Reproduction, hormones and plant hormones Flashcards

Question

The role of the placenta in transfer of substances?

Answer 1

The distance between foetal and maternal blood is very small, as little as 5 micrometres. The cells that separate the maternal and foetal blood form the placental barrier. This must be selectively permeable, allowing some substances to pass but not others. Glucose, amino acids, oxygen and all other substances required by the foetus pass from maternal to foetal blood. Carbon dioxide, urea and other waste products pass from foetal to maternal blood. Diffusion, facilitated diffusion, osmosis and endocytosis are all used in these exchanges for specific substances. The blood passes from the placenta to the body of the foetus in the umbilical vein. The foetus can be retained in the uterus to a much later stage of development in placental mammals than in mammals such as kangaroos which don’t develop a placenta.

Answer 2

Puberty is development from childhood to sexual maturity. When the brain determines that the time for puberty has arrived, the peptide hormones GnRH (gonadotropin-releasing hormone), is synthesised by neurons into the bloodstream. Secretion continues through puberty and adulthood. GnRH stimulates secretion of FSH and LH by the pituitary gland. These two hormones are gonadotropins because they cause changes in the gonads- testes and ovaries.

Answer 3

In boys, FSH stimulates testis growth and LH stimulates secretion of the steroid hormone testosterone from the testes. Testosterone causes the development of the male secondary sexual characteristics in boys during puberty such as enlargement of the penis, growth of pubic hair and deepening of the voice due to growth of the larynx.

Answer 4

In girls, FSH stimulates the development of follicles in the ovary. The follicle wall secretes the steroid hormone oestradiol. LH causes the development of the follicle wall into the corpus luteum after ovulation, which continues the secretion of oestradiol. Oestradiol causes the development of secondary sexual characteristics in females during puberty such as enlargement of the uterus, development of the breasts and growth of pubic and underarm hair.

Answer 5

Secretion of hCG throughout pregnancy stimulates continuing secretion of oestradiol and progesterone. These two steroid hormones are initially secreted by the corpus luteum, but after about 10 weeks of pregnancy the placenta takes over. They are essential to prevent miscarriage and allow the pregnancy to continue. Oestradiol and progesterone prevent degeneration of the uterus lining. Progesterone prevents contractions of the muscular outer wall of the uterus (myometrium) by inhibiting oxytocin secretion by the pituitary gland. Concentrations of both steroid hormones gradually rise, but near the end of a pregnancy the foetus signals to the placenta to stop secreting progesterone, so the concentration starts to decline. This allows the pituitary gland to start secreting oxytocin. Oxytocin stimulates contraction of the muscles in the myometrium. The contractions are detected by stretch receptors, which signal to the pituitary gland to increase oxytocin secretion (positive feedback). Increased oxytocin concentration makes the contractions more frequent and more vigorous, causing more oxytocin secretion. This is an example of positive feedback feedback. In this case, it has the advantage of causing a gradual increase in the myometrial contractions, allowing the baby to be born with the minimum intensity of contraction. The baby is pushed out through the cervix and vagina. The umbilical cord is broken and the baby takes its first breath and achieves physiological independence from its mother.

Answer 6

Hormone replacement therapy (HRT) is used to relieve menopausal symptoms. It supplements levels of the oestradiol and progesterone that naturally decrease as a woman approaches the menopause. HRT can relieve symptoms such as hot flushes, night sweats, mood swings, vaginal dryness and reduced sex drive. Many of these symptoms pass after a few years, but they can be unpleasant and HRT offers relief to many women. It can also reduce osteoporosis (loss of bone density) which is more common after the menopause.

Answer 7

- Anthers produce pollen containing male gametes. Diploid cells inside the anther divide by meiosis to produce four haploid cells, each of which develops into a pollen grain. The nucleus inside a pollen grain divides by mitosis to produce the male gametes. Pollen grains develop a thickened wall for protection during pollination - Ovules are ovoid structures inside the ovary. One cell in the centre of the ovule grows particularly large and then divides by meiosis. One of the haploid nuclei produced divides three times by mitosis to produce eight haploid nuclei, one of which becomes the female gamete (egg nucleus).

Answer 8

- Pollination is the transfer of pollen from an another to a stigma. The pollen is usually moved by wind or by animals - Fertilisation happens inside an ovule. Each pollen grain that lands on the stigma grows a tube d own the style to the ovary, to transport the male gametes from the pollen grain. Once inside the ovary, the pollen tube grows to one ovule and digests a route into it. When the tube reaches the centre of the ovule where the female gamete (egg) is located, the male gametes are released and fertilisation occurs, resulting in the production of a zygote. - Embryo development- the zygote divides repeatedly by mitosis and the cells produced develop into an embryo, with an embryo root, embryo shoot and either one or two embryo leaves (cotyledons). The outer layers of the ovule develop into the seed coat.

Answer 9

The type of reproduction that happens inside flowers is sexual. Many flowers are hermaphroditic because they have both male and female parts, so can act as both a male and female parent. In some cases, hermaphrodite plants can act as both the male and female parent at the same time, by self-pollination and self-fertilisation. Even so, this is not asexual reproduction as all the key events of sexual reproduction happen: meiosis, gamete production and fertilisation.

Answer 10

Cross-pollination is the transfer of pollen from the anther of one plant to the stigma of another. An outside agent is needed to transfer the pollen, usually an animal or the wind. Cross-pollination leads to fusion of male and female gametes from different plants, so promotes genetic variation and therefore evolution, which is essential in an ever changing environment. Many plants are hermaphroditic; they produce both pollen with male gametes and ovules with female gametes, making self pollination possible, causing inbreeding. In most human societies there are taboos against closely related individuals reproducing. This is because of high rates of miscarriage and genetic disease in offspring in such cases. Inbreeding causes the chance of a rare recessive allele in one ancestor being inherited twice by an individual, thus causing a genetic disorder. The offspring of two genetically identical plants tend to be healthy and grow strongly (hybrid vigour). Natural selection therefore tends to favour plants that reproduce by cross pollination, and mechanisms have evolved to promote it and reduce the change of inbreeding by self pollination. Some methods include: separate male and female plants so athers and carpels are on different plants (Ginkgo biloba or urtica dioica) separate male and female flowers on the same plant so anthers and carpels arent in the same flower (zea mays or betula papyifera) stigmas and pollen on anthers mature at different times (protandry = pollen first, protogyny = stigma first) (Nelumbo nucifera, digitalis purpuera)

Answer 11

- Petals- large and brightly coloured to advertise the flower, act as a landing stage and guide the insects movements in relation to the position of the anthers and stigma. Petals in many insect-pollinated plants release a scent to allow insects to find the flower more easily - Pollen grains- spiky so they stick to the anther initially, and then to the visiting insect. The grains can be relatively large as they do not need to be light to be blown by wind. - Anther- positioned to deposit pollen on the insect - Stigma- positioned where a visiting insect will brush past it and sticky so pollen from it adheres. - Nectaries- glands that secrete nectar; nectaries are positioned deep inside the flower so insects can only reach them by brushing past the anthers and stigma

Answer 12

Despite adaptations that reduce the chance of self-pollination the stigma of hermaphrodite plants may receive pollen from that plants own anthers. In many plants this pollen fails to germinate, or the pollen tube. stops growing before it reaches the ovary. This protection against inbreeding due to a single plant acting as both male and female parent is self-incompatibility. It has evolved more than once and there are different mechanisms, but there is always a genetic bias, with alternative alleles of one or more genes. Plants with the same self incompatibility alleles cannot produce offspring together.

Answer 13

Seeds are often transported long distances from the parent plant. This is seed dispersal. It helps to spread the species and reduces competition between offspring and parent. Seed dispersal and pollination are separate processes in the sexual life cycle of plants. Seed dispersal is the function of the fruit, which develops from the ovary of the flower. The structure of fruits is very variable and corresponds to the seed dispersal strategy: - fleshy and attractive for animals to eat - feathery or winged to catch the wind - covered in hooks that catch onto the coats of animals - explosive seed germination follows dispersal - it happens when the conditions are suitable. There must be water, oxygen and warmth - food reserves inside the seed are mobilised by being digested and transferred to the growing embryo; the embryo shoot grows upwards and the embryo root downwards.

Answer 14

Plants control the direction of growth of their roots and shoots. These parts of the plant have meristems at their tips, where cell division and enlargement occur. Roots and their shoots become curved if one side grows more quickly than the other side. This type of unequal growth happens in response to external stimuli perceived by the plant, such as the direction of gravity or sunlight. Differential growth responses to directional stimuli are called trophic responses or tropisms. - positive tropism is growth towards the stimulus - negative tropism is growth away from the stimulus Most roots are positively graviotropic (geotropic). This means they grow downwards, in the same direction as gravity. Most shoots are positively phototropic and negatively gravitropic. This means they grow towards the source of light and, in darkness, they grow upwards in the opposite direction of gravity. The roots and shoots of plants do not all show the same tropic responses. You can investigate these responses qualitatively- by recording observations in drawings or photos- or quantitatively. The angle of curvature of a root or shoot that has carried out a trophic response is an example of a quantitative measure.

Answer 15

If one side of a shoot receives a higher intensity of light than the opposite side, the shoot responds with positive phototropism. The response to lateral light is differential growth, with the sunnier side growing more slowly than the shadier side. When the shoot has curved towards the direction of maximum light intensity, growth becomes equal on all sides, so the shoot carries on growth in that direction. The benefit of positive phototropism is obvious: it increases the amount of light absorbed by a shoots leaves for use in photosynthesis. This is particularly important where there is competition with other plants for light and where the brightest light is to one side rather than directly above. A pigment detects light intensities in the shoot tip and auxin (growth promoting phytohormone) is then redistributed in the shoot, causing differential growth.

Answer 16

A hormone is a chemical message that is produced and released in one part of an organism to have an effect in another part of the organism. Plants and animals produce, transport and use hormones in different ways, so plant hormones are called phytohormones. A variety of chemicals rare used as phytohormones by plants, with roles in the control of growth, development and responses to stimuli.

Answer 17

1. Growth Phytohormones. can promote or inhibit growth, by affecting rates of cell division and cell enlargement. For example, gibberellin promotes stem growth. Wheat and other crop plants have been bred with short stems by introducing alleles which make the plant less responsive to gibberellin. 1. Development Phytohormones can promote or inhibit aspects of development- for example, whether a but starts to grow to produce a side shoot, or whether the apex of a stem produces more leaves or changes to produce flowers. Fruit ripening is promoted by the phytohormone ethylene in many plants. 1. Responses to stimuli Tropic responses are controlled by phytohormones. Tendrils of climbing plants respond to touch stimuli by coiling around a potential support. Communication using electrical signals is used for rapid responses, such as the capture of an insect by a Venus flytrap plant. However, a phytohormone called jasmonic acid triggers the subsequent secretion of enzymes to digest the fly.

Answer 18

Auxin can enter cells by passive diffusion if its carboyxl group (COOH) is uncharged. The cytoplasm of plant cells is alkaline, so auxin is deprotonated (H+) from its carboxyl group, and becomes negatively charged (COO-). This traps the auxin inside the cell. However, auxin efflux carriers can pump negatively charged auxin molecules across the plasma membrane into the surrounding cell wall. The cell wall is acidic, so the auxin reverts to its uncharged state. It can then diffuse into an adjacent cell. To transport auxin across a tissue, efflux carriers are moved to the same end of each cell. Auxin is therefore pumped in the same direction by all the cells in the tissue, generating a concentration gradient.

Answer 19

Auxin (a plant hormone) promotes the synthesis of proton pumps (H+-ATPases) within the cell. These proton pumps are inserted into the plasma membrane of plant cells. The pumps actively transport H+ ions from the cytoplasm into the cell wall, resulting in acidification of the cell wall. This acidic environment weakens the hydrogen bonds between the cellulose microfibrils in the cell wall, a process that loosens the structure of the wall. As a result, water enters the cell by osmosis due to the lower osmotic potential in the vacuole, causing the cell to swell and expand. The swelling of the cell wall allows for cell elongation and growth.

Answer 20

- Auxin is produced in shoot tips and transported in the phloem down stems and into roots. - Cytokinin, another type of phytohormone, is produced in root tips and transported in xylem up roots and into stems. The amounts of auxin and cytokinin produced in a plant provide a means of balancing root and stem growth. They work together to stimulate some processes (synergism) and have opposite effects in other cases (antagonism) Auxin produced by the shoot tip inhibits growth of axillary buds nearby, so the main shoot grows strongly, without competition from side branches. If the main shoot of a plant is eaten by an animal, auxin concentrations decrease, which allows one or more nearby axillary buds to develop into branches of the stem, replacing the lost main shoot. These side shoots produce auxin, which stimulates the root growth needed to supply water and nutrients and inhibits development of more axillary buds.

Answer 21

cell division in root and shoot tip meristems (synergism) cell growth in root and shoot tip meristems (synergism) both auxin and cytokinin promote these action

Answer 22

development of new roots and branching of roots (antagonism): this is promoted by auxin and inhibited by cytokinin branching of stems by the growth of axillary buds (antagonism): this is inhibited by auxin and promoted by cytokinin

Answer 23

Succulent fruits such as peaches ripen when the seeds inside are fully developed. Animals are then attracted to eat the fruits and disperse the seeds. During fruit ripening, the flesh becomes softer, acids and starch are converted to sugar and the skin colour changes and volatile scents are produced to advertise the fruit to animals. Many fruits are stimulated to ripen by ethylene (ethene) and produce this phytohormone as they ripen. so there is a positive feedback mechanism that promotes rapid ripening. Ethylene is volatile, so can diffuse through the air from ripening fruits to other fruits, initiating their ripening. This helps to synchronise the ripening of fruits, increasing the attractiveness of a plant with fruits to animals and encouraging the dispersal of the seeds.

Reproduction, hormones and plant hormones Flashcards

(47 cards)