Science - B5

Cells are building blocks of all living things. All cells contain DNA which is a nucleic acid.
DNA are in the form of a double helix and contain the genetic code. They determine the genetic development of cellular organisms.

All cells contain organelles. In general-
Plant cells have: cell wall, permanent vacuole, cytoplasm, nucleus, ribosomes, chloroplast and cell membrane.
Animal cells have: cytoplasm, ribosome membrane, ribosomes, cell membrane and a nucleus.

Definitions:
Cell wall - made of cellulose, strengthens the cell and keeps the shape
Vacuole - filled with cell sap, supports the cell
Cytoplasm - most chemical reactions take place here (may contain mitochondria- where respiration takes place)
Nucleus - controls what cell does, contains genetic info, ribosomes attatched to it
Ribosomes - where protein synthesis occurs, interpret coded instructions
Cell membrane - controlls movement in and out of the cell
chloroplast - contains chorophyll which absorbs light energy to make food

Mitosis is the process where a cell divides to produce 2 new cells which identical sets of chromosomes as the parent cell, and all the neccesary organelles. Mitosis creates new cells for growth and repair.
The cells go through a cycle of growth and division, and the cycle repeats until a cell can no longer divide (approx 70 divisions). When a cell enters the growth phase of a cycle, the number of organelles increase and chromosomes are copied as the cell is preparing to divide. Copies need to be made otherwise daughter cells would be incomplete and unable to function.

Meiosis happens in testes and ovaries. This cell division creates gametes (sex cells). Gametes contain half the number of chromosomes as the parent cell. Chromosomes relicate and divide. Then the chromosome copies seperate and divide again, to leave 4 new cells.

During fertilisation, a male gamete (sperm) and female gamete (egg) fuse together to form a zygote. It is important that cells made in meiosis have half the number of chromosomes as the paren cell so when 2 gametes fuse, they will have the right amount of chromosomes as a normal body cell (46).  

The zygote divides by mitosis to form a cluster of cells (embryo). All cells contain the same DNA. Eventually, the cells specialise to form bone, muscle, nerve cells etc. The cells of the embryo have to ensure different tissues develop at the correct stages.

The embyo comtinues to develop by mitosis, from 1 cell, to 2, 4, 8, 16, 32 etc...). An adult human contains approx 100trillion cells.

Meiosis and sexual reproduction produce variation between offspring as when gametes fuse, genetic info from 2 individuals if combined. For each gene, just one of each parents 2 alleles is passed on. This means that each offspring can have a different combination of alleles from each parent.

Growth and development in organisms are governed by genes present on the chromosomes in each cell in the nucleus. Genes determine characteristics eg-eye colour, by providing instructions for the creation of proteins, and therefore, control the whole organism.

The instructions are in a code of four bases, which hold 2 strands of the molecule together. Bases always pair in the same way. Adenine (A) pairs with thymine (T) and Cytosine (C) pairs with Guanine (G).

Chromosomes and genes always stay inside the nucleus. But protein production happens outside the nucleus, in the cytoplasm. Therefore, there is a way of transporting infor in genes, to the cytoplasm.
DNA cannot leave a cell as it is too large. Relevant sections of DNA is unzipped and the info is copied onto smaller molecules calles messanger RNA (mRNA). These molecules leave the nucleus and go to the ribosomes where they use the info to create proteins.
The order of bases in a gene determines the order in which amino acids are joined to make a particular protein. A group of 3 base pairs codes for an amino acid and is called a triplet code.

In human embryos all cells remain identical and unspecialised up to the 8 stage cell. They could develop into anything at this point - neurons, blood cells, liver cells, muscle cells etc. They are called embryonic stem cells.

At 16 stage cells cells begin to specialise and form different types of tissues. They contain the same genes, but the cells position is different.

The only genes that are active in a cell are the ones that are needed by the cell. The cell only makes the proteins it needs to carry out it's role. Some proteins are needed by every cell, while some proteins are only needed in specific cells.

Stem cells could potentially be used to help treat diseases and disorders and repair damaged tissues in the body. There are lots of benefits in stem cell technology but it is also a very ethical issue.
There are three sources of stem cells - embroyo's, blood from umbilical cord and adult stem cells. Only embryonic stem cells are completely unspecialised.

Therapeutic cloning involves taking a nucleus from an egg cell and replacing it with a nucleus from a patient's cell. The cell is then stimulated so it divides. At the 8 cell stage, cells will be unspecialised so they can be removed. But this is highly ethical as it is effectively cloning the adult patient.

Adult stem cells are unlike embryonic stem cells as they will only produce cells of a certain type, and genes would need to be re-activated. The problem is that there is only a small number of stem cells in the body so they are difficult to harvest.

Scientists, to date, have succesfully grown ears, skin and a bladder using a patient's cells. They can be grown in a lab or on a host animal to maintain a blood supply during the growth. As they are the patient's own cells, the patient will not reject them.

Plant cells, as with animal cells, divide by the process of mitosis. Then the cells specialise into roots, leaves, flowers etc. Plants continue to grow in height and width throughout their lives.

Plant growth happens in sites called meristems (sites where unspecialised cells divide repeatedly by mitosis). Then the cells become specialised in relation to the function they will have in the plant.
There are 2 types of meristems. Meristems that result in increased girth=lateral. Whereas meristems that result in increased height and longer roots=apical.

Cells from meristems are unspecialised and develop into anything the plant needs, when it needs it. Clones of plants can be produced by taking a cutting with a meristem plant. The cutting then grows roots and all the other things the plant needs using it's unspecialised cells. The clone will have the same genetic info as the parent did.

Unspecialised plant cells can also grow into a range of other tissues such as xylem and phloem.

Xylem are used by the plant to transport water and soluble mineral salts from roots to the stem and leaves. This replaces water that was lost during transpiration and photosynthesis.

Phloem are the tubes in plants which transport dissolved food to the whole plant for respiration or storage.

Xylem and Phloem are created by adding certain plant hormones to unspecialised plant cells.

There are a wide variety of plant hormones. The main group are called auxins. Auxind mainly affect cell division at the tip of a shoot. Auxins are growth hormones in plants.

Responding to stimuli:
All living organism have sensitivity and respond to changes in their environment. Plants can respond to external stimuli (such as gravity and light) by changing the direction of growth.

Plants respond to light by phototropism. Plants grow toward light. Fir example, say there is a plant with a light source in it's right. The hormone auxin does not like being in the light so will go to the darkest place, in this case on the left of the plant. As auxin is the growth hormone and is on the left of the plant, the left of the plant will then grow much more that the right of the plant. This will mean that the plant will start to bend toward the right, closer to the light source. 
If a light source is directly overhead, the distribution of auxin will be equal on both sides of the plant, and therefore grow straight.