The notes hereby are created as a summary of IB Biology HL Unit 1 topic — cell biology. The notes are gathered mainly from Pearson book as well as other supplementary sourcers.
IB BIOLOGY UNIT 1: CELL BIOLOGY
CELL THEORY
● The main principles of cell theory are:
1. All organisms are composed of one or more cells.
2. Cells are the smallest units of life.
3. All cells come from pre-existing cells.
● Certain types of cells do not conform to the cell theory:
● Striated Muscle Fibers: Muscle cells fuse to form fibers that are very long and hence, they
have multiple nuclei despite being surrounded by a single continuous membrane. This
challenges the idea that cells always function as autonomous units and be able to carry out
activities only for their own survival.
● Aseptate Fungal Hyphae: Fungi may have structures called hyphae, which are separated into
cells by internal walls. Some fungi are not partitioned by internal walls and hence, have a
continuous cytoplasm with multiple nuclei along the length of the hyphae. This challenges the
idea that living structures are composed of discrete cells.
● Giant Algae: Certain species of unicellular algae may grow to very large sizes. This challenges
the idea that larger organisms are always made of many microscopic cells.
● Development of the cell theory occurred largely as a result of advancements in the field of microscopy:
● Robert Hooke — Used light microscope to look at cork and saw tiny chambers which he named
"cell"
● Anton van Leeuwenhoek — Was first to see living microscopic organisms, which he called
"animalcules", while observing pond water
● Matthias Schleiden — Stated that "All living plants are made of cells." and is credited for
developing the first two principles of cell theory, along with Schwann
● Theodore Schwann — Stated that "All living animals are made of cells." and is credited for
developing the first two principles of cell theory, along with Schleiden
● Rudolph Virchow — Stated that "Where a cell exists, there must have been a pre-existing
cell" and is credited for developing the third principle of the cell theory
BIOGENESIS
● Biogenesis describes the principle that living things only arise from other living things by reproduction.
● The law of biogenesis is largely attributed to Louis Pasteur, who demonstrated that bacterial growth in
nutrient broths was due to contamination by pre-existing cells.
1. Broths were stored in vessels that contained long
tubings that did not allow external dust particles to
pass.
2. The broths were boiled to kill any microorganisms
present in the growth medium.
3. It was observed that growth occurred in the broth
only if the flask was open, exposing the contents to
contaminants from outside. From this, it was
concluded that bacterial growth did not
spontaneously occur.
, ENDOSYMBIOSIS
● Eukaryotic cells are believed to have evolved from early prokaryotes that were engulfed by
phagocytosis.
● The engulfed prokaryotic cell remained undigested as it contributed new functionality such as the ability
to photosynthesize to the engulfing cell.
● Over generations, the engulfed cell lost some of its independent utility as the eukaryotic cell was
providing it with protection and hence, the engulfed cell eventually became a supplemental organelle.
● Mitochondria and chloroplasts are both organelles that are thought to have arisen by endosymbiosis.
The evidence that supports the theory are as follows:
● double membrane structure;
● possession of their own naked and circular DNA;
● possession of their own 70S ribosomes;
● size near to that of prokaryotic cells';
● reproduction by a fission-like process independent of the host cell;
● susceptibility to be affected by antibiotics suggesting bacterial origins.
FUNCTIONS OF LIFE
● All living things, whether they are unicellular or multicellular, carry out 7 basic functions to survival:
● Metabolism: Living things undertake essential chemical reactions
● Reproduction: Living things produce offspring, either sexually or asexually
● Response: 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 gasses with the environment
● Growth: Living things can move and change shape or size
● Viruses are not considered living organisms because they cannot carry out functions of life on their own
without a host cell.
● As unicellular organisms are composed of a single cell, this cell must be able to carry out all the life
functions:
● PARAMECIUM (Heterotroph)
● Metabolism: Food particles are enclosed within small
food vacuoles that contain enzymes for digestion.
● Reproduction: Paramecium divides asexually through
binary fission although horizontal gene transfer can
also occur via conjugation.
● Response: Paramecia are surrounded by small hairs
called cilia which allow them to move.
● Homeostasis: Contractile vacuoles are involved in
the maintenance of homeostasis by keeping the
water balance.
● Excretion: Solid wastes are removed via an anal
pore, while liquid wastes are pumped out via
contractile vacuoles.
● Nutrition: Paramecia engulf food via a specialized
membranous feeding groove called cytostome.
, ● SCENEDESMUS / CHLORELLA (Autotroph)
● Metabolism: Chlorophylls allow organic molecules to be produced via photosynthesis.
● Reproduction: Daughter cells form through asexual division of the parent cell.
● Response: Scenedesmus may exist as unicells or form colonies for protection.
● Nutrition & Excretion: Scenedesmus exchange gasses and other essential materials via
diffusion.
SURFACE AREA : VOLUME
● The SA:V ratio effectively limits the size of the cells up until the point that they can still carry out the
functions of life.
● As a cell grows, volume (units3) increases faster than surface area (units2), leading to a decrease in
surface area to volume ratio with increasing size.
● Growing cells tend to divide and remain small in order to maintain a high SA:V ratio suitable for survival
as:
● the cell cannot get enough material with increasing size as there will be less space available in
the cell and the gates on the plasma membrane will not be enough to get the necessary
materials in;
● the nucleus loses control over the cell functions as so many materials accumulate in the cell.
MICROSCOPES & MAGNIFICATION
● There are two main types of microscopes: light microscope and electron microscope.
Light Microscope Electron Microscope
use lenses to bend light use electromagnets to focus electrons
provide significantly greater
magnify images up to 2000x magnifications and resolutions up to
500000x
can be used to view living or dead can be used to view dead specimens in
specimens in natural color monochrome (not in natural color)
inexpensive to purchase and operate expensive to purchase and operate
complex and lengthy specimen
simple and easy specimen preparation
preparation
● To calculate the linear magnification of a drawing or image, the following equation should be used:
Magnification = Image Size / Actual Size
● The relative sizes of biological materials are as follows:
● Eukaryotic Cell ~ 100 μm
● Organelle ~ 10 μm
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