B2.3 Cell specialisation

explain how unspecialised cells form following fertilisation

fertilisation = fusion of female & male gametes to produce zygote

zygotę divides repeatedly to form embryo

early stage embryo = unspecialised cells

embryo grows → cells differentiate

early stage embryos:

• develop morphogens → signalling chemicals that form gradients

• concentration of morphogens determines cell’s position and pathway of differentiation

• morphogens regulate gene expression

outline properties of stem cells

self replicating - stem cells divide endlessly to produce more stem cells

undifferentiated - stem cells can differentiate alone different pathways

explain function of stem cell niches - use bone marrow and hair follicles as examples

embryonic cells = stem cells

stem cell niche - specific location in tissue that provides right microenvironment for stem cells to:

• remain inactive and undifferentiated

• rapidly proliferate and differentiate when needed

examples:

• bone marrow: contains haematopoietic stem cells that differentiate into blood cells & platelets, receives 15% cardiac output to supply oxygen, amino acids and nutrients

• hair follicles: contain stem cells at base that divide to promote hair growth, blood capillaries supply essential nutrients

size differences in various human cells

• Male gametes : 3 μm wide, 50 μm long – size helps swimming.

• Red blood cells : 7 μm in diameter, 1–2 μm thick – small size helps fit in capillaries and maximises surface area to volume ratio.

• White blood cells : B lymphocytes are 10–12 μm when inactive; plasma cells grow to 30 μm when active (antibody production).

• Female gametes : 110 μm – large size stores nutrients for embryo.

• Neurons: 20 μm diameter, but axons can be over 1 metre long.

• Striated muscle fibres: 20–100 μm in diameter, over 100 mm in length

discuss relationship between cell size and SA - in terms of movement of material across a cell

• As cell size increases, the surface area to volume ratio decreases.

• The rate of material entering/leaving the cell depends on surface area.

• The rate of material use and production depends on volume

identify & describe cell types that increase SA:V

1. Flattening of shape:

• Red blood cells : Flattened disc shape increases surface area.

• Type I pneumocytes: Extremely flat cells in the lungs.

2. Microvilli:

• Found on epithelial cells (e.g. kidney nephrons).

• Increase surface area for reabsorption of glucose and other substances.

3. Invagination of membrane:

• Plasma membrane folds inward to form tubules/folds/sacs.

• Example: Kidney tubule cells have basal channels for pumping Na⁺ and co-transporting glucose.

describe specific adaptations that cells in lung, heart, muscle and gonads have undergone to perform their functions

Lung cells:

• Type I pneumocytes: Form 95% of alveolus wall; very thin and permeable for gas exchange.

• Type II pneumocytes: Form 5% of wall (but more numerous); contain vesicles with fluid to moisten alveoli and surfactant to reduce surface tension.

Muscle cells:

• Cardiac muscle cells: Branched, with intercalated discs for signal transmission; 50–100 μm long with one nucleus per cell.

• Striated muscle cells: Long, unbranched, cylindrical, no intercalated discs; up to 30,000 μm long and multinucleated.

Gonad cells:

• Sperm cells: 3 μm wide, 50 μm long; flagellum for movement, mitochondria near tail, streamlined with minimal organelles, acrosomal vesicle to penetrate egg.

• Egg cells: 120 μm; spherical, non-motile, many organelles and storage structures. Vesicles block entry of more than one sperm.

  • Sperm are produced throughout life; eggs are made before birth.

  • Both are haploid (contain one set of chromosomes)