantages & Disadvantages of Asexual Reproduction

Answer 3

Question 4

Sexual Reproduction

Answer 4

Sexual reproduction is a process involving the fusion of the nuclei of two gametes (sex cells) to form a zygote (fertilised egg cell) and the production of offspring that are genetically different from each other

Fertilisation is defined as the fusion of gamete nuclei

Question 5

Gametes & Zygotes

Answer 5

A gamete is a sex cell (in animals: sperm and ovum; in plants pollen nucleus and ovum)

Gametes differ from normal cells as they contain half the number of chromosomes found in other body cells – we say they have a haploid nucleus

This is because they only contain one copy of each chromosome, rather than the two copies found in other body cells

In human beings, a normal body cell contains 46 chromosomes but each gamete contains 23 chromosomes

When the male and female gametes fuse, they become a zygote (fertilised egg cell)

This contains the full 46 chromosomes, half of which came from the father and half from the mother – we say the zygote has a diploid nucleus

Question 6

Advantages & Disadvantages of Sexual Reproduction

Answer 6

Most crop plants reproduce sexually and this is an advantage as it means variation is increased and a genetic variant may be produced which is better able to cope with weather changes, or produces significantly higher yield

The disadvantage is that the variation may lead to offspring that are less successful than the parent plant at growing well or producing a good harvest

Question 7

Flowers & Pollination

Answer 7

Flowers are the reproductive organ of the plant

They usually contain both male and female reproductive parts

Plants produce pollen which contains a nucleus inside that is the male gamete

Unlike the male gamete in humans (sperm), pollen is not capable of locomotion (moving from one place to another)

This means plants have to have mechanisms in place to transfer pollen from the anther to the stigma

This process is known as pollination and there are two main mechanisms by which it occurs: transferred by insects (or other animals like birds) or transferred by wind

The structure of insect and wind-pollinated flowers are slightly different as each is adapted for their specific function

Question 8

Parts of a Flower

Answer 8

Question 9

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Question 10

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Question 11

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Question 12

The pollen produced by insect and wind-pollinated flowers is also different:

Answer 12

Insect-pollinated flowers produce smaller amounts of larger, heavier pollen grains that often contain spikes or hooks on the outside so they are better able to stick to insects

Wind-pollinated flowers produce large amounts of small, lightweight pollen grains that are usually smooth

Question 13

Answer 13

A

Question 14

Self and Cross-Pollination

Answer 14

Cross-pollination occurs when the pollen from one plant is transferred to the stigma of another plant of the same species

This is the way most plants carry out pollination as it improves genetic variation

Occasionally, the pollen from a flower can land on its own stigma or on the stigma of another flower on the same plant – this is known as self-pollination

Self-pollination reduces genetic variety of the offspring as all the gametes come from the same parent

Lack of variation in the offspring is a disadvantage if environmental conditions change, as it is less likely that any offspring will have adaptations that suit the new conditions well

On the other hand, cross-pollination relies completely on the presence of pollinators and this can be a problem if those pollinators are missing (eg the reduction in bee numbers is of great importance to humans as bees pollinate a large number of food crops)

Question 15

Fertilisation occurs when

Answer 15

a pollen nucleus fuses with an ovum nucleus in the ovule

Question 16

Fertilisation

Answer 16

As the pollen has no ‘tail’ to swim to the ovary, instead it grows a pollen tube

This only happens if it has landed on the right kind of stigma (ie of the same species as the flower the pollen came from)

The nucleus inside the pollen grain slips down the tube as it grows down the style towards the ovary

The ovary contains one or more ovules which each contain an ovum with a female nucleus that a male pollen nucleus can fuse with

Once the nuclei (pl) have joined together, that ovule has been fertilised and a zygote has been formed

The zygote will start to divide and eventually form a seed with the ovule

As different plants have different numbers of ovules, this explains why different fruits (which develop from the ovary) have different numbers of seeds (which develop from the ovules)

Question 17

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Question 18

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Question 19

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Question 20

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Question 21

Gametes and Fertilisation

Answer 21

Fertilisation is the fusion of the nuclei from a male gamete (sperm cell) and a female gamete (egg cell)

It occurs in the oviducts

Gametes have adaptations to increase the chances of fertilisation and successful development of an embryo

Question 22

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Question 23

sperm cell structure

Answer 23

Question 24

An egg cell structure

Answer 24

Question 25

Adaptations of Gametes Explained

Answer 25

Question 26

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Question 27

The Placenta

Answer 27

During the gestation period the foetus develops and grows by gaining the glucose, amino acids, fats, water and oxygen it needs from the mother’s blood The bloods run opposite each other, never mixing, in the placenta The foetus’s blood connects to and from the placenta by the umbilical cord The mother’s blood also absorbs the waste from the foetus’s blood in the placenta; substances like carbon dioxide and urea are removed from the foetus’s blood so that they do not build up to dangerous levels Movement of all molecules across the placenta occurs by diffusion due to difference in concentration gradients The placenta is adapted for this diffusion by having a large surface area and a thin wall for efficient diffusion The placenta acts as a barrier to prevent toxins and pathogens getting into the foetus’s blood Not all toxin molecules or pathogenic organisms (such as viruses, eg rubella) are stopped from passing through the placenta (this usually depends on the size of the molecule) This is why pregnant women are advised not to smoke during pregnancy as molecules like nicotine can pass across the placenta After the baby has been born, the umbilical cord is cut – this does not hurt as there are no nerves in it, just two blood vessels It is tied off to prevent bleeding and shrivels up and falls off after a few days leaving the belly button behind The placenta detaches from the uterus wall shortly after birth and is pushed out due to contractions in the muscular wall of the uterus – known as the afterbirth

Question 28

Growth & Development of the Fetus

Answer 28

After fertilisation in the oviduct, the zygote travels towards the uterus This takes about 3 days, during which time the zygote will divide several times to form a ball of cells known as an embryo In the uterus, the embryo embeds in the thick lining (implantation) and continues to grow and develop The gestation period for humans is 9 months Major development of organs takes place within the first 12 weeks, during which time the embryo gets nutrients from the mother by diffusion through the uterus lining After this point the organs are all in place, the placenta has formed and the embryo is now called a fetus The remaining time is used by the fetus to grow bigger in size The fetus is surrounded by an amniotic sac which contains amniotic fluid (made from the mother’s plasma) This protects the fetus during development by cushioning it from bumps to the mother’s abdomen The umbilical cord joins the fetus’s blood supply to the placenta for exchange of nutrients and removal of waste products

Question 29

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Question 30

Stages of Birth

Answer 30

Amniotic sac breaks Muscles in the uterus wall contract Cervix dilates (gets wider) Baby passes out through the vagina Umbilical cord is tied and cut Afterbirth is delivered

Question 32

Antenatal Care

Answer 32

Antenatal (before birth), care is the name given to the care and advice given to expectant mothers along with checks on fetal growth and development Whilst pregnant, expectant mothers are given advice on: diet including the need to take folic acid to prevent developmental issues with the fetus and the importance of a balanced diet exercise to stay fit health precautions such as avoiding infections, tobacco, alcohol and other drugs

Question 33

Breastfeeding

Answer 33

During pregnancy the mammary glands enlarge and become prepared to secrete milk Shortly after birth, the mother will be stimulated to release milk due to the sucking action of the baby at the breast Some mothers struggle to breastfeed successfully and so may feed the baby using formula milk in a bottle

Question 34

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Question 35

Secondary Sexual Characteristics

Answer 35

Primary sexual characteristics are present during development in the uterus and are the differences in reproductive organs etc between males and females Secondary sexual characteristics are the changes that occur during puberty as children become adolescents They are controlled by the release of hormones – oestrogen in girls and testosterone in boys

Question 36

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Question 37

Some changes occur to both boys and girls including

Answer 37

growth of sexual organs and growth of body hair Emotional changes also occur due to the increased levels of hormones in the body These include more interest in sex and increased mood swings

Question 38

The menstrual cycle is controlled by hormones released from

Answer 38

the ovary and the pituitary gland in the brain

Question 39

oestrogen and progesterone in the Menstrual Cycle

Answer 39

Oestrogen levels rise from day 1 to peak just before day 14 This causes the uterine wall to start thickening and the egg to mature The peak in oestrogen occurs just before the egg is released Progesterone stays low from day 1 – 14 and starts to rise once ovulation has occurred The increasing levels cause the uterine lining to thicken further; a fall in progesterone levels causes the uterine lining to break down (menstruation / ‘period’)

Question 40

FSH and LH

Answer 40

FSH (Follicle Stimulating Hormone) is released by the pituitary and causes an egg to start maturing in the ovary It also stimulates the ovary to start releasing oestrogen The pituitary gland is stimulated to release Luteinising Hormone (LH) when oestrogen levels have reached their peak LH causes ovulation to occur and also stimulates the ovary to produce progesterone

Question 41

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Question 42

Interaction between all four of the menstrual cycle hormones:

Answer 42

The pituitary gland produces FSH which stimulates the development of a follicle in the ovary An egg develops inside the follicle and the follicle produces the hormone oestrogen Oestrogen causes growth and repair of the lining of the uterus wall and inhibits production of FSH When oestrogen rises to a high enough level it stimulates the release of LH from the pituitary gland which causes ovulation (usually around day 14 of the cycle) The follicle becomes the corpus luteum and starts producing progesterone Progesterone maintains the uterus lining (the thickness of the uterus wall) If the ovum is not fertilised, the corpus luteum breaks down and progesterone levels drop This causes menstruation, where the uterus lining breaks down and is removed through the vagina – commonly known as having a period If pregnancy does occur the corpus luteum continues to produce progesterone, preventing the uterus lining from breaking down and aborting the pregnancy It does this until the placenta has developed, at which point it starts secreting progesterone and continues to do so throughout the pregnancy

Question 44

Birth control methods are important in

Answer 44

keeping family sizes small and in limiting the increase in human population

Question 45

Humans can use ...methods to prevent a pregnancy

Answer 45

mechanical, chemical, surgical and natural contraceptive

Question 46

Some birth control methods also give protection from ...

Answer 46

sexually transmitted infections

Question 47

Natural

Answer 47

Abstinence avoiding sexual intercourse completely Rhythm method avoiding sexual intercourse during the fertile period of the menstrual cycle when ovulation occurs the exact time ovulation happens can be worked out by monitoring body temperature and quality of cervical mucus this is the least reliable method of birth control

Question 48

Chemical

Answer 48

IUD / IUS an intrauterine device or intrauterine system is a small device fitted inside the uterus by a doctor or nurse it releases sex hormones which thicken the mucus produced in the cervix, making it difficult for sperm to swim into the uterus it also thins the lining of the uterus, making it more difficult for a fertilised egg to implant an IUD also interferes with passage of sperm through the uterus, in which way it is acting as a barrier method of birth control Contraceptive pill, implant, injection may contain just progesterone or a mixture of progesterone and oestrogen very effective when taken regularly the hormones can also be delivered from a small skin implant or an injection, both of which last several months and increase the effectiveness as they remove the risk of forgetting to take a pill regularly

Question 50

Barrier

Answer 50

These all work by preventing sperm from reaching the egg Condom latex sheath worn over the penis prevents sperm entering the vagina as ejaculate remains in condom also protects against STIs Femidom latex sheath inserted into the vagina prevents entry of sperm into the vagina Diaphragm a rubber cap that fits over the entrance to the cervix prevents entry of sperm into uterus often used with a spermicide (cream which kills sperm)

Question 51

Surgical

Answer 51

Vasectomy the sperm ducts are cut, meaning that no sperm is present in the semen when ejaculation occurs very effective but difficult to reverse Female sterilisation (tubal ligation) the oviducts are cut or tied off, preventing eggs from reaching the uterus or sperm from reaching the eggs very effective but difficult to reverse

Question 52

Answer 52

Question 53

How do Contraceptive Hormones in The Pill Work?

Answer 53

They work by mimicking some of the hormone levels during pregnancy By raising the levels of progesterone and oestrogen, the uterus lining is maintained and development of another egg cell is prevented This means that sex at any time of the month cannot cause pregnancy as no egg is released to be fertilised

Question 55

In situations where couples find it difficult to conceive,... can improve their chances

Answer 55

fertility treatments

Question 56

Artificial Insemination (AI)

Answer 56

If the male is not producing healthy sperm, donor sperm can be used The sperm are placed into the female’s vagina at the fertile point in her menstrual cycle Social issues to consider include: The male must be able to cope with the fact that the child is not biologically his Sperm donors may father multiple children who are not able to know their parentage

Question 57

Fertility Drugs

Answer 57

This method is used when the female is not producing enough eggs Hormones, including FSH, are given to her to stimulate egg production Social issues to consider include: Several eggs can be released at once so this increases the chance of multiple births (twins or triplets etc) In Vitro Fertilisation (IVF) If the female cannot conceive naturally even after taking fertility drugs, or if there are issues with both male and female fertility in a couple, IVF can be used This involves fertility drugs being given to the female to stimulate egg production before they are harvested from the ovary The eggs are then inseminated in a petri dish using sperm from the male (‘in vitro’ means ‘in glass’) and, once embryos have formed, they are placed back into the uterus of the female Several embryos are implanted to increase the chance of one developing further Social issues to consider include: IVF is relatively expensive and not all couples can afford it As several embryos are implanted, the risk of multiple births is quite high Some women use IVF to get pregnant at a later age than they would be able to conceive naturally

Question 58

Unprotected sexual intercourse can lead to

Answer 58

the transfer of pathogens via exchange of body fluids

Question 59

Unprotected sexual intercourse can lead to the transfer of pathogens via exchange of body fluids Infections passed on in this way are known as

Answer 59

sexually transmitted infections (STIs

Question 60

An example of an STI is

Answer 60

s HIV, the virus that usually leads to the development of acquired immunodeficiency disease (AIDS)

Question 61

HIV can also be spread via

Answer 61

sharing needles with an infected person, blood transfusions with infected blood and from mother to fetus through the placenta and mother to baby via breastfeeding

Question 62

How HIV Affects the Immune System

Answer 62

Immediately after infection, people often suffer mild flu-like symptoms These symptoms pass and for a period of time infected people might not know they are infected The virus infects the lymphocytes of the body’s immune system Normally lymphocytes seek out and destroy pathogens that enter the body However HIV avoids being recognised and destroyed by lymphocytes by repeatedly changing its protein coat It then infects a certain type of lymphocyte and uses the cells to multiply This reduces the number of lymphocytes, and also the number of antibodies that can be made This decreases the body’s ability to fight off infection, eventually leading to AIDS

Question 63

Controlling the Spread of STIs

Answer 63

The spread of STIs such as HIV are best controlled by: Limiting the number of sexual partners an individual has Not having unprotected sex, but making sure to always use a condom Getting tested if unprotected sex or sex with multiple partners has occurred Raising awareness by education programmes