6.2 Sexual Reproduction Chapter 6 notes

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

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

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

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

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

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

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

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...650 g and 35 cm by the end third trimester: continued growth, especially of the brain