OCR biology cells

unit 1

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  • Created by: michaela
  • Created on: 04-10-11 19:35

Magnification and resolution

MAGNIFICATION-the degree to which the size of an image is larger than the object itself.

RESOLUTION-the degree to which it is possible to distinguish between two objects that are very close together.

                         Light Microscope                  SEM                      TEM    

Manification      x1500                                   x100,000               x500,000

Resolution        200nm                                  0.1-0.2nm              0.1-0.2nm

mm-->nm = x1000        

The IAM triangle

actual size=image size÷magnification

magnification=image size÷actual size

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cell organelles

RER-studded with ribosomes, transports proteins that were made on the attached ribosomes.

SER-flattened membrane bound sacs, no ribosomes, makes lipids.

Golgi apparatus-membrane bound flattened sacs, receives proteins from the ER and modifies them before packaging them into vesicles and they are transported.

Mitochondria-two membranes-cristae (folded), matrix, ATP production.

Chloroplasts-flattened sacs called thylakoids, stack is called a granum, site of photosynthesis.

Lysosomes-spherical sacs surrounded by a single membrane, contain digestive enzymes to break down materials.

Ribosomes-tiny organelles, some in cytoplasm and some bound to the RER. Two subunits, site of protein synthesis, mRNA is used to assemble proteins from amino acids.

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cell organelles continued..

Centrioles-small tubes of protein fibres (microtubules), come in pairs, take part in cell division-form spindles and move the chromosomes during nuclear division.

Nucleus-largest organelle-surrounded by a nuclear envelope. Nuclear pores go right through the envelope-large molecules can pass through. There is a dense spherical structure called a nucleolus inside the nucleus. This houses all of the genetic material. Chromatin inside (contains DNA and proteins)-has instructions for making proteins. Nucleolus makes RNA and ribosomes.

Roles of different organelles in the production and secretion of proteins:

RER-transports proteins made on the attached ribosomes.

Ribosomes-site of protein synthesis.

Golgi apparatus-modifies and packages proteins into vesicles.

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animal cell


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plant cell

Plant Cell Diagram (http://www.celldiagram.net/wordpress/wp-content/uploads/plant-cell1.gif)

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eukaryotic and prokaryotic cells

Eukaryotic cells-have a nucleus, can contain two membranes, have membrane bound organelles, large ribosomes, linear chromosomes (separate strands) of DNA, DNA surrounded by a membrane, ATP production takes place in the mitochondria, larger, cell wall made of cellulose.

Prokaryotic cells-no nucleus, one membrane, no membrane bound organelles, small ribosomes, DNA in single loop free in liquid, ATP in infolded regions, smaller, murein cell wall.

PHOSPHOLIPID BILAYER-basic structural component of plasma membranes. Two layers of phospholipid molecules-creates a barrier to many molecules.

5 roles of membranes: separate cell contents from cytoplasm, separate cell contents from outside environment, cell recognition and signalling, holding the components of somes metabolic pathways in place, regulating the transport of materials into and out of cells.

All membranes are permeable to water molecules because water molecules can diffuse through the phospholipid bilayer.

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fluid mosaic model

FLUID MOSAIC-a bilayer of phospholipid molecules, protein molecules and proteins which are embedded in the bilayer.

Glycoproteins and glycolipids-carbohydrates attached to the proteins or phospholipids. They are involved in cell signalling and bind cells together in tissues. They have a specific shape complementary to a trigger molecule, binds to the receptor in the glycoprotein/lipid.

Cholesterol-gives mechanical stability-fits between fatty acid tails. Makes the barrier more complete.

Channel proteins-allows the movement of some substances across the membrane e.g. glucose.

Carrier proteins-actively move substances across the membrane using ATP.

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fluid mosaic model

Increasing temperature=more kinetic energy, so molecules move faster, leaky membranes. Allows substances that would not normally do so to enter or leave the cell.

CELL SIGNALLING-communication between cells where receptors recognise a signal and initiate a response.

Hormones-chemical messengers, bonds to a receptor on a target cell surface membrane as the two have complementary shapes.

Insulin-triggers internal responses when it attaches, so cells have more glucose channels present, enabling the cell to take up more glucose from blood, reducing blood glucose levels.

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passive processes


  • Diffusion- is the net movement of molecules from an area of high concentration to low concentration.
  • Diffusion occurs because all molecules have kinetic energy which makes them move about at random. after diffusion has occurred molecules reach an equilibrium. where they are evenly spread in a certain area of gas or liquid.
  • Facilitated diffusion- aided by transport carrier or channel molecules. no metabolic energy required.

Factors that affect the rate of diffusion

  • Steepness of concentration gradient- higher concentration gradient.. increased rate of diffusion
  • Temperature- high temp.. increase in diffusion 
  • Surface area- greater surface area.. increase in diffusion 
  • Nature of molecules, e.g. size/ distance- size.  smaller molecules.. increase in diffusion.  distance. thicker membranes.. slower diffusion
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active processes


the movement of molecules or ions across membranes, which uses ATP to drive protein 'pumps' within the membrane. Each protein pump carries a specific molecule. Go in opposite direction to the concentration gradient, and the rate is faster.

what happens to ATP during active transport?

  • ATP---ADP+P
  • chemical when broken down releases energy

where is active transport important?

  • when molecules need to move against a concentration gradient
  • bulk transport of materials

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active processes


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