Biology

Chapter 5.2

Plant Cells

Concepts/Ideas/Facts:

1. plastids are remarkable for the ease with which they can change from one type to another.
2. most plant cells contain hundreds of thousands of mitochondria
3. mitochondria congregate where energy is need and are in constant motion
4. ribosomes in plastids and mitochondria are similar to ribosomes in eukaryotes.
5. membranes are dynamic, mobile structures s that continuously change their shape and surface area.
6. plant cells have both mitochondria and chloroplasts

Definitions:

1. Plasmalemma or cell membrane – outer cell membrane that controls passage of materials into and out of a cell and makes it possible for the cell to differ biochemically and structurally from its surrounding.
2. Phytoalexins – antibiotics in plants that are toxic to pathogens.
3. Respiration – respiration involves the release of energy from organic molecules and its conversion to molecules of ATP.  It occurs in the mitochondria.
4. Haploid – half the number of chromosomes found in gametes
5. Diploid – full compliment or number of chromosomes found in gametes
6. Protoplasm – all the various the contents of the cell
7. Microbodies – organelles bounded by a single membrane
8. Endomembrane System – endoplasmic reticulum, dictyosomes (golgi apparatus), and the plasma membrane together.

Characteristic of Prokaryotes (Bacteria and Cynobacteria):

1. lack nuclei – DNA is not surrounded by a membranous envelope
2. single, circular molecule of DNA which is not associated with histone proteins to form chromosomes
3. no specialized membrane-bound structures (organelles)
4. Prokaryotes are classified on the basis of their biochemical features
5. no sexual reproduction
6. no microtubules
7. no 9-plus 2 flagella

Characteristic of Eukaryotes:

1. DNA is surrounded by two membranes, the nuclear envelope
2. DNA associated with histones, combined with proteins is located in chromosomes
3. have membrane surrounded organelles
4. sexual reproduction
5. intergrated mulitcellularity
6. have microtubules
7. have no 9-plus 2 flagella

The Plant Cell:

1. Cell Wall – rigid outer membrane of a plant cell, which limits the size of the protoplast.  It plays an important role in the absorption, transport, and secretion of substances in plants and may play an active role in defense against bacterial and fungal pathogens.  The long, thin cellulose molecules are united into microfibrils.  The cell wall consists of:
1. Microfibrils – united molecules of cellulose in the cell wall of plants
2. Cellulose – major component of plant cell walls made up of repeating molecules of glucose attached end to end.
3. Micelle – smaller parts of the microfibril with a crystalline structure
4. Macrofibrils – coiled strands of microfibrils which give cellulose great strength
5. Matrix – non-cellulosic molecules which link substances in the cell wall
6. Hemicelluloses – polysaccharide matrix molecule
7. Pectic Substances (pectins) – other non-cellulosic matrix substance
8. Lignin – rigid substance in plant cell walls
9. Cutin, suberin, waxes – protective fatty substances found in the outer walls of plants which function to reduce water loss.
10. Primary pit-fields – thin areas of the primary wall where plasmodesmata aggregate
11. Plasmodesmata – cytoplasmic threads, which connect the living protoplasts of adjacent cells.  They are narrow, plasma-membrane lined canals in cell walls.
12. Desmotubule – tubule of endoplasmic reticulum that traverses plasmodesmata

2. Cell Wall Layers/Structures:
1. Primary Wall – outer cell wall formed first, which may contain hemicellulose, pectin, glycoproteins, and enzymes, but are primarily made of pectic polysaccharides.  Actively dividing cells, such as cells involved in healing and regeneration, commonly have only primary walls.
2. Middle Lamella  (intercellular substance) – region of union of the primary walls of adjacent cells made up of pectic substances.  The process of lignification begins in the middle lamella and spreads to the primary wall and then to the secondary wall.
3. Secondary Wall – wall laid down by the protoplast of the cell on the inner surface of the primary wall.  Secondary walls contain more cellulose that primary walls and pectin substances are lacking.
4. Primary Pit-Fields – thin areas of the primary wall where plasmodesmata aggregate
5. Pit Membrane – the middle lamella and two primary walls between the two pits
6. Pit-Pari – two opposite pits plus the membrane
3. Protoplast – unit of protoplasm inside a plant cell wall, which consists of cytoplasm and a nucleus.
4. Cytoplasm – includes distinct membrane-bound entities (organelles), systems of membranes (endoplasmic reticulum), and non-membranous entities  (ribosomes, actin filaments, microtubules).
5. Cytosol or Ground Substance – “cellular soup” or cytoplasmic matrix in which membrane systems are suspended.  The ground substance is electrically negative to both the aqueous medium outside the cell and the solution inside the vacuole (cell sap).
6. Plasma Membrane – separates the cytoplasm from the cell wall.
1. mediates the transport of substances into and out of the protoplast
2. coordinates the synthesis and assembly of cell wall microfibers (cellulose)
3. translates hormonal and environmental signals involved in the control of cell growth and differentiation
7. Vacuole – liquid (cell sap) filled cavity bounded by a single membrane within the cytoplasm of plant cells.  Different vacuoles may have different functions.  The principle function of the vacuole is to maintain tissue rigidity (turgor).  Vacuoles contain salts and sugars as well as some dissolved proteins.  Crystals, called druses, may also form in vacuoles.  Vacuoles act as a storage area for reserve proteins in seeds and they remove toxic substances such as nicotine and alkaloid from the cytoplasm.  They are often the site of pigment deposition such as the water-soluble anthocyanins (blue, violet, purple, scarlet, and dark red colors), which are responsible for the various colors of flowers, fruits, and vegetables.  Vacuoles are involved with the breakdown of macromolecules and the recycling of their components within the cell.  Entire organelles, such as plastids and mitochondria, may be deposited and degraded in vacuoles.  This digestive process is comparable to that of lysosomes in animal cells. 
8. Tonoplast – single membrane than surrounds a vacuole
9. Cytoplasmic Streaming (Cyclosis)– the continuous movement or streaming of the cytoplasm within a plant cell.  This seems to facilitate the exchange of materials within a cell.
10. Nucleus – the brains of the cell
1. controls the ongoing activities of the cell by determining which protein molecules are produced and when they are produced
2. stores the genetic information, passing it on to daughter cells during cell division.
11. Nuclear Envelope – in eukaryotes, pair of membranes that surround the nucleus.  The nuclear envelope may be considered a specialized, locally differentiated portion of the endoplasmic reticulum.
12. Nuclear Pores – circular openings, 30 to 100 nanometers in diameter, that join the inner and outer nuclear membrane which provide a direct passageway through the nuclear envelope for the exchange of materials between the nucleus and the cytoplasm.
13. Nucleoplasm – nuclear “cellular soup” or nuclear ground substance
14. Chromatin – made up of DNA combined with large amounts of proteins called histones.
15. Chromosomes – the condensed chromatin created during nuclear division which carry the hereditary information.
16. Somatic Cell – body cell (non-gamete)
17. Gametes – reproductive cells
18. Diploid – full compliment or number of chromosomes found in somatic cells (body cells)
19. Nucleolus (nucleoli) – holds the chromosomes within the nucleus.  It is the site of ribosome assembly.
20. Plastids – organelles with a double membrane and a pigment containing ground substance (stroma).  Plastids are classified on the basis of the kinds of pigment they contain.  Plastids reproduce by fission, the process of dividing into equal halves.
21. Stroma – ground substance found in plastids of plant cells
22. Chloroplasts – semi-autonomous photosynthetic organelles that contain chlorophyll and carotenoid pigments.  Chloroplasts are disk-shaped and measure about 4 to 6 micrometers in diameter, and are located in the cytoplasm along cell walls, with their broad surfaces facing the wall surface.  They often contain starch grains and small droplets of lipids (oil).  They are also involved in amino acid synthesis and fatty acid synthesis and provide temporary storage of starch.
23. Chromoplasts – pigment plastids that lack chlorophyll but synthesis and retain carotenoid pigments which are responsible for flower, fruit, old leaf, and some root color.
24. Leucoplasts – non-pigmented plastids that synthesize starch or other substances including oils and proteins.  Upon exposure to light they may turn into chloroplasts.
25. Protoplasts – small, colorless or pale green, undifferentiated plastids that occur in meristematic cells of roots and shoots.  Protoplasts are precursors of other plastids.
26. Prolamellar Bodies – arrested protoplasts (due to the lack of light, which are semicystalline bodies that are composed of tubular membranes.  When exposed to light the prolamellar membrane develop into thylakoids.
27. Etioplasts – forming in leaf cells grown in the dark they are plastids containing prolamellar bodies.  When exposed to light they turn into chloroplasts when the prolamellar bodies develop into thylakoids.
28. Amyloplasts – type of leucoplast that stores starches usually found in storage organs such as potato tubers.
29. Mitochondria – semiautonomous organelles containing the necessary components for the synthesis of some of their own proteins and the powerhouse of the cell where respiration occurs.  Respiration involves the release of energy from organic molecules and its conversion to molecules of ATP.
30. Cristae (crista) – folded and pleated inner membrane of the mitochondria
31. Druses – aggregate crystals within a vacuole composed of calcium oxalate
32. Microbodies – spherical organelles with a granular interior bounded by a single membrane which lack DNA and ribosomes.  They are associated with segments of the endoplasmic reticulum.
33. Peroxisomes – microbodies associated with glycolic acid metabolism, which is associated with photorespiration.
34. Glyoxysomes – contain enzymes necessary for the conversion of fats to carbohydrates during the germination of seeds.
35. Cell Sap – the liquid contained in a vacuole, which is usually slightly acidic and contains salts an sugars.
36. Endoplasmic Reticulum (ER) – complex three-dimensional membrane system of parallel membranes with a narrow, transparent space, or lumen, between them.  It seems to be continuous with the outer membrane of the nuclear envelope and it acts as a channeling system for materials, such as lipids and proteins, to different parts of the cell.  Endoplasmic reticulum of adjacent cells in interconnected by way of cytoplasmic threads, (plasmodesmata), which traverse their common cell walls. The ER is the principle site of membrane synthesis within the cell.

a.       Rough Endoplasmic Reticulum – endoplasmic reticulum bearing ribosomes is the major site of protein synthesis.

b.   Cisternae – flattened sacs formed of endoplasmic reticulum

c.   Smooth Endoplasmic Reticulum – usually a tubular form of endoplasmic reticulum that lacks ribosomes.  It is found in cells that secrete lipids.

36. Dictyosomes (Golgi Apparatus or Golgi Bodies) – groups of flat, disk-shaped sacs, or cisternae, which are often branched into a complex series of tubules at their margins.  They are involved in secretion and cell wall synthesis particularly the carbohydrate portion of the cell wall.
37. Glycoproteins – carbohydrate-protein compounds, which are important cell wall constituents.
38. Cytoskeleton – complex network of protein filaments that extends throughout the ground substance and is involves in many processes, including cell division, growth and differentiation, and the movement of organelles from one location to another within the cell.
1. Microtubules – long, thin cylindrical structures built up of subunits of protein called tubulin.  These subunits are arranged in a helix to from 13 rows or “protofilaments” aligned in parallel around a hollow core.  Thus they are polar and have plus and minus ends.  The function of microtubules is to insure orderly growth of the cell wall
2. Actin Filaments (microfilaments)– made of the contractile protein, actin, they are polar structures involved in cell wall deposition, tip growth of pollen tubes, nuclear migration following cell division, and cytoplasmic streaming.
39. Ergastic Substances – passive products of the protoplast, some are storage products and others are waste products.  They are such materials as starch, crystals, resins, gums, tannins, lipid droplets, anthocyanin pigments, and protein bodies.  They are found in cell walls, ground substance, organelles, and vacuoles.
40. Primary Metabolites- substances that play a basic role in cell metabolism such as starch and sugars.
41. Secondary Metabolites – play no apparent function in the plant’s primary metabolism such as tannins.
42. Flagella (flagellum) – long, either alone or in small number, hair-like structures that extend from the surface of many eukaryotic cells.  In plants, flagella are only found sex cells (gametes) and then only in those plants that have motile sperm, including the mosses, liverworts, ferns, cycads, the Ginko biloba tree.  Flagella grow out of basal bodies.  Each flagella has an outer ring of nine pairs of microtubules which surround two more microtubules in the center.  Movement is by a sliding microtubule mechanism in which the outer pairs move past one another without contracting.  This causes bending in the flagella which in turns causes movement.
43. Tinsel Flagella – flagella bearing one or two rows of minute, lateral appendages.
44. Whiplash Flagella –flagella having only a straight “tail” with no added appendages.
45. Cilia (cilium) – shorter, multiple number, hair-like structures that extend from the surface of many eukaryotic cells.  Cilia grow out of basal bodies.

Structure of Chloroplasts:

1. Stroma – the ground substance or “soup” of a chloroplast
2. Thylakoids – elaborate system of membranes in the form of flattened sacs that traverse the stroma.  Thylakoids of various grana are interconnected by other thylakoids, called stroma thylakoids.
3. Grana (Granum) – stacks of disk-like thylakoids embedded with chlorophyll and carotenoid pigments.
4. Lamellae (stroma thylakoids) – traverse the stroma and interconnect the grana
5. Nucleoids – clear, grana-free regions containing circular, histone-free DNA
6. Ribosomes – small particles consisting of equal amounts of proteins and RNA.  They are the sites linking amino acids to form protein (protein synthesis).  They occur freely in the cytoplasm, attached to the endoplasmic reticulum, or in the nuclei.  Membrane bound ribosomes and free ribosomes are both structurally and functionally identical, differing from one another only in the proteins they synthesize.
7. Polysomes or polyribosomes – clusters or aggregates of ribosomes actively involved with protein synthesis.
8. Outer Membrane – permeable outer protective membrane
9. Inner Membrane – invaginates to form thylakoid disks
10. Intermembrane Space – space between the outer membrane layer and the inner membrane layer

Comparison of Chloroplasts to Bacteria:

1. both contain one or more nucleoids, clear, grana-free regions containing DNA
2. DNA in both is in circular form and is not associated with histones
3. in both, ribosomes are about two-thirds as large as the cytoplasmic ribosomes of eukaryotes.
4. in both, ribosome production is inhibited by anti-biotics, unlike eukaryotic ribosomes.
5. both divide by fission (equal halves)

Structure of Mitochondria: power house of the cell

1. two memebrane structure
2. cristae (crista)– extensively folded and pleated inner membrane
3. stroma – ground substance or liquid matrix
4. ribosomes – protein synthesis
5. mitochondrial DNA which occurs as a circular molecule in nucleoids
6. nucleoids – contains DNA

Plastid Development Cycle:

Eoplast à  amyloplast à pregranal plastid (etioplastid) à chromoplast

Comparison of Plant Cells to Animal Cells:

1. plants have cell wall, animals do not
2. plants have plastids, animals do not
3. plants have vacuoles, plants don’t
4. plant cells usually lack centrioles

Cell Theory: emphasizes the basic similarity of all living systems and brings an underlying unity to varied studies involving many different kinds of organisms.

1. cells are the structural and functional units of life.
2. cell is the smallest unit of life
3. all cells come from cells