Overview: The Fundamental Units of Life

           All organisms are made of cells

           The cell is the simplest collection of matter
that can be alive

           Cell structure is correlated to cellular function

           All cells are related by their descent from earlier cells

 

Concept 6.1: Biologists use microscopes and the tools of biochemistry to study cells

           Though usually too small to be seen by the unaided eye, cells can be complex

Microscopy

           Scientists use microscopes to visualize cells too small to see with the naked eye

           In a light microscope (LM), visible light is passed through a specimen and then through glass lenses

           Lenses refract (bend) the light, so that the image is magnified

           Three important parameters of microscopy

         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

           LMs can magnify effectively to about 1,000 times the size of the actual specimen

           Various techniques enhance contrast and enable cell components to be stained or labeled

           Most subcellular structures, including organelles (membrane-enclosed compartments), are too small to be resolved by an LM

           Two basic types of electron microscopes (EMs) are used to study subcellular structures

           Scanning electron microscopes (SEMs) focus a beam of electrons onto the surface of a specimen, providing images that look 3-D

           Transmission electron microscopes (TEMs) focus a beam of electrons through a specimen

           TEMs are used mainly to study the internal structure of cells

           Recent advances in light microscopy

         Confocal microscopy and deconvolution microscopy provide sharper images of three-dimensional tissues and cells

         New techniques for labeling cells improve resolution

Cell Fractionation

           Cell fractionation takes cells apart and separates the major organelles from one another

           Centrifuges fractionate cells into their component parts

           Cell fractionation enables scientists to determine the functions of organelles

           Biochemistry and cytology help correlate cell function with structure

 

Concept 6.2: Eukaryotic cells have internal membranes that compartmentalize their functions

           The basic structural and functional unit of every organism is one of two types of cells: prokaryotic or eukaryotic

           Only organisms of the domains Bacteria and Archaea consist of prokaryotic cells

           Protists, fungi, animals, and plants all consist of eukaryotic cells

Comparing Prokaryotic and Eukaryotic Cells

           Basic features of all cells

         Plasma membrane

         Semifluid substance called cytosol

         Chromosomes (carry genes)

         Ribosomes (make proteins)

           Prokaryotic cells are characterized by having

         No nucleus

         DNA in an unbound region called the nucleoid

         No membrane-bound organelles

         Cytoplasm bound by the plasma membrane

           Eukaryotic cells are characterized by having

         DNA in a nucleus that is bounded by a membranous nuclear envelope

         Membrane-bound organelles

         Cytoplasm in the region between the plasma membrane and nucleus

           Eukaryotic cells are generally much larger than prokaryotic cells

           The plasma membrane is a selective barrier that allows sufficient passage of oxygen, nutrients, and waste to service the volume of every cell

           The general structure of a biological membrane is a double layer of phospholipids

           Metabolic requirements set upper limits on the size of cells

           The surface area to volume ratio of a cell is critical

                       As the surface area increases by a factor of n2, the volume increases by a factor of n3

           Small cells have a greater surface area relative to volume

A Panoramic View of the Eukaryotic Cell

           A eukaryotic cell has internal membranes that partition the cell into organelles

           Plant and animal cells have most of the same organelles

 

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

           The nucleus contains most of the DNA in a eukaryotic cell

           Ribosomes use the information from the DNA to make proteins

The Nucleus: Information Central

           The nucleus contains most of the cell’s genes and is usually the most conspicuous organelle

           The nuclear envelope encloses the nucleus, separating it from the cytoplasm

           The nuclear membrane is a double membrane; each membrane consists of a lipid bilayer

           Pores regulate the entry and exit of molecules from the nucleus

           The shape of the nucleus is maintained by the nuclear lamina, which is composed of protein

           In the nucleus, DNA is organized into discrete units called chromosomes

           Each chromosome is composed of a single DNA molecule associated with proteins

           The DNA and proteins of chromosomes are together called chromatin

           Chromatin condenses to form discrete chromosomes as a cell prepares to divide

           The nucleolus is located within the nucleus and is the site of ribosomal RNA (rRNA) synthesis

Ribosomes: Protein Factories

           Ribosomes are particles made of ribosomal RNA and protein

           Ribosomes carry out protein synthesis in two locations

         In the cytosol (free ribosomes)

         On the outside of the endoplasmic reticulum or the nuclear envelope (bound ribosomes)

 

Concept 6.4: The endomembrane system regulates protein traffic and performs metabolic functions in the cell

           Components of the endomembrane system

         Nuclear envelope

         Endoplasmic reticulum

         Golgi apparatus

         Lysosomes

         Vacuoles

         Plasma membrane

           These components are either continuous or connected via transfer by vesicles

The Endoplasmic Reticulum: Biosynthetic Factory

           The endoplasmic reticulum (ER) accounts for more than half of the total membrane in many eukaryotic cells

           The ER membrane is continuous with the nuclear envelope

           There are two distinct regions of ER

         Smooth ER, which lacks ribosomes

         Rough ER, surface is studded with ribosomes

Functions of Smooth ER

           The smooth ER

         Synthesizes lipids

         Metabolizes carbohydrates

         Detoxifies drugs and poisons

         Stores calcium ions

Functions of Rough ER

           The rough ER

         Has bound ribosomes, which secrete glycoproteins (proteins covalently bonded to carbohydrates)

         Distributes transport vesicles, proteins surrounded by membranes

         Is a membrane factory for the cell

The Golgi Apparatus: Shipping and Receiving Center

           The Golgi apparatus consists of flattened membranous sacs called cisternae

           Functions of the Golgi apparatus

             Modifies products of the ER

             Manufactures certain macromolecules

             Sorts and packages materials into transport vesicles

Lysosomes: Digestive Compartments

           A lysosome is a membranous sac of hydrolytic enzymes that can digest macromolecules

           Lysosomal enzymes can hydrolyze proteins, fats, polysaccharides, and nucleic acids

           Lysosomal enzymes work best in the acidic environment inside the lysosome

           Some types of cell can engulf another cell by phagocytosis; this forms a food vacuole

           A lysosome fuses with the food vacuole and digests the molecules

           Lysosomes also use enzymes to recycle the cell’s own organelles and macromolecules, a process called autophagy

Vacuoles: Diverse Maintenance Compartments

           A plant cell or fungal cell may have one or several vacuoles, derived from endoplasmic reticulum and Golgi apparatus

           Food vacuoles are formed by phagocytosis

           Contractile vacuoles, found in many freshwater protists, pump excess water out of cells

           Central vacuoles, found in many mature plant cells, hold organic compounds and water

The Endomembrane System: A Review

           The endomembrane system is a complex and dynamic player in the cell’s compartmental organization

 

Concept 6.5: Mitochondria and chloroplasts change energy from one form to another

           Mitochondria are the sites of cellular respiration, a metabolic process that uses oxygen to generate ATP

           Chloroplasts, found in plants and algae, are the sites of photosynthesis

           Peroxisomes are oxidative organelles

The Evolutionary Origins of Mitochondria and Chloroplasts

           Mitochondria and chloroplasts have similarities with bacteria

         Enveloped by a double membrane

         Contain free ribosomes and circular DNA molecules

         Grow and reproduce somewhat independently in cells

           The Endosymbiont theory

         An early ancestor of eukaryotic cells engulfed a nonphotosynthetic prokaryotic cell, which formed an endosymbiont relationship with its host

         The host cell and endosymbiont merged into a single organism, a eukaryotic cell with a mitochondrion

         At least one of these cells may have taken up a photosynthetic prokaryote, becoming the ancestor of cells that contain chloroplasts

Mitochondria: Chemical Energy Conversion

           Mitochondria are in nearly all eukaryotic cells

           They have a smooth outer membrane and an inner membrane folded into cristae

           The inner membrane creates two compartments: intermembrane space and mitochondrial matrix

           Some metabolic steps of cellular respiration are catalyzed in the mitochondrial matrix

           Cristae present a large surface area for enzymes that synthesize ATP

Chloroplasts: Capture of Light Energy

           Chloroplasts contain the green pigment chlorophyll, as well as enzymes and other molecules that function in photosynthesis

           Chloroplasts are found in leaves and other green organs of plants and in algae

           Chloroplast structure includes

         Thylakoids, membranous sacs, stacked to form a granum

         Stroma, the internal fluid

           The chloroplast is one of a group of plant organelles, called plastids

Peroxisomes: Oxidation

           Peroxisomes are specialized metabolic compartments bounded by a single membrane

           Peroxisomes produce hydrogen peroxide and convert it to water

           Peroxisomes perform reactions with many different functions

           How peroxisomes are related to other organelles is still unknown

 

Concept 6.6: The cytoskeleton is a network of fibers that organizes structures and activities in the cell

           The cytoskeleton is a network of fibers extending throughout the cytoplasm

           It organizes the cell’s structures and activities, anchoring many organelles

           It is composed of three types of molecular structures

         Microtubules

         Microfilaments

         Intermediate filaments

Roles of the Cytoskeleton: Support and Motility

           The cytoskeleton helps to support the cell and maintain its shape

           It interacts with motor proteins to produce motility

           Inside the cell, vesicles can travel along “monorails” provided by the cytoskeleton

           Recent evidence suggests that the cytoskeleton may help regulate biochemical activities

Components of the Cytoskeleton

           Three main types of fibers make up the cytoskeleton

         Microtubules are the thickest of the three components of the cytoskeleton

         Microfilaments, also called actin filaments, are the thinnest components

         Intermediate filaments are fibers with diameters in a middle range

Microtubules

           Microtubules are hollow rods about 25 nm in diameter and about 200 nm to 25 microns long

           Functions of microtubules

         Shaping the cell

         Guiding movement of organelles

         Separating chromosomes during cell division

Centrosomes and Centrioles

           In many cells, microtubules grow out from a centrosome near the nucleus

           The centrosome is a “microtubule-organizing center”

           In animal cells, the centrosome has a pair of centrioles, each with nine triplets of microtubules arranged in a ring

Cilia and Flagella

           Microtubules control the beating of cilia and flagella, locomotor appendages of some cells

           Cilia and flagella differ in their beating patterns

           Cilia and flagella share a common structure

         A core of microtubules sheathed by the plasma membrane

         A basal body that anchors the cilium or flagellum

         A motor protein called dynein, which drives the bending movements of a cilium or flagellum

           How dynein “walking” moves flagella and cilia

         Dynein arms alternately grab, move, and release the outer microtubules

         Protein cross-links limit sliding

         Forces exerted by dynein arms cause doublets to curve, bending the cilium or flagellum

Microfilaments (Actin Filaments)

           Microfilaments are solid rods about 7 nm in diameter, built as a twisted double chain of actin subunits

           The structural role of microfilaments is to bear tension, resisting pulling forces within the cell

           They form a 3-D network called the cortex just inside the plasma membrane to help support the cell’s shape

           Bundles of microfilaments make up the core of microvilli of intestinal cells

           Microfilaments that function in cellular motility contain the protein myosin in addition to actin

           In muscle cells, thousands of actin filaments are arranged parallel to one another

           Thicker filaments composed of myosin interdigitate with the thinner actin fibers

           Localized contraction brought about by actin and myosin also drives amoeboid movement

           Pseudopodia (cellular extensions) extend and contract through the reversible assembly and contraction of actin subunits into microfilaments

           Cytoplasmic streaming is a circular flow of cytoplasm within cells

           This streaming speeds distribution of materials within the cell

           In plant cells, actin-myosin interactions and sol-gel transformations drive cytoplasmic streaming

Intermediate Filaments

           Intermediate filaments range in diameter from 8–12 nanometers, larger than microfilaments but smaller than microtubules

           They support cell shape and fix organelles in place

           Intermediate filaments are more permanent cytoskeleton fixtures than the other two classes

 

Concept 6.7: Extracellular components and connections between cells help coordinate cellular activities

           Most cells synthesize and secrete materials that are external to the plasma membrane

           These extracellular structures include

         Cell walls of plants

         The extracellular matrix (ECM) of animal cells

         Intercellular junctions

Cell Walls of Plants

           The cell wall is an extracellular structure that distinguishes plant cells from animal cells

           Prokaryotes, fungi, and some protists also have cell walls

           The cell wall protects the plant cell, maintains its shape, and prevents excessive uptake of water

           Plant cell walls are made of cellulose fibers embedded in other polysaccharides and protein

           Plant cell walls may have multiple layers

         Primary cell wall: relatively thin and flexible

         Middle lamella: thin layer between primary walls of adjacent cells

         Secondary cell wall (in some cells): added between the plasma membrane and the primary cell wall

           Plasmodesmata are channels between adjacent plant cells

The Extracellular Matrix (ECM) of Animal Cells

           Animal cells lack cell walls but are covered by an elaborate extracellular matrix (ECM)

           The ECM is made up of glycoproteins such as collagen, proteoglycans, and fibronectin

           ECM proteins bind to receptor proteins in the plasma membrane called integrins

           Functions of the ECM

         Support

         Adhesion

         Movement

         Regulation

Cell Junctions

           Neighboring cells in tissues, organs, or organ systems often adhere, interact, and communicate through direct physical contact

           Intercellular junctions facilitate this contact

           There are several types of intercellular junctions

         Plasmodesmata

         Tight junctions

         Desmosomes

         Gap junctions

Plasmodesmata in Plant Cells

           Plasmodesmata are channels that perforate plant cell walls

           Through plasmodesmata, water and small solutes (and sometimes proteins and RNA) can pass from cell to cell

Tight Junctions, Desmosomes, and Gap Junctions in Animal Cells

           At tight junctions, membranes of neighboring cells are pressed together, preventing leakage of extracellular fluid

           Desmosomes (anchoring junctions) fasten cells together into strong sheets

           Gap junctions (communicating junctions) provide cytoplasmic channels between adjacent cells

The Cell: A Living Unit Greater Than the Sum of Its Parts

           Cells rely on the integration of structures and organelles in order to function

           For example, a macrophage’s ability to destroy bacteria involves the whole cell, coordinating components such as the cytoskeleton, lysosomes, and plasma membrane