IB BIOLOGY- SL
CELL THEORY.
Many scientists have contributed to developing the three main principles of cell theory. These are
. all organisms are composed of cells;
. cells are the smallest unit of life;
. all cells arise from pre-existing cells.
This theory has amassed tremendous creditability, largely through the use of microscope- an important tool
.
Robert Hooke first described cells in 1665 while observing cork cells with a microscope he built himself.
A few years later, Antonie van Leeuwenhock observed the first living cells and referred them as “animalcules”
In 1838 botanist Mathias Schleiden stated that plants are made of ‘independent, separate beings’ called cells.
One year later, the zoologist Theodor Schwannn made a similar statement about animals.
The second principle continues to gain support today, as we have not been able to find any living entity that is not made of at least one cell.
There is evidence for the third part of the cell theory. Some of the classic experiments in biology showed that spontaneous generation of life is impossible. The first cells must have been formed in the origin of life from non cellular material, but today there is no evidence that cells can be formed except by cell division.
For example, sterilized soup in an open container decays because bacteria float in.
Sterilized soup in a sealed container does not decay as no bacteria are present.
Some very famous scientists, such as Louis Pasteur in the 1860s, have performed experiments to support the last principle. After sterilizing chicken broth by boiling, Pasteur showed that living organisms would not spontaneously reappear. Only after exposing to pre-existing cells was life able to re-establish itself in the sterilized chicken broth.
UNICELLULAR ORGANISMS:
Some organisms such as Amoeba, chlorella, and Euglena have only one cell. This single cell has to carry out all functions of life.
Chlorella
Metabolism: chemical reactions that occur within an organism.
Growth: increase in size. Growth maybe limited but is always evident in one way or other.
Reproduction: producing offspring, involves hereditary molecules that can be passed to offspring.
Response: reacting to stimuli.
Homeostasis: refers to maintaining a constant internal environment. Temperature and acid base balance.
Nutrition: obtaining food.
Limiting cell size:
The surfaces area to volume ratio effectively limits the size of cells. In the cell, the rate of heat and waste production and rate of resource consumption are functions (depend ) of volume. Most of the chemcial reactions occur in the interior of the cell and its size affects the rate of these reactions. The surface of the cell, the membrane, control what material move in and out of the cell. Cells with more surface area per unit volume are able to move material in and out of the ell, for each unit volume of the cell.
As the width of an object such as cell increases, the surface area also increases but as a much slower rate than the volume.
This is shown by the following table in which you can see that the volume increases by a factor calculated by cubing the radius; at the same time, the surface area increases by a factor calculated by squaring the radius.
Cell radius / 0.25 units / 0.5 units / 1.25 unitsSurface area / 0.79 units / 3.14 units / 7.07 units
Volume / 0.06 units / 0.52 units / 1.77 units
Surface area: Volume / 13.17: 1 / 6.04:1 / 3.99 : 1
This means that large cell has relatively less surface area to bring in needed materials and to rid the cell of waste, than a small cell. Because of this cells are limited as to the size they can attain and still be able to carry out the functions of life. Thus large cells do not have larger cells, they have more ells. Cells that are larger in size have modifications that allow them to function efficiently. This accomplished by shape changes such as from spherical to long and thin.
MULTICELLULAR ORGANISMS:
Multicellular organisms consist of many cells. These cells do not have to carry out many different functions. Instead they can become specialized for one particular function and carry out very efficiently. Cells in multicellular organisms therefore develop in different ways. This is called differentiation.
Multicellular organisms are said to show emergent properties. This means that the whole organism is more than the sum of its parts, because of complex interactions between cells.
DIFFERENTIATION:
Cells in multicellular organisms develop in different ways and can therefore carry out different functions. This is called differentiation.
The cells need different genes to develop in different ways. Each cell has all of these genes, so could develop in one way, but it only uses the genes that it needs to follow its pathway of development. Once a pathway of development has begun in a cell, it is usually fixed and the cell cannot change to follow a different pathway. The cell is said to be committed.
The drawings below show three of the hundreds of different types of differentiated cells in humans.
Heart muscle tissue:
All heart muscle cells contain structures made from protein fibers that are used to contract the cell and help to pump the blood in the heart.
STEM CELLS: Stem cells are defined as cells that retain the ability to divide and differentiate into various cell types.
Plant contain such cells in region of meristematic tissue. They occur in the region of root tip and shoot tip and are composed of rapidly reproducing cells that produce new cells capable of becoming various types of tissue within the root or stem.
Human embryos consists entirely of stem cells in their early stages, but gradually the cells in the embryos commit themselves to a pattern of differentiation. Once committed, a ell may still divide several more times, but all the cells formed will differentiate in the same way and so they are no longer stem cells.
Small number of embryonic cells remain as stem cells however and they are still present in the adult body. They are found in most human tissues, including bone marrow, skin and liver. They give some human tissues considerable powers of regeneration and repair.
The stem cells in other tissues only allow limited repair- brain, kidney and heart for example.
There has been great interest in stem cells because of their potential for tissue repair and for treating a variety of degenerative conditions.
For example, Parkinson’s disease, multiple sclerosis and strokes all involve the loss of neurons or other cells in the nervous system. Although still only at the research stage, there is the potential to use stem cells to replace them.
A problem discovered early in the research was that stem cells cannot be distinguished by their appearance. They can only be isolated from other cells on the basis of their behavior.
THERAPEUTIC USE OF STEM CELLS:
Some of the most promising research has been directed towards growing large number of embryonic stem cells in culture so that they could be used to replace differentiated cells lost due to injury and disease.
This involved therapeutic cloning.
Parkinson’s disease and Alzheimer’s disease are caused by loss of brain cells, and it is hoped that implanted stem cells could replace many of these lost brain cells thus relieving the disease symptoms.
Certain forms of diabetes deplete the pancreas of essential cells and it is hoped that stem cell implant in this organ could have positive effects.
There is a type of stem cell treatment that has been proceeding successfully in human for many years. Besides embryonic or pluriopotent stem cells, there are tissue specific stem cells. These stem cells reside inside certain tissue cell types and can only produce new cells of that particular tissue. For example, blood stem cells have been routinely introduced into human s to replace the damaged bone marrow of some leukemia patients.
Ethical issues:
The embryonic stem cells come from embryos often obtained from laboratories carrying out in-vitro fertilization. To gather these cells involves death of the embryo and opponents argue that this represents the taking of a human life. On the other hand, it is argued that this research could result in the significant reduction of human suffering and is therefore totally acceptable.
One therapeutic use of stem cells:
1. The placenta and umbilical cord of a baby is used as a source of stem cells. At the end of childbirth, the placenta is taken and placed on a stand , with the umbilical cord hanging down from it. Blood drains out of the umbilical cord and is collected- about 100cm3. The cord blood contains many hematopoietic cells. These cells can divide and differentiate into any type of blood cells.
2. Red blood ells are removed from the cord blood and the remaining fluid is then tested to find its tissue type, checked for disease causing organisms and stored in liquid nitrogen, in a special bank of cord blood.
3. Cord blood can be used to treat patients, especially children, who have developed certain forms of leukemia. This is a cancer in which the cells in bone marrow divide uncontrollably, producing far too many white blood cells. The patient’s tissue type is matched with cord blood in the bank. If suitable cord blood is available, the patient is given chemotherapy drugs that kill bone marrow cells, including the cells causing the leukemia.
4. The selected cord blood is taken from the bank, thawed and introduced into patient’s blood system, usually via a vein in the chest or arm. The hematopoietic stem cells establish themselves in the patient’s bone marrow, where they divide repeatedly to build up a population of bone marrow cells to replace those killed by the chemotherapy drugs.
Mom Stacy Trebing, rolls Katie around on her IV pole
to entertain her during a blood transfusion in 2004.
(Newsday / David L. Pokress / July 15, 2004)