1. 6.2 Sexual Reproduction
Chapter 6 notes

2. Sexual reproduction
Mating – process by which gametes arrive in the same place at the same time; many animals have set times of the year that they mate in order to give birth at the most optimal time for survival

3. Methods of fertilization
Cell division after fertilization only occurs if there is enough nutrients, warm enough so that proteins and enzymes work properly, moisture so that the embryo doesn’t dry out and protection
1. External fertilization  
sperm cell and egg cell unite outside the body of the parents; common in animals that live in the water and plants like mosses and ferns 
advantages: little energy is required to find a mate, large numbers of offspring are produced at one time so that more can survive to become adults and offspring are usually widely spread out so they don’t compete with their parents for food  
disadvantages: gametes and developing young are unprotected and many don’t survive to adulthood

4. More methods 2. Internal fertilization
Sperm cells are deposited inside the female’s body where they meet an egg cell; only one sperm fertilizes an egg and the others are prevented by chemical reactions 
Advantages: embryo develops and is protected inside the mother’s body, most mammals continue to protect their young for a period of time so more offspring survive 
Disadvantages: requires more energy to mate and lower production of zygotes

5. Yet more methods 3. Pollination
In plants, internal fertilization occurs through pollination where male gametes are in pollen and are transferred from the male part of a plant to the female part of a plant 
Pollen tubes carry the sperm cells to the ovules where the egg cells are contained 
Colourful flowers attract bees and other insects to help transfer the pollen around to other plants

6. Still more methods 4. Pollen transport
Some flowering plants have no petals and release the pollen into the air 
Seeds are enclosed in fruit, nuts or cones and can be transported far away 
Other plants don’t have flowers and sperm and egg cells are produced in cones

7. Embryonic development
In humans it takes place in the first two months after fertilization
By the end of the first week, the zygote has divided many times into a ball called a morula 
By the end of the second week the ball is hollow and called a blastula 
The cells are known as embryonic stem cells which can grow into any type of cell

8. Embryonic development cont.
Next the blastula cells organize themselves into three layers (called the gastrula):
the outside layer called the ectoderm becomes the skin and nervous system 
the middle layer called the mesoderm becomes the kidneys, muscles, blood vessels, reproductive organs and bones
the inner layer called the endoderm becomes the lungs, liver, and the lining of the digestive system

9. Fetal development
Differentiation (cell layers forming organs, etc) happens for a period of 38 weeks
first trimester : all organ systems begin to develop, brain and spinal cord develop, bone cells form and the embryo is called a fetus
second trimester: rapid growth g and 35 cm by the end
third trimester: continued growth, especially of the brain