Revision:AQA AS BIO- Cells and movement in and out of them

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Cells and Movement in and out of them

Investigating structure of cells:

Cell; Units of life; Microscopic

Microscopy

Instrument of magnification Works with a simple convex glass lens

Light microscope

More effective- than a convex lens

However

Long wavelength – magnification is limited Only distinguishes between 2 objects if over 0.2µm apart (µm= 10-6 x 2m= 0.000002m)

Electron microscope

Overcomes limitations of light microscope. As can distinguish 2 objects as close together as 0.1nm (nm= 10-9 x 1m= 0.000000001m). Uses beams of electrons instead of light. This has a shorter wavelength which improves its resolution.

Magnification

Magnification= enlargement scale of an image compared to its object. An object is material placed under the microscope. An image is the what is seen through the lens. Magnification=(Size of image)/(Size of object ) Or Size of object=(Size of image)/(Magnification ) Ensure units of length are equal.

Resolution- aka resolving power

Greater resolution= greater clarity & precision. Resolution= minimum distance between 2 objects in order for them to be viewed as separate. Dependant on wavelength and form of radiation used- (light= 0.2µm) (electron= 0.1nm). Shorter wavelength= greater resolution.

However

Increasing resolution doesn’t improve magnification. It increases the size of image. Once limit of resolution is overcome, with further magnification image becomes blurry.

Cell fractionation

To study structure and function of organelles. Essential to isolate large quantities of organelles. Cell fractionation= process of breaking cells in order to release its contents- organelles- and separating them out.

Preparation

The tissue must be placed in a cold, isotonic buffered solution. Cold- to reduce enzyme activity as it may break down organelles. Isotonic- prevent organelles from bursting/ shrinking from osmotic gain/loss of water. Buffered- maintain a constant pH.

2 stages:

Homogenisation. Cells are broken up using a blender- releases organelles. Resultant fluid= homogenate. Homogenate- filtered- removes debris/ non-homogenised cells. Ultracentrifugation- high speed spinning machine. Centrifugal force separates the filtered homogenate into fragments. Test tube 1= slow speed spin= heaviest organelles=nuclei (1000 gforce). Fluid above= supernatant- removed into test tube 2. Supernatant= spun again= next heaviest sinks= mitochondria (3500 gforce). Process repeated- lysosomes (16500) – ribosomes (100,000). Improves biological knowledge.

The Electron Microscope

Short wavelength= higher resolution- 0.1nm (2000 x better then light). Negatively charged electrons- beam can be focused using electromagnets. Near vacuum needed- electrons absorbed by air. Transmission electron microscope- TEM. 0.1nm resolution (subject to prep problems). Complete vacuum needed- living orgs cannot be observed. Electron gun= electron beam- focused using condenser electromagnet. Specimen must be extremely thin- electrons to pass= bright. Parts that absorb electrons= dark. Image appears on a screen- photomicrograph. Artifact may be present due to prep/ how it is cut. May not be valid image. Flat 2d black & white image. Series of photomicrographs= 3d image possible. Slow and complicated process. Extensive preparation: Killing- dead sample needed- vacuum. Fixing- prevents sample from distorting. Staining- increases colour and contrast as bio samples are transparent. Dehydrating- preserves sample and allows it to be embedded. Scanning electron microscope- SEM. 20nm resolution- 10 x better then light. Same limitations as TEM except… Specimens don’t need to be extremely thin- electrons do not pass through. Beam directed from above- then below. The sample is coated in heavy metals= electrons to scatter. contours are distinguished. 3d image can be produced via computer analysis.

Structure of an epithelial cell

Cell= metabolic compartment Contains chemical processed Internal components designed for its particular purpose ultra structure observation of organelle proportions and size reflects cell function e.g. mitochondrion^ number= ^ energy/metabolic rate epithelial cell= eukaryotic located- S. intestines purpose- absorb and secrete

Eukaryotic cells= membrane bound organelles

distinct nucleus 10- 20m diameter contains hereditary material- chromosome form control centre for cell activity- through mRNA production and protein synthesis- ribosomes, ribosomal mRNA nuclear envelope double membrane- outer= continuous with ER- ribosomes on surface controls passage of material in and out contains reactions nuclear pores 3000 approx in total – 40- 100nm diameter Exchange of large moles E.g. messenger RNA out of nucleus nucleoplasm granular jelly-like= bulk of nucleus nucleolus- spherical body manufactures ribosomal RNA assembles ribosomes chromatin diffused form- when not replicating hereditary material mitochondrion rod shaped 1-10m length number/ size- dependant on metabolic activity of cell muscle/ epithelial cells will have higher demand for energy= ^ mitochondrion numbers site of respiration- Krebs cycle, oxidative phosphorylation pathway responsible for ATP production- from carbohydrates energy- fuels metabolic processes- e.g. active transport double membrane surrounds organelle outer- controls entry/ exit inner folded into cristae cristae Shelf like extensions ^SA- for enzyme attachment in respiration reactions Site of ETC matrix semi rigid material- contains proteins, lipids and DNA traces Endoplasmic reticulum- ER Synth, storage and transportation Continuation of outer nuclear membrane 3D Sheet- like membrane in the cytoplasm Encloses flattened sacs- cisternae Quantity dependant on the need to manufacture/store proteins, lipids and carbs E.g. liver and secretary cells 2 forms- Rough ER Ribosomes on outer surface More sheet-like appearance

Function:

^ SA for protein/glycoprotein synth Transportation pathway of material e.g. proteins through cell Smooth ER No ribosomes More tubular appearance

Function: Synth, stores and transports lipids and carbs Golgi Apparatus Occurs in most eukaryotic cells Similar structure to SER- however more compact ^ development in cells with a secretory function e.g. epithelial cells

Function: Transport store and modifies lipids and proteins Protein modification= protein + carb (non-protein)= glycoproteins Lipid modification Production of secretory enzymes E.g. pancreatic juice Secrete carbohydrates E.g. cellulose for cell wall manufacture Form lysosomes Proteins and lipids from ER are passed through Golgi in a strict sequence modification Organisation- sorting destination Transportation- via vesicles Cisternae Stacked membranes Vesicles small round hollow structures- pinch off from cisternae ends Known as a lysosome when enzyme filled transports the organised molecules Moves to cell surface- fuse with membrane and release contents to outside Lysosomes vesicles from golgi apparatus that contain enzymes e.g. protease and lipase- approx 50 enzymes per lysosome up to 1m diameter isolates potentially harmful enzymes from the cell releases enzyme content outside the cell or to a phagocytic vesicle within cell abundant in secretory and phagocytic cells

function:

breaks down ingested material of phagocytic cells e.g. white blood cells releases enzymes to the outside of a cell (exocytosis) in order to destroy digests worn out organelles so the chemicals can be recycled completely breaks down cells after death (autocytiosis) Ribosomes Small cytoplasmic granules Found in cytoplasm or associated to the membrane of RER Important role in protein synthesis 2 types: 80s- found in eukaryotes- 25 70s- found in prokaryotes - >25 Structure 2 subunits- large and small Each contains ribosomal mRNA and proteins quantity found universally in all cells 25% of a cells dry mass Small size- large quantity Microvilli structure finger-like projections on epithelial cells function ^ SA- therefore more efficient absorption

Lipids Consist of H, C, O2 (smaller prop compared to carbs) Insoluble in water Soluble in alcohols and acetones Main group= Triglycerides (fats and oils), phospholipids and waxes Roles Main= plasma membrane Membrane flexibility Controls movement of lipid-soluble material over membrane Energy source When oxidised= >2x KJ as equal mass of carbs Waterproofing Insoluble Waxy cuticle- plants and insects- conserves water Oily secretions- sebaceous glands in skin- mammals Insulation Fat= slow heat conductor- under body surface- retains body heat Protection Surrounds delicate organs e.g. kidneys- cushioning Triglycerides Three fatty acids combined- via condensation reactions-with glycerol Glyceride mole= constant Fatty acids= variable- Over 70 varieties Carboxyl group -COOH which has a hydrocarbon attached. Saturated if chain has no ‘carbon=carbon’ double bond C linked to max number of H- saturated with H Mono-unsaturated if chain has single a double bond Poly-unsaturated if the chain has >1 double bond Double bonds cause the molecules to bend- they cannot pack closely together= liquid= oils Fats= solid (saturated fatty acids) at room temp (10-200c) Oils= liquid (unsaturated fatty acids) Phospholipids Similar structure to lipids- however 1 fatty acid is replaced by a phosphate mole Fatty acid tails- hydrophobic (repels water- attracts fat) Phosphate head- hydrophilic (attracts water-repels fat ) Phospholipids have 2 ends which behave differently= polar Phospholipids placed in water will position themselves so that phosphate head interacts with the water and the fatty acid tails are far away from the water Test for lipids- The emulsion test Using a clean test tube mix 2cm3 test sample with 5cm3 ethanol Shake thoroughly- to dissolve any lipids in sample Add 5cm3 water and shake gently Cloudy-white= lipids present Lipids in the sample have been dispersed in the water= emulsion Light passing through is refracted as it passes from oil to water droplet= cloudy appearance As a control use water instead of test sample- final solution should be clear ell-surface membrane All cell membranes- even around organelles are plasma membranes Cell-surface plasma membrane surrounds the entire cell Forms a boundary between environment and cell cytoplasm Controls movement of material in/out of the cell structural molecules in plasma membrane Phospholipids Form a bilayer sheet outer layer= hydrophilic heads point outwards- interacts with water in the cells environment inner layer= hydrophilic heads point downwards - interacts with water in cytoplasm 2 sets of hydrophobic tails point central - protected from water either side of the phosphate heads

Function:

Allows passage of lipid-soluble material via membrane Prevents passage of water-soluble material Adds flexibility to membrane Proteins Randomly embedded in the in phospholipid bilayer Extrinsic On surface/partially embedded Mechanical support Act as cell receptors= recognition site- in conjunction with glycoproteins- for molecules e.g. hormones Intrinsic Extend across the width fully embedded Act as carriers- transport water-soluble material across Allow active transport- ion channel for sodium, potassium… Form recognition sites Help cells adhere together Act as receptors e.g. for hormones Fluid mosaic model Molecules ^ are integrated together Fluid- flexible membrane, as the individual phospholipids move relative to each other Mosaic- proteins embedded in the bilayer- vary in shape, size and pattern Diffusion

Defined:

Net movement of molecules/ ions from a region of high concentrated to a low concentration down a concentration gradient in order to reach dynamic equilibrium- no net movement Exchange of substances between cells and environment absorption Passive process- energy comes from the natural kinetic energy of the particle Random movement Particles bounce of each other and other objects- sides of vessels Particle concentration within a contained space particles naturally distribute themselves evenly= diffusion only occurs between different concs of the same substance Rate of diffusion Dependant on: Concentration gradient strength Greater the difference in particle concentration on either side of an exchange surface= faster rate Area Larger SA= more rapid diffusion Thickness of exchange surface Thinner= faster rate Diffusion is proportional to: (SA × difference in concentration)/(length of diffusion path) However the rate is dependant on the structure of the plasma membrane- no of pores size/nature of molecule- small= faster and fat-soluble and polar molecules are quicker than water – soluble or non-polar Facilitated diffusion passive- kinetic energy down a conc gradient occurs at specific points along a plasma membrane usually very slow diffusion of water-soluble moles if not facilitated protein channel molecules (water filled)- opens and closes for specific molecules- very selective control allow passage of water-soluble ion/molecules e.g. glucose/ amino acids carrier proteins spans the entire width of the bilayer specific molecules can bind to it and alter the shape to allow passage down a conc gradient- passive process

Osmosis

defined: the passage of water molecules from a region of high water potential to low via a partially permeable membrane across plasma membranes- permeable to molecules e.g. water and small moles but impermeable to large moles Water potential = pressure created by water molecules Measured in units of pressure- kPa- kilopascals Pure water = = zero= under conditions of 250c & 100 kPa Solutions Solute= dissolved in a solvent Solute + solvent= solution Solutes + Pure water [0]= lower  A solution= >0 More solute added= conc solute : water=  Water moves: high low conc (more negative number) Experiments Study of cell/ tissue  Place in series of diff  solutions If no net movement=  solution matches cells/tissue Animal cells E.g. red blood cells Contain a variety of solutes dissolved in the watery cytoplasm salts, sugar and minerals usually bathed in the same  in rbc= bathed in blood plasma if external is higher (less -) = rbc bursts haemolysis contents e.g. organelles and haemoglobin are released plasma membrane of cell is broken if external is lower (more -) = rbc shrinks and shrivels the cells content= conc Plant cells structure central vacuole watery solution solutes= salts, sugars, organic acids protoplast outer cell membrane nucleus cytoplasm inner membrane cellulose cell wall inelastic tough permeable to moles of all sizes Unable to alter the external  which is usually pure water= o  Equal external = less than 0 no pressure of the protoplast against the cell wall begins to pull away= incipient plasmolysis Higher external  Usually placed in near pure water- gains water via osmosis Causes protoplasts to swell and press against the cell wall= turgid The cell wall neither breaks or expands= kPa build up Prevents further entry of water in the roots lower  loss of own water Cell volume  Protoplast completely pulled away from the cell wall= cell becomes plasmolysed Active transport Defined movement of molecules/ ions from a region of low to high concentration against a concentration gradient via carrier molecules- act as pump requires energy- ATP selective process process uses ATP directly to move molecules= direct active transport

or

over a conc gradient set up by direct active transport= co-transport single molecule- direct active transport carrier protein spanning the cell-surface membrane accepts the molecule on one side molecule binds to the proteins channel receptors ATP binds to the protein on the inside of the cell Hydrolysis= ADP +phosphate Releases energy to change the shape of the protein to open at the other side of the membrane Molecule detaches and is released to the opposite side of the membrane ATP is releases and hydrolysed = ADP + P Protein returns to original shape