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Topic 1 – Cell Biology

Cell Theory
The three principles of cell theory:
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Cells are the smallest unit of life that can exist independently
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This is because muscle fibres are large structures surrounded by a single membrane and
contain many nuclei, and it is not clear whether giant algae arose because one cell grew or if it was
because cell walls failed to form
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It is the interaction
of the component parts of a complex structure
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Magnification: the ability to make objects larger
Resolution: the ability to distinguish between objects that are close together
Light microscopes can magnify up to 1,500 times
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Micrometres (μm) are the units used to measure the size of cellular structures
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1mm = 1000μm
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I is the size of the image on the page, M is the magnification, A is the actual size of the image
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To convert mm to μm, multiply by 1000
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Topic 1 – Cell Biology

Limiting Cell Size
The metabolic rate of a cell is proportional to the volume of the cell
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The rate at which substances cross the membrane depends on the cell’s surface area
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Surface area to volume ratio limits the size of cells because a big cell would have a small surface area
so rate of diffusion into the cell would be slower
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Cell Reproduction and Differentiation
Cells become specialised to perform a specific function – they become differentiated
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Different proteins are produced, which is why
they differentiate
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Stem Cells
Stem cells are unspecialised cells
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Cord blood can be used to treat Leukaemia because it contains haematopoietic stem cells that can
differentiate into any type of cell
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Using embryonic stem cells reduces human suffering and they are easier to get hold of, as well as
being able to differentiate into any type of cell
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Stargardt’s disease causes a loss of central and colour vision
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Prokaryotic Cells
Prokaryotic cells do not have a definite nucleus or membrane-bound organelles, such as mitochondria
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Genes code for bacterial
proteins
Gelatin-like Capsule: to protect and
prevent certain things from entering
70S Ribosomes: the site of protein
synthesis
Flagellum and Pili: to help the cell
move
Cell wall and cell membrane: to
prevent things from entering and
leaving the cell
Mesosome: the site of respiration
Plasmid: to contain certain genes for antibiotic resistance

Eukaryotic Cells
Eukaryotic cells have membrane bound organelles and a nucleus
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Exocrine cells secrete enzymes
that are released into the small intestine
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The hydrophilic head is made of a phosphate molecule and the hydrophobic tail is made
of fatty acids (this means they are amphipathic)
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Cholesterol restricts movement of phospholipids and reduces the fluidity of the
membrane
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The Davson-Danielli model was the first idea proposed about the structure of the phospholipid
membrane
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Topic 1 – Cell Biology
Roles of proteins in the membrane:
Glycoproteins: used for cell-to-cell communication
Insulin receptor: an integral protein that is a hormone receptor
Cadherin: an integral protein used for cell-to-cell adhesion
Cytochrome C: a peripheral protein used for electron transport
Cytochrome Oxidase: an integral protein that is an immobilised enzyme
Nicotinic acetylcholine receptor: an integral protein that is both a receptor for a neurotransmitter and
a channel for facilitated diffusion of sodium ions
Calcium pump: an integral protein for active transport of calcium ions
Integral channel and carrier proteins transport molecules across the membrane
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Transport Across Cell Membranes
Passive transport does not require ATP, and the particles move down a concentration gradient from a
high concentration to a low concentration, e
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diffusion, facilitated diffusion, and osmosis
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Diffusion
Diffusion is the passive movement of particles from a region of higher concentration to a region of
lower concentration as a result of the random motion of particles
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A bigger
difference gives a faster rate of diffusion
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g
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There are four factors that affect
diffusion: concentration gradient, surface
area, temperature, and the distance over
which diffusion occurs
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The carrier protein then
transports the substance into the
membrane
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g
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Topic 1 – Cell Biology
Osmosis
Osmosis is the passive movement of water molecules from a region of lower solute concentration (less
negative water potential) to an area of higher solute concentration (more negative water potential)
through a partially permeable membrane
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Water potential: how easy it is for the water to move about
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Osmolarity: the number of moles of solute particles per volume
of solution
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An isotonic solution has the same water potential as the cell –
it will move into and out of the cell equally
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A hypotonic solution has a less negative water potential than the cell – it will
move into the cell
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The protein pumps have a specific tertiary structure so
they will only transport molecules with a complimentary
shape
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Energy is released from the cell
and the substance is transported to the opposite side of
the membrane
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Exocytosis:
Proteins produced by ribosomes pass through the RER
and are then modified by the golgi body, which
releases them as vesicles
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The membrane flattens out
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A
droplet of fluid becomes enclosed when a vesicle is
pinched off
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Origin of Cells
The Endosymbiotic Theory
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Symbiosis is two organisms living together
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The smaller cell
performs useful functions for the larger cell
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Aerobic respiration is more efficient
than anaerobic respiration, which gives the larger cell a competitive advantage
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A heterotrophic cell took in a smaller photosynthetic bacterium, which supplied it with organic
compounds, making it an autotroph
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Evidence for the Endosymbiotic Theory
Mitochondria and chloroplasts grow and divide like cells, they have a loop of naked DNA, and they
have double membranes
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Cell Division and the Cell Cycle
The cell cycle has three stages:
G1 – The first growth phase
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S – Synthesis phase
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G2 – The second growth phase
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These occur in interphase, then mitosis happens (nuclear division)
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Cyclins are a family of proteins that control the progression of cells through the cell cycle by activating
Cdk enzymes (cyclin-dependent kinase enzymes)
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Mitosis
Mitosis occurs during growth, repair, and asexual reproduction
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Topic 1 – Cell Biology
IPMAT:
Interphase
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No distinct chromosomes visible
DNA replicates
Proteins and organelles synthesis

Prophase
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Chromosomes condense and become visible
Each chromosome consists of two identical chromatids joined at the
centromere

Metaphase
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Nuclear membrane breaks down
Spindle fibres form across the cell
Each chromosome moves to the equator of the cell and attaches
to the spindle by its centromere

Anaphase
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Centromeres split as spindle fibres contract
Sister chromatids separate and move to opposite poles of the cell

Telophase
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Cytokinesis occurs
Nuclear membranes reform
Nuclei form

Supercoiling: the repeated coiling of DNA molecules (around proteins called histones) to make the
chromosomes shorter and wider
Spindle: made of contractile protein filaments produced by the centrioles
Mitotic index = number of cells in mitosis / total number of cells

Cancer and Its Treatment
Cancer results from damage to the genes that control the cell cycle and mitosis
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Treatment of cancer may involve blocking part of the cell cycle or a stage of mitosis
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Environmental factors can increase the rate of mutations (mutagenic agents)
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Cancer occurs when proto-oncogenes, that stimulate cell division, become mutated and turn into
oncogenes
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When both of these factors combine, cell division occurs rapidly and
uncontrollably
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