Cell Structure

• All life on earth exists as cells. These have basic features in common (cytoplasm and cell membrane). Differences between cells are due to the addition of other sub-cellular features. This provides indirect evidence for evolution.
• Prokaryote cells are smaller and without a nucleus compared to eukaryote cells that have a nucleus and are bigger. Eukaryotic cells include animal, plant, fungi and protists and are living.
• what sub-cellular structures in eukaryotes are not organelles?

  cell surface membrane, cell wall, cytoplasm
• what sub-cellular structures in eukaryotes are not membrane bound?

  ribosomes
• what sub-cellular structures in eukaryotes are single membrane bound?

  endoplasmic reticulum, Golgi body, permanent vacuole, lysosomes
• what sub-cellular structures in eukaryotes are double membrane bound organelles?

  nucleus, mitochondria, chloroplasts
• cell surface membrane


  This is a thin, flexible layer round the outside

  of all cells made of phospholipids and

  proteins. It separates the contents of the cell from the

  outside environment, and controls the entry

  and exit of materials and is partially permeable.
• nucleus


  This is the largest organelle. It is surrounded by a nuclear envelope, which is a double membrane with nuclear pores – large holes containing proteins that control the exit of substances from the nucleus.

  The interior is called the nucleoplasm, which is full of chromatin – the DNA/protein complex. During cell division the chromatin becomes

  condensed into discrete observable chromosomes. The nucleolus is a dark region of chromatin (made of DNA and histones), involved in making ribosomes.
• mitochondrion


  This is where aerobic respiration takes place in

  all eukaryotic cells (anaerobic respiration

  takes place in the cytoplasm). Mitochondria release energy (in the form of ATP). Mitochondria are surrounded by a double

  membrane: the outer membrane is simple and quite permeable, while the inner membrane is highly folded into cristae, which give it a large surface area. The space enclosed by the inner membrane is called the mitochondrial matrix, and contains small circular strands of DNA. The inner membrane is studded with stalked particles, which are the enzymes that make ATP.
• lysosomes


  These are small membrane-bound vesicles (and can fuse with other membranes) formed from the RER containing digestive (hydrolytic) enzymes (lysozymes).They are used to break down unwanted chemicals, toxins, organelles or even whole cells, so that the materials may be recycled by the Golgi vesicles.
• chloroplasts (in photosynthetic cells)


  They are flattened discs, bounded by two

  membranes, which are highly selective. Inside is a membrane of many flattened sacs called thylakoids, which in places form stacks called grana. Grana contain chlorophyll molecules which absorb light for photosynthesis. The membrane system is surrounded by the stroma- the site of the enzymes used to make sugars and starch using photosynthesis.
• ribosomes


  These are the smallest and most numerous of

  the cell organelles, and are the sites of

  protein synthesis. Ribosomes are either found free in the cytoplasm, where they make proteins for the cell's own use, or they are found attached to the rough endoplasmic reticulum, where they make proteins for export from the cell. All eukaryotic ribosomes are of the larger, "80S", type. Prokaryotic ribosomes are smaller, “70S” type.
• endoplasmic reticulum (ER)


  This is a series of membrane channels involved

  in synthesising and transporting materials.

  Rough Endoplasmic Reticulum (RER) is

  studded with numerous ribosomes, which give

  it its rough appearance. The ribosomes

  synthesise proteins, which are processed in

  the RER (e.g. by modifying the polypeptide

  chain, or adding carbohydrates), before

  being exported from the cell via the Golgi

  body.

  Smooth Endoplasmic Reticulum (SER) does

  not have ribosomes and is used to process

  materials, mainly lipids as well as carbohydrates, needed by the cell.
• Golgi body


  A series of flattened membrane bound vesicles, formed from the endoplasmic reticulum. Its job is to package and transport proteins from the RER to the cell membrane for export. Parts of the RER containing proteins fuse with one side of the Golgi body membranes, while at the other side small vesicles bud off and move towards the cell membrane, where they fuse, releasing their contents by exocytosis (cell release contents through cell membrane, but is not diffusion). Makes lysosomes.
• Cell wall (in plants, algae and

  fungi)

  The cell wall is quite thin. It is not rigid like a brick wall, but it does give the cell strength and support. Water enters a leaf cell by osmosis and fills the vacuole. The water pushes against the wall and makes the cell firm. The cell wall is strong enough to prevent the cell from bursting. It is made from cellulose microfibrils running through a matrix of other complex polysaccharides such as pectin.
• permanent cell vacuole (in plants)


  A sac bounded by a single membrane called the tonoplast. It contains cell sap, a concentrated solution of various molecules such as mineral salts, sugars, pigments, organic acids and enzymes. Acts as storage for substances including waste products.
• prokaryotic cells


  Prokaryotes are bacteria. They are very small, single-celled organisms with no nucleus and no membrane bound organelles. A typical bacterial cell has a cell wall, containing the glycoprotein murein, a cell membrane, DNA as a single circular molecule which is free in the cytoplasm and is not associated with proteins and small 70S ribosomes.

  Some also have one or more plasmids (tiny loops of DNA), a capsule surrounding the cell and one or more flagella.
• Functions of organelles found in all prokaryotic cells

  Cell surface membrane - controls entry and exit of substance into and out of cell.

  Circular DNA - this carries genes for the proteins the cell needs. The DNA is not complexed with protein.

  Food reserve granule.

  Ribosomes (70S) - for protein synthesis.

  Cytoplasm.

  Cell wall made of glycoprotein murein.
• Functions of organelles found in some prokaryotic cells

  Plasmids - a small circular piece of DNA which carries genes additional to those in the main genetic material (e.g. antibiotic resistance).

  Slimy capsule - stores waste, protects against drying out.

  Flagella - for locomotion (movement).
• Viruses are acellular and non living. Sometimes they are referred to as particles. A virus does not have a cell surface membrane or cytoplasm.


  A typical virus does have a simple structure containing DNA or RNA (but never both), a protein coat and attachment proteins.
• Cell Fractionation means separating different parts and organelles of a cell, so that they can be studied in detail. All the processes of cell metabolism (such as respiration or photosynthesis) have been studied in this way. The most common method of fractionating cells is to use differential centrifugation.


  1 - cut tissue in ice-cold isotonic buffer

  Ice-cold to slow enzyme reactions, to prevent digestion of organelles.

  Isotonic, to stop osmosis, so organelles don't burst or shrink.

  Buffer to stop pH changes, so that proteins are not denatured.

  2 - grind tissue in a blender to open cells
• Step 3 of differential centrifugation

  Filter using filter paper or a sieve. This removes debris (e.g. give example using the context of the question) so the filtrate is now a cell-free extract and is capable of carrying out most of the normal cell reactions.
• Steps 4 and 5 of differential centrifugation

  4 - centrifuge filtrate at low speed to pellet (the bit at the bottom that's been seperated out) nuclei which can be resuspended (can be removed from the supernatant using a buffer or centrifuge solution) - the densest organelle gets pushed to the bottom first

  5 - centrifuge supernatant (liquid still in the centrifuge tube) at medium speed to pellet mitochondria and chloroplasts which can be resuspended.
• Steps 6, 7 and 8 of differential centrifugation

  6 - Centrifuge supernatant at high speed to pellet ER, Golgi and other membrane fragments, which can be resuspended

  7 - Centrifuge supernatant at very high speed to pellet ribosomes which can be resuspended

  8 - supernatant is now an organelle free cytoplasm
• Microscopes, define magnification and resolution

  Magnification simply indicates how much bigger the image is of that the original object.

  Magnification = increases the apparent size of an object

  Resolution = the ability to distinguish between 2 adjacent points/ resolve objects that are close together

  There was a considerable period of time during which the

  scientific community distinguished between artefacts (something that isn't part of the sample but has been added as part of the stain) and cell organelles.
• Resolution in magnification

  Resolution is the smallest separation at which two separate objects can be distinguished and is therefore a distance. The resolution of a microscope can be improved by using a shorter wavelength of light, which is why some have blue filters. A longer wavelength of light = lower resolution
• List the standard SI units of measurements used in microscopy


  metre : m : 1m

  millimetre : mm : 10^-3m

  micrometre : μm : 10^-6m

  nanometre : nm : 10^-9m

  picometre : pm : 10^-12m

  multiply by 1000 each time
• Electron microscopes

  Electrons microscopes use a beam of electrons rather than light to 'illuminate' the specimen. Electrons behave like waves and can easily be produced (using a hot wire), focused (using electromagnets) and detected (using a phosphor screen or photographic film). A beam of electrons has an effective wavelength of less than 1nm, so can be used to resolve small sub-cellular ultrastructure. The development of the electron microscope in the 1930s revolutionised biology, allowing organelles such as mitochondria, ER and membranes to be seen in detail for the first time.
• The problems with electron microscopy

  There must be a vacuum inside an electron microscope (so the electron beam isn’t scattered by air molecules), so it can't be used for living organisms. Specimens must be very thin, so are embedded in plastic for support, so can't be manipulated under the microscope. Specimens can be damaged by the electron beam, so delicate structures and molecules can be destroyed. Specimens are usually transparent to electrons, so must be stained with an electron-dense chemical (usually lead or gold).
• The two types of electron microscopes

  Transmission electron microscopes (TEM) work much like a light microscope, transmitting a beam of electrons through a thin specimen and then focusing the electrons to form an image on a screen or on film. This is the most common form of electron microscope and has the best resolution (<1nm).

  Scanning electron microscopes (SEM) scan a fine beam of electron

  onto a specimen and collect the electrons scattered by the surface. This has poorer resolution, but gives excellent 3-dimentional images of surfaces.
• Calculating magnification

  magnification = image length

  ------------

  actual length

  image and actual length must be in the same units
• Calculating magnification with bar length

  actual size = image length

  ------------

  bar length (measured with your own ruler)

  multiply this by bar scale (the number on the bar given)

  
  

  image and bar length must be in the same units, the actual size will come out as the same units as the bar scale