B.2.3. Flashcards
(8 cards)
Stem cell properties
Stem cells are unspecialised cells that can divide endlessly and differentiate along different pathways. When stem cells divide they differentiate and are no longer stem cells. Stem cells remain undifferentiated or partially differentiated and they are capable of differentiating in different pathways.
Pneumocyte adaptations
Type I pneumocytes are extreme long and thin cells that are adapted for oxygen and carbon dioxide diffusion in the alveoli. As the capillary is also only one cell thick, the distance between air and blood is smaller and therefore increasing the rate of gas exchange.
Type II pneumocytes are more numerous although they only occupy 5% of the surface area, they contain many mitochondria unlike type I. These cells contain lamellar bodies that secrete a fluid to keep the surface of gas exchange moist, this fluid is called surfactant.
Differentiation
Differentiation is the development of cells into specialised cells that differ from other cells by performing different and specific functions. During the early stages of the embryo the cells are unspecialised and by the activation of different genes they become specialised to perform a specific function.
As all the cells of multicellular organisms have the same set of genes what differs between specialised cells is gene expression, that being the switch to turn on or off specific genes and not others. This is conducted thanks to chemical signals called morphogens, their concentration determines cell position and cell differentiation. The closer to a higher concentration of morphogens like in the source cell or morphogens will activate specific genes while further away from the source cell where there are less morphogens other genes will be expressed.
types of stem cells (stem cell potency)
Stem cells have different types of potency, not all can differentiate into all different types of cells.
- Totipotent cells can differentiate into any kind of cells, these are found in the early stages of the embryo.
- Then once the embryos keep developing and cells begin differentiating one way or another, stem cells turn from totipotent to pluripotent, these are found in the blastocyst, they can differentiate into a large range of cells but not all types of cells.
- Multipotent stem cells are cells that remain in the adult that can differentiate into a limited range of cell types, for instance haematopoietic cells in the bone marrow.
Embryonic stem cells have more potency but there are ethical issues associated with their use as the embryo is created then destructed to be used, adult stem cells have less rejection chances but lower potency. Although potency can be increased by nuclear reprogramming but it is expensive and difficult.
Surface area to volume ratio
As the area of the cell increases the volume increases faster. This creates a limiting factor for the cell’s size. In the cytoplasm there are a large number of chemical reaction that happen for cellular metabolism. The rate of this reaction is directly proportional to the cell volume, and it depends on the substances that move though the cell membrane and removing the waste products generated. But the rate of movement of substances moving between the cytoplasm and the outside (through the plasma membrane) is dependant on the surface area to have contact with its surrounding.
The surface area of the cell affects the rate of material and gas exchange. When the cell increases in size so does the metabolism activity therefore more substances need to be taken in and more waste products need to be taken out.
Therefore a single cell is limited in its growth, the bigger it grows the smaller the SA:V ratio becomes. To solve this organisms become multicellular:
- Smaller multicellular organisms have less surface area exposed to the surrounding but because their bodies have such a small volume, any temperature change that happens on the surface is spread quickly throughout the body.
- As the transfer of energy is fast, small animals have high metabolism rates to compensate the loss of heat.
- Larger multicellular animals tend to isolate their core from the environment and have a lower environment.
stem cell niches
Embryonic stem cells are located in the embryo; specifically in the blastocyst, during the first 5 days after fertilisation.
Adult stem cells are located in adult tissues. The precise place where they are located is called a stem cell niche. These locations have special conditions which maintain stem cells inactive for long time but can proliferate and differentiate when it is required.
- Bone marrow: are made of haematopoietic cells that can differentiate into huge range of blood cells.
- Hair follicles: the hair follicles contain epidermal stem cells that are involved in hair growth, skin innervation, and wound repair.
Adaptations in cells for specialisation
The size of a cell is an adaptation to the specific function they perform.
- Egg cells are extremely large to allow large quantities of food to be stored in the cytoplasm.
- Sperm cells are extremely small and narrow which reduce resistance and allow them to swim more easily
- White blood cells become larger when activated, the extra volume is due to na increase in cytoplasm, rER and Golgi body for antibody synthesis.
- Red blood cells are small and indent on both sides. The size allows to easily diffuse through capillaries and the shape helps to have a large SA:V ratio to increase the rate of oxygen exchange.
- Motor neurons can be even 1 meter long although only 20 micro meters thick and allows for signals to be carried from the central nervous system to the muscle extremely fast.
- Striated muscle fibres are large cells up to 10cm long with a diameter of 20-100 micro meters, this allows them to exert a greater force and contract by a greater length than smaller muscle cells.
Proximal convoluted tubule cells adaptations
