Topic 2: Cells

• Function of the nucleus

  1. Controls cell's activities - Controls entry and exit of materials, and contains nuclear reactions.

  
  

  2. Retains genetic material in the form of DNA and chromosomes.
• Structure of Mitochondria

  1. Double membrane surrounding organelle - controls entry and exit of material.

  
  

  2. Cristae - extensions of the inner membrane, providing a large surface area for the attachment of enzymes and other proteins during respiration.

  
  

  3. Matrix - contains proteins, lipids, ribosomes and DNA matrix contains enzymes for cell respiration.
• Function of the Mitochondria

  site of aerobic respiration
• Structure of Chloroplasts

  Chloroplast envelope - double plasma membrane, highly selective

  
  

  2. Grana - stacks of disc-shaped thylakoid membrane.

  
  

  3. Thylakoids - contain chlorophyll used in photosynthesis, can be linked by lamellae to other grana.

  
  

  4. Stroma - fluid-filled matrix that contains starch grains.
• Functions of Chloroplasts

  site of photosynthesis
• Structure of Endoplasmic Reticulum

  Rough and Smooth ER have folded membranes called cisternae

  
  

  
  

  RER have ribosomes on the cisternae
• Function of endoplasmic reticulum

  RER – protein synthesis

  
  

  SER- synthesises and stores lipids and carbohydrates
• Structure of Golgi Apparatus

  Flattened sacs and folded membranes making cisternae
• Function of Golgi Apparatus

  Processes, packages and modifies proteins

  
  

  Stores lipids and proteins and transports them out of the cell

  
  

  Vesicle- pinches off cisternae and the modified molecule can be transported.
• Structure of Lysosomes

  Vesicles/Bags of digestive enzymes
• Functions of Lysosomes


  Hydrolyse phagocytic cells

  
  

  Completely break down dead cells (autolysis)

  
  

  Exocytosis- release enzymes outside of cell to destroy material

  
  

  Digest worn out organelles for the reuse of materials
• Structure of Ribosomes

  Small organelle made up of proteins and rRNA (ribosomal RNA)

  
  

  Eukaryotic cells have large ribosomes

  
  

  Prokaryotic cells, mitochondria and chloroplasts have smaller ribosomes
• Functions of Ribosomes

  Carry out protein synthesis
• Structure of Cell Wall

  Plants- made of microfibrils of cellulose

  
  

  Fungi- made of chitin, a nitrogenous polysaccharide
• Function of Cell Wall

  Provides structural strength to the cell
• Structure of Vacuoles

  1. Fluid-filled sac bounded by a single membrane.

  
  

  2. Single membrane around it called tonoplast.
• Function of vacuole


  Makes cells turgid and therefore provides support

  
  

  Temporary store of sugars and amino acids

  
  

  The pigment may colour petals to attract pollinators
• Define Tissue

  
  

  Give an example

  A collection of similar cells that work together to perform a specific function.

  
  

  Example = epithelial tissue - consists of sheets of cells, lining the surfaces of organs, often having a protective or secretory function.
• Define eukaryotic cell

  A larger cell with a true nucleus that is bounded by a nuclear membrane/nuclear envelope.
• Define prokaryotic cell

  A smaller cell which has no true nucleus or nuclear envelope. Organelles are not membrane bound
• Can you describe how prokaryotic cells differ from eukaryotic cells?

  Prokaryotic cells:

  - have no nucleus

  - smaller 70S ribosomes

  - cytoplasm lacks membrane-bound organelles

  - much smaller cell

  - cell wall contains murein (a glycoprotein)

  - plasmids may be found in prokaryotic cells

  
  

  Eukaryotic Cells:

  - have nucleus

  - 80S ribosomes

  - membrane-bound organelles

  - larger cell

  - cell wall made of cellulose or chitin

  - no plasmids
• Can you list other features of prokaryotic cells?

  Prokaryotic cells can have:

  - one or more plasmids

  - a slime capsule surrounding the cell

  - one or more flagella
• What is cell fractionation?

  A method used to isolate different organelles so they can be studied. This enables the individual organelle structures to be studied in an electron microscope
• What are the steps of cell fractionation

  - Homogenisation

  - Filtration

  -Ultracentrifugation
• Why should the solution be isotonic?

  water potential is the same as the solution which prevents osmosis that could shrink or burst organelles
• Why should the solution be buffered?

  keeps pH constant and avoids damaging the protein structures of the organelle
• Why should the solution be ice cold?

  reduces enzyme activity that might damage organelles
• How does an optical microscope work?

  A beam of light is condensed to create the image as visible light passes and is bent through the lens system to enable the user to see the specimen
• What are the properties of an optical microscope?

  - the specimen can be alive

  
  

  - Individual cells are transparent, and their components are not distinguishable unless they are coloured with special stains.

  
  

  Staining usually kills the cells.

  
  

  Small organelles are not visible, but living samples can be examined and a colour image is obtained
• What are the uses of an optical microscope?

  Most student microscopes are classified as light microscopes.

  
  

  Max resolution is 0.2 micrometres.

  
  

  The nucleus and mitochondria can be seen with a light microscope.

  
  

  The max magnification is around x1,500.
• How is an electron microscope used?

  A beam of electrons that are condensed using electromagnets creating the image. This allows higher magnification and higher resolving power, allowing more detail to be seen with black and white images

  
  

  Samples must be in a vacuum and non-living
• What are the uses of an electron microscope?


  Max resolution of 0.0002 micrometres. Around 1000 times more than light microscopes.

  
  

  Max magnification is around x1,500,000.
• What are the two types of electron microscopes?

  transmission (TEM) and scanning (SEM) electron microscopes.
• How does an SEM work?

  A beam of electrons moves back and forth across a cell’s surface, creating details of cell surface characteristics.

  
  

  SEMs knock electrons off the specimen and these electrons come together to form an image.

  
  

  SEM images can be three-dimensional.
• What are the properties of an SEM?

  -The specimens do not need to be thin, as the electrons are not transmitting through. -Can produce a 3D image
• How does a TEM work?

  Extremely thin specimens are stained and placed in a vacuum

  
  

  The electron beam penetrates the cell and provides details of a cell’s internal structures.

  
  

  TEMs use electromagnets to focus the electron beams and are high resolution microscopes.

  
  

  In thin specimens, you can see the internal structures of organelles such as chloroplasts.
• What are the steps in calibrating the eyepiece graticule?

  Line up the stage micrometre and eyepiece graticule whilst looking through the eyepiece

  
  

  Count how many divisions on the eyepiece graticule fit into one division on the micrometre scale

  
  

  For example, each division on the micrometre is 10, so this can be used to calculate one division on the eyepiece graticule at that current magnification. 10/2=5 micrometres
• Can you describe the pros and cons of optical microscopes?

  Pros:

  - cheap

  - images in colour

  - no training required

  - live specimens

  
  

  Cons:

  - low magnification x1500

  - low resolution

  - 2D images
• Can you describe the pros and cons of transmission electron microscopes?

  Pros:

  - high resolution images

  - high magnification

  - visible internal structures

  
  

  Cons:

  - expensive

  - training is required

  - no colour images

  - 2D images

  - only thin specimens
• Can you describe the pros and cons of scanning electron microscopes?

  Pros:

  - 3D images

  - high magnification

  - high resolution

  - thick specimens

  
  

  Cons:

  - expensive

  - training is required

  - no colour images