2.1.1 Cell Structure

2.1.1 a) Microscopes!

• Magnification - number of times larger an image appears, compared to the size of the object
• Resolution - the ability to distinguish two objects from each other
• There are 4 types of microscope: Optical, Laser-scanning, Tranmission Election and Scanning electron microscopes.
• Optical microscpe; Magnification: x1500, Resolution 200nm
• Pros: relatively cheap, easy to use, portable, can use whole living specimens
• Cons: Limited magnification and resolution
• Laser Scanning Microscope: use laser light to scan an object and build up a computer image point by point. Used in medicine and biological research
• Magnification: x2000
• Pros: High resolution & contrast, Depth selectivity, whole live specimens,
• Cons: Low resolution and low magnification compared to electron microscopes

Diapositiva 2

• Electron microscopes:
• Transmission electron microscope: Specimen is dehydrated and stained. Beams of electrons passed through the specimen. Some electrons pass through and hit a screen. Creates a 2D grey-scale image
• Magnification: x2 000 000 Resolution: 0.2nm
• Pros: Very high magnification and resolution - good for looking at internal structures of cells
• Cons: expensive, large, requires specialist training to use, can't look at live specimens
• Scanning electron microscope: Specimen is put into a vacuum and coated with a fine film of metal. Electrons bounce off a specimens surface onto a screen. Produces a 3D image and computer can add (false) colours.
• Magnification: x200 000 Resolution: 10nm
• Pros: 3D image, High resolution
• Cons: Costly, specimen needs to be in a vacuum, requires specialist training, large

Preparing slides...

• Stains are coloured chemicals that bind to molecules in or on the specimen making them easier to see.
• Differential staining: stains which bind to specific cell structures so that structures can be easily identified
• Examples of differential staining: acetic orcein binds to DNA and stains chromosomes dark red; eosin stains cytoplasm
• Magnification: Image size / object size
• Eyepiece graticule: a measuring device which is placed in the eyepiece of a microscope and acts as a ruler when viewing an object under the microscope
• The scale is arbitrary - it represents different lengths depending on the magnification. It has to be calibrated using a stage graticule.
• Stage graticule: precise measuring device which is put on the microscope stage and used to calibrate an eyepiece graticule. The ruler is 1mm long and split into 100 divisions, making each division 10um (or 0.01mm)

Cell structure

Chloroplasts

• Found only in plants and some protoctists. Abundant in leaf cells,.
• 4-10um long, surrounded by a double membrane. Inner membrane is continuous with stacks of flattened membrane sacs called thylakoids which contain chlorophyll. A stack of thylakoids is called a granum. Fluid-filled matrix is called the stroma. Chloroplasts contain loops of DNA and starch grains.
• Site of photosynthesis. Light-dependent stage takes place in the chlorophyll. in the thylakoid membranes. Light-independent stage takes place in the stroma.
• 

Nucleus, Nuclear Envelope & Nucleolus

• 
• Nucleus - stores genome, transmits genetic information, provides instructions for protein synthesis, makes ribosomes
• Nuclear envelope - double membrane which surrounds the nucleus to separate the contents of nucleus from the rest of the cell. It has nuclear pores which let substances such as mRNA through.
• Nucleolus contains RNA and is where ribosomes are made -
• Chromatin is the genetic material (DNA wound around histone proteins)

Rough Endoplasmic Reticulum (RER)

• 
• A system of membranes, containing fluid-filled cavities (cisternae) that are continuous with the nuclear membrane,, coated with ribosomes.
• Intracellular transport system - the cisternae form channels to transport proteins which are synthesised in the ribosomes. They are then actively passed into the cisternae to be transported to the Golgi apparatus for modification.

Smooth Endoplasmic Reticulum (SER)

• A system of membranes, containing fluid-filled cavities (cisternae) that are continuous with the nuclear membrane. (No ribosomes)
• Contains enzymes which catalyse reactions to synthesise lipids.
• 

Golgi Apparatus

• Stack of membrane-bound flattened sacs. Secretory vesicles bring materials to and from the Golgi apparatus.
• Modification of proteins from the ER and packaging of proteins into vesicles to be transported around the cell
• 

Mitochondria

• Spherical, rod shaped or branched structure, 2-5 um long. Surrounded by a two membranes, with a fluid-filled space between them. Inner membrane is highly folded into cristae. Inner part of mitochondria is a fluid-filled matrix.
• Site of ATP production during aerobic respiration..
• 

Lysosomes

• Small sacs surrounded by a single membrane containing powerful digestive enzymes.
• Lysosomes are abundant in phagocytic cells such as neutrophils and macrophages that digest invading pathogens
• Keep the powerful enzymes separate from the rest of the cell. Can engulf old cell organelles and foreign matter and then return it to the cell for reuse.
• 

Flagella and Cilia

• Protrusions from the cell, surrounded by a cell surface membrane. It has a 9+2 arrangement - 9 microtubules around the edge in a ring and then a 2 microtubules in the centre.
• Cilia move liquid outside the cell (i.e. movement of mucus in the airways). Flagella move the whole cell (i.e. sperm).
• 

Ribosomes

• Small spherical organelles 20nm in diameter. Made of ribosomal RNA. Made in nucleolus. Consists of two subunits. Some are free in cytoplasm and some are attached to the ER.
• Ribosomes bound to RER synthesise proteins which will be exported outside the cell. Free ribosomes synthesise proteins which will be used inside the cell.
• 

Centrioles

• Made of two bundles of microtubules (made from tubulin protein subunits forming a cylinder) at right angles to each other.
• During cell division, the spindle is formed from the centrioles. The spindle attaches to chromosomes and motor proteins pull the chromosomes to opposite ends of the cell. Centrioles also form cilia and flagella.
• 

Cellulose Cell Wall

• Plant cell wall, made from bundles of cellulose fibres.
• Provide strength & support, maintains cell's shape, permeable, prevent cells bursting when turgid
• 

Importance of the cytoskeleton

• Give support and mechanical strength to cell
• Keeps the cell's shape stable
• Allows cell movement by making up the cilia, flagella and centrioles
• Help organelle movement
• Forms the spindle and enables chromosomal movement
• Anchor nucleus within the cytoplasm

Interrelationship between organelles

• Making and Secreting Proteins:
• 1. Gene with coded instructions for a protein is in the chromatin in the nucleus
• 2. Transcribed into a length of messenger RNA and many copies are made
• 3. mRNA passes out of the nuclear pores in the nuclear envelope and goes to the ribosomes
• 4. mRNA instructions are translated by the ribosomes and the protein is assembled
• 5. Protein molecule pass into the cisternae of the rough endoplasmic reticulum
• 6. Vesicles which hold the protein are pinched off from the RER and move to Golgi apparatus by the cytoskeleton
• 7. Vesicles fuse with the Golgi apparatus where the protein may be modified for release
• 8. Vesicles are pinched off the Golgi apparatus and pass to the plasma membrane
• 9. Vesicles fuse with the plasma membrane and the protein is released outside the cell.

Prokaryotic Vs Eukaryotic Cells

• Similarities:
• Plasma membrane
• Cytoplasm
• Ribosomes
• DNA and RNA
• Differences:
• Much smaller
• Less developed cytoskeleton with no centrioles
• No nucleus
• No membrane-bound organelles
• Smaller ribosomes
• Naked DNA (no histones) and loop DNA (rather than linear)
• Divide by binary fission, not mitosis