CELLS,MEMBRANES AND DIVISION
MAGNIFICATION AND SIZE OF ORGANELLES
Actual size = image size divided by magnification
Magnification = image size divided by actual size
Image size =actual size x magnification
Magnification -> the number of times lager an image appears, compared with the size of the object
Photomicrograph -> photographs of an image seen using an optical microscope
Maximum magnification = x1500
OPTICAL MICROSCOPES
ADVANTAGES: Relatively cheap, easy to use, portable (able to be used in field and lab), able to study whole living organisms
DISADVANTAGES:Limited magnification (x1500) cannot view small organelles like ribosomes, limited resolutions structures closer together than 200 nanometers willl appear as one object
Total magnification = objective lens x eyepiece
usually 4 objective lens, x4, x10, x40, x100 (oil immersion)
ELECTRON MICROSCOPE
DISADVANATGES: large and very expensive, great deal of skill and training required to use, specimens have to be dead as viewed while in a vacum
MICROMETRE: used fro cells, animal cells are 20-40 micrometers long, large organelles
NANOMETRE (nm): used for small organelle sizes, large molecules, cell membrane 10nm wide, viruses up to 100nm diameter
STRUCTURE AND FUNCTION OF THE MEMBRANES
A cell need to be able to import substances it needs to survive and to export waste materials and substances that are needed outside the cells
There are several methods by which substances (molecules and ions) can cross the cell membrane: diffusion and facilitated diffusion, osmosis, active transport, and bulk transport.
Diffusion -> the net movement of particles down a concentration gradient from a region of a high concentration to a region of low concentration until equilibrium is reached. No ATP is expended during diffusion.
The steeper then concentration gradient, the faster diffusion takes place (fast rate of diffusion), less steep concentration gradient the slower rate of diffusion
Polarity and diffusion
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Small, non-polar molecules such as oxygen and carbon dioxide rapidly diffuse across a membrane.
Small, polar molecules, such as water and urea, can also diffuse across, but much more slowly.
Special water channel proteins called aquaporins are present in cells that require a high rate of water movement.
Charged particles (ions) cannot diffuse across a membrane, even if they are very small.
Lipid based molecules - as the membrane is made up of phospholipids, fat-soluble molecules can also pass across the membrane by diffusion as they dissolve in the lipid bilayer. • Steroid hormones are lipid-based and so diffuse through membranes into cells.
other factors that affect the rate of diffusion
moving/stirring
size of molecule
temperature
Membrane proteins
channel proteins -> Allow ions with an electrical charge and surrounded by water to pass through
carrier proteins -> By changing their shape can carry specific molecules across the membrane e.g. glucose
other proteins -> May be attached to the carrier proteins and function as enzymes, antigens or receptor sites for complementary-shaped signalling chemicals such as hormones
osmosis -> the diffusion of water.it is the net movement of water molecules from a region of high-water potential to a region of low water potential, across a partially permeable membrane.
solvent -> the liquid a solute dissolve in
solution -> the combination of a solute and a solvent
solute -> a substance that can dissolve
Water potential
Water potential is denoted by the symbol Ψ and is measured in kilopascals (kPa).
Pure water has the highest water potential and has a value of 0 kPa. Solutions have a lower water potential than pure water and have a negative water potential.
Partially permeable barriers
Some very small molecules diffuse though the membrane
Some substances dissolve in the lipid layer and pass through
Some substances pass through special channel proteins or are carried by carrier proteins
Roles of membranes at the surface
The plasma membrane (or cell surface membrane): separates cell's components from its external environment, regulates transport