Some living organisms are made up of just one cell. These are called UNICELLULAR organisms. All seven life processes can take place inside the one cell. Most organisms are MULTICELLULAR - they are made up of many cells. The different cells share the work of the whole organism. Each cell has its own particular job to do, and is SPECIALISED to do that job. The cells have different shapes and sizes (adaptations) to help them carry out their specific functions.
Cells doing the same job are often grouped together. A group of cells like this is called a TISSUE. For example, muscle tissue is made up of identical muscle cells. Muscle tissue contracts (gets shorter), to move other parts of the body. Epithelial cells (skin cells) work together as epithelial tissue to provide coverings for various parts of the body.
An ORGAN is made up of different tissues that work together to do a particular job. For example, muscle tissues and epithelial tissues work together as the organ called the heart. Organs work together in ORGAN SYSTEMS. Different organ systems carry out different jobs and work to make up the whole organism.
Cell ---> Tissue ---> Organ (eg: lung) ---> Organ system (eg: respiratory system)
All organisms start life as a ZYGOTE (a fertilised egg cell). A zygote grows by dividing itself (cell division) and making new cells in a process called MITOSIS. A zygote divides to produce two daughter cells. Each daughter cell is identical to the cell they came from and to each other. The daughter cells divide again to produce two daughter cells each, and so on. This group of cells is called an EMBRYO. As the embryo develops, one cell may change its shape and structure to become adapted to do a particular job. In other words, it becomes DIFFERENTIATED to carry out a particular function and the cell is then specialised.
Two specialised cells in the blood are red blood cells and white blood cells. Each one has its own particular shape and structure to help carry out its function. Red blood cells CARRY OXYGEN to all cells in the body. The shape of a red blood cell is adapted to help its function. It has a large surface area, allowing efficient absorption of oxygen.
White blood cells are an important part of the IMMUNE RESPONSE. When a pathogen enters the body, white blood cells act in specific ways to kill the pathogen. For example, they may change shape to engulf the pathogen and kill it or they may produce antibodies that kill or disable the pathogen. To do this, they need to have lots of energy and be able to produce chemicals.
Red and white blood cells are specialised cells with very different functions.
How cells differentiate
A specialised cell will start to carry out its particular function when it has the size, shape or chemicals that allow it to work. The size, shape and chemicals of a cell are determined by genes in the DNA of the nucleus. All body cells contain the same genes in their DNA. They undergo differentiation because of the turning on and off of genes in their DNA. Genes that are turned off will no longer instruct the cell what to do. The genes that are turned on (ACTIVE) control how the cell behaves and looks. Cells that are not specialised are able to carry on dividing and growing to form more identical cells.
Stem cell therapy is being used to treat arthritis in dogs, but humans will have to wait many years for the same treatment to become available. This is because using stem cells from human tissues raises a lot of ethical and moral questions. The stem cells for the treatment of dogs come from their body fat and it causes no lasting damage to the dog when they are harvested.
In most species, sexual reproduction involves the joining of SEX CELLS from two parents. In male animals, the sex cells are called SPERM and in female animals the sex cells are called EGGS. During sexual reproduction, the sperm cell and the egg cell join together. This is called FERTILISATION. The fertilised egg cell is called a ZYGOTE. The zygote will divide many times to become an EMBRYO. Eventually the embryo will develop into an individual organism.
The importance of stem cells
When the embryo is at the eight-cell stage, all the cells are identical to each other and have the ability to become any type of cell. These cells are called EMBRYONIC STEM CELLS. When a stem cell divides, each new cell has the potential either to remain as a stem cell or become differentiated to become another type of cell with a more specialised function, such as a muscle cell, a red blood cell, or a nerve cell.
As the embryo develops, its differentiated cells form tissues and organs. Sometimes undifferentiated cells can be found among differentiated cells in a tissue or organ. These are called ADULT STEM CELLS and they can later differentiate to become one of the specialised cells of that tissue or organ. For example, adult stem cells found in bone marrow can divide and differentiate to become any type of blood cell, such as a red blood cell or any of the different types of white blood cell.
Adult stem cells usually differentiate into the types of cell found in the tissue, where they are located. Thus, a liver stem cell is only able to differentiate into a liver cell type but not into a nerve cell. Stem cells are important for living organisms for many reasons. In the embryo, stem cells give rise to the entire body of the organism, including all of the many specialised cell types and organs such as the heart, lung, skin, sperm, eggs and other tissues. In some adult tissues, such as bone marrow, adult stem cells replace cells that are lost through normal wear and tear, injury or disease.
Stem cells in plants
The ability to reproduce themselves by cell division and to become differentiated to any type of cell also makes stem cells extremely interesting to scientists. They offer potential new treatments to replace cells lost to diseases and injuries. Stem cell research may help develop alternatives to organ transplantation as well as look at the effects of new drugs and help us to understand the causes of birth defects.
Plant cells, like animal cells, have specialised features for different functions. Some of the specialised cells found in plants are described below:
PALISADE CELLS: are found near the top of leaves so they get a lot of light from the Sun. They are rectangular and full of chloroplasts. Lots of palisade cells make up palisade tissue.
XYLEM CELLS: are adapted to carry water and mineral salts to where they are needed in the plant. Dead xylem cells join together to form long tubes known as xylem tissue.
PHLOEM CELLS: carry dissolved substances such as sugars and amino acids to every part of the plant. They are adapted to carry out this function by joining other phloem cells to make long thin tubes.
Cell division in plants
Just like animal cells, plant cells that have already been differentiated are no longer able to divide. Only the cells in special regions called MERISTEMS are available to divide. The function of cells in the MERISTEM is similar to that of stem cells found in animals. These are cells that remain unspecialised and are able to divide to form identical unspecialised cells. Some of the cells resulting from this division remain in the meristem and continue to produce more cells, while others differentiate and are incorporated into tissues and organs of the growing plant.
A cell in the meristem divides by MITOSIS to become two identical daughter cells. The two daughter cells are separated by a cell plate that later becomes a cell wall for each cell. The daughter cells may either divide further or they may differentiate into specialised cells.
Meristems are found in the tops of shoots and roots as well as in buds and flowering parts of the plant. Unlike animals, which stop growing when they become adult, plants grow throughout their entire life.
The cells of the meristem divide to increase height and girth of the plant and the length of the roots, as well as developing into the leaves, flowers and branches of the plant. During their life, plants can regrow whole organs such as leaves and roots if they are damaged.
It is possible to produce new individuals from certain plants by putting the cut end of a shoot into water or moist earth. Roots grow from the base of the stem into the soil while the shoot continues to grow and produce leaves. The ability of plants to reproduce this way is how most of our fruit plants and houseplants are grown.
Gardeners often dip cuttings into rooting powder that contains a chemical which promotes the production of roots from the stem. Given the right conditions, the cutting can grow into a whole plant. This new plant is an exact copy, or CLONE, of the parent plant that provided the cutting. Geraniums are commonly grown from cuttings.
Growing plant cuttings is a lengthy and expensive business. Cuttings can only be taken in particular seasons, otherwise they will fail to grow. Plant cuttings can take many months to root and grow into full plants. To save time and money, horticulturalists use another method of cloning plants called TISSUE CULTURE.
Tissue culture involves taking small iece of plant tissue from a root or stem and treating it with enzymes to separate the cells. The cells are then placed individually on nutrient jelly containing plant hormones that promote the growth of roots, stems and leaves. This gives a large number of plants in a short time. These new plantlets will eventually become big enough to be grown in compost.
Tissue culture using meristem cells
Take cells from a plant root or stem ---> Use enzymes to separate the cells ---> Place individual cells on nutrient jelly to grow into plantlets ---> Transfer planlets to compost to grow into identical plants.
Tissue culture involves the use of almost any tissue of a plant and gives rise to a large number of plants that are clones of the parent plant. However, sometimes irregularities may take place that cause the plants to become variants, genetically different to the parent plant.
If the plant to be cloned is valuble, novel or has desirable features, it can be cloned using meristem cells. This method of propagation is used to produce multiple copies of clones of the parent plant. The cells of the meristem are dividing rapidly and have the ability to develop into any cell in plant tissue. These cells are all exact clones of the parent plant, so each individual plant produced from these cells is genetically identical.
Examples of plant hormones that promote growth are AUXINS. Hormones are chemical messages that are produced in one part of an organism and are transported in very small quantities to another part to help co-ordinate and control the growth of other cells. Unlike animal hormones, which target specific cells, plant hormones seem to affect every cell in the plant. Auxins affect cells by causing changes in cell division and elongation of the cell.
A plant needs light, carbon dioxide and water to be able to make food (PHOTOSYNTHESISE). Light helps a plant shoot tip to develop the green coloured chemical called chlorophyll. The chlorophyll absorbs the light energy used for photosynthesis, so without chlorophyll, a plant cannot photosynthesise. Without photosynthesis, a plant would not be able to grow.
A plant growing in plenty of light will grow straight. If light falls on only one side of the plant, the plant will grow towards the light. Plants do not "bend" towards the light, they grow towards it.
When a seedling starts to grow, its shoots always grow towards light. This growth response to light is called PHOTOTROPISM. Shoots growing towards light are said to be POSITIVELY PHOTOTROPIC. By growing towards the source of light, a shoot brings its leaves into the best situation for photosynthesis. Similarly, flowers are brought into the best situation where they can be seen and pollinated by insects.
The explanation for phototropism required creative hypotheses, observations and testing. This scientific process led to the discovery of auxin, a plant growth hormone, and its action in light. Auxin is naturally found in the tips of shoots and roots. In the shoots, auxin causes growth of cells in the shoot tip. In the roots, auxin has the OPPOSITE EFFECT. The auxin slows down growth in the growing root.
The phototropic response in plants is due to the effects of auxin. When a shoot gets light from one side, most auxin is found in the shaded part of the shoot tip. Auxin seems to enable to cells walls to be stretched more easily by the pressures developed in the vacuoles. This makes the cells on the shaded side expand and elongate. The shoot then curves towards the light.
1. Auxin is mainly found on the shaded side. Cells on the shaded side elongate more and cells on the sunny side elongate less. Not much auxin is found on the light side. Auxin made in shoot tip.
2. The shoot grows towards the light.
There are a fixed number of chromosomes in each species. Human body cells each contain 46 chromosomes (or 23 pairs). This means that there are 46 chromosomes in each of your kidney cells, in every skin cell, nerve cell, or so on. New cells are needed for the organism to grow. They are also needed to replace cells that are worn out or damaged. The new cells must be the same as the original cells so that they can do the same job. The cell division that takes place in the normal body cells and produces identical daughter cells is called MITOSIS. In mitosis, a cell divides by splitting into two new DAUGHTER cells. These daughter cells are identical to each other and the parent cell. This means they each have the same number of chromosomes as each other and the parent cell.
Stages in mitosis
Before a cell can divide in mitosis, it must make a copy of its chromosomes to give to the daughter cells.
The term "CELL DIVISION" is misleading since it implies that the cell must halve to produce two daughter cells. In reality, there is a phase of cell growth before the cell can divide. These two phases of the cell are represented by the cell cycle.
The first phase of the cell cycle is cell growth. During this phase, the cell grows, accumulates nutrients for the second phase of the cell cycle and makes copies of its chromosomes. At the end of this phase, there is an increase in the number of organelles (the specialised structures within the cell) that are going to take part in the process of mitosis.
The second phase of the cell cycle is mitosis. During this phase, the original chromosomes and their copies are divided into identical sets in two nuclei. This is followed immediately by a process in which the nuclei, cytoplasm, organelles and cell membrane are divided into two cells containing equal amounts of each of these components.
Once the parent cell has completely divided, each daughter cell enters the first phase of the cell cycle and it begins again.
Stages of meiosis
We already know that each cell in the body of a human has 46 chromosomes in its nucleus. A human embryo results from the joining of the sperm nucleus and the egg nucleus.
We have seen that mitosis is the type of cell division that takes place during the growth of an organism or when replacing dead or old cells.
There is another type of cell division, called MEIOSIS. Meiosis takes place in the testes of a male and in the ovaries of a female. Meiosis is a cell division that results in the formation of sex cells (GAMETES). The gametes of a male animal are called sperm cells and the gametes of a female are called ova - which mature later to become egg cells. Gametes are different from body cells as they contain only half the number of chromosomes in a body cell.
Four chromosomes in two pairs are part of the cell, then the chromosomes are copied (just as in mitosis), then the cell divides into two... and immediately into two again. This produces four gametes (egg of sperm), each with half the number of chromosomes.
Sperm cells and egg cells join in fertilisation to become a zygote. In a human zygote, there are 46 chromosomes in total. Half of these (23) come from the mother and the other 23 come from the father. If human sex cells had 46 chromosomes each, then a zygote would end up with 92 chromosomes. All humans have 46 chromosomes in their body cells; therefore a zygote with 92 chromosomes would not work.
Chromosomes and DNA
As a result of meiosis, gametes contain only half the full chromosome number. So when human gametes join together at fertilisation, the zygote formed contains the right number of chromosomes (46).
MITOSIS results in two cells with the same number of chromosomes as the parent cell; MEIOSIS results in four gametes with half the number of chromosomes as the parent cell.
Inside nearly all cells there is a nucleus, which contains long thread-like structures called CHROMOSOMES. Chromosomes occur in pairs. There are different numbers of pairs for different species of living organism. Humans have 23 pairs of chromosomes in their cell nuclei, kangaroos have 6 pairs and apples have 17 pairs. Chromosomes are made up of a long molecule called DNA. A DNA molecule is made of thousands of single DNA "units".
A single unit of DNA is made up of three molecules: a phosphate, a sugar and a BASE. The sugar and phosphate molecules join up and form the "backbone" of the DNA strand. The bases are attached to the sugar molecules. A DNA molecule is actually two strands of molecules facing each other in such a way that it looks like a ladder. The sugars and phosphates are the uprights of the ladder and the bases are like the rungs. The strands of the DNA molecule are twisted around each other form form a DOUBLE HELIX - like a spiral staircase. Pairs of chemicals called bases hold the two strands of the DNA molecule together by forming cross links.
Genes and base pairs
A DNA unit has one of four different bases, referred to as A, T, C and G. Each unit is called a NUCLEOTIDE. A molecule of DNA is made up of two strands joined together by the bases of a unit on each side of the strand. Each base is held together in a BASE PAIR and the bases always pair up the same way.
A pairs with T
C pairs with G
Proteins determine what a cell will look like and what it will do. These proteins are determined by genes.
GENES are sections of a DNA molecule.
A DNA molecule is made up of thousands of genes arranged like beads on a necklace. The sequence of bases in each gene contains a particular set of instructions, usually coding for a specific protein. This sequence of bases on DNA that code for a protein is called the GENETIC CODE.
DNA is only found inside the nucleus of a cell, but the cell structures that make protein are found outside the nucleus in the CYTOPLASM of the cell. The proteins produced in a cell determine what the cell will look like and what its job will be. Since DNA controls the making of protein, there needs to be a way to get information from the nucleus to the structures that make the protein.
DNA codes for protein but stays INSIDE THE NUCLEUS. The proteins are made in the cytoplasm.
Cell membrane controls the movement of substances in and out of the cell.
Nucleus carries genetic information.
Cytoplasm where many chemical reactions happen.
Genetic information is carried on chromosomes.
Each cell in the body makes proteins. DNA controls the making of protein but the actual assembly of the protein is carried out in the cytoplasm.
Getting the message across
DNA cannot leave the nucleus, so another molecule has to carry the instructions from the DNA to small structures in the cytoplasm, called RIBOSOMES, that make proteins. This "messenger" molecule copies instructions found in the DNA and carries it out of the nucleus to the ribosome.
Chromosomes, genes and DNA do not leave the cell nucleus. DNA makes a copy of itself called mRNA. The mRNA moves out of the nucleus into the cytoplasm to the ribosome.
To make a protein, lots of AMINO ACIDS have to join together. The type and sequence of the amino acids determines what the protein will be. A set of three bases in a DNA strand carries the code for one amino acid. This means that the sequence of bases in DNA determines the order in which amino acids are arranged to make a protein.
Each gene codes for a particular combination of amino acids, which makes a specific protein. By determining which proteins are produced in a cell, the genetic code of the genes also determines the structure and function of the cell. This sequence is decided by the genetic code of the sections of DNA (genes). The molecule called MESSENGER RNA (mRNA) copies the base sequence of the DNA and carries it to the ribosomes. The mRNA attracts another molecule (tRNA) which gives up the amino acid that it carries. The amino acids then bond together to form a long chain - the protein.
All body cells in a human contain 46 chromosomes. These many chromosomes have thousands of genes that are able to code for many different proteins. If all these codes were active, the cell would not be able to carry out any function normally, and it would be wasteful in terms of space, energy and materials. This is why many genes in body cells are not active; they are said to be SWITCHED OFF. This way, the cell only produces the proteins it needs to function. Specialised cells make only those proteins that allow them to carry out the functions they are adapted to perform.
Only some genes are active in a cell.
Up to the eight-cell stage, when the cells of an embryo divide by mitosis, two identical daughter cells are produced. Beyond this stage, after each cell division there are two paths that each daughter cell may follow.
The cell may divide again by mitosis to form identical daughter cells. The daughter cells from this division can then divide by mitosis or undergo differentation.
The cell may undergo differentation to become a specialised cell. It will then start making specific proteins and will undergo a dramatic change in size and shape.
Stem cell therapy
Once a cell is specialised, some of the genes are no longer active.
Specialised cells produce specific proteins because the genes coding for these proeins are ACTIVE or SWITCHED ON. Embryonic stem cells are able to differentiate into any type. This is because any of the genes in their chromosomes are able to be switched on. In specialised cells, only the genes needed for that particular cell are switched on. For example, muscle cells make specific proteins that allow the cell to contract.
The ability of embryonic stem cells to differentiate into any type of cell makes them important in scientific research looking at replacing damaged or diseased tissue.
Stem cells are cells that have the potential to develop into any type of specialised cell.
Currently, if a brain cell or nerve cell dies, there is a very rare possibility of it being replaced by another normally functioning cell. With a few exceptions such as liver and skin cells, the human body cannot replace cells that have become damaged or died.
Stem cells have given scientists the key to developing brain and nervous tissue to replace damaged or diseased tissue. This may develop further into the potential to grow whole organs to replace damaged or diseased hearts, lungs, kidneys or any other organs.
Stem cell cloning
Embryonic stem cells come from embryos at the eight-cell stage. They have the ability to become any type of cell in the body. They are used in research to help scientists develop new cells to replace damaged or diseased cells. Scientists use stem cell research to develop these cells to become whole organs. Using stem cells in such a way to replace injured or diseased organs will help reduce the need for organ transplants from other donors.
Adult stem cells come from bone marrow and umbilical cords of newborns. These cells can also be differentiated to some kinds of cells, but these types of cells are limited. For example, stem cells from the bone marrow can only differentiate to become blood cells. Scientists are developing new ways to use these cells to become all types of cells. Stem cells can grow into muscle, blood and nerve cells, among others.
There is a huge ethical debate surrounding the use of stem cells in research. Using embryonic stem cells means that the embryo is not able to develop into a fetus. Many people, including scientists, treat this as a destruction of life and raise moral questions about whether this is right. To replace the use of embryonic stem cells that specialise into specific cells, scientists have developed a procedure called THERAPUTIC CLONING in which they remove a nucleus from an egg cell and replace it with a nucleus from a body cell. This has the advantage that the cells produced will have identical genes. For example, new brain cells can be developed by using the nucleus of the patient's normal brain cells.
Reactivating inactive genes
However, there are some ethical issues raised by theraputic cloning too. Not all countries allow their scientists to use this procedure. The UK is one of a handful of countries in Europe that has been allowed use of theraputic cloning techniques to develop stem cell research by the European Union. One of the biggest ethical issues is about who donates the eggs for this procedure.
Once a cell differentiates, only those genes needed by that particular cell remain active, the other genes become inactive (switched off). For a differentiated cell to revert back to stem cell, the genes that become inactive have to be reactivated again. Under normal conditions, specialised cells cannot become stem cells again.
In 2006, Japanese scientists managed to transform cloned mouse skin cells to look and behave like embryonic stem cells. In other words, they were able to take a cell that had differentiated back to its original state as a stem cell. This stem cell had the potential to differentiate again and become any type of cell.
Armed with this ability, scientists are now exploring new ways to use these stem cells. However, there are ethical considerations involved too, as these cells have the potential to develop into a human embryo, producing a clone of the donor. Many nations are already prepared for this by having legislation in place that bans human cloning.