Eukaryotic cells

• Cell surface membrane

  The structure of the cell surface membrane - although the structure looks static, the phospholipids and proteins forming the bilayer are constantly in motion
• Cell surface membrane

  • All 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
• Cell wall


  • Found in plant cells but not in animal 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
• Plasmodesmata


  Narrow threads of cytoplasm (surrounded by a cell membrane) that connect the cytoplasm of neighbouring plant cells
• Nucleus

  • Present in all eukaryotic cells (except red blood cells)
  • 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 (eg. 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 slightly wound around proteins called histones
  • 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
• Mitochondria


  • The site of aerobic respiration within all eukaryotic cells
  • Mitochondria are just visible with a light microscope
  • Surrounded by double-membrane with the inner membrane folded to form cristae
  • 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)
• Chloroplasts


  • Found in plant cells
  • Larger than mitochondria, also 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)
  • The light-dependent stage of photosynthesis 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
• Ribosomes


  • Found freely in the cytoplasm of all cells or as part of the rough endoplasmic reticulum in eukaryotic cells
  • Each ribosome is a complex of ribosomal RNA (rRNA) and proteins
  • 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
  • Site of translation (protein synthesis)
• Rough Endoplasmic Reticulum (RER)

  • Found in plant and animal cells
  • Surface covered in ribosomes
  • Formed from continuous folds of membrane continuous with the nuclear envelope
  • Processes proteins made by the ribosomes
• Smooth Endoplasmic Reticulum (ER)


  • Found in plant and animal cells
  • Does not have ribosomes on the surface, its function is distinct to the RER
  • Involved in the production, processing and storage of lipids, carbohydrates and steroids
• Golgi apparatus (golgi complex)

  • Found in plant and animal cells
  • Flattened sacs of membrane similar to the smooth endoplasmic reticulum
  • 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
• Large permanent vacuoles


  • A sac in plant cells surrounded by the tonoplast, selectively permeable membrane
  • Vacuoles in animal cells are not permanent and small
• Vesicles


  • Found in plant and animal cells
  • A membrane-bound sac for transport and storage
• Lysosomes


  • 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)
• Centrioles


  • Hollow fibres made of microtubules
  • 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
• 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
• Cilia


  • Hair-like projections made from microtubules
  • Allows the movement of substances over the cell surface
• Flagella


  • Found in specialised cells
  • Similar in structure to cilia, made of longer microtubules
  • Contract to provide cell movement for example in sperm cells