Sexual Reproduction Two Different Parents: Sexual Reproduction
Sexual reproduction occurs with two different parents which is
different from asexual reproduction which happens with just one
parent. Sexual Reproduction: The Source of Variation Overview of
Sexual Reproduction In sexual reproduction, a male provides sperm
which fertilizes one or more eggs of a female. To make an
offspring, one sperm fuses with one egg to form a fertilized egg or
zygote. The Chromosome Number in Sexual Reproduction Each
reproductive cell or gamete (sperm and egg) has one set of
chromosomes (n). When they fuse to form a zygote, they make a cell
with two sets of chromosomes (2n). Egg Cells Control What Sperms
May Fertilize Them In each species (kind) of organism, their eggs
make special proteins and sugars on their cell membranes. For a
sperm to be allowed into an egg cell, its proteins and sugars must
match the proteins and sugars of the egg cell. Urchin Egg Cells
Recognize Their Own Type of Sperm Two species of sea urchin, green
and purple live in the same environment. These sea urchins release
their sperm and eggs into the surrounding water. Sperm of purple
sea urchins is not recognized by eggs of green sea urchins. Mating
Mating is the process by which a male places sperm near the eggs of
a female of the same species. Internal Fertilization Internal
fertilization is when sperm are placed inside the body of a female.
External Fertilization External fertilization is when the sperm and
eggs are released outside the bodies of organisms to meet in the
outside environment rather than inside one of the parents bodies.
Sexual Reproduction in Plants Many plants use sexual reproduction
through cones or flowers. Male reproductive cells are formed from
the pollen of plants and the egg cells are located in cones or
flowers. Pollination In conifers and flowering plants, male
reproductive cells (pollen) fertilize eggs (in cones or in flower
ovaries). The transfer of pollen from anthers (male organs) to the
pistil (female organs) is called pollination. Pollination and
Double Fertilization When pollen lands on a pistil, it grows a tube
down through the pistil towards the ovule in the ovary. As it
grows, it produces two sperms. One sperm fertilizes the endosperm
nuclei in the ovule and the other sperm fertilizes the egg nucleus
in the ovule. This is a double fertilization. The Importance of
Double Fertilization The sperm and endosperm nuclei develop into
the food source of a seed. The sperm fusing with the egg develops
into a tiny embryo plant in the seed What Flower Parts Develop Into
The ovary of a flower becomes a fruit. The ovules inside the flower
ovary develop into the seeds. Each ovule has endosperm nuclei that
become (when fertilized) the food source in the seed and an egg
which develops (when fertilized) into a tiny embryo plant. Examples
of Flowers and Fruits Human Development After Fertilization The
fertilized egg (zygote) divides 4 or 5 times until it forms a ball
of cells called the morula. The cells in the morula are stem cells
which can change into any kind of cell. The morula continues to
divide and forms a hollow ball of cells called the blastula. Human
Development From Blastula to Gastrula The hollow ball of cells
(blastula) begins to form an indentation which becomes a two cell
layer ball of cells called the gastrula. From Two Cell Layers to
Three The gastrula then develops outgrowths of cells that move to
make a layer between the inner and outer layer. Now the embryo has
three cell layers. The middle layer is called mesoderm, the outer
layer is called ectoderm and the inner layer is called endoderm.
Our Bodies Come From An Embryo With 3 Layers The three layers all
human bodies develop from are ectoderm, mesoderm, endoderm. Which
Body Parts Come From Each Layer? The endoderm layer becomes the
lining of the digestive system. The mesoderm layer becomes our
skeleton, muscles, kidneys and blood. The ectoderm layer becomes
the skin and nervous system. Differentiation As cells divide
throughout the embryo, they start to change their shapes and take
on special jobs they become different. This process of becoming a
specialized cell in both structure (shape) and function (jobs
performed) is called differentiation. The Placenta Nourishes the
Fetus The embryo and fetus develops an umbilical cord attached to
an organ called the placenta. The placenta is attached to the
inside of a mothers womb (uterus) and transfers food and oxygen to
the fetus as well as taking wastes away like carbon dioxide and
ammonia. The mother supplies the placenta with food and oxygen and
also takes the wastes of the fetus away. Human Development in
Trimester One A trimester is three months of time. During the first
3 months (12 weeks), a human develops from a zygote to a blastula
to an embryo and then into a fetus. A fetus heart forms in 3-4
weeks as does its brain and spinal cord. All organs are formed by
12 weeks, 3 months. 8 weeks (2 months) The Second Trimester (Months
4-6) A fetus grows rapidly during the second trimester. By 5
months, the mother can feel the baby moving and the babys eyelids
open. By month 6, the baby can survive outside the mother with
specialized care. Birth at Around 9 Months After about 9 months of
development, a baby passes through its mothers pelvis. When the
babys chest is exposed to air, the baby begins to breathe using its
own lungs. End of Presentation A A
