topic 2A - cells & transport Flashcards
cell surface membrane
-controls the exchange of materials between the inside & outside of the cell
-partially permeable
-formed from a phospholipid bilayer
cell wall
-permeable to most substances
-offers structural support to cell
nucleus
-contains chromatin (genetic material of the cell) from which chromosomes are made
-separated from the cytoplasm by a nuclear envelope (has many pores)
chromosomes
made of sections of linear DNA wound around proteins called histones
mitochondria
-site of aerobic respiration
-surrounded by double-membrane, inner membrane folds to form cristae
-the matrix formed by the cristae contains enzymes needed for respiration
-small circular pieces of DNA and ribosomes are found in the matrix
chloroplast
-larger than mitochondria
-surrounded by a double-membrane
-thylakoids containing chlorophyll stack to form grana
-grana are joined together by lamellae
-site of photosynthesis
-contains small circular pieces of DNA and ribosomes
ribosome
-formed in nucleolus
-made of almost equal amounts of RNA and protein
-free in the cytoplasm/part of the RER in eukaryotic cells
-site of translation
endoplasmic reticulum
RER:
-surface covered in ribosomes
-formed from continuous folds of membrane
-processes proteins made by the ribosomes
ER:
-no ribosomes
-involved in the production, processing and storage of lipids, carbohydrates
golgi apparatus
-flattened sacs of membrane
-modifies proteins and lipids before packaging them into golgi vesicles
large permanent vacuole
(+ in animals)
-sac in plant cells surrounded by the tonoplast
-selectively permeable membrane
-vacuoles in animal cells are small and not permanent
vesicle
membrane-bound sac for transport and storage
lysosome
-specialist forms of vesicles
-contain hydrolytic enzymes
-break down waste materials
microvilli
increase the surface area for absorption
cilia
-hair-like projections made from microtubules
-allows the movement of substances over the cell surface
flagella
-made of microtubules
-rotate in whip like motion to move cell
tissue
group of cells that work together to perform a particular function
organs
different tissues work together to form organs
prokaryotic cells: key facts
-smaller than eukaryotic cells
-cytoplasm lacks membrane-bound organelles
-ribosomes are smaller than those in eukaryotic cells
-no nucleus
-a cell wall that contains murein
plasmids
-small circular loops of DNA
-contain genes that can be passed between prokaryotes
slime capsule
-final outer layer
-helps to protect bacteria from drying out and from attack
structures that are always present in prokaryotic cells:
-cell wall
-cell membrane
-cytoplasm
-circular DNA
-ribosomes
structures that are sometimes present in prokaryotic cells:
-flagellum
-capsule
-infolding of cell surface membrane (may allow photosynthesis)
-plasmid
-pili (for attachment to other cells/surfaces, involved in sexual reproduction)
what are viruses?
-not alive
-much smaller than prokaryotic cells
-all viruses are parasitic, they can only reproduce by infecting living cells and using their ribosomes to produce new viral particles
structure of viruses
-a nucleic acid core (their genomes are either DNA or RNA)
-a protein coat (capsid)
-some viruses have an an envelope formed from the phospholipids of a cell they were made in
-legs for attaching
magnification
how many times bigger the image produced by the microscope is than the real-life object you are viewing
resolution
the ability to distinguish between objects that are close together
what do optical microscopes use?
use light to form an image
resolution of optical microscopes
-using light, can’t distinguish between two objects that are closer than half the wavelength of light
low resolution
-maximum resolution of around 0.2 micrometres
what can optical microscopes be used to observe?
-eukaryotic cells, their nuclei (possibly mitochondria and chloroplasts)
-living specimens
maximum magnification of optical microscopes:
1500x
what do electron microscopes use?
-use electrons to form an image
resolution of electron microscopes
-high resolution
-a beam of electrons has a much smaller wavelength than light
-maximum resolution of 0.0002 µm
what can electron microscopes be used to observe?
small organelles
what is the maximum magnification of electron microscopes?
×1,500,000
what are the two types of electron microscopes?
transmission electron microscopes:
(TEMs)
scanning electron microscopes:
(SEMs)
how do TEMS work?
-use electromagnets to focus a beam of electrons
-beam of electrons is transmitted through the specimen
-denser parts of the specimen absorb more electrons & appear darker on the final image produced
advantages of TEMs
-give high-resolution micrographs
-allows small organelles to be seen
disadvantages of TEMs
-can only be used with very thin specimens
-cannot be used to observe live specimens (as there is a vacuum inside a TEM, all the water must be removed from the specimen and so living cells cannot be observed)
-the lengthy treatment required to prepare specimens means that artefacts can be introduced (artefacts look like real structures but are actually the results of preserving and staining)
-do not produce a colour image
how do SEMS work?
-scan a beam of electrons across the specimen
-beam bounces off the surface of the specimen and the electrons are detected, forming an image
-this means SEMs can produce 3D images that show the surface of specimens
advantages of SEMs:
-can be used on thick or 3-D specimens
-allow the 3-D structure of specimens to be observed
disadvantages of SEMs:
-lower resolution images than TEMs
-cannot be used to observe live specimens
-no colour image
electron microscope vs light microscope (basic differences)
-large and can’t be moved (e) v small and easy to carry (l)
-vacuum needed (e) v no vacuum needed (l)
-complicated sample preparation (e) v simple sample preparation (l)
-over 500,000x magnification (e) v up to 2,000x magnification
-specimens are dead (e) v specimens can be living (l)
cell fractionation
the process of separating cell organelles from each other
homogenisation steps
-the sample of tissue must be placed in a cold, isotonic buffer solution
-the tissue-containing solution is then homogenised using a homogeniser
that grinds the cells up
why is the solution ice-cold?
to reduce the activity of enzymes
why is the solution isotonic?
to prevent the shrinkage or expansion of the organelles via osmosis
why is the solution buffered?
to prevent organelle proteins from becoming denatured
what is the objective of homogenisation?
to break the membrane of the cells and access the organelles
steps of filtration:
-the homogenate (containing the homogenised cells) is then filtered through a gauze to separate out any large debris
-this leaves a filtrate that contains a mixture of organelles
steps of ultracentrifugation
1) the filtrate is placed into a tube and the tube is placed in a centrifuge (machine that separates materials by spinning)
2) the filtrate is first spun at a low speed, the largest, most dense organelles settle at the bottom of the tube & form a thick sediment (pellet)
3) the rest of the organelles stay suspended in the solution above the pellet (supernatant)
4) the supernatant is drained off and placed into another tube, which is spun at a higher speed, heavier organelles settle at the bottom of the tube, forming a new pellet and leaving a new supernatant
5) the new supernatant is drained off and placed into another tube, which is spun at an even higher speed. this process is repeated at increasing speeds until all the different types of organelle present are separated out (or just until the desired organelle is separated out)
6) each new pellet formed contains a lighter organelle than the previous pellet
order of organelle collection in ultracentrifugation
nuclei
chloroplasts
mitochondria
lysosomes
endoplasmic reticulum
ribosomes
cell cycle
the regulated sequence of events that occurs between cell divisions
3 phases of the cell cycle:
interphase
nuclear division (mitosis)
cell division (cytokinesis)
what happens in interphase?
the cell increases in mass and size and carries out its normal cellular functions
3 phases of interphase:
G1 phase
S phase
G2 phase
what happens in the G1 phase
cells make the proteins required for growth (eg: enzymes)
what happens in the S phase?
the DNA in the nucleus replicates
what happens in the G2 phase?
the cell continues to grow and the new DNA that has been synthesised is checked and any errors are usually repaired
what is mitosis?
nuclear division where two genetically identical daughter nuclei are produced
why is mitosis useful?
-lets unicellular zygotes grow into multicellular organisms
-damaged tissues can be repaired by mitosis
asexual reproduction
offspring are genetically identical to the single parent used to produce them
prophase
-chromosomes condense and are visible
-the two centrosomes move towards opposite poles
-spindle fibres begin to emerge from the centrosomes
-the nuclear envelope breaks down
metaphase
-centrosomes reach opposite poles
-chromosomes line up at the equator
-spindle fibres (protein microtubules) reach the chromosomes and attach to the centromeres
anaphase
-the sister chromatids separate at the centromere
-spindle fibres begin to shorten
-the chromosomes are pulled to opposite poles by the spindle fibres
telophase
-chromosomes arrive at opposite poles and begin to decondense
-nuclear envelopes reforms around each set of chromosomes
-the spindle fibres break down
cytokines
the cytoplasm divides & two genetically identical cells form
how do cancers start?
when changes happen in the genes that control cell division (mutation)
how is mitosis affected in cancer?
uncontrolled mitosis, which forms a tumour
what is an oncogene
a mutated gene that causes cancer
are mutations common?
yes
do mutations usually lead to cancer?
no:
-most mutations either result in early cell death or result in the cell being destroyed by the body’s immune system
(most cells can be easily replaced, so these events usually have no harmful effect on the body)
what are carcinogens?
any agents that may cause cancer
examples of carcinogens:
-UV light
-tar in tobacco smoke
-x-rays
which tumours don’t _____
benign tumours don’t spread from their original site & don’t cause cancer
which tumours invade neighbouring tissue?
malignant (cause cancer)
what do malignant tumours do to organs/tissue?
interfere with the normal functioning of the organ / tissue in which they have started to grow
metastasis
tumour cells can break off the tumour and travel through the blood to form secondary growths in other parts of the body
why is metastasis dangerous?
it can be very difficult to detect, locate and remove secondary cancers
stages in the development of cancer
1) oncogenes arise due to carcinogens
2) cancerous cell does not respond to signals from other cells & continues to divide
3) cancerous cells aren’t removed by the immune system
4) the tumour gets bigger due to absorption of nutrients leading to rapid mitosis
5) tumour is supplied with blood, if the tumour is malignant, tumour cells spread in blood to other parts of the body
6) metastasis (tumour cells invade other tissues, secondary tumours form throughout the body)
what mutated gene do most people with cancer posses?
a mutated p53 gene (a gene that helps to control cell growth)
can viruses cause cancer?
yes, some oncoviruses cause cancers
how do most current cancer treatments work?
by controlling the rate of mitosis
methotrexate
inhibits the synthesis of DNA nucleotides in cells
vincristine and taxol
prevent the formation of the mitotic spindle
process of binary fission:
1) the single, circular DNA molecule undergoes DNA replication
2) any plasmids undergo DNA replication
3) the parent cell divides into two cells, with the cytoplasm halved between the two daughter cells
4) the two daughter cells each contain a single copy of the circular DNA molecule and a variable number of plasmids
processes of viral replication
1) the virus attaches to the surface of the host cell
2) the viral DNA/RNA is injected into the host cell
3) viral DNA/RNA replicates, new viral proteins are made (new capsids)
4) new viral particles are assembled
5) host cell lyses (bursts) releases the newly made virus
6) host cell is destroyed as new virus leaves
what does the cell surface membrane create?
-an enclosed space separating the internal cell environment from the external environment
phospholipid monolayer (structure)
-if phospholipids are spread over the surface of water
-they form a single layer with the hydrophilic phosphate heads in the water and the hydrophobic fatty acid tails sticking up away from the water
phospholipid bilayer (structure)
two layers of phospholipids; their hydrophobic tails facing inwards and hydrophilic heads outwards
role of phospholipids
1) ensures needed water-soluble molecules cannot leak out of the cell and unwanted water-soluble molecules cannot get in
2) can be chemically modified to act as signalling molecules by:
↳ moving within the bilayer to activate other molecules (eg. enzymes)
↳ being hydrolysed which releases smaller water-soluble molecules that bind to specific receptors in the cytoplasm
why does the fluid mosaic model describe cell membranes as fluid
-the phospholipids and proteins can move around via diffusion
-the phospholipids mainly move sideways, within their own layers
why does the fluid mosaic model describe cell membranes as mosaic
-different molecules of different sizes look like a mosaic from above
structure of cholesterol
-cholesterol molecules have hydrophobic tails & hydrophilic heads
-fit between phospholipid molecules & oriented in the same way
-absent in prokaryotes membrane
role of cholesterol in the membrane
1) cholesterol molecules sit in between the phospholipids, stopping them from packing too closely together (when temperatures are low; this stops membranes from freezing and fracturing)
2) interaction between cholesterol and phospholipid tails stabilises the cell membrane at higher temperatures by stopping the membrane from becoming too fluid
↳ cholesterol molecules bind to the hydrophobic tails of phospholipids, stabilising them and causing phospholipids to pack more closely together
3) increases strength and stability of membranes; without it membranes would break down and cells burst
structure of glycolipids
(lipids with carbohydrate chains attached)
-carbohydrate chains project out into whatever fluid is surrounding the cell -found on the outer phospholipid monolayer
role of glycolipids in the membrane
-glycolipids contain carbohydrate chains that exist on the surface which enables them to act as receptor molecules
↳ allows glycolipids to bind with certain substances at the cell’s surface
-some act as cell markers or antigens, for cell-to-cell recognition
structure of glycoproteins
-proteins with carbohydrate chains attached
-these carbohydrate chains also project out into whatever fluid is surrounding the cell (they are found on the outer phospholipid monolayer)
role of glycoproteins
-glycolipids contain carbohydrate chains that exist on the surface which enables them to act as receptor molecules
↳ allows glycolipids to bind with certain substances at the cell’s surface
-some act as cell markers or antigens, for cell-to-cell recognition
what are the three types of receptor cells?
1) signalling receptors for hormones and neurotransmitters
2) receptors involved in endocytosis
3) receptors involved in cell adhesion and stabilisation (as the carbohydrate part can form hydrogen bonds with water molecules surrounding the cell)
structure of proteins in the membrane
can be intrinsic:
embedded in the membrane of phospholipids
OR
can be extrinsic:
found on the outer surface of the membrane
when are membranes less fluid?
1) when there is an increased proportion of saturated fatty acid chains, the chains pack together tightly and therefore there is a high number of intermolecular forces between the chains
2) at lower temperature as the molecules have less energy and therefore are not moving as freely which causes the structure to be more closely packed
when are membranes more fluid?
1) an increased proportion of unsaturated fatty acid chains as these chains are bent, which means the chains are less tightly packed together and there are less intermolecular forces
2) at higher temperatures, the molecules have more energy and therefore move more freely, which increases membrane fluidity
what is diffusion?
the movement of a substance from a region of its higher concentration to a region of its lower concentration
what happens after diffusion?
molecules reach an equilibrium, they are evenly spread within a given volume of space
which factors affect the rate of diffusion?
-concentration gradient
-temperature
-surface area
-properties of molecules/ions
how does concentration gradient affect the rate of diffusion?
-the difference in the concentration of the substance on the two sides of the surface
-a greater difference in concentration means a greater difference in the number of molecules passing in two directions
how does temperature affect the rate of diffusion?
-molecules and ions have more energy at higher temperature
-the move faster leading to a higher rate of diffusion
how does surface area affect the rate of diffusion?
-the greater the surface area, the more molecules/ions can cross the surface at any particular moment -> diffusion is faster
-surface area can be increased by folding
how do the properties of ions or molecules affect the rate of diffusion?
-large molecules diffuse slower than smaller ones, they need more energy to move
-uncharged molecules diffuse directly across the phospholipid bilayer
what is facilitated diffusion?
-for charged substances that cannot diffuse through the phospholipid bilayer of cell membranes
-these substances can only cross the phospholipid bilayer with the help of certain proteins (channel or carrier)
what are channel proteins?
-water-filled pores that are gated
-part of the channel protein on the inside can move to close or open the pore
-substances move along conc gradient
what are carrier proteins?
-can switch between two shapes
-the binding site of the carrier protein is open to one side of the membrane first, and then open to the other side of the membrane when the carrier protein switches shape
-the direction of movement depends on their conc on each side of the membrane
what is osmosis?
the diffusion of water molecules from a dilute solution / high water potential to a more concentrated solution / low water potential) across a partially permeable membrane
what is water potential?
the tendency of water to move out of a solution
what is the water potential of pure water
0kPa
why can water pass through the phospholipid bilayer?
although water molecules are polar, they can still pass through the bilayer because of their small size
the more negative the water potential, the _____ the water potential
lower
what will happen if a plant cell is placed in pure water or a dilute solution?
-water will enter the plant cell through its cell surface membrane by osmosis
-the volume of the plant cell increases
-the expanding protoplast pushes against the cell wall and pressure builds up inside the cell (the inelastic cell wall prevents the cell from bursting & stops too much water entering)
-when a plant cell is fully inflated with water, it is fully turgid
why is turgidity important for plants?
-if all the cells in a plant are firm then l support and strength are provided for the plant
-the plant can upright with its leaves held out to catch sunlight
-if plants do not receive enough water the cells cannot remain turgid and the plant wilts
what will happen if a plant cell is placed in a concentrated solution?
-water will leave the plant cell through its partially permeable cell surface membrane by osmosis
-as water leaves the vacuole of the plant cell, the volume of the plant cell decreases
-the protoplast gradually shrinks and no longer exerts pressure on the cell wall
-as the protoplast continues to shrink, it begins to pull away from the cell wall
(plasmolysis)
why are loses and gains of water more severe with animal cells?
animal cells do not have a supporting cell wall
what happens if an animal cell is placed in a solution with a lower water potential than the cell?
-water will leave the cell through its partially permeable cell surface membrane by osmosis and the cell will shrink and shrivel up
what happens if an animal cell is placed in a solution with a higher water potential than the cell?
-water will enter the cell membrane by osmosis
-the cell will continue to gain water by osmosis until the cell membrane is stretched too far and the cell bursts (cytolysis)
isotonic
the solution outside of the cell has the same solute concentration as the inside of the cell
