Main differences between plant and animal cells: Plant cells have: cell walls, a large central vacuole, plastids and turgor pressure. Animal cells have a lysosome (related to vacuole) and centrioles (function in organizing microtubules).
The Cell Wall The primary cell wall is capable of rapid expansion during growth. The secondary cell wall is deposited within the primary cell wall after the cell reaches mature size. From Biochemistry & Molecular Biology of Plants, Buchanan, Gruissem and Jones
Some cells have thick secondary walls such as stone cells in coconut shells (b)
Cell wall components: Cellulose - a polysaccharide composed of 1,4-linked β-d-glucose residues. Synthesized in the plasma membrane by cellulose synthase. Hemicellulose - branched polysaccharides that are structurally homolgous to cellulose because they have a backbone composed of 1,4-linked β -D-hexosyl residues (mannose, galactose, xylose, etc). Synthesized in the Golgi. Pectin - a family of complex polysaccharides that all contain 1,4-linked α-d-galacturonic acid. Synthesized in the Golgi. Structural proteins : hydroxyproline-rich glycoproteins (HRGPs), proline rich proteins (PRPs) and glycine-rich proteins (GPGs). Synthesized at rough endoplasmic reticulum (RER), modified in the Golgi. From: The Complex Carbohydrate Research Center http://www.ccrc.uga.edu
Sites of synthesis of cell wall components plasma membrane: cellulose callose Golgi: Pectins, hemicelluose such as xyloglucans, and glycosylation of cell wall proteins Rough endoplasmic reticulum (rer): cell wall structural proteins and enzymes such as hydrolases
Plasma membrane serves as a semi-permeable barrier. Transporters (pumps, channels, etc.) control what crosses the membrane. Extracellular environment Some are glycosylated PHOSPHOLIPID BILAYER PUMPS AND CHANNELS Cytoplasm SENSORY PROTEINS RECEPTORS STEROL The PM is the outermost surface of the protoplast.
Cellulose synthase is an integral membrane protein that uses cytosolic UDP-glucose as a substrate to synthesize cellulose that it deposits extracellularly.
Other cell wall components such as hemicellulose and pectin are synthesized in the Golgi and delivered to the cell wall by exocytosis. Exocytosis: fusion of a vesicle with the cell membrane, releasing the contents of the vesicle to the cell exterior.
How do proteins get from the cytoplasm into the lumen of the ER? They are translocated across the membrane either post translationally or co-translationally. Then they can enter the secretory pathway.
ER, Endoplasmic Reticulum - synthesis of the endomembrane system (lipid synthesis) - protein synthesis (on RER): both membrane proteins and soluble proteins - oil body synthesis, protein body synthesis - ER is continuous with the nuclear envelope - ER is continuous through plasmodesmata - RER has associated Ribosomes that translate mrna into protein
Plasmodesmata, connections between plant cells. E.R. E.R. lumen Cytoplasm plasma membrane Cell wall plasmodesmal proteins Cytoplasm
Plasmodesmata connect the cytoplasm of adjacent cells, the continuous cytoplasm is called the symplast. The space outside of cells is called the apoplast or cell wall space.
one pore nuclear envelope 1 µm 0.2 µm lipid bilayer facing the nucleoplasm nuclear envelope lipid bilayer facing the cytoplasm pore complex that spans both bilayers Fig. 3-8, p. 37
Golgi (also called dictyosome): - synthesis of cell wall polysaccharides (you should know which ones) - glycosylation of proteins - preparation of secretory vesicles destined for the plasma membrane Camillo Golgi (1843-1926) was an Italian cell biologist and microscopist who was the first to identify the Golgi apparatus in 1898.
Proteins that are destined for the plasma membrane are synthesized on rough ER (RER) then can modified in the Golgi, packaged into secretory vesicles and sent to the PM. Soluble secreted proteins are generally synthesized on RER, inserted in to the lumen of the ER, and then sorted to the PM in secretory vesicles. Cell Wall Polysaccharides (hemicellulose, pectin) are synthesized in the Golgi and sorted to the PM for secretion.
Vacuoles - vacuoles allow plant cells to be large (without making a large volume of cytoplasm). - storage of ions, sugars, amino acids, etc. - digestion, the vacuole is analogous to the animal lysosome and contains proteases, nucleases, glycosidases, lipases etc. - defense, sequestration of toxic compounds, pigments (flower color). Figure 3.16: Vacuole pigments in petals.
The Cytoskeleton two types of protein fibers: actin microfibers and microtubules. They function in moving things around in cells. Microtubules: composed of α and β tubulin. Actin microfibers composed of G-actin
Motor proteins use the energy of ATP hydrolysis to move directionally on the cytoskeleton. Myosin is a motor protein that moves on actin microfibers. Kinesin and Dynein motor proteins move in different directions on microtubules.
Example: actin functions in chloroplast orientation in response to light quality. Dim light Bright light
Movie S1, Related to Figure 1. Chloroplast Movements in Response to a Blue-Light Microbeam in Wild-Type Arabidopsis and thrumin1-1 Mutants. During exposure to a blue-light microbeam (blue circle), chloroplasts (red) in wild-type mesophyll cells moved away from the light and returned after the microbeam was turned off. Chloroplast movement was impaired in the thrumin1-1 mutant in response to microbeam treatment.
Chloroplast structure two outer membranes thylakoids stroma
Mitochondria and chloroplasts are semi-autonomous: - both divide by fission - both contain circular chromosomal DNA, located in nucleoids within the stroma/matrix. - they contain ribosomes, trnas, etc. - they depend on the import of nuclear encoded proteins for many functions.
Chloroplast structure
A chloroplast
Close-up of stacked thylakoids and starch granules in the stroma.
Chloroplasts are one type of plastid
Dividing mitochondrion
Chloroplasts: function in photosynthesis: light reactions, dark reactions. Starch synthesis and storage, chlorophyll and fatty acid synthesis. Mitochondria: Main function is respiration (conversion of energy stored in sugars to energy stored in ATP).