Introduction of cells

The Cell Theory

Metabolism: chemical reactions inside the cell, including the cell respiration to release energy
Reproduction: producing offspring either sexually or asexually
Growth: an irreversible increase in size
Response: the ability to react to changes in the environment
Excretion: getting rid of the waste products of metabolism
Nutrition: obtaining food, to provide energy and the materials needed for growth
Homeostasis: keeping conditions inside the organism within tolerable limits

Substances move into and out of cells through the plasma membrane at the surface of the cell rate of substances across the membrane depends on SA
if the ratio is small (elephant):
- substances will not enter the cell as quickly as they are required
- waste products will accumulate because they are produced more rapidly than excreted
- cells may overheat because the metabolism produces heat faster than it is lost over the cell surface

organisms consisting of a single mass of cells, fused together, are multicellular
example of a multicellular organism: Caenorhabditis elegans
- the adult body is 1mm long and is made up of exactly 959 cells.
- it feeds on the bacteria that cause decomposition
- has a mouth, pharynx, intestine, and anus
- hermaphrodite (contains both male and female reproductive organs
- has neurons

Emergent properties

individual cells in a group can organize themselves and interact with each other to form a living organism with distinctive overall properties. (1+1>2)
properties that arise from the interaction of component parts

the development of cells in different ways performs different functions
a group of cells specialize in the same way to perform the same function is called tissue
becoming specialized makes the cell more efficient than they had many different roles
thus multicellular organisms are able to carry out multiple and complex functions because different cells responsible for different jobs

- differentiated cells get exactly the same set of genes
- 25000 genes in the human genome are all present in every body cell
- gene is being expressed means gene is being used, which can switch specific gene to make a protein or other gene product
- cell differentiation happens because a different sequence of genes is expressed in different cell types

stem cells can divide again and again to produce copious quantities of new cells, therefore they are useful for the growth of tissues or the replacement of cells that have been lost or damaged
Stem cells are not fully differentiated. They can differentiate into different cell types. Scientists now add cytokines to ESCs to make them differentiate into the specific cell type
2 characteristics
- self-renewal: the ability tounmergo numerous cycles of cell division while maintaining an unspecialized state
- potency: the ability to differentiate into specialized cells

Totipotent (zygote) = can develop into any type of cell;

Pluripotent (embryonic stem cell) = can develop into most types of cells;

Multipotent (adult stem cell...) = can develop into a restricted range of cells;

Unipotent = can develop into one type of cell

Stargardt's macular dystrophy
- the genetic disease develops in children between 6-12
- Caused by the death of photoreceptor cells in the central portion of the retina called the macula.
- The macula is responsible for sharp central vision — for tasks like reading, watching television, and looking at faces. It is also important for color vision.
- Symptoms are reduced central vision and reduced color vision, side vision is usually kept

image size = actual size ✖️ magnification

~~Resolution-~~the ability to distinguish between two separate objects depends on the wavelength of EM radiation used

The specimen must be kept in a vacuum – air molecules would collide with electrons.
Dense parts of the specimen absorb electrons – heavy metal stains increase the density gradient.
The image is viewed on a screen or focussed onto photographic film to produce an electron micrograph.
Advantages
- Very high resolution (0.5 nm)

Disadvantages
- The specimen must be dead. Specimen deteriorates. Preparation may damage the specimen. Preparation is difficult & time-consuming. Very expensive.

The Electron beam is scanned across the surface of the specimen, electrons are reflected and collected.

Forms a 3D image on a cathode ray tube.

The image is viewed on a screen or focussed onto photographic film to produce an electron micrograph.

Advantages
- Excellent 3D images.
- Surface structures are shown.
- Larger samples can be examined.

EM’s have a much greater resolution so are used to investigate the ultrastructure of a cell.
LMs are used to get an overall picture. They are also much quicker, easier, and cheaper to use and can be used to view living specimens.