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The evolution of multicellular organisms allowed cell specialisation and cell replacement.
Main topics
● Cell theory
● Multicellular organisms and emergent properties
● Surface area to volume ratio
● Cell differentiation and specialised tissues
● Stem cells
Objectives
● State what is meant by 'cell theory' and examine evidence for it.
● Explain how surface area to volume ratio limits size.
● Examine unicellular and multicellular organisms.
● Compare types of microscope and calculate magnification and size.
● According to the cell theory, living organisms are composed of cells.
● Organisms consisting of only one cell carry out all functions of life in that cell.
● Surface area to volume ratio is important in the limitation of cell size.
● Multicellular organisms have properties that emerge from the interaction of their cellular
components.
● Specialised tissues can develop by cell differentiation in multicellular organisms.
● Differentiation involves the expression of some genes and not others in a cell's genome.
● The capacity of stem cells to divide and differentiate along different pathways is necessary in
embryonic development. It also makes stem cells suitable for therapeutic uses.
Applications
, ● Questioning the cell theory using atypical examples, including striated muscle, giant algae and
aseptate fungal hyphae.
● Investigation of functions of life in Paramecium and one named photosynthetic unicellular organism.
● Use of stem cells to treat Stargardt’s disease and one other named condition.
● 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.
Skills
● Use of a light microscope to investigate the structure of cells and tissues, with drawing of cells.
● Calculation of the magnification of drawings and the actual size of structures and ultrastructures
shown in drawings or micrographs.
THE CELL THEORY
All living organisms are composed of cells.
The origins of cell theory
The discovery of cells was linked to the development of the microscope.
,Cell theory states that:
● all living organisms are made of one or more cells
● cells are the smallest units of life.
● cells can only be formed from the division of other cells.*
*cell theory refutes spontaneous generation
All cells exhibit the features of living organisms:
M Movement Travel from one place to another
R Respire Release energy from food
S Sensitive Respond to stimuli or their environment
G Grow Increase in size or mass
R Reproduce Produce offspring
E Excrete Remove toxic waste products
N Nutrition Either make their own food or eat other organisms
(C) Control Keep their internal environment within fine limits
The cell theory debate
For Against
The microscope has shown all living things to be Skeletal muscle is made of large fibres and have
made of cells or cell products. multiple nuclei.
Cells removed from tissue can grow independently for Fungal hyphae have multiple nuclei and are not
a short amount of time. clearly divided.
Nothing smaller than a cell has been found to be able Giant algae can grow to 100mm despite having
to carry out all the functions of life. just 1 nucleus.
Experiments by Pasteur and Redi proved Life may have originated from smaller organelles
spontaneous generation of cells did not occur. (mitochondria & chloroplasts) that then became
encapsulated within cells.
, Phloem sieve cells have few internal organelles
and their metabolism is controlled by companion
cells.
Exceptions to the cell theory
1. Striated muscle fibres:
○ Muscle cells fuse to form fibres that may be very long (>300mm).
○ They have multiple nuclei despite being surrounded by a single, continuous plasma membrane.
○ Challenges the idea that cells always function as autonomous units.
2. Aseptate fungal hyphae:
○ Fungi 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.
○ Challenges the idea that living structures are composed of discrete cells.
3. Giant algae:
○ Certain species of unicellular algae may grow to very large sizes (e.g. Acetabularia > 7 cm in
length).
○ Challenges the idea that larger organisms are always made of many microscopic cells.
UNICELLULAR ORGANISMS
Organisms consisting of only one cell carry out all functions of life in that cell.
Unicellular organisms carry out at least seven functions of life:
SURFACE AREA TO VOLUME RATIO
Surface area to volume ratio is important in the limitation of cell size.
A cell's:
● Rate of metabolism is a function of its volume (larger cells need more energy to sustain essential
functions)
● 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 the ratio is too small:
○ substances will not enter the cell as quickly as they are required and waste products will
accumulate because they are produced more rapidly than they can be excreted.
, ○ cells may overheat because the metabolism produces heat faster than it is lost over the cell's
surface.
MULTICELLULAR ORGANISMS
Multicellular organisms have properties that emerge from the interaction of their cellular components.
Emergent properties
● arise from the interaction of the component parts of a complex structure.
● a whole is greater than the sum of its parts.
● allows multicellular organisms to complete functions that individual cells cannot.
CELL DIFFERENTIATION AND GENE EXPRESSION
Specialised tissues can develop by cell differentiation in multicellular organisms.
Differentiation involves the expression of some genes and not others in a cell's genome.
Differentiation
● 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).
● caused by the activation of different instructions (genes) within a given cell by chemical signals.
Cell packaging
Within the nucleus of a eukaryotic cell, DNA is packaged with proteins to form chromatin:
● Active genes: is transcribed.
○ usually packaged in an expanded form (euchromatin).
● Inactive genes: not transcribed.
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