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Learning outcomes
Learning outcomes
1.1.U1 According to the cell theory, living organisms are composed of cells.
1.1.U2 Unicellular organisms carry out all functions of life.
1.1.A1 Questioning the cell theory using atypical examples, including striated muscle, giant algae and aseptate fungal hyphae.
1.1.A2 Investigation of functions of life in Paramecium and one named photosynthetic unicellular organism.
1.1.NOS1 Looking for trends and discrepancies- although most organisms conform to cell theory, there are exceptions.
(U - understanding; A - applications; NOS - nature of science; S - skills)
Cell theory
Cell theory
Living organisms are composed of cells.
Living organisms are composed of cells.
The internal structure of living organisms is very intricate and is built up from very small individual parts. Organs such as the kidney and the eye are easily visible. If they are dissected we can see that large organs are made of a number of different tissues, but until microscopes were invented little or nothing was discovered about the structure of tissues.
The three tenets of the cell theory are as described below:
All living organisms are composed of one or more cells.
The cell is the basic unit of structure and organization in organisms.
Cells arise from pre-existing cells.
All living organisms are composed of one or more cells
All living organisms are composed of one or more cells
The cell is the basic unit of structure and organization in organisms
The cell is the basic unit of structure and organization in organisms
Cells arise from pre-existing cells
Cells arise from pre-existing cells
The history of the formation of cell theory
The history of the formation of cell theory
1590
Zacharias Jansen invents the compound microscope
1663
Robert Hooke discovers cells
1674
Anton van Leeuwenhoek discovers unicellular organisms
1838
Matthias Schleiden realizes all plants are made of cells
1839
Theodore Schwann realizes all animals are made of cells
1855
Rudolf Virchow proposed that all cells come from other cells
1886
Ernst Abbe and Carl Zeiss invent the
modern compound microscope
1935
The scanning electron microscope was invented
Seven main signs of a living organism
Seven main signs of a living organism
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
Mnemonic: MR SHENG
Metabolism
Reproduction
Sensitivity
Homeostasis
An example of unicellular organisms showing all the signs of a living organism
An example of unicellular organisms showing all the signs of a living organism
Paramecium (heterotroph)
Paramecium (heterotroph)
Paramecia are surrounded by small hairs called cilia which allow it to move (responsiveness)
Paramecia engulf food via a specialised membranous feeding groove called a cytostome (nutrition)
Food particles are enclosed within small vacuoles that contain enzymes for digestion (metabolism)
Solid wastes are removed via an anal pore, while liquid wastes are pumped out via contractile vacoules (excretion)
Essential gases enter (e.g. O2) and exit (e.g. CO2) the cell via diffusion (homeostasis)
Paramecia divide asexually (fission) although horizontal gene transfer can occur via conjugation (reproduction)
Scenedesmus (autotroph)
Scenedesmus (autotroph)
Scenedesmus exchange gases and other essential materials via diffusion (nutrition / excretion)
Chlorophyll pigments allow organic molecules to be produced via photosynthesis (metabolism)
Daughter cells form as non-motile autospores via the internal asexual division of the parent cell (reproduction)
Scenedesmus may exist as unicells or form colonies for protection (responsiveness)
Certain types of cells / tissues do not conform to a standard notion of what constitutes a cell:
Certain types of cells / tissues do not conform to a standard notion of what constitutes a cell:
Striated muscle fibres
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
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
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
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