1. Sexual Reproduction

2. Two Different Parents: Sexual Reproduction
Sexual reproduction occurs with two different parents which is different from asexual reproduction which happens with just one parent.

3. Sexual Reproduction: The Source of Variation

4. 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.

5. 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).

6. 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.

7. 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.

8. Mating
Mating is the process by which a male places sperm near the eggs of a female of the same species.

9. Internal Fertilization
Internal fertilization is when sperm are placed inside the body of a female.

10. 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.

11. 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.

12. 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.

13. 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.

14. 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

15. 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.

16. Examples of Flowers and Fruits

17. Examples of Flowers and Fruits

18. Examples of Flowers and Fruits

19. Examples of Flowers and Fruits

20. Examples of Flowers and Fruits

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