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Cells

You need to be able to use your knowledge of the structure of cells and the function of the organelles to interpret electronmicrographs of cells. In particular the epithelial cells lining the small intestine. These are cells which are specialised for absorbing the products of digestion. They will therefore have a large surface area provided by the microvilli and due to the need for active transport across their cell membranes they will contain a large number of mitochondria providing them with ATP. Epithelial cells also secrete enzymes and other proteins. This means that they will have a large and visible endoplasmic reticulum, Golgi apparatus to allow protein production and secretion.

Below is a list of the most important organelles that you are required to know about. It is worth you using textbooks and perhaps even other websites such as www.cellsalive.com to look at actual images of different types of cells and their organelles.

Nucleus

  • Contains DNA
  • DNA arranged into long thin threads known as chromosomes
  • In most cells the chromosomes are arranged in homologous pairs
  • Surrounded by nuclear envelope
  • This has pores to allow communication between the nucleus and cytoplasm

Plasma Membrane

  • Sea of phospholipids - arranged as a bilayer
  • Intrinsic and extrinsic proteins float within the phospholipids
  • Selectively permeable barrier - controls movement of substances between the internal and external environments

Microvilli

  • Adaptation of cells to increase surface area for absorption or secretion
  • Found on epithelium of the small intestine

Lysosomes

  • Formed by the golgi apparatus
  • Contain digestive enzymes - proteases and lipases
  • Important to protect the cell from the effect of these enzymes before they are released at the cell surface membrane or into a phagocytic vesicle

Mitochondria

  • 1µm in diameter and 7µm in length
  • Mostly protein, but also contains some lipid, DNA and RNA
  • Power house of the cell
  • Energy is stored in high energy phosphate bonds of ATP
  • Mitochondria convert energy from the breakdown of glucose into adenosine triphosphate (ATP)
  • Responsible for aerobic respiration
  • Metabolic activity of a cell is related to the number of cristae (larger surface area) and mitochondria
  • Cells with a high metabolic activity (e.g. heart muscle) have many well developed mitochondria

Ribosomes

  • 20-30nm in size
  • Small organelles often attached to the ER but also found in the cytoplasm
  • Large (protein) and small (rRNA) subunits form the functional ribosome
    • Subunits bind with mRNA in the cytoplasm
    • This starts translation of mRNA for protein synthesis (assembly of amino acids into proteins)
  • Free ribosomes make proteins used in the cytoplasm. Responsible for proteins that
    • go into solution in cytoplasm or
    • form important cytoplasmic, structural elements
  • Ribosomal ribonucleic acid (rRNA) are made in nucleus of cell

Endoplasmic Reticulum (ER)

  • Rough ER
    • Have ribosomes attached to the cytosolic side of their membrane
    • Found in cells that are making proteins for export (enzymes, hormones, structural proteins, antibodies)
    • Thus, involved in protein synthesis
    • Modifies proteins by the addition of carbohydrates, removal of signal sequences
    • Phospholipid synthesis and assembly of polypeptides
  • Smooth ER
    • Have no ribosomes attached and often appear more tubular than the rough ER
    • Necessary for steroid synthesis, metabolism and detoxification, lipid synthesis
    • Numerous in the liver

Golgi Apparatus

  • Stack of flattened sacs surrounded by membrane
  • Receives protein-filled vesicles from the rough ER (fuse with Golgi membrane)
  • Uses enzymes to modify these proteins (e.g. add a sugar chain, making glycoprotein)
  • Adds directions for destination of protein package - vesicles that leave Golgi apparatus move to different locations in cell or proceed to plasma membrane for secretion
  • Involved in processing, packaging, and secretion
  • Other vesicles that leave Golgi apparatus are lysosomes

Techniques used in Cell Biology

  • Microscopy
    • Magnification → increases the size of an object
    • Resolution/resolving power → ability to distinguish between adjacent points
  • Calculating magnification
    • X = size of picture (measure the size of the diagram in the question)
    • Y = size of object in real life (often given in exam question)
    • Make sure Y has the same unit as X!
      • If X = mm and Y = μm
      • Convert mm to μm = X * 1000
    • Magnification = Xμm / Yμm

Feature

Optical microscope

Electron microscope

Radiation

Light

Electrons

Magnification

400x (max1500)

≈500 000x

Resolution

2µm

1nm / 0,001µm
Electrons have a small wavelength
Thus, higher resolution

Vacuum in microscope

Absent

Present

Specimen is

- Alive or dead
- Stained

- Dead (vacuum!)

Transmission microscope:
Electrons pass through internal
structure of specimen

Scanning microscope:
Beams of electrons are reflected
off specimens surface. Allows a
three dimensional view

Cell Fractionation

To study the function of individual organelles large numbers of isolated organelles need to be obtained. Cell fractionation is used to gather these organelles.

It is worth looking on the internet or in your text books for a step by step diagram of the process to use alongside this explanation.

  • The tissue from which the organelles are to be harvested from is firstly placed in a COLD, ISOTONIC, BUFFERED solution
    • The solution is cold to minimise enzyme activity
    • The solution is isotonic to prevent organelle damage due to osmotic water gain or loss
    • The solution is buffered to maintain a constant pH
  • The solution containing the cells is then HOMOGENISED in a blender to release organelles from the cells. After homogenising, the fluid is known as the HOMOGENATE, it is now FILTERED to remove any large pieces of cell debris
  • The filtered homogenate is then centrifuged in an ultracentrifuge at progressively greater speeds in order to separate the different components. When spun in the centrifuge at a low speed, the largest organelle - the nucleus will be forced to the bottom of the tube and form a pellet.
  • The SUPERNATANT (fluid above the pellet) now contains cell components too small to sediment at this speed. This fluid is centrifuged at a higher speed to form another pellet which will contain organelles such as mitochondria
  • The supernatant can then again be centrifuged at an even higher speed to separate out even smaller organelles
  • Once the organelle that is to be studied has been extracted from the homogenate it can be resuspended in distilled water to make it easier to use in experiments