What is a microscope?
An instrument which magnifies the image of an object.
What is the equation for magnification?
Size of image/Size of object
The minimum distance apart 2 objects can be in order for them to appear as separate items.
Define cell fractionation.
The process where cells are broken up and the different organelles they contain are separated out.
Why is a cold, isotonic, buffered solution used in cell fractionation?
COLD- to reduce enzyme activity that may break down organelles ISOTONIC- prevent organelles bursting or shrinking as a result of osmotic gain or loss of water. Same water potential as original tissue. BUFFERED- to maintain a constant pH
What happens during the homogenation stage of cell fractionation?
Cells broken up by homogeniser. Releasing organelles. The resultant fluid (homogenate), is then filtered to remove any complete cells and large pieces of debris
What happens during the ultracentrifugation stage of cell fractionation?
Fragments in filtered homogenate are separated in a machine (CENTRIFUGE) This spins tubes of homogenate at v. high speed in order to create a centrifugal force.
-Spun at low speed
-Heaviest organelles (NUCLEI) are forced to the bottom of the tube, form a thin sediment.
-Fluid at top (SUPERNATANT) removed, leaving sediment of nuclei.
-Supernatant transferred to another tube and spun at faster speed.
-Next heaviest organelles removed at bottom (MITOCHONDRIA)
-Process continued, with increase of speed each time.
Why do light microscopes have relatively low resolution?
Long wavelength of light.
What are the two main advantages of an electron microscope?
- Electron beam has v. short wavelength (good resolving power) -Can be focused using electromagnets
Explain how a transmission electron microscope works.
Electron gun that produces a beam of electrons that is focused onto the specimen by a condenser electromagnet. The beam passes through a thin section of the specimen. Parts of specimen absorb electrons so appear dark, other parts allow electrons to ass through and so appear bright. Image produced on a screen and this can be photographed to give a PHOTOMICROGRAPH. Get 2D image.
What are the main limitations of TEM and SEM.
-Vacuum (no living samples)
-Complex staining process is required to get a BLACK AND WHIT IMAGE.
-Extremely thin specimen. (TEM)
-Image may contain ARTEFACTS- Not part of natural specimen.
Explain how a scanning electron microscope works.
Directs beam of electrons to surface of specimen from above, beam passed back and forth across a portion of specimen in a regular pattern. Electrons are scattered by the specimen and the pattern of this scattering depends on the contours of the specimen surface. 3D image. LOWER RESOLVING POWER THAN TEM.
What is the ultrastructure of a cell?
The internal structure of a cell that suits its job.
What are the different structures of the nucleus. (5)
-NUCLEAR ENVELOPE- Double membrane surrounding nucleus. Outer membrane continuous with endoplasmic reticulum. Controls entry and exit of materials in and out of nucleus. -
NUCLEAR PORES- Allow passage of large molecules (e.g. RNA) out of nucleus.
-NUCLEOPLASM- Granular, jelly like material that makes up bulk of nucleus.
-CHROMATIN- DNA found within the nucleoplasm. Form chromosomes take when not dividing.
-NUCLEOLUS- small spherical body within nucleoplasm. Manufacture ribosomal RNA and assembles ribosomes.
What are the functions of the nucleus? (3)
-Acts as control centre of cell through production of mRNA and hence protein synthesis.
-Retain genetic material of cell in form of DNA and chromosomes.
-Manufacture ribosomal RNA and ribosomes.
What are the different structures of mitochondrion? (3)
-DOUBLE MEMBRANE- surround organelle, outer controlling entry and exit of material. Inner folded to form extensions (CRISTAE)
-CRISTAE- Shelf like extensions of inner membrane. Provide large SA for attachment of enzymes involved in respiration.
-MATRIX- remainder of mitochondrion. Semi-rigid material containing protein, lipids and traces of DNA that allows mitochondria to control the production of their own proteins. Enzymes involved in respiration found here.
What are the functions of mitochondria?
Site of respiration
Production of energy-carrier molecule ATP from carbohydrates.
Describe structure and function of Rough Endoplasmic Reticulum.
Ribosomes present on outer surfaces of the membranes
-Provides large SA for synthesis of proteins and glycoproteins
-Provide pathway for the transport of materials, especially proteins, throughout the cell.
Describe structure and function of Smooth Endoplasmic Reticulum.
Lacks ribosomes on its surface and is often more tubular in appearance.
-Synthesise, store and transport lipids
-Synthesise. store and transport carbohydrates.
Describe structure of Golgi Appartatus
Similar to SER in structure except more compact.
Consists of a stack of membranes that make up flattened sacs, or CISTERNAE, with small rounded hollow structures called vesicles.
What are the functions of the golgi apparatus?
Proteins and lipids produced by ER are passed through Folgi in strict sequence.
Golgi modifies these proteins, often adding non protein components (e.g. carbs). Form glycoproteins.
Labels them, allowing them to be accurately sorted and sent to their correct destinations.
Once sorted, modified proteins and lipids are transported in vesicles which are regularly pinched of from ends of golgi cisternae.
Vesicles move to cell surface, where they fuse with the membrane and release their contents to outside.
Also forms lysosomes.
How are lysosomes formed?
When vesicles produced by Golgi contain enzymes such as proteases and lipases.
What is the function of lysosomes?
-Break down material ingested by PHAGOCYTIC cells (e.g. white blood cells)
-Release enzymes to outside of cell in order to destroy material around the cell.
-Digest worn out organelles so that useful chemicals can be reused.
-Completely break down cells after they have died.
What two types of ribosomes are there and where are they found?
80S type- eukaryotic cells (25nm diameter)
70S type- prokaryotics cells (smaller)
What is the function of ribosomes?
What are microvilli?
Finger like projections of epithelial cell that increase SA to allow more efficient absorption.
What characteristics do lipids share?
-They contain carbon, hydrogen and oxygen
-The proportion of oxygen to carbon and hydrogen is smaller than in carbohydrates.
-Insoluble in water
-Soluble in organic solvents such as alcohol and acetone.
What are the 5 roles of lipids?
Main role in the plasma membranes. Phospholipids contribute to the flexibility of membranes and the transfer of lipid-soluble substances across them.
-WATERPROOFING- Insoluble in water.
-INSULATION-Fats are slow conductors of heat.
-PROTECTION- Fat often stored around delicate organs.
Why are triglycerides called so?
Each have 3 fatty acids combined with glycerol
Created in a condensation reaction.
Where do the differences in properties of fats and oils come from?
variations in the fatty acids, glycerol molecule the same.
When is a fatty acid SATURATED/UNSATURATED?
Saturated when no C=C bonds
Unsaturated when C=C bond present.
Single double bond- mono-unsaturated
More than 1 double bond- polyunsaturated.
How are phospholipids different to lipids?
One fatty acid replaced with PHOSPHATE MOLECULE.
Which part is the hydrophillic part and which part is the hydrophobic part?
HYDROPHILLIC- phosphate (attract)
HYDROPHOBIC- fatty acids (repel)
What are molecules with to ends (poles) that behave differently called?
How is a phospholipid structured?
CH2COO- fatty acid
CHCOO- fatty acid(repel) (hydrophillic)
What is the test for lipids?
-Add 5 cm3 ethanol to sample being tested.
-Shake thoroughly to dissolve lipid
-Add 5cm of water shake
-Cloudy/white indicates presence of lipid.
-As a control repeat with water instead of sample.
What are the membranes around and within cells called?
What is the cell surface membrane?
Plasma membrane that surrounds cells and forms boundary between cell cytoplasm and the environment.
Allows different conditions to be established inside and outside a cell.
Controls movement of substances in and out of the cell.
How are phospholipids arranged to make up the cell surface membrane?
- One layer of phospholipids has hydrophillic heads pointing inwards (water in cell cytoplasm)
-Other layer has hydrophillic heads pointing outwards (interacting with water surrounding cell.
-Hydrophobic tails of both layers point into centre of membrane- protected from water on both sides.
What are the functions of phospholipids in the membrane/
-Allow lipid soluble substances to enter and leave cell.
-Prevent water- soluble substances entering and leaving the cell
-Make membrane flexible.
How are extrinisic proteins embedded in the phospholipid bilayer and what is their function?
occur either on surface or partly embedded, never extend completely across.
Act either to give mechanical support to membrane or, in conjunction with glycolipids, as cell receptors fro molecules such as hormones.
How are intrinsic proteins embedded in the phospholipid bilayer sheet and what is their function?
Completely span bilayer sheet from one side to another.
Some act as carriers to transport water-soluble material across the membrane while others are enzymes.
What are the functions of the proteins in the membrane? (6)
-Act as carriers transporting water soluble substances across membrane
-Allow active transport across membrane by forming ion channels.
-form recognition sites by identifying cells
-help cells adhere together -act as receptors
What is the name of the structure of the cell surface membrane. Why is it called this?
FLUID- Individual phospholipid molecules can move relative to one another. Flexible structure constantly changing shape.
MOSAIC- proteins are embedded in bilayer vary in shape, size and pattern.
What kind of transport is diffusion? Active or passive?
Passive- the energy comes from the natural, inbuilt motion of particles rather than from an external source.
The net movement of molecules or ions from a region where they are more highly concentrated to one where their concentration is lower.
What is dynamic equilibrium?
When the amount of particles on each side are the same but individual particles are continuously changing position.
What 3 factors affect the rate of diffusion?
CONCENTRATION GRADIENT- the greater the difference in concentration on either side of the exchange surface, the faster the rate of diffusion.
AREA- The larger the area of an exchange surface, the faster.
THICKNESS- the thinner the exchange surface, the faster the re of diffusion.
What is diffusion proportional to?
surface area X difference in concentration/ length of diffusion path
What two factors also affect diffusion in cells?
THE NATURE OF THE PLASMA MEMBRANE- its composition and number of pores
THE SIZE AND NATURE OF DIFFUSING MOLECULE- smaller molecules diffuse faster than bigger ones. Fat soluble faster than water soluble. Polar faster than non- polar
Explain facilitated diffusion.
-Occurs down a concentration gradient
-Occurs at specific points on the plasma membrane where there are special protein molecules.
-These proteins form water filled channels across the membrane.
-Allow water-soluble ions to pass through. Such molecules would usually diffuse v. slowly.
-The channels are selective, each only opening in presence of a specific ion. If not present, channel stays closed.
-Control over entry and exit of substances.
Explain facilitated diffusion involving carrier protein.
-Carrier proteins span plasma membrane.
-When a molecule (e.g. glucose) that is specific to the protein is present, it binds with the protein.
-This causes the protein to change shape in a way that the molecule is released to the inside of the membrane.
-No external energy needed.
-High conc. - > Low conc.
the passage of water from a region where it has a higher water potential to a region where it has a lower water potential through a partially permeable membrane.
What is a solute?
Any substance that is dissolved in a solvent.
What is water potential?
The pressure created by water molecules. Pure water is said to have a WP of 0
How is water potential affected by the addition of a solute?
Addition of solute to pure water will lower its water potential. Water potential of solution always less than 0. More solute added the lower (more -ve) its water potential.
How can you find out the water potential of cells or tissues?
Place them in a series of solutions with different WPs.
No gain/loss of water, water potential inside cells must be same as external solution.
What happens in cells if water is too high/low or equal?
High- burst because of thin/ unstretchy membranes. Swells and becomes TURGID
Equal- Nothing, no change, INCIPIENT PLASMOLYSIS
Low- water leaves and cell becomes shrivelled. Shrinks. PLASMOLYSED.
Define active transport.
The movement of molecules or ions into or out of a cell from a region of lower concentration to a region of higher concentration using energy and carrier molecules.
How does active transport differ from passive forms of transport? (4)
-Metabolic energy (ATP) needed.
-Against a concentration gradient.
-Carrier proteins which act as pumps involved.
-Very selective process.
How does active transport use ATP?
In one of two ways:
1. Using ATP directly to move molecules
2. Using a concentration gradient that has already been set up by direct active transport.
Explain active transport.
-Carrier proteins span cell-surface membrane and accept molecules or ions to be transported on one side of it.
-Molecule/ion binds to receptors on the channels of carrier protein.
-On inside of cell ATP binds to protein, causing it to split into ADP and a phosphate molecule. Which causes protein molecule to change shape and opens on other side of membrane.
-Molecules/ions released on other side of membrane.
-Phosphate molecule released from protein and recombines with ADP to form ATP during respiration.
-Causes protein to revert to original shape. Sometimes more than one molecule/ ion moved in same direction at same time. Occasionally, molecule/ion moved into cell at same time as different one is being pumped out.
How is the small intestine adapted for absorption?
Walls folded in to villi.
Thin walls, lined with epithelial cells, on the other side of which is a rich network of blood capillaries.
Villi increase SA- increase rate of absorption. Lined with microvilli, finger like projections of cell-surface membrane.
Where are the villi situated in the small intestine and how do their properties increase efficiency of absorption?
Between lumen (cavity) of intestines and the blood and other tissues in body.
-Increase SA for diffusion
-V. thin wall
-Able to move and so help to maintain a diffusion gradient
-Well supplied with blood vessels so blood can carry away absorbed molecules and maintain a diffusion gradient.
Explain the role of diffusion in absorption in the small intestine.
Carbohydrates being digested continuously so normally greater concentration of glucose within small intestine than in blood.
THEREFORE A DIFFUSION GRADIENT.
Blood constantly being pumped by the heart, glucose absorbed into it is continuously being removed by the cells as they use it up during respiration.
Helps maintain a diffusion gradient.
Also villi contain muscles which regularly contract, mixing contents of small intestine.
Ensures that as glucose is absorbed from food adjacent to villi, new glucose rich food replaces it.
Explain the role of active transport in absorption in the small intestine.
If equilibrium is reached from diffusion, by active transport glucose can still be absorbed so all glucose can be absorbed into blood.
Actual mechanism of glucose from small intestine to blood is called co-transport (two molecules involved).
Glucose drawn into cells along with sodium ions that have been actively transported out by SODIUM-POTASSIUM PUMP.
Explain sodium-potassium pump and involvement in active transport when absorbing glucose from small intestine to blood.
-Sodium ions transported out of epithelial cells, by S-P pump, into blood. Takes place in one type of protein-carrier molecule found in cell-surface membrane of epithelial cells.
-Now much higher conc. of sodium ions in lumen than inside epithelial cells
-Sodium ions diffuse into epithelial cells down conc. gradient through a different type of protein carrier (co-transport protein).
-As sodium ions flood back through second carrier protein, they couple with glucose molecules which are carried into cell with them.
-Glucose passes into blood plasma by facilitated diffusion using another type of carrier.
Describe structure of a bacterial cell. (9)
Small cell wall made up of peptidoglycan (polymer).
Many further protect themselves by secreting a capsule of mucilaginous slime around cell wall.
Cell surface membrane
Store food as glycogen granules or oil droplets
Genetic material in form of circular strand of DNA.
Separate from this are smaller circular pieces of DNA called PLASMIDS (may give bacterium resistance to harmful chemical)
How does the cholera bacterium cause disease?
-Almost all bacterium digested are killed off by acidic conditions in stomach. Few may survive
-When reach s. intestine they use their flagella to propel themselves (corkscrew like fashion) through mucus lining of intestinal wall.
-Start to produce toxic protein which has two parts.
- binds to specific carbohydrate receptor on cell- surface membrane. Only epithelial cells of s. intestine have these so cholera only affects this region of body.
TOXIC- enters epithelial cells. Causes ion channels of cell-surface membrane to open, so CHLORIDE IONS flood out into lumen of intestine.
-Loss of chloride ions from epithelial cells raises their water potential, while increase in lumen lowers its water potential. Water therefore flows from cells into lumen.
-Loss of ions from epithelial cells establishes conc. gradient. Ions therefore move by diffusion into epithelial cells from surrounding tissues, incl. blood. This then creates a water potential gradient hat causes water to move by osmosis from the blood and other tissues into the intestine.
What are the main symptoms of cholera?
Diarrhoea and dehydration Some people show few symptoms Some people are just carriers
How is cholera transmitted?
Ingestion of water, or food (more rare), that has been contaminated with faecal matter containing the pathogen.
What are the features of a prokaryotic cell?
No nucleus, only a diffuse are of nuclear material with no nuclear envelope.
Circular strands of DNA but no chromosomes.
No membrane-bounded organelles
Smaller ribosomes (70S type)
No endoplasmic reticulum or golgi apparatus and lysosomes
Cell wall made of peptidoglycan
What are the feature of a eukaryotic cells?
Distinct nucleus, with nuclear envelope
Membrane- bounded organelles, such as mitocondria chloroplast in plant cells
Larger ribosomes (80S type)
Endoplasmic reticulum resent along with golgi apparatus and lysosomes
Cell wall made from glucose (where present)
What features do eukaryotic and prokaryotic cells share?
Cell surface membrane
If present- flaggelum
What causes diarrhoea?
-Damage to epithelial cells linig the intestine
-Loss of microvilli due to toxins
-Excessive secretion of water due to toxins.
What is the result of diarrhoea
Excessive fluid is lost from the body and/or insufficient fuid is taken in to make up for loss. Dehydration results and this could be fatal.
Why is just drinking water not sufficient for rehydration?
-Water is not being absorbed from the intestine.
-Drinking of water does not replace electrolytes (ions) being lost from epithelial cells in intestine.
How can an oral rehydration solution be taken?
-Intravenously but requires trained personel
How does oral rehydration solution work?
Makes use of alternative carrier proteins in the cell surface membranes of the epithelial cells that transport sodium ions.
As sodium ions are absorbed, water potential of the cells falls and water enters the cells by osmosis.
What does a rehydration solution need to contain and why?
Water- rehydrate tissues
Sodium- to replace the sodium ions lost from the epithelium of the intestine and to make optimum use of alternative sodium carrier proteins.
Glucose- to stimulate the uptake of sodium ions from the intestine and to provide energy.
Potassium- to replace lost potassium ions and to stimulate appetite
other electrolytes- such as chloride and citrate ions, to help prevent electrolyte imbalance.
How are oral rehydration solutions made readily available for masses?
They can be packaged as a powder, which can be made into solution with boiling water. Administered with minimal training.
How should ORS be taken?
Given regularly , in large amounts, throughout illness.