Magnification and Resolution
the degree to which the size of an image is larger than the object itself. Numerically, it is the imae size divided by the actual size of the oject, measured using the same units. It is usually expresses as x10, x1.5, etc.
the degree to which it is possible to distinguish between two objects that are very close together. The highter the resolution, the greater the detail you can see.
Actual Size = Image Size/Magnification
Magnification = Image Size/Actual Size
Transmission Electron Microscope (TEM)
- The electron beam passes through a very thin prepared sample
- Electrons pass through the denser parts of the sample less easily, so giving some contrast
- The final image is 2D
- The magnification possible is x500,000
Scanning Electron Microscope (SEM)
- The electron beam is directed onto a sample. The electrons don't pass through the specimen
- They 'bounce off' the sample
- The final image is a 3D view of the surface of the sample
- The magnification possible is x100,000
EMs produce black, white and grey images. 'False-colour' can be added
Cells contain a network of fibres made of protein. These fibres keep the cell's shape stable by providing an internal framework called the cytoskeleton
Some of the fibres (called actin filaments) are like the fibres found in muscle cells. They are able to move against each other. These fibres cause the movement seen in some white blood cells. They also move some organelles around inside cells.
There are other fibres known as microtubules. These are cylinders about 25nm in diameter. They are made of a protein called tubulin. Microtubules may be used to move a microorganism through a liquid, or to waft a liquid past the cell. Other proteins present on the microtubules move organelles and other cell contents along the fibres. This is how chromosomes are moved during mitosis. It is how vesicles move from the endoplasmic reticulum to the Golgi apparatus. These proteins are known as microtubule motors. They use ATP to drive these movements.
Flagella (undulipodia) and cilia
In eukaryotes (organisms that have cells with nuclei) flagells and cilia are structurally the same. They are hair-like extensions that stick out from the surface of cells. Each one is made up of a cylinder that contains nine microtubules arranged in a circle. There are also two microtubules in a central bundle. Undulipodia are longer than cilia.
- Undulipodia usually occur in ones or twos in a cell
- Cilia often occur in large numbers on a cell
Vesicles and Vacuoles
- Vesicles are membrane-bound sacs found in cells. They are used to carry many different substances around cells
- In plant cells, the large cell vacuole maintains cell stability. It is filled with water and solutes so that it pushes the cytoplasm against the cell wall, making the cell turgid. If all the plant cells are turgid, this helps to support the plant. This is especially important in non-woody plants.
Endoplasmic reticulum (ER)
Rough ER transports proteins that were made on the attached ribosomes. Some of these proteins may be secreted from the cell. Some will be placed on the cell surface membrane.
Smooth ER is involved in making the lipids that the cell needs.
Golgi apparatus recieves proteins from the ER and modifies them. It may add sugar molecules to them. The Golgi apparatus then packages the modified proteins into vesicles that can be transported. Some modified proteins may go to the surface of the cell so that they may be secreted
Mitochondria are the site where adenosine triphosphate (ATP) is produced during respiration. ATP is sometimes called the universal energy carrier because almost all activities that need energy in the cell are driven by the energy released by ATP
The basic structure of a phospholipid molecule is that the phosphate 'head' is hydrophilic while the two fatty acid 'tails' are hydrophobic. These properties come from the way charges are distributed across the molecule.
Molecules with charges that are evenly distributed around the molecule do not easily dissolve or mix with water e.g oil and water
If phospholipid molecules are mixed with water, they form a layer at the water surface. The phosphate heads stick into the water, white the fatty acid tails stick up out of the water.
If phospholipid molecles are completely surrounded by water, a bilayer can form.
The fluid mosaic model
fluid mosaic refers to the model of cell membrane structure. The lipid molecules give fluidity and proteins in the membrane give it a mosaic (patchwork) appearance. The molecules can move about.
Glycoproteins and glycolipids
Some of the phospholipid molecules making up the bilayer, and some of the proteins found in the membrane, also have small carbohydrate parts attached to them. Where phospholipid molecules have a carbohydrate part attached they are called glycolipids. Where protein molecules have a carbohydrate part attached they are called glycoproteins.
cells communicate with one another by signals. Many molecules act as signals - some signal during processes taking place inside cells; others signal from one cell to others. Cytokines are an example of cell signals.
Water always moves from a region of high water potential to a region of lower water potential - it moves from a region of high concentration of 'free' water molecules to a region of lower concentration.
solute - a solid that dissolves in a liquid
solvent - a liquid that dissolves solids
solution - a liquid containing dissolved solids
- in pure water a plant cell takes water in until it becomes turgid - the plant cell wall prevents it from bursting. Animal cells burst open because they take in too much water, the cell is haemolysed.
- in a concentrated sugar solution (low water potential) water moves out of cells. In plant cells the cell membrane pulls away from the cell wall, the cell is plasmolysed. Animal cells shrink and appear wrinkled, it is crenated.
The cell cycle
the cell cycle describes the events that take place as one parent cell divides to produce two new daughter cells which then each grow to full size. For some organisms, the cell cycle is the life cycle, and each daughter cell is a new single-celled organism.
The cell cycle is divided into stages:
- interphase - DNA replicates in this stage
- mitosis - the nucleus divides and chromatids separate
- cytokinesis - the cytoplasm divides or cleaves
- growth phase - each new cell grows to full size
mitosis refers to the process of nuclear division where two genetically identical nuclei are formed from one parent nucleus. This process involves:
- prophase - replicated chromosomes supercoil (shorten and thicken)
- metaphase - replicated chromosomes line up down the middle of the cell. spindle fibres are formed.
- anaphase - the replicas of each chromosome are pulled apart from each other towards opposite poles of the cell
- telophase - two new nuclei are formed
Organising the organism
A collection of cells that are similair to each other and perform a common function. They may be found attached to each other, but not always. Examples include xylem and phloeam in plants; epithelial and nervous tissues in animals
A collection of tissues working together to perform a particular function is called an organ. Examples include leaves of plants and the liver in animals
an organ system is made up of a number of organs working together to perform an overall life function. Examples include the excretory system and the reproductive system.