Surface area to volume ratio
Surface area to volume ratio is important in the limitation of cell size
Rate of metabolism –mass or volume
Rate of material exchange—surface area
Small size of cell – high SA:Vol ratio—rate of material exchange >metabolic rate ( suitable for
survival)
Large size of cell –low SA: Vol ratio—metabolic rate> rate of material exchange( cells
eventually die)
Stem Cells
Totipotent – Can form any cell type, as well as extra-embryonic (placental) tissue (e.g. zygote)
Pluripotent – Can form any cell type (e.g. embryonic stem cells)
Multipotent – Can differentiate into a number of closely related cell types (e.g. haematopoeitic
adult stem cells)
Unipotent – Can not differentiate, but are capable of self renewal (e.g. progenitor cells, muscle
stem cells)
Examples of Stem Cell Therapy
1. Stargardt’s Disease
2. Parkinson’s Disease
Ethics of the therapeutic use of stem cells from specially created embryos, from the umbilical
cord blood of a new-born baby and from an adult’s own tissues
Prokaryotic
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Archaebacteria – found in extreme environments like high temperatures, salt concentrations or
pH (i.e. extremophiles)
Eubacteria – traditional bacteria including most known pathogenic forms (e.g. E. coli, S.
aureus, etc.)
Eukaryotic Cells
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Protista – unicellular organisms; or multicellular organisms without specialised tissue
Fungi – have a cell wall made of chitin and obtain nutrition via heterotrophic absorption
Plantae – have a cell wall made of cellulose and obtain nutrition autotrophically (via
photosynthesis)
Animalia – no cell wall and obtain nutrition via heterotrophic ingestion
Universal Organelles (prokaryote and eukaryote):
Ribosomes
Cytoskeleton
Plasma membrane
Structure: Phospholipid bilayer embedded with proteins (not an organelle per se, but a vital
structure)
Function: Semi-permeable and selective barrier surrounding the cell
Eukaryotic Organelles (animal cell and plant cell):
Nucleus
Endoplasmic Reticulum
Golgi Apparatus
Mitochondrion
Peroxisome
Centrosome
Plant Cells Only
Chloroplast
Vacuole (large and central)
Function: Maintains hydrostatic pressure (animal cells may have small, temporary vacuoles)
Cell Wall
Animal Cells Only
Lysosome
Structure: Membranous sacs filled with hydrolytic enzymes
Function: Breakdown / hydrolysis of macromolecules (presence in plant cells is subject to
debate)
Vacuole (large and central)
Structure: Fluid-filled internal cavity surrounded by a membrane (tonoplast)
Function: Maintains hydrostatic pressure (animal cells may have small, temporary vacuoles)
Peroxisome
Structure: Membranous sac containing a variety of catabolic enzymes
Function: Catalyses breakdown of toxic substances (e.g. H2O2) and other metabolites
Ribosomes
Structure: Two subunits made of RNA and protein; larger in eukaryotes (80S) than prokaryotes
(70S)
Function: Site of polypeptide synthesis (this process is called translation)
Cytoskeleton ( Filamentous structure)-in the cytoplasm
Cytosol(fluid portion of the cytoplasm is the cytosol)
Plasma membrane
Phospholipids form bilayers in water due to the amphipathic properties of phospholipid
molecules.
• Membrane proteins are diverse in terms of structure, position in the membrane and
function
Cholesterol is a component of animal cell membranes
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Cholesterol interacts with the fatty acid tails of phospholipids to moderate the
properties of the membrane:
Cholesterol functions to immobilise the outer surface of the membrane, reducing
fluidity
It makes the membrane less permeable to very small water-soluble molecules that
would otherwise freely cross
It functions to separate phospholipid tails and so prevent crystallisation of the
membrane
It helps secure peripheral proteins by forming high density lipid rafts capable of
anchoring the protein
Drawing of the fluid-mosaic model
Particles move across membranes by simple diffusion, facilitated diffusion, osmosis and active
transport
semi-permeable
selective
Passive Transport
Active Transport
Diffusion
Osmosis
Osmolarity is a measure of solute concentration, as defined by the number of osmoles of a
solute per litre of solution (osmol/L)
Estimating Osmolarity
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The osmolarity of a tissue may be interpolated by bathing the sample in solutions
with known osmolarities
The tissue will lose water when placed in hypertonic solutions and gain water when placed in
hypotonic solutions
Water loss or gain may be determined by weighing the sample before and after bathing in
solution
Tissue osmolarity may be inferred by identifying the concentration of solution at which there is
no weight change (i.e. isotonic)
Application:
• Tissues or organs to be used in medical procedures must be bathed in a solution with the
same osmolarity
as the cytoplasm to prevent osmosis
Facilitated diffusion is the passive movement of molecules across the cell membrane via the
aid of a membrane protein
odium-potassium pump and potassium channels for
facilitated diffusion in axons
Active Transport
Vesicular transport
The fluidity of membranes allows materials to be taken into cells by endocytosis
or released by exocytosis
Endocytosis -The process by which large substances (or bulk amounts of smaller
substances) enter the cell without crossing the membrane
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An invagination of the membrane forms a flask-like depression which envelopes the
extracellular material
The invagination is then sealed off to form an intracellular vesicle containing the
material
There are two main types of endocytosis:
Phagocytosis – The process by which solid substances are ingested .
Pinocytosis – The process by which liquids / dissolved substances are ingested .
Exocytosis
The process by which large substances (or bulk amounts of small substances) exit the
cell without crossing the membrane
Vesicles (typically derived from the Golgi) fuse with the plasma membrane, expelling
their contents into the extracellular environment.
Cyclins
Cyclins are involved in the control of the cell cycle
Cyclin concentrations need to be tightly regulated in order to ensure the cell cycle progresses in
a proper sequence
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Different cyclins specifically bind to, and activate, different classes of cyclin dependent kinases
Cyclin levels will peak when their target protein is required for function and remain at lower
levels at all other times
Cancer Development
Mutagens, oncogenes and metastasis are involved in the development of primary and
secondary tumours
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Proto-oncogenes code for proteins that stimulate the cell cycle and promote cell growth and
proliferation
Tumour suppressor genes code for proteins that repress cell cycle progression and promote
apoptosis