2.1.6-(i) the organisation of cells into tissues, organs and organ systems

Tissues= groups of similar cells that work together to perform a similar function

e.g. xylem and phloem, muscular tissue, mesophyll in plants, epithelial tissue, Epidermal tissue (Human skin, and the waxy covering of some plants)

Organs= collection of tissues that work towards a specific function

2.1.6- (h) how cells of multicellular organisms are specialised for particular functions

 Neutrophils:

- Phagocytic leukocytes which are involved in one of the early lines of defence against pathogens by removing bacteria that have entered the body

- They have a nucleus with 2-5 lobes and contain organelles called lysosomes (contain digestive enzymes)

Squamous epithelial cells:

- are round and flat with a small, centrally located nucleus

- the cells fit together to form a covering/lining

- When arranged together in a single layer, they facilitate diffusion in tissues such as- gas exchange in lungs, nutrient and waste exchange in blood capillaries.

Ciliated epithelial cells:

- Have cilia on the apical surface of each cell

E.g. the pseudo stratified column or epithelia in the respiratory tract

-the cilia enhance the movement of mucus and trapped particles out of the respiratory tract through a wafting motion
- This helps protect the body from pathogens and harmful material that has been inhaled.

Guard cells:

- surround the stomatal pore and control the opening and closing of the stomata

- the osmotic state within the guard cells determines their turgor; When the guard cells are flaccid, the stomata close/ when the guard cells are turgid, the stomata open.

- Turgor is regulated by the active transport of ions, such as K+ and H+ across the plasma membrane

Sperm cells:

- Male reproductive cells (gametes) which fuse with an egg cell during fertilisation to form a zygote
- To ensure the zygote has one complete diploid set of chromosomes both egg and sperm cells are haploid and only one sperm fuses with one egg.
- the acrosome in the head of the sperm contains digestive enzymes which degrade the zona pellucida surrounding the egg cell, allowing the plasma membrane to fuse with the egg plasma membrane.

Root hair cells:

- specialised to increase the efficiency of water absorption and the uptake of minerals required for the plants survival. 

- the root hairs are long projections that increase the surface area that plants can use to absorb water and minerals

- Are underground so do not contain chloroplasts.

2.1.6- (g) the main stages of meiosis

Meiosis is a form of cell division that gives rise to genetic variation. The main role of meiosis is production of haploid gametes from diploid cells, through two divisions. The steps involved are:

Meiosis 1:

Prophase 1

- the chromosomes super coil, the nuclear envelope breaks down, and the spindle fibres form from the centrioles (allowing genetic material to be moved around the cell.)

-Then, the chromosomes undergo a process called crossing over= the exchange of sections of DNA between chromatids on homologous chromosomes (explained at bottom)

Metaphase 1

- Chromosomes line up at the equator of the cell (center) 

- Chromosomes are attached to the spindle fibres by centromeres

- Each chromosome lies next to its homologous pair at the equator.

- Within this homologous pair, the chromosomes are randomly organised on either side of the equator- this is independent assortment.

Anaphase 1

- Each chromosome is pulled apart to opposite ends (poles) of the cell

- One of the pair of homologous chromosomes will end up on each side randomly

Telophase 1 + Cytokinesis

- Animal cells reform a nucleus for a short period of time after anaphase 

- Also undergo cytoplasm splitting to form two new cells.

Meiosis 2

After Meiosis 1, we are left with two sets of segregated chromosomes

Prophase 2

- The DNA supercoils, the nucleus disintegrates and new spindle fibres form.

Metaphase 2

- the chromosomes line up at the centre through the attachment of spindle fibres to their centromeres

- Chromatids are randomly assorted to either side of the equator

Anaphase 2

- The genetically different chromatids are randomly segregated to opposite poles

Telophase 2

- Nuclei starts to reform+ cell splits

END OF MEIOSIS 

Crossing over

= the exchange of sections of DNA between chromatids on homologous chromosomes in Prophase 1 of Meiosis. 

- The homologous chromosomes line up and twist around each other, with the same genes in the same place.

- This wrapping can cause sections of chromatid to break off, which than can then be swapped over to the other homologous chromosome.

- This means that whilst crossed over chromosomes have the same genes as before, the alleles are shuffled around to different chromosomes- this leads to genetic variation.

2.1.6-(f) the significance of meiosis in life cycles

 Sexual reproduction:

- Gametes are the sex cells (e.g. sperm cells and egg cells) that fuse together during sexual reproduction.

- Gametes are haploid (contain half chromosomes)

- When the gametes fuse together, a new diploid organism is formed.

- An individual inherits half of their total chromosomes from each parent

Gamete production:

- haploid cells are produced from diploid cells via meiosis

Genetic Variation:

- Produces four genetically different daughter cells 

- Each one receives a different combination of chromosomes

- Increases genetic diversity this way

2.1.6- (e) the significance of mitosis in life cycles

Growth:

- Multicellular organisms must grow through increasing their cell count

- Mitosis allows somatic(body) cells to reproduce and form clones that perform the exact same function within the body as parent cells.

Tissue Repair:

- When damaged or injured an organism can replace the lost cells via mitosis

- Structures such as skin and bones can be reconstructed with new cells as well as lost fluids such as blood can be replenished

Asexual Reproduction:

- single celled organisms reproduce via mitosis as cellular and organismal levels of organisation are the same for them- reproduction of entire organisms

- Mitosis produces identical cells so reproduction produces clonal organisms. E.g. entire colonies of bacteria are likely to be identical to each other

- Asexual reproduction only requires one parent cell.

2.1.6-(d) sections of plant tissue showing the cell cycle and stages of mitosis PAG

 This can be considered a classroom PAG, but is also useful knowledge.

Preparation of stained squashes of cells from root tips

(Meristem tissue that can divide by mitosis for growth)

1) Sample preparation- wear gloves and forceps to handle the tips, which must be actively growing. Place into 5M hydrochloric acid, and rinse in cold water.

2) Cut the root tips- place onto microscopic slide

3) Staining- add 2-3 drops of stain and leave for two minutes. Use a mounted needle to spread the root tips into a thin layer.

4)Viewing the sample- Place the slide of the stage using the lowest power lens. On a lens power of x400, it should be possible to clearly see the chromosomes in the dividing cells. 

2.1.6- (c) the main stages of mitosis

The role of mitosis is to produce identical daughter cells for growth and the asexual reproduction of cells. 

In Mitosis, chromosomes go through interphase, prophase, metaphase and anaphase.

1)Interphase

- The cell prepares to divide, and DNA is replicated by semi-conservative replication. There are now two copies of every chromosome.

- Organelles are also replicated

- More ATP is produced to be used in cell division

2)Prophase

-The nuclear envelope and nucleus break down, and subsequently disappear. Chromosomes are left floating in the cytoplasm.

- The chromosomes coil more tightly (becoming shorter and fatter). They can be seen with an optical microscope.

- Small protein bundles called centrioles move to opposite poles of the cell for mitotic spindle formation.

3)Metaphase

- the chromosomes line up along the equator of the cell (mid line) and attach to the spindle fibres via centromeres

-at this stage, the chromosomes are maximally condensed

4)Anaphase

- The chromosomes break into two chromatids. The sister chromatids separate at the centromere.

-The spindles contract and pull the chromatids to each pole of the cell.

5)Telophase

- The chromatids reach opposite poles and decondense, becoming chromosomes again.

- Nuclear envelopes form around the chromosomes so there are now two nuclei

- The cytoplasm splits and two daughter cells are formed- identical to the original and to each other

2.1.6-(b) how the cell cycle is regulated

 Regulation of the cell cycle

Internal checkpoints are important.

- G1 is the restriction checkpoint (point where a cell becomes committed to the cycle), where DNA is checked to make sure there is no damage, and the cell is checked to ensure it is the correct size and has enough energy reserves (nutrients).

- G2/M is the second checkpoint, chromosomes are checked to make sure they have all been replicated, and none of it is faulty+ cell size is checked again

- M- sister chromatids are checked to confirm attachment to spindles are suitable.

2.1.6- (a) the cell cycle

The Cell Cycle has three major phases: Interphase, the Mitotic phase and Cytokinesis

- Interphase = the cell grows and DNA is replicated (prepares to divide)

- Mitosis = cell division occurs via prophase, metaphase, anaphase and telophase

- Cytokinesis = the parent and replicated organelles move to opposite poles of the cell and the cytoplasm divides resulting in two daughter cells.

Processes that take place during Interphase

Divided into three growth stages:

G1- the cell grows 

S- DNA synthesis occurs

G2- cell grows more, before mitotic phase begins (produces any more molecules it needs to divide)

2.1.1- (b) the preparation and examination of microscope slides for use in light microscopy

 

2.1.1- (a) the use of microscopy to observe and investigate different types of cell and cell structure in a range of eukaryotic organisms