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CHAPTER 4

GENERAL FEATURES

OF CELLS

Prepared by

Brenda Leady, University of Toledo

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Cell theory

1. All living things are composed of one or more cells

2. Cells are the smallest units of living organisms

3. New cells come only from pre-existing cells by cell division

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Microscopy

MagnificationRatio between the size of an image produced

by a microscope and its actual size Resolution

Ability to observe two adjacent objects as distinct from one another

ContrastCan be enhanced using dyes

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Light microscopeUses light for illuminationResolution 0.2 µm

Electron microscopeUses an electron beamResolution 2 nm

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Electron microscopes

Transmission electron microscopy (TEM)Thin slices stained with heavy metalsSome electrons are scattered while others

pass through to form an image Scanning electron microscopy (SEM)

Sample coated with heavy metalBeam scans surface to make 3D image

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Prokaryotic cells

Simple cell structure Lack a membrane-enclosed nucleus 2 categories- bacteria and archaea

Both smallBacteria- abundant, most not harmfulArchaea- less common, often found in

extreme environments

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Typical bacterial cell

Plasma membrane- barrier Cytoplasm- contained inside plasma

membrane Nucleoid- region where genetic material

found Ribosomes- involved in protein synthesis

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Many structures are located outside the plasma membraneCell wall- support and protectionGlycocalyx- traps water, protectionAppendages- pilli (attachment), flagella

(locomotion)

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Eukaryotic cells DNA housed inside nucleus Eukaryotic cells exhibit compartmentalization Organelle- subcellular structure or membrane-

bounded compartment with its own unique structure and function

Shape, size, and organization of cells vary considerably among different species and even among different cell types of the same species

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The Proteome Determines the Characteristics of a Cell How does a single organism produce different types

of cells? Identical DNA in different cells but different proteomes The proteome of a cell determines its structure and

function Gene regulation, amount of protein, amino acid

sequence of a particular protein and protein modification can influence a cell’s proteome

Proteomes in healthy cells are different from the proteomes of cancerous cells

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Cytosol

Region of a eukaryotic cell that is outside the cell organelles but inside the plasma membrane

Cytoplasm includes everything inside the plasma membraneCytosol, the endomembrane system and the

semiautonomous organelles

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Metabolism

Cytosol is central coordinating region for many metabolic activities of eukaryotic cells

Catabolism- breakdown of a molecule into smaller components

Anabolism- synthesis of cellular molecules and macromolecules

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Translation

Process of polypeptide synthesis Information within a gene is ultimately

translated into the sequence of amino acids in a polypeptide

Ribosome- site of synthesis Transfer RNA (tRNA)- brings amino acids Messenger RNA (mRNA)- information to make

a polypeptide

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Cytoskeleton

Network of three different types of protein filaments Microtubules

Dynamic instability Centrosome or microtubule-organizing center in animals

Intermediate filaments Tend to be more stable than microtubules and actin

filaments, which readily polymerize and depolymerize Actin filaments

Also known as microfilaments

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Motor Proteins Category of cellular proteins that use ATP as a source of

energy to promote movement Three domains called the head, hinge, and tail Walking analogy

Ground is a cytoskeletal filament, your leg is the head of the motor protein, and your hip is the hinge

Three different kinds of movements Motor protein moves the cargo from one location to another Motor protein can remain in place and cause the filament to move Motor protein attempting to walk (both the motor protein and

filament restricted in their movement) exerts a force that causes the filament to bend

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Flagella and cilia

Flagella usually longer than cilia and present singly or in pairs

Cilia are often shorter than flagella and tend to cover all or part of the surface of a cell

Share the same internal structure Microtubules, dynein, and axoneme Movement involves the propagation of a

bend, which begins at the base of the structure and proceeds toward the tip

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Sheetz and Spudich Show That Myosin Walks Along Actin Filaments Early researchers proposed the sliding-filament model

based on work with living cells in vivo 1983, Michael Sheetz and James Spudich devised an

approach to study myosin in vitro Nitella cells were used as a source of actin filaments Myosin was purified and attached to a fluorescently

labeled bead Confirmed that myosin is a motor protein that uses ATP

to walk along actin filaments All that is needed for movement are actin, myosin, and ATP

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Endomembrane system

Network of membranes enclosing the nucleus, endoplasmic reticulum, Golgi apparatus, lysosomes, and vacuoles

Also includes plasma membrane May be directly connected to each other or

pass materials via vesicles

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Nuclear envelope

Double-membrane structure enclosing nucleus

Outer membrane of the nuclear envelope is continuous with the endoplasmic reticulum membrane

Nuclear pores provide passageways Materials within the nucleus are not part of

the endomembrane system

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Nucleus

ChromosomesComposed of DNA and proteinChromatin

Primary function involves the protection, organization, and expression of the genetic material

Ribosome assembly occurs in the nucleolus

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Endoplasmic reticulum

Network of membranes that form flattened, fluid-filled tubules or cisternae

ER membrane encloses a single compartment called the ER lumen

Rough endoplasmic reticulum (rough ER) Studded with ribosomes Involved in protein synthesis and sorting

Smooth endoplasmic reticulum (smooth ER) Lacks ribosomes Detoxification, carbohydrate metabolism, calcium balance,

synthesis and modification of lipids

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Golgi apparatus

Also called the Golgi body, Golgi complex, or simply Golgi

Stack of flattened, membrane-bounded compartments, which are not continuous with the ER

Vesicles transport materials between stacks Three overlapping functions

Secretion, processing, and protein sorting

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Lysosomes Contain acid hydrolases that perform

hydrolysis Many different types of acid hydrolases to

break down proteins, carbohydrates, nucleic acids, and lipids

AutophagyRecycling of worn-out organelles through

endocytosis

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Vacuoles

Functions of vacuoles are extremely varied, and they differ among cell types and even environmental conditions

Central vacuoles in plants for storage and support

Contractile vacuoles in protists for expelling excess water

Phagocytic vacuoles in protists and white blood cells for degradation

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Plasma membrane

Boundary between the cell and the extracellular environment

Membrane transport in and out of cellSelectively permeable

Cell signaling using receptors Cell adhesion

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Semiautonomous organelles

Can grow and divide to reproduce themselves, but they are not completely autonomous because they depend on other parts of the cell for their internal components

Mitochondria, chloroplasts, and peroxisomes

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Mitochondria

Outer and inner membrane Intermembrane space and mitochondrial matrix

Primary role is to make ATP Also involved in the synthesis, modification,

and breakdown of several types of cellular molecules

Can also generate heat in brown fat cells

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Chloroplasts Photosynthesis

capture light energy and use some of that energy to synthesize organic molecules such as glucose

Found in nearly all species of plants and algae

Outer and inner membrane with an intermembrane space

Third membrane, the thylakoid membrane, forms flattened tubules that stack to form a granum (plural, grana)

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Peroxisomes

Relatively small organelles found in all eukaryotic cells

Origin remains controversial General function to catalyze certain chemical

reactions, typically those that break down molecules by removing hydrogen or adding oxygen

Reaction by-product is hydrogen peroxide (H2O2) Catalase breaks down H2O2 without forming

dangerous free radicals

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