Examples of Specialised Cells (HL) (HL IB Biology)

• There are millions of alveoli in the lungs which collectively provide maximum surface area for gas exchange by diffusion
• In addition to this, the alveolar walls (also called alveolar epithelium) are only one cell thick which provides a short diffusion distance
• Two different cell types make up the tissue of the alveolar epithelium

Type I Pneumocytes

• Type I pneumocytes are extremely thin alveolar cells which make up the majority of the alveolar epithelium
  • They are adapted to maximise the rate of gas exchange by providing a short diffusion distance
  • The capillary walls are also only one cell thick which means there is usually less than 0.5μm between the air in the alveoli and the blood, this maximises the rate of diffusion

Structure and Adaptations of Type I Pneumocytes Diagram

The thin type I pneumocyte cells and the thin capillary walls provide a short diffusion distance to maximise gas exchange

Type II Pneumocytes

• Type II pneumocytes are rounded cells which possess many secretory vesicles (lamellar bodies) which secrete a solution that coats the epithelium of the alveoli
• They occupy a much smaller proportion of the alveolar epithelium than the type I pneumocytes, around 5%
• The solution released by type II pneumocytes contains pulmonary surfactant
  • Pulmonary surfactant has hydrophobic tails and hydrophilic heads 
  • The molecules form a monolayer with the hydrophobic tails facing the alveolar air
• Pulmonary surfactant reduces surface tension, maintaining alveolar shape and preventing the alveoli sacs sticking together
  • This prevents the alveoli, and therefore the lungs, from collapsing
• The solution also aids gas exchange
  • The layer of moisture provided by the solution allows oxygen to dissolve before it diffuses into the blood
  • Carbon dioxide diffuses from the moist surface before it is removed in exhalation

Arrangement of Pulmonary Surfactant Produced by Type II Pneumocytes Diagram

The type II pneumocyte cells in the alveoli produce a solution containing pulmonary surfactant which reduces surface tension

Type I and Type II Pneumocytes Diagram

The alveolar epithelium is made up of type I and type II pneumocyte cells

• Muscles in the body that are attached to the skeleton and aid movement are called skeletal muscles
• Other muscle types include:
  • Cardiac muscle which is found in the heart
  • Smooth muscle is found in the blood vessels and organs

Striated Muscle Fibres

• Skeletal muscle is striated as it has a stripy appearance when viewed under a microscope
• Striated muscle cells are bundled up into fibres which are surrounded by a single plasma membrane called the sarcolemma
• The fibres are highly specialised cell-like units
• Fibres are referred to as being cell-like due to features which distinguish their ultrastructure from that of other more traditional cells
  • Each muscle fibre contains:
    • An organised arrangement of contractile proteins in the cytoplasm
    • Many nuclei – this is why muscle fibres are not usually referred to as cells
    • Specialised endoplasmic reticulum called the sarcoplasmic reticulum (SR) which stores calcium and conveys signals to all parts of the fibre at once using protein pumps in the membranes
    • Specialised cytoplasm called the sarcoplasm contains mitochondria and myofibrils
      • The mitochondria carry out aerobic respiration to generate the ATP required for muscle contraction
      • Myofibrils are bundles of actin and myosin filaments, which slide past each other during muscle contraction
• The sarcolemma (muscle fibre membrane) has many deep tube-like projections that fold in from its outer surface
  • These are known as transverse system tubules or T-tubules
  • These run close to the SR

Diagram to show the Structure of Striated Muscle Fibres

The ultrastructure of striated muscle and of a section of muscle fibre

Cardiac Muscle

• Cardiac muscle is only present within the heart
• It is a type of specialised striated muscle with the following properties:
  • It is myogenic, meaning that it can contract without external stimulation via nerves or hormones. This allows the heart to beat at its own regular intervals (the length of the intervals can be regulated by the nervous system and endocrine system)
  • It does not tire or fatigue so it can contract (beat) continuously throughout an individual's life
  • The cardiac muscle fibres form a network that spreads through the walls of the atria and ventricles
  • Cardiac muscle fibres are connected to each other via specialised branched connections called intercalated discs, this feature allows the contraction to spread more quickly across the chambers of the heart
  • There is a large number of mitochondria present in the muscle fibres. These are needed to provide the large quantity of ATP needed for continual contraction
  • Contractile myofibrils are present in cardiac muscle in the same way that they are in skeletal fibres

Diagram to show the Structure of Cardiac Muscle Cells

The structure of cardiac muscle. There is only one nucleus per cell.

Sperm Cells

• Sperm and ova are examples of specialised cells, meaning that their structure aids their function
• Special features of sperm cells that relate to function include
  • A haploid nucleus, contained within a streamlined head, that can fuse with an ovum nucleus to form a diploid zygote
  • An acrosome containing digestive, or hydrolytic, enzymes to aid entry into the ovum through the zona pellucida
  • Many mitochondria (within the middle piece) for the release of energy to aid movement
  • A flagellum made of protein microtubules to aid movement

Diagram to show the Structure of a Sperm Cell

Sperm cells are specialised to enable movement towards and entry into the ovum

Egg Cells

• Special features of ova that relate to function include
  • A haploid nucleus that can fuse with a sperm cell nucleus to form a diploid zygote
    • The final stage of meiosis is only completed after fertilisation
  • A surrounding jelly layer, or zona pellucida, that can harden to prevent polyspermy (when the ovum is penetrated by more than one sperm, this can affect embryo development)
  • Follicle cells which nourish and protect the ova
  • A series of vesicles, or cortical granules, containing digestive enzymes that are released into the zona pellucida to prevent polyspermy
  • A cytoplasm rich in nutrients for the developing embryo after fertilisation

Diagram to show the Structure of an Egg Cell

Ova are specialised to prevent polyspermy and aid development of the embryo