Chapter 6 Cells In 1665 Robert Hooke first

Chapter 6: Cells • In 1665, Robert Hooke first described cells using the light microscope. • Two classifications of cells: Prokaryotes Eukaryotes

Prokaryotic Found only in Eubacteria and Archaebacteria (formerly Monera) No true nucleus; lacks nuclear envelope Genetic material in the nucleiod region No membrane bound organelles

Eukaryotic • Found in Kingdoms Protista, Fungi, Plantae, and Animalia • True nucleus; bound by nuclear envelope • Genetic material within the nucleus • Contains cytoplasm with cytosol and membrane-bound organelles

• Cytoplasm – entire region between the nucleus and cell membrane(eukaryotic) • Cytosol – Semi-fluid medium found in the cytoplasm(all cells) Size ranges of cells: Most bacteria are 1. 0 – 10. 0 m Most eukaryotic cells are 10. 0 – 100. 0 m

• The importance of compartmental organization: The average eukaryotic cell has a thousand times more volume but only a hundred times more surface area than prokaryotic cells. Eukaryotic cells compensate for the small surface area to volume ratio using internal membranes.

Organelle – a structure within a cell that serves a specific function Organelles include: 1. Nucleus 2. Ribosomes (non membranous, not a true organelle) 3. Endoplasmic reticulum 4. Golgi apparatus 5. Lysosomes 6. Vacuoles 7. Peroxisomes 8. Mitochondria 9. Chloroplasts

Nucleus –contains most of the genes that control the entire cell. 1) enclosed by a nuclear envelope two lipid bilayers, each with their own specific proteins. 2) contains most of the cell’s DNA, which is organized with histone proteins into complexes called chromatin.

Each species has a characteristic chromosome number Humans= 46 chromosomes (except gametes) The nucleus controls protein synthesis in the cytoplasm:

Nucleolus dark spot(s) in a nucleus It synthesizes and assembles r. RNA, used to form ribosomes out in the cytoplasm. Ribosome – a cytoplasmic organelle with no membrane which is the site of protein synthesis. They are complexes of RNA and proteins * Cells with high rates of protein synthesis have a particularly high number of ribosomes. human liver cell has a few million

Ribosomes function either free in the cytosol or bound to rough endoplasmic reticulum. Bound and free ribosomes are structurally identical and interchangeable. Most proteins made by free ribosomes will remain and function in the cytosol. Most proteins made by bound ribosomes are destined for membrane export.

Endomembrane System: • Biologists consider many membranes of the eukaryotes to be interrelated either directly (through physical contact) or indirectly (through vesicles). • Vesicles are membrane-enclosed sacs that are pinched off portions of membranes moving from one membrane to another. The endomembrane system includes: nuclear envelope endoplasmic reticulum golgi apparatus vacuoles plasma membrane (not actually an endomembrane, but related to the endomembrane system)

Endoplasmic reticulum (ER) – Extensive membranous network of tubules and sacs (cisternae) which separate its internal lumen (cisternal space) from the cytosol. There are two distinct regions: smooth ER(lacks ribosomes) & rough ER

Functions of Smooth ER: • Synthesizes lipids, phospholipids, and steroids Mammalian sex hormones and steroids secreted by adrenal medulla. • Participates in carbohydrate metabolism Smooth ER in liver contains an embedded enzyme that catalyzes the final steps in the conversion of glycogen to glucose • Detoxifies drugs and poisons • Stores calcium ions necessary for muscle contraction example: Sarcoplamic Reticulums

Rough ER and Protein Synthesis: • cytoplasmic side is studded with ribosomes. • continuous with the outer membrane of the nuclear envelope • Manufactures secretory proteins and phospholipid membrane

• Ribosomes attached to rough ER synthesize secretory proteins • Growing polypeptide is threaded through ER membrane into the lumen (cisternal space) • Protein folds into its native conformation • If destined to be a glycoprotein, enzymes localized in the ER membrane catalyze the covalent bonding of an oligosaccharide to the secretory protein • Protein departs in a transport vesicle pinched off from transitional ER adjacent to the rough ER

• Golgi Apparatus: an organelle made of stacked, flattened membranous sacs (cisternae) that modifies, stores, and routes products of the ER many transport vesicles leave the ER and travel to the Golgi apparatus.

• The Golgi apparatus has two poles (cis and trans face): 1) The cis face receives products by accepting transport vesicles from the ER 2) The trans face pinches off vesicles from the Golgi and transports molecules to other sites. • cisternae between the cis and trans face contains unique combinations of enzymes. Golgi products in transit from one cisternae to the next are carried in transport vesicles.

• Lysosome: an organelle which is a membrane-enclosed bag of hydrolytic enzymes that digest all major classes of macromolecules. Enzymes include lipases, carbohydrases, proteases, and nucleases • Lysosomal membrane performs two important functions: a)Sequesters potentially destructive hydrolytic enzymes from the cytosol. b)Maintains the optimal acidic environment for enzyme activity by pumping H+’s inward from the cytosol to the lumen

• Lysosomes probably pinch off from the trans face of the Golgi apparatus. Functions of Lysosomes: 1) Intracellular digestion Phagocytosis 2)Recycle cell’s own organic material 3)Programmed cell destruction

Vacuole: organelle which is a membrane-enclosed sac that is larger than a vesicle (transport or lysosome) Vacuole types and functions: • Food vacuole – site of intracellular digestion in some protists and macrophages • Contractile vacuole – pumps excess water from cell (found in some freshwater protozoa) • Central vacuole – large vacuole found in most mature plant cells. - Stores organic compounds (e. g. protein storage in seeds) - Collects dangerous metabolic by-products from the cytoplasm(acts as lysosome) - Contains soluble pigments in some cells (e. g. red and blue pigments in flowers) - May protect the plant from predators by containing poisonous or unpalatable compounds

Peroxisomes: membrane-bound organelles in most eukaryotic cells that contain specialized teams of enzymes ex) breakdown of fatty acids into (acetyl Co. A) which enters Kreb’s Cycle (mitochondria) H 2 O 2 is produced as a side product-but catalase is present in peroxisomes too Also involved in drug detox

• Mitochondria and Chloroplasts Similarities • Enclosed by double membranes • not part of the endomembrane system • Contain ribosomes and some DNA that program a small portion of their own protein synthesis (rest are imported from cytosol and under nuclear control)

• Mitochondria – organelles which are the sites of cellular respiration(#varies per cell)

Structure of mitochondria: • The smooth outer membrane is highly permeable to small solutes • The convoluted inner membrane contains embedded enzymes that are involved in cellular respiration.

Intermembrane space • between inner and outer membranes with a solute composition like the cytosol, Mitochondrial matrix • enclosed by the inner mitochondrial membrane both contain enzymes needed for cellular respiration

Chloroplast – a chlorophyll-containing organelle which is the site of photosynthesis. • Found in eukaryotic algae, leaves, and other green plant organs. • Are dynamic structures that change shape, move, and divide.

Structure of Chloroplast: 1) Intermembrane space 2) Thylakoids (grana) The thylakoid membrane contains chlorophyll and separates the thylakoid space (inside) from the stroma(fluid-outside)

Plant Cell Walls Thick includes cellulose fibers embedded in a matrix of other polysaccharides and proteins. Has membrane-lined channels called plasmodesmata which connect the cytoplasm of neighboring cells.

Plant cell walls function to: protect plant cells maintain their shape prevent excess water intake (maintain turgor pressure)

Primary vs. secondary cell walls 1)primary – first-formed - flexible, external 2) secondary – deposited last -inside of primary wall -durable, less flexible -can be more than one *Between two adjacent cells, the middle lamella is found and consists of pectins

Cytoskeleton – a network of fibers throughout the cytoplasm that forms a dynamic framework for support and movement. Provides mechanical support and helps maintain cell shape Enables a cell to change shape three types of fibers: microtubules (thickest), microfilaments (thinnest), and intermediate filaments (intermediate in diameter).

Microtubules are hollow fibers found in the cytoplasm of all eukaryotic cells. Constructed from globular proteins called tubulin that consist of one -tubulin and tubulin molecule. Functions of microtubules: 1) Cellular support 2) Tracks for organelle movement 3) Separation of chromosomes (division) 4) Make up centrioles in animal cells 5) Make up cilia and flagella(locomomotor)

Microfilaments – solid rods built from globular protein monomers called G-actin. (helical formation) Function of microfilaments: 1) Participate in muscle contraction Actin(thin filaments) interacts with thick myosin filaments (involves ATP) 2) Provide cellular support 3) Responsible for cytokinesis in animal cells and pseudopodia in ameobas

Intermediate filaments – intermediate diameter Specialized for bearing tension (framework for cytoskeleton) May help fix organelle position (e. g. nucleus) Keratin proteins

Extracellular matrix – meshwork of macromolecules outside the plasma membrane of animal cells. locally secreted by cells composed mostly of glycoproteins (a lot of collagen!) provides support and anchorage for cells

Intercellular Junctions Intercellular junctions in plants: Plasmodesmata Intercellular junctions in animals: Tight junctions(prevent leakage) Desmosomes(prevent separation) Gap junctions(rapid ion exchange)