Eukaryotic Cell Ultrastructure

• Nucleuspart 1


  • Present in all eukaryotic cells (except RBCs), the nucleus is relatively large and 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
• Nucleus part 2

  • Usually, at least one or more darkly stained regions can be observed – these regions are individually termed ‘nucleolus’ (plural: nucleoli) and are the sites of ribosome production
• Nucleus function

  • controls the cell's activities
  • stores the organism's genome
  • DNA contains instructions to make proteins
  • transmits genetic information
• Rough endoplasmic reticulum

  -a series of flattened sacs enclosed by a membrane with ribosomes on the surface. RER folds and processes proteins made by the ribosomes

  -a system of membranes, containing fluid-filled cavities (Cisternae) that are continuous with the nuclear membrane

  • Found in eukaryotic cells
  • Large surface area covered by ribosomes -> assemble amino acids into proteins -> actively pass through the membrane into the cisternae and transported to the Golgi apparatus for modification and packaging
  • intracellular transport system: cisternae form channels for transporting substances
• Smooth endoplasmic reticulum

  -a system of membranes, containing fluid-filled cavities (cisternae) that are continuous with the nuclear membrane

  • Found in eukaryotic cells
  • Does not have ribosomes on the surface
  • Involved in the production, processing and storage of lipids, carbohydrates and steroids
  • contains enzymes that catalyse reactions involved with lipid metabolism e.g. synthesis of cholesterol, lipids, steroid hormones
  • involved with absorption, synthesis and transport of lipids
• Golgi apparatus

  -a series of fluid filled, flattened and curved sacs with vesicles surrounding the edges

  • Found in eukaryotic cells
  • produces lysosomes
  • Modifies proteins and lipids before packaging them into Golgi vesicles
  • The vesicles then transport the proteins and lipids to their required destination
  • Proteins that go through the Golgi apparatus are usually exported (e.g. hormones such as insulin), put into lysosomes (such as hydrolytic enzymes) or delivered to membrane-bound organelles
• Ribosomes

  -small spherical organelle around 20nm in diameter

  -made of ribosomal RNA

  -made in the nucleolus, as two separate sub units which pass through the nuclear envelope and into the cell cytoplasm then combine

  -80S ribosomes (composed of 60S and 40S subunits) are found in eukaryotic cells

  -70S ribosomes (composed of 50S and 30S subunits) in prokaryotes, mitochondria and chloroplasts

  • no membrane
  • those bound to the RER synthesise proteins
  • those free in the cytoplasm are the site of assembly of proteins used inside the cell
• Mitochondria

  -may be spherical, rod-shaped or branched (2-5 micrometres long)

  -surrounded by a double membrane with inner one folded into cristae

  • The site of aerobic respiration within all eukaryotic cells, mitochondria are just visible with a light microscope
  • The matrix formed by the cristae 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)
  • self replicating
  • abundant in cells where much metabolic activity takes place e.g. liver cells
• Lysosomes

  -round organelle formed from the Golgi apparatus surrounded by a single membrane

  -abundant in phagocytic cells such as neutrophils and macrophages

  • Specialist forms of vesicles which contain hydrolytic enzymes (enzymes that break biological molecules down)
  • Break down waste materials such as worn-out organelles
  • Used extensively by cells of the immune system and in apoptosis (programmed cell death)
  • keep hydrolytic enzymes separate from the rest of the cell
  • can engulf old cell organelles and foreign matter, digest them and return the digested components to the cell for reuse
• Chloroplast part 1

  -Found in plant cells and some protoctists

  -> mitochondria, also surrounded by a double-membrane- inner membrane is continuous with stacks of flattened membrane sacs called thylakoids

  -thylakoids containing chlorophyll stack to form structures called grana

  -Grana are joined together by lamellae (thin and flat thylakoid membranes)

  -contain loops of DNA and ribosomes used to synthesise proteins needed in chloroplast replication and photosynthesis

  -contains starch grains
• Chloroplast part 2

  • site of photosynthesis:
  • The light-dependent stage takes place in the thylakoids
  • The light-independent stage (Calvin Cycle) takes place in the stroma
• Plasma membrane


  -cells are surrounded by a cell surface 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 cell membrane is formed from a phospholipid bilayer of phospholipids spanning a diameter of around 10 nm

  • regulates the movement of substances into and out of the cell
  • receptor molecules so which allows it to respond to chemicals like hormones
• Centrioles

  -Hollow cylinders containing a ring of microtubules at right angles to each other

  -microtubules are made of tubulin protein subunits

  • Two centrioles at right angles to each other form a centrosome, which organises the spindle fibres during cell division
  • Not found in flowering plants and fungi
  • multiply and line up beneath the cell surface membrane to form cilia
• Cell wall

  -Found in plant cells

  -Cell walls are formed outside of the cell membrane and offer structural support to cell

  • 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
  • maintains cell shape
  • permeable and allows solutions to pass through
• Cilia

  -small, hair like structures found on the surface membrane of some animal cells

  -in cross section, they have an outer membrane and a ring of nine pairs of protein microtubules inside, with two microtubules in the middle

  -formed from centriole

  • microtubules allow the cilia to move ->allows the cell to move substances along the cell surface
• Vacuole


  -A sac in plant cells surrounded by the tonoplast, selectively permeable membrane

  -Vacuoles in animal cells are not permanent and small

  • filled with water and solutes and maintains cell stability because when full it pushes against the cell wall, making the cells turgid
• Vesicles

  -Found in plant and animal cells

  -a small fluid-filled sac in the cytoplasm, surrounded by a membrane

  -some formed by the Golgi apparatus or ER whereas some at the cell surface

  • transports substances in and out of the cell and between organelles
• Microtubules

  -found in all eukaryotic cells

  -makes up the cytoskeleton of the cell about 25 nm in diameter

  -made of α and β tubulin combined to form dimers, the dimers are then joined into protofilaments

  -thirteen protofilaments in a cylinder make a microtubule

  • The cytoskeleton is used to provide support and movement of the cell
• Microvilli


  -found in specialised animal cells

  -Cell membrane projections

  • Used to increase the surface area of the cell surface membrane in order to increase the rate of exchange of substances
• Cytoskeleton part 1

  -a network of protein filaments within the cytoplasm that move organelles from place to place within the cell has:

  -rod-like microfilaments (around 7nm in diameter) made of subunits of the protein actin

  -intermediate filaments about 10nm in diameter

  -straight, cylindrical microtubules, made of protein subunits called tubulin about 18-30nm in diameter

  -cytoskeletal motor proteins, myosins, kinesins and dyneins are molecular motors. they are also enzymes and have a site that binds to and allows hydrolysis of ATP as their energy source
• Cytoskeleton part 2

  • microfilaments give support and mechanical strength, cell movement and stable shape
  • Microtubules form a track along which motor proteins transport organelles in the cell
  • form the spindle before a cell divides. These spindle threads enable chromosomes to be moved within a cell.
  • make up the cilia, undulipodia and centrioles
  • Intermediate filaments anchor the nucleus within the cytoplasm
  • extend between cells in some tissues, between special junctions enabling cell signalling and allowing cells to adhere to a basement membrane, therefore stabilising tissues
• Ribosomal RNA



  -a single-stranded polynucleotide that is synthesised at high speed in the nucleolus (the largest structure within the nucleus). After transcription, the rRNAs associate with the ribosomal proteins to form ribosomal subunits that will later assemble to form a functioning ribosome.
• What is a membrane bound organelle?

  A membrane-bound organelle means that the sub-cellular structure is covered by a membrane that will be similar in structure to the cell surface membrane. The membrane keeps each structure within a discrete compartment and therefore separates it from the other structures. These are only found in eukaryotic cells.
• DNA

  -linear and tightly wound around histone proteins to form the thread-like structures called chromosomes