Chapter 4.9

1. Chapter 4.9 Cell Surface Specializations AP Biology Fall 2010

2. Objectives • Understand the difference between primary and secondary cell walls • Describe the extra cellular matrix • Distinguish between the different cell junctions in plant and animal cells

3. Eukaryotic Cell Walls • Cell Walls: carbohydrate framework for mechanical support in bacteria, protistans, fungi, and plants • Not found in animals

4. Eukaryotic Cell Walls • Cell wall protects and physically supports cell • Imparts shape to cell • Wall is porous so water and solutes can move to and from plasma membrane • Different plant cells form one or two walls around plasma membrane

5. Primary Wall • In growing plant parts, bundles of cellulose strands form a primary cell wall that is pliable enough to allow enlargement under pressure • Sticky primary walls cements abutting cells together • Cells that only have a primary wall retain capacity to divide or change shape

6. Secondary Wall • More layers are deposited on the inside of the primary wall to form the secondary wall • Many cells stop enlarging when they are mature • Reinforces cell shape

7. Cell Wall

8. Cell Wall

9. Cell Wall • Lignin composes up to 25% of secondary wall in woody plants • Makes plant parts stronger, more waterproof, and less inviting to insects • Cell secretions form a protective covering in plants, called cuticle that functions to reduce water loss and protect exposed surfaces

10. Matrixes Between Animal Cells • Animal cells have no cell walls • Intervening between many of them are extracellular matrixes (ECM) made of cell secretions and of materials from surroundings • ECM usually carbohydrates and proteins organized to provide structural support

11. Cell Junctions • Cell interacts with the outside world at its plasma membrane • In multicelled species, structures extend into neighbouring cells or into a matrix • Cell junctions: molecular structures where a cell sends or receives signals or materials, or recognizes and glues itself to cells of the same type

12. Cell Junctions • Plants • Tiny channels called plasmodesmata cross the adjacent primary cell walls and connect the cytoplasm

13. Cell Junctions • Animal cells display 3 types of junctions: • Tight Junctions: occur between cells of epithelial tissues in which cytoskeletal strands of one cell fuse with strands of neighbouring cells, causing an effective seal • Link cells of most body tissues

14. Cell Junctions 2. Adhering Junctions: like spot welds at the plasma membranes of two adjacent cells that need to be held together during stretching, as in the skin and heart

15. Cell Junctions • Gap Junctions: small, open channels that directly link the cytoplasm of adjacent cells • Function as open channels for rapid flow of substances • In heart muscles

16. Cell Junctions

17. Review • List the three types of cell junctions. • What are the differences between the cell junctions? • What is the difference between primary and secondary cell walls?

18. Answers • Cell junctions: tight junctions, adhering junctions, gap junctions • Tight junctions link cells of most body tissues, provide a seal. Adhering junctions link cells to allow stretching. Gap junctions connect adjacent cells, function as open channels for flow of substances. • Primary cell walls are bundles of cellulose strands that allow enlargement. Secondary cell walls reinforce cell shape.