Modules 1 : Cells 1

Light microscope

• Resolution: 200nm, two objects closer than this are seen as one. The limit is due to the magnitude of the wavelength of light. Two objects can only be distiguished if light waves can pass between them.
• Magnification: up to a maximum of x1500

Magnification : The number of times greater an image is than the object.

Resolution: The ability to distinguish two separate points as distinct from each other.

• Staining is needed in light microscopes because alot of biological material is not coloured, so you can't see the detail. Also some material distorts when you try to cut it into thin sections.
• cm,mm,mircometre, nm go down in dividing by 1000

• Actual size = Image size / Magnification
• Magnification = Image size / Actual size

Two types of electron microscope:

• Transmission (TEM):
  • Magnification: x500000
• Scanning (SEM):
  • Magnification: x100000
• Resoultion: 0.1nm, beam of electrons wavelength 0.004nm

Mircotubules and microfilaments make up the cytoskeleton that:

• supports animal cells (plants cell wall)
• moves organelles around eukaryotic cells
• moves cytoplasm, eg. during cell migrations in development;phagocytosis; cytokinesis

• Microtubles know as microtubule motors move organelles and other cell contents along fibres it is how:
  • Chromosomes are moved during mitosis
  • vesciles move from the ER to the golgi apparatus
• They use ATP to drive these movements.

Undulipodia and cilia:

• Cilia
  • Short, made up of a cylinder that contains nine mircotubles arranged in a circle.
  • in ciliated epithelial tissue, the sweeping movements of the cilia move substances such as mucus across surface of cells.
• Undulipodia
  • Long, structurally same, forms tail of a sperm cell

Organelles surrounded by membranes:

• The nucleus:
  • Nuclear envelope: A double membrane around the nucleus containing muclear pores which allow exchange between the nucleus and the cytoplasm
  • Nucleus: Contains DNA responsible for the individual characteristics of each cell. DNA is similar in all cells but depending on which type of cell it is, some genes maybe turned on or off. Division of the nucleus proceeds cell division
  • Nucleolus: Inside the nucleus. Produces ribosomes which leave the nucleus to take positions on the rough endoplasmic reticulum
• Lysosomes: Contains hydrolytic enzymes which in a white blood cell, digest bacteria. Enzymes are contained in the lysosomes to prevent necrosis (cell death)

• Endoplasmic reticulum (ER):
  • Rough endoplasmic reticulum: Transports proteins that were made on the attached ribosomes.
  • Smooth endoplasmic reticulum: Carries out synthesis of lipid
• Golgi apparatus: A stack of membrane bound vesicles. Packages macromolecules for transport around the cell
• Mitochondria: A double membrane bound organelle which is involved in aerobic respiration. Inner membrane forms folds called cristae to increase surface area of membrane. An the cristae, glucose is combined with oxygen to produce ATP.
• Chloroplasts: Contain chlorophyll. Have a double outer membrane. Within the stroma there are other membrane structures called grana where photosynthesis takes place

Without membranes surrounding them:

• Centriole: A hollow cylinder about 0.4µm long formed from a ring of microtubules which are used to grow the spindle fibres used in nuclear division
• ribosomes : Some are in cytoplasm and some bound to ER. The site of protein synthesis, act as an assembly line where coded information (mRNA) from the nucleus is used to assemble proteins from amino acids.

Organelles at Work

Division Labour:

• Instructions to make the hromone are in the DNA in the nucleus.
• Instructions known as genes, gene is a chromosome.
• Nucleus copies the instructions in the DNA into a molecule called mRNA.
• mRNA leaves nucleus through nuclear pore and attaches to ribosome. In this case attached to the rough ER
• Ribosomes reads the instructions to use the codes to assemble the hormone (protein).
• Protein is pinched off in a vesicle and transported to the golgi apparatus.
• Golgo apparatus packages the protien and may also modify it so that it is ready for release.
• Packaged protein in vesicle is moved to the cell surface membrane, where it is secrected

• Cell surface (plasma) membranes surround all cells and control what enters and leaves. Membranes divide up the cytoplasm of eukaryotic cell into separate.

The roles of membrane:

• Separating cell comtents from the outside environment
• separating cell components from cytoplasm
• cell recognition and signalling
• holding the components of some metabolic pathways in place
• regulating the transport of materials into or out of cells

Within cells

• provide a large surface for holfing the enzymes and coenzymes for forming ATP in mitochondria ans chloroplasts.

Why fluid mosaic?

• fluid: phospholipids are liquid - think of a membrane as like a thin layer of oil
• Mosaic: this is a picture mabe of many pieces of tile. Proteins are like the pieces of tile surrounded by phospholipuds,which are like cement holding everything together.

• Phospholipids:
  • form a bilayer that acts as a barrier between cytoplasm and cell exterior
  • are fluid, so components, can move within the membrane
  • are parmeable to non-polar molecules eg. O2, CO2
  • are permeable to small polar molecules such as, H2O, CO2
  • are impermeable to ions and large polar molecules such as sugars and amino acids.
• Cholesterol:
  • stabilises the phospholipid bilayer by binding to polar heads and non-polar tails of phospholipid.
  • controls fluidity by preventing phospholipids solidifying at low temperatures and becoming too fluid at high temperatures.
  • reduces permeability to water, ions and polar molecules.
• Proteins:
  • are transmembrane proteins acting as channels and carries
  • are receptors for chemicals made by other cells, eg. hormones.

• Glycollipids and glycoprotiens:
  • are carbohydrate chains only found on exterior surface of cell membrane
  • act as receptors for signalling molecules (eg. hormones) and for drugs
  • act as cell surface markers that identify the cells to others
  • are involved in "sticking" cells to one another (cell adhesion)

Effects of temperature:

• Increasing temperature gives molecules mopre kinetic energy, so they move faster. This increased movement of phospholipids and other components make membranes leaky and allows substances enter and leave membrane that shouldn't.

• Neurones:
  • Neurones transmit information elctrically over long distances. At the end of each neurone is a swelling that contains vesicles of a chemical trasmitter substance known as a neurotransmitter.
  • When a nerve impulse reaches the end of the neurone, these vesicles travel to the membrane and release molecules of neurotrasmitter by exocyytosis across the synapse.
  • On the next neurone, or on an effector, there are receptor molecules that detect the neurotransmitter molecules.
• Hormones:
  • Endocrine cells secrete hormones into the bloodstream. Hormones have effects on their target cells, which are thr only cells that detect the hormone and respond to it.
  • Receptor molecules on the cell surface or inside the cell combine with the hormone, triggering a series of events leading to the cell's response.

• Local hormones:
  • Some cells release molecules that travel short distances to adjoining cells. Examples of these local hormones are histamine and cytokines.
  • During an immune response, some lymphocytes release cytokines that stimulate other lymphocytes to divde by mitosis and secrete antibodies; cytokines also stimulate phagocytes to become more active.

• The hormones insulin and glucagon do not cross cell membranes as they are large, water-soluble molecules. Glycoprotein receptor molecules on the cell surface detect insulin.
• Steriods (eg. testosterone) are fat-soluble, so they can cross phospholipid bilayers and interact with receptors inside cells.