B5 REVISION

Overview

Cells, Tissues and Organs

• Groups of specialised cells working together are called tissues and groups of tissues working together are called organs.

Mitosis

• Mitosis is the process where cells divide to produce two new identical sets of chromosomes to the parent cell.

• The purpose of mitosis is to produce new cells for growth and repair to replace old tissue.

Fertilisation

• When a sperm is fertilised by an egg it becomes a Zygote.

• The Zygote then divided by mitosis to form a cluster of new cells called embryo.

• Up to and including the eight cell stage, all the cells are identical and can produce any sort of cell required by the organism including neurons, blood cells liver cells etc. These care known as EMBRYONIC STEM CELLS.

Fertilisation cont.

• At the 16 cell stage (4 days after fertilisation) most of the new cells in the embryo are starting to specialise and form different types of tissue.

• Some cells remain unspecialised, these are known as adult stem cells, these become specialised at a later stage. But unlike embryonic stem cells, adult stem cells cannot become any type of cell.

Plant Meristem

• Plants have cells that are like stem cells in humans and animals. These are known as Meristems. Only cells with meristems can divide repeatedly.

• Cells in the meristem are unspecialised but they can develop into any type of plan cell. Under normal conditions that the tissues such as xylem and phloem could be formed as well as organs.

Xylem

• Xylem is made of specialised cells to transport water and soluble mineral salts from the plant roots to the stem and leaves, and to replace water lost during transpiration and photosynthesis.

Phloem

• Made of specialised cells to transport dissolved food made by photosynthesis throughout the plan for respiration and storage.

Xylem and Phloem

• When a stem is deliberately cut, special plant hormones can be added. These can send messages to the meristems to start producing roots. As the cutting already has a stem and leaves it will grow into a clone of the parent plant.

Auxins

• There are a wide variety of plant hormones but the main one in Auxins. Auxins mainly effect cell division at the tip of a shoot because this is where the meristems are. Just under the tip, cells grow in the presence of Auxins causing the stem or root to grow longer.

Phototropism

• Phototropism is a response by the plant to light. A plants survival depends on its ability to photosynthesise, so therefore plants need strategies in intensity. Therefore plants grown towards the light source.

How Phototropism works

• The cells furthest away from the light source grow more, due to the presence of auxin which is sensitive to light. Auxin is produced at the shoot and tip and migrates down the shoot.

• If a light source shines onto a shoot at an angle, the auxin facing the light moves to the side furthest away.

How Phototropism works cont.

• As a result the concentration of auxin on the side furthest away from the light and the shoot beings to bend towards the light.

• If the tip of the shoot is covered with an opaque material or is removed the plant will continue to grow upwards as if the light source was not there.

• But if it is covered with a transparent cap it will still grow towards the light source.

Mitosis

• Mitosis leads to the production of two new cells which are identical to each other and to the parent cell. Mitosis only takes place when the cells are ready to divide.

Cell Cycle

• Consist of the growth stage where the cell gets bigger and number of organelle increases, then a synthesis stage where DNA is copied followed by another short growth stage immediately before mitosis.

Higher Tier

• Both new cells need to have a full compliment of organelles and DNA to function properly. Therefore the number of organelles needs to increase and the DNA has to be copied.

• The chromosomes are copied when two strands of each DNA molecules separate and new strands form alongside them.

Meiosis

• Meiosis only takes place in the testes and ovaries. This is a special type of cell division that produces gametes. (eggs and sperm) for sexual reproduction.

• Gametes contain half the number of chromosomes of the parent cell. This is important because gametes fuse the number of chromosomes will increase back the full number.

DNA

• DNA is a nucleic acid found in the nucleus and is in the form of a double helix. The bases are always pairs:

A = TG = C

DNA cont.

• The DNA has sequences of genes which are codes for proteins. However the proteins themselves are manufactured in the cytoplasm of the cell.

• The DNA molecule is too large to leave the cell so the relevant section of the DNA is unzipped and the instructions are copied onto smaller molecules which can pass through the nuclear membrane of the nucleus into the cytoplasm.

Gene Switching

• All body cells including stem cells contain exactly the same genes. All cells have to produce proteins for growth and respiration. This means that certain genes will be switched on in all cells. The genes that are not needed will be switched off.

• When a stem cell becomes specialised the genes for proteins specific to the new cell type will be switched on.

Ethical decisions

• To produce large numbers of stem cells needed for treatments, it is necessary to clone cells from 5 day old embryos.

• The stem cells are collected when the embryo is made up of approximately 150 cells. The rest of the embryo is destroyed, this is an ethical issue.