Notas de biologia

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  • Publicado : 8 de enero de 2012
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Unicellular organisms:
Some organisms consist of only one cell. This cell, therefore has to carry out all the functions of life. These are:
1) Nutrition.
2) Metabolism.
3) Growth.
4) Sensitivity.
5) Homeostasis.
6) Reproduction.
Therefore these cells are more complex than most of the cells in multicellular organisms.

Sizes of cells:
Scientists uses the SI units:
Metres (m),Millimetres (mm), Micrometers (um) and nanometres (nm).

Magnification and scale bars:
Magnification= Size of image .
Actual size of specimen.

Surface area to volume ratios in cells:
The rate of these reactions (the metabolic rate of cell) is proportional to the volume of the cell.
The surface area to volume ratio of a cell is therefore very important. Ifthe ratio is too small then substances will not enter the cell as quickly as they are required and waste products will accumulate because they are produced more rapidly than they can be excreted.
Surface area to volume ratio is also important to heat production and loss. If the ratio is too small then the cells may overheat because the metabolism produces heat faster than it is lost over thecells surface.

Multicellular organisms and cell differentiation:
Some unicellular organisms live together in colonies. Each colony consists of a ball made of a protein gel, with 500 or more identical cells attached to its surface. Although the cells co-operate they are not fused to form a single cell mass and so do not form a single organism.
The development of cells in different ways to performdifferent functions is called differentiation.

Stem cells:
Stem cells are defined as cells that have the capacity to self-renew by cell division and to differentiate.

Therapeutic use of stem cells:
The greatest success so far in the therapeutic use of stem cells involves bone marrow transplants.

Extracellular components:
The plasma membrane of a cell is usually considered to be thebarrier between the inside of the cell and the outside. However, many cells produce components that pass through the membrane and form part of the structure outside. These are called extracellular components. Anything inside the plasma membrane is intracellular.

Prokaryotic Cells:
First organisms to evolve on earth, still have simplest cell structure.
Mostly small in size, unicellular and arefound almost everywhere.
Cell wall, Ribosome’s, Plasma Membrane, Cytoplasm, Nucleiod, Pili, Flagella.

Eukaryotic Cells:
Much more complicated internal structure than prokaryotic cells. They have nucleus and organelles in the cytoplasm with single or double membranes. Each organelle have a distinctive structure and function.
Nucleus, Rough endoplasmic reticulum, Golgi apparatus,Lysosomes, Mitochondria, Free ribosome’s.

Membrane structure:
Not composed entirely of phospholipids- they also contain proteins.
Some of these are embedded in the phospholipids and are called integral proteins. Others are more loosely attached to the surface of the membrane and are called peripheral proteins.
Some proteins in plasma membranes have short chains of sugar moleculesattached on the outside of the membrane, but not to the side facing the cytoplasm. These are called glycoprotein.

More particles move from an area of high concentration to low concentration than vice versa. There is therefore a net movement from the higher concentration to the lower concentration.
Diffusion is an effective method for moving particles a few micrometers, but it is tooslow over great distances.

Simple diffusion across membranes:
Simple diffusion across membranes involves particles passing between the phospholipids in the membrane. It can only happen if the phospholipids bilayer is permeable to the particles. The centre of the membrane is hydrophobic, so ions with positive or negative charges cannot easily pass through. Polar molecules, which have partial...
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