Plant Cells & Tissues

1. Plant Cells & Tissues

2. Plant Cells • Plant Cells have a few unique organelles: • Vacuoles • Plastids (e.g., chloroplasts) • Cell wall

3. Vacuoles • Definition: membranous sacs filled with water that serve a variety of functions: • Storage of “food”, water, oil, pigments, toxins, & waste). • Helps to maintain turgor pressure. • Young plant cells often contain many small vacuoles, but as the cells mature, these unite to form a large central vacuole.

4. Plasmolysis • Definition: the contraction of cells within plants due to the loss of water through osmosis. • It is when the cell membrane “shrinks away” from the cell wall and the vacuole collapses. • Occurs when water is drawn out of the cell (high water concentration  low water concentration). • Causes plants to wilt.

5. Plastids • Definition: large organelles that are associated with pigments and other storage products (e.g., starch). • A typical plant cell will usually have 20-40 plastids. • Several Types: • Chloroplasts • Amyloplasts/Leucoplasts • Chromoplasts

6. Chloroplasts • Definition: plastids that capture light energy from the sun to make “food”; contain chlorophyll (Elodea)

7. Amyloplasts • Definition: colorless plastids that store starch • Leucoplasts are pretty similar. • Found in things like potatoes & onions

8. Chromoplasts • Definition: plastids that make & store carotenoid pigments • Found in red & yellow peppers, tomatoes, flower petals … • When leaves change color in autumn, it is the result of the conversion of chloroplasts into chromoplasts.

9. Cell Wall • Definition: a flexible outer barrier made of cellulose fibrils that surrounds plant cells. • Functions: • Protection & Support • Determines plant shape & size • Maintains turgor pressure • Regulates movement in and out • Cell-cell communication

10. Plasmodesmata • Definition: small tubes that connect plant cells to each other, establishing living bridges between cells. • Allow certain molecules to pass directly from one cell to another. • These channels, penetrate the cell wall and are lined with plasma membrane, uniting all connected cells with essentially one continuous cell membrane.

11. Stomata • Definition: pores found in the underside of plant leaves in the epidermis that allow for the exchange of gases (CO2) and H20. • The stomata are flanked by two half moon shaped guard cells, which contains chloroplasts and regulates the opening and closing of the stoma. • Normally stomata open when the light strikes the leaf in the morning and close during the night.

12. Pea leaf stoma (SEM x 3, 520)

13. Trichomes • Definition: epidermal hairs or scales found on nearly all plants. • Functions: • Glandular secretions • Absorb nutrients • Protect against predators • Reduce evaporation • Reflect solar radiation • Collect water

14. Cell Division • The mechanism that allows the nuclei of cells to split and provide each daughter cell with a complete set of chromosomes during cellular division. • Permits plants to grow & develop.

15. Garlic Root Tip • In plants cell division occurs in tissues called meristems, which are often found in root tips. Apical meristem Root cap

16. Phases of Cell Division • Interphase • G1 phase • S phase • G2 phase • Mitosis: • Prophase • Metaphase • Anaphase • Telophase • Cytokinesis

17. Interphase • Cellular growth • Preparation for mitosis

18. Mitosis

19. Prophase • Appearance of string-like chromosomes due to condensing of chromatin.

20. Metaphase • Chromosomes line up in the middle of the cell (equator)

21. Anaphase • Chromosomes separate and move to opposite ends of the cell (poles).

22. Telophase & Cytokinesis • Two new nuclei are formed and the cytoplasm is beginning to separate.

23. Tissues

24. Plant Tissues • Definition: a group of cells that forms a structural and functional unit. • Three tissue systems in plants: • Ground tissue: variety of functions including support and photosynthesis • Vascular tissue: “Plumbing system” throughout plants; conducts water, food (CHO), and dissolved minerals; also strengthens and supports the plant. • Dermal tissue: Provides covering for the plant body.

25. SIMPLE TISSUES

26. The Ground Tissue System

27. Parenchyma cells • Definition: a simple tissue composed of spherical-shaped cells; most common type of tissue (most of apples & potatoes); found throughout plant • Have thin primary walls • Living, metabolizing tissue • Functions: • Photosynthesis & Respiration • Storage (starch, oil droplets, water, and salts) • Wound healing & regeneration

28. Collenchyma • Definition: a simple tissue found beneath the epidermis in young stems and in large veins of leaves (“strings” in celery). • Provides a flexible support system (like our skeletal system). • Found uniformly throughout the plant. • Elongated cells that have unevenly thickened cell walls (especially in the corners) • Alive at maturity

29. Sclerenchyma • Definition: a simple tissue specialized for structural support; occurs in many areas • Two types: • Fibers • Sclereids • Have both primary and secondary cell walls. • At functional maturity cells are often dead.

30. Fibers • Long, tapered cells that often occur in groups or clumps (abundant in the wood and bark of flowering plants) • Found in leaves & stems. • Primary function is support. These sclerenchyma fibers clearly show the thick cell walls. Notice the pits (identified by the arrows) which allow materials to pass from one fiber to another

31. Sclereids • Short, cuboidal cells common in shells of nuts and pits of fruits (peaches, cherries) • Found in all plant parts, usually clustered. • Primary function is for protection. The red cells in this picture are sclereids from a pear. Notice the irregular shape. The surrounding cells are parenchyma cells.

32. COMPLEX TISSUES

33. The Vascular Tissue System

34. Two Complex Tissues • Xylem • Phloem • Both are continuous throughout the plant body.

35. Xylem • Conducts water and dissolved minerals from the roots and the stems and leaves & provides support. • Composed of four types of cells: • Tracheids* • Vessel elements* • Parenchyma cells • Fiber (*Both of these conduct water!)

36. Tracheids • Tracheids are typically found in gymnosperms and seedless vascular plants and aid in transporting water. • The hollow cells that are stained red are tracheids from a fern.

37. Vessel Elements • Vessel elements are typically found in angiosperms and aid in transporting water. • Upper right is a long section of vessel elements showing the perforation plate • Lower right is a cross section of vessel elements.

38. Phloem • Conducts food materials, carbohydrates, in solution and provides support. • Composed of four different cells: • Sieve-tube members (carry food) • Companion cells (aid STM) • Schlerenchyma fibers (support) • Parenchyma cells (storage)

39. Sieve-tube members (STM) • Long, thin cells that are stacked end-to-end to form extended sieve tubes. • Some of the most-specialized cells in nature that are alive, but lose their organelles at maturity. • The end of the cells are called sieve plates. • Adjacent to STM are companion cells, which assist in the STM by moving food into the sieve-tube for transport.

40. Sieve-tube Members (STM) This is a sieve tube member from milkweed. Notice how STM’s are connected by sieve plates (identified by the arrows).

41. Sieve-tube Members (STM) This picture show several sieve tube member in cross section. Notice the perforated sieve plate near the top that would connect two sieve tube members.

42. Sieve-tube Members (STM) Sieve tube members can secrete a protein called callose to plug the seive plates and stop the flow of food if there is damage in the phloem. This picture shows an example.

43. Companion Cells • Companion Cells are alive and regulate the movement of material through sieve tubes. • The arrows point out several companion cells adjacent to the sieve tube members. • Notice the sieve plate as well.

44. Dermal Tissue System

45. Dermis • In herbaceous plants the dermis is a single layer of cells called the epidermis. Epidermis

46. Epidermis • Comprised of two types of cells: • Parenchyma cells (storage) • Guard cells (together create stoma) • Epidermal parenchyma contain no chloroplasts, so they appear transparent allowing light to penetrate into interior photosynthetic tissues. This picture shows the single layer of parenchyma cells that makes up the epidermis and also a very thick cuticle.

47. Epidermis • Functions: • Absorption of water • Secretion of cuticle • Protection • Control of gas exchange

48. Epidermis--Cuticle • This picture shows the single layer of parenchyma cells that makes up the epidermis and also a very thick cuticle. Cuticle Parenchyma cells

49. Cuticle • A waxy layer that is secreted by epidermal cells of stems and leaves. • Functions to prevent water loss. • Also prevents important gases like CO2 from diffusing into leaf/stem.

50. Epidermis—Guard Cells • This picture shows the single layer of parenchyma cells that makes up the epidermis along with the cuticle and a pair of guard cells creating a stoma.