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A Tour of the Cell
Chapter 7
Figure 7.1 The size range of cells.
Figure 7.3 Cell Fractionation
Biochemical approach in cell biology. Goal is to take cells apart, so that organelles can
be separated and their functions can be studied.
PROCESS:
•homogenization – disruption of cells
•low speed in centrifuge – separates into pellet and supernatant
•increase speed with each step, collect smaller and smaller organelles
Cells – The Basic Unit of Life
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CELLS ARE THE BASIC UNIT OF STRUCTURE
AND FUNCTION FOR LIVING THINGS.
ALL LIVING THINGS ARE MADE OF CELLS.
CELLS ARISE FROM PRE-EXISTING CELLS.
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Collectively, these are known as the Cell Theory!
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Prokaryotes vs. Eukaryotes
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Prokaryotes:
 NO NUCLEUS, but do have nucleoid region with DNA present
 Small and Simple – few organelles
 Have cell membranes and cytoplasm
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Ex. Bacteria
Eukaryotes:
 Contain nuclei
 Contains organelles that perform specialized functions
 Unicellular or multicellular
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Ex. Plant and animal cells
Figure 7.4 A Prokaryotic Cell
Lacking a true nucleus and the other membrane-enclosed organelles of the
eukaryotic cell, the prokaryotic cell is much simpler in structure.
Only organisms of the domain BACTERIA and ARCHAEA have prokaryotic
cells.
Figure 7.7 Overview of an Animal Cell
Figure 7.8 Overview of a Plant Cell
Basic Cell Parts: Cell Membrane
http://bcs.whfreeman.com/thelifewire/content/chp04/0402001.html
 Cell
membrane –
provides barrier between internal and
external environment of cell
 is semi-permeable (some things can go
in, some cannot; some things can exit,
some never can)
 made up of phospholipid bilayer with
proteins embedded that allow for needed
passage of large molecules
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Function of Cell Membrane
• Major job of cell membrane is to maintain the
cell’s environment – establish homeostasis!
Problem: Surface Area to Volume Ratio
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Surface area acts as limiting factor in size of
cell because is a two dimensional unit.
Volume is three dimensional, so increases
more quickly than the surface area can
accommodate.
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Larger organisms do not generally have LARGER
CELLS, simply MORE CELLS!
Why are cells
microscopic?
Surface Area to Volume Ratio Problem

Metabolic requirements impose upper
limits on cell size.
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As an object increases in size, its volume grows
proportionately more than surface area (area2 and
volume3)…
So…the smaller the object, the greater
its ratio of surface area to volume.
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As an object increases in size its volume increases as the
cube of its linear dimensions while surface area increases
as the square.
As these cubes illustrate the surface area to volume ratio of
a small object is larger than that of a large object of similar
shape. This ratio limits how large cells can be.
Figure 8.6 The detailed structure of an animal cell’s plasma membrane, in
cross section
Figure 7.6 The Plasma Membrane
Fluid Mosaic Model
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Cell membrane and embedded proteins are
not locked into position – they flow against
one another as the cytoplasm and the
external liquid environment dictate.
Figure 8.7 The Structure of a Transmembrane Protein
Other Components of the Cell Membrane
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Cholesterol – helps to stabilize the phospholipids
Proteins in Cell Membrane
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Channel proteins – act as passageways through the
phospholipid bilayer for large things:
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Receptor proteins – receive info about the environment
outside the cell and transmit it into the cell – no physical
molecule passes through this, just INFO:
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These are integral proteins – penetrate the hydrophobic core of the
lipid bilayer.
may be integral or peripheral proteins – not embedded in the
bilayer, can be extensions of integral proteins
Marker Proteins – identify the type of cell using
carbohydrate chains (glycoproteins)
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may also have glycolipids that are not attached to proteins
Cytoplasm
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CYTOPLASM includes the entire region
between the nucleus and the cell membrane!

The semi-fluid substance that fills this area is
called CYTOSOL, and this is what the
organelles are suspended in.
Cell Wall
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Found in plant cells (another barrier in
ADDITION to the cell membrane).
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Protects the cell.
Gives support to cell.
Made of polysaccharide called cellulose.
Is very porous and allows molecules to pass
through, but is NOT SELECTIVELY
PERMEABLE!!!
Organelles
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Control:
*Nucleus
*Centrosome
Assembly, Transport, and Storage:
*Endoplasmic reticulum
*Ribosomes
*Golgi apparatus
*Vacuoles
*Lysosomes
*Leucoplasts
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(plant and animal)
(plant and animal)
(plant and animal)
(plant and animal)
(plant and animal)
(plant -1 large, and animal many)
(animal)
(plant only)
Energy transformations:
*Chloroplasts and Chromoplasts
*Mitochondria
(plant only)
(plant and animal)
Some Useful Animation Tutorials
• http://bcs.whfreeman.com/thelifewire/conte
nt/chp04/0402001.html
• www.cellsalive.com
Nucleus
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Contains most of eukaryotic cell’s genetic library
Largest organelle
Enclosed by nuclear envelope or membrane, which is a double
membrane – each of which is a lipid bilayer!!!
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Nuclear envelope has pores in it
Nuclear lamina lines nuclear side of envelope – a net-like array
of protein filaments that maintain nuclear shape.
 May also be a nuclear matrix that extends into the entire nucleus
Contains inactive DNA – chromatin
 When gets ready to divide, chromatin condenses into
chromosomes
Directs protein synthesis by synthesizing mRNA and sending to
ribosomes in the cytoplasm
 DNA  mRNA  protein (transcription and translation)
Figure 7.9 The nucleus and its envelope
Nucleolus
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Prominent structure in non-dividing nucleus
Components of ribosomes are synthesized
here (ribosomal RNA and ribosomal subunits)
Ribosomes
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Sites of PROTEIN SYNTHESIS
Are made of rRNA and protein
Made of two subunits –
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one acts as attachment point for beginning of translation,
other acts as “cap” to prevent mRNA from moving until tRNA has
brought in appropriate amino acid
Cells with high rates of protein synthesis have MANY
ribosomes (human pancreas cell has MILLIONS of
ribosomes)
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There are “free” ribosomes in cytosol that make proteins for the
cell in which they reside
Ribosomes that are attached to endoplasmic reticulum (bound) are
making proteins for packaging and export
Figure 7.10 Ribosomes
Endomembrane System
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Many of the different membranes of the eukaryotic cell are
part of an ENDOMEMBRANE SYSTEM.
Membranes in cell are not identical in structure or function
(modifications are present according to job)
Includes:
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nuclear envelope
endoplasmic riticulum
Golgi apparatus,
Lysosomes
Vacuoles
plasma membrane
Figure 7.16 Review: relationships among organelles of the endomembrane
system
Endoplasmic Reticulum
• A membrous system of channels
and flattened sacs that traverse
the cytoplasm
• 2 varieties:
– Rough ER: the site of protein
synthesis resulting from the attached
ribosomes
– Smooth ER: assists in the synthesis
of steroid hormones and other lipids
• Also connects rough ER to the Golgi
apparatus and carries out various
detoxification processes in liver
• Smooth and rough E.R. are
actually connected, not distinct,
separate sections
Golgi Apparatus
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Packages substances produced in the rough ER
and secretes them to other cell parts or to the cell
surface for export.
Has cis (entrance) side and trans (exit) side
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Golgi will modify products as needed – gives more variety
by removing some monomers and substituting others
Knows what to do by using molecular identification
tags (like phosphate groups); even adds molecules
on their membranes that may recognize “docking
sites” on organelle surfaces or on the cell
membranes of other cells
Figure 7.12 The Golgi apparatus
Lysosomes
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Membrane-bounded sac of hydrolytic enzymes – are the
principle site of intracellular digestion
Different lysosomes break down each of the major classes of
macromolecules – proteins, polysaccharides, fats, nucleic
acids
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Work best at pH of 5
Using active transport to maintain this – pumps hydrogen ions
from cytosol into itself
Used in autophagy – recycle the cell’s own organic material
for use
Can also be used in programmed destruction of cells by
lysosomal enzymes – ex. Tadpole loses tail
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Related diseases – Pompe’s and Tay-Sachs (pg. 122)
Figure 7.14 The formation and functions of lysosomes (Layer 1)
Figure 7.14 The formation and functions of lysosomes (Layer 2)
Figure 7.14 The formation and functions of lysosomes (Layer 3)
Figure 7.17 The mitochondrion, site of cellular respiration
Figure 7.18 The chloroplast, site of photosynthesis
Animated Tutorial – Endosymbiotic
Theory
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http://www.sumanasinc.com/webcontent/ani
mations/content/organelles.html
Peroxisomes
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Job is to generate and degrade hydrogen
peroxide—contain enzymes that transfer
hydrogen from various substrates and make
H2O2 as a by-product
Also detoxify alcohol in liver cells
H2O2 is toxic, but peroxisomes contain
enzymes that convert it to water.
Cytoskeleton
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Network of fibers extending into cytoplasm of
cell
Provides structural support, and aids in cell
motility and cell regulation
Made up of microtubules (thickest),
microfilaments (thinnest), and intermediate
filaments (see page 127)
Table 7.2 The structure and function of the cytoskeleton
Centrosome and Centrioles
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Microtubules often grow out of centrosome
(central area of cell)
Within the centrosome are centrioles, each
composed of nine sets of triplet microtubules
arranged in a ring; CENTRIOLES aid in
chromosome separation
Figure 7.22 Centrosome containing a pair of centrioles
Cilia and Flagella
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The movement of these two locomotor
appendages is controlled by microtubules
Cilia are short projections, flagella are much
longer
Movement may not be for entire organism;
may be part of a larger unit – ex. Cilia lining
windpipe propel foreign substances out…
See page 129 for diagrams
Figure 7.23 A comparison of the beating of flagella and cilia
Figure 7.24 Ultrastructure of a eukaryotic flagellum or cilium
Dynein
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Large protein that makes up the motor
molecules that extend from each microtubule
doublet to the next
Responsible for the bending and movement
of cilia and flagella
See page 130
Figure 7.25 How dynein “walking” moves cilia and flagella
Actin and Myosin
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Actin is a globular protein; makes up
microfilaments
Myosin is a protein that acts as a motor
molecule – “walking” along the actin filaments
Both aid in use of pseudopodia and
cytoplasmic streaming
Extracellular Matrix
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Found in animal cells
Main ingredients are glycoproteins
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most abundant one is collagen
Collagen fibers are embedded in a network woven
from proteoglycans – lots of these in carbohydrates
Fibronectins – glycoproteins that bind to receptor
proteins called integrins
Integrins span the membrane, bind on cytoplasmic
side to microfilaments of cytoskeleton
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See page 133 for role of ECM in regulating cell behavior!
Cell Surfaces and Junctions
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Cell wall – plant cells (much thicker than plasma
membrane, contains microfibrils made of
cellulose)
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Protects plant cell
Maintains shape
Prevents excessive water uptake
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primary cell wall -- in young plant cells, thin and flexible (when
mature, hardening materials are added for strength)
middle lamella – between cell walls of adjacent cells, contains
pectins (thick polysaccaride)
secondary cell wall – may be added when plant is older, found
between plasma membrane and primary wall, consists of
several laminated layers (Ex. Wood)
Figure 7.28 Plant cell walls
plasmodesmata – plants; channels that allow cytosol to pass through
and connect the living contents of adjacent cells (see page 134)
Animal Cell Junctions
Neighboring cells often adhere, interact, and
communicate through special patches of direct
physical contact…intracellular junctions help
integrate cells!
See Text Figure 7.30
tight junctions – animals
– membranes of
neighboring cells are
actually fused; prevent
leakage of extracellular
fluid across a layer of
epithelial cells
desmosomes – animals;
“anchoring junctions” –
function like rivets,
fastening cells together in
strong sheets (are
reinforced by intermediate
filaments made of keratin)
gap junctions – animals;
“communicating junctions”
provide cytoplasmic
channels between
adjacent animal cells –
see page 134