Cell Division

Specification:

• Understand that division of a diploid cell by mitosis produces two cells which contain identical sets of chromosomes
• Understand that mitosis occurs during growth, repair, cloning and asexual reproduction
• Understand that division of a cell by meiosis produces four cells, each with half the number of chromosomes, and that this results in the formation of genetically different haploid gametes

Mitosis:

• A cell nucleus divides into two identical 'daughter' nuclei.
  
• The daughter cells produced by mitosis are genetically identical to the parent cell.
• Mitosis is important for the growth of an organism, for repair of worn-out parts of the body and for asexual reproduction in plants.
  
  Stages in mitosis:

1. A pair of chromosomes in the nucleus.
2. Each chromosome makes an identical copy of itself.
3. Cell starts to divide into two. each daughter cell has a complete set of chromosomes.
4. Two new daughter cells, each identical to the parent cell they divided from.
   
   Meiosis:

• 2 cell divisions, producing 4 sex cells, daughter nuclei are haploid (contains half the number of chromosomes as the parent nucleus).
  
• Meiosis produces haploid gametes.
• Meiosis results in variations in the gametes produced.
• Variations occur due to crossing over and also due to independent assortment of chromosomes.
• Independent assortment of chromosomes means one chromosome from each pair can combine with either chromosome of the other pair. This results in four different gametes being produced from two pairs of chromosomes. --since fertilisation is random (any sperm fuses with any egg), such variations in the gametes produce variations in the offspring. 

Mitosis	Meiosis
Daughter cells contain same number of chromosomes as parent cell	Daughter cells contain half the number of chromosomes as parent cell
Pairing of homologous chromosomes doesn't occur	Homologous chromosomes pair at prophase I
No crossing over	Crossing over may occur
Daughter cells are identical to parent cell	Variations occur in daughter cells
Two daughter cells are produced from one parent cell	Four daughter cells are produced from one parent cell
Involves only one nuclear division	Involves two nuclear divisions
Occurs in normal body cells (somatic cells) during growth or repair of body parts	Occurs in the gonads (Sex organs) during gamete formation

Determining Sex of Offspring

Specification:

• Recall that the sex of a person is controlled by one pair of chromosomes, XX in a female and XY in a male
• Describe the determination of the sex of offspring at fertilisation, using a genetic diagram

 

Definitions:

Autosomes/Autosomal Chromosomes: Chromosomes in a cell other than the sex chromosomes

Sex chromosomes:

• Human males have an X chromosome and a much shorter Y chromosome in each somatic cell. The X chromosome contains more genes than the Y chromosome.
• Females have a pair of X chromosomes.
• Human somatic cells have 22 pairs of autosomal chromosomes and a pair of sex chromosomes, X and Y.
   
  Determining Sex:
• During gamete production, the female gametes or eggs contain an X chromosome each.
• However, the male will produce two types of sperm-one containing an X chromosome, the other, the Y chromosome.
• When male and female gametes fuse during fertilisation, there is a 50% chance that the offspring could be male or female.
• A baby girl inherits one X chromosome from her mother and another from her father.
• A baby boy receives an X chromosome from his mother and a Y chromosome from his father.
  
   
  
   
  
     

Nutrition and Transport in Plants etc.

2.17 describe the process of photosynthesis and understand its importance in the conversion of light energy to chemical energy
2.18 recall the word equation and the balanced chemical symbol equation for photosynthesis 


Overall equation for photosynthesis:


*Ignore the second equation!! It is unbalanced, and is there just to illustrate the thought process as you convert from a word equation to a symbol equation.*

What conditions are essential for photosynthesis?

· sunlight

· carbon dioxide

· chlorophyll

· a suitable temperature

· water

Photosynthesis depends on enzyme reactions in the chloroplasts. Remember the effect of temperature on enzyme activity-enzymes have optimum temperatures that vary between different organisms. (Optimum temperature=this is the temperature at which the enzyme is most active, catalysing the largest number of reactions per second.) --Certain enzymes in plants have a high optimum temperature. E.g. the optimum temperature of the enzyme papain found in papaya is about 65°C.

How do guard cells control the size of stomata?

In sunlight:

·         The concentration of potassium ions (K⁺) increases in the guard cells

·         Chloroplasts in the guard cells photosynthesise. The light energy is converted into chemical energy used to pump potassium ions into the guard cells from neighbouring epidermal cells. This lowers the water potential in the guard cells.

·         Water from neighbouring epidermal cells enters guard cells by osmosis so that they swell and become turgid.

·         The guard cells have a thicker cellulose wall on one side of the cell (the side around the stomatal pore). Hence, the swollen guard cells become more curved and pull the stoma open.

At night:

·         The potassium ions accumulated in the guard cells during the day diffuse out of the guard cells.

·         This increases the water potential in the guard cells and water leaves them by osmosis.

·BlogThis!

Heredity

This is elaborating a bit on Genes and Inheritance, with more info about the key words to further understanding.


Chromosome
This is a rod-like structure visible in the nucleus during cell division. It is made up of the molecule, deoxyribonucleic acid (DNA). Chromosomes carry the information for making new animal or plant bodies. This information is carried in DNA. Each chromosome may carry many genes along its length.

Gene
A gene is a unit of inheritance. It is a small segment of DNA in a chromosome where specific genetic (hereditary) information is stored. Each gene has a specific function. E.g. there is a gene which determines the height of the pea plant, another determines the colour of their flowers, and another for the shape of the seeds. The place on the chromosome where the gene is located is called the gene locus (plural: loci).

Allele
Each gene can have different forms. Different forms of the same gene are called alleles. E.g. the gene for the height of the pea plant has 2 alleles: short and tall. The gene for flower colour in pea plants also has 2 alleles: purple and white. The alleles occupy the same relative positions on a pair of homologous chromosomes. (i.e. chromosomes that have the same genes at the same loci but possibly different alleles.) Letters are usually used to represent alleles. E.g. the dominant allele for tallness in pea plants may be designated T and the recessive allele for dwarfness, t.

Homologous chromosomes
Homologous chromosomes:

• exist in pairs. One chromosome in the pair comes from the male parent and the other from the female parent.
• are similar in shape and size (with the exception of the sex chromosomes X and Y)
• have exactly the same order or sequence of gene loci. The alleles in these gene loci may not be the same. 

Phenotype

This refers to a trait which can be seen, e.g. the outward appearance or visible trait of an organism. Therefore, tallness in pea plants is a phenotype. 

Genotype

The genetic makeup of an organism. (Basically the combination of genes in an organism.)

• An organism is said to be homozygous for a trait if the 2 alleles controlling the trait are the same, e.g. TT (homozygous dominant) or tt (homozygous recessive) 
• An organism is heterozygous for a trait if the 2 alleles controlling the trait are different, e.g. Tt.

Dominant allele

This expresses itself and gives the same phenotype in both the homozygous and heterozygous conditions, e.g. tall plants have the TT or Tt genotype (phenotype: tall)

Recessive allele

Does not express itself in the heterozygous conditions. Only expressed in the homozygous condition (tt). Thus pea plants will only be dwarf if they have the 'tt' genotype whereas a tall pea plant may have either the TT or Tt genotype. 

Monohybrid Inheritance

Mendel's monohybrid experiments-named after Gregor Mendel, a 19th century Austrian monk who first explained how heredity might work. He carried out breeding experiments on garden peas (Pisum sativum). 

For his experiments, Mendel selected several varieties of garden pea plants with one pair of contrasting characteristics or traits such as: 

• plants that were either very tall or very short (dwarf)
• plants that had either red or white flowers
• plants that produced either yellow or green seeds; and
• plants the produced either round or wrinkled seeds

Inheritance involving only one pair of contrasting traits is called monohybrid inheritance.

(Remember: alleles, which always occur in pairs, control the inheritance of various characteristics. Genes are always at the same position (locus) on homologous chromosomes.)

Mendel also used pure-bred varieties of pea plans. Pure-bred plants are plants which, when self-fertilised, produce offspring (progeny) that resemble their parent. For example, when tall plants self-fertilise, they produce tall offspring. 

Mendel's monohybrid experiments:

1. In one experiment, Mendel crossed or cross-pollinated tall pea plants with pollen from dwarf plants and vice versa.
2. Mendel planted the seeds from the cross and observed the characteristics of the resulting hybrids, which were tall. (TT x tt, results would be Tt, so offspring are tall) A hybrid is the offspring from 2 different varieties or species. Mendel called these hybrids the First Filial Generation or F
   ₁ 
    generation. 
3. He allowed the F
   ₁
     hybrids to self-fertilise and produce new seeds. These seeds gave rise to the F₂ (second filial) generation. In the F₂ generation, a ratio of 3 tall plants to one dwarf plant was observed. (3:1 ratio we talked about in class is hence derived)

Notes: 

In all his experiments, Mendel observed that one trait always appeared in the F

₁

 hybrids (e.g. tall plants), Mendel called this trait dominant. The other trait (dwarf) seemed to disappear or 'recede'. Mendel called this trait recessive. The recessive trait reappeared in about one quarter of the total number of F2 offspring. (3:1 ratio, the 1 out of four is a homozygous recessive.) 

The Eye

Updated: 20/03/13
Note: According to an anonymous comment (see below), knowledge of the fovea and optic disc is now required so please beware of that. I did this blog according to the 2009 syllabus, which was what I did, so I am unaware of the differences in the new syllabus. And plus I've finished IGCSEs... so yeah. Thanks to the anonymous tipper!! :) 

This section focuses on the eye-its structure and function. I'll do things according to the syllabus, so it's WHAT YOU NEED TO KNOW. (for DOUBLE AWARD)

Specification 2.87 describe the structure and function of the eye as a receptor

Tip: Know about the cornea, iris, lens, pupil, retina and optic nerve mainly. You may be asked to label a diagram and possibly answer a few 1-2 mark questions on the functions of these parts. You should know the positions of the rest, but they aren't the main important bits you need to know.. 

. 

1. Sclera: the tough outer coat of the eye, which is the visible, white part of the eye. It protects the eyeball from mechanical damage. 

2. Cornea: at the front of the eye the sclera becomes a transparent 'window' which is the cornea-this lets light into the eye, refracting or bending the light rays into the eye. This plays a key part in the focusing of an image on the retina. 

3. Choroid: this is the middle layer of the eye (between the sclera and the retina), it is black, preventing reflection of light in the interior of the eyeball. (It also contains blood vessel that bring oxygen and nutrients to the eyeball and remove metabolic waste products.)

4. Ciliary body: this contains a circular ciliary muscle (just call it ciliary muscle) which is attached to the lens with the suspensory ligaments. These play a huge role in accomodation-which is basically changing the shape of the lens to focus light onto the retina so an image may be formed. 

5.Iris: this is in front of the lens, it is a circular diaphragm controlling the amount of light entering the eye.

6. Pupil: this is basically a hole/opening in the iris to let light through.

7. Retina: this is inside the choroid layer, it is a light-sensitive membrane with neurones and photoreceptor cells. There are 2 types of photoreceptor cells: rods and cones. Cones enables us to see colours in bright light while rods enable us to see in black and white in dim light. The photoreceptors are connected to the nerve-endings from the optic nerve. 

8. Macula: NOT NEEDED. Fyi, the fovea is in the centre of the macula. Basically, this is where visual perception is most acute.

9. Optic nerve: A nerve that transmits nerve impulses to the brain when the photoreceptors in the retina are stimulated. 

10. Optic disc: NOT NEEDED. Fyi, this connects the retina to the optic nerve. 

11. Vitreous humour: A transparent, jelly-like substance. This keeps the eyeball firm and helps to refract light onto the retina too. 

12. Aqueous humour: A transparent, watery fluid. This keeps the front of the eyeball firm and helps to refract light into the pupil. 

13. Canal of Schlemm: NOT NEEDED. Fyi, this is basically just a channel in the eye that collects aqueous humour and moves it into the bloodstream.

14. Lens: A transparent, circular, biconvex structure. It is elastic and changes it shape or thickness to refract light onto the retina. 

15. Conjunctiva: this is a thin transparent membrane covering the sclera in front. It is a mucous membrane, it secretes mucus, thus keeping the front of the eyeball moist.