Chapter 6A Tour of the Cell

The Importance of Cells

All organisms are made of cellsThe cell is the simplest collection of living

matter; the smallest unit of life.

Figure 6.1

Microscopy

To study cells, biologists use microscopes and the tools of biochemistry

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

Types of microscopes◦Light microscopes◦Electron microscopes

Microscopy

Magnification – The ratio of an object’s image size to its actual size

Resolution – The measure of clarity of the object’s image; the minimum distance two distinct points can be separated by and still be distinguished as two points.

Microscopy

Light microscopes (LMs)◦Pass visible light through a specimen◦Magnify structures w/ lenses◦Limited magnification; cannot resolve detail

finer than 2 μm, or 200 nm.◦Up to 1000x magnification

Microscopy

Different types of microscopes◦Can be used to

visualize different sized cellular structures

◦Have other disadvantages and advantages associated with them

1 m

0.1 nm

10 m

0.1 m

1 cm

1 mm

100 µm

10 µ m

1 µ m

100 nm

10 nm

1 nm

Length of somenerve and muscle cells

Chicken egg

Frog egg

Most plant and Animal cells

Smallest bacteria

Viruses

Ribosomes

Proteins

Lipids

Small molecules

Atoms

NucleusMost bacteriaMitochondrion

Lig

ht m

icro

sco

pe

Ele

ctro

n m

icro

sco

pe

Ele

ctro

n m

icro

sco

pe

Figure 6.2

Human height

Measurements1 centimeter (cm) = 102 meter (m) = 0.4 inch1 millimeter (mm) = 10–3 m1 micrometer (µm) = 10–3 mm = 10–6 m1 nanometer (nm) = 10–3 mm = 10–9 m

MicroscopyTECHNIQUE RESULT

Brightfield (unstained specimen). Passes light directly through specimen. Unless cell is naturally pigmented or artificially stained, image has little contrast. [Parts (a)–(d) show a human cheek epithelial cell.]

(a)

Brightfield (stained specimen). Staining with various dyes enhances contrast, but most staining procedures require that cells be fixed (preserved).

(b)

Phase-contrast. Enhances contrast in unstained cells by amplifying variations in density within specimen; especially useful for examining living, unpigmented cells.

(c)

50 µm

Figure 6.3

Differential-interference-contrast (Nomarski). Like phase-contrast microscopy, it uses optical modifications to exaggerate differences indensity, making the image appear almost 3D.

Fluorescence. Shows the locations of specific molecules in the cell by tagging the molecules with fluorescent dyes or antibodies. These fluorescent substances absorb ultraviolet radiation and emit visible light, as shown here in a cell from an artery.

Confocal. Uses lasers and special optics for “optical sectioning” of fluorescently-stained specimens. Only a single plane of focus is illuminated; out-of-focus fluorescence above and below the plane is subtracted by a computer. A sharp image results, as seen in stained nervous tissue (top), where nerve cells are green, support cells are red, and regions of overlap are yellow. A standard fluorescence micrograph (bottom) of this relatively thick tissue is blurry.

50 µm

50 µm

(d)

(e)

(f)

Microsopcy

Electron microscopes (EMs) focus a beam of electrons either through or over the surface of a cell.◦Scanning Electron Microscope – Electrons pass

over surface of cell; resulting image is a three dimensional view of the surface of a cell.

◦Transmission Electron Microscope – Electrons pass through a cell (slice); resulting image shows internal structures of cell.

MicroscopyScanning electron microscope (SEM)

◦detail surface of a specimen (3D)

TECHNIQUE RESULTS

Scanning electron micro-scopy (SEM). Micrographs takenwith a scanning electron micro-scope show a 3D image of the surface of a specimen. This SEM shows the surface of a cell from a rabbit trachea (windpipe) covered with motile organelles called cilia. Beating of the cilia helps moveinhaled debris upward toward the throat.

(a)

Cilia1 µm

Figure 6.4 (a)

Microscopy

Transmission electron microscope (TEM)◦details internal ultrastructure of cells

Transmission electron micro-scopy (TEM). A transmission electron microscope profiles a thin section of a specimen. Here we see a section through a tracheal cell, revealing its ultrastructure. In preparing the TEM, some cilia were cut along their lengths, creating longitudinal sections, while other cilia were cut straight across, creating cross sections.

(b)

Longitudinalsection ofcilium

Cross sectionof cilium

1 µm

Figure 6.4 (b)

Microscopy

Biological samples stained with heavy metals

Electrons on cell surface become excited and can be detected electronically.

Uses electromagnets as lenses rather than glass.

Microscopy

Cytology – The study of cell structure

Brainstorm: Compare and contrast the different types of microscopes.What are some advantages to each type?What are some disadvantages?

Microscopy

What type of microscope would be used to study:◦Changes in shape of a living white blood cell◦Details of the surface structure of a hair◦The detailed structure of an organelle?

Isolating Organelles by Cell FractionationCell fractionation

◦Takes cells apart & separates organelles from 1 another

◦Centrifuge fractionates cells into their component parts

Tissuecells

Homogenization

Homogenate1000 g(1000 times theforce of gravity)

10 min Differential centrifugationSupernatant pouredinto next tube

20,000 g20 min

Pellet rich innuclei andcellular debris

Pellet rich inmitochondria(and chloro-plasts if cellsare from a plant)

Pellet rich in“microsomes”(pieces of plasma mem-branes andcells’ internalmembranes)

Pellet rich inribosomes

150,000 g3 hr

80,000 g60 min

Figure 6.5

In the original experiments, the researchers used microscopy to identify the organelles in each pellet, establishing a baseline for further experiments. In the next series ofexperiments, researchers used biochemical methods to determine the metabolic functions associated with each type of organelle. Researchers currently use cell fractionation to isolate particular organelles in order to study further details of their function.

RESULTS

Figure 6.5

Prokaryotic and Eukaryotic Cells

Review: What are the main differences between eukaryotic cells and prokaryotic cells?

Create a venn diagram to compare and contrast the structures that are found in eukaryotic and prokaryotic cells. Define all of the terms (including a description of the function of organelles). Using those definitions, write a brief explanation of the different capabilities of each type of cell.

Comparing Prokaryotic & Eukaryotic CellsPlasma membraneCytoplasmCytosolChromosomesRibosomes

Brainstorm: What is the function of ribosomes?

Hint: Look at the word. What other term does it remind you of?

Prokaryotic Cells

“Pro” + “karyon” (kernel)No nucleusDNA is found in the nucleoidNucleoid is not bound by a membrane

(b) A thin section through the bacterium Bacillus coagulans (TEM)

Pili: attachment structures onthe surface of some prokaryotes

Nucleoid: region where thecell’s DNA is located (notenclosed by a membrane)

Ribosomes: organelles thatsynthesize proteins

Plasma membrane: membraneenclosing the cytoplasm

Cell wall: rigid structure outsidethe plasma membrane

Capsule: jelly-like outer coatingof many prokaryotes

Flagella: locomotionorganelles ofsome bacteria

(a) A typical rod-shaped bacterium

0.5 µmBacterialchromosome

Figure 6.6 A, B

Eukaryotic Cells

“Eu” + “karyon”Membrane bound nucleus

◦Nuclear envelope

Prokaryotes vs Eukaryotes

Brainstorm: Why does the size of the cell matter?

Prokaryotes vs Eukaryotes

A smaller cell ◦Has higher surface area to volume ratio◦Makes it easier to transport enough through

membraneSurface area increases whiletotal volume remains constant

5

11

Total surface area (height width number of sides number of boxes)

Total volume (height width length number of boxes)

Surface-to-volume ratio (surface area volume)

6

1

6

150

125

12

750

125

6

Figure 6.7

Eukaryotic Cells

Plasma membrane acts as a selective barrier (semipermeable)◦Allows passage of nutrients

& wasteCarbohydrate side chain

Figure 6.8 A, B

Outside of cell

Inside of cell

Hydrophilicregion

Hydrophobicregion

Hydrophilicregion

(b) Structure of the plasma membrane

Phospholipid Proteins

TEM of a plasmamembrane. Theplasma membrane,here in a red bloodcell, appears as apair of dark bandsseparated by alight band.

(a)

0.1 µm

Eukaryotic CellsMembrane bound organelles

◦Compartmentalize the cell◦Membranes participate in metabolic activity

Brainstorm: Why is it important for eukaryotic cells to be compartmentalized?

Eukaryotic Cells

Plant cells vs animal cells◦Have most of the same organelles◦More similar to each other than either are to

prokaryotic cells

Practice: Just like you did for prokaryotes vs eukaryotes, create a venn diagram comparing and contrasting plant and animal cells. Focus on organelles, and include a key that defines each of the terms.

A animal cell

Rough ER Smooth ER

Centrosome

CYTOSKELETON

Microfilaments

Microtubules

Microvilli

Peroxisome

Lysosome

Golgi apparatus

Ribosomes

In animal cells but not plant cells:LysosomesCentriolesFlagella (in some plant sperm)

Nucleolus

Chromatin

NUCLEUS

Flagelium

Intermediate filaments

ENDOPLASMIC RETICULUM (ER)

Mitochondrion

Nuclear envelope

Plasma membrane

Figure 6.9

A plant cell

In plant cells but not animal cells:ChloroplastsCentral vacuole and tonoplastCell wallPlasmodesmata

CYTOSKELETON

Ribosomes (small brwon dots)

Central vacuole

Microfilaments

Intermediate filaments

Microtubules

Rough endoplasmic reticulum Smooth

endoplasmic reticulum

ChromatinNUCLEUS

Nuclear envelope

Nucleolus

Chloroplast

PlasmodesmataWall of adjacent cell

Cell wall

Golgi apparatus

Peroxisome

Tonoplast

Centrosome

Plasma membrane

Mitochondrion

Figure 6.9

Eukaryotic Cells

Genetic instructions are housed in the nucleus and carried out by the ribosomes

Nucleus◦Genetic library of the cell◦Bound by nuclear envelope (double membrane)◦Usually the largest, most obvious organelle◦Protein studded surface Pore complex

Nuclear Lamina – maintains shapeChromosomes – made of chromatinNucleolus – cite of rRNA synthesis

Eukaryotic Cells nuclear envelope

◦Encloses the nucleus, separates contents from cytoplasm

Figure 6.10

Nucleus

NucleusNucleolus

Chromatin

Nuclear envelope:Inner membraneOuter membrane

Nuclear pore

Rough ER

Porecomplex

Surface of nuclear envelope.

Pore complexes (TEM). Nuclear lamina (TEM).

Close-up of nuclearenvelope

Ribosome

1 µm

1 µm0.25 µm

Eukaryotic Cells

Ribosomes◦Made of ribosomal RNA (rRNA)

and protein◦Carry out protein synthesis (translation)◦Free ribosomes – suspended in cytosol◦Bound ribosomes – attached to rough ER or

nuclear envelope◦Ribosomes can alternate roles

ER

Endoplasmic reticulum (ER)

Ribosomes Cytosol

Free ribosomes

Bound ribosomes

Largesubunit

Smallsubunit

TEM showing ER and ribosomes Diagram of a ribosome

0.5 µm

Figure 6.11

Eukaryotic Cells

Endomembrane system Regulates protein traffic and performs

metabolic functions in the cell◦Protein synthesis and transport◦Metabolism of lipids◦Detoxification

Structural differences mediate function◦Endoplasmic reticulum, Golgi apparatus,

lysosomes, vacuoles, plasma membrane.

The Endoplasmic Reticulum: Biosynthetic Factory

Endoplasmic reticulum accounts for more than half of the total membrane in eukaryotic cells

Net-like structure made up of membranous tubules and cisternae (sacs of liquid)

Rough ER – Covered in ribosomesSmooth ER – Lacks ribosomes

ER membrane◦continuous with/attached to the nuclear envelope

Smooth ER

Rough ER

ER lumenCisternae

RibosomesTransport vesicle

Smooth ER

Transitional ER

Rough ER 200 µm

Nuclearenvelope

Figure 6.12

Functions of Smooth ER

◦Synthesizes lipids◦Metabolizes carbohydrates◦Stores Ca+

Releases into cytosol in response to nerve impulse, causing muscle contraction

◦Detoxifies poison (adds hydroxyl groups to drugs)

Functions of Rough ER

◦Studded with ribosomes◦Makes proteins and membranes, distributed by

transport vesicles (sacs of membrane) Secreted proteins Membrane-bound proteins

◦Glycoproteins – secretory proteins, covalently bonded to carbohydrates

◦Synthesizes membranes Phospholipids Embeds proteins directly into membrane

The Golgi Apparatus: Shipping and Receiving Center Golgi apparatus

◦gets transport vesicles from rough ER◦flattened membranous sacs called cisternae◦Modifies products of rough ER◦Makes certain macromolecules

http://www.hhmi.org/bulletin/winter-2014/express-shipping

https://www.youtube.com/watch?v=jazs6sRQ2og

http://www.ted.com/talks/david_bolinsky_animates_a_cell

Golgiapparatus

TEM of Golgi apparatus

cis face(“receiving” side ofGolgi apparatus)

Vesicles movefrom ER to Golgi Vesicles also

transport certainproteins back to ER

Vesicles coalesce toform new cis Golgi cisternae

Cisternalmaturation:Golgi cisternaemove in a cis-to-transdirection

Vesicles form andleave Golgi, carryingspecific proteins toother locations or tothe plasma mem-brane for secretion

Vesicles transport specificproteins backward to newerGolgi cisternae

Cisternae

trans face(“shipping” side ofGolgi apparatus)

0.1 0 µm16

5

2

3

4

Golgi apparatus

Figure 6.13

Lysosomes: Digestive CompartmentsMembranous sacsFull of digestive enzymesInternally acidicSynthesized by the rough ER, processed

by the GolgiPhagocytosis – intracellular digestion

◦Cells engulf smaller objects (sometimes organisms), forming a food vacuole

◦Food vacuole fuses with lysosome

Lysosomes

Figure 6.14 A(a) Phagocytosis: lysosome digesting food

1 µm

Lysosome containsactive hydrolyticenzymes

Food vacuole fuses with lysosome

Hydrolyticenzymes digestfood particles

Digestion

Food vacuole

Plasma membraneLysosome

Digestiveenzymes

Lysosome

Nucleus

Lysosomes

Autophagy- self eating (vesicle surrounds damaged organelle)

Figure 6.14 B

(b) Autophagy: lysosome breaking down damaged organelle

Lysosome containingtwo damaged organelles 1 µ m

Mitochondrionfragment

Peroxisomefragment

Lysosome fuses withvesicle containingdamaged organelle

Hydrolytic enzymesdigest organellecomponents

Vesicle containingdamaged mitochondrion

Digestion

Lysosome

Vacuoles: Diverse Maintenance CompartmentsPlant or fungal cells have at least one

vacuoleFood vacuoles – surround ingested

materials so that they can be broken downContractile vacuoles – pump excess water

out of a cellCentral vacuole – enclosed by tonoplast,

also works to maintain homeostasis.Cell sap – solution inside of the central

vacuole

Central Vacuole

Central vacuole

Cytosol

Tonoplast

Centralvacuole

Nucleus

Cell wall

Chloroplast

5 µmFigure 6.15

The Endomembrane System: A ReviewCompartmental organization of a cellSystem of interconnected membranesFunction, composition, and contents

modified with progression from ER to vacuoles

Brainstorm: How do transport vesicles serve to integrate the endomembrane system?

Plasma membrane expandsby fusion of vesicles; proteinsare secreted from cell

Transport vesicle carriesproteins to plasma membrane for secretion

Lysosome availablefor fusion with anothervesicle for digestion

4 5 6

Nuclear envelope isconnected to rough ER, which is also continuouswith smooth ER

Nucleus

Rough ER

Smooth ERcis Golgi

trans Golgi

Membranes and proteinsproduced by the ER flow inthe form of transport vesiclesto the Golgi Nuclear envelop

Golgi pinches off transport Vesicles and other vesicles that give rise to lysosomes and Vacuoles

1

3

2

Plasmamembrane

Relationships within the endomembrane system

Figure 6.16

Mitochondria and Chloroplasts

Change energy from one form to anotherMitochondria

◦Sites of cellular respirationChloroplasts

◦Only in plants & plantlike protists; photosynthesis

Mitochondria

Chemical energy conversion – site of cellular respiration

Generates ATP from fuel sources (sugars, fats, etc…)

Double membrane◦Outer = smooth◦ Inner = convoluted; infoldings called cristae

Intermembrane space and mitochondrial matrixNumber of mitochondria related to metabolic

activity of a cell

Mitochondria; enclosed by 2 membranes◦smooth outer membrane◦inner membrane folded into cristae

Mitochondrion

Intermembrane space

Outermembrane

Freeribosomesin the mitochondrialmatrix

MitochondrialDNA

Innermembrane

Cristae

Matrix

100 µm

Figure 6.17

Brainstorm: Why is the inner membrane folded?

Chloroplasts: Capture of Light EnergyPlastids – family of plant organelles

◦Amyloplast – colorless; stores starch◦Chromoplast – pigmented; gives color to

flowers◦Chloroplast

Chloroplast◦has chlorophyll – green pigment◦Enzymes and molecules that function in

photosynthetic production

Chloroplasts

Leaves and green algaeChloroplast structure:

◦ Thylakoids- membranous sacs; flattened◦ Stroma- internal fluid; contains chloroplast DNA

Chloroplast

ChloroplastDNA

RibosomesStroma

Inner and outermembranes

Thylakoid

1 µm

GranumFigure 6.18

Peroxisomes: Oxidation

◦ Make H2O2 (hydrogen peroxide) and convert it to H2O◦ Single membrane; transfers hydrogen◦ Break down fatty acids to transfer to mitochondria

ChloroplastPeroxisome

Mitochondrion

1 µm

Figure 6.19

Brainstorm: What role woulda peroxisome in a liver cellhave?

The Cytoskeleton

A network of fibers that organizes structures and activities in the cell

Figure 6.20

Microtubule

0.25 µm MicrofilamentsFigure 6.20

Roles of the Cytoskeleton: Support, Motility, and Regulation

Cytoskeleton◦Gives mechanical support to the cell◦involved in cell motility, using motor proteins

VesicleATP

Receptor formotor protein

Motor protein(ATP powered)

Microtubuleof cytoskeleton

(a) Motor proteins that attach to receptors on organelles can “walk”the organelles along microtubules or, in some cases, microfilaments.

Microtubule Vesicles 0.25 µm

(b) Vesicles containing neurotransmitters migrate to the tips of nerve cell axons via the mechanism in (a). In this SEM of a squid giant axon, two  vesicles can be seen moving along a microtubule. (A separate part of the experiment provided the evidence that they were in fact moving.)Figure 6.21 A, B

3 types of fibers make up the cytoskeleton

Table 6.1

The wacky history of cell theory - Lauren Royal-Woods | TED-Ed

Microtubules

1.) Microtubules◦Shape the cell◦Guide movement of organelles◦Help separate chromo copies (mitosis)

Centrosomes and Centrioles

Centrosome- “microtubule-organizing center”

Cilia and Flagella

Cilia & flagella◦have specialized arrangements of microtubules◦locomotor appendages of some cells

Brainstorm: How are cilia and flagella similar? How are they different?

Flagella beating pattern

(a) Motion of flagella. A flagellum usually undulates, its snakelike motion driving a cell in the same direction as the axis of the flagellum. Propulsion of a human sperm cell is an example of flagellatelocomotion (LM).

1 µm

Direction of swimming

Figure 6.23 A

Ciliary motion

(b) Motion of cilia. Cilia have a back- and-forth motion that moves the cell in a direction perpendicular to the axis of the cilium. A dense nap of cilia, beating at a rate of about 40 to 60 strokes a second, covers this Colpidium, a freshwater protozoan (SEM).

Figure 6.23 B

15 µm

Cilia and flagella share a common ultrastructure

(a)

(c)

(b)

Outer microtubuledoublet

Dynein arms

Centralmicrotubule

Outer doublets cross-linkingproteins inside

Radialspoke

Plasmamembrane

Microtubules

Plasmamembrane

Basal body

0.5 µm

0.1 µm

0.1 µm

Cross section of basal body

Triplet

Figure 6.24 A-C

The protein dynein◦Is responsible for the bending movement of cilia

and flagella

Microtubuledoublets ATP

Dynein arm

Powered by ATP, the dynein arms of one microtubule doublet grip the adjacent doublet, push it up, release, and then grip again. If the two microtubule doublets were not attached, they would slide relative to each other.

(a)

Figure 6.25 A

Outer doubletscross-linkingproteins

Anchoragein cell

ATP

In a cilium or flagellum, two adjacent doublets cannot slide far because they are physically restrained by proteins, so they bend. (Only two ofthe nine outer doublets in Figure 6.24b are shown here.)

(b)

Figure 6.25 B

Localized, synchronized activation of many dynein arms probably causes a bend to begin at the base of the Cilium or flagellum and move outward toward the tip. Many successive bends, such as the ones shown here to the left and right, result in a wavelike motion. In this diagram, the two central microtubules and the cross-linking proteins are not shown.

(c)

1 3

2

Figure 6.25 C

Microfilaments (Actin Filaments)

Microfilaments are twisted double chains of actin◦Actin = globular protein◦Branching network connected by proteins◦Tension bearing (pulling forces)◦Connected with myosin (motor protein) ◦Ameoboid movement (pseudopodia) and

cytoplasmic streaming

Microfilaments

0.25 µm

Microvillus

Plasma membrane

Microfilaments (actinfilaments)

Intermediate filaments

Figure 6.26

Microfilaments

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

Actin filament

Myosin filament

Myosin motors in muscle cell contraction. (a)

Muscle cell

Myosin arm

Figure 6.27 A

Amoeboid movement◦Involves the contraction of actin & myosin

filamentsCortex (outer cytoplasm):gel with actin network

Inner cytoplasm: sol with actin subunits

Extendingpseudopodium

(b) Amoeboid movementFigure 6.27 B

Cytoplasmic streaming◦Is another form of locomotion created by

microfilaments

Nonmovingcytoplasm (gel)

Chloroplast

Streamingcytoplasm(sol)

Parallel actinfilaments Cell wall

(b) Cytoplasmic streaming in plant cellsFigure 6.27 C

Intermediate Filaments

Support cell shapeFixes organelles in spaceNo set structure or function (diverse class

of cytoskeletal elements)◦Constructed from proteins from specific family

(e.g. keratin)More permanent than microtubules and

microfilaments

Cell Walls of Plants

An extracellular structure of plant cells that distinguishes them from animal cells

Cellulose fibers embedded in other polysaccharides and protein

Middle lamina (sticky)Multiple layers

◦Primary cell wall◦Secondary cell wall

The Extracellular Matrix (ECM) of Animal CellsAnimal cells do not have a cell wallInstead are connected by extracellular

matrixComposed mostly of glycoproteins

◦Collagen◦Proteoglycans◦Fibronectin

Connected to proteins embedded in plasma membrane called integrins

Collagen

Fibronectin

Plasmamembrane

EXTRACELLULAR FLUID

Micro-filaments

CYTOPLASM

Integrins

Polysaccharidemolecule

Carbo-hydrates

Proteoglycanmolecule

Coreprotein

Integrin

Figure 6.29

A proteoglycan complex

Extracellular Matrix

Functions◦Support◦Adhesion◦Movement◦Regulation

Intercellular Junctions

Plants: Plasmodesmata

Plasmodesmata◦channels in plant cell walls

Interiorof cell

Interiorof cell

0.5 µm Plasmodesmata Plasma membranes

Cell walls

Figure 6.30

Animals: Tight Junctions, Desmosomes, and Gap Junctions

Animals have 3 types of intercellular junctions◦Tight junctions◦Desmosomes◦Gap junctions

Types of intercellular junctions in animals

Tight junctions prevent fluid from moving across a layer of cells

Tight junction

0.5 µm

1 µm

Spacebetweencells

Plasma membranesof adjacent cells

Extracellularmatrix

Gap junction

Tight junctions

0.1 µm

Intermediatefilaments

Desmosome

Gapjunctions

At tight junctions, the membranes ofneighboring cells are very tightly pressedagainst each other, bound together byspecific proteins (purple). Forming continu-ous seals around the cells, tight junctionsprevent leakage of extracellular fluid acrossA layer of epithelial cells.

Desmosomes (also called anchoringjunctions) function like rivets, fastening cellsTogether into strong sheets. IntermediateFilaments made of sturdy keratin proteinsAnchor desmosomes in the cytoplasm.

Gap junctions (also called communicatingjunctions) provide cytoplasmic channels fromone cell to an adjacent cell. Gap junctions consist of special membrane proteins that surround a pore through which ions, sugars,amino acids, and other small molecules maypass. Gap junctions are necessary for commu-nication between cells in many types of tissues,including heart muscle and animal embryos.

TIGHT JUNCTIONS

DESMOSOMES

GAP JUNCTIONS

Figure 6.31

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

5 µ

m

Figure 6.32