Lecture 6

Cell structure I

LIF101

21-1-2014Tuesday Dr. Jonaki Sen

First biological observation made through a microscope

Galileo Galilei

In the early 1600s Galileo made a device which is one of the earliest versions of a microscope and recorded the observations he made of the geometric patterns of insect eyes.

Robert HookeCurator of Instruments for the Royal Society of England (mid 1600s).

He looked through microscope at a thin slice of cork from a mature tree and found tiny compartments which he named “cellulae” (Latin for small rooms).

Did not know that these were dead cells since he was unaware that cells could be living. Also observed other living plant cells “fill’d with juices”.

Hooke’s compound microscope Hooke’s drawing of cell walls from cork

Antony van LeeuwenhoeckA Dutch shopkeeper (late 1600s) very skilled in constructing lenses and with very keen vision.

Observed what he called as “very small animalcules, the motion of which was very pleasing to behold”

He observed protistans, sperm and even a bacterium.

One of Leeuwenhoeck’s sketches of sperm

Called an opaque spot he noticed in several cells as the nucleus

Robert Brown (In the 1820s)

Theodor Schwann (1839)Animals as well as plants are made of cells and cell products-and even though cells are part of a whole organism, to some extent they have an individual life of their own.

Rudolf Virchow (1849)Studied the cell’s growth and reproduction and concluded that every cell comes from a cell that already exists.

Matthias Schleiden (1838)Each plant cell must develop independently even though it is a part of the plant.

Cell theory• Every organism is composed of one or more cell

• The cell is the smallest unit having the properties of life

• The continuity of life arises directly from the growth and division of single cells.

Cells differ a lot with respect to size, shape and activities.

However, all cell have the following:1) A plasma membrane2) A region of DNA3) A region of cytoplasm

Plasma membraneThis thin membrane maintains the distinct identity of the cell. It separates the metabolic activities of the cell from random events in the environment. It does not isolate the interior of the cells as there are signals and substances constantly moving across it in a controlled manner.

DNA-containing regionDNA, with its heritable instructions occupies part of the cell interior along with molecules that can copy or read the instructions.

CytoplasmCytoplasm is the rest of the cell other than the plasma membrane and the DNA-containing region. It consists of a semi-fluid matrix and other components such as ribosomes.

Cytoplasm

Cell sizeMost cells are not visible to the naked eye. Some notable exceptions are, the yolk of bird eggs, cells in red parts of watermelon and fish eggs (caviar).

Why are most cells so small?

The smaller the cell the higher the surface to volume ratio which facilitates the exchange of material between the cell and its environment.

An spherical objects volume increases with the cube of its diameter but its surface area increases only with the square. As the diameter of a growing cell expands the cells volume increases faster than its surface area does.

If a round cell’s diameter increases to four times the original it’s volume increases to 64 times the original but the surface area is only sixteen times. Now each unit area of the cell’s membrane must serve four times as much cytoplasm as before. Past a certain size the inward flow of nutrients and the outward flow of waste will not be fast enough and the cell will die.

Diameter (cm) 2 3 6

Surface area (cm2) 12.6 28.2 113

Volume (cm3) 4.2 14.1 113

Surface to volume ratio 3:1 2:1 1:1

Cell Size

Microscope : The tool used to visualize and study cells

Light microscope: visible light is passed through the specimen and then through glass lenses which magnify the image and project it to the eye.

The light microscope cannot resolve objects smaller than 200 nm, the size of a small bacterium.

Magnification: The ratio of an object’s image size to its real size

Resolution: Is a measure of clarity of the image; it is the minimum distance that two points can be separated and still be distinguished as two points

Resolution is inversely related to the wavelength of the radiation used for imaging.

A light microscope and its parts

The light microscope cannot resolve objects smaller than 200 nm, the size of a small bacterium. That is because the smallest wavelength of visible light is about 400 nm. If a cell structure is less than one half of a wavelength long it will not be able to disturb the rays of light streaming past and thus it will not be visible.

Rod-shaped bacterial cells on the tip of a household pin shown at increasingly higher magnification

200 μm 1 μm40 μm

Different types of light microscopy

Electron microscopy:

An electron beam is aimed at a thin section of the specimen.

The beam is focused by using electromagnets as lenses.

The specimen is stained by atoms of heavy metals which attach to certain substructures inside the cells, making them more electron dense. The electrons passing through are scattered more in the denser regions and fewer electrons are transmitted.

The image is created by the pattern of transmitted electrons.

Theoretically the electron microscope can achieve a resolution of .002 nm but practically 2nm is the limit for biological samples.

Electron microscope

Electron micrographs

Scanning electron micrograph(SEM)Shows the 3D image of the surface of the specimen

Transmission electron micrograph(TEM) Shows a thin section of the specimen.

Light microscopy

Electron microscopy

Different microscopes reveal different characteristics of the same organism (green algae)

Cell Types• Prokaryotic = Before the nucleus • Eukaryotic = Truly nuclear

Only bacteriaAll other cell types

Prokaryotic cell: bacterium

Do not have a nucleus i.e. no membrane between the region of DNA and the cytoplasm.Do not have any membrane bound organelles.

Eukaryotic Cells• Nucleus bound by membrane• Include fungi, protists, plant, and

animal cells• Possess many membrane-bound

organelles

Protozoan

Major features of eukaryotic cellsOrganelles and their main functions:

Non-membranous structures and their main functions:

Nucleus

Endoplasmic reticulum

Golgi body

Various vesicles

Mitochondria

Localizing the cell’s DNA

Routing and modifying polypeptide chains. Synthesis of lipids

Modifying, sorting and shipping of proteins and lipids

Transport , storage and digestion of various substances

Producing ATP molecules in an efficient manner

Ribosomes

Cytoskeleton

Assembling polypeptide chains

Imparting shape and organization to the cell and its contents. Movement of the cell and its internal structures.

Overview of an animal cell

Overview of a plant cell

The NucleusThe nucleus is about 5m in size in most eukaryotic cells

Functions of the nucleus:

The nucleus houses the DNA which contains all the genetic information of the cell.

The nucleus directs protein synthesis by synthesizing messenger RNA according to the instructions provided by the DNA.

The nucleus enclosed by a double membrane separated by a space of 20 to 40 nm (nuclear envelop).

The nuclear pore is lined by a intricate protein structure called the nuclear pore complex which regulates the entry and exit of macromolecules and particles.

Except the pores the nuclear side of the envelop is lined by the nuclear lamina, a net like array of protein filaments that maintains nuclear shape

The nuclear envelope

A prominent structure in the non-dividing nucleus.

It appears as a mass of densely stained granules and fibers joining parts of the chromatin.

Ribosomal RNA is synthesized and ribosomes are assembled here.

Nucleolus

http://www.cbs.dtu.dk/staff/dave/roanoke/nucleolus.gif

The Nucleus: Genetic Library of the Cell

Within the nucleus the DNA is organized into discrete structures called chromosomes. Each chromosome is composed of mix of DNA and protein called chromatin. Chromatin appears diffuse in a non-dividing cell but I condenses into chromosomes when the cell prepares for cell division.

http://micro.magnet.fsu.edu/cells/nucleus/images/chromatinstructurefigure1.jpg

Important pointsWhat is the Cell theory?

What are the major parts of a cell?

Why are most cells of a certain size? Why does size matter?

What are the differences between prokaryotic and eukaryotic cells?

What is resolution and what is magnification in a microscope?

What are the features of a light and an electron microscope? What is the utility of each?

What are the major features of eukaryotic cells ?Compare and contrast between a plant and an animal cell.

What are the major parts of nucleus and what are the functions of each?