Investigating Cell Structure
Magnification:How many times bigger the image on a microscope is when compared to the object
Magnification = image size/object size
Resolution: The ability to distinguish between two points on an image.
Resolving power depends on wavelength/form of radiation.
The Electron Microscope
The Electron Microscope
Uses electrons insted of light - smaller wavelength so higher resolution.
A vacuum is required and a black and white picture is produced.
Specimens must be dead.
- Transmits THROUGH specimen
- Denser parts absorb more electrons so show darker on image
- High resolution
- Only used for thin specimens
- Produces a Photomicrograph
- Only see cross-section
- Transmits ACROSS specimen
- Reflected electrons gather in cathode - forms TV image
- Can produce 3D image
- Lower resolution than Transmission
Cell Fractionation: Cells broken up & their organelles are separated out
Homogenization: Breaking cells up using a homogeniser forming a liquid homogenate. Then filtered to remove whole cells/debris
Ultracentrifugation: Fragments in homogenate separated in an ultracentrifuge (homogenate in tubes spun so fast a centrifugal force is formed)
- Filtrate tube put in ultracentrifuge (slow)
- Nuclei forced to bottom of tube - thin sediment (pellet) formed
- Supernatant removed. (left with just nuclei sediment) then transferred & spun faster
- Mitochondria forced to bottom
- Supernatant removed, transferred and spun faster
- And so on
Nuclei - 1000g-force - 10mins
Mitochondria - 3500g-force - 10 mins
Lysosomes - 16500g-force - 20 mins
Ribosomes - 100000g-force - 60mins
Nuclei - Mitochondria - Lysosomes - Endoplasmic Reticulum - Ribosomes
- System of membranes enclosing fluid filled space
- No ribosomes on surface
- Synthesises, stores & transports lipids and carbs
- Covered in Ribosomes
- Large surface area for Protein and glycoprotein synthesis
- folds & processes proteins made in ribosomes
- Pathway for material transportation
- Cells that make & store lots of carbs,lipids and proteins have lots of ER e.g. liver & secretory cells
- contain C, O and H
- Insoluble in water but soluble in oranic solvents e.g. alcohol & acetone
Triglycerides: 3 fatty acids and glycerol
Phospholipids: 2 fatty acids, glycerol and a phosphate group
- Energy source
- Waterproofing - insoluble in water
- Insulation - fats= slow heat conductors. Stored beneath body's surface
- Protection - stored around delicate organs
Emulsion test for Lipids
- Dissolve in Ethanol
- Add Water
- If Lipids present - white emulsion formed
Acid COOH group
Hydrocarbon chain (represented by 'R') - R.COOH
- Contain one (mono) or more (poly) double bonds. These double bonds mean chains can't lie together, giving them low melting points
- Liquid at room temp
- No double bonds
- Hold max. possible number of hydrogens
- High melting point
- Usually solid at room temp
Happens for each chain - 3 water molecules produced per triglyceride
Hydrolysis Reaction - opposite. Add water breaking chemical bonds in trigylceride producing gylcerol & 3 fatty acids
- Surround all living cells
- Controls how/if substances move in/out of a cell
- Similar membranes surround some organelles within the cells e.g. nuclei
- Phospholipids form a thin, flexible sheet
- Proteins float in sheet
- Carbohydrates extend out from the Proteins
Membranes - Phospholipids
- Hydrophilic phosphate heads facing outwards (interacts with water)
- Hydrophobic fatty acid tails facing inwards (repelled by water - mixes with fats)
- Arranged in a Bilayer
- Animal cell membranes also contain cholesterol to link the fatty acids together and give stability.
Membranes - Proteins
Proteins in membranes can slide around the membrane quickly but can't flip from one side to the other.
There are two types of Proteins in Membranes:
- Intrinsic - Can span from one side of the membrane to the other
Function: Act as carriers to transport water soluble materials
Help cells adhere together
Provide structural support
Allow active transport across membrane (ion channel formation)
- Extrinsic - Can sit on one of the bilayer surfaces or be partially embedded. Never extend across completely.
Function: Mechanical support to membrane
Cell receptors (with glycolipids) for molecules e.g. hormones
Membranes - Carbohydrates and Glycoproteins
Carbohydrates - extend out of proteins in the the membrane
- A protein with a carbohydrate is called a glycoprotein
- This coating of glycoproteins act as receptors and recognition sites.
- Recognition site for toxins etc.
Membranes often described as 'Fluid' and 'Mosaic'
- Fluid - phospholipids are constantly moving
- Mosaic - protein molecules are scattered through the layer.
The net movement of molecules down a concentration gradient across a partially permeable membrane.
"Passive" because no energy is needed
Lipid soluble/small molecules can diffuse easily and directly through the bilayer.
Cells cannot control diffusion.
3 factors effect rate of diffusion
- Conc. gradient - steeper the gradient, faster the diffusion
- Surface Area - Larger SA, faster diffusion
- Thickness of surface - faster if surface is thin.
Facilitated Diffusion is the transport of substance across a membrane by a transmembrane protein (intrinsic)
Transmembrane proteins are specific so substances can only cross if the correct protein is present.
- Have binding site for marticular molecule
- flip between states meaning that the sites alternate between being open to one side of the membrane, and being open to the other.Substance binds on one side and is released on the other
- Water-filled pore/channel in the membrane
- Allows ions to diffuse across
- Most can be opened or closed allowing the cell to control entry/exit of ions
Osmosis is the net movement of water molecules up a concentration gradient
Water potential is the pressure created by water molecules - the measure of the extent to which a solution gives out water.
- The higher the number of water molecules, the higher the water potential
- Pure water has a water potential of 0
Absorption in the Small Intestine
Villi and Microvilli aid absorption by
- Increasing SA for diffusion
- Thin walls reduce distance over which diffusion takes place
- Able to move - help maintain diffusion gradient
- Well supplied with blood vessels - blood can carry away absorbed molecules, maintaining diff. gradient.
Blood always flowing - microvilli moving - high conc. gradient absorbtion of glucose takes place in villi.
Lumen - hollow cavity inside a tubular structure
Sodium ions -> Epithelial cells to blood via Active transport
Potassium ions -> Blood to Epithelial cells via Active Transport
Sodium ions -> Lumen to Epithelial cells via Active Transport
Stage 1: Diffusion of Glucose from Epithelial Cells into Blood.
- Carbohydrates break down by hydrolysis giving higher conc. of glucose, which then diffuses into the blood
- This conc. becomes lower in the lumen than the blood - diffusion stops
Stage 2: Active Transport
- Sodium ions actively transported into blood via sodium-potassium pumps.
- Conc. gradient is created (higher conc. sodium ions in lumen than inside the cell)
- Ions diffuse from lumen to cell by sodium-glucose co-transporter proteins which carry glucose and sodium into the cells, increasing the glucose concentration inside the cell
- Glucose then diffuses out of the cell into the blood by facilitated diffusion.
A cell lacking a nucleus and membrane bound organelles
Bacterial Cells have:
- Cell Wall - Physical barrier that protects against mechanical damage and excludes certain substances
- Cell Surface Membrane - Differentially permeable layer controlling chemical entry/exit
- Capsule - layer of slime that protects bacterium from other cells and helps bacteria stick together
- Circular DNA - Contains genetic information for bacterial cell replication
- Flagella - used for locomotion (movement). Corkscrew shape & spinning base allows it to spin through fluid
- Plasmid - Contains genes aiding bacteria survival in extreme conditions (looks like a ring)
- Pilli - Help bacterium stick to surfaces
Cholera & ORS
Bacterium causes disease by:
- producing a toxin
- causes ion channels in intestinal epithelial cells to open
- chloride ions enter intestine
- lowers water potential
- water enters intestine by osmosis
- watery diarrhoea
Oral Rehydration Therapy
- Replaces fluids lost from diarrhoea
- Replaces water and electrolytes (ions)
- more than one type of carrier protein on cell surface membrane of epithelial cells that absorb Na+ ions
ORS replaces water by increasing uptake of sodium ions/glucose/sugars by co-transport proteins. This therefore lowers water potential in cells, so water moves from the intestines into the cells by osmosis.
More on ORS
Boiled water is used to kill bacteria/sterilise it
- mainly effects kids - consent needed
- Blind clinical trials (eliminate bias) - some given standard, some given new so can be compared - don't know which they get.
- Risk of patient dying if they don't know if new ORS is better
Early solutions w/ excess sodium had side effects, so they tried more glucose and less sodium but the extra glucose lowered the water potential in the lumen of the small intestine, drawing more water out of epithelial cells. Lowering glucose reduces energy being supplied as it acts as a respiratory substrate. Starch was used in place of glucose (large, insoluble, no osmotic effect). - broken down in small intestine by amylase & maltase -> glucose monomers. ORS was made that released glucose at optimum rate after experimenting with different concentrations.
Rice starch - readily available worldwide & is nutritionally valuable but rice flour is viscous and hard to swallow, so its partly digested with amylase first.