cells, bio mols and membranes

what is magnification?

how many times larger an image is than an object

what is resolution?

the ability to distinguish between two separate points

how to calibrate the graticule?

• fix the stage micrometer into place on the stage

• look through the eyepiece to line up the micrometer and graticule

• count the number of graticule divisions that fit into one micrometer division

• use formula: gractiule division = size of one micrometer division/number of graticule divisions

steps of using a light microscope

• clip prepared slide onto stage

• select objective lens with lowest power

• use coarse focus to bring stage just below objective lens

• look down eyepiece and use coarse focus to move stage downwards until image is roughly in focus

• use fine focus to make image clearer

• if high magnification is needed then swap to a more powerful objective lens and refocus

why do electron microscopes have a better resolution than light microscopes?

electrons have a shorter wavelength than light

what are artefacts in terms of microscopes?

visible details than aren’t part of the specimen being observed such as air bubbles or fingerprints

how do transmission electron microscopes work?

they use electromagnets to transmit a beam of electrons through a specimen - the denser parts absorb more electron and so appear darker in the image

how do scanning electron microscopes work?

they scan a beam of electrons across the surface of a specimen and use the reflected electrons to form an image

advantages of SEM over TEM

• SEMs can be used on thicker specimens

• SEMs produce a 3D image

magnification of each microscope

Light: x1500

TEM: x1,500,000

SEM: x1,500,000

resolution of each microscope

light: 0.2 micrometers

TEM: 0.5 nanometers

SEM: 5 nanometers

structure of the nucleus

• contains genetic information in the form of chromosomes

• contains a structure known as a nucleolus

• surrounded by a nuclear envelop (double membrane) containing pores

functions of the nucleus

• controlling the cell’s activities: contains DNA with instructions to produce proteins

• synthesis of ribosomes: the nucleolus makes ribosomal RNA

• exchange between nucleus and cytoplasm: substances can enter or leave the nucleus via the nuclear pores

structure of the cell-surface membrane

• found on the surface of animal cells

• mainly made up of lipids and protein

functions of the cell-surface membrane

• controls movement of substances into and out of the cell: the membrane is partially permeable

• cell signalling: receptors can detect signals from other cells

structure of the mitochondria:

• contains an enzyme-rich liquid known as the matrix

• surrounded by a double membrane in which the inner membrane is folded to form structures known as cristae

• contain their own DNA and ribosomes

function of mitochondria

site of aerobic respiration - produces ATP

structure of ribosomes

• made up of proteins and rRNA

• consists of a large and small subunit

• not surrounded by a membrane

function of ribosomes

site of protein synthesis: involved in the process of translation

structure of Golgi apparatus 

• contains fluid-filled, membrane-bound sacs known as cisternae 

• contain smaller vesicles

functions of the Golgi apparatus

• process and package lipids and proteins: carried out by the cisternae

• store and transport lipids and proteins: carried out by the vesicles

• synthesis lysosomes: specialised vesicles

structure of the rough endoplasmic reticulum

• contains a network of membranes enclosing a fluid-filled space, known as cisternae

• the surface of the cisternae is covered with ribosomes

function of the RER

synthesis and transport of proteins: proteins are made using the ribosomes

structure of the smooth endoplasmic reticulum

same as rough endoplasmic reticulum without the ribosomes

function of the SER

synthesis, storage and transport of lipids and carbohydrates: such as cholesterol and steroid hormones

structure of lysosomes

• contain hydrolytic enzymes

• surrounded by a membrane to keep enzymes separate from the cytoplasm of the cell

functions of the lysosomes

• digest pathogens: this process uses enzymes

• breaks down waste material: this material includes old organelles and cells

what does the cytoskeleton do?

it is present throughout the cytoplasm and provides structure and support to cell - consists of 3 components

cytoskeleton component: microfilaments

• made of the protein actin

• involved in cell movement and locomotion such as crawling and contraction

cytoskeleton component: microtubules

• made of the protein tubulin

• form a scaffold-like structure throughout the cell

• form the main component of the mitotic spindle, used for cell division

• acts as tracks for intracellular transport of vesicles and organelles

cytoskeleton component: intermediate filaments

• maintain the position of organelles within the cell

• responsible for the mechanical strength of certain tissues such as skin and hair

structure of cell wall

• made of cellulose

• contains channels known as plasmodesmata

functions of cell wall

• supports the cell: contents of the cell press against the wall to make it rigid

• prevents the cell from bursting: the cell wall can withstand high osmotic pressure 

• allows exchange of substances between cells: plasmodesmata connects neighbouring cells 

structure of chloroplasts:

• contain fluid-filled sacs known as thylakoids which are stacked up to form grana

• surrounded by a double membrane enclosing a fluid known as stroma

• contain their own DNA and ribosomes

function of chloroplasts

site of photosynthesis: these reactions take place in the grana and stroma

structure of the vacuole

• contains cell sap: solution fo salts and sugars 

• surrounded by a selectively permeable membrane known as a tonoplast

function of vacuole

helps to maintain pressure within the cell which keeps the cell rigid and stops the plant from wilting

algal cells

• contain the same organelles as plant cells

• contain chloroplasts with different shapes to those found in plant cells

fungal cells 

• contain most of the organelles found in plant cells

• do not contain chloroplasts 

• cell wall made of chitin rather than cellulose

how are prokaryotes different from eukaryotes?

• smaller

• do not contain nucleus

• do not contain membrane-bound organelles

• cell wall: made of murein (peptidoglycan)

• ribosomes: have 70S which is smaller than 80S in eukaryotes

• genetic material found in large circular strand of DNA and extra genes (antibiotic resistance) in smaller loops called plasmids

capsules

• extra layer around cell wall in prokaryotes

• structure made of polysaccharides 

• protects cells from attack by antibiotics or WBCs

flagellum

• long hair like structure in some prokaryotes

• attached to cell membrane and rotates to push cell through environment

pili

• short hair like structure on cell surface of prokaryotes

• used for attaching prokaryotic cells to other cells or surfaces

what substances have only CHO present?

glucose, starch, glycogen, triglyceride

what substances have only CHONS present?

amino acids, protein

what substances have only CHONP present?

DNA, RNA

monomer and polymer of carbohydrates

monosaccharides and polysaccharides

monomer and polymer of proteins

amino acids and polypeptides

monomer and polymer of nucleic acids

nucleotides and poly nucleotides

what is a condensation reaction?

the removal of water to form a chemical bond between two molecules

what is a hydrolysis reaction?

the addition of water to break a chemical bond between two molecules

what is an inorganic ion?

an ion that does not contain carbon

why is water a dipolar molecule?

• shared electron are pulled towards oxygen atom

• therefore the oxygen atom has a slightly negative charge 

• and the hydrogen atoms have a slightly positive charge 

• so water has both positive and negative poles

what causes water to have its useful properties?

• positive and negative ends cause the molecules to interact with one another

• the positive hydrogen attracts towards the negative oxygen of another molecule

• this is called a hydrogen bond

why is water a solvent?

• substances within cells are ionic compound

• when ionic compounds are added to water they are split apart

• the negative oxygens attract the positive ions

• the positive hydrogens attract the negative ions

• each ion is surrounded by water molecules and the compound dissolved

why is it useful for water to be the universal solvent?

• it dissolves more substances than any other liquid

• most biological reactions take place in solution (cytoplasm of cells)

• dissolved substances can be transported around the body (ions in blood plasma)

why is water a temperature buffer?

• high specific heat capacity 

• the hydrogen bonds can absorb lots of energy before being broken 

• this means water is resistant to rapid changes in temperature 

• many organisms are made of water so this allows their bodies to stay stable

why is water a cooling mechanism?

• water has a high latent heat of vaporisation

• so lots of energy is required to break the hydrogen bonds to change it from a liquid to a gas

• useful for organisms as they can use evaporation of water to cool down without losing too much water

• evaporating water takes heat energy away from the surface

why is water a useful habitat?

• high specific heat capacity and high latent heat of vaporisation

• does not change temperature or evaporate easily

• provides a stable environment

• at low temperature water freezes

• molecules are held further apart making it less dense than water

• this causes ice to float forming an insulating layer at the surface of ponds

• the water below does not freeze which allows organisms to survive

why is water a metabolite?

• involved in many chemical reactions inside organisms

• hydrolysis, condensation, photosyntheis

why is water a useful transport medium?

• water has cohesion: tendency for molecules to stick together

• water has adhesion: tendency to stick to other materials

• strong adhesion and cohesion allows water to flow through organisms and carry substances with it

• eg allows water to move through xylem

• when water molecules meet air they create high surface tension which is strong enough to support small organisms

role of carbohydrates

• energy supply for cells

• energy storage

• structural components

• cellular recognition (glycoproteins help cells identify each other)

• building blocks fro biological molecules

disaccharide examples

• maltose

• sucrose

• lactose

name 2 pentose sugars

• ribose

• deoxyribose

properties and uses of glucose

• soluble: hydroxyl groups can form hydrogen bonds with water so can be transported around organisms

• bonds store lots of energy: energy is released when the bonds are broken§

what is maltose made of?

2 glucose molecules

what is sucrose made of?

one glucose and one fructose

what is lactose made of?

one glucose and one galactose

name the two chains that starch comes in

amylose and amylopectin

structure of amylose

• long, unbranched chain of alpha glucose

• joined by 1-4 glycosidic bonds

• angles of bonds cause chain to coil into helix

• compact structure

structure of amylopectin

• long, branched chain of alpha-glucpse

• joined by both 1-4 and 1-6 glycosidic bonds

• side branches allow enzymes to hydrolyse alpha-glucose monomers

what features of starch make it a good store of energy?

• insoluble: does not affect water potential so water does not enter cells by osmosis

• large: cannot diffuse out of cells

• many side branches: allows enzymes to hydrolyse the glycosidic bonds to rapidly release glucose

• coiled: makes it compact so lots of glucose can be stored in a small space

• hydrolysis releases alpha-glucose monomers: used in respiration

structure of glycogen

• alpha glucose monomers

• joined by 1-4 and 1-6 glycosidic bonds

• highly branched chains

why is glycogen a good energy store?

• insoluble: does not affect the water potential of cells so water does not enter cells by osmosis

• compact: lots of glucose can be stored in a small space

• more highly branched than starch: enzymes can easily hydrolyse the glycosidic bonds to rapidly release glucose

• large: cannot diffuse out of cells

• hydrolysis releases alpha-glucose monomers: used in respiration

structure of cellulose

• beta glucose

• every other molecule is inverted so hydroxyl groups are close enough to react 

• long, straight, unbranched chains

• each hydrogen bond is weak but number of bonds provides strength 

what are microfibrils and macrofibrils?

• microfibrils are bundles of cellulose chains which have been tightly cross linked by hydrogen bonds

• macrofibrils are microfibrils joined together which make strong cellulose fibres

what counts as a reducing sugar?

• all monosaccharides

• some disaccharides (maltose and lactose)

what counts as a non-reducing sugar?

• some disaccharides (sucrose)

• all polysaccharides

what is a reducing sugar?

a sugar that gets oxidised in a chemical reaction

colours that determine concentration of reducing sugar - no to high

• blue

• green

• orange

• brick-red

how to test for non reducing sugars

• carry out reducing sugars test and get negative results

• add 2cm³ of food sample to 2cm³ dilute HCl

• heat mixture in gently boiling water for 5 minutes

• acid will hydrolyse the disaccharides into monosaccharides

• neutralise mixture with sodium hydrogencarbonate solution

• retest mixture with reducing sugar test

• now get brick-red precipitate if non-reducing sugars present

formula of all disaccharides

C12H22O11

what are lipids made of?

CHO

how do lipids differ from carbohydrates?

lipids contain a much lower proportion of oxygen than carbohydrates

what are the roles of lipids?

• energy supply (oxidised to provides energy to cells)

• structural components (phospholipids used in cell membranes)

• waterproofing (insoluble lipids are used to form water-resistant barriers)

• insulation (lipids can help retain heat)

• protection (delicate organs are surrounded by a layer of fat)

what are most lipids made of?

fatty acids and an alcohol

what properties do lipids with saturated fatty acids have?

• higher melting points

• solid at room temperature

what happens to the lipid chain when it contains unsaturated fatty acids?

the chains kink

properties of lipids with unsaturated fatty acids?

• lower melting point

• liquid at room temperature

test for lipids

• place food sample in test tube

• add 2cm³ ethanol

• shake

• add 2cm³ distilled water

• milky white emulsion appears if lipids are present

what is the use of triglycerides?

energy store in animals, plants and some bacteria

what features allow triglycerides to store energy efficiently?

• long hydrocarbon tails (carbon-hydrogen bonds can be broken to release energy)

• low mass to energy ratio (lots of energy can be stored in a small volume)

• insoluble (do not affect the water potential of cells as they are large and non-polar)

• high ratio fo hydrogen to oxygen atoms (will release water when oxidised)

what are phospholipids used for?

structural component of the cell membrane

parts of phospholipids

• hydrophilic head - contains glycerol and phosphate

• hydrophobic tail - 2 fatty acids

explain the arrangement of the phospholipid bilayer

• double layer where hydrophilic heads are facing outwards and the hydrophobic tails are facing inwards

• this creates a hydrophobic centre in the bilayer so that water-soluble substances cannot pass through

what is cholesterol and what does it do?

• type of lipid known as a sterol

• used by animal cells to increase the stability of the cell membrane

• used to make vitamin D, steroid hormones and bile

is cholesterol polar or not?

polar

what are the dimers of amino acids called?

dipeptides