3.1 AS Unit F211: Cells, Exchange and Transport
Module 1: Cells
Cells are the basic units of all living things. Organisms function because of communication and co-operation between specialised cells.
Cell division is a fundamental process, necessary for reproduction, growth and repair.
1.1.1 Cell Structure
Context and exemplification
The cell is the basic unit of all living things.
An understanding of how to use a light microscope is developed along with an understanding of why electron microscopes are so important in biology.
Careful observation using microscopes reveals details of cell structure and ultrastructure and provides evidence to support hypotheses regarding the roles of cells and organelles.
Candidates should be able to:
(a) state the resolution and magnification that can be achieved by a light microscope, a transmission electron microscope and a scanning electron microscope;
|Type Of Microscope||Resolution||Magnification|
(b) explain the difference between magnification and resolution;
Resolution is the ability to distinguish between two objects that are very close together. This is useful for looking at objects in detail, whereas magnification is the size of an object’s image in relation to the size of the original object. Magnification can be increase almost indefinitely but is limited to a limit of useful magnification by its resolution.
(c) explain the need for staining samples for use in light microscopy and electron microscopy;
Most parts of cells are transparent to both electrons and photons, so you have to stain the sample to add contrast to be able to distinguish different parts. For light microscopes they use a dye, such as methylene blue and eosin. This is taken up by some parts of the object more than others and this contrast makes different parts look different and let them show up.
In electron microscopes heavy metals are used such as lead and these large metal ions scatter the electrons creating contrast and allowing you to see the specimen clearly.
(d) calculate the linear magnification of an image (HSW3);
Magnification=Image size/Object Height
(e) describe and interpret drawings and photographs of eukaryotic cells as seen under an electron microscope and be able to recognise the following structures: nucleus, nucleolus, nuclear envelope, rough and smooth endoplasmic reticulum (ER), Golgi apparatus, ribosomes, mitochondria, lysosomes, chloroplasts, plasma (cell surface) membrane, centrioles, flagella and cilia;
(f) outline the functions of the structures listed in (e);
The nucleus is the biggest organelle it is bounded by the nuclear envelope, which is made of two plasma membranes, and contains the chromatin, which is the extended form the chromosomes take during interphase, and nucleolus makes ribosomes. It has nuclear pores in its nuclear envelope.
Both Rough Endoplasmic Reticulum (RER) and Smooth ER are a system of flattened membrane bound sacs called cisternae in tube and sheet shapes. The membranes are continuous with the nuclear envelope. RER has ribosomes on a surface and produces proteins, whereas smooth ER do not have ribosomes but are the site of lipid and steroid synthesis. Golgi apparatus is a stack of membrane bound sacs called cisternae, continuously being formed at one end and being budded off at the other end into vesicles. It is used in secretion, lysosome formation and in the addition of sugars to proteins to form glycoproteins. Ribosomes are formed from a large and a small subunit. They are made of roughly equal parts of protein and RNA and are used in protein synthesis. They are found on RER, lie free in the cytoplasm and smaller ones are found in mitochondria and chloroplasts. They form polysomes (or polyribosomes) which re collections of ribosomes strung along messenger RNA. Mitochondria are the sites of aerobic respiration. Specifically it occurs on the cristae where the ATPsynthase enzymes used to produce it are found. It is surrounded by a mitochondrial envelope, with the inner wall folded into shapes forming cristae and is filled with a fluid called matrix. In this matrix you can find some circular DNA and also some phosphate granules. These were originally separate organisms but have now become part of our cells according to the endosymbiotic theory. Lysosomes are sphrerical sacs bound by a single membrane containing (hydrolytic) enzymes that can be used in apoptosis and breaking down various structures. Chloroplasts are similar to mitochondria in that they are organelles whose origin is described by the endosymbiotic theory. These are the sites of photosynthetic reactions. It is enclosed by an envelope (double membrane) and it has coin-like disc shaped membranes called thylakiod membranes. These are stacked up in the chloroplasts in grana. These grana (stacks) are linked by lamellae which are thin, flat pieces of thylakiod membranes. The grana contain chlorophyll and this is where the light dependant reactions of photosynthesis are carried out. In these reactions the light energy from sunlight is used to split water to form hydrogen ions. These are then used to make ATP and reduced NADP (or NADPH). Then these are used in the light independent stage of photosynthesis in the stroma to reduce CO2 to glucose. The purpose of the cell surface (plasma) membrane is (TRRACC) Transport – channel proteins, Recognition and Receptors – Glycolipids and glycoproteins, Adhesion- glycoproteins and glycolipids, Compartmentalisation – entire membrane, Control of incoming and outgoing substances – phospholipids. Centrioles are small hollow cylinders containing a ring of microtubules (tiny protein cylinders) at 90o to each other and these are used to form the spindles used in the separation of chromosomes in nuclear division. Flagella are similar to cilia and they have two microtubules in the centre and nine pairs around the edge which slide across each other to let it move. Flagella are to move the cell itself. This requires ATP. In eukaryotic cells however these should be known as Undulipodia. Flagella are in prokaryotic cells and are just a spiral of protein attached to a spinning disc at the base. Cilia are similar to Undulipodia, also having two microtubules in the middle and nine pairs round the outside. These are to move substances along past the cell.
(g) Outline the interrelationship between the organelles involved in the production and secretion of proteins (no detail of protein synthesis is required);
This process is an example of Division of Labour. A gene from the chromatin in the nucleus is copied into mRNA (messenger RNA). This is called transcription. Then this copy passes out of a Nuclear pore and into the cytosol (which is the word for the fluid alone whereas cytoplasm includes all the organelles) and to one of the ribosome, which is either located in the RER or the cytoplasm. Then the mRNA is “read” and the instructions translated into a polypeptide chain. The next stage is known as Post-Translational Modification and it involves finishing the final protein. This can be done by adding sugars to create glycoproteins ect. This takes place in the Golgi apparatus, and it travels to fuse with Golgi in a membrane bound vesicle that is pinched off from the ER. After the stage the finished protein travels off in another vesicle, either to another part of the cell or fuses with the cell surface membrane to secrete the protein.
(h) explain the importance of the cytoskeleton in providing mechanical strength to cells, aiding transport within cells and enabling cell movement;
The cytoskeleton is a network of protein fibres this give the cell stability by providing mechanical strength and allow it to move. Eukaryotic cells have the proteins arranged in microtubules (small protein tubes) and microfilaments (small solid strands). The cytoskeleton provides an internal framework for the cell, preserving its shape and holding the organelles in position. Parts of the cytoskeleton, fibres called Actin Filaments are similar to fibres found in muscle cells and are able to slide against each other and so are responsible for the movement of some of the organelles and also the movement and change in shape of White Blood Cells (phagocytes). They are used to power cilia and flagella.
(i) compare and contrast, with the aid of diagrams and electron micrographs, the structure of prokaryotic cells and eukaryotic cells;
|Prokaryotic Cells||Eukaryotic Cells|
|DNA is circular.||DNA is linear.|
|Small cells (less than 2 μm across)||Larger cells (2-200 μm)|
|Small Ribosomes||Larger ribosomes|
|No nucleus- DNA free in the cytoplasm.||Nucleus present-DNA inside nucleus.|
|Cell wall made polysaccharide, not cellulose or chitin.||No cell wall in animals, cellulose cell wall in plants and chitin cell wall in fungi.|
|Few organelles, no mitochondria.||Many organelles, mitochondria present.|
|Photosynthesis (if carried out) is on a mesosome (an in-folding of the cell surface membrane)||Photosynthesis (if carried out) is in chloroplasts.|
|Has pili for adhesion.||Has glycoproteins and glycolipids for adhesion.|
|Has plasmids||Does not have plasmids.|
|No internal plasma membranes.||Has internal plasma membranes.|
(j) compare and contrast, with the aid of diagrams and electron micrographs, the structure and ultrastructure of plant cells and animal cells.
|Has chloroplasts||Does not.|
|Cellulose cell wall||No cell wall|
|Has a tonoplast surrounding a permanent vacuole filled with cell sap.||Only has temporary vacuoles.|
|Has plasmodesmata with a middle lamella between the cell walls of different cells.||Does not.|
|Does not have centrioles.||Has centrioles.|
|Does not have lysosomes.||Has lysosomes.|
|Everything else both have.|