Module 2: Section 6 - Cell Division and Cellular Organisation

M phase - mitosis and cytokinesis

G0 - Triggered by a checkpoint chemical at early G1, cell may undergo apoptosis, differentiation or senescence

G1 - cell grows and new organelles and proteins are made, controlled by p53

G1 checkpoint - the cell checks that the chemicals needed for replication are present and for any damage to the DNA before entering S-phase

S-phase - cell replicates its DNA, ready to divide by mitosis

G2 - cell keeps growing and proteins needed for cell division are made

G2 checkpoint - the cell checks whether all the DNA has been replicated without any damage - if it has, the cell can enter mitosis 

Interphase - cell carries out its normal functions, but also prepares to divide - DNA is unravelled and doubled, organellles are replicated so it has spare ones, and its ATP content is increased

Prophase - chromosomes condense, spindle forms, nuclear envelope breaks down

Metaphase - chromosomes (each with 2 chromatids) line up along the equator and attach to the spindle by their centromeres. At the metaphase checkpoint, the cell checks that all the chromosomes are attached to the spindle before mitosis can continue

Anaphase - centromeres divide, separating each pair of sister chromatids. Spindles contract, pulling chromatids to opposite ends of the cell, centromere first

Telophase - chromosomes reach opposite poles, they uncoil and become long and thin, nuclear envelop forms around each group of chromosome

Cytokinesis - cytoplasm divides. In animal cells, a cleavage furrow forms to divide the cell memnbrane. In plant cells, an end plate forms where the equator was and a new plasma membrane and cell wall are laid down either side. There are now two genetically identical daughter cells. Cytokinesis usually begins in anaphase and ends in telophase - separate process to mitosis 

Asexual reproduction

• Single celled protoctists, plants and yeast divide by mitosis to produce new individuals
• Rarer in animals 

Growth

• All multicellular organisms grow by producing more cells which are genetically identical to each other and to the parent cell from which they arose by mitosis

Tissue repair

• Wounds heal when growth factors, secreted by platelets and macrophages and damaged cells of the blood-vessel walls, stimulate the proliferation of endothelial and smooth muscle cells to repair damaged blood vessels 
• New cells need to perform the same function – so need to be identical

Adult stem cells and stem cells in umbilical cord blood are multipotent - there is a limit to what they can become 

Embryonic stem cells are pluripotent - can differentiate into any type of cell, with some exceptions) 

Totipotent stem cells are the first stem cells made after fertilisation - can differentiate into any type of cell

Stem cells divide to become new cells, which then become specialised through differentiation

In animals, adult stem cells are used to replaced damaged cells eg. to make new skin or blood cells

Plants are always growing, so stem cells are needed to make new shoots and roots throughout their lives

Stem cells in plants can differentiate into various plant tissues, including xylem and phloem

Stem cells are also able to divide to produce more undifferentiated cells, ie. they can renew themselves

Treating Alzheimer's

• With Alzheimer's, nerve cells in the brain die in increasing numbers, resulting in severe memory loss
• Researchers are hoping to use stem cells to regrow healthy nerve cells in people with Alzheimer's

Treating Parkinson's

• People with Parkinson's suffer from tremors they can't control
• The disease causes the loss of a particular type of nerve cell found in the brain which releases dopamine - needed to control movement
• Transplanted stem cells may help to regenerate the dopamine-producing cells

Stem cells are also used by scientists researching developmental biology ie. how organisms grow and develop

Studying stem cells can help us to understand more about things like developmental disorders and cancer

Paliside mesophyll cells 

• Many chloroplasts to absorb a lot of sunlight and has thin walls for diffusion of carbon dioxide
• Contain cytoskeleton threads and motor proteins to move the chloroplasts towards the area of greatest light intensity

Root hair cells

• Hair like projections provide a large surface area for absorption 
• Have a thin, permeable cell wall for entry of water and ions - carrier proteins for ions
• Cytoplasm contrains extra mitochondria to provide the energy for active transport

Guard cells

• Use light energy to produce ATP, which actively transports potassium ions from surrounding epidermal cells into the guard cells
• Lowers their water potential so water enters guard cells from neighbouring epidermal cells by osmosis
• Guard cells swell, flexible tips bulge and thickened inner wall remains rigid - causes the stoma to enlarge

A tissue is a group of cells that are specialised to work together to carry out a particular function

Animal tissues:

• Squamous epithelium is a single layer of flat cells lining a surface, found in many places including the alveoli in the lunds
• Ciliated epithelium is a layer of cells covered in cilia, found on surfaces where things need to be moved eg. in the trachea or fallopian tubes
• Muscle tissue is made up of muscle fibres, three types: smooth, cardiac and skeletal

Plant tissues:

• Xylem tissue transports water arounf the plant and supports the plant, contains hollow xylem vessels and living parenchyma cells
• Phloem tissue transports sugars around the plant, arranged in tubes and is made up of sieve cells, companion cells and some ordinary plant cells. Each sieve cell has end walls (sieve plates) with tiny holes in them (plasmodesmata) so sap can move easily through them

Xylem

1. Meristem cells produce small cells that elongate

2. Lignin is deposited in their cell walls to reinforce and waterproof them; however this also kills the cells

3. The ends of th cells break down so that the xylem forms continuous columns with wide lumens to carry water and other dissolved minerals

Phloem

1. Meristem cells produce cells that elongate and line up end-to-end to form a long tube

2. Sieve tubes lose most of their organelles, and sieve plates develop between them

3. Companion cells retain their organelles and continue metabolic functions to provide ATP for the active loading of sugards into the sieve tubes

An organ is a group of different tissues that work together to perform a particular function

The lungs - they contain squamous epithelial tissue (in the alveoli) and ciliated epithelial tissue (in the bronchi etc.). They also have elsatic connective tissue and vascular tissue (in the blood vessels)

Leaves - they contain palisade tissue for photosynthesis, as well as epidermal tissue (to prevent water loss from the leaf), and xylem and phloem tissues in the veins 

Organs work together to form organ systems - each system has a particular function

The respiratory system is made up of all of the organs, tissues and cells involved in breathing. The lungs, trachae, larynx, nose, mouth and diaphragm are all part of the repiratory system

The circulatory system is made up of all of the organs involved in blood supply. The heart, arteries, veins and capilaries are all parts of this system