Cell Structure and Function
First Glimpse of The Cell
• 1662 – Robert Hooke
– English Scientist
– One of the first microscopists.
– Looked at thin slices of cork with a compound microscope
and called the outside walls “cells”– b/c they looked like
rooms that monks live in.
• 1668 – Anton Van Leeuwenhoek
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Dutch Draper
Received no higher education or university degrees
First to view living cells – “animalcules”, sperm, blood, etc.
He also examined bacteria from the scrapings of his own
teeth!
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 animal tissue is 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.
BASIC CHARACTERISTICS OF CELLS
1. Maintain a homeostatic condition
2. Take up nutrients, digest them, and excrete
waste products
3. Take up O2/CO2 and release CO2/O2
4. Maintain water and salt content
5. Grow, reproduce, and move
6. Respond to external stimulation
7. Expend energy to carry out activities
8. Inherit genetic programs from parent and
pass onto offspring
9. Die
This picture is a human white blood cell is trapping bacterial cells. This type of cell defends
the body against pathogens by engulfing them, delivering them to the lysosome of the cells,
and destroying them with the help of the lysosomal enzymes
Prokaryotic Cells
- before nucleus
• SIMPLE- capable of much less complex
activities
• CONTAIN MUCH LESS GENETIC INFO
• NO MEMBRANE BOUND NUCLEUShouses genetic info in the nucleoid
• LACK ORGANELLES (aside from ribosomes)
• Represent the vestiges of an EARLY STAGE
IN EVOLUTION
• SMALL- seldom reach diameters greater than
a few μm (micrometers)
• Up to 700 million could fit on the head
of a thumbtack.
• EXAMPLE- Bacteria
Eukaryotic Cell
– true nucleus
• More COMPLEX
• TRUE NUCLEUS- where genetic
information is housed and surrounded by
a complex membranous envelope
• MANY ORGANELLES- cytoplasm is filled
with organelles that are specialized for
various activities
• LARGE- cells range in diameter from 10100 μm
• EXAMPLE: yeast, ameoba,, red blood ,
and liver
Just How Small Are We Talking?
• The smallest objects that the unaided human eye
can see are about 0.1 mm long.
– you might be able to see an ameoba or a human egg,
without using magnification.
• Smaller cells are easily visible under a light microscope.
• To see anything smaller than 500 nm, you will need an
electron microscope.
• Most cells < 50 m
• Micrometer or micron (m) = 1000 mm
• Nanometer (nm)= 1000 m
• Angstrom – Å
• 1 Å = 0.1 nm = 1.0 x 10-4 m = 1.0 x 10-8 cm
Cell scale demo
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?
Applying S.A: V ratio to LIfe:
If Shaq, the 7-foot tall, 300-pound, basketball player for the Lakers
were twice as tall, would he be twice as good a ball player?
•The same surface/volume ratio principle illustrated with the
cubes applies to Shaq.
•In general, if his height was doubled and his proportions
remained geometrically similar,
• then his surface area would quadruple,
• However, his volume and mass would octuple!
• He would weigh roughly 2400 pounds!
•Not only would Shaq no longer be able to rebound, but, like
the landlubber blue whale he would be crushed under his own
weight. His bones would no longer be able to support him.
From: http://invsee.asu.edu/Modules/size&scale/unit4/unit4.htm#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
• Sperm cells lack almost all organelles and
have a streamline structure and flagella for
motility.
A Brief Tour of a Eukaryotic Cell
• Cool Cell Animation
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.
•The hydrophobic ends of the lipids create a non-polar
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.
The Nucleus
Roger, Headquarters
•
•
•
•
•
•
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
Red blood cells do not have a nucleus
– This maximizes the space available for hemoglobin
– They do, however, develop from bone marrow cells that
DO have a nucleus. They lose it once they mature.
Nuclear Structure
• Nuclear envelope
– A 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
• 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
• Highway of the cell
– System of passageways that allow materials to be
channeled to different locations within the cell
• Lipid bilayer with embedded proteins
Smooth ER
• Site of membrane phospholipid synthesis
• Highly
in pancreas and salivary glands
Roughdeveloped
ER
Rough or Smooth?
• 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
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
• Cluster on the ER to make protein for export
– Free ribosomes make proteins for use within the cell
• 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)
Mitochondria- cont.
• 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
Mitochondria Structure
Electron Microscope View
Centrioles
Microtubule Assembly Centers
• Centrioles – help to assemble microtubules
• Help assemble spindle fibers which move and
align chromosomes during cell division
• Found only in animal cells
Cytoskeleton
•
Three main types of components
1) Microtubules – composed of the protein tubulin
2) Microfilaments – contractile protein actin
3) Intermediate filaments – variety of proteins
• Carry out many functions for the cell
1) Maintain cell shape
2) Anchor organelles within the cytoplasm
3) Help in cell movement
4) 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
– 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