Studied by 4 people
Cell Definition
The basic unit of all living things. It is surrounded by a cell membrane, contains genetic material (DNA) and contains cytoplasm (which contains organelles)
What are the 2 main ideas that were discovered about cells.
All organisms are made up of cells (the basic unit of life)
All cells come from pre-existing cells by the process of cell division
Why is the cell membrane important?
It separates the activity going on inside the cell from the environment outside.
It controls the exchange of substances between the cell and the external environment.
It is partially permeable (it can act as a barrier but also allow the exchange of substances)
What are the 2 types of cells and their definitions?
Eukaryotic cells: A cell that contains a nucleus and other membrane-bound organelles.
Prokaryotic cells: A cell that doesn’t contain a nucleus or other membrane-bound organelles.
Definition of organelle.
A structurally and functionally distinct part of the cell.
Describe a eukaryotic cell.
This is the larger and more complex type of cell.
They contain a nucleus that is surrounded by 2 membranes.
The nucleus contains the DNA (genetic material).
Describe a prokaryotic cell.
This is the smaller and simpler type of cell.
It contains no nucleus, where the genetic material (DNA) is apparently free in the cytoplasm
DNA isn’t surrounded by membranes.
Definition of nucleus.
A relatively large organelle present in eukaryotic cells, but absent from prokaryotic cells. It contains the genetic material (DNA) of the cell, and therefore controls the activities of the cell. It is surrounded by 2 membranes that together makes up the nuclear envelope.
State the 2 types of microscope and the key difference between both.
Light microscope; uses light as it’s source of radiation.
Electron microscope; using electrons as it’s source of radiation
Structures in an animal cell that can be seen under a light microscope.
Cytoplasm
Cell membrane
Nucleus (contains nuclear envelope, chromatin and nucleolus - both of which are deeply staining)
Golgi apparatus
Centriole
Mitochondria
Structures in a plant cell that can be seen under a light microscope.
Cytoplasm
Cell membrane
Nucleus (contains nuclear envelope, chromatin and nucleolus - both of which are deeply staining)
Golgi apparatus
Mitochondria
Chloroplast
Cell wall
Plasmodesmata
Vacuole
Tonoplast
Definition of cell surface membrane
A very thin membrane (7nm) that surrounds all cells. It is partially permeable and controls the exchange of substances between the cell and its environment.
Describe the nucleus (using the terms chromatin, nucleolus and chromosomes).
A large structure that stains intensely.
The deeply staining material is chromatin (a mass of coiled threads)
These threads collect together during nuclear division and form chromosomes.
Chromatin contains DNA that contains a set of instructions (genes) that control the activities of the cell.
A more staining structure present in the nucleus is the nucleolus - made of loops of DNA from several chromosomes to make ribosomes.
Define chromatin.
A material from which chromosomes are made, consisting of DNA, proteins and small amounts of RNA. It is seen as patches/fibres under a microscope when stained.
Why must cells be stained?
Many cell contents are transparent and colourless. They must be stained to be seen.
In animal cells, chromatin is particularly staining, and in plant cells chloroplasts already contain chlorophyll (green pigment), the are visible without staining.
Define chromosomes.
In the nucleus of eukaryotic cells made of tightly coiled chromatin (DNA, proteins and RNA), seen during cell division.
Circular DNA is now used to express the the circular strand of DNA present in prokaryotes.
Define nucleolus.
A small structure (1/more found in the nucleus), which is deeply staining. It’s function is to manufacture ribosomes using information in it’s own DNA.
Define cytoplasm.
Contents of a cell excluding the nucleus.
Describe cytoplasm and explain how it relates to its function.
An aqueous material that contains many small structures floating around in it (called organelles).
Organelles usually are surrounded by a single/ double membrane (called an envelope)
This allows its activities to be separated from the surrounding cytoplasm.
This compartmentalisation allows the efficient working of the cell as it is a complex structure.
Define mitochondria.
An organelle in eukaryotic cells in which aerobic respiration takes place.
State the function of Golgi apparatus (for 1 mark)
A membranous structure that collects and processes molecules (particularly proteins).
State the differences between plant and animal cells.
Centrioles are present in animal but absent from plant cells
*The rest of the structures are present in plant but absent from animal cells.
Cell walls and Plasmodesmata
Vacuoles (with it’s tonoplast)
Chloroplasts
Define cell wall.
A wall that surrounds prokaryote, fungal and plant cells. It contains a strengthening material that protects the cell from mechanical damage, supports it and prevents bursting from osmosis if placed in a dilute solution.
Describe the cell wall.
It’s an extra structure which is outside the cell membrane.
The wall is relatively rigid because of the cellulose fibres (a strengthening polysaccharide)
The cell wall gives the cell a definite shape.
It prevents the cell from bursting when water enters by osmosis, allowing large pressures to develop in the cell.
Cell walls can be made extra strong by reinforcing it with more cellulose/lignin.
Cellulose is freely permeable, allowing the free movement of molecules and ions through to the cell membrane.
How are plant cells linked?
Plant cells are linked through pores in the cell walls containing fine strands of cytoplasm. They are lined with cell membrane. The movement through pores is thought to be controlled by the structure of them.
Define plasmodesmata.
A pore-like structure that is present in the plant cell wall. Plasmodesmata of neighbouring cells line up to form tube-like pores through the cell walls, this allows the controlled passage of materials from one cell to the other. The pores contains ER and are lined with cell membrane.
Define vacuole.
An organelle found in eukaryotic cells. Large, permanent central vacuoles are found in plant cells where it has a variety of functions such as the storage of biochemicals such as water, salts, sugar and waste products. Temporary vacuoles such as phagocytic vacuoles in phagocytes may form in animal cells.
Describe vacuoles and their functions.
They are sac-like structures surrounded by a single membrane.
Temporary vacuoles form in animal cells, but large, permanent central vacuoles are present in mature plant cells.
The vacuole is surrounded by a single membrane called the ‘tonoplast’.
Which controls the exchange between the vacuole and cytoplasm.
The fluid in the vacuole contains: pigments, mineral salts, sugar, water, waste products, enzymes, oxygen and carbon dioxide.
Vacuoles help regulate the osmotic properties of the cells.
Define chloroplasts.
An organelle bounded by an envelope (surrounded by 2 membranes) in which photosynthesis takes place.
Describe chloroplasts (the simple description).
Chloroplasts are found in cells in the green parts of plants (mostly in leaves).
They are relatively large organelles and so can be seen under a light microscope.
There are small grains called grana present (stacks of membranes inside chloroplasts).
Grana contains a green pigment called chlorophyll, which absorbs light during the process of photosynthesis.
Formula for magnification.
How can the size of specimens (like cells) be calculated?
Using an eyepiece graticule (small scale placed in the microscope eyepiece).
Count the number of divisions (it’s called eyepiece units)
This is calibrated using stage micrometer.
Magnification definition.
The no. times larger an image is compared to the actual object.
Resolution definition.
The ability to distinguish between 2 objects that are close together. The higher the resolution, the greater the detail can be seen.
The maximum resolution of a light microscope? Explain what that means.
200nm. This means that if objects are closer together than 200nm, they can’t be distinguished as separate.
Explain how the resolution of a light microscope is calculated.
Visible light has wavelengths ranging from 400-700nm. The brain distinguishes between these wavelengths as different colours.
400nm = violet
700nm = red
The greater the energy of the wave, the higher the frequency, the shorter the wavelength.
Half the wavelength of the radiation is the limit of resolution.
What is the problem with the light microscope?
If we use violet light (of 400nm wavelength) objects that are larger than 200nm in diameter can interfere with the radiation. However, objects smaller than 200nm don’t have any effect on the light waves.
Why are electron microscopes better options?
We must use radiation that has a shorter wavelength than visible light.
We use electrons (rather than UV/X-rays). They are -vely charged particles that orbit the nucleus of an atom. When metals become very hot, some of its e- gain so much energy that they escape from their orbits.
Free e- act like electromagnetic radiation, they have a shorter wavelength than visible light (high energy).
Electrons are better radiation:
wavelengths are very short
they are -vely charged and can be focused easily by electromagnets
They have a resolution of 0.5nm
What are the 2 different types of electron microscopes and what is the difference.
Transmission Microscope (TEM):
The beam of electrons is passed through the specimen before being viewed.
Only electrons that are transmitted can be seen (this means only thin sections of specimens can be seen). It also means that the inside of cells can be seen.
Scanning Microscope (SEM):
The electron beam is used to scan the surfaces of structures and only the reflected beam is observed. Therefore a 3D appearance is achieved. However, it can’t achieve the same resolution of TEM.
How are specimens viewed under an electron microscope?
Electron beams can’t be seen, so the electron beam must be projected onto a fluorescent screen.
Areas hit by by the electrons shine brightly to give a black and white picture.
The stains used to improve the contrast of biological specimens contains heavy metal, which stops the passage of electrons.
The more densely stained parts of specimens appear more blacker.
The e- beam must be in a vacuum, because if the e- collided with air molecules, they would scatter, making it impossible to get a clear picture.
Water also boils at room temperature in a vacuum, which means all specimens must be dehydrated before being placed in the microscope.
Only dead /non-living material can therefore be used.
Define microvilli.
Small, finger-like extensions of cells which increases the surface area of the cell for more efficient absorption/secretion.
This is common for animal cells such as epithelial cells.
Define nuclear envelope.
The 2 membranes, situated close together that surrounds the nucleus and is perforated with pores.
Define nuclear pores.
Pores found in the nuclear envelope that controls the exchange of materials (like mRNA) between the nucleus and cytoplasm.
Describe the nuclear envelope.
The nucleus is surrounded by 2 membranes to form the nuclear envelope.
The outer membrane is continuous with the endoplasmic reticulum.
The envelope has many small pores (nuclear pores). This allows and controls the exchange of materials between the nucleus and the cytoplasm.
Materials leaving the nucleus includes: mRNA, tRNA and ribosomes for protein synthesis
Materials entering the nucleus includes: proteins, nucleotides, ATP and hormones (T3-thyroid)
Why does the nucleus control the activities of the cell?
The nucleus contains genetic material, which is made of DNA. DNA is organised into functional units called genes. Genes control the activities of the cell and inheritance. Therefore, nucleus controls cell activities.
Describe chromosomes (what they’re made of).
DNA molecules are very long, they must be folded into a more compact shape in order to prevent strands from being tangled.
They are folded into a compact shape by combining them with proteins (histones).
The combination of DNA + proteins = chromatin. Chromatin also contains amounts of RNA.
Chromosomes are therefore made of chromatin.
When a cell is about to divide, what divides first and why?
The nucleus divides first, so that each new cell has its own nucleus.
Describe nucleolus.
The nucleolus is a rounded structure and is deeply staining. One or more may be present in the nucleus.
It contains a core of DNA from one or more chromosomes.
Around the core is less dense regions.
State and expand on the function of the nucleolus.
The main function of the nucleolus is to manufacture ribosomes using information from its own DNA.
The core contains the genes necessary to code for ribosomal RNA (rRNA) - the form of RNA used in manufacturing ribosomes.
It also contains the genes to code for tRNA.
The less dense regions are where ribosomal subunits are assembled - combining rRNA and ribosomal proteins from the cytoplasm.
*The more ribosomes the cell makes, the larger the nucleolus.
Is the nucleolus always present? Expand.
No. The nucleolus only comes together during the manufacture of ribosomes.
The nucleolus separates when (like during nuclear division) ribosome synthesis ceases.
Define endoplasmic reticulum.
A network of flattened sacs running through the cytoplasm of eukaryotic cells. Molecules, particularly proteins can be transported through the cell inside the sacs, separate from the cytoplasm. The ER is continuous with the outer membrane of the nuclear envelope.
Describe the ER and how it relates to it’s function.
There are 2 ERs:
Rough ER ( is covered in ribosomes)
Smooth ER
The membranes of the ER form flattened compartments (sacs) called cisternae. This way, the processes that take place in the ER can be separated from the cytoplasm.
Molecules (proteins in particular) can be transported through cell in ER.
ER continuous with outer membrane of nuclear envelope.
State the functions of the Rough ER
RER is covered in ribosomes - the site for protein synthesis.
It is a part of the secretory pathway for proteins.
Proteins can be secreted out of the cell, be part of the cell membrane or remain in the cell.
The secretory pathway is as follows:
Ribosome → ER → Golgi apparatus → lysosome/secretory vesicle.
Post-translational modifications of proteins (e.g form di-sulfide bonds) to form tertiary/quaternary structures of proteins.
State the functions of the Smooth ER
Smooth appearance due to the lack of ribosomes on its surface.
It makes lipids (like cholesterol for the cell membrane)
It makes steroids (such as reproductive hormones: oestrogen and testosterone)
Major storage site for Ca ions (SER is abundant in muscle cells as Ca ions are involved in muscle contraction)
It is involved in drug (in liver) and carbohydrate metabolism.
Define ribosomes.
Tiny organelles found in abundance in cells. In prokaryotic cells, they are 20nm in diameter. In eukaryotic cells, they are 25nm in diameter. They are involved in the process of protein synthesis.
Describe ribosomes and how it relates to its function.
Small + not visible in light microscope.
Consists of a small and large subunit.
They are made of roughly equal amounts (by mass) of protein and rRNA.
Its 3D structure allows all interacting molecules involved in protein synthesis to come together in 1 place.
mRNA, tRNA, amino acids and regulatory proteins
Describe the Golgi apparatus (GA).
A stack of flattened sacks - cisternae
The stack is constantly formed in one end from vesicles that bud off from the ER.
Then the GA is broken down at the other end to form Golgi vesicles.
The stack of sacs + vesicles = GA
Define Golgi apparatus.
An organelle found in eukaryotic cells, consisting of flattened sacs (cisternae) constantly forming in 1 ed (cis) and breaking up into Golgi vesicles in the other (trans). The Golgi apparatus chemically modifies the molecules it transports.
The difference in ribosomes between prokaryotes and eukaryotes, and explain the units used.
Eukaryotes = 80S ribosomes (slightly bigger)
Prokaryotes = 70S ribosomes
S units = Svedberg units
S units = how rapidly substances sediment in an ultracentrifuge (very fast)
The faster they sediment, the higher the S number.
Define Gogi vesicles (GV).
They carry its contents from the GA to other parts of the cell (usually the cell membrane for secretion).
Expand on the GA’s role in the secretory pathway.
The GA collects and processes molecules (particularly proteins from RER).
After processing the molecules are transported via GV, to other parts of cell/out of cell.
Releasing the molecules from cell = secretion
The pathway followed by the molecules = secretory pathway.
State 5 functions of the GA.
Golgi vesicles make lysosomes
Sugars are added to proteins to make molecules called glycoproteins.
Sugars are added to lipids to make glycolipids. Glycoproteins + glycolipids are important parts of cell membranes (used in cell signalling).
During plant cell division, Golgi enzymes are involved in the synthesis of new cell walls.
In gut + gas exchange system, goblet cells release a substance called mucin from the GA. Mucin is one of the main components of mucus.
Define lysosomes.
A spherical organelle found in eukaryotic cells that contains digestive (hydrolytic) enzymes. It has a destructive function, like the removal of old cell organelles.
Difference of lysosomes in animal and plant cells.
Lysosomes are simple structures - they are sacs surrounded by a single membrane.
In animal cells they are 100-500nm.
In plant cells, the large central (+ permanent) vacuole can act as a lysosome, though there are animal-like lysosomes present in its cytoplasm too.
State the function of lysosomes and how it is adapted to carry out its function.
Lysosomes are responsible for the breakdown of unwanted structures and substances (old cell organelles/whole cells)
To carry this function out:
It contains digestive enzymes (hydrolase- hydrolysis)
Protease, lipase, nuclease (break down proteins, lipids and nucleic acids)
The enzymes are surrounded by the single membrane of the lysosome to prevent damage.
Hydrolysis works best in an acidic environment, so the inside of the lysosome has a low pH.
The enzymes are synthesised in the RER and delivered to lysosome via GV.
What are the 4 different categories of activities of lysosomes?
Getting rid of unwanted cell components:
Lysosomes engulf + destroy components such as molecules and organelles in cell.
Endocytosis:
Material taken into cell via endocytosis (phagocytosis), lysosomes fuse with endocytic vacuoles formed and release the enzymes to digest the contents.
Exocytosis:
Lysosomal enzymes released from cell for extracellular digestion (replacement of cartilage by bone during development + acrosome (a type of lysosome) in sperm head to digest layers of cells around egg).
Self digestion:
Lysosome contents released into cytoplasm, resulting in whole cell being digested (tadpole tail reabsorbed during metamorphosis/ uterus returning to usual size after pregnancy/ after death when cell membranes lose its partial permeability).
Describe the structure of mitochondria.
1000nm in diameter + sausage shaped
Surrounded by 2 membranes (mitochondrial envelope).
Inner membrane folded forming finger-like cristae projecting into the interior of mitochondia
The interior of mitochondria = matrix.
Space between 2 membrane = intermembrane space.
Those cells which have high demand for energy (does more aerobic respiration) contains larger numbers of mitochondria.
Define cristae.
Folds of inner membrane of mitochondrial envelope, where particles of ATP synthase + electron transport chains associated with aerobic respiration are found.
State the functions of mitochondria and expand on them.
Carry out aerobic respiration
Reactions take place from which energy is released from energy-rich molecules (sugars + fats).
Most of the energy is transferred to molecules of adenosine triphosphate (ATP)
ATP = energy-carrying molecule found in all living cells (universal energy currency)
The reactions take place in the matrix + cristae (contains enzymes + electron carriers).
ATP leave mitochondria once made, and since is is small and soluble, it diffuses quickly to all parts of cell where energy is required.
The energy is released by breaking down ATP to ADP (hydrolysis)
ADP recycled back to ATP in mitochondria during aerobic respiration.
Why are mitochondria and chloroplasts thought to be of prokaryotic origins.
Contains 70S ribosomes which are found in prokaryotes.
Contains small circular DNA molecules
They undergo binary fission
The DNA and ribosomes of mitochondria and chloroplasts are still active and are responsible for the coding and the synthesis of certain vital proteins.
Define microtubule.
Tiny tubes made of a protein called tubulin. They are found in most eukaryotic cells and are responsible for a variety of functions:
Cell support
Determining cell shape
The ‘spindle’ in spindle fibres when the chromatids of chromosomes separate during nuclear division is made of microtubules.
State the role of microtubules in determining cell shape.
Microtubules together with actin filaments make up the cytoskeleton of the cell (a structural component) that helps determine the shape of the cell.
Describe mmicrotubules.
They are made up of a protein called tubulin.
There are 2 forms of tubulin (alpha and beta)
Both alpha and beta tubulin combine to form dimers.
Dimers are joined end-to-end to form protofilaments (polymerisation)
13 protofilaments line up alongside each other in a ring, to form a hollow cylinder.
There is a helical pattern formed by neighbouring alpha and beta tubulin molecules.
5 functions of microtubules.
Mechanical support
Secretory vesicles and other organelles/cell components move along the outside surface of microtubules - this forms an intracellular transport system.
During nuclear division, microtubules are used in the separation of chromatids/chromosomes.
Microtubules form the structure of centrioles
They form an essential part of the structure of cilia/flagella, for the mechanism of beating movements.
What are MTOCs?
Microtubule Organising Centres
They are responsible for the assembly of microtubules from tubulin molecules.
They can be formed/broken down there according to need.
Define centrioles.
One of 2 cylindrical structures found just outside the nucleus in a region called the centrosome in animal cells. They are also found at the bases of cilia and flagella (basal bodies). They are made of microtubules.
Define centrosomes.
The main Microtubule Organising Centre in animal cells, consisting of 2 centrioles at right angles to each other.
Describe a centriole.
A short hollow cylinder of around 500nm long, made up of microtubules.
It is made of 9 triplets of microtubules, where each triplet consists of 1 complete microtubule and 2 partial microtubules.
State the functions of centrioles and centrosomes.
Centrosomes act as the MTOCs for the assembly of microtubules that make up spindles during nuclear division.
centrioles are required for the production of cilia/flagella - they are found at the bases and is known as the ‘basal body’.
In this case the centrioles act as MTOCs, where microtubules extend from the cilia/flagella, which are important in the beating movement of them.
Definition of cilia and flagella.
Cilia:
Whip-like structures that project from the cells of many animal cells and unicellular organisms. They beat, causing locomotion/movement of fluid across it’s surface.
Flagella:
The same definition as cilia. They are actually identical in structure to cilia, but are longer.
Each are surrounded by an extension of the cell membrane.
Describe the structure of Cilia.
Composed of over 600 polypeptides.
This complexity results in very fine control of how they beat.
They consist of 2 central microtubules and are surrounded by 9 doublets around the outside. ‘9 + 2’ structure.
Each microtubule doublet consists of an A and B microtubule.
The wall of the A microtubule is a complete ring with 13 protofilaments.
The wall of the B microtubule is incomplete with only 10 protofilaments.
Each A microtubule has inner and outer arms made of a protein called dynein.
The dynein arms connect with the B microtubule or neighbouring MTs during beating.
There would be 2 rows of several hundred dynein arms.
The whole cylindrical structure inside the cell membrane = axoneme.
At the base of cilia/flagella = basal body identical to centriole - centrioles replicate themselves to form basal body.
How is the beating mechanism of cilia/flagella caused?
By dynein arms making contact with and moving along neighbouring microtubules. This produces a force which causes the cilia to beat.
As neighbouring MT doublets slide past each other, the sliding motion is converted into bending by other parts of cilium.
Describe the structure of chloroplasts.
They have an elongated shape with a diameter of around 3000-10,000nm.
They are surrounded by 2 membranes which together forms the chloroplast envelope.
It has a membrane system of fluid-filled sacs called ‘thylakoids’. On the membranes is a green photosynthetic pigment called chlorophyll
Thylakoids stack up to form grana.
Around the grana is the stroma, in which starch grains ans lipid droplets may be seen.
It also contains its own 70S ribosomes and circular DNA.
Explain the function of chloroplasts in relation to its structure.
Chloroplasts carry out photosynthesis
Which is why photosynthetic pigments (chlorophyll) can be found on the membranes of thylakoids.
Chlorophyll absorbs light during the light-dependent stage of photosynthesis (1st stage).
The light-independent stage of photosynthesis (2nd stage) uses the energy + reducing power generated in stage 1 to convert CO2 into sugars.
This takes place in the stroma.
sugars can be stored as starch grains in stroma.
To make membranes, there are lipid reserves in the stroma (in the form of lipid droplets). Or they could have been formed from the breakdown of internal membranes over time.
They have their own protein synthesising machinery: 70S ribosomes + circular DNA
Describe the structure of cell walls and explain how it relates to its function.
The first walls (primary wall) first formed is relatively rigid
Consists of parallel strands of cellulose (polysaccharide) running in a matrix of pectin.
Cellulose is relatively rigid + high tensile strength (difficult to break + stretch)
Prevents bursting when water enters by osmosis in a dilute solution.
More layers are added onto primary wall for extra strength (secondary layer).
In a given layer, the cellulose fibres are parallel to each other
But fibres of diff layers run in diff directions to make it strong.
Some walls are given extra strength by adding more cellulose/lignin.
Lignin adds compressional + tensile strength.
State 7 functions of cellulose.
Mechanical strength + support for both individual cells + plant as a whole.
Involves lignification for support
Turgid leaves relying on cell walls.
Prevents bursting by osmosis in a dilute solution
Different orientations of cellulose fibres helps determine the shapes of the cells as they grow.
Interconnected cell walls allowing the apoplastic pathway for the transport of water and minerals.
Living connections between neighbouring cell walls (plasmodesmata), allowing the symplastic pathway for the transport of water and minerals.
Cell walls in root endodermis impregnated with suberin - waterproof substance providing a barrier to the movement of H2O - controlling uptake of water and minerals.
Epidermal cells often have a waterproof layer of waxy cutin (forming the cuticle) on the outer surface of walls to prevent the loss of water via evaporation.
State the 6 functions of the vacuole in plants.
Provides support
Solution in vacuole is conc. Water enters vacuole via osmosis.
This inflates vacuole, causing a buildup of pressure. Inflated cells = turgid (supports stems & leaves)
Lysosomal activity (contains hydrolase - acts as lysosome).
Contains secondary metabolites:
not essential for growth/development - required for survival.
pigments to create colours in flowers/fruits - attracts insects
latex of opium poppy = morphine (from which heroin + opium is obtained)
contains alkaloids that deters herbivores from eating them.
Food reserves
Waste products
Growth in size:
osmotic uptake of water increases the volume of plant cells during growth.
What are the structures always found in bacteria?
Cell wall (made of peptidoglycan)
cell membrane
Circular DNA
70S ribosomes
cytoplasm
What are the structures sometimes found in bacteria?
Flagellum (for locomotion - simple structure)
Capsule (slime layer for extra protection)
Mesosome (infolding of cell membrane forming photosynthetic membrane/nitrogen fixation.
Plasmid (small circle of DNA)
Pili (attachment to other cells/surfaces or for sexual reproduction).
What is similar and different about a bacterial cell wall?
What other structures are similar?
The cell wall provides mechanical support and prevent bursting via osmosis just like in plant cells.
However, it is made of peptidoglycan (murein; a polysaccharide combined with amino acids)
They are:
Cell membrane
Cytoplasm (but no double membrane structures)
Ribosomes (but only 70S)
Circular DNA (found in region called nucleoid that contains proteins and small amounts of RNA.
Not surrounded by double membrane and there can be more than 1 copy of the DNA molecule.
What structures are different in a bacterial cell?
Flagellum:
Much simpler than eukaryotic flagellum
simple hollow cylinder made of identical proteins - rigid, doesn’t bend.
It is wave shaped and works by rotating its base like a propeller (bacterium moves forward in a corkscrew-shaped motion)
Mesosome
Cell membrane folds inwards to form an extra surface on which biochemical reactions take place.
Contains photosynthetic pigments, nitrogen fixation can take place.
Capsule
Bacteria is surrounded by an extra layer outside cell wall - can be capsule/slime layer.
Capsule = polysaccharides. Slime is easily washed off.
Helps prevent bacteria from drying out + can protect from antibiotics/phagocytosis.
Plasmid
small circular piece of DNA that can give resistance to antibiotics.
they copy themselves independently of the chromosomal DNA
they can spread rapidly from 1 DNA to another.
they aren’t associated with protein (naked DNA)
Pili
Fine protein rods - for attachment and interaction with different surfaces and cells.
Allows the transfer of genes (like plasmids) from one bacterium to another.
Describe virus structure and explain why they cannot be considered as living.
They are particles smaller than bacteria
They cannot be considered as living as they don’t have a cell structure.
No cell membrane/cytoplasm with ribosomes
Consists only of a molecule of DNA/RNA
A protein coat called a capsid made of proteins called capsomeres.
Some viruses have a membrane-like outer layer made of phospholipids and some proteins may project from the envelope.
What are viruses?
Viruses can be considered parasitic because they can only reproduce by infecting and taking over living cells.
The virus DNA/RNA takes over the protein synthesising machinery of the host cell to make new virus particles.
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