Eukaryotic Cell Structure: Comparisons & Atypical Examples (HL IB Biology)

• Eukaryotic cells exist in four kingdoms
  • The animal kingdom
  • The plant kingdom
  • The fungal kingdom
  • The protist (protoctista) kingdom
• The cells of each of these possess unique characteristics and structures that contribute to their differences

Differences in eukaryotic cell structure

Cell walls

• Animal cell do not have a cell wall
• Plant cell walls are composed of the polysaccharide cellulose
• Fungal cell walls are made up mainly of glucans, chitin and glycoproteins

Vacuoles

• Vacuoles can be present in animal cells but they tend to be small, temporary and numerous when present with unique functions
• Plant cells have large permanent vacuoles used for the storage of various substances
• Like animal cells, fungal cells can contain vacuoles but they are small and non-permanent

Chloroplasts

• Animal cells do not have chloroplasts
• Plant cells possess many chloroplasts used for the production of carbohydrates through photosynthesis
• Fungal cells do not have chloroplasts

Presence of centrioles

• Animal cells do contain centrioles used in the role of microtubule organisation during cell division
• Plant cells do not possess centrioles
• Fungal cells do not possess centrioles

Presence of cilia and flagella

• Animal cells can have cilia and flagella, associated with a basal body (a protein structure from which the cilia are assembled), and are used in various functions such as the movement of an egg cell through the oviduct or the movement of fluids in the respiratory tract
• Plant cells do not contain cilia or flagella
• True fungi do not contain cilia or flagella

Other differences

• Animal and fungal cells store their carbohydrates as glycogen, whereas plants so carbohydrates as starch
• Animal cells are flexible as they lack a rigid cell wall, whereas plant cells have a fixed shape. Fungal cells, although they have a cell wall, can be flexible and their shape may vary

• Some eukaryotic cells have a very unique or atypical structure which enables them to carry out specialised functions
• The number of nuclei can be used to illustrate atypical examples
• Skeletal muscle, aseptate fungal hyphae, red blood cells and phloem sieve tubes are examples of cells/tissue with structures that question the integrity of the cell theory

Atypical examples

Striated muscle fibres

• Striated muscle fibres (fused muscle cells) are:
  • Longer than typical cells (up to 300 mm in length in comparison to a cardiac muscle cell which has a length of 100 - 150 µm)
  • Have multiple nuclei surrounded by a single membrane (sarcolemma) 
  • Striated muscle cells are formed from multiple cells which have fused together (which is how they have many nuclei rather than one) that work together as a single unit
  • These features challenge the concept that cells work independently of each other even in a multicellular organism

Aseptate fungal hyphae

• Fungi have many long, narrow branches called hyphae
• Hyphae have cell membranes, cell walls and some have septa
• Aseptate fungal hyphae do not have septa, thus these cells are multinucleated with continuous cytoplasm
• The cells have no end walls making them appear as one cell

Red blood cells

• Red blood cells, a type of animal cell, are unique in that they do not contain a nucleus
• The reason for this is to enable the cell to carry a large volume of the oxygen binding pigment haemoglobin
• The biconcave shape of red blood cells means they have maximum surface area to improve their oxygen carrying capacity

Phloem sieve tubes

• These serve a plant by transporting dissolved substances, such as sucrose, around the plant
• These unique tissues have no end cell wall and lack many cell organelles such as nuclei, mitochondria and ribosomes
• Because of the lack of their own organelles, sieve tube elements can only survive due to the presence of companion cells which sit alongside next to the sieve tube elements and help to maintain the cytoplasm of the sieve tubes