Investigating Cells

1. Investigating Cells Cell Division G Davidson

2. Cell Division • Single celled animals and plants simply divide into 2 identical, but smaller, cells when they are fully grown. • Cell division occurs in 2 separate phases: • The nucleus divides to form 2 identical nuclei which move apart in the cell. • The whole cell divides into 2 new, but smaller, cells. G Davidson

3. Cell Division G Davidson

4. Cell Division • In plant cells a new cellulose wall forms between the 2 new nuclei. • In animal cells the cell divides into 2 by the membrane becoming pinched off between the 2 nuclei. G Davidson

5. Cell Division G Davidson

6. Root Growth • The growth of an animal or plant body involves 2 distinct processes • Cell division. • Cell enlargement. • Growth only takes place in restricted parts of a plant such as the tips of roots and stems. • It occurs throughout the bodies of most animals. G Davidson

7. Root Growth • There are different types of cells and shapes of cells in plant roots. • The base of the root is the root cap which protects the root as it grows. • Just behind this is the root tip where cell division occurs and new cells are made. • Behind this is the area of CELL ELONGATION where growth is occurring. G Davidson

8. Cell elongation Root tip Root Cap Root Growth G Davidson

9. Root Growth • In the root tip the cells look different because their nuclei have been replaced with thread-like structures called CHROMOSOMES. G Davidson

10. Observing Chromosomes • When plant and animal cells divide, the chromosomes in their nuclei always pass through the same series of changes. • The name MITOSIS is given to this kind of cell division. • At the end of mitosis, 2 cells are produced which have the identical number of chromosomes to the original cell. G Davidson

11. Observing Chromosomes • Chromosomes appear as double threads, made up of two chromatids. • When separation occurs the single chromatids are pulled to opposite ends of the cell and pass into the nuclei of the new cells which are to be formed. G Davidson

12. Observing Chromosomes • At a stage between the end of one cell division and the start of the next, the chromosomes themselves are duplicated. • This restores once more the original structure of 2 chromatids per chromosome. G Davidson

13. Observing Chromosomes • The cells of every kind of animal and plant contain a fixed number of chromosomes within their nuclei. • These chromosomes are grouped into identical pairs. G Davidson

14. Observing Chromosomes • All the cells of an organism have the same number of chromosomes, except the gametes (sex cells). • E.g. • Onion – 8 pairs • Man – 23 pairs Privet hedge – 23 pairs • Chimp – 24 pairs • Dog – 39 pairs G Davidson

15. Observing Chromosomes • If 2 kinds of organism have the same number of chromosomes, it is clear that some extra factor must make them different. • Each pair of chromosomes carries information in the form of a code. • Each piece of information is called a GENE. • This coded information is unique to every individual plant or animal. G Davidson

16. Cell Growth • Individual cell cannot grow indefinitely, getting bigger and bigger. • There is a maximum size to which they can grow. • As cells get bigger, the ratio of their surface area to their volume changes. • This affects their ability to take in sufficient raw materials from their surroundings. G Davidson

17. Height Breadth Length Cell Growth If we imagine a cell takes the form of a cube, we can calculate the surface area, the volume, and, therefore, the SA:V ratio. G Davidson

18. Cell Growth We first need to calculate the surface area: Surface area = Length x Breadth x number of sides Surface area is measured in cm2 We now need to calculate the volume: Volume = Length x breadth x height Volume is measured in cm3 G Davidson

19. Cell Growth We can now work out the ratio: Surface area : Volume e.g. if the cube has sides 1cm long: Surface area = 1 x 1 x 6 So S.A. = 6 cm2 Volume = 1 x 1 x 1 So V = 1 cm3 S.A. : V Therefore: =6 : 1 G Davidson

20. Cell Growth G Davidson

21. Cell Growth G Davidson

22. Mitosis G Davidson

23. Mitosis Prophase Anaphase Mitosis Animation Metaphase Telophase G Davidson

24. The Cell as a Factory • A cell can be thought of as being like a factory which manufactures a range of different products. Raw Materials Products CELL Energy Waste G Davidson

25. The Cell as a Factory • All of the substances that the cell needs to take in or get rid of, must pass through the membrane at the surface of the cell. • As the volume of the cell increases, so the manufacturing capacity increases. • However as this increases the surface area : volume ratio is decreasing and it can’t take in enough raw materials to work properly. • This is why cells have maximum size to which they can grow. G Davidson

26. Chromosomes • The chromosomes control all the processes which occur in a cell. • In order to do this, chromosomes must contain a store of information and be able to use this information. G Davidson

27. Chromosomes • This storage of information is very similar to a computer database. • A cell’s records are in the form of packets of information arranged along the length of each chromosome. • Each packet of information is called a gene. G Davidson

28. Chromosomes • Each chromosome carries a number of genes. • In order to function properly the cell must have a complete information store. G Davidson

29. Chromosomes • If any part of the store is missing, or if any of the information is incorrect, then the cell or even the entire organism will not be able to function correctly. • It is essential that the information stored in all the chromosomes is copied accurately before the cell divides so that complete and correct information is passed on to the new cells. G Davidson