1. Cellular Differentiation
DNA and the Genome
Key Area 4a
Cellular Differentiation

2. Cellular differentiation
Learning Intentions
By the end of this topic you should be able to:
Cellular differentiation
Explain what ‘cellular differentiation’ means
Define ‘meristem’
Define ‘stem cell’
Give 3 examples of specialised plant cells and describe the types of genes that are expressed in each
Describe the process of differentiation into specialised cells from meristems in plants
Describe the process of differentiation into specialised cells from embryonic and tissue (adult) in animals

3. Cellular Differentiation

4. Differentiation
The process by which a cell expresses certain genes to produce proteins characteristic for that type of cell. This allows the cell to carry out specialised functions

5. Genetic control of Differentiation
In multicellular organisms, every body cell has all of the genes required to produce the whole organism.
By the process of mitosis the original zygote has divided time and time again to produce each and every body cell. Certain genes are switched on in all cells.
As each cell develops it becomes specialised (differentiated) to perform a specific function. In each type of differentiated cell only the genes needed will continue to operate, coding for the required proteins. All other genes will remain switched off.

6. Cellular Differentiation Plants

7. Plant Growth In plants growth only occurs at meristems.
Meristems are regions of unspecialised cells in plants that can divide (self-renew) and/or differentiate
There are two types of meristem:
apical meristems allows primary growth
lateral meristems allows secondary growth

8. Apical Meristems: root tip & shoot tip
An increase in length at a root or shoot requires:
formation of new cells at the apical meristem by mitosis
elongation of the new cells – cells become longer
These cells then undergo…
vacuolation – a vacuole forms within each cell
differentiation – each cell becomes specialised as part of permanent tissues e.g. xylem, phloem
Image source:

10. Growth of a Shoot Terminal bud (containing apical meristem)
Region of mitosis & cell division
Region of elongation & vacuolation
Region of differentiation
Permanent tissues
lateral bud (containing
apical meristem)
Terminal bud (containing apical meristem)
Images source:

11. Growth of a Root Root hair Permanent tissues Region of differentiation
Region of mitosis & cell division
Region of elongation & vacuolation
Region of differentiation
Permanent tissues
Protective root cap
Only area that increases in length
Image source:

12. Specialised Plant Cells
Phloem
Companion cell
Sieve
tube
Xylem
Xylem vessel
Lignin
Hollow tube
Images source:

13. Transverse section of a stem
Secondary Growth
…also known as secondary thickening, is the development of extra xylem in a perennial plant (plant which grows year after year) which makes the stem increase in girth (thickness).
This extra xylem is produced by a lateral meristem called cambium.
xylem
phloem
cambium
Vascular bundle
cortex
Transverse section of a stem
Image source:

14. Xylem
Vascular Bundle
Cambium
Phloem
Lateral meristems consist of cambium cells which are situated in vascular bundles between xylem and phloem tissue (phloem to the outside, xylem to inside)

15. Ring of Cambium
Lateral meristems increase the thickness of a stem in a process known as secondary thickening. This process takes places in 2 main stages:-
Stage 1
Cambium cells divide rapidly by mitosis and cell division producing new cells to form a complete ring of cambium

16. Ring of Phloem
Ring of Cambium
Ring of Xylem
Stage 2
The ring of cambium divides rapidly by mitosis and cell division producing new cells which differentiate into new xylem to the inside and phloem to the outside

17. Xylem is produced every year leading to the formation of annual rings
Rings of Xylem
Year 3
Year 4
Xylem is produced every year leading to the formation of annual rings

18. Annual Rings
Annual rings are can be used to age a tree. Each ring is the ring of woody xylem vessels formed in one year
The activity of the cambium varies with the seasons so within each ring the spring wood and autumn wood can be identified:
spring & summer - cambium forms large xylem vessels (transport water & nutrients)
autumn - cambium is less active and the xylem vessels formed are smaller
winter - cambium is inactive.

19. Annual Rings: Thickness
The thickness of annual rings depend on the growing conditions of the year.
Images source:
Wide ring – mild temperatures in spring & summer
- lots of sunshine
- lots of rainfall
Narrow ring – poor growing conditions
e.g. cold weather / drought / disease
The ‘grain’ of wood cut longitudinally is the pattern of annual rings.
Images source:

20. Differentiation in Animals
Stem Cells
Differentiation in Animals

21. Differentiation in animals
Each human life begins life as a fertilised egg (zygote) and divides to become approximately 37.2 trillion cells ( ) with almost 200 different cell types.
All of these cells originate from STEM CELLS

22. Stem cells Reproduce themselves by repeated mitosis and cell division.
Stem cells are unspecialised cells in animals that can divide (self-renew) and/or differentiate. This means they can:
Reproduce themselves by repeated mitosis and cell division.
Differentiate into specialised cells when required to do so

23. Stem cells
There are different sources of stem cells in animals, the main 2 being:
Embryonic stem cells which have the potential to differentiate into any type of cell found in the human body(pleuripotent)
Tissue (adult) stem cells that have a limited power of differentiation (multipotent)