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6.2 Sexual Reproduction
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Sexual reproduction brings non-identical gametes together to form a new organism. Advantage: Offspring are genetically different from parents so may be better suited to survive new diseases, environmental changes, or threats.
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Three stages:
Mating - process by which gametes are bought together at same place & same time
Fertilization - process by which egg & sperm join to form a new organism
Development - process by which an organism develops from zygote to embryo to fetus
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4. Methods of Fertilization
Two types: External or Internal Fertilization
Note: To produce a successfully developing embryo, certain conditions must be met:
Embryo must have enough nutrients.
Temperature must not be too cold or too hot.
There must be enough moisture so the embryo does not dry out.
Embryo must be protected from predators & environmental elements that could potentially harm it.
See page 207

5. External Fertilization
In external fertilization, sperm & egg join outside parents
Advantages
Very little energy required to mate
Large numbers of offspring produced
Offspring can be spread widely in the environment so less competition between each other & parents
Disadvantages
Many gametes will not survive
Many eggs will not be fertilized
Must be in a moist/wet environment
Offspring are often not protected by parents, so many of them die
Frog Eggs - GNU Free Doc Photo
See pages
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6. Internal Fertilization
In internal fertilization, sperm and egg join inside parents & embryo is nourished inside mother
Advantages
Embryo is protected
Can reproduce in dry environments
Offspring more likely to survive, as many species will protect their them while they mature
Disadvantages
Much more energy required to find mate
Fewer zygotes produced, resulting in less offspring
More energy required to raise and care for offspring
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Pollination
See pages
Sexual reproduction in many types of plant
Internal fertilization
pollen (male gametes) from the male reproductive part is transferred to the female reproductive part
Pollen can be carried by wind, water, or organisms.
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In Flowering Plants
The flower contains male & female reproductive structures
The stamen (male reproductive organ) produces pollen which transfers to the stigma (female reproductive organ)
A pollen tube forms to deliver the sperm cells to egg cells
Egg cells are contained within ovules within an ovary
Zygote develops into an embryo which is nourished by food stored within the seed
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In Conifers
Cones contain male & female reproductive structures
Pollen from male cones is carried by wind to female cones
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Stages of Development
Stage 1 –Germinal Period
End of the first week - ball of cells called morula
By end of second week it is a hollow ball called a blastula
Cells at this stage are stem cells, & have the ability to develop into any kind of cell
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In humans, first days
From fertilization (zygote) to morula to blastula implantation
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13. Stage 2 – Embryotic Period
Embryonic development = early development of an organism
in humans, implantation to 8 weeks
“critical stage”
Vulnerable to outside influences
Probability of structural defects are at their greatest
18 days
See pages
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In this stage, a gastrula forms
3 germ layers:
ectoderm (skin, nerves)
mesoderm (muscles, bones)
endoderm (lungs, liver, digestive system lining)
The cell layers now differentiate into the organs and tissues of a baby
3rd week: head, blood vessels, brain
4th week: heart begins to beat
5-8 weeks: hands, feet, fingers, & toes develop; kidney & liver function, bones
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Stage 3 – Fetal Period
In humans, 9 weeks to birth
Continued development
Weight gain
Facial features
Active movement
Rapid rate of growth
Brain development
See pages
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16. The cell layers now differentiate into the organs & tissues of a baby
This is divided into 3 trimesters:
First Trimester (0-12 weeks)
Organ systems begin to develop & form. Bone cells form.
Second Trimester (12-24 weeks)
Rapid growth from weeks.
Third Trimester (24+ weeks)
Continued growth, especially of brain. Fat begins to deposit at 32 weeks to keep baby warm at birth.

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Take the Section 6.2 Quiz
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