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IB Cell Biology Notes
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CELL BIOLOGYCELLS
- Organelles are structures that perform specific functions: most surrounded by membrane
- Some membranes form channels which help transport substances from one part of the
cell to another part of the cell
Accepted parts of modern cell theory:
➢ Cell is fundamental unit of structure and function in living organisms
➢ All cells arise from pre-existing cells by division
➢ Energy flow occurs within cells
➢ Cells contain hereditary information (DNA) which is passed from cell to cell during cell
division
➢ All cells are basically the same in chemical composition in organisms of similar species
➢ All known living things are made up of one (unicellular organism) or more cells
(multicellular)
➢ The activity of an organism depends on the total activity of independent cells
Exceptions:
➢ Viruses are considered alive by some, but they are not made up of cells - by definition of
cell theory, they are not alive
➢ The first cell didn’t originate from a pre-existing cell - there was no exact first cell since
the definition of cell is imprecise
➢ Mitochondria and chloroplasts have their own genetic material, and reproduce
independently from the rest of the cell
Magnification = size of image (drawing size)/size of specimen (actual size)
Micrometers to mm
Caveats to the Cell Theory
Striated muscle fibers:
➔ Type of tissue used to change position of the body
➔ Building blocks of this tissue = muscle fibers
➔ Have their own genetic material and energy release system
➔ But muscle fibers aren’t typical, they’re larger than animal cells
➔ Cells fuse to form long fibers with multiple nuclei
➔ CHALLENGES THE CELL THEORY:
◆ Cells always function autonomously
◆ Have multiple nuclei
Aseptate fungal hyphae:
➔ Fungi have narrow thread-like structure called hyphae
, ➔ In some fungi, hyphae is divided into cell-like sections by cross walls called septa (but
there are no in aseptate fungi)
➔ Each hypha is an uninterrupted tube-like structure with many nuclei on them
➔ Some fungi don’t have walls separating cells in hyphae; continuous cytoplasm
➔ CHALLENGES THE CELL THEORY:
◆ Living things are made of discrete cells - absence of division
Giant Algae (unicellular):
➔ Algae organisms that feed on photosynthesis and store genes in nuclei (but simpler than
plants in structure)
➔ Many consist of one microscopic cell (unicellular algae) → basis of most marine food
chains
➔ Giant algae (acetabularia) can grow to 100mm, even though they are only one cell with
one nucleus
➔ CHALLENGES THE CELL THEORY:
◆ Larger organisms are always made of small cells - but it can be seen from the eye
CELL SIZE LIMITATION
● Cells need to be small
○ Their surface area/volume ratio is large
● Why?
○ Metabolic rate
■ Big cells have more room for chemical reactions. BUT…
● Need more resources (IN)
● Create more waste and heat (OUT)
■ surface/volume ratio decreases as cells gets bigger
● Bigger volume = less cell membrane surface for transport
○ Communication
■ Distance from nucleus to other organelles is shorter in small cells
UNICELLULAR ORGANISMS
★ Only have one cell
★ Include both prokaryotes and some eukaryotes
★ Prokaryote (P)
○ Organism made of a single prokaryotic cell (first cells on Earth)
○ Types: Bacteria and archaea
○ No nucleus to hold DNA -- Single loop of DNA floats in the cell’s cytoplasm
○ No membrane bound organelle and no mitochondria
○ Have pilli (strings on the diagram) and flagella (tail)
○ Outer components
■ Cell wall (contain peptidoglycan) and cell membrane (common in all)
■ Additional layer = capsule (not found in all) → allows it to cling to surfaces
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