Physics Made Easy

Reproduction

Asexual reproduction: only one individual involving, so no mixing of genetic information, with clones produced (e.g. mitosis). Evolution is slow, with variation only occurring due to mutation; therefore a population produced by asexual reproduction.

On the positive side, no fertilisation is required, avoiding the need to find a mate, allowing for reproduction outside the growing season (in plants) and leading to a rapid increase in population numbers.

Examples of asexual reproduction in animals

  1. Budding: individual grows out of parents.
  2. Parthenogenesis (‘virgin birth’): unfertilised egg spontaneously develops into new individual.
  3. Regeneration of damaged/fragmented body, e.g. into new individuals.

Examples of asexual reproduction in plants

  1. Cuttings: fragments of plant can regenerate into whole individuals.
  2. Rhizomes: underground stems produce new plants.
  3. Corns, bulbs and tubers: modified shoots adapted for food storage can divide to produce more individuals.

Sexual reproduction: one or two individuals, usually of the same species are involved. Gametes (sex cells) are formed by meiosis, and genetic information is then recombined at fertilisation, leading to variation. This means that populations can evolve and adapt more readily to changes, but at the same time sexual reproduction is a slower process; plants can only reproduce during the growing season, whilst animals are vulnerable during copulation and when giving birth.

External fertilisation occurs in an aquatic environment. Sperm and eggs are both released into water; the sperm are mobile and can swim to the eggs and fertilise them. Large numbers of gametes must be produced as many sperm will not reach the eggs and some will be eaten.

Internal fertilisation: as some creatures evolved to live on land, external fertilisation could no longer take place; instead, specialised organs and complex behavioural patterns evolved to allow copulation. Fertilisation takes place within the female, where the gametes are less vulnerable and more likely to meet.

In birds, the fertilised egg develops inside a waterproof, protective covering- safer than external fertilisation, but still vulnerable. In mammals, the embryo develops inside the female, who gives birth to live young.

Structure of testes: a single testis contains many seminiferous tubules. The walls of these tubules are lined with germinal epithelial cells, which will eventually form spermatozoa (male gametes). Between these tubules are blood capillaries, connective tissue and interstitial cells which secrete male sex hormones.

Spermatogenesis (formation of male gametes in mammals) takes 8-9 weeks. It is a continuous process that occurs from puberty until death, and is controlled by the hypothalamus and pituitary gland.

  1. Germinal epithelial cells (diploid) undergo many mitotic divisions (multiplication).
  2. This results in many primary spermatocytes (diploid), which undergo some growth before moving towards the lumen of the tubules.
  3. The primary spermatocytes undergo meiosis I to form short-lived secondary spermatocytes (haploid).
  4. Secondary spermatocytes undergo meiosis II to form spermatids (haploid).
  5. Spermatids develop tails and mature to form spermatozoa (haploid).

Sertoli cells: cells which span the depth of the walls of the seminiferous tubules. Mature spermatozoa have their heads in Sertoli cells, which nourish them. Sertoli cells also secrete the fluid which fills the lumen and phagocytose (eat) foreign particles.

Epididymus: spermatozoa are stored here after they become detached from Sertoli cells. If they are not ejaculated, they will degenerate and become reabsorbed.

Structure of the ovaries: there is a layer of germinal epithelium over the surface. Within, there are thousands of follicles in various stages of development (only a few hundred complete development); female gametes are protected inside the follicle. Between these follicles is a fibrous tissue called the stroma.

Oogenesis (formation of female gametes in mammals): in humans, this starts at puberty and occurs once every 28 days (or thereabouts) until the menopause. Unsurprisingly, this process is also controlled by the hypothalamus and pituitary gland.

  1. Germinal epithelial cells (diploid) undergo mitosis (multiplication) before birth, forming many oogonia.
  2. The oogonia undergo significant growth, forming primary oocytes protected in primary follicle.
  3. Primary oocytes undergo meiosis I, forming a large secondary oocyte plus a small polar body with little cytoplasm, both protected inside a Graafian follicle.
  4. If fertilisation occurs, then meiosis II will also take place, forming a mature ovum and three small polar bodies. Otherwise, the follicle ruptures and only a corpus luteum remains (ovulation). As the mature ovum retains most of the cytoplasm during meiosis, it possesses all the nutrients it requires within the cell.

Movement of sperm during and after ejaculation: sperm are moved by muscular activity. Fluid is added from seminal vesicles, prostate and Cowpers glands to form semen. The fluid is alkaline to neutralise urea and acidic conditions in the vagina, and also contains nutrients. The sperm are now activated, increasing fertility.

About 3cm3 is deposited at the top end of the vagina, where sperm can survive for 1-2 days. There, they swim to the oviducts, aided by cilia in the uterus which ‘waft’ them in the right direction. It takes 4-8 hours to reach the oviducts.

Capacitation: enzymes in the uterus remove the layer of glycoprotein around the sperm to enable fertilisation. This process takes 6-7 hours.

Fertilisation: sperm come into contact with the secondary oocyte by random movement. When the head of the sperm reaches the zona pellucida (jelly-like glycoprotein coating of the oocyte), the acrosome opens, releasing enzymes which soften the glycoprotein so that the head can pass through; the tail is discarded. Vesicles in the outer part of the secondary oocyte’s cytoplasm discharge their contents by exocytosis in the space between the plasmamembrane and the jelly coating to prevent other sperm from entering.

The secondary oocyte undergoes its second meiotic division to produce the ovum. The ovum nucleus fuses with the sperm nucleus at fertilisation.

Structure of sperm cell:

sperm1.jpg

Structure of secondary oocyte:
oocyte.jpg

Female reproductive cycle is divided into three main stages.

  1. Follicular stage is controlled by FSH (follicle stimulating hormone), which is released by the pituitary gland. FSH stimulates the maturation of a primary oocyte in the ovaries into a Graafian follicle containing a secondary oocyte.

Follicle cells secrete oestrogen which stimulates the growth of endometrium (uterus lining) and blood supply. Oestrogen also stimulates the pituitary to secrete LH (luteinising hormone) and inhibits the production of FSH.

  1. Ovulation: The mature Graafian follicle secretes oestrogen until the LH stored in the pituitary gland is released. LH causes ovulation- the Graafian follicle bursts and the secondary oocyte is released. Cilia at the entrance of the oviducts “waft” the secondary oocyte into the oviduct. The remains of the Graafian follicle are known as the corpus luteum, which produces some oestrogen and a lot of progesterone.
  2. Luteal phase: Increasing progesterone levels inhibit FSH and LH, whilst maintaining the endometrium and blood supply to uterus. The corpus luteum eventually shrivels without FSH and LH to maintain it; progesterone and oestrogen levels fall, lifting the inhibition on FSH production, so the cycle can start again.

Menstruation: when progesterone levels fall, blood vessels in the uterus lining contract, cutting off the blood supply so that the cells of the uterus lining die. The blood vessels then dilate to push the dead cells away from the wall, causing menstruation.

Summary diagram of female reproductive cycle:
female-reproductive-cycle.jpg

Oestrus: behavioural and physiological changes that occur in many female mammals during ovulation, as an indication that mating at this time is most likely to lead to fertilisation; too long before ovulation and the sperm will die before the egg has matured, too late and the egg will have died without encountering any sperm.

Benefits of oestrus: many females are only fertile for a short period of time- oestrus gives males a clear indication of this time so that the female is more likely to mate and conceive.

Oestrus may occur at a certain time of year -triggered by day length or lunar cycle- to ensure that young are born in a season where they can be provided for, or so that all females of a herd bear young at the same time (safety in numbers- the majority of offspring should survive).

Detecting oestrus in farm animals ensures that farmers know when mating is likely to lead to conception, so they can make arrangements for mating or artificial insemination to take place during that time. For example, female pigs in oestrus show the following signs: red, swollen vulva, mucous produced from vagina, characteristic grunt or roar, sniffing of genital areas of other pigs and “standing reflex” (they stand rigid when pressure is applied to the lower back, especially if a boar is present).

Causes of infertility

  1. Males: low sperm count or abnormal sperm, blocked vas deferens (sperm cannot leave testes) or impotence (failure to get an erection).
  2. Females: poor ovulation (on average, ovulation takes place in eight out of ten cycles.

Treatment of infertility in females

  1. Test for ovulation by looking for high progesterone levels in the second half of the cycle (due to corpus luteum).
  2. Ovulation is usually prevented by low FSH levels (follicle does not develop). Treatment with a drug called clomiphine prevents FSH inhibition by oestrogen secreted by any developed follicle cells that have not been ovulated. Natural FSH levels should now get high enough to allow the reproductive cycle to occur.
  3. In extreme cases FSH may be given, but this often leads to multiple births.

Contraception

  1. The combined oral contraceptive pill mimics pregnancy. It keeps progesterone levels high to inhibit FSH production and thus follicle development. The oestrogen concentration is kept as low as possible to reduce the risk of thrombosis, strokes and heart attacks.
  2. The mini-pill: with this, ovulation still occurs, but thick mucus is constantly produced to prevent sperm from getting through. It is believed that it may prevent the division of the secondary oocyte or implantation of the fertilised egg.

Early embryo development

  1. Cleavage: the fertilised egg divides rapidly to form a ball of cells (blastula), but there is no overall growth. Up to the eight cell stage, individual cells still have the potential to become separate organisms.

Once in a particular position in a blastula, genetic information is “switched on” or “off”, so that each cell will now develop into a particular type of tissue.

  1. Gastrulation: cells begin to migrate towards the centre of the ball, and begin to differentiate into the tissues they will become. The ball of cells is now as gastrula.
  2. Implantation: the remnants of the follicles produce human chorionic gonadotrophin (HCG) which prevents the corpus luteum from degenerating. This maintains high progesterone levels, ensuring that the uterus lining does not break down and inhibiting FSH production. The gastrula implants in the uterus and the placenta begins to develop; pregnancy has started.

Structure of the placenta: a series of very thin membrane layers which are very close together. The surface area is increased by villi bearing microvilli. Blood on the maternal side is at high pressure, whilst blood on the foetal side is at low pressure; the two bloods never come into direct contact.

Flower structure in dicotyledonous plants
dicotyledonous-plant.jpg

Formation of mature pollen grains

  1. Numerous diploid pollen mother cells inside the anther (sporophyte generation) undergo meiosis.
  2. Each pollen mother cell produces a tetrad of four haploid microspores (gametophytes) which develop into mature pollen grains (haploid).
  3. The pollen grain nucleus undergoes mitosis to form two haploid nuclei within the grain- the pollen tube nucleus and the generative nucleus.
  4. During fertilisation, the generative nucleus will undergo mitosis to form two male gametes (more on this later).

The anther bursts open to release the mature pollen grains (dehiscence). Note that the nuclei are the gametes, not the pollen grain.

Formation of mature embryosac

  1. Within the ovule, one cell from the nucellus (a mass of diploid cells) enlarges to form an embryosac mother cell.
  2. Embryosac mother cell undergoes meiosis to form four haploid megaspores, three of which are degenerate.
  3. The remaining megaspore enlarges to form the embryosac (gametophyte).
  4. The embryosac nucleus undergoes mitosis to form two nuclei at opposite ends of the embryosac.
  5. Each nucleus undergoes two mitotic division to form four haploid nuclei at each end of the embryosac.
  6. Two of these nuclei migrate to the middle of the embryosac, leaving us with three nuclei at the north end which play no part and two polar nuclei in the centre. At the south end, the middle nucleus becomes the ovum (gamete), whilst the other two nuclei may produce enxymes which dissolve tissue to allow pollen tube entry.

Insect (entomophilous) pollination: the flower is shaped to ensure pollen brushes off onto the insect and can be transferred to the stigma of another flower. Flowers ‘advertise’ by having large numbers of petals, often brightly coloured with a pleasant smell. Insects are tempted into the flower by nectar or the pollen itself; pollen grains are sticky or spiky so that they will attach to the insect.

Wind (anemophilous) pollination: anthers ripen in dry weather, and large amounts of light pollen are produced (a lot will be wasted). Anther bursts open to release pollen; stigmas are often feathery and hang outside the flower to catch the pollen as it blows past.

Reproduction

Asexual reproduction: only one individual involving, so no mixing of genetic information, with clones produced (e.g. mitosis). Evolution is slow, with variation only occurring due to mutation; therefore a population produced by asexual reproduction.

On the positive side, no fertilisation is required, avoiding the need to find a mate, allowing for reproduction outside the growing season (in plants) and leading to a rapid increase in population numbers.

Examples of asexual reproduction in animals

  1. Budding: individual grows out of parents.
  2. Parthenogenesis (‘virgin birth’): unfertilised egg spontaneously develops into new individual.
  3. Regeneration of damaged/fragmented body, e.g. into new individuals.

Examples of asexual reproduction in plants

  1. Cuttings: fragments of plant can regenerate into whole individuals.
  2. Rhizomes: underground stems produce new plants.
  3. Corns, bulbs and tubers: modified shoots adapted for food storage can divide to produce more individuals.

Sexual reproduction: one or two individuals, usually of the same species are involved. Gametes (sex cells) are formed by meiosis, and genetic information is then recombined at fertilisation, leading to variation. This means that populations can evolve and adapt more readily to changes, but at the same time sexual reproduction is a slower process; plants can only reproduce during the growing season, whilst animals are vulnerable during copulation and when giving birth.

External fertilisation occurs in an aquatic environment. Sperm and eggs are both released into water; the sperm are mobile and can swim to the eggs and fertilise them. Large numbers of gametes must be produced as many sperm will not reach the eggs and some will be eaten.

Internal fertilisation: as some creatures evolved to live on land, external fertilisation could no longer take place; instead, specialised organs and complex behavioural patterns evolved to allow copulation. Fertilisation takes place within the female, where the gametes are less vulnerable and more likely to meet.

In birds, the fertilised egg develops inside a waterproof, protective covering- safer than external fertilisation, but still vulnerable. In mammals, the embryo develops inside the female, who gives birth to live young.

Structure of testes: a single testis contains many seminiferous tubules. The walls of these tubules are lined with germinal epithelial cells, which will eventually form spermatozoa (male gametes). Between these tubules are blood capillaries, connective tissue and interstitial cells which secrete male sex hormones.

Spermatogenesis (formation of male gametes in mammals) takes 8-9 weeks. It is a continuous process that occurs from puberty until death, and is controlled by the hypothalamus and pituitary gland.

  1. Germinal epithelial cells (diploid) undergo many mitotic divisions (multiplication).
  2. This results in many primary spermatocytes (diploid), which undergo some growth before moving towards the lumen of the tubules.
  3. The primary spermatocytes undergo meiosis I to form short-lived secondary spermatocytes (haploid).
  4. Secondary spermatocytes undergo meiosis II to form spermatids (haploid).
  5. Spermatids develop tails and mature to form spermatozoa (haploid).

Sertoli cells: cells which span the depth of the walls of the seminiferous tubules. Mature spermatozoa have their heads in Sertoli cells, which nourish them. Sertoli cells also secrete the fluid which fills the lumen and phagocytose (eat) foreign particles.

Epididymus: spermatozoa are stored here after they become detached from Sertoli cells. If they are not ejaculated, they will degenerate and become reabsorbed.

Structure of the ovaries: there is a layer of germinal epithelium over the surface. Within, there are thousands of follicles in various stages of development (only a few hundred complete development); female gametes are protected inside the follicle. Between these follicles is a fibrous tissue called the stroma.

Oogenesis (formation of female gametes in mammals): in humans, this starts at puberty and occurs once every 28 days (or thereabouts) until the menopause. Unsurprisingly, this process is also controlled by the hypothalamus and pituitary gland.

  1. Germinal epithelial cells (diploid) undergo mitosis (multiplication) before birth, forming many oogonia.
  2. The oogonia undergo significant growth, forming primary oocytes protected in primary follicle.
  3. Primary oocytes undergo meiosis I, forming a large secondary oocyte plus a small polar body with little cytoplasm, both protected inside a Graafian follicle.
  4. If fertilisation occurs, then meiosis II will also take place, forming a mature ovum and three small polar bodies. Otherwise, the follicle ruptures and only a corpus luteum remains (ovulation). As the mature ovum retains most of the cytoplasm during meiosis, it possesses all the nutrients it requires within the cell.

Movement of sperm during and after ejaculation: sperm are moved by muscular activity. Fluid is added from seminal vesicles, prostate and Cowpers glands to form semen. The fluid is alkaline to neutralise urea and acidic conditions in the vagina, and also contains nutrients. The sperm are now activated, increasing fertility.

About 3cm3 is deposited at the top end of the vagina, where sperm can survive for 1-2 days. There, they swim to the oviducts, aided by cilia in the uterus which ‘waft’ them in the right direction. It takes 4-8 hours to reach the oviducts.

Capacitation: enzymes in the uterus remove the layer of glycoprotein around the sperm to enable fertilisation. This process takes 6-7 hours.

Fertilisation: sperm come into contact with the secondary oocyte by random movement. When the head of the sperm reaches the zona pellucida (jelly-like glycoprotein coating of the oocyte), the acrosome opens, releasing enzymes which soften the glycoprotein so that the head can pass through; the tail is discarded. Vesicles in the outer part of the secondary oocyte’s cytoplasm discharge their contents by exocytosis in the space between the plasmamembrane and the jelly coating to prevent other sperm from entering.

The secondary oocyte undergoes its second meiotic division to produce the ovum. The ovum nucleus fuses with the sperm nucleus at fertilisation.

Structure of sperm cell:

Female reproductive cycle is divided into three main stages.

  1. Follicular stage is controlled by FSH (follicle stimulating hormone), which is released by the pituitary gland. FSH stimulates the maturation of a primary oocyte in the ovaries into a Graafian follicle containing a secondary oocyte.

Follicle cells secrete oestrogen which stimulates the growth of endometrium (uterus lining) and blood supply. Oestrogen also stimulates the pituitary to secrete LH (luteinising hormone) and inhibits the production of FSH.

  1. Ovulation: The mature Graafian follicle secretes oestrogen until the LH stored in the pituitary gland is released. LH causes ovulation- the Graafian follicle bursts and the secondary oocyte is released. Cilia at the entrance of the oviducts “waft” the secondary oocyte into the oviduct. The remains of the Graafian follicle are known as the corpus luteum, which produces some oestrogen and a lot of progesterone.
  2. Luteal phase: Increasing progesterone levels inhibit FSH and LH, whilst maintaining the endometrium and blood supply to uterus. The corpus luteum eventually shrivels without FSH and LH to maintain it; progesterone and oestrogen levels fall, lifting the inhibition on FSH production, so the cycle can start again.

Menstruation: when progesterone levels fall, blood vessels in the uterus lining contract, cutting off the blood supply so that the cells of the uterus lining die. The blood vessels then dilate to push the dead cells away from the wall, causing menstruation.

Summary

Oestrus: behavioural and physiological changes that occur in many female mammals during ovulation, as an indication that mating at this time is most likely to lead to fertilisation; too long before ovulation and the sperm will die before the egg has matured, too late and the egg will have died without encountering any sperm.

Benefits of oestrus: many females are only fertile for a short period of time- oestrus gives males a clear indication of this time so that the female is more likely to mate and conceive.

Oestrus may occur at a certain time of year –triggered by day length or lunar cycle- to ensure that young are born in a season where they can be provided for, or so that all females of a herd bear young at the same time (safety in numbers- the majority of offspring should survive).

Detecting oestrus in farm animals ensures that farmers know when mating is likely to lead to conception, so they can make arrangements for mating or artificial insemination to take place during that time. For example, female pigs in oestrus show the following signs: red, swollen vulva, mucous produced from vagina, characteristic grunt or roar, sniffing of genital areas of other pigs and “standing reflex” (they stand rigid when pressure is applied to the lower back, especially if a boar is present).

Causes of infertility

  1. Males: low sperm count or abnormal sperm, blocked vas deferens (sperm cannot leave testes) or impotence (failure to get an erection).
  2. Females: poor ovulation (on average, ovulation takes place in eight out of ten cycles.

Treatment of infertility in females

  1. Test for ovulation by looking for high progesterone levels in the second half of the cycle (due to corpus luteum).
  2. Ovulation is usually prevented by low FSH levels (follicle does not develop). Treatment with a drug called clomiphine prevents FSH inhibition by oestrogen secreted by any developed follicle cells that have not been ovulated. Natural FSH levels should now get high enough to allow the reproductive cycle to occur.
  3. In extreme cases FSH may be given, but this often leads to multiple births.

Contraception

  1. The combined oral contraceptive pill mimics pregnancy. It keeps progesterone levels high to inhibit FSH production and thus follicle development. The oestrogen concentration is kept as low as possible to reduce the risk of thrombosis, strokes and heart attacks.
  2. The mini-pill: with this, ovulation still occurs, but thick mucus is constantly produced to prevent sperm from getting through. It is believed that it may prevent the division of the secondary oocyte or implantation of the fertilised egg.

Early embryo development

  1. Cleavage: the fertilised egg divides rapidly to form a ball of cells (blastula), but there is no overall growth. Up to the eight cell stage, individual cells still have the potential to become separate organisms.

Once in a particular position in a blastula, genetic information is “switched on” or “off”, so that each cell will now develop into a particular type of tissue.

  1. Gastrulation: cells begin to migrate towards the centre of the ball, and begin to differentiate into the tissues they will become. The ball of cells is now as gastrula.
  2. Implantation: the remnants of the follicles produce human chorionic gonadotrophin (HCG) which prevents the corpus luteum from degenerating. This maintains high progesterone levels, ensuring that the uterus lining does not break down and inhibiting FSH production. The gastrula implants in the uterus and the placenta begins to develop; pregnancy has started.

Structure of the placenta: a series of very thin membrane layers which are very close together. The surface area is increased by villi bearing microvilli. Blood on the maternal side is at high pressure, whilst blood on the foetal side is at low pressure; the two bloods never come into direct contact.

Functions of the placenta: the placenta acts as the exchange surface between the mother and the developing foetus. Exchange is rapid but selective, using the mechanisms of diffusion and active transport; nutrients pass into the embryo, waste products pass out.

Foetal haemoglobin has a higher affinity for oxygen than the mother’s haemoglobin, so oxygen is taken up easily. Some antibodies pass through, giving the foetus passive immunity. Viruses and chemicals can sometimes pass through as well (not desirable).

In early foetal development, the placenta stores glycogen for emergency use by the foetus if necessary. The placenta also becomes an endocrine organ, secreting oestrogen and progesterone in place of the corpus luteum.

Flower structure in dicotyledenous plants

Formation of mature pollen grains

  1. Numerous diploid pollen mother cells inside the anther (sporophyte generation) undergo meiosis.
  2. Each pollen mother cell produces a tetrad of four haploid microspores (gametophytes) which develop into mature pollen grains (haploid).
  3. The pollen grain nucleus undergoes mitosis to form two haploid nuclei within the grain- the pollen tube nucleus and the generative nucleus.
  4. During fertilisation, the generative nucleus will undergo mitosis to form two male gametes (more on this later).

The anther bursts open to release the mature pollen grains (dehiscence). Note that the nuclei are the gametes, not the pollen grain.

Formation of mature embryosac

  1. Within the ovule, one cell from the nucellus (a mass of diploid cells) enlarges to form an embryosac mother cell.
  2. Embryosac mother cell undergoes meiosis to form four haploid megaspores, three of which are degenerate.
  3. The remaining megaspore enlarges to form the embryosac (gametophyte).
  4. The embryosac nucleus undergoes mitosis to form two nuclei at opposite ends of the embryosac.
  5. Each nucleus undergoes two mitotic division to form four haploid nuclei at each end of the embryosac.
  6. Two of these nuclei migrate to the middle of the embryosac, leaving us with three nuclei at the north end which play no part and two polar nuclei in the centre. At the south end, the middle nucleus becomes the ovum (gamete), whilst the other two nuclei may produce enxymes which dissolve tissue to allow pollen tube entry.

Insect (entomophilous) pollination: the flower is shaped to ensure pollen brushes off onto the insect and can be transferred to the stigma of another flower. Flowers ‘advertise’ by having large numbers of petals, often brightly coloured with a pleasant smell. Insects are tempted into the flower by nectar or the pollen itself; pollen grains are sticky or spiky so that they will attach to the insect.

Wind (anemophilous) pollination: anthers ripen in dry weather, and large amounts of light pollen are produced (a lot will be wasted). Anther bursts open to release pollen; stigmas are often feathery and hang outside the flower to catch the pollen as it blows past.

Mechanisms to ensure cross-pollination: plants ‘prefer’ cross pollination because it increases the variation of offspring (they get genetic information from two individuals instead of just one). There are three main mechanisms.

  1. Protandry: stamens develop and shed pollen from anther before stigmas develop in any given flower head.
  2. Protogyny: stigma develops before stamen in any given flower head.
  3. Heterostyly: flowers have two different forms (polymorphism) e.g. primroses

Insect visits thrum-eyed form, pollen brushes off onto its body. If it then subsequently visits the pin-eyed form, the pollen brushes off onto the stigma -> thrum-eyed flowers pollinating pin-eyed flowers.

Fertilisation in plants

  1. Pollen lands on stigma.
  2. Pollen grain absorbs water from stigma by osmosis. It swells and bulges out to form a pollen tube which penetrates the stigma, and grows down the style to the ovule.
  3. The tube nucleus “leads the way”, controlling the tube’s growth. The generative nucleus divides by mitosis to form two male gametes.
  4. The pollen tube enters the ovule through a gap (micropyle) and reaches the embryosac. A double fertilisation now occurs.
  5. One male gamete fuses with the egg cell nucleus to form a diploid cell that will become the embryo. The other male gamete fuses with the two polar nuclei to form a triploid nucleus that will become the endosperm (food store).

Development of embryo: embryo itself consists of tiny root (radicle) and tiny shoot (plumule). Endosperm accumulates and is stored instead a seed, ready to feed plant at germination (e.g. maize) or earlier (e.g. pear, which has further food stores in cotyledons).

Formation of seed and fruit: after fertilisation, the ovule is strengthened by cutin and lignin to form the testa (tough seed coat). The ocary becomes the fruit.

Dispersion of seeds

  • Fruits swell up and become fleshy; animals eat them, they pass through the digestive system and are deposited as waste.
  • Dispersion by wind or explosive mechanisms such as seed pods bursting open.
  • Some seeds have hooks or are sticky- they attach to animal fur and are deposited elsewhere.

Seeds must be dispersed to avoid intraspecific competition with the parent plant, which is better established and would out-compete its own seedlings.

Photoperiodism is the mechanism by which plants know when to flower. Plants contain a light sensitive pigment called phytochrome (a protein) which exists in two forms- photoisomers- PR(660) which absorbs red light of wavelength 660nm and PFR(730) which absorbs ‘far red’ light of wavelength 730nm.

Daylight contains more red than far red light, so during the day, PR -> PFR. At night, PFR slowly reconverts to PR. The critical factor in determining the relative levels of the two isomers is the duration of the dark period.

Long-day plants (e.g. petunia, spinach, radish, lettuce) only flower if a sufficient proportion of phytochrome is PFR. Therefore, the period of uninterrupted darkness must be less than a certain critical length to prevent the conversion of all the PFR back to PR.

Short-day plants (e.g. chrysanthemum, poinsettia, orchid) only flower if a sufficient proportion of phytochrome is PR (it is thought PFR inhibits flowering in this case). The period of uninterrupted darkness must be longer than a certain critical length to ensure enough PFR is converted back to PR.

Day neutral plants (e.g. geraniums, tomatoes, cucumbers, snapdragons) appear to be indifferent to day length and flower at any time of year.

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Details of mechanism: detected by leaves- only one leaf needs to be exposed to light to activate mechanism. Once day length is within critical limits, a message is transmitted to leaf buds, some of which respond by changing into flower buds- it is believed the messenger is a chemical named “florigen”, although this has not yet been chemically identified.

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Commercial implications:

  1. Short day plants- keep in darkness for long periods to ensure flowering.
  2. Long day plants- interrupt darkness with a flash of red light to rapidly convert PR to PFR, keeping PFR levels high and thus ensuring flowering.
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