M2:S1 Cell structure

• Prokaryotic organisms


  Prokaryotic cells (single-celled organisms)
• Eukaryotic organisms


  Made up of eukaryotic cells
• Both prokaryotic and eukaryotic cells contain organelles
• Organelles


  Parts of cells, each with a specific function
• Eukaryotic cells are complex and include all animal and plant cells
• Prokaryotic cells are smaller and simpler, e.g. bacteria
• Animal cell


  • Plasma (cell surface) membrane
  • Rough endoplasmic reticulum
  • Lysosome
  • Ribosome
  • Nuclear envelope
  • Nucleolus
  • Nucleus
  • Smooth endoplasmic reticulum
  • Golgi apparatus
  • Cytoplasm
  • Mitochondrion
• Plant cell


  • Plasma (cell surface) membrane
  • Smooth endoplasmic reticulum
  • Chloroplast
  • Cytoplasm
  • Rough endoplasmic reticulum
  • Plasmodesma
  • Nuclear envelope
  • Nucleolus
  • Nucleus
  • Mitochondrion
  • Ribosome
  • Golgi apparatus
  • Vacuole
  • Cell wall
• Plasma (cell surface) membrane


  The membrane found on the surface of animal cells and just inside the cell wall of plant cells and prokaryotic cells. It's made mainly of lipids and protein.
• Cell wall


  A rigid structure that surrounds plant cells. It's made mainly of the carbohydrate cellulose.
• Nucleus


  A large organelle surrounded by a nuclear envelope (double membrane), which contains many pores. The nucleus contains chromatin (which is made from DNA and proteins) and a structure called the nucleolus.
• Lysosome


  A round organelle surrounded by a membrane, with no clear internal structure. Contains digestive enzymes.
• Ribosome


  A very small organelle that either floats free in the cytoplasm or is attached to the rough endoplasmic reticulum. It's made up of proteins and RNA. It's not surrounded by a membrane.
• Rough Endoplasmic Reticulum (RER)


  A system of membranes enclosing a fluid-filled space. The surface is covered with ribosomes.
• Smooth Endoplasmic Reticulum (SER)


  Similar to rough endoplasmic reticulum, but with no ribosomes. Synthesises and processes lipids.
• Vesicle


  A small fluid-filled sac in the cytoplasm, surrounded by a membrane. Transports substances in and out of the cell (via the plasma membrane) and between organelles.
• Golgi Apparatus


  A group of fluid-filled, membrane-bound, flattened sacs. Vesicles are often seen at the edges of the sacs. It processes and packages new lipids and proteins. It also makes lysosomes.
• Mitochondrion


  They're usually oval-shaped. They have a double membrane - the inner one is folded to form structures called cristae. Inside is the matrix, which contains enzymes involved in respiration.
• Chloroplast


  A small, flattened structure found in plant cells. It's surrounded by a double membrane, and also has membranes inside called thylakoid membranes. These membranes are stacked up in some parts of the chloroplast to form grana. Grana are linked together by lamellae-thin, flat pieces of thylakoid membrane.
• Centriole


  Small, hollow cylinders, made of microtubules (tiny protein cylinders). Found in animal cells, but only some plant cells.
• 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.
• Flagellum


  Flagella on eukaryotic cells are like cilia but longer. They stick out from the cell surface and are surrounded by the plasma membrane. Inside they're like cilia too-two microtubules in the centre and nine pairs around the edge.
• Protein Production in a Cell

  1. Proteins are made at the ribosomes

  2. Ribosomes on the rough ER make proteins that are excreted or attached to the cell membrane

  3. Free ribosomes in the cytoplasm make proteins that stay in the cytoplasm

  4. New proteins produced at the rough ER are folded and processed in the rough ER

  5. Proteins are transported from the ER to the Golgi apparatus in vesicles

  6. At the Golgi apparatus, the proteins may undergo further processing

  7. The proteins enter more vesicles to be transported around the cell
• Cytoskeleton


  • The organelles in cells are surrounded by the cytoplasm
• Protein Production in a Cell

  1. Proteins are made at the ribosomes

  2. Ribosomes on rough ER make proteins that are excreted or attached to cell membrane

  3. Free ribosomes in cytoplasm make proteins that stay in cytoplasm

  4. New proteins produced at rough ER are folded and processed (e.g. sugar chains added)

  5. Proteins transported from ER to Golgi apparatus in vesicles

  6. At Golgi apparatus, proteins may undergo further processing (e.g. sugar chains trimmed or more added)

  7. Proteins enter more vesicles to be transported around the cell
• Glycoproteins (found in mucus)


  • Move to cell surface and are secreted
• The Cytoskeleton


  • Organelles in cells are surrounded by cytoplasm, which has a network of protein threads called the cytoskeleton
  • In eukaryotic cells the protein threads are arranged as microfilaments (small solid strands) and microtubules (tiny protein cylinders)
• Functions of the cytoskeleton

  • Support the cell's organelles, keeping them in position
  • Help to strengthen the cell and maintain its shape
  • Responsible for the movement of materials within the cell
  • Cause the cell to move (e.g. movement of cilia and flagella)
• The cytoskeleton is dynamic (constantly changing), which allows it to respond to changes in the cell and carry out its functions
• The assembly of microtubules and microfilaments, and the movement of materials along them, requires energy from respiration
• Microtubules and microfilaments can be prevented from functioning using respiratory inhibitors
• Prokaryotes


  • Extremely small cells (less than 2 um diameter)
  • DNA is circular
  • No nucleus-DNA free in cytoplasm
  • Cell wall made of a polysaccharide, but not cellulose or chitin
  • Few organelles and no membrane-bound organelles, e.g. no mitochondria
  • Flagella (when present) made of the protein flagellin, arranged in a helix
  • Small ribosomes
• Eukaryotes


  • Larger cells (about 10-100 μm diameter)
  • DNA is linear
  • Nucleus present-DNA is inside nucleus
  • No cell wall (in animals), cellulose cell wall (in plants) or chitin cell wall (in fungi)
  • Many organelles-mitochondria and other membrane-bound organelles present
  • Flagella (when present) made of microtubule proteins arranged in a '9+2' formation
  • Larger ribosomes
• Prokaryotes like bacteria are roughly a tenth the size of eukaryotic cells
• Normal microscopes aren't really powerful enough to look at the internal structure of prokaryotic cells
• Magnification


  How much bigger the image is than the specimen (the sample you're looking at)
• Resolution


  How well a microscope distinguishes between two points that are close together
• Light microscopes have a lower resolution than electron microscopes, with a maximum resolution of about 0.2 micrometres (um)
• The maximum useful magnification of a light microscope is about x 1500
• Laser Scanning Confocal Microscopes

  • Use laser beams to scan a specimen, usually tagged with a fluorescent dye
  • The laser causes the dye to fluoresce, and the light is focused through a pinhole onto a detector
  • The pinhole blocks out-of-focus light, producing a much clearer image than a normal light microscope
  • Can be used to look at objects at different depths in thick specimens