DP Biology HL D3.1 Reproduction

DP Biology HL D3.1 Reproduction - Full notes

RevisionPrep

DP Biology HL | D3.1 Reproduction

DP BIOLOGY · THEME D: CONTINUITY AND CHANGE

D3.1

Reproduction

Full notes for HL · IB-style

Contents

1	Sexual and asexual reproduction
2	Human reproductive systems
3	The menstrual cycle and its hormones
4	Fertilization and in vitro fertilization
5	Sexual reproduction in flowering plants
6	Pollination, fertilization and seed dispersal
7	Gametogenesis: spermatogenesis and oogenesis (HL only)
8	Pregnancy: hCG, the placenta and birth (HL only)
9	Sexual and asexual reproduction
10	Gametes and fertilization
11	The male reproductive system
12	The female reproductive system
13	The menstrual cycle: hormones
14	Negative and positive feedback in the cycle
15	Fertilization in humans
16	In vitro fertilization (IVF)
17	Sexual reproduction in flowering plants
18	Fertilization and seed development
19	Self- and cross-pollination
20	Seed dispersal and germination
21	Puberty
22	Mixed review: worked problems
23	Additional exam-style practice
24	Gametogenesis (HL)
25	Common misconceptions and links
26	Exam-style question with markscheme
27	Check your understanding
28	Summary and topic map

These are full teaching notes. Read each section in order, try the Think about it question before you read on, and do every Quick check before moving to the next section.

Before you start

Rate yourself on each statement now, and again after you've worked through the notes. Shade R (not yet), A (getting there) or G (confident).

I can…	R	A	G
compare sexual and asexual reproduction
describe the male and female reproductive systems
explain the roles of FSH, LH, oestradiol and progesterone in the menstrual cycle, including feedback
explain fertilization and the use of hormones in IVF
describe the structure of a flower
explain pollination, fertilization, seed development and dispersal
explain methods of promoting cross-pollination
compare spermatogenesis and oogenesis
explain the roles of hCG, the placenta and oxytocin in pregnancy and birth

Sections marked HL only (Sections 7 and 8) are Additional Higher Level content. Every other section is common to SL and HL.

Guiding questions

How do organisms produce offspring? How do hormones control the human menstrual cycle?

How this topic is examined

D3.1 is examined through hormone graphs of the menstrual cycle, diagrams of reproductive systems and flowers, IVF questions, and comparisons of sexual and asexual reproduction. HL questions include gametogenesis diagrams and pregnancy hormones.

Name each hormone, the gland that secretes it, its target and its effect.

Command term	What the examiner wants
State / Identify	a brief answer with no explanation needed
Outline	a brief account or summary
Describe	a detailed account, including trends or features, without reasons
Explain	a detailed account including causes, reasons or mechanisms
Distinguish / Compare and contrast	give the differences (and similarities), point by point
Calculate	a numerical answer with working and units
Deduce / Suggest	reach a conclusion from data; propose a reasonable explanation
Evaluate / Discuss	weigh strengths and limitations and reach a judgement

How to use these notes

Each section teaches one part of the syllabus in full. You'll see these boxes throughout; each has one job:

Box	What it's for
Think about it	a question to get you thinking before a section; the answer comes in the section
Key point	the one idea you must not forget
Common mistake	an error examiners see every year, and how to avoid it
In the real world	where the biology shows up outside the classroom
Try this	something quick you can do yourself to test the idea
Worked example	full solutions with IB-style marks (M1 method, A1 answer, R1 reasoning)
Quick check	short questions to test yourself at the end of each section

1 Sexual and asexual reproduction

Two strategies: copying or mixing.

Think about it

Many plants can reproduce both from seeds and by sending out runners. Why might it be useful to have both options?

Asexual reproduction involves one parent and produces offspring that are genetically identical to the parent and to each other (clones), by mitosis. Examples: binary fission in bacteria, budding in yeast, runners in strawberries, tubers in potatoes. It is fast, needs no partner, and passes on a successful combination of genes to all offspring. But all offspring are equally vulnerable to a new disease or environmental change.

Sexual reproduction involves the fusion of two haploid gametes, usually from two parents, to form a diploid zygote. Meiosis and fertilization produce offspring that are genetically different from each other and their parents. It is slower and needs energy to find mates, but the genetic variation it creates increases the chance that some offspring survive changes in the environment, and allows natural selection to act.

	Asexual	Sexual
Parents	one	usually two
Gametes	none	haploid gametes fuse
Cell division	mitosis	meiosis and mitosis
Offspring	genetically identical	genetically varied
Advantage	fast, no mate needed	variation helps adaptation

2 Human reproductive systems

Structures for producing, transporting and receiving gametes.

Male reproductive system: the testes produce sperm and the hormone testosterone; they are in the scrotum, which keeps them slightly cooler than body temperature. Sperm are stored in the epididymis, then travel through the sperm duct (vas deferens). The seminal vesicles and prostate gland add fluid, forming semen, which provides nutrients (fructose) and an alkaline medium. Semen leaves through the urethra in the penis.

Female reproductive system: the ovaries contain follicles that produce eggs (oocytes) and the hormones oestradiol and progesterone. Each month an egg is released into the oviduct (fallopian tube), where fertilization occurs. The uterus has a thick lining (endometrium) where an embryo implants and develops. The cervix at the base of the uterus leads to the vagina.

Structure	Function
Testis	produces sperm and testosterone
Epididymis	stores sperm; sperm mature
Sperm duct	carries sperm to urethra
Seminal vesicle, prostate	secrete fluid for semen
Ovary	produces eggs, oestradiol and progesterone
Oviduct	carries egg; site of fertilization
Uterus	implantation and development of embryo

3 The menstrual cycle and its hormones

Four hormones coordinate ovulation and the uterus lining.

The menstrual cycle, about 28 days, prepares the body for pregnancy each month. It is controlled by four hormones: two from the pituitary gland and two from the ovaries.

Follicle-stimulating hormone (FSH) from the pituitary stimulates the development of follicles in the ovary and their secretion of oestradiol. Oestradiol causes the endometrium to thicken and, at low levels, inhibits FSH (negative feedback). As the follicle grows, oestradiol rises to high levels, which stimulates a surge of luteinizing hormone (LH) (positive feedback). The LH surge triggers ovulation (release of the egg) at about day 14.

After ovulation, LH stimulates the empty follicle to develop into the corpus luteum, which secretes progesterone (and some oestradiol). Progesterone maintains the thickened endometrium and inhibits FSH and LH (negative feedback), so no new follicles develop. If fertilization does not occur, the corpus luteum degenerates, progesterone falls, the endometrium breaks down and is shed (menstruation), and FSH is no longer inhibited, so a new cycle begins.

Figure 1. Hormone levels during the menstrual cycle.

Hormone	Secreted by	Main effects
FSH	pituitary	follicle development; oestradiol secretion
LH	pituitary	surge triggers ovulation; corpus luteum forms
Oestradiol	follicle	thickens endometrium; low: inhibits FSH; high: stimulates LH surge
Progesterone	corpus luteum	maintains endometrium; inhibits FSH and LH

Key point

Oestradiol is the switch: low levels give negative feedback on FSH; high levels give positive feedback, causing the LH surge and ovulation.

Quick check

1. What triggers ovulation?

2. What secretes progesterone?

3. Why does menstruation occur?

Answers: 1. LH surge 2. corpus luteum 3. progesterone falls when the corpus luteum degenerates

4 Fertilization and in vitro fertilization

Fusion of gametes, inside or outside the body.

Fertilization is the fusion of a sperm nucleus with an egg nucleus, forming a diploid zygote. In humans it occurs in the oviduct. The sperm's acrosome releases enzymes that digest the outer layer of the egg; the sperm and egg membranes fuse; the egg then prevents other sperm entering (the cortical reaction); and the two haploid nuclei combine.

In vitro fertilization (IVF) helps couples who cannot conceive naturally. The steps are: (1) the woman takes drugs to suppress her natural cycle (stopping FSH and LH release); (2) she is given large doses of FSH to stimulate many follicles to develop (superovulation); (3) an injection of hCG (acting like LH) triggers egg maturation; (4) eggs are collected from the ovaries; (5) eggs are mixed with sperm in a dish (or a sperm is injected directly, ICSI); (6) after a few days, one or two embryos are transferred to the uterus; (7) progesterone is given to support the endometrium.

Quick check

1. Where does fertilization normally occur?

2. Why is FSH given during IVF?

Answers: 1. in the oviduct 2. to stimulate many follicles to develop so several eggs can be collected

5 Sexual reproduction in flowering plants

Flowers are adapted for pollination.

In flowering plants, the flower is the reproductive organ. The male parts are the stamens, each made of an anther (which produces pollen grains containing male gametes) on a filament. The female parts form the carpel (pistil): the stigma (which receives pollen), the style and the ovary, which contains ovules (each with a female gamete). Petals attract pollinators in insect-pollinated plants; sepals protect the bud.

Part	Function
Anther	produces pollen
Filament	supports anther
Stigma	sticky surface that receives pollen
Style	connects stigma to ovary
Ovary	contains ovules; becomes the fruit
Petal	attracts pollinators
Sepal	protects the flower bud

6 Pollination, fertilization and seed dispersal

From pollen grain to new plant.

Pollination is the transfer of pollen from an anther to a stigma. Cross-pollination (between different plants) increases genetic variation; self-pollination (within one plant) does not. Many plants promote cross-pollination: stamens and stigmas mature at different times, or are positioned so self-pollination is difficult, or the plant is self-incompatible (pollen from the same plant cannot fertilize it).

Pollen is carried by animals (insects, birds, bats: flowers are colourful, scented, with nectar) or by wind (flowers small, dull, with large feathery stigmas and lots of light pollen). Many plants depend on mutualism with pollinators (C4.1); declining bee populations threaten crops.

Fertilization: a pollen grain on a compatible stigma grows a pollen tube down the style to an ovule; a male gamete travels down the tube and fuses with the female gamete. The ovule develops into a seed (containing the embryo and a food store), and the ovary develops into a fruit.

Seed dispersal spreads seeds away from the parent, reducing competition and colonizing new areas: by wind (dandelion), animals (fruits eaten, or hooked seeds), water (coconut), or explosive pods. Germination requires water, oxygen and a suitable temperature.

Quick check

1. Define pollination.

2. Give two ways plants promote cross-pollination.

3. Why is seed dispersal important?

Answers: 1. transfer of pollen from anther to stigma 2. different maturation times; self-incompatibility; flower structure 3. reduces competition with parent; colonizes new areas

7 Gametogenesis: spermatogenesis and oogenesis (HL only)

Making sperm and eggs.

HL only

This section is Additional Higher Level content. SL students do not need it.

Spermatogenesis occurs in the seminiferous tubules of the testes. Germ cells (spermatogonia) divide by mitosis, then grow into primary spermatocytes, which undergo meiosis I to form secondary spermatocytes and meiosis II to form spermatids, which differentiate into sperm. Sertoli cells nourish developing sperm. The process is continuous from puberty, producing millions of sperm daily, with four sperm from each primary spermatocyte.

Oogenesis occurs in the ovaries. It begins before birth: oogonia divide by mitosis and grow into primary oocytes, which begin meiosis I but stop at prophase I. From puberty, each month one primary oocyte completes meiosis I, producing a large secondary oocyte and a small polar body (unequal cytokinesis, keeping most cytoplasm in the egg). The secondary oocyte is released at ovulation and completes meiosis II only if fertilized, producing a second polar body. So one egg is produced from each primary oocyte.

	Spermatogenesis	Oogenesis
Location	seminiferous tubules	ovaries
Products per primary cell	four sperm	one egg (plus polar bodies)
Cytokinesis	equal	unequal
Timing	continuous from puberty	begins before birth; one per month from puberty
Gamete size	small, motile	large, with food store

8 Pregnancy: hCG, the placenta and birth (HL only)

Hormones maintain pregnancy and trigger birth.

HL only

This section is Additional Higher Level content. SL students do not need it.

After fertilization, the embryo develops into a ball of cells (blastocyst) and implants in the endometrium about 7 days later. It secretes human chorionic gonadotropin (hCG), which maintains the corpus luteum, so progesterone secretion continues and the endometrium is not shed. Pregnancy tests detect hCG in urine.

The placenta develops from embryonic and maternal tissue. Fetal blood in villi comes close to maternal blood, allowing exchange of oxygen, nutrients, CO₂ and wastes without the bloods mixing. From about 12 weeks, the placenta itself secretes progesterone and oestradiol, taking over from the corpus luteum.

At the end of pregnancy, progesterone falls and oxytocin from the pituitary stimulates uterine contractions. Contractions stretch the cervix, which stimulates more oxytocin release: a positive feedback loop that intensifies contractions until birth.

Quick check

1. Why is hCG important in early pregnancy?

2. Why is birth an example of positive feedback?

Answers: 1. maintains corpus luteum, so progesterone continues 2. contractions trigger more oxytocin, which increases contractions

9 Sexual and asexual reproduction

Advantages of each.

Feature	Asexual	Sexual
parents	one	two (usually)
offspring	genetically identical clones	genetically varied
speed	rapid	slower
advantage	efficient in stable conditions	variation helps adaptation to change
examples	bacteria, strawberry runners	humans, flowering plants

10 Gametes and fertilization

Haploid to diploid.

Gametes are haploid (n) cells produced by meiosis. Fertilization fuses male and female gametes to form a diploid (2n) zygote. In humans, n = 23 and 2n = 46.

WORKED EXAMPLE 1 Chromosome number (basic)

A species has 2n = 38. How many chromosomes are in its sperm?

Solution

19A1

11 The male reproductive system

Structures and functions.

Structure	Function
testis	produces sperm and testosterone
epididymis	stores sperm; sperm mature
sperm duct (vas deferens)	carries sperm to urethra
seminal vesicle and prostate	secrete fluid with nutrients (fructose) and alkaline fluid
urethra	carries semen out of the body
penis	delivers sperm into vagina

12 The female reproductive system

Structures and functions.

Structure	Function
ovary	produces eggs, oestradiol and progesterone
oviduct	site of fertilization; carries egg to uterus
uterus	site of implantation and fetal development
cervix	entrance to uterus; dilates at birth
vagina	receives sperm; birth canal

13 The menstrual cycle: hormones

FSH, LH, oestradiol, progesterone.

FSH from the pituitary stimulates follicle development and oestradiol secretion. Rising oestradiol thickens the endometrium and, at high levels, triggers an LH surge (positive feedback), causing ovulation around day 14. The corpus luteum secretes progesterone, maintaining the endometrium. If no pregnancy occurs, progesterone falls, and menstruation begins.

WORKED EXAMPLE 2 LH surge (exam level)

Explain the cause and effect of the LH surge.

Solution

High oestradiol stimulates LH secretion (positive feedback)R1

LH triggers ovulation and corpus luteum formationR1

Quick check

1. Which hormone maintains the endometrium after ovulation?

Answers: 1. progesterone

14 Negative and positive feedback in the cycle

Hormonal control.

Progesterone and oestradiol inhibit FSH and LH secretion (negative feedback), preventing new follicles from developing in the second half of the cycle. High oestradiol before ovulation switches to positive feedback on LH.

Quick check

1. Why do hormonal contraceptives prevent ovulation?

Answers: 1. synthetic hormones inhibit FSH and LH by negative feedback

15 Fertilization in humans

Sperm meets egg.

Sperm swim through the cervix and uterus into the oviduct. The acrosome reaction releases enzymes that digest the zona pellucida; a sperm membrane fuses with the egg membrane. The cortical reaction hardens the zona pellucida, preventing polyspermy.

WORKED EXAMPLE 3 Polyspermy (standard)

Explain how the egg prevents fertilization by more than one sperm.

Solution

Cortical granules release enzymes after fusionR1

Zona pellucida hardensR1

16 In vitro fertilization (IVF)

Assisted reproduction.

IVF: hormone treatment stimulates multiple follicles; eggs are collected and fertilized with sperm in a dish (or by sperm injection, ICSI); embryos are cultured for a few days and one or two are transferred to the uterus.

WORKED EXAMPLE 4 Hormones in IVF (standard)

Why are FSH injections given in IVF?

Solution

Stimulate many follicles to developR1

So several eggs can be collectedR1

In the real world

Since the first IVF baby was born in 1978, more than 10 million babies have been born using IVF.

17 Sexual reproduction in flowering plants

Flowers, pollen and ovules.

Flowers contain stamens (anther and filament; produce pollen) and carpels (stigma, style, ovary containing ovules). Pollination transfers pollen from anther to stigma, by insects, wind or animals.

WORKED EXAMPLE 5 Insect pollination (standard)

Give two features of insect-pollinated flowers.

Solution

Brightly coloured petals and scentA1

Nectar; sticky pollen; stigma inside flowerA1

18 Fertilization and seed development

From flower to seed.

A pollen tube grows down the style; the male nucleus fertilizes the egg in the ovule. The ovule develops into a seed (embryo, food store, testa); the ovary develops into a fruit, which helps seed dispersal.

Quick check

1. What develops from the ovary after fertilization?

Answers: 1. the fruit

19 Self- and cross-pollination

Promoting variation.

Cross-pollination between different plants increases genetic variation. Plants prevent self-pollination by self-incompatibility genes, separate male and female flowers, or maturing anthers and stigmas at different times.

Quick check

1. Why is cross-pollination advantageous?

Answers: 1. increases genetic variation; reduces inbreeding

20 Seed dispersal and germination

Spreading offspring.

Seeds are dispersed by wind, animals, water or explosive mechanisms, reducing competition with the parent. Germination requires water, oxygen and a suitable temperature; water activates enzymes such as amylase to mobilize food stores.

WORKED EXAMPLE 6 Germination (standard)

State three conditions needed for germination.

Solution

Water, oxygen, suitable temperatureA2

21 Puberty

Hormonal changes.

Puberty is triggered when the hypothalamus increases GnRH secretion, stimulating FSH and LH. Testosterone (males) and oestradiol (females) cause secondary sexual characteristics.

Quick check

1. Name the hormone from the hypothalamus that starts puberty.

Answers: 1. GnRH

22 Mixed review: worked problems

Bringing the ideas together.

WORKED EXAMPLE 7 Mixed problem (exam level)

Explain the roles of FSH and LH in the menstrual cycle.

Solution

FSH stimulates follicle development and oestradiolR1

LH surge causes ovulationR1

LH stimulates corpus luteum to secrete progesteroneR1

23 Additional exam-style practice

Paper 1 and Paper 2.

WORKED EXAMPLE 8 Paper 1 question (standard)

Which hormone surge causes ovulation? A. FSH B. LH C. progesterone D. oestradiol

Solution

BA1

WORKED EXAMPLE 9 Paper 2 question (exam level)

Compare sexual and asexual reproduction.

Solution

Asexual: one parent, identical offspring, fastR1

Sexual: two parents, varied offspring, slowerR1

24 Gametogenesis (HL)

Spermatogenesis and oogenesis.

Spermatogenesis in seminiferous tubules produces four sperm per meiosis, continuously from puberty. Oogenesis produces one egg and polar bodies per meiosis; it begins before birth and is completed only if fertilization occurs. Unequal cytokinesis gives the egg most cytoplasm.

WORKED EXAMPLE 10 Comparing (exam level)

Compare spermatogenesis and oogenesis.

Solution

Both involve mitosis, growth and meiosisR1

Sperm: 4 per meiosis, continuous; egg: 1 per meiosis with polar bodies, cyclicalR1

25 Common misconceptions and links

Students often think…	Actually…
asexual reproduction is always worse	it is fast and efficient in stable environments
oestradiol only has negative feedback	high oestradiol triggers the LH surge (positive feedback)
pollination and fertilization are the same	pollination transfers pollen; fertilization fuses gametes
oogenesis produces four eggs	it produces one egg and polar bodies

How D3.1 connects to the rest of the course

Topic	Connection
D2.1 Meiosis	gamete production
C2.1 Signalling (HL)	steroid hormones
D3.3 Homeostasis	feedback
D3.2 Inheritance	fertilization combines alleles
C4.1 Communities	pollinator mutualism

26 Exam-style question with markscheme

Try this under exam conditions (about 18 minutes) before reading the markscheme.

Figure 2. Hormone levels in the menstrual cycle.

(a) Identify the hormone that peaks at day 14. [1]

(b) Explain the role of oestradiol in the first half of the cycle. [3]

(d) Outline why FSH is injected during IVF. [2]

(e) Compare sexual and asexual reproduction. [3]

(f) Compare spermatogenesis and oogenesis. [3]

Markscheme

Part	Answer	Marks
(a)	LH	A1
(b)	thickens endometrium	R1
	low levels inhibit FSH	R1
	high levels stimulate LH surge	R1
(c)	corpus luteum degenerates	R1
	no hCG to maintain it	R1
(d)	stimulates many follicles	R1
	several eggs collected	R1
(e)	one vs two parents	A1
	identical vs varied offspring	A1
	mitosis vs meiosis and fertilization	A1
(f)	four sperm vs one egg	A1
	equal vs unequal cytokinesis	A1
	continuous vs monthly / begins before birth	A1

Examiner's comments. In (b), the best answers explained both the negative and the positive feedback roles of oestradiol.

27 Check your understanding

Section A: multiple choice

1. Ovulation is triggered by

A. a surge of LH B. a fall in FSH C. progesterone D. menstruation

2. Progesterone is secreted by

A. the pituitary B. the uterus C. the corpus luteum D. the follicle before ovulation

3. FSH stimulates

A. ovulation B. follicle development C. menstruation D. endometrium shedding

4. Fertilization in humans occurs in the

A. uterus B. ovary C. vagina D. oviduct

5. The anther produces

A. ovules B. pollen C. nectar D. seeds

6. Asexual reproduction produces offspring that are

A. genetically identical B. genetically varied C. haploid D. always weaker

7. hCG maintains

A. the follicle B. the pituitary C. the placenta only D. the corpus luteum

8. Oogenesis produces

A. four eggs B. two eggs C. one egg and polar bodies D. sperm

Section B: structured

9. Explain the roles of hormones in the menstrual cycle. [6]

10. Outline the stages of IVF. [4]

11. Explain how flowers promote cross-pollination. [3]

12. Explain the roles of hCG and the placenta in pregnancy. [4]

Answers

1. A 2. C 3. B 4. D 5. B 6. A 7. D 8. C

9. FSH follicle; oestradiol endometrium and feedback; LH surge ovulation; progesterone maintains lining; negative feedback; fall causes menstruation.

10. Suppress cycle; FSH injections; hCG; egg collection; fertilization in dish; embryo transfer; progesterone.

11. Different maturation times; positioning; self-incompatibility; animal or wind pollination.

12. hCG maintains corpus luteum and progesterone; placenta exchanges materials and later secretes progesterone.

28 Summary and topic map

Idea	Remember
Sexual vs asexual	variation vs clones
Male system	testes, epididymis, sperm duct, glands
Female system	ovaries, oviducts, uterus
Menstrual cycle	FSH, LH, oestradiol, progesterone; feedback
IVF	suppress, FSH, hCG, collect, fertilize, transfer
Flowers	anther, stigma, ovary
Plant reproduction	pollination, fertilization, seeds, dispersal
HL	spermatogenesis vs oogenesis; hCG; placenta; oxytocin

Before the exam, make sure you…

•link each hormone to its source, target and effect
•explain both types of feedback in the cycle
•distinguish pollination from fertilization
•compare sexual and asexual reproduction

Reproduction on one page

Cover this page with a sheet of paper and try to rebuild it from memory. Whatever you can't recall is what to revise next.

Figure 3. Topic map for Reproduction.

revisionprep.comIB-style notes · question bank · mock papers