Eukaryotic Cell Structure (SL IB Biology)

Structure of eukaryotic cells

• Eukaryotic cells have a more complex ultrastructure than prokaryotic cells
• The cytoplasm of eukaryotic cells is divided up into membrane-bound compartments called organelles
• The compartmentalisation of the cell is advantageous as it allows:
  • enzymes and substrates to be available at higher concentrations
  • damaging substances to be kept separated, e.g. digestive enzymes are stored in lysosomes so they do not digest the cell
  • optimal conditions to be maintained for certain processes, e.g. optimal pH for digestive enzymes
  • the numbers and locations of organelles to be altered depending on requirements of the cell

Animal cell diagram

Animal cells contain membrane-bound compartments, e.g. the nucleus and mitrochondria

Plant cell diagram

Plant cells are also eukaryotic cells, and have additional features when compared to animal cells, e.g. a cell wall and chroroplasts

Organelles

Plasma membrane

• All cells are surrounded by a plasma membrane which controls the exchange of materials between the internal cell environment and the external environment
  • The membrane is described as being ‘partially permeable’
• The plasma membrane is formed from a bilayer of phospholipids spanning a diameter of around 10 nm

The cell surface membrane, or plasma membrane, controls the passage of substances into and out of cells

Nucleus

• Present in all eukaryotic cells (except red blood cells), the nucleus is a large organelle that is separated from the cytoplasm by a double membrane (the nuclear envelope) which has many pores
• Nuclear pores are important channels for allowing mRNA and ribosomes to travel out of the nucleus, as well as allowing enzymes (e.g. DNA polymerases) and signalling molecules to travel in
• The nucleus contains chromatin (the material from which chromosomes are made)
  • Chromosomes are made of sections of linear DNA tightly wound around proteins called histones
• Usually at least one or more darkly stained regions can be observed within the nucleus; these regions are individually termed the ‘nucleolus’ (plural: nucleoli) and are the sites of ribosome production

The nucleus of a cell contains chromatin (a complex of DNA and histone proteins) which is the genetic material of the cell

Rough endoplasmic reticulum

• Found in plant and animal cells
• Surface covered in ribosomes (80S)
• Formed from folds of membrane continuous with the nuclear envelope; these flattened membrane sacs are called cisternae
• Processes proteins made by the ribosomes
• The proteins synthesised by the ribosomes move to the cisternae, bud off into vesicles that carry the proteins to Golgi apparatus before being secreted out of the cell

The rough endoplasmic reticulum (RER) has ribosomes on its surface

Ribosomes

• 80S ribosomes are found freely in the cytoplasm or as part of the rough endoplasmic reticulum in eukaryotic cells
• Each ribosome is a complex of ribosomal RNA (rRNA) and proteins
• Constructed in the nucleolus
• Site of translation (which is part of protein synthesis)

Ribosomes are formed in the nucleolus and are composed of almost equal amounts of RNA and protein

Mitochondria

• The site of aerobic respiration within all eukaryotic cells, mitochondria (singular mitochondrion) are just visible with a light microscope
• Surrounded by a double-membrane with the inner membrane folded to form cristae
• The matrix contains enzymes needed for aerobic respiration, producing ATP
• Small, circular pieces of DNA (mitochondrial DNA) and ribosomes are also found in the matrix (needed for replication)

The inner mitochondrial membrane has protein complexes vital for the later stages of aerobic respiration embedded within it

Golgi apparatus

• Flattened sacs of membrane called cisternae
• Modifies proteins and lipids before packaging them into Golgi vesicles
• The vesicles then transport the proteins and lipids to their required destination
• Proteins that pass through the Golgi apparatus are usually:
  • exported, e.g. hormones such as insulin
  • put into lysosomes, such as hydrolytic enzymes
  • delivered to membrane-bound organelles

The Golgi apparatus has a distinctive appearance due to the arrangement of cisternae from which it is formed

Vesicles

• Membrane-bound sacs for transport and storage, e.g. Golgi vesicles transport proteins from the Golgi apparatus around the cell
• Lysosomes are specialised vesicles that contain hydrolytic enzymes
  • The role of lysosomes is to break down waste materials such as worn-out organelles
  • Lysosomes are used extensively by cells of the immune system and in apoptosis (programmed cell death)

The structure of the vesicle

Microtubules

• Make up the cytoskeleton of the cell and are about 25 nm in diameter
• The cytoskeleton is used to provide support and movement to the cell
• Made of α and β tubulin proteins combined to form dimers, the dimers are then joined into protofilaments

Microtubules make up the cell cytoskeleton

Plant cell structures

Chloroplasts

• Larger than mitochondria
• Surrounded by a double-membrane
• Membrane-bound compartments called thylakoids containing chlorophyll stack to form structures called grana
• Grana are joined together by lamellae (thin and flat thylakoid membranes)
• Chloroplasts are the site of photosynthesis:
  • The light-dependent stage takes place in the thylakoids
  • The light-independent stage (Calvin Cycle) takes place in the stroma
• Also contain small circular pieces of DNA and ribosomes used to synthesise proteins needed in chloroplast replication and photosynthesis

Chloroplasts are found in the green parts of a plant; the green colour is the result of the photosynthetic pigment chlorophyll

Large permanent vacuole

• A sac in plant cells surrounded by the tonoplast, which is a selectively permeable membrane
• Animal cells can contain vacuoles, but they are temporary and small

Plant cells contain large, permanent vacuoles

Cell wall

• Found in plant cells but not in animal cells
• Formed outside of the cell membrane and offer structural support to the cell
  • Cell walls are extra-cellular, so are not technically cellular organelles
• Structural support is provided by the polysaccharide cellulose in plants, and peptidoglycan in most bacterial cells
• Narrow threads of cytoplasm (surrounded by a cell membrane) called plasmodesmata connect the cytoplasm of neighbouring plant cells

The cell wall is freely permeable to most substances (unlike the plasma membrane)