Summary
IB Biology: Cell Biology Summary
Cell Biology (unit 1) Summary
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1.1 INTRODUCTION TO CELLSLight Microscopes
★ Use lenses to bend light and magnify images by a
factor of roughly 100-fold
★ Can be used to view living specimens in natural
colour
★ Chemical dyes and fluorescent labelling may be
applied to resolve specific structures
Electron Microscopes
★ Use electromagnets to focus electrons resulting in
significantly greater magnifications and resolutions
★ Can be used to view dead specimens in monochrome
(although false colour rendering may be applied)
★ Transmission electron microscopes (TEM) pass
electrons through specimen to generate a
cross-section
★ Scanning electron microscopes (SEM) scatter
electrons over a surface to differentiate depth and
map in 3D
The cell theory states that:
1. All living things are composed of cells (or cell
products)
2. The cell is the smallest unit of life
3. Cells only arise from pre-existing cells
Certain types of cells / tissues do not conform to a standard notion of what
constitutes a cell:
★ Striated muscle fibres:
○ Muscle cells fuse to form fibres that may be very long (>300mm)
○ Consequently, they have multiple nuclei despite being surrounded by a single,
continuous plasma membrane
○ Challenges the idea that cells always function as autonomous units
★ Aseptate fungal hyphae:
○ Fungi may have filamentous structures called hyphae, which are separated into
cells by internal walls called septa
,○ Some fungi are not partitioned by septa and hence have a continuous cytoplasm
along the length of the hyphae
○ Challenges the idea that living structures are composed of discrete cells
★ Giant Algae
○ Certain species of unicellular algae may grow to very large sizes (e.g.
Acetabularia may exceed 7 cm in length)
○ Challenges the idea that larger organisms are always made of many microscopic
cells
All living things carry out 7 basic functions integral to survival:
★ Metabolism – Living things undertake essential chemical reactions
★ Reproduction – Living things produce offspring, either sexually or asexually
★ Sensitivity – Living things are responsive to internal and external stimuli
★ Homeostasis – Living things maintain a stable internal environment
★ Excretion – Living things exhibit the removal of waste products
★ Nutrition – Living things exchange materials and gases with the environment
★ Growth – Living things can move and change shape or size
Cells need to produce chemical energy (via metabolism) to survive and this requires
the exchange of materials with the environment
★ The rate of metabolism of a cell is a function of its mass / volume (larger cells
need more energy to sustain essential functions)
★ The rate of material exchange is a function of its surface area (large membrane
surface equates to more material movement)
As a cell grows, volume (units3) increases faster than surface area (units2), leading to
a decreased SA:Vol ratio
★ If metabolic rate exceeds the
rate of exchange of vital
materials and wastes (low
SA:Vol ratio), the cell will
eventually die
★ Hence growing cells tend to
divide and remain small in order to maintain a high SA:Vol ratio suitable for
survival
Magnification = Image size (with ruler) ÷ Actual size (according to scale bar)
Differentiation is the process during development whereby newly formed cells
become more specialised and distinct from one another as they mature. All cells of
, an organism share
an identical genome
– each cell contains
the entire set of
genetic instructions
for that organism.
The activation of
different instructions
(genes) within a
given cell by
chemical signals
will cause it to differentiate
Within the nucleus of a eukaryotic cell, DNA is packaged with proteins to form
chromatin
★ Active genes are usually packaged in an expanded form called euchromatin that
is accessible to transcriptional machinery
★ Inactive genes are typically packaged in a more condensed form called
heterochromatin (saves space, not transcribed)
Differentiated cells will have different regions of DNA packaged as euchromatin and
heterochromatin according to their specific function
There are four main types of stem cells present at various stages of human
development:
★ 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)
Stem cells are necessary for embryonic development as they are an undifferentiated
cell source from which all other cell types may be derived. Cell types that are not
capable of self-renewal (e.g. amitotic nerve tissues) are considered to be non-stem
cells. As these tissues cannot be regenerated or replaced, stem cells have become a
viable therapeutic option when these tissues become damaged.
★ Stargardt’s Disease
An inherited form of juvenile macular degeneration that causes progressive vision
loss to the point of blindness
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