Ib biology: B2.3 Cell Specialisation

mitosis

duplication which has no variation

cells in zygote

unspecialised stem cells

what are stem cells

Stem cells are unspecialised. They have the potential to differentiate into many other types of cells. There are many kinds of stem cells with varying abilities to differentiate.

sperm cells size and reason

50 micrometers, but head and midpiece are actually very small, measuring about 5 micrometers in length. that is because they need to move.

egg cell size and reason

100 micrometers, larger to store nutrients for the developing embryo.

white blood cells size and reason

Varies, typically around 12-17 micrometers; larger to effectively fight infections and respond to pathogens.

iPS cells

induced pluripotent stem cells: specialised cell that were treated with a reprogramming factor and that have then produced pluripotent stem cells

What needs to be able to get in and out of cells

• water

• carbon dioxide ( out)

• oxygen

• amino acids

• fatty acids

• glycerol

• heat

required for processes such as respiration, protein synthesis…

morula

solid ball of embryonic cells of about 16 to 32 cells

blastocyst

a few days after the zygote has become a morula, it becomes a blastocyst.

it is hollow inside, and has an outer layer of cells called trophoblasts, and inside there is an inner cell mass

the trophoblasts will develop into the placenta and the inner cell mass will become the embryo

morphogen gradients

• early on in the pregnancy, certain cells secrete molecules called morphogens

• as the molecules diffuse outward of their source, a concentration difference is formed in the neighbouring cells

• the further away from the morphogen secreting cells, the lower the morphogen concentration

• the cells can detect that through receptors on it’s surface and develops accordingly

Totipotent stem cells

• Can differentiate into any type of cell including placental cells.

• Can give rise to a complete organism.

• the first cells formed following fertilisation of an egg cell

Pluripotent stem cells

• Can differentiate into all body cells, but cannot give rise to a whole organism.

• embryonic stem cells of the blastocyst

Multipotent stem cells

• Can differentiate into a few closely related types of body cell.

• umbilical cord stem cells

Unipotent stem cells

Can only differentiate into their associated cell type. For example, liver stem cells can only make liver cells.

stem cell niche in embryos ; what is is and what are it’s effects

the “microenvironment” which defines how a stem cell will develop.

it can be because of cell to cell interactions or because of interactions with the extracellular matrix ( which can have signalling molecules )

thus some cells can be instructed to remain dormant, others into multiplying, and other into differentiating into another kind of cells

main stem cell niches

• blood

• skin

• intestine

• brain

• muscle cells

blood stem cell niche

• in bone marrow: it has haematopoietic (immature stem cells which form the blood cells) and supportive cells

• the supportive cells regulate the haematopoietic cells

hair stem cells

• hair follicle stem cells are in “the bulge”

• other cells are responsible for the breakdown of old hairs and it’s maintenace

reasons for size difference between sperm cell and egg cell

• human egg cell is that size as it contains all nutrients necessary for early development of the embryo;

• the sperm cell doesn’t need such nutrients and thus remains small

examples of very long cells

• neurons in the sciatic nerve and the longest in the human body: the axons can be longer than a meter

• their goal is to be an effective mechanism for communication between the spinal chord and more distant parts of the body

example of the smallest cells and why

red bloods cells/ erythrocytes : 7.5µm in diameter and 2µm in thickness.

have no nucleus to have more space for haemoglobin for binding oxygen to transport around the body

have a very flexible membrane which allows them to be repetitively deformed and sprung back into shape

surface area to volume ratio

the larger the volume, the smaller the surface area to volume ratio; thus the harder if is for the necessary nutrients to diffuse inside the cell and for excretory products to exit the cell

examples of how a better SA:V ratio is achieved

• smaller volume - prokariotic cells: no specialised trasnport mechanisms

• flattening - epitheal cells: they also have microvilly which further increase surface area

• invagination of membranes - mitochondria ( cristae )

specific adaptation in the lungs

type I pneumocytes: very thin and long cells in the alveoli which allows for short diffusion distances to capillaries. make up 95% of alveolar surface

type II pneumocytes: make up 5% of alveolar surface; the type of the cells protrude in the alveolar space; they secrete surfactant into the alveolar space.

macrophages: phagocise any pathogens

surfactant released by type II pneumocytes

• decreases surface tension ( prevents the cells from sticking together ) and thus prevents lungs from collapsing

• a phospholipid- also allows gases to dissolve and diffuse more quickly

specific adaptation in the skeletal muscles

made up of repeating units called sarcomeres which gives the muscles a striated appearance. unlike cardiac muscles, they are unbranched. this allows very precise control of voluntary muscle contractions.

each muscle fibre is called a syncytium; it’s precursors have fused together and thus the fibre is multi-nucleated. it allows the tissue to conduct more protein synthesis and repair

specific adaptation in the heart cells

made up of repeating units called sarcomeres which gives the muscles a striated appearance. heart cells are called cardiomyocytes. they are large ( 150µm) and only have a single nucleus

they are connected to other cardiomyocytes via gap junctions at intercalated discs. have only one nucleus. the muscles fibres there are branching so they can connect with the neighbouring cardiomyocytes. this allows for electrical impulses to pass , and thus allows for all cells to contract together in a synchronised manner

specific adaptation in the egg cells

• egg cells are the largest human cells and it’s cytoplasm is rich in lipids and polysaccharides.

• It has many layers of specialised cell coverings preventing more than one sperm fertilising the egg.

• since egg cells are very costly to produce in terms of energy, only a few are produced

specific adaptation in the sperm cells

• sperm have evolved to be motile; so they have a lot of mitochondria in order to provide energy for the flagellum.

• in the head of the sperm, a small cytoplasm contains a haploid nucleus

• the head of the sperm has an acrosome cap which contains digestive enzymes used to penetrate the egg’s “zona pellucida”

• there is no energy storage in the sperm cells, so it is suppllied in the seminal fluid

potency of stem cells

• can be used therapeutically to treat diseases by replacing damaged cells with healthy ones

• embryonic stem cells have greater potency, but there are ethical issues associated with their use

• adult stem cells have less ethical issues and lower chance of graft regection ( as use of own cells)

• induced pluripotent stem cells: best BUT very expensive

hair stem cell niche

• contain a range of multipotent epidermal stem cells and are responsible for cyclic bouts of hair growth, skin innervation, vascularisation and wound repair

• could be harvested to help burn victims

How do the following optimise SA to vol ratio:

• dividing yeast cells

• root with root hair cells

• cactus

• small intestine- villi and microvilli

• yeast: grows up to a certain point, and then cells divide to increase SA to vol ratio

• the hair cells provide extra SA without a lot of extra volume so that more nutrients can be absorbed

• develop structures without leaves, so SA is reduced and less water loss occurs through evapotranspiration through pores of leaves

• the villi and microvilli in the stomach increases the SA necessary for absorption of nutrients into bloodstream: increase SA 40 times

proximal convoluted tubules

• proximal convoluted tubule epithelial cells have microvilli to absorb nutrients needed in the direction of the lumen

• a lot of mitochondria are found in the cells, allowing for active transport if ions and other substances

• invaginations on other side if cells allowing for nutrients to get out