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Summary of IB Biology Topic 1 (Cell Biology)
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Summary of IB Biology Topic 1 (Cell Biology)
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1.1 - Introduction to CellsCell Theory
Principles of the cell theory:
• All living things are composed of cells (or cell products)
• The cell is the smallest unit of life (unicellular organisms are the smallest organisms capable of
independent life)
• Cells only arise from pre-existing cells
Additionally:
• Cells always function as autonomous units
• Living structures are composed of discrete cells
• Larger organisms are always made up of microscopic cells (cells are small)
Evidence:
Microscopes have increased man's ability to visualise tiny objects;
All living things when viewed under a microscope have been found to be made of cells and cell
products (e.g. hair)
Experimental Evidence:
Cells removed from tissues can survive independently for short periods of time;
Nothing smaller than a cell has been found to be able to live independently;
Experiments by Pasteur have demonstrated that cells cannot grow in sealed and sterile
conditions;
Exceptions to the cell theory:
• Striated muscle bre (enable body movement)
• Multi-nucleated (due to fusing of cells).
• Challenges: cells always function as autonomous units.
• Aseptate fungal hyphae
• Continuous cytoplasm (not divided into discrete cells)
• Challenges: living structures are composed of discrete cells.
• Giant Algae
• Unicellular, yet very large
• Challenges: Larger organisms are always made up of microscopic cells (cells are small)
Cell Theory History
Timeline:
• 1590 - Janssen: rst compound microscope
• 1665 - Hooke: discovered cells
• 1673 - Leeuwenhoek: discovered microorganisms (‘animalcules’)
• 1838 - Schleiden: “All living plants are made of cells”, cell is smallest unit
• 1839 - Schwann: “All living animals are made of cells, cell is smallest unit
• 1855 - Virchow: “Where a cell exists, there must have been a pre-existing cell”
Functions of life
(Mrs. Heng)
• Metabolism: undertakes essential chemical reactions
• Eg…. Below
• Reproduction: Produce o spring, either sexually or asexually
• Sensitivity: Responsive to internal and external stimuli
• Homeostasis: Maintain a stable internal environment
• Excretion: Exhibit the removal of waste products (from metabolic processes)
fifi ff
, • Nutrition: Obtain food for energy for growth (exchange materials and gases with the
environment)
• Growth: able to change shape and irreversibly increase in size and change
Functions of life in unicellular organisms:
• Paramecium (heterotroph)
• Cilia (sensitivity)
• Cytostome: feeding groove (nutrition)
• Digestive enzymes (metabolism)
• Anal pore for solid waste and contractile vacuoles for liquid waste (excretion)
• Cell di usion (homeostasis)
• Divide asexually (reproduction)
• Scenedesmus (autotroph)
• Di usion (nutrition and excretion)
• Photosynthesis (metabolism)
• Internal asexual division of parent cells (reproduction)
• May form colonies for protection (sensitivity)
Surface Area to Volume Ratio
Energy production via metabolism requires the exchange of material between the internal and
external environment.
• Rate of metabolism = mass/volume (larger cells need more energy)
• Rate of material exchange = surface area (larger membrane = more material movement)
Cells have a high SA/V ratio:
• If metabolic rate exceeds the rate of material exchange (low SA/V ratio), the cell will eventually
die.
small cells have larger ratio of surface area to volume (than larger
cells):
surface area must be large enough to absorb nutrients e.g. oxygen;
surface area must be large enough to excrete waste products; e.g.
CO2
need for materials is determined by (cell) volume;
cell size is limited (by SA / vol ratio) so cells divide when they reach
a certain size (there is a limit to their volume);
this allows enough di usion across the membrane to meet the
needs of the cell.
As volume increases, SA/V ratio decreases; hence, cells divide to remain small.
Cells are perfectly sized; SA/V ratio is high, but volume is still small enough to ensure convenient
distances.
Utilising the ratio:
• Intestinal tissue - may form a ru ed structure to increase surface area
• Alveoli - have microvilli which increase total surface area.
Cell Scale
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, Magnification
Equation:
Magni cation = Image size (with a ruler) / Actual size (according to scale bar)
Drawing microscopic structures:
• Title: name of the specimen
• Magni cation or scale
• Label identi able structures (do not manipulate drawings)
Microscopes
Two main types of microscopes:
• Light microscopes
• Use lenses to bend light
• Can view living specimens in natural colour
• Often use dyes
• Electron microscopes
• Use electromagnets to focus electrons
• Can view dead specimens in monochrome
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