biology igcse

• Hydrophytes: live wholly or partly submerged in water.
• Features of Hydrophytes:

  -Leaves are highly divided to create a large surface area for absorption and photosynthesis

  - Lack of xylem tubes

  -no stomata underside of leaves Stomata are on the upper surface have a thick waxy layer to repel water
• Xerophytes live in deserts where water is scarce, and evaporation is rapid or in windy habitats
• Features of Xerophytes:

  Deep roots reach the water

  underground Leaves reduced spines with minimum surface area for transpiration

  leaf hairs and stomata sunk in pits to trap moist air

  Waxy leaf cuticle

  impermeable water Stomata open at night and close at midday when evaporation is highest
• Adaptive feature: an inherited feature that helps an organism to survive and reproduce in its environment
• Process of Selective Breeding: Selecting by individuals with desirable features Crossing three individuals to produce the next generation Selection of offspring showing the desirable features
• Artificial Selection: breeds organisms with valued characteristics together to produce offspring that share those valuable characteristics.
• Adaptation: the process of natural selection by which populations become more suited to their environment over many generations.
• Codominance: when both alleles in heterozygous organisms contribute to the phenotype
• Sex-linked characteristic: a characteristic in which the gene responsible is located on a sex chromosome, making it more
• Variation: differences between individuals of the same species
• Continuous variation: results in a range of phenotypes between two extremes; examples include body length and body mass
• Discontinuous variation: results in a limited number of phenotypes with no intermediates
• Genotype: the genetic makeup of an organism in terms of the alleles present
• Phenotype: the observable features of an organism
• Meiosis: Reduction division in which the chromosome number is halved from diploid to haploid
• Mitosis: The nuclear division gives rise to genetically identical cells
• Mitosis is needed for: Growth,in animals. Repair damaged tissues, Replacement of worn-out cells, Asexual reproduction in plants
• Homozygous: having two identical alleles of a particular gene (e.g. TT or gg)
• Heterozygous: having two different alleles of a particular gene (e.g. Tt or Gg)
• Oestrogen is secreted by the ovaries. It stops FSH from being produced - so that only one egg matures in a cycle, and it stimulates the pituitary gland to release the hormone LH.
• Progesterone is a hormone secreted by ovaries. It maintains the lining of the uterus during the middle part of the menstrual cycle and pregnancy
• (FSH) is secreted by the pituitary gland. It causes an egg to mature in an ovary and stimulates ovaries to release the hormone oestrogen.
• (LH): is also secreted by the pituitary gland and causes mature eggs to be released from the ovary.
• DNA: controls cell function by controlling the production of proteins, including enzymes, membrane carriers and receptors for neurotransmitters
• Protein synthesis has two stages:

  Transcription (rewriting the base code of DNA into bases of RNA) Translation (using RNA base sequence to build amino acids into a sequence in a protein)
• How proteins are made:

  the gene coding for the protein remains in the nucleus

  messenger RNA (mRNA) is a copy of a gene mRNA molecules are made in the nucleus and move to the cytoplasm

  the mRNA passes through ribosomes

  the ribosome assembles amino acids into protein molecules
• Chromosomes: made of DNA, which contains genetic information in the form of genes
• Gene: a length of DNA that codes for a protein
• Allele: an alternative form of a gene
• Haploid nucleus: a nucleus containing a single set of unpaired chromosomes
• Diploid nucleus: a nucleus containing two sets of chromosomes
• Self Pollination: the transfer of pollen grains from the anther of a flower to the stigma of the same flower or a different flower on the same plant.
• Cross-pollination: the transfer of pollen grains from the anther of a flower to the stigma of a flower on a different plant of the same species.
• Fertilisation: The fusion of the nuclei from a male gamete (sperm) and a female gamete (egg cell).
• Asexual Reproduction: the process resulting in the production of genetically identical offspring from one parent.
• Sexual reproduction: a process involving the fusion of the nuclei of two gametes (sex cells) to form a zygote and the production of offspring that are genetically different from each other
• Pollination: transfer of pollen grains from the male part of the plant (anther of stamen) to the female part of the plant (stigma).