Cell Structure

• Magnification

  How much bigger the image is than the object/specimen

  
• Resolution

  The minimum distance between 2 objects where they can still be viewed as separate

  
• Light microscopes

  Light microscopes can magnify up to x2000. They can be used on living cells (we can witness processes like cell division). However, sometimes a stain is needed which kills cells.

  - Inexpensive to buy and operate

  - Small and portable

  - Sample preparation doesn't usually lead to distortion

  - Vacuum is not required

  
• Electron microscopes

  A beam of electrons with a very short wavelength is used to illuminate the specimen, this means that more cell ultrastructure can be seen. However, the interior of an electron microscope has a vacuum meaning that living cells cannot be viewed.

  - Sample preparation often distorts material and creates artefacts

  - Expensive to buy and operate

  - Large and needs to be installed

  - Vacuum is required

  - Specimens are dead

  
• What are the 2 types of electron microscopes

  Transmission electron microscope (TEM) and scanning electron microscope (SEM)

  
• Transmission Electron Microscope (TEM)

  A beam of electrons passes through the specimen. Some parts of the specimen absorb the electrons, making them appear darker. They are cut thinly. TEM has a higher resolution and magnification than SEM. Produces a 2D image. They show detailed internal structures of the cell

  
• Scanning Electron Microscope (SEM)

  A beam of electrons is sent across the surface of the specimen and the reflected electrons are collected. They do not have to be cut thinly. SEM has a lower resolution than TEM. Produces a 3D image

  
• What are artefacts?

  Visible structures caused by preparation of a specimen (it is NOT a feature of the specimen). Examples of artefacts include, loss of continuity of the membrane, empty spaces in the cytoplasm and distortion of organelles
• What are the 4 main types of sample preparation?

  - Dry mount

  - Wet mount

  - Squash slide

  - Smear slide
• Dry mount

  Solid specimens can be viewed as whole or sectioned (slicing them into very thin slices using a sharp blade). The sample is then placed in the centre of the slide and a coverslip (protects the sample) is placed over it.

  Hair, dust, pollen can be viewed as whole whereas muscle/plant tissue must be sectioned

  
• Wet mount

  Solid specimens are suspended in a liquid (e.g. water/immersion oil). A coverslip is placed at an angle with a mounting needle (to prevent air bubbles) and viewed under a LIGHT microscope. Aquatic organisms are viewed this way

  
• Squash slide

  Prepare a wet mount and then using lens tissue, gently press down on the coverslip. However, this can damage the coverslip so 2 microscope slides can be used to squash the sample in between. This can be used to observe a root tip squash sample when observing mitosis

  
• Smear slide

  Using the edge of a slide, smear the sample to create a thin, even coating. The coverslip is then placed. Most soft samples/specimens are prepared using smear slides, for example, root tip for cell division or examining blood cells in a blood sample.

  
• Magnification (equation)

  image size/actual size

  
• Why are stains used in microscopy?

  Stains increase contrast as different cell components within a cell take up a stain to different degrees. Stains also make it easier to distinguish between cell components
• Differential staining

  A technique using multiple stains to stain different parts of a cell in different colours.

  
• What are 2 positively charged dyes that are used in differential staining?

  Crystal violet and methylene blue.

  - They attract negatively charged material in the cytoplasm which leads to the staining of cell components
• What are 2 negatively charged dyes that are used in differential staining?

  Nigrosine and Congo red

  - They are repelled by the negatively charged cytosol. They create a stained background, leaving the cells unstained. This is called the negative stain technique
• Gram staining technique

  Separates bacteria into 2 groups: Gram-positive bacteria and gram-negative bacteria.

  1) Crystal violet is applied to the bacterial specimen on the slide, then iodine is applied which fixes the dye. The slide is then washed with alcohol to remove any unbound stain

  2) Gram-positive bacteria retains the stain as it has thicker peptidoglycan cell walls and appears blue/purple under a light microscope. Whereas gram-negative bacteria, loses the stain as it has thinner cell walls. It is then applied with a safranin dye (known as a counterstain) and appears red under a light microscope.
• Why are Gram-negative bacteria not susceptible to the antibiotic penicillin?

  Gram-negative bacteria are not susceptible to penicillin as they have thinner cell walls. Whereas gram-positive bacteria are and this inhibits the formation of the cell wall.

  (Being able to distinguish between 2 types of bacteria helps medics to prescribe the appropriate antibiotic to patients with bacterial infections)
• Scientific Drawings

  - Draw in pencil

  - Title the diagram to indicate what the specimen is

  - State the magnification

  - Annotate cell components, cells and sections of tissue visible

  - Do not sketch. Only use solid lines that do not overlap

  - Do not colour in or shade
• Animal Cell

  
• Nucleus

  - The nucleus is surrounded by the nuclear envelope. The nuclear envelope has a double membrane structure

  - The nuclear envelope has nuclear pores which allows molecules to enter and leave the cell (e.g. mRNA)

  - The nucleus has linear chromosomes which a protein-bound

  - The nucleolus is a small area within the nucleus which is the site of RNA production and makes ribosomes

  FUNCTION:-

  - Site of DNA replication and transcription

  - Site of ribosome synthesis

  
• Flagella

  - Whip-like structure (not present on all eukaryotic cells)

  FUNCTION:-

  - For mobility and sometimes as a sensory organelle for chemical stimuli

  
• Cilia

  - Hair-like projections (not present on all eukaryotic cells)

  FUNCTION:-

  - Cilia can be mobile or stationary

  - Mobile cilia helps move substances in a sweeping motion

  - Stationary cilia are important in sensory organs (e.g. nose)

  
• Centrioles

  - Made of microtubules

  - Occur in pairs to form a centrosome

  FUNCTION:-

  - Involved in the production of spindle fibres and organisation of chromosomes in cell division

  
• Cytoskeleton

  - A network of fibres found in the cytoplasm

  - Consists of microfilaments, microtubules and intermediate fibres

  FUNCTION:-

  - Provides mechanical strength to the cell and helps maintain the shape/stability of the cell

  - Microtubules are responsible for cell movement

  - Microfilaments are responsible for creating a scaffold-like structure

  - Intermediate fibres are responsible for mechanical strength

  
• Endoplasmic Reticulum (RER AND SER)

  - Both RER and SER have folded membranes (cisternae)

  - RER have ribosomes on the cisternae

  FUNCTION:-

  - The RER is the site of protein synthesis and transports proteins

  - The SER is the site of synthesis of lipids and carbohydrates and stores them

  
• Golgi apparatus and vesicles

  - The Golgi apparatus has a similar structure to the SER (they have folded membranes, forming cisternae)

  - Secretory vesicles pinch of the cisternae

  FUNCTIONS:-

  - Transports, modifies and stores lipids

  - Forms lysosomes

  - Adds carbohydrates to proteins to form glycoproteins

  - Finished products are released in secretory vesicles which fuses with the plasma membrane and the contents of the vesicle are released

  
• Lysosomes

  - Bag of digestive enzymes

  FUNCTIONS:-

  - Hydrolyses phagocytic cells

  - Completely breaks down dead material

  - Exocytosis: Releases enzymes outside the cell to destroy material

  
• Mitochondria

  - Double membrane structure

  - Has inner membrane folding (cristae)

  - Has a fluid centre called the mitochondrial matrix

  - Contains a small amount of DNA called mitochondrial DNA

  FUNCTION:-

  - Site of aerobic respiration and ATP production

  
• Ribosomes

  - Small and made up of 2 sub-units of proteins and RNA

  - 80S ribosomes (large ribosomes) are found in eukaryotic cells

  - 70S ribosomes (small ribosomes) are found in prokaryotic cells, mitochondria and chloroplasts

  FUNCTION:-

  - Site of protein synthesis

  
• Chloroplasts

  - Surrounded by a double membrane

  - Contains thylakoids (internal network of membranes that form flattened sacs)

  - Fluid-filled stroma which contains enzymes for photosynthesis

  FUNCTION:-

  - Site of photosynthesis

  
• Cell Wall

  - In plant and fungi cells

  - Plants: Made of microfibrils of the cellulose polymer

  - Fungi: Made of chitin

  FUNCTION:

  - Provides structural strength to the cell

  
• Plasma Membrane

  - Found in all cells

  - Made up of the phospholipid bilayer - molecules embed and attach on the outside (proteins, carbohydrates, cholesterol)

  FUNCTION:-

  - Controls the entrance and exit of molecules

  
• Production and Secretion of Proteins

  1) Polypeptide chain is synthesised on the RER

  2) The polypeptide chain moves to the cisternae in the RER where it is modified and packaged into vesicles to be sent to the Golgi apparatus for further modification

  3) At the Golgi apparatus, the proteins are further modified and packaged into secretory vesicles

  4) The secretory vesicles carrying the proteins move to the cell surface (plasma) membrane where it fuses and releases its protein content by exocytosis.
• Differences between eukaryotic and prokaryotic cells

  Prokaryotic cells are:

  - Much smaller

  - No membrane-bound organelles

  - 70S ribosomes

  - DNA is not contained in the nucleus

  - Has a cell wall made of murein

  - May contain plasmids, a capsule around the cell and flagella

  - No nucleus, no mitochondria, no RER, no Golgi apparatus, no chloroplasts
• Capsule

  Slimy layer of protein. It prevents bacteria from drying out and protects bacteria against the host's immune system
• Flagella

  Rotates to enable the bacteria to move
• Plasmids

  Additional small, circular loops of DNA which carry few genes (e.g. for antibiotic resistance)