2. CELL THEORY
All organisms consist of cells or the product of cells and
all cells arise from pre-existing cells.
The cell is the unit of structure and function of most
organisms – they are the building blocks of life. They :
- respond to stimuli
- reproduce
- move
- transform energy for perform functions
- produce waste materials
3. While organisms vary in size and complexity. Large
organisms are multicellular while the smallest are
unicellular.
1665 – Robert Hooke used the first recorded magnifying
device to observe cork cells.
1667 – Leeuwenhoek used a small bead of glass to
examine material from his mouth which he called
“animalcules” or bacteria.
4. Two main types of microscopes
Electron microscope
Beams of electrons pass through a vacuum and then
through a thin layer of tissue. Electrons have a much
shorter wavelength compared to visible light they provide
much better detail or resolution (ability to separate
objects)
X 100,000 magnification
5. Light microscope
Relies on light passing through a very thin section of
tissue. Combination of “eye piece” and “objective” lens
X 1500 magnification
Micrometre (μm) unit of length used to measure cells. If
structures are measured within a cell scientists use
nanometre (nm)
Bacteria - 1 μm
Human red blood cell - 10μm
Human liver cell - 50μm
Plant mesophyll cell - 100μm
Viruses – 20-400nm (not considered a cell)
11. Surface Area To Volume Ratio
The SA:V ratio explains the exchange of materials
between external and internal environment.
Cells which are microscopic in size allow the required
substances and waste products to easily diffuse.
12. As a cell grows in size it will have relatively less
surface area compared to its volume and is
thought to be a stimulus for it to divide
13. The shape of a cell also affects SA:V ratio. The
human red blood cell is a bi-concave disc which
maximises surface area to volume increasing its
capacity to exchange oxygen and carbon dioxide
as well as allowing for flexibility in the
capillaries.
15. Prokaryotic cells
Do not have defined nucleus, DNA is found as a
concentrated mass or a small circle of DNA
known as a plasmid
Have existed for millions of years, relatively
unspecialised and do not contain membrane
bound organelles
Eg bacteria
16. Some prokaryotes have extensively folded
plasma membranes using their large surface
areas for respiration, photosynthesis
(chemosynthesis) and DNA replication. They
contain ribosomes enabling protein synthesis.
They are very small and as such have a very
good SA:V ratio.
17. Eukaryotic cells
Cells that make up large complex multicellular
organisms are eukaryotic. Highly specialised,
larger in size and contain a membrane bound
nucleus and specialised membrane organelles
20. Nucleus
Membrane bound and contains chromosomes
which controls activities
of the cell
Mitochondria
Site of aerobic respiration, enables cell to release
energy from organic molecules for cellular
purposes
22. Chloroplasts
Contain the green pigment chlorophyll which allows for
photosynthesis, converting inorganic materials to organic
molecules using light
23. Vacuoles
Membrane bound structures filled with fluid
which contains dissolved sugars or salts. In plants
cells contain large vacuoles surrounded by
membrane. They are a storage site for solutes,
cell turgidity and maintaining water balance.
Unicellular cells that live in freshwater have
“contractile” vacuoles which pump out water
which enters via osmosis
Animal cells contain “food vacuoles” that store
food particles acquired by phagocytosis
24. Endoplasmic Reticulum
Internal folded membranes that connect nucleus with cell
membrane: Two types
1. Smooth ER – no attached ribosomes, involved with
synthesis and metabolism of fluids
2. Rough ER – attached ribosomes, synthesised proteins
are modified by ER.
25. Golgi Bodies (Apparatus)
Found in all eukaryotic cells, consist of
flattened discs surrounded by spherical vesicles.
Involved in packaging and transporting sugar-
coated proteins (glycoproteins) to the outside of
the cell.
26.
27. Cell Membrane
The cell membrane is found in all living cells and controls
the entry and exit of materials in out of the cell. It
consists of two layers of phospholipid molecules
embedded with proteins. There have been various models
put forward over the last 100 years to explain its
structure
28. Fluid Mosaic Model
Fluid
Refers to the ability of proteins and lipids to move
sideways through within the structure, giving it a fluid
nature
Mosaic
Refers to the variety of protein molecules that are
embedded or attached to the phospholipid bi-layer. These
proteins determine the function of the cell membrane
29. Types of proteins inside Mosaic layer
1. channel type proteins which span the
membrane
Passive – diffusion
Active – active transport
2. receptor type proteins bound to the surface
that are involved in cellular communication
- recognition of antigens and hormones
3. proteins on underside of membrane held in
place by cytoskeleton
30. Fluid Mosaic model explains the “semi-
permeable” nature of the cell membrane.
31. The Fluid Mosaic Model
• The phospholipid bi-layer consists of a double layer of
phospholipid molecules.
• The phospholipid molecules have a hydrophilic end
(water loving) and a hydrophobic end (water hating)
Hydrophilic Phosphate
head
Hydrophobic hydrocarbon tail
34. Endocytosis & Exocytosis
Movement of larger molecules such as proteins or
polysaccharides and fluids.
Exocytosis
Substances passing out of cell. Molecules packaged by
Golgi bodies and ER such as hormones, enzymes.
Endocytosis
Substances passing into the cell. Two types
1. Phagocytosis - solid particles “eating”
2. Pinocytosis – liquid particles “cell drinking”
36. Cytoskeleton
Internal framework of microtubules and micro fibres.
Organised by the centrosome or “micro-tubule organising
centre” (MTOC)located near the nucleus.
Three functions
1. Support
Maintains shape of animal cells in absence of cell wall.
Organelles are supported and attached to cytoskeleton in
the cytoplasm
37. 2. Movement
-movement of vesicles in side cell as in exocytosis
-movement of chromosomes during cell division
-movement of cilia and flagella
-contraction and expansion of muscle cells
3. Regulation
Relay of messages from external environment to cells
interior
38.
39. Prokaryotes vs Eukaryotes
Prokaryotes
• Smaller (1 – 10um)
• Circular DNA
• No Nucleus (nuclear Membrane)
• Little internal organisation
• No membrane-bound organelles
• Single chromosome
• Most have a cell wall (made of
peptidoglycan)
• Some can obtain energy by
photosynthesis
Eukaryotes
• Larger (10 – 100um)
• Linear DNA
• Contains a nucleus
• High level of internal organisation
• Contains membrane bound
organelles
• Two or more chromosomes
• Cell wall (if present) made of
cellulose
• Obtain energy by photosynthesis
or ingesting other living organisms
40. Cells ensure that the composition of their internal
environment is carefully regulated to ensure that
chemical concentrations are kept relatively stable.
Cells manufacture organic molecules from their
interaction with inorganic molecules (and other organic
molecules)
The cell membrane regulates and controls this exchange
41.
42. Living cells require a constant supply of energy to carry
out processes.
Inside cells chemical reactions are occurring at all times
and these require a stable internal environment
(Homeostasis)
i.e. concentrations of substrates (O2) and products (urea,
CO2)
Reactions inside cells will also be adversely affected with
changes in pH or temperature
43. Several characteristics will determine movement across
the membrane
1. Size of the molecule
2. Chemical composition of the molecule. Chemicals
which dissolve are more likely to move easily across
the membrane
3. Shape of the molecule – cell membrane can
discriminate and select specific molecules i.e. glucose
44. Examples of selective exchange:
CO2 / O2 easily move through cell membrane because of
their small size
Glucose and water do not readily move across, they
require specific protein channels or carrier proteins
Proteins and polysaccharides are too large to cross the
membrane unless they move via endocytosis or
exocytosis.
46. Diffusion
The random movement of a
molecules from a region of high
concentration to a region of
low concentration. It is a
passive process which requires
no energy.
48. Osmosis
Diffusion of water across a semipermeable
membrane from a high concentration to a low
concentration.
Water is a solvent, the materials dissolved in it, is
the solute and the combined mixture is referred
to as the solution
49. Isotonic – same solute concentration as the cell
Hypotonic- a lower concentration as the cell
Hypertonic – a higher solute concentration as the cell
51. Osmosis has a more drastic effect on animal cells than
plant cells Why?
Animal cells do not have a cell wall and therefore are
prone to gaining (bursting) or losing water (shrivel),
particularly if they exist in fresh water.
E.g. Protozoa – Euglena (contractile vacuole expels excess
water)
Plant cells are strengthened by the cell wall-rarely
destroyed by osmosis.
52. Active Transport
In some situations molecules are required to move against
a concentration gradient – this requires energy
Transport proteins in the cell membrane actively move
specific substances across the membrane from a high to
low concentration
ATP provides this energy
54. Active Transport
Transmission of nerve signals involves
exchange of sodium and potassium ions
by diffusion across the cell membrane.
When fatigued it is necessary to pump
these ions against a concentration
gradient back to the other side
55. Active Transport
Another example is the absorption of glucose. If the
concentration in the gut is higher than the blood it will
diffuse, but if it is lower, it will only move by active
transport.
Why?
Fish actively absorb ions through their gills to replace
solutes lost in large volumes of their urine
57. Facilitated Diffusion
This is where some larger molecules which
cannot diffuse through the membrane are
helped through the membrane by special
proteins.
They are facilitated through.
Substances like:
• Glucose
• Amino acids
Facilitated diffusion is still passive (substances
move along a concentration gradient from high
to lower concentrations)
