3.2.1 cell structure and 3.2.3 transport across cell membranes Flashcards
define the term of eukaryotic cells
DNA is contained in a nucleus , contains membrane-bound specialised organelles
define the term of prokaryotic cells
single celled organism without a nucleus or membrane bound organelles
state the relationship between a system and specialised cells
specialised cells = tissues that perform specific function = organs made of several tissue types = organ systems
cell membrane function
- mainly made of lipids/proteins
- regulates movement of substances in and out of the cell by being selectively permeable
- has receptor molecules = allows it to respond to chemicals e.g. hormones
- involved in cell signalling/ recognition
structure of the nucleus
- surrounded by nuclear envelope (semi permeable double membrane)
- nuclear pores in the nuclear envelope
- dense nucleolus
function of the nucleus
- controls cells activities (by controlling the transcription of DNA)
- DNA contains instructions to make proteins
- pores allow substances (e.g. RNA) to move between the nucleus and the cytoplasm
- nucleolus makes ribosomes
description of mitochondrion
- oval shaped
- double membrane
- inner membrane is folded to form cristae
- inside is the matrix = contains enzymes involved in respiration
function of mitochondrion
-site of aerobic respiration which produces ATP (common energy source in the cell)
chloroplast description
- double membrane
- has thylakoid membranes inside (when stacked up form grana)
- grana linked together by lamellae ( thin/flat pieces of thylakoid membrane)
chloroplast function
- where photosynthesis takes place
- some parts of photosynthesis happen in the grana and some in the stroma (thick fluid found in chloroplasts)
golgi apparatus description
- group of fluid filled membrane bound flattened sacs
- vesicles often seen at the edges of the sacs
golgi apparatus function
- processes and packages new lipids/proteins
- makes lysosomes
golgi vesicle structure
- small fluid filled sac in cytoplasm surrounded by a membrane and produced by the golgi apparatus
golgi vesicle function
- stores lipids/proteins made by the golgi apparatus and transports them out of the cell via the cell surface membrane
lysosome description
- round organelle surrounded by a membrane
- no clear internal structure
lysosome function
- contains hydrolytic enzymes
- kept separate from cytoplasm by surrounding membrane and used to digest invading cells/ break down worn out components of the cell
ribosome description
- very small organelle that floats free in cytoplasm or attached to RER
- made up of proteins and RNA
- not surrounded by a membrane
ribosome function
- where proteins are made
rough endoplasmic reticulum description
- system of membranes enclosing a fluid filled space
- surface covered with ribosomes
rough endoplasmic reticulum function
folds/processes proteins that have been made at the ribosomes
smooth endoplasmic reticulum description
- similar to RER but with no ribosomes
smooth endoplasmic reticulum function
synthesis and processes lipids
cell wall description
- rigid structure surrounding the cell
- plants/algae= made of cellulose
- fungi = made of chitin
- bacteria = made of murein
cell wall function
- supports cell
- prevents them changing shape
cell vacuole description
- membrane bound organelle found in cytoplasm
- contains cell sap (weak solution of sugars/salts)
- surrounding membrane is called the tonoplast
cell vacuole function
- helps maintain pressure inside cell/ keep cell rigid
- involved in isolation of unwanted chemicals inside the cell
common cell adaptations
- folded membrane/ microvilli increase surface area e.g. for diffusion
- many mitochondria = lots of ATP for active transport
- walls one cell thick = reduces distance of diffusion pathway
prokaryotes - capsule
- made up of secreted slime
- helps protect bacteria from attack by cells of the immune system
prokaryotes - plasmids
- small loops of DNA, aren’t part of the main circular DNA molecule
- contain genes e.g. for antibiotic resistance and can be passed between prokaryotes
prokaryotes - DNA
- floats free in the cytoplasm
- circular DNA, present as one long coiled up strand
- not attached to any histone proteins
prokaryotes - flagellum
- long hair like structure, rotates to give movement
compare eukaryotes and prokaryotes
both have:
- cytoplasm
- cell surface membrane
- ribosomes
contrast eukaryotes and prokaryotes (eukaryotes)
- larger cells/often multicellular
- always have organelles/nucleus
- linear chromosomes associated with histones
- larger ribosomes (80s)
- mitosis/ meiosis = asexual/sexual
- always cytoskeleton
contrast eukaryotes and prokaryotes (prokaryotes)
- small cells/unicellular
- no membrane bound organelles/nucleus
- circular DNA not associated with proteins
- smaller ribosomes (70S)
- binary fission (always asexual reproduction)
- sometimes cytoskeleton
are viruses cells?
no: they are acellular and non-living, no cytoplasm, can’t self-reproduce, no metabolism
structure of a viral particle
- linear genetic material (DNA/RNA) and viral enzymes
- surrounded by capsid (protein coat) with attachment proteins sticking out of it
structure of an enveloped virus
- simple virus surrounded by matrix proteins
- matrix protein surrounded by envelope derived from cell membrane of host cell
- attachment proteins on surface
role of capsid
- protects nucleic acid from degradation by restriction endonucleases
- surface sites enable viral particle to bind to and enter host cells or inject their genetic material
role of attachment proteins
enable viral particle to bind to complementary sites on host cell : entry via endosymbiosis
describe how optical microscopes work
- lenses focus rays of light and magnify the view of a thin slice of specimen
- different structures absorb different amounts and wavelengths of light
- reflected light is transmitted to the observer via the objective lens and eyepiece
preparing microscope slides
- use a pipette to add a small drop of water onto the centre of the slide
- use tweezers to place a thin section of your specimen on top of the water drop
- add a drop of stain e.g. iodine
- add the cover slip (stand the slip upright on the slide, next to the water droplet. carefully tilt and lower it to avoid air bubbles)
advantages/limitations of using a light microscope
+ can show living structures
+ affordable apparatus
- 2D image
- lower resolution (maximum of 0.2 micrometres), can’t see smaller subcellular structures e.g. ribosomes
- lower magnification (*1500)
describe how a transmission electron microscope (TEM) works
- pass a high energy beam of electrons through thin slice of specimen
- more dense structures appear darker since they absorb more electrons
- focus image on fluorescent screen/photographic plate using magnetic lenses
advantages and limitations of TEMs
+ electrons have shorter wavelength than light = high resolution, ultrastructure visible
+ high magnification (*500000)
- 2D image
- requires a vacuum can’t show living structures
- extensive preparation may introduce artefacts
- no colour image
describe how a scanning electron microscope (SEM) works
scan a beam of electrons across the specimen. this knocks off electrons from the specimen, which are gathered in a cathode ray tube to form an image.
advantages and limitations of using SEMs
+ 3D image
+ can be used on thick specimens
+ electrons have shorter wavelength than light = high resolution
- requires vacuum, can’t show living structures
- only shows outer surface
define magnification
factor by which the image is larger than the actual specimen
define resolution
smallest separation distance at which 2 separate structures can be distinguished from one another
how do you use an eyepiece graticule and stage micrometer to measure the size of a structure?
- place micrometer on stage to calibrate eye piece graticule
- line up scales on graticule and micrometer. count how many graticule divisions are in 100 micrometres on the micrometer
- length of 1 eyepiece division= 100 micrometres / number of divisions
- use calibrated values to calculate actual length of structures
how do you calculate actual size?
actual size = image size / magnification
units
millimetre
micrometre
nanometre
going down: *1000
going up: /1000
cell fractionation stages
- homogenisation (breaking up the cells)
- filtration ( getting rid of the big bits)
- ultracentrifugation (separating the organelles)
homogenisation
- done e.g. by vibrating the cells or grinding them up in a blender
filtration
homogenised cell solution filtered through gauze, separates large cell/tissue debris e.g. connective tissue from organelles
ultracentrifugation
pour mixture into a tube and put it in a centrifuge and spin at a low speed. heaviest organelles go to the bottom of the tube 1st (pellet), rest of the organelles stay suspended above (supernatant). Pour supernatant into different tube and spin in the centrifuge again at a higher speed. Again the pellet/supernatant will form. repeat over and over again at higher speeds until all the organelles are separated out
name the organelles from most dense to least dense (ultracentrifugation)
nuclei, chloroplasts, mitochondria, lysosomes, RER, plasma membrane, SER, ribosomes
why are fractionated cells kept in an ice cold, buffered, isotonic solution?
isotonic - prevent osmotic lysis/ shrinking of organelles
buffered- maintaining constant pH
ice cold- reduce the activity of enzymes that breakdown organelles, slow action of hydrolase enzymes
how is the cell membrane structure described?
- as a fluid mosaic model, partially permeable (lets some molecules through but not others)
phospholipids in cell surface membrane
- from continuous bilayer
- this bilayer is fluid because the phospholipids are constantly moving
what 2 types of proteins do cell surface membranes have?
extrinsic (on inner or outer surface only) and intrinsic (span the whole membrane)
functions of extrinsic proteins in cell surface membrane
binding sites/receptors e.g. for hormones
antigens (glycoproteins)
bind cells together
involved in cell signalling
functions of intrinsic proteins in cell surface membrane
electron carriers (respiration/photosynthesis)
channel proteins (facilitated diffusion)
carrier proteins (facilitated diffusion/active transport)
cholesterol function in cell surface membrane
gives membrane stability
present in all cell membranes except bacteria
fits in-between phospholipids molecules to increase rigidity and stability by reducing lateral movement of other molecules (e.g. phospholipids)
hydrophobic so pulls together the phospholipid tails/creates further barrier to polar substances moving through the membrane
functions of glycolipids in cell surface membrane
covalently bond to polysaccharide (carbohydrate). carbohydrate portion extends outside the cell
usually on outside of membrane, used for recognition and attachment to other cells to form tissues
functions of carrier and channel proteins in cell surface membrane
assist and control the movement of water soluble ions and certain molecules across the membrane
functions of receptor proteins in cell surface membrane
recognise and bind with specific molecules outside the cell e.g. hormones and neurotransmitters
enzymes in cell surface membrane
some enzymes are located in the cell membrane
functions of glycoproteins in in cell surface membrane
a protein joined to a polysaccharide
some for recognition/some for antigens
some proteins are involved in active transport
they transport molecules or ions against a concentration gradient
phospholipids structure
form a barrier to dissolved substances
-hydrophilic head, hydrophobic tail
- hydrophobic tail has a saturated fatty acid (straight) and an unsaturated fatty acid (kinked)
- centre of bilayer is hydrophobic = membrane doesn’t allow water soluble ions (e.g ions/polar molecules) to diffuse through it
small non polar substances e.g.CO2 or H2O can diffuse through it
what to factors affect membrane permeability?
temperature and use of a solvent
how do temperatures below 0 degrees C affect membrane permeability?
phospholipids have little energy = can’t move very much, are closely packed together/ membrane is rigid
channel/carrier proteins denature (lose structure/function) = increasing membrane permeability
ice crystals may form = pierce the membrane so it is highly permeable when it thaws
how do temperatures between 0 and 45 degrees C affect membrane permeability?
phospholipids can move around/aren’t packed as tightly together - the membrane is partially permeable
as temp increases, phospholipids move more (more kinetic energy) = increased membrane permeability
how do temperatures above 45 degrees C affect membrane permeability?
phospholipid bilayer starts to melt, membrane = more permeable
water inside cell expands = puts pressure on membrane
channel/carrier proteins denature so can’t control what enters or leaves the cell = membrane permeability increases
how do solvents affect membrane permeability?
non polar solvents e.g. alcohol/ethanol can insert themselves into the bilayer
molecules can form hydrogen bonds with a phospholipid molecule near the ester bonds
pushes phospholipids out of their orderly placement increasing permeability
solvents can also denature proteins by disrupting bonds
required practical 4: investigating cell membrane permeability
method:
1. Cut beetroot into 6-10 identical cubes using a scalpel.
2. Wipe/rinse to clean off any pigment released as a result.
3. If investigating temperature: place each of the cubes of beetroot in an equal
volume of distilled water (5-15ml).
4. Place each test tube in a water bath at a range of temperatures (30-80°C).
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5. If investigating concentration of solvents: create a dilution series of ethanol
using distilled water. Ethanol concentrations should range from 0-100% ethanol.
6. Leave the samples for 20 minutes - the pigment will leak out of the beetroot.
7. Set the colorimeter to a blue filter and zero using a cuvette with distilled water.
8. Filter each sample into a cuvette using filter paper.
9. Measure the absorbance for each solution. A higher absorbance indicates
higher pigment concentration, and hence a more permeable membrane
what is simple diffusion?
the net movement of particles from an area of high concentration to an area of low concentration until equilibrium is reached. It is a passive process
how does concentration gradient affect the rate of diffusion?
the higher it is, the faster the rate of diffusion
diffusion slows down over time
how does the thickness of the exchange surface affect the rate of diffusion?
the thinner the exchange surface, the faster the rate of diffusion (particles have a shorter distance to travel)
how does the surface area affect the rate of diffusion?
the larger the surface area, the faster the rate of diffusion
what is facilitated diffusion?
small + polar molecules can diffuse directly through the membrane however larger molecules e.g. amino acids/glucose or charged particles e.g. ions/polar molecules would diffuse slowly as they are water soluble and the centre of the bilayer is hydrophobic. Therefore these particles that would take longer to diffuse do it with the help of carrier/channel proteins (facilitated diffusion)
how do carrier proteins work in facilitated diffusion?
move large molecules across the membrane
- large molecule attaches to carrier protein in the membrane
- then the protein changes shape
- this releases the molecule on the opposite side of the membrane
how do channel proteins work in facilitated diffusion?
form pores in the membrane for charged particles to diffuse
through
different channel proteins facilitate the diffusion of different charged particles
how do the number of channel/carrier proteins affect the rate of facilitated diffusion?
once all the proteins in the membrane are in use , facilitated diffusion can’t happen any faster even if you increase the concentration gradient
what is Fick’s law?
rate of diffusion is proportional to:
(surface area*difference in concentration) / diffusion distance
define osmosis
net movement of a solvent (usually water) from a high to low water potential across a semi permeable membrane until equilibrium is reached (passive process)
what is water potential?
- potential of water molecules to diffuse out of or into a solution/pressure created by water molecules
- measured in Pa
what is the water potential of pure water
at 25 degrees C and 100 kPa = 0
solutes and water potential
adding more solute decreases water potential
water potential of any solution is always negative
more negative water potential = stronger concentration of solutes in the solution
cells in isotonic solution (osmosis)
two solutions have same water potential
no net movement of water
cells in hypotonic solution
- cell placed in solution that has a higher water potential outside
- plant = protoplast swells
how does water potential gradient affect osmosis?
higher water potential gradient = faster rate of osmosis
as it takes place, difference in water potential on either side of the membrane decreases = rate of osmosis levels off over time
how does the thickness of the exchange surface affect the rate of osmosis?
thinner exchange surface = faster rate of osmosis
how does the surface area affect the rate of osmosis?
larger surface area = faster rate of osmosis
investigating water potential - required practical 3
● Paper towels
Method
1. Make a series of dilutions of 1M sucrose solution. These should be at 0.0, 0.2,
0.4, 0.6, 0.8 and 1.0M sucrose. Dilute using distilled water.
2. Measure 5cm3 of each dilution into separate test tubes.
3. Use a cork borer to cut out six potato chips and cut down the sections into
identically sized chips. Dry each chip using a paper towel to remove excess
water but do not squeeze.
4. Weigh each before the start of the experiment.
5. Place a potato chip in each test tube (one per sucrose concentration) and leave
for 20 minutes.
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6. Remove each potato chip, dry gently using paper towel, and weigh them in turn.
7. Calculate the percentage change in mass for each sucrose solution.
define active transport
the movement of molecules/ions across a plasma membrane from an area of low to high concentration using energy from ATP (produced by aerobic respiration in mitochondria). involves carrier proteins + co-transporters
what happens to ATP for active transport to occur?
ATP undergoes a hydrolysis reaction, splitting into ADP and P which releases energy so that the solutes can be transported
carrier proteins in active transport
a molecule attaches to the carrier protein, the protein changes shape, and this moves the molecule across the membrane , releasing it on the other side
co-transporters in active transport
- they are type of carrier protein
- they bind 2 molecules at a time. the concentration gradient of one of the molecules is used to move the other molecule against its own concentration gradient
- substances bind to complementary intrinsic protein
- symport: transports substances in same direction
- antiport; transports substances in opposite direction e.g. sodium potassium pump
co transport and the absorption of glucose:
step 1
sodium ions actively transported out of the epithelial cells in the ileum into the blood by a sodium potassium pump
this creates a concentration gradient- higher concentration of sodium ions in the lumen of the ileum than inside the cell
co transport and the absorption of glucose:
step 2
causes sodium ions to diffuse from the lumen of the ileum into the epithelial cell, down their concentration gradient, (done via the sodium-glucose co-transporter proteins)
co-transporter carries glucose into the cell with the sodium, so concentration of glucose inside the cell increases
co transport and the absorption of glucose:
step 3
glucose diffuses out of the cell and into the blood (down its concentration gradient) through a protein channel by facilitated diffusion
