Growth and development
Growth is a permanent increase in size of an organism. Growth involves cell division by mitosis and the
manufacture of more protoplasm and extracellular materials eg; mineral of bones, fibres or connective
tissue. Development is a change in form and complexity of an organism. Development usually involves
cell specialization (differentiation).
Methods of measuring growth
1.
2.
3.
4.
5.
6.
7.
8.
Length –stems and roots
Height – small plants and humans
Wet mass – animals
Dry mass – germinating seeds
Surface area- leaves
Number of leaves from seedling to fruiting plant
Circumference – stems
Number of individuals – population
Mitosis
Mitosis is a type of cell division in which two identical cells are formed from a parent cell. Mitosis is the
way in which any cell, plant or animal, divide when an organism is growing, or repairing a damaged part
of its body.
Stages of mitosis:
a. Interphase – when a cell is not dividing, no chromosomes can be seen clearly in the nucleus.
They are there, but so long and thin they are invisible
b. Prophase – the chromosomes get short and fat, so they can now be seen with a light
microscope. Each chromosome appears as a double stranded structure. Each strand is called a
chromatid and they are exactly like each other and the original single chromosome from which
they came.
c. Metaphase – the nucleus membrane vanishes. The chromosome line up on the equator of the
spindle
d. Anaphase – the centromere of each chromosome splits, so the two chromotids separate. The
chromotids move away from each other, along the spindle fibres.
e. Telophase – the chromotids arrive at opposite ends of the cell and form into groups. A nucleus
membrane appears round each other. The spindle fibres fade away. Each group is exactly the
same as the other
f. Late telophase – the chromosomes become long and thin again, so that they are invisible. The
cytoplasm divides forming two daughter cells. Each of the two cells and the original mother
have the same genes.
In plants mitosis occurs only in the meristems which are at the tip of the shoots, and just behind the tips
of the roots.
Control of growth in animals and plants
Most growth in animals is controlled by hormones (see excretion handout for hormones that control
growth). Chemicals such as auxins (hormone also check excretion handout on response in plants) and
gibberellins control growth in plants. Gibberellins cause the plants to grow tall.
SEXUAL AND ASEXUAL REPRODUCTION
Asexual reproduction
Asexual reproduction involves one parent only. All the offspring produced asexually from one parent are
produced by mitosis and are identical and are also collectively called a clone. There are different forms
of asexual reproduction
1. Binary fission, e.g. amoeba, bacteria. The parent cell splits into two identical daughter cells by
mitosis.
2. Vegetative propagation, e.g. herbaceous perennial plants. Plants are formed from vegetative
(perennating) organs at the beginning of each growing season, e.g. rhizomes, stems, tubers,
corms, bulbs. They also form out of growths of the old plant, e.g. runners, plantlets.
3. Artificial vegetative propagation; cuttings: parts of plants which, given suitable conditions,
develop roots and shoots to become new plants.
 Sugar cane: pieces of stem, two internodes are placed horizontally on the soil. Roots and
shoots develop at the nodes.
 Yam: pieces of yam tuber, called yam setts, each with a lateral bud ‘eye’ are planted, a
new plant is produced by each bud
 Hibiscus: pieces of stem with two or three leaves at the top are planted; roots grow
from the cut ends.
 Grafting: a shoot, the scion, from a young plant bearing large fruits or flowers are
attached to the stem of a vigorous, established root system, the stock. The shoot grows
into a new plant which has beneficial characteristics of both stock and scion. E.g. citrus,
mango and rose trees.
4. Cloning in plants and animals- an experimental procedure. Single cells, taken from certain plants
and given suitable nutriments and hormones, have been induced to divide, by mitosis, to
produce new plants identical to each other. E.g. carrots and orchids. Nuclei taken from cells of a
frog’s intestine and introduced into the ova whose nuclei has been destroyed by ultra violet
light, will cause the ova to grow into tadpoles and frogs identical to each other and to the frog
from which the nuclei were taken. Similar techniques might be successful and useful in higher
vertebrates, e.g. cloning of prize bulls and racehorses. Cloning may eventually be successful in
humans but this opens serious moral questions.
Parthenogenesis
Sometimes an egg will develop into a new organism without being fertilized. This is called
parthenogenesis. Aphids (greenfly) can reproduce in this way. When they have plenty of food, the
female aphids produce large numbers of eggs. The eggs are not fertilized. They stay inside the
female’s body, where they grow into young aphids. The young are then born, fully developed.
Sexual reproduction in humans
Sexual reproduction involves two parents. Two gametes (reproductive cells) which are produced by
meiosis (one from each parent) fuse during fertilization, to form a single celled zygote. This divides by
mitosis to form an embryo and eventually an adult. Offspring produced sexually show characteristics of
both parents. Humans have 46 chromosomes in each of their body cells except in their gametes which
contain 23, half the amount in each cell. This is because when fertilization occurs and the zygote form
both gametes containing 23 chromosome from each parent will give 46 the amount of chromosome
each cell should contain, the offspring will have half of each parents genes however in genetics we will
see that this is a random process and that the offspring may be similar to one parent or share
characteristics of both.
The structure of male and female reproductive organs
The production of ova (eggs) and sperms
Ova
At birth, each female ovary contains many thousands of potential egg cell or ova; each is surrounded by
a fluid filled space forming a primary follicle. Potential ova begin developing into mature ova at puberty
(about 12-14 years), about 450 mature between puberty and menopause (about 45 years), one per
month. During development, a potential ovum undergoes meiosis; one cell produced becomes a mature
ovum and is released from the ovary during ovulation, the other three degenerate.
Sperms
Sperm cells are produced continually from puberty in sperminiferous tubules in the testes. Cells in the
tubule walls undergo meiosis and all the cells produced become mature sperms
Table comparing the ova and sperm
The menstrual cycle
This is a cycle of about 28days comprising ovulation, the release of an ovum from the ovary and
menstruation, the shedding of the blood vessels build up in the uterus 14 days after ovulation if
fertilization has not occurred. The cycle is controlled by hormones. First, a follicle develops inside an
ovary. The developing follicle secretes a hormone called oestrogen. The oestrogen makes the lining of
the uterus grow thick and spongy. When the follicle is fully developed, ovulation takes place. The follicle
stops secreting oestrogen. It becomes a corpus luteum and starts to secrete progesterone. Progesterone
keeps the uterus lining thick, spongy and well supplied with blood, in case the egg is fertilized. If there is
no fertilization the corpus luteum degenerates and the lining of the uterus breaks down. Menstruation
happens and a new follicle develops to start the process once more. If fertilizations occur however, the
corpsus luteum continues to produce progesterone until the embryo sinks into the uterus wall. The
placenta develops and continues to secrete progesterone so there is no menstruation during pregnancy.
When the embryo is implanted into the placenta it is now a fetus. The placenta is very important; it
allows the exchange of material between mother and child (though the umbilical cord) such as nutrients
and oxygen and removal of waste. It protects the child from pathogens and chemicals, it also allow s for
both mother and child with different blood types to interact safely. The fetus is also surrounded by a
membrane called the amnion and within this is the amniotic fluid which helps prevent mechanical shock
and helps maintain a constant environment around the fetus. The umbilical cord is the passage through
which any exchange occurs between mother and child.
Birth control or Contraception
Contraception means avoiding conception, or not producing a child. Not having sexual intercourse is the
most certain method. There is also the ‘safe period’ during menstruation where the chances of having
intercourse when the sperm will not fertilize the egg. This will include regularizing the cycle because the
sperm can survive a few days. This method is very risky and should not e practiced because it does not
prevent the spread of sexually transmitted diseases. Other methods of contraceptives are
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Barrier methods- usually some kind of barrier prevents sperm from reaching the egg. These
would include a condom (not 100% effective against aids or preventing pregnancy). There are
both male and female condoms.
Sterilization – operations can be performed on men to prevent sperm passing to the penis
(vasectomy; cutting sperm ducts), and on women to prevent ova passing to the uterus (tubal
ligation; the fallopian tubes are cut and tied). This is the most effective.
Hormonal method- women take pills containing hormones to inhabit ovulation. This is known as
birth control pills or contraceptive pills and is very effective against pregnancy when used right,
second to sterilization.
Mechanical method- the IUD (intra-uterine device) or ‘coil’ is inserted in the uterus and prevents
an embryo from implanting in the uterine lining. Same as the barrier method in effectiveness.
Sexual reproduction in plants
Plants produce specialized, haploid sex cells in their flowers. The male sex cells are the pollen grains and
the female sex cells are ova. Just as in animals, the male sex cells must be transferred to the female sex
cells. This is called pollination. Pollination is carried out either by wind or insects. Following pollination,
fertilization takes place and the zygote formed develops into a seed, which, in turn, becomes enclosed in
a fruit. The sex cells are produced by meiosis in structures in the flower. Pollen grains are produced in
the anthers of the stamens. The ova are produced in ovules in the ovaries. In pollination, pollen grains
are transferred from the anthers of a flower to the stigma. If this occurs within the same flower it is
called self- pollination. If the pollen grains are transferred to a different flower, it is called crosspollination. Pollination can occur by wind or by insect in either case. Plants that are wind pollinated
produce flowers with a different structure to those of insect pollinated flowers.
Diagram of wind pollinated and insect pollinated
Table summarizing the main differences between wind and insect pollinated flowers
Feature of flower
Type of flower
Insect pollinated
Wind pollinated
Position of stamen
Enclosed within the flower
so that the insect must
make contact
Exposed so that wind can easily
blow pollen away
Position of stigma
Enclosed within flower so
that insect must make
contact
Exposed to catch pollen blowing
in the wind
Type of stigma
Sticky so pollen grains
attach from insects
Feathery, to catch pollen grains
blowing in the wind
Size of petals
Large to attract insects
Small
Colour of petals
Brightly coloured to attract
insects
Not brightly coloured
Nectarines
Present- nectar is a ‘reward’ Absent
for insects
Pollen grains
Small, sticky grains to stick
to insects’ bodies
Larger, inflated grains to carry in
the wind.
Fertilization in plants
Pollination transfers the pollen grain to the stigma, however for fertilization to occur; the nucleus of the
pollen grain must fuse with the nucleus of the ovum, which is inside an ovule in the ovary. To transfer
the nucleus to the ovum, the pollen grain grows a tube, which digests its way through the tissue of the
style and into the ovary. Here it grows around to the opening in an ovule. The tip of the tube dissolves
and allows the pollen grain nucleus to move out of the tube and into the ovule. Here it fertilizes the
ovum nucleus.
Seed and fruit formation
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The zygote develops into an embryonic plant with small root (radicle) and shoot (plumule)
The other contents of the ovule develop into cotyledons which will be a food store for the young
plant when the seed germinates
The ovule wall becomes the seed coat or testa
The ovary wall becomes the fruit coat; this can take many forms depending on the type of fruit
Dispersal of fruit and seed
Dispersal ensures the survival of the species. If all the seeds produced by the plant germinate in the
same area then the competition for food and resources would be high resulting in some plants getting
fewer resources than others. This also makes the species more susceptible to diseases and predators.
Seeds can be dispersed by wind, water and animals. When animals eat the fruit they pass the
undigested seeds in different areas. Also seeds may become stuck on their bodies. The coconut palm is
dispersed by water
Germination
Micropyle- tiny holes in testa
Radical- eventually grows into a root
Plumule- eventually grows into a shoot
Cotyledons- where food is stored such as starch and proteins, it also contains enzymes
Testa- tough, protective covering, stops the embryo from being damaged and also prevents bacteria and
fungi from entering the seed
hilum- where the seed was joined into the pod
When a seed germinates, it first takes up water through the micropyle. As the water goes into the
cotyledons, they swell. Eventually, they burst the testa and the radical emerges, lateral roots begin to
grow. The testa falls off, root system forms. The Cotyledon come above ground, turn green and
photosynthesize. The plumule opens out and grows into a shoot.
Once there is sufficient water, the enzyme in the cotyledons become active. Amylase begins to break
down the stored starch molecules to maltose. Proteases break down the protein molecules to amino
acids. Maltose and amino acids are soluble, so they dissolve in water. They diffuse to the embryo plant,
which uses the foods for growth.
When a seed first begins to germinate, it increases in weight. This is because it absorbs water from the
soil. The seed like all living organisms get its energy by breaking down glucose in respiration. Quite a lot
of glucose from the stored starch will be used up in respiration, so the seed loses weight. After a few
days, the plumule of the seed grows above the surface of the ground. The plant can now make its own
food faster than it is using it up. It begins to increases in weight. During germination the radical and the
plumule grow length ways, quickly becoming the root and shoot of the young plant. Roots and shoots
continue to grow throughout the life of the plant.
Primary growth- when young plants begin to grow, its shoots and roots mainly grow longer. Growth in
length occurs in regions called apical meristems at the tip of shoots and roots.
Secondary growth- some plants such as trees have stems and roots which grow wider. Cambium is a
meristemic cell and can divide by mitosis. In stems cell dividion occurs in the cambium cells adding
phloem to the outside and xylem to the inside. Xylem is added in rings, each ring represents a season
growth. The rings may be used to determine the age of a woody stem.
Meiosis