BIO146

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Chapter 6 - Tour of the Cell
Chapter 6 - Tour of the Cell

6.1 Biologists use microscopes and biochemistry to study cells

prokaryotic cell - the DNA is concentrated in a region that is not membrane-enclosed — nucleoid

Microscopy

Light microscope (LM) - visible light is passed through the specimen and then through glass lenses

organelle - the membrane-enclosed structures within eukaryotic cells

disadvantage - any image that is above the visible light, we as humans cannot see

electron microscope (EM) - focuses a beam of electrons through the specimen or onto its surface

scanning electron microscope (SEM) - especially useful for detailed study of the topography of a specimen

transmission electron microscope (TEM) - used to study the internal structure of cells; to see the internal of the organelles

magnification - the ratio of an object’s image size to its real size

resolution - the measure of the clarity of the image, or the minimum distance of two distinguishable points

contrast - visible differences in parts of the sample (staining & iris light)

Cell Fractionation

cell fractionation (细胞分级分离) - takes cells apart and separates major organelles and other subcellular structures from one another

enables researchers to prepare specific cell components in bulk and identify their functions

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6.2 Eukaryotic cells have internal membranes that compartmentalize their functions

organisms of the domains Bacteria and Archaea consist of prokaryotic cells

organisms of the domain Eukarya - protists, fungi, animals, and plants — all consist of eukaryotic cells

Comparing Prokaryotic and Eukaryotic Cells

cytosol - sub cellular components are suspended; jellylike substance

eukaryotic cell - most of the DNA is in an organelle called nucleus (bounded by a double membrane)

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cytoplasm - the interior of either type of cell

→ in eukaryotic cells, this term refers to only the region between the nucleus and the plasma membrane

→ membrane-bounded structures are absent in almost all prokaryotic cells

plasma membrane - functions as a selective barrier that allows passage of enough oxygen, nutrients, and wastes to service the entire cell

square micrometer of membrane, only a limited amount of a particular substance can cross per second, so the ratio of surface area to volume is critical

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A Panoramic View of the Eukaryotic Cell

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6.3 The eukaryotic cell’s genetic instructions are housed in the nucleus and carried out by the ribosomes

The Nucleus: Information Central

nucleus - contains most of the genes in the eukaryotic cell (some genes are located in mitochondria and chloroplasts)

nuclear envelope - encloses the nucleus, separating its contents from the cytoplasm

nuclear lamina - a netlike array of protein filaments (in animals cells, called intermediate filaments) maintains the shape of the nucleus by mechanically supporting the nuclear envelope

chromosomes - structures that carry the genetic information

→ each chromosome contains one long DNA molecule associated with many proteins, including small basic proteins called histones

chromatin - The complex of DNA and proteins making up chromosomes

→ when a cell is not dividing, stained chromatin appears as a diffuse mass in micrographs, and the chromosomes cannot be distinguished from one another

nucleolus - appears through the electron microscope as a mass of densely stained granules and fibers adjoining part of the chromatin

Ribosomes: Protein Factories

Ribosomes - complexes made of ribosomal RNAs and proteins, are the cellular components that carry out protein synthesis

→ ribosomes are not membrane bounded so that they are not considered organelles

ribosomes build proteins in two cytoplasmic region

free ribosomes - suspended in the cytosol

bound ribosomes - attached to the outside of the endoplasmic reticulum or nuclear evelope

→ bound and free ribosomes are structurally identical, and ribosomes can play either role at different times

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6.4 The endomembrane system regulates protein traffic and performs metabolic functions

Endomembrane system - includes the nuclear envelope, the endoplasmic reticullum, the Golgi apparatus, lysosomes, various kinds of vesicles and vacuoles, and plasma membrane

→ carries out a variety of tasks in the cell, including synthesis of proteins, transport of proteins into membranes and organelles or out of the cell, metabolism and movement of lipids

→ the membranes of this system are related with eitherthrough direct phyiscal continuity or by the transfer of membrane segments as tiny vesicles (sacs made of membrane)

The Endoplasmic Reticulum: Biosynthesis Factory

endoplasmic reticulum (ER) - an extensive network or membranes that it accounts for more than half the total membrane in many eukaryotic cells (connects to the nuclear envelope)

→ consists of a network of membranous tubules and sacs called cisternae

→ the ER membrane separates the internal compartment of ER, called the ER lumen(cavity) or cisternal space, from the cytosol

→ the ER membrane is continuous with the nuclear envelope, the space between the two membranes of the envelope is continuous with the lumen of the ER

Smooth ER - outer surface lacks ribosomes

Rough ER - studded with ribosomes on the outer surface of the membrane

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Functions of Smooth ER

1. Enzymes of the smooth ER are important in the synthesis of lipids, including oils, steroids, and new membrane phospholipids

steroids are produced by smooth ER in animal cells

2. Other enzymes of the smooth ER help detoxify drugs and poisons, especially in live cells

detoxification usually involveds adding dydroxyl groups to drug molecules, making them more water-soluble and easier to flush from the body

3. the smooth ER also stores calcium ions

→ Ex. the smooth ER membrane pumps calcium ions from the cytosol into the ER lumen. When a muscle cell is stimulated by a nerve impulse, calcium ions rush back across the ER membrane into cytosol and trigger contraction of the muscle cell

→ In different cell types, release of calcium ions from the smooth ER triggers different responses <Ex. secretion of vesicles carrying newly synthesized proteins>

4. Metabolizes carbohydrates

Functions of Rough ER

many cells secrete proteins that are produced by ribosomes attached to rough ER

many secretory proteins are glycoproteins (polysacchoride)- proteins with carbohydrates covalently bonded to them

the carbohydrates are attached to the proteins in the ER lumen by enzymes built into the ER membrane

after secretory proteins are formed, the ER membrane keeps them separate from proteins (that are produced by ribosomes) in the cytosol

secretory proteins depart from the ER wrapped in the membranes of vesicles that bud like bubbles from a specialized region called transitional ER

→ vesicles in transit from one part of the cell to another are called transport vesicles

Rough ER is a membrane factory for the cell

→ it grows in place by adding membrane proteins and phospholipids to its own membrane

The Golgi Apparatus: Shipping and Receiving Center

After leaving the ER, many transport vesicles travel to the Golgi apparatus

products of the ER, such as proteins, are modified and stored and then sent to other destinations

→ the Golgi apparatus is especially extensive in cells specialized for secretion

Golgi apparatus consists of a group of assoicated, flatened membranous sacs — cisternae

Golgi apparatus are engaged in the transfer of material between parts of the Golgi and other structures

The two sides of a Golgi stack are referred to as the cis face(on the same side) and the trans face (on the opposite side)

→ the cis face is usually located near the ER (”receiving” side of Golgi apparatus)

→ the trans face gives rise to vesicles that pinch off and travel to other sites (”shipping” side of Golgi apparatus)

In addition, Golgi apparatus also manufactures some macromolecules

→ many polysaccharides secreted by cells are Golgi products

→ nonprotein Golgi products that will be secreted depart from the trans face of the Golgi inside transport vescles that eventually fuse with the plasma membrane

Lysosomes: Digestive Compartments

lysosome - membranous sac of hydrolytic enzymes that many eukaryotic cells use to digest (hydrolyze) macromolecules

lysosome enzyme works the best in the acidic environment found in lysosomes

hydrolytic enzymes and lysosomal membrane are made by rough ER and then transferred to the Golgi apparatus for further processing

phagocytosis - amoebas and many other unicellular protists eat by engulfing smaller organisms or food particles

→ digestive products include simple sugars, amino acids, and other monomers, pass into the cytosol and become nutrients for the cell

autophagy - lysosomes use their hydrolytic enzymes to recycle the cell’s own organic material

→ during autophagy, a damaged organelle or small amount of cytosol becomes surrounded by a double membrane (of unknown origin), and a lysosome fuses with the outer membrane of this vesicle

Vacuoles: Diverse Maintenance Compartments

vacuoles - large vesicles derived from the endoplasmic reticulum and Golgi apparatus

vacuolar membrane is selective in transporting solutes

→ the solution inside a vacuole differs in composition from the cytosol

Food vacuoles - formed by phagocytosis

contractile vacuoles - pump excess water out of the cell, to maintain a suitable concentration of ions and molecules inside the cell; many unicellular protists living in fresh water have this

→ In plants and fungi, certain vacuoles carry out enzymatic hydrolysis

central vacuole - develops by the coalescence of smaller vacuoles; mature plant cells generally contain it

→ the solution inside the central vacuole is called cell sap, the plant cell’s main repository of inorganic ions, including potassium and chloride

→ enlarge as the vacuole absorbs water, enabling the cell to become larger with a minimal investment in new cytoplasm

6.5 Mitochondria and chloroplasts change energy from one form to another

mitochondria and chloroplasts are teh organelles that convert energy to forms that cells can use for work

mitochondria - are the sites of cellular respiration, the metabolic process that uses oxygen to drive the generation of ATP by extracting energy from sugars, fats, and other fuels

chloroplasts - found in plants and algae, are the sites of photosynthesis

The Evolutionary Origins of Mitochondria and Chloroplasts

endosymbiont theory - led from mitochondira and chloroplasts display similarities with bacteria

Mitochondria: Chemical Energy Conversion

Mitochondria are found in almost all eukaryotic cells, including those of plants, animals, fungi, and most protists.

The outer membrane is smooth, but the inner membrane is convoluted, with infolding called cristae

intermembrane space — the narrow region between the inner and outer membranes

mitochondrial matrix — enclosed by the inner membrane; the second compartment

→ the matrix contains many different enzymes as well as the mitiochondrial DNA and ribosomes

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Chapter 6 - Tour of the CellChapter 6 - Tour of the Cell