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Cell Structure and Function
Pattern #3: Life Needs an
Inside and an Outside
How Small is Small
Unit Outline
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Background
Cell Theory
Prokaryotes vs. Eukaryotes
Surface Area vs. Volume
Form vs. Function
Cell Membrane
Cytoplasma
Nucleus
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Endoplasmic Reticulum
Ribosomes
Golgi Apparatus
Lysosomes
Vacuoles
Mitochondria
Cytoskeleton
Endosymbiosis
First Glimpse of The Cell
• 1665 – Robert Hooke
– One of the first microscopists–wrote Micrographia
– Thin slices of cork, completely enclosed “cells”
• 1677 – Anton Van Leeuwenhoek
– First to view living cells – animalcules
– Discovered bacteria in the human mouth
Birth of Cell Theory
• 1831 – Robert Brown
– Discovered nucleus in plant cells
• 1838 - Matthias Schleiden
– All plants are composed of cells
• 1839 – Theodor Schwann
– All animals and tissues are composed of cells
• 1855 – Robert Remak and Rudolf Virchow
– Cells come from other living cells
Cell Theory
1) All organisms are composed of one or more
cells.
2) Cells are the smallest living things, the basic
units of organization for all organisms.
3) Cells arise only by the division of a previously
existing cell.
Prokaryotic Cells
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No true nucleus
Still carry out all life functions
Bacteria, cyanobacteria
Cell membrane and sometimes cell wall
Few/no organelles (ribosomes)
Movement by flagella
Eukaryotic Cell
• True nucleus
• Membrane bound organelles
• Compartmentalizaiton
Limits to Cell Size
• Communication
• Diffusion/Transportation
• Surface Area to Volume ratio
– Smaller cells have more surface area per unit
volume
– Larger cells must import/export more
materials through the cell membrane
– Volume increases at a cubic rate, surface area
at a squared rate
The Bigger They Come The
Harder to They are to Maintain
Cubic Cell
Spherical Cell
Cell
Radius
(mm)
Surface
Area
(mm2)
Volume
(mm3)
S.A.:Vol
Cell
Radius
(mm)
Surface
Area
(mm2)
Volume S.A.:Vol
(mm3)
1
6
1
6:1
1
12.56
4.18
3:1
2
24
8
3:1
2
50.24
33.49
1.5:1
3
54
27
2:1
3
113
113
1:1
5
150
125
1.2:1
5
314
523
0.6:1
Some cells are much larger than
others. Given the constraints
imposed by the S.A. to volume
ratio, how would you expect the
level of activity in large cells to
compare with that in small cells?
Form Follows Function
• Nerve cells are long and skinny to
transmit messages
• Red Blood Cells are close to spherical to
maximize S.A. to volume
• Skin cells fit together tightly
Just How Small Are We
Talking?
• Micrometer or micron - m
• Nanometer – nm
• Angstrom – Å
• 1 Å = 0.1 nm = 10-4 m = 10-8 cm
• Most cells < 50 m
Plasma Membrane
Consistent from Bacteria to Mammals
1) Forms a protective outer barrier for the
cell
2) Helps maintain a constant internal
environment
3) Regulates exchange of substances in and
out of the cell
Fluid-Mosaic Model
1972 – Singer and Nicolson
The membrane is made of a phospholipid bilayer that
is viscous and free to move. Globular proteins are
embedded in the bilayer and move about as well.
The Hydrophobic ends of the lipids create a nonpolar region within the membrane. This region
impedes the passage of all water soluble molecules.
Hydrophilic heads exist at the inner and outer
surfaces and allow specific chemical interactions to
take place.
Membrane Structure
• Lipid bilayer
• Transmembrane proteins
• Network of supporting fibers
– Shape and structure
– scaffolding
• Exterior proteins and glycolipids
– “sugar coating” acts as cell identity markers
– Glycoproteins – self recognition
– Glycolipids – tissue recognition
Cytoplasm
The material within a cell excluding the
nucleus
The cytoplasm of most eukaryotic cells is
filled with membranous structures that extend to
every nook and cranny of the cell’s interior.
While the membranes of the cytoplasm have
the same basic structure, the particular proteins
embedded in the lipid bilayer vary and give
specialized functions.
The membranes of the cytoplasm form a
highly interdependent network within the cell.
The Nucleus
Roger, Headquarters
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Genetic headquarters
Largest and most easily seen organelle
Repository of genetic information
Discovered by Robert Brown – 1831
Fungi and other groups may have >1 nucleus
Mammalian erythrocytes (RBC) lose nucleus
when mature
Nuclear Structure
• Nuclear envelope
– Two phospholipid bilayers
– Nuclear pores
• Membranes pinch together, filled w/ proteins that restrict
movement
• Proteins moving into the nucleus
• RNA and RNA complexes to be exported into the cytoplasm
• Nucleolus
– Site of intensive rRNA synthesis
• Nucleoli
– Tiny granules that are precursors to ribosomes
• Nucleoplasm
– Semifluid area that organizes the contents and provides sites
of attachment for enzymes in DNA duplication
Nuclear Shots
Nucleus
Nucleus
Nuclear
Diagram
Pore
Pores
Liver
Cell with
Nucleus
Chromosomes
Packaging DNA
• Stored as thin strands (chromatin) except
for cell division
– Allows access to genetic information
• During cell division DNA coils around
histones in a condensed forms called
chromosomes
– After cell division chromosomes uncoil and
can’t be seen with a light microscope
Endoplasmic Reticulum
ER every night of the week
• Located within the cytoplasm – little net
• Highway of the cell
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– System of passageways that allow materials to be
channeled to different locations within the cell
Smooth ER
Cell is organized by endomembrane system
ER – lipid bilayer with embedded proteins
Rough
ER
Site
of membrane
phospholipid synthesis
Highly developed in pancreas and salivary glands
Rough or Smooth – It’s All ER?
• Rough ER
– Studded with ribosomes
– Site of protein synthesis and segregation
– Proteins can be used within the cell or exported
outside of the cell
• Smooth ER
– Found in lesser quantities
– May be responsible for synthesis of steroids
– Break down lipids and toxins in the liver
Golgi Apparatus (Bodies)
Delivery System of the Cell
• Discovered in 1898 by Camillo Golgi
• Flattened stacks of membranes thought to form from
vesicles produced by the RER
• Abundant in glandular cells – secretions
• Collection, packaging, and distribution
• Proteins from ER are modified
– Short sugar chains are added (glycoproteins and glycolipids)
• Two distinct sides
– “cis” end accepts protein packages from ER
– “trans” end secretes membrane bound vesicles to be expelled
from the cell (exocytosis)
exocytosis
Trans
Cisternae
Cis
Ribosomes
• Site of protein synthesis
• They are not membrane bound
– Eukaryotic ribosomes slightly larger than prokaryotic
• Consists of small and larger subunits
– rRNA and about 50 structural proteins
• Cluster on the ER to make protein for export
– Free ribosomes make proteins for use within the cell
• Functional only when attached to mRNA in the
cytoplasm
• Ribosomal subunits are manufactured in the
nucleolus
Lysosomes and Vesicles
• Vesicles transport materials in and out of the cell
– Exocytosis
– Endocytosis – phago- (solid) and pino- (liquid)
• Lysosomes – membrane bound digestive vesicles that
arise from the golgi bodies
• Lysosomes contain a concentrated mix of digestive
enzymes
– Catalyze breakdown of protein, NA’s, lipids, carbo’s
– Recycle old organelles – mitochondria replaced every 10 days
• Lysosomes in metabolically inactive eukaryotic cells
dissolve cells from the inside out
Vacuoles
• Large fluid containing sacs
• In plants they may occupy more than 90 percent of
the cell’s volume
• Bounded by a single membrane
• In addition to water the vacuole may contain gases
(O2, N2, and/or CO2), acids, salts, sugars, pigments
• In plants the vacuole keeps toxins separate from the
rest of the cell and maintain internal pressure which
aids in the support of the plant
Mitochondria
Powerhouse of the Cell
• Site of aerobic respiration
• Energy released and ATP produced
• Inner membrane (cristae) houses the electron
transport system
• Mitochondria have their own DNA (mDNA)
• Cells do not produce new mitochondria during cell
division
– Mitochondria divide and partition between new cells
Mitochondria
(continued)
• Could have been a bacteria-like organism
incorporated into another cell 1.5 bya
• Mitochondria are particularly numerous
in muscle cells
• All mitochondria of offspring is maternal
– Mitochondria of sperm remain outside
fertilized egg
– mDNA is inherited maternally
How can maternal
mitochondria be
useful to scientists?
Mitochondria Structure
Electron Microscope View
Centrioles
Microtubule Assembly Centers
• Centrioles – help to assemble microtubules
• Some centrioles contain DNA which helps
control synthesis of structural proteins
• Help assemble spindle fibers which move and
align chromosomes during cell divison
Cytoskeleton
• Three main types of components
1) Microtubules – composed of the protein tubulin
2) Microfilaments – contractile protein actin
3) Intermediate filaments – variety of proteins
• Interconnected to form an elaborate
network
• Carry out many functions for the cell
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Maintain cell shape
Anchor organelles within the cytoplasm
Help in cell movement
Help to organize the internal contents of the cell
Cytoskeleton Fibers
• Actin filaments
– About 7 nm in diameter
– 2 protein chains loosely twined together
– Contraction, “pinching”, and cellular extension
• Microtubules
– About 25 nm in diameter
– A ring of 13 protein protofilaments
– Cell movement, transport of materials witin the cell
• Intermediate filaments
– About 8-10 nm in diameter
– The most durable element of the cytoskeleton
– Structural stability
Cell Movement
• Cell motion is tied to the movement of actin
filaments, microtubules or both
• Actin filaments can form and dissolve very
rapidly allowing cells to change shape quickly
• In cells treated with drugs that make
microfilaments dissolve all cell locomotion
stops
Some Crawl, Some Swim
• Some cells use a pseudopod (false foot)
– Cytoplasmic oozing forces a “foot” out in a
certain direction, the cell then drags itself
• Some cells use cilia or flagella to swim
– Whip-like flagella and shorter cilia both have a 9 +
2 structure of microtubules in eukaryotes
– The beating or turning of these structures propels
the cell
Endosymbiosis
• Proposes that today’s eukaryotic cells
evolved by a symbiosis in which one
species of prokaryote was engulfed by and
lived inside another species of prokaryote
• Mitochondria and chloroplasts are
thought to be two prime examples of this
theory
– Double membranes
– Both contain circular DNA similar to bacteria
– Mitochondria divide by simple fission
Sources
Brum, Gilbert D., L. McKane, and G. Karp. 1994.
Biology: Exploring Life, 2d ed. New York: Wiley.
Raven, Peter H. and G.B. Johnson. 1999. Biology,
5th ed. New York: McGraw-Hill.
http://cellsalive.com/
http://gened.emc.maricopa.edu/bio/bio181/BIOBK
/BioBookTOC.html
http://www.pbrc.hawaii.edu/~kunkel/gallery