Cell specialisation

development of specialised cells

• specialisation occurs after fertilisation to allow development of different tissues within embyo

stem cells

• a cell that can divide by mitosis an unlimited number of times

• each new cell has potential to remain a stem cell, or develop into specialised cell by differentiation

stem cell niches

• some stem cells remain in specific locations in human body, this is stem cell niche

• their presence gives capacity for these tissues to regenerate and repair

• niche must have ability to maintain inactive state of stem cell or ability to stimulate stem cell proliferation and differentiation

examples of stem cell niches

• bone marrow

  • provides a niche for stem cells which are used to replace red blood cells, white blood cells & platelets

  • important for continual production of these cells which are required indefinitely

• hair follicle

  • niche located at root of hair, where hair is anchored to skin

  • stem cells here promote continual hair growth

stem cell potency

• ability to differentiate is known as potency

• 4 types of potency

  • totipotency:

    • can differentiate into any cell type found in an embryo, and extra-embryonic cells (placenta cells)

    • a zygote is totipotent

  • pluripotency:

    • can differentiate into any cell type found in embryo, but not extra embryonic

  • multipotency:

    • adult stem cells that can differentiate into closely related cell types e.g in bone marrow

  • unipotency:

    • adult stem cells that can only differentiate into their own lineage

cell size and specialisation

• red blood cells: small for movement through narrow capillaries

• active white blood cells: larger than inactive to allow space for RER and golgi apparatus

• sperm cells: long for movement towards egg, narrow streamlined heads to reduce resistance

• egg cell: largest volume of all cells to allow stored food reserves

• nerve cell: large cell body to allow protein synthesis, maintain structure of axon for delivery of impulses

• muscle cells: larger than normal cells, length and diameter designed to exert force during muscle contraction

surface area to volume ratio

• metabolic reactions rely on materials being exchanged across membrane surface

• metabolic requirements of cell will vary depending on volume of cytoplasm, larger volume will have higher metabolic requirements

• as cell increase in size, SA:V ratio decreases as there is less SA in relation to vol of organism

• increase in vol will increase cell metabolic requirements, but its ability to carry out exchange with environment does not increase at this rate

constraint on cell size

• single celled organism have high SA:V ratio, means they can survive by simple diffusion at cell surface

  • metabolic requirements are low, SA large enough for sufficient rate of exchange, small vol means diffusion distance to all organelles is short

• SA:V ratio decreases as cells get larger, so cells cannot grow bigger indefinitely

  • metabolic requirements are higher, SA doesn’t increase at same rate as metabolic requirements, not large enough for sufficient rate of exchange, large vol means diffusion distance to all organelles is long

• once SA:V ratio becomes too small, growth stops and cells divide, making multicellular organisms

NOS models

• models are simplified versions of complex systems

• SA:V relationship can be modelled using cubes of different side lengths

increasing SA:V ratio of cells

• red blood cell:

  • flattened biconcave shaped to maximise SA and minimise vol

• proximal convoluted tubule cells:

  • responsible for reabsorption of vital substances

  • micro villi and invaginations maximise SA

pneomocyte adaptations

• alveoli in lungs maximise SA for gas exchange

• thin alveolar walls for short diffusion distance

• 2 different cell types make up the tissue of alveolar epithelium

  • more than one cell type present because different adaptations required for overall functions of the cell

type 1 pneumocytes

• extremely thin alveolar walls, make up majority of alveolar epithelium

  • adapted to maximise gas exchange by short diffusion distance

  • capillary walls only one cell thick

type 2 pneumocytes

• occupy much smaller proportion of alveolar epithelium than type 1 pneumocytes

• rounded cells possessing many secretory vesicles, which secrete solution that coats the epithelium of alveoli

• solution contains pulmonary surfactant that reduces surface tensions, maintaining alveolar shape, and preventing alveoli sacs from sticking together

• layer of moisture aids gas exchanged by allowing oxygen to dissolve before it diffuses into blood

adaptations of muscle cells

• both striated muscle fibres and cardiac muscle have:

  • contractile myofibrils

  • large numbers of mitochondria to supply ATP needed for muscle contraction

striated muscle fibres

• unbranched

• very long

• considered cell-like because it’s non-traditional

  • multi-nucleated

  • much larger than normal cell

cardiac muscle cells

• branched: each cell is connected to several others

• shorter in length

sperm cell

• haploid nucleus contained with a streamlined head that can fuse with ovum nucleus to form diploid zygote

• mitochondria for release of energy to aid movement

• flagellum to aid movement

egg cell

• haploid nucleus that can fuse with sperm cell to form diploid zygote

• very large as they contain food reserves for early development of embryo

• jelly layer that can harden to prevent polyspermy

• follicle cells which nourish and protect the egg