II

22-07-06

A.Rameshwar

Lesson Objectives

• Cell theory
• Cell as a structural unit
• Cell as a functional unit
• Structure of cells
• Cells are a bunch of membranes
• What symbiont theory is about?
• What are the techniques of microscopy

and centrifugation?

.

Robert Hooke

1665

coined the word `cell’ for the box-like structures
of cork

M J Schleiden

a lawyer turned botanist

&

Theodore Scwann

an animal embryologist

met at dinner party in 1837

•1839

formally espoused cell theory

•the cells are of universal occurrence

and

are

•

the basic units of an organism

1849

Cell division described

1858

Rudolph Virchow forcefully

states the paradigm of cell theory

``

All cells come from previous cells’’

Omnis cellula e cellula

Thus: 

all individuals derived from a
single celled organism are
related &

all cells in a multi-celled organism
are descended from one fertilized
egg.

.

•CELL THEORY is to Biology

as

Atomic Theory is to

physics 

Tenets of the Cell Theory

• all known living things

(systems)

are

composed

of cells

• cells

arise

only from other cells

     (not spontaneously - experiments of Redi
and Pasteur)

 

• all

energy flow

(metabolism & biochemistry)

of life occurs within cells

• cells contain

DNA

as genetic info -

(for

subsequent generations)

 

Some important facts about cells

:

        - their

diversity

: many sizes, shapes, and types

(165+ in humans)

        - their

similarity

: have same basic structural

plan

                  - surrounded by

cell membrane

                  - contain

nuclei

     (bacteria =

genophore

)

                  - similar sub-cell parts (

ORGANELLES

)

                  - all contain

same macromolecules

Different Shapes &
Forms

Like a `gare‘

or donut

Scanning electron micrograph of

human red blood cells

Sickle cells

Cell shapes

- conifer early wood

.

• Mechanism of division was selected

during evolution

• Cell has volume (3-D)
• Surface membrane is measured in

square units (2-D)

• Growing beyond limits cause inadequate

supply of nutrition from environment

• Division solves the problem

Why are cells so small?

a. Efficiency:  smaller cells have

larger surface area

b. Specialization: Having

numerous small cells permits
specialization

•As an object increases in size its volume increases as the cube of its
linear dimensions while surface area increases as the square

Surface area = L X B Square of two linear dimensions

Volume = L X B X H Cube of three linear dimensions

•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

How large cells can

be?

Getting around this problem

Divide to become

multicelled -

Embryo

long and thin or skinny

and flat

many protists, nerve cells,

muscle cells

Smaller Particles React Faster Than

Larger Particles

in Chemical Reactions

• Granular sugar dissolves easier than sugar

cubes

• Everything that the cell needs or has to get rid
of has to go through the cell membrane, the

amount of which relates to the surface area

• Surface must allow sufficient exchange to

support the contents of the cell

• stops growing when its surface area is

insufficient to meet the demands of the cell's
volume

An Early Cell

Microscope

• Scientific discoveries follow the

construction of tools

• Microscope has had a profound influence

on the development of biology

• Light microscope

: Limited resolution ~ 0.2

μm, magnification not > 1500 times

• Electron microscope

: Magnification

500,000 times- a pin head>1 Km

diameter!
Cell ultrastructure
Drawbacks

Human eye

limit

Relative Sizes of Cells &

Components

limit of resolution of ~ 200 nm
(0.2 µm)

due to the wavelength of light
(0.4-0.7 µm). Cells observed
under a light micro-scope can be
alive, or fixed and stained

The light microscope

limit of resolution of ~ 2nm.

due to limitations of the lens used
to focus electrons onto the sample.

looks at replicas of dead cells, after
fixation and heavy metal ion
staining

e

-

are scattered as they pass

through a thin section of the
specimen, & then detected and
projected onto an image on a
fluorescent screen.

The Transmission
Electron

Microscope (TEM)

more relevant for

biological samples

also has a limit of 2nm

Like the TEM, allows to look at

replicas

of dead cells, after fixation and

heavy

metal ion staining

electrons reflected off the surface of

the specimen

The Scanning Electron Microscope

(SEM)

Cells Have Evolved Two

Basic Architectural

Plans

Cells without a nucleus

Cells with a nucleus

“karyon” = kernel or nut; “pro” = before ; “eu” = good or true; • Eukaryotes - “cells with a true

nucleus”

• Prokaryotes - “cells with no defined nucleus”

Prokaryotes

• Cells whose nucleus

is not enclosed in a membrane

• Lack other organelles too

• First to evolve,

The oldest accepted

prokaryote fossils date to 3.5 billion years

• Contain

a circular chromosome

Plasmids with satellite DNA

Ribosomes

food storage granules

Prokaryotic cell-
Bacterium

A general prokaryotic cell

Eukaryotes

• Cells whose nucleus is enclosed in a

membrane

• Evolved later.

Eukaryotic fossils date to between 750

million years and possibly as old as 1.2-1.5 billion years.

Mango Fruit & Cell

• What will be the

nucleus
counterpart?

• Fruit skin is like ?

Cell as seen with a light microscope

Fine structure of an animal cell- based on
electron microscope

studies

Eukayotic cell as seen with an electron
microscope

P

L

A

N

t

C

e

l

l

Organelles

• Mitochondria and chloroplasts are

energy-transducing organelles

• Both seem to share many properties

with bacteria:

– contain 70S ribosomes (whereas rest of

eukaryote cells contain 80S ribosomes)

– divide by binary fission
– contain circular DNA without nucleus,

etc.

Endosymbiont Theory

Symbiosis = "living together“

Lynn Margulis

1971

• Chloroplasts and Mitochondria

(& undulipodia)

derived

from ancient colonization of large bacteria (became

the eucaryotic cell) by smaller bacteria

• Host cells acquired respiration from the precursor

of the mitochondrion, and oxygenic photosynthesis

from the precursor of the chloroplast

• Also acquired much of their genetic information
• Eventually, organelles lost ability to exist as

separate organisms, cannot have independent

existence from cell

Some protists

E

n

d

o

S

Y

m

b

I

o

si

s

Why double membrane of chloroplast?

Outer membrane of

organelle originally

belonged to the host

cell.

Invagination

Images with electron microscope

Mitochondrium

Cutaway view of
mitochondrion

Inner membrane more
active

Centrifugation

• A centrifuge is a device for

separating particles from a solution

according to their size, shape,

density, viscosity of the medium and

rotor speed

• In biology, the particles are usually

cells, sub cellular organelles, viruses,

large molecules such as proteins and

nucleic acids

• Instrumental in working out the

functions of organelles

Centrifuge

.

CELLS Alive!

Tutorial

http://www.cellsalive.com/

is a highly visual site, where you'll find movies and animated

illustrations on cell processes, parasites, penicillin and more.

• cell.de

http://www.cells.de/index_e.htm

Online-Service for Cell Biology includes digital media

in internet quality and further information for university and
high school on cellular and molecular biology. The IWF -

Institute for Scientific Film, Göttingen

(Germany) prepares

educational

media about cellular and molecular biology

didactically and technically for different media carriers.

• C. elegans Movies

http://www.bio.unc.edu/faculty/goldstein/lab/movies.html

A visual introduction to C. elegans and its development. This page

has links to movies made by C.elegans researchers worldwide.

• Biology Mad

http://www.biologymad.com/

This website is mainly aimed for students studying AQA (spec. A)

Biology in the UK. It is informative, beautifully designed and

easy to use.

Further Help

.

• Virtual Cell

http://personal.tmlp.com/Jimr57/tour/cell/cell.htm

• Comparison of Prokaryote, Animal and Plant Cells

http://www.wiley.com/legacy/college/boyer/0470003790/animations/cell_structure/c

ell_structure.htm

• Interactive Cell Models

http://www.cellsalive.com/cells/3dcell.htm

• Organize It!

http://biologyinmotion.com/organize-it/

• MODELING LIMITS TO CELL SIZE

This investigation provides students with a 'hands-on' activity that simulates the

changing relationship of Surface areas -to- Volume for a growing cell.

http://www.accessexcellence.org/AE/AEC/AEF/1996/deaver_cell.html

• Endosymbiotic Theory

http://dekalb.dc.peachnet.edu/~pgore/students/w96/joshbond/symb.htm

• A Short History of the Microscope

http://www.microscope.org/micro/sm101.htm

• Centrifugation

http://ntri.tamuk.edu/centrifuge/centrifugation.html

Review Questions

1.

Which of these is not a type of cell? a) bacterium; b) amoeba; c) sperm;

d) virus

2.

The oldest fossil forms of life are most similar to _____. a) animals; b)

modern bacteria; c) archaebacteria; d) fungi

3.

A prokaryotic cell would not have which of these structures? a)

ribosome; b) nucleus; c) cell membrane; d) cell wall

4.

Ribosomes are cellular structures involved in ____. a) photosynthesis; b)

chemosynthesis; c) protein synthesis; d) carbohydrate synthesis

5.

The earliest microscopes used to image the specimens. a) high energy

electron beams; b) interatomic forces; c) low energy electron beams; d)

light

6.

Plant cells have ___ and ___, which are not present in animal cells. a)

mitochondria, chloroplasts; b) cell membranes, cell walls; c)

chloroplasts, nucleus; d) chloroplasts, cell wall

7.

The ___ is the membrane enclosed structure in eukaryotic cells that

contains the DNA of the cell. a) mitochondrion; b) chloroplast; c)

nucleolus; d) nucleus

8.

The mitochondrion functions in ____. a) lipid storage; b) protein

synthesis; c) photosynthesis; d) DNA replication; e) ATP synthesis

9.

Which of these cellular organelles have their own DNA? a) chloroplast;

b) nucleus; c) mitochondrion; d) all of these

For answers click on notes