B2.3 Cell Specialisation

Describe the process of unspecialised cells becoming specialised

• After fertilisation, fertilised egg (zygote) divides into many unspecialised cells (called embryonic stem cells)

• These cells start off w/ the same DNA & genes 

  • But due to chemical gradients, cells can turn on/off genes to make proteins for specific functions (differentiation)

  • These gradients cause cells in different parts of embryo to receive different signals → guides cells to specialise in correct location

Define cell differentiation

The process where unspecialised cells become specialised to carry out distinct functions

Describe two properties of stem cells

1) Unlimited division - Has capacity to keep dividing & making cells 

2) Differentiation - Depending on type, can develop into different specialised cells 

Define a stem cell niche

A specialised microenvironment in tissues where adult stem cells live and are controlled

Describe the functions of stem cell niches (5)

• It sends signals that:

1) Support self-renewal (including proliferation = rapid increase of cells) 

2) Maintain quiescence (resting, inactive state) 

3) Trigger differentiation (for tissue repair/function)

4) Protect stem cells from stress/damage

Describe examples of stem cell niches (2)

1) Bone marrow - Maintains a balance between stem cell storage & production of blood cells

2) Hair follicle - Stores stem cells that control hair growth, rest, & regrowth cycles

Define potency

The ability of a stem cell to develop into different types of specialised cells

Describe the stem cell types (location, what they can become, & example) 

Stem Cell Type (by 📉 potency)	Location	Can become	Example
Totipotent	Very early embryo (zygote & first cell divisions)	Any body cell & placenta	A fertilised egg (zygote)
Pluripotent	Early-stage embryo (blastocyst)	Any body cell (not placenta)	Embryonic stem cells
Multipotent	Adult tissues (e.g. bone marrow, skin)	Related cells within one system/tissue	Blood stem cells (RBCs, WBCs, platelets)

Describe sperm cell’s size/structure & the link to its function (2)

Size: ~5-10 μm in diameter (very small) 

Structure → Function: 

• Small size (minimal cytoplasm) + flagellum → quick movement through fluids & towards egg

Describe egg cell’s size/structure & the link to its function (2)

Size: ~100 μm (1 of largest human cells)

Structure → Function: 

• Large size + round → nutrient storage for early development

Describe RBC’s size/structure & the link to its function (4)

Size: ~6-8 μm in diameter

Structure → Function: 

Small, biconcave disc shape + no nucleus → 

• Maximises surface area for oxygen binding & gas exchange 

• Can fit through small capillaries 

Describe WBC’s size/structure & the link to its function (2)

Size: ~12-17 μm in diameter

Structure → Function: 

• Irregular & variable (enlarges during protein synthesis) + nucleus → recognising pathogens & produce immune responses 

Describe granule cell’s size/structure & the link to its function (2)

Size: ~4-5 μm in diameter (very small)

Structure → Function: 

• Small size → allows packing of millions of cells

Describe motor cell’s size/structure & the link to its function (2)

Size: Axon up to 1 m long

 

Structure → Function: 

• Long length → transmit signals to muscles over long distances

Describe motor cell’s size/structure & the link to its function (3)

Size: Diameter of 30-40 μm, up to 40 mm in length

Structure → Function: 

• Long, thick size → generate powerful contractions to move bones 

• Multinucleated→ greater protein synthesis capacity 

What happens in surface area vs volume in cells? (2)

• Surface area = where exchange occurs

• Volume = where materials are used/produced

What happens to SA:V ratio as cell size grows? (3)

• As cells grow, volume increases faster than surface area

• This decreases the SA:V ratio

  • This affects how well substances move in/out of cells

NOS: Describe how cubes can model SA:V relationship (3 + table)

• Cubes of different side lengths serve as models to simplify & visualise SA:V relationship

• Although real cells have irregular shapes & internal complexity, scale factors are the same  

  • Allows understanding that bigger size = lower SA:V

Cube side length	Surface area  (6 × side²)	Volume (side³)	SA:V ratio
1 unit	6	1	6 : 1
2 units	24	8	3 : 1
3 units	54	27	2 : 1

What are the common cell adaptations to increase SA:V ratio>

1) Flattening - Reduces thickness, spreading cell to expose more surface 

2) Microvilli - Finger-like projections multiplying cell’s surface

3) Invaginations (infoldings) - Folds on the cell membrane that expand surface inward

4) Biconcave shape - Increases external surface area, while keeping low volume 

How do Erythrocytes (type of RBC) adapt to increase SA:V ratio? (2)

Adaptation: Biconcave disc-shape

Function: Maximizes SA & provides a high SA:V ratio for rapid oxygen exchange 

How do Proximal Convoluted Tubule Cells (in Kidney Nephron) adapt to increase SA:V ratio? (5)

Adaptations: 

• Microvilli (on apical surface) - Increase SA facing filtrate 

  • Allows efficient reabsorption into cell

• Basal infoldings (on underside) - Increase SA for exchange w/ blood capillaries 

  • Provide space for mitochondria to fuel active transport 
    

→ they both maximise efficiency in reabsorbing valuable substances back into bloodstream 

Define pneumocyte

Specialised epithelial cells lining the alveoli of the lungs  

Describe the structure & function of type I pneumocytes (2)

Structure: Extremely thin & flat, making up most of the alveolar surface

Function: Minimises diffusion distance for oxygen & carbon dioxide into bloodstream 

Describe the structure & function of type II pneumocytes (6)

Structure: Cuboidal cells w/ many secretory vesicles

Function: 

1) Secretes pulmonary surfactant to support lung function:

• Reduces surface tension in alveoli

• Prevents alveolar collapse during exhalation

• Keeps alveoli open & functional 

2) Can divide & replace damaged Type I cell

Describe the adaptations of skeletal muscle cells (3)

• Branching - Unbranched

• Length - Very long

• Nuclei - Multiple nuclei per cell (at edges)

Describe the adaptations of cardiac muscle cells (3)

• Branching - Branched

• Length - Short 

• Nuclei - 1-2 nuclei per cell (at centre)

What is the shared adaptation of cardiac & striated muscle cells? (2)

Myofibrils - 

• Contractile units within both cardiac & striated muscle cells that are composed of actin & myosin

  • These proteins form sarcomeres (repeating structures responsible for muscle contraction) → giving them striated appearances

Are skeletal muscle fibres cells? (3)

• Skeletal muscle fibres are cells, but its features make it atypical:

  • It is multinucleated

  • Very long → formed by fusion of multiple myoblasts (embryonic cells)

Define gametes

Specialised reproductive cells that fuse during fertilisation to form a zygote 

Describe adaptations of sperm cells, in relation to their functions (6)

Adaptation	Function
Small size	Enables fast movement over long distances
Streamlined shape	Reduces water resistance, aids fast swimming
Flagellum (tail)	Propels the sperm using whip-like motion
Mitochondria (midpiece)	Provides ATP for tail movement
Acrosome	Contains enzymes to penetrate egg’s outer layers
Haploid nucleus	Carries 23 chromosomes

Describe adaptations of egg cells, in relation to their functions (6)

Adaptation	Function
Large size	Contains nutrients for embryo development
Zona pellucida	Protects egg & blocks multiple sperm entry
Cortical granules	Harden zona after fertilisation to prevent polyspermy
Cytoplasm rich in organelles	Supports early embryo growth
Haploid nucleus	Carries 23 chromosomes