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AQA Biology A-level Topic 2: Cells - Coggle Diagram
AQA Biology A-level Topic 2: Cells
Microscopes
light microscopes use a pair of
Convex lenses
that can resolve images that are
0.2um
apart.
This is due to the
wavelength of light
and therefore restricts the resolution.
Light Microscopes
Resolution
= minimum distance apart that two objects can be distinguished as separate objects in an image. The greater the resolution the clearer the image will be.
Magnification
= size of image/size of real object
Resolution and Magnification
Electron microscopes can reach a far smaller resolution at
0.1nm
apart, henceforth Electron microscopes can be used to look at objects that are much smaller and closer together.
Transmission Electron Microscope
- beam of electrons passes through a thin section of specimen. Areas that absorb the electrons appear darker on the electron micrograph.
Scanning Electron Microscope
- in a scanning electron microscope a beam of electrons passes across the surface and scatter. The pattern builds up a 3D image depending on the contours pf the specimen.
Electron Microscopes
Limitations:
The whole system must be in a vacuum so
living specimens cannot be observed
.
A
complex staining process
is required.
Specimens must be
very thin
, so electrons can pass through.
SEM had a lower resolving power than TEM
Cell Fractionation and Ultracentrifugation
Cell Fractionation
- is the process in which different parts of the organelles of a cell are separated. The most common method is
differential centrifugation
Process of homogenation
The cells are first blended in an
homogeniser
forming the
resultant fluid
called the
homogenate
. The homogenate is then placed in a
centrifuge
and spun at a slow speed.
Heaviest organelles, the
nuclei
, are forced to the bottom of tube where a pellet forms.
The fluid that forms at the top is known as the
supernatant
, after each spin this is removed, leaving just the sediment. The supernatant is then transferred to another tube and spun at a slightly higher speed.
The process continues and each time the homogenate is spun the next heaviest organelle forms in the pellet.
Order of organelles:
Nuclei
Mitochondria and chloroplasts
Pieces of membranes
Ribosomes
Homogenate
Homogenate at the beginning is placed in a
cold, buffered solution of the same water potential
. This prevents the organelles bursting under
osmotic pressure
, to
inactivate enzymes
from breaking down organelles and so the
PH
does not fluctuate.
Cell Division
Cell cycle
Mitosis
- mitosis produces identical cells, there are four stages of mitosis:
prophase, metaphase, anaphase and telophase
.
Interphase
- during this stage the cell
grows and prepares to divide
- chromosomes and some other organelles are replicated, chromosomes also begins to condense.
cytokinesis
- during cytokinesis the parent and replicated organelles move to opposite sides of the cell and the
cytoplasm divides
this producing two daughter cells.
Binary Fission
The process by which prokaryotic cells divide is known as
Binary Fission
The
circular DNA
in the cells replicates and both copies attach to the cell membrane. Plasmids also replicate.
The cell membrane then begins to grow between the two DNA molecules and begins to
pinch inwards
, dividing the cytoplasm in two.
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Growth:
all cells produced are identical so organisms can grow during mitosis.
Repair:
all cells produced are identical so organisms can replace dead tissues using mitosis.
Mitosis
- importance:
Reproduction:
some single-celled organisms, reproduce by dividing into two identical daughter cells.
Mitosis
- process:
Prophase
- Spindle fibres appear and chromosomes condense.
Prometaphase
- Spindle fibres attach to chromosomes.
Metaphase
chromosomes align in centre of cell.
Anaphase
- centromeres divide and sister chromatids move to opposite sides of cell.
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Viruses
As
viruses are non-living
they do not undergo cell division - following the injection of their nucleic acids into another cell, the infected host cell replicates the virus particles.
Cell Structure
Ultrastructure of eukaryotic cells
Nucleus
is a
double membrane
containing
3000 nuclear pores
that enables molecules to enter and leave. It also contains
chromatin
and a
nucleolus
which is the site of ribosome production. A substance known as
nucleoplasm
makes up the bulk of the cell.
Rough endoplasmic reticulum
is a series of flattened sacs enclosed by a membrane with ribosomes on the surface. It
folds and processes
proteins.
Smooth endoplasmic reticulum
is a system of membrane bound sacs. SER
produces and processes lipids
.
Golgi apparatus
is a series of fluid filled, flattened and curved sacs with vesicles surrounding the edges. Golgi apparatus
processes and packages proteins and lipids
. It also
produces lysosomes
.
Mitochondria
are oval-shaped, bound by a membrane called the envelope. The inner membrane is folded to form projections called
cristae
with a
matrix
on the inside containing the enzymes needed for
respiration
.
Centrioles
are hollow cylinders containing a ring of
microtubules
arranged at right angles to each other. Centrioles are involved in producing
spindle fibres
for
cell division
.
Ribosomes
are composed of two sub units and are the sight of
protein production
.
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Prokaryotic cell structure
cell wall
- a rigid outer covering made of
peptidoglycan
.
Capsule
- protective slimy layer which helps the cell to
retain moisture
and
adhere
to surfaces.
Mesosomes
- infoldings in the inner membrane which
contain enzyme required for respiration
.
Flagellum
- a tail like structure which
rotates to move the cell
.
pili
- hair like structures which attach to other bacterial cells.
Ribosomes
- site of
protein production
.
Plasmid
- circular piece of DNA.
Viruses
Viruses
are
non-living
structures which consist of
nucleic acid
enclosed in the protective protein coat called the
capsid
, sometimes with lipid layer called
envelope
.
Biological membranes
Partially Permeable membrane
The cell membrane is comprised of phospholipids in which the
hydrophilic
head (phosphate group) points outwards and they
hydrophobic tail
(2 fatty acid chains). This allows lipid soluble molecules to pass through, but
not water soluble
molecules.
Structure
Function:
The main function of the membrane is controlling the movement of substances in and out of the cell.
It also contains receptors for other molecules which allows adjacent cells to stick together.
Prevents the leakage of water from cells.
Components:
Proteins
- are present to aid movement across the membrane, provide mechanical support and act in conjunction with glycolipids as receptors. Can be
intrinsic
(include carrier proteins) or
extrinsic
(act as receptors).
Glycolipids
- made up of
carbohydrate bound to lipids
and act as
cell surface receptors
for certain molecules.
Cholesterol
- makes the membrane more
rigid and reduce lateral movement
of phospholipids. Also
prevents the leakage and dissolved ions
from the cell as it is very
Hydrophobic
.
Glycoproteins
- made up of
carbohydrates attached to extrinsic proteins
and act as a
cell surface receptor
and a neurotransmitter (molecules used by nervous system for message transmission between neurons).
Movement of molecules:
Diffusion
- the
passive
movement of small, non-polar, lipid soluble molecules such as CO2 and O2 down the concentration gradient and directly through the phospholipid bilayer.
Facilitated Diffusion
- requires a
channel protein
in the cell membrane to transport polar, charged and water soluble molecules across the membrane.
Osmosis
- diffusion of
water molecules
from area of high water potential to area of low.
Active transport
- transport of molecules against the concentration gradient, requires
ATP
and
carrier proteins
.
Exocytosis
-
vesicles
containing
large molecules
are fused with the membrane and released from the cell.
Endocytosis
- Particles are engulfed my
cell surface membrane
and form vesicles which are transported around cell.
Co-transport
Co-transport is the coupled transport of two molecules through the cell membrane via a
carrier
protein.
Co-transport in ileum of gut
:
Sodium ions are
actively transported
out of the epithelial cells, by the
sodium-potassium pump
, into the blood.
Henceforth, there is a much
higher concentration
of sodium ions in the lumen of the ileum than inside the epithelial cells.
Sodium ions diffuse into the epithelial cells through a
co-transport protein
(carrier protein). Either Amino acid molecules or
glucose
is diffused into the cell with them.
The Glucose/ amino acid molecules then pass into
blood plasma
by
facilitated diffusion
.
Sodium is moving
down its concentration gradient
, however, glucose is moving up its. The energy for this
active transport
is provided indirectly by the sodium concentration gradient.
Rate of gas exchange:
The
surface area
of the surface increases.
The
diffusion gradient
becomes more steep.
The
diffusion distance
decreases.
The
temperature
increases.
Immunity
Active Immunity
- from the production of antibodies by the immune system in response to the presence of an antigen.
Natural Active Immunity
- arises from being exposed to an antigen through disease.
Artificial Active Immunity
is acquired through vaccinations.
Passive Immunity
- results from the introduction of antibodies from another person or animal.
Natural Passive Immunity
- result of crossing mothers antibodies through placenta or breast milk.
Artificial Passive Immunity
- where antibodies are injected into the body.
Vaccine
Vaccines
are a way of introducing a pathogen into the body in order to produce an immune response.
The pathogen may be
dead or inactivated
. The
antigens
on surface membrane will produce a immune response.
Example of
active immunity
and results in the creation of
memory B cells
which trigger a rapid secondary response.
Vaccines aren't always useful in
preventing a disease outbreak
. This is because the antigen on the surface of the pathogen can change (example is
influenza
).
Success of programme:
Expense of the vaccine.
Severity of the side effects.
Ease of production, transportation and administration.
Number of people who need to be vaccinated for
her immunity
Herd Immunity
Herd immunity
is that enough people are vaccinated in the population then eventually the
pathogen
won't be able to be transmitted.
This means that people who aren't
vaccinated
are protected by those around them.
Bacteria and Viruses
Differences:
Bacteria are
Prokaryotic
cells - their genetic information is stored in the form of a circular strand of DNA. Viruses consist of
nucleic acids
enclosed in a protein coat.
Viruses require a
host
to survive whereas bacteria do not require this.
Viruses are
significantly smaller
than bacteria.
Bacteria have a cell membrane, cell wall and cytoplasm as well as other organelles such as ribosomes, plasmids, flagellum and pili whereas viruses possess no such structures.
Tuberculosis
Caused by a bacteria called
Mycobacterium tuberculosis
which infects
phagocytes
in the lungs.
First infection is
symptomless
as the infected phagocytes are sealed in
tubercles
as a result of the
inflammatory response
in the lungs.
Bacteria lay
dormant
in the tubercles as they are not destroyed by the immune system due to the tubercles being covered in a
thick waxy coat
.
When the immune system becomes weakened, the bacteria become active again and
destroy lung tissue
, thus leading to: breathing problems, coughing, weight loss as well as fever.
HIV
Human immunodeficiency virus
which causes
AIDs
and is an example of a viral infection.
The first symptoms of HIV include
fevers, tiredness and headaches
. After several weeks HIV antibodies appear in blood thus making a person HIV positive.
After this period, the symptoms disappear until the
immune system becomes weakened
again leading to AIDs. Symptoms of AIDs include:
weight loss, diarrhoea, dementia, cancers and opportunistic infections
.
Immune Response
Physical Barriers
Skin
which is a tough physical barrier consisting of
keratin
.
Stomach acid
(hydrochloric acid), which kills bacteria.
Gut and skin flora
- natural bacterial flora
competes with pathogen
for food and space.
Non-specific immune response
Inflammation
-
histamines
released by damaged white blood cells cause
vasodilation
which increases the flow of blood to the infected area and the
permeability
of blood vessels. Antibodies, white blood cells and plasma leak into the infected tissues.
Lysozyme action
- lysozymes are
enzymes
found in the secretions such as tears and mucus which kill bacterial cells by damaging their cell wall.
Interferon
- interferons prevent viruses spreading by stopping protein synthesis in viruses.
Phagocytosis
- process in which white blood cells engulf pathogens. The enclose them in a
phagocytic vacuole
with a
lysosome
. After the pathogen is destroyed, its
antigens
are presented on the surface of the cell. This activates the immune response if the antigen is recognised as
foreign
.
Phagocyte
is attracted to the pathogen by chemical products of the pathogen. It moves towards the pathogen along a
concentration gradient
.
The
phagocyte
has several receptors on its cell-surface membrane that attach to chemicals on the surface of the pathogen.
Lysosomes
within the phagocyte migrate towards the
phagosome
formed by engulfing the bacteria.
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PHAGOCYTOSIS
The Specific Immune Response
The specific immune response
is antigen specific and produces response specific to one type of pathogen only. Relies on
lymphocytes produced in the bone marrow:
B cells
mature in the bone marrow and are involved in the
humoral response
T cells
move from the bone marrow to the thymus gland where they mature, are involved in
cell mediated response
.
Memory Cells
are cells which replicate themselves when exposed to an invading pathogen and remain in
lymph nodes
for decades searching for the same antigen. Results in a
faster immune response
.
T helper
cells
stimulate B cells
and
Tkiller
cells to divide.
T killer
cells
destroy pathogen
.
B effector or plasma
cells are antibody producing cells,
Cell Mediated Response
Foreign microbe attracts phagocyte so microbe with
antigens
enters.
Phagocyte
ingests and engulfs microbe with antigens.
Antigens
are processed and presented on
cell surface membrane
to
T cell
.
T cell
produces clones.
Killer T cell
Directly destroys antigens.
Helper T cells
Stimulates T and B cells.
Memory T cell
Remembers antigen for future.
Humoral response
A
B cell
is triggered when it encounters its matching antigen.
The
B cell
engulfs the antigen and digests it.
It displays
antigen
fragments bound to its unique
MHC molecules
.
The combination of antigen and
MHC
attracts the help of a mature complementary
T cell
.
Cytokines
secreted by the
T cell
help the
B cell
to multiply and mature into
antibody-producing
plasma cells
.
Antibodies
are then released into the blood and lock onto matching
Antigens
, which are then destroyed by
phagocytes
and
white blood cells
.
Antibodies
In the
humoral response
plasma cells produce
antibodies
.
These are made of
four polypeptide
chains forming a
Y shaped structure
.
The structure of an antibody is only
complementary to a single antigen
. Work by forming an
antigen-antibody
complex which serves as
markers for phagocytes
to destroy attached cells.
Two binding sites
means that antibodies can clump pathogens together in
agglutination
making it easier to be destroyed.
Structure
2 light
chains +
2 heavy
chains
Constant
region +
variable
region.
Receptor
binding site.
Antigen
binding site.