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Biology IB notes
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Lecture notes of 64 pages for the course AQA Biology at Abacus College, Oxford ()
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Introduction to Cells (1.1):The Cell Theory:
Organs are made up from tissues, but cells are too small to be observed without a microscope.
The cell theory was developed when similar features were noticed in the different cells of many organisms.
Unicellular organisms: organisms that consist of one cell.
Multicellular organisms: organisms that are composed of many cells.
Cells have several features in common, such as:
Every cell is surrounded by a membrane, which separates the cell's contents from everything else outside.
Cells contain genetic material which stores all of the functions needed for cellular activity (DNA).
Many of the cellular activities are chemical reactions, catalyzed by enzymes produced inside the cell.
Cells have their own energy release system that powers all of its activities.
There are some exceptions to the cell theory as some organisms' cells are not typical.
Using Light Microscopes
Drawing Cells:
Use a sharp pencil with a hard lead to draw single sharp lines.
Join up lines carefully to form continuous structures such as cells.
Draw lines freehand, but use a ruler for labelling lines.
Calculation of Magnification and Actual Size:
A typical school microscope has three levels of magnification:
x40 (low power).
x100 (medium power).
x400 (high power).
Magnification = size of image / actual size of specimen.
Testing the Cell Theory:
There are several atypical examples that contradict the cell theory, including:
Striated muscle fibers:
o They are extremely long in comparison to other cells (about 3cm long in humans).
o They have many nuclei, sometimes several hundreds.
Hyphae:
o Hyphae make up fungi, and are sometimes divided into cell-like structures by walls called septa.
o Some hyphae don’t have septa (e.g.: aseptate hyphae), making them seem as continuous structures
with multiple nuclei.
Algae:
o Giant algae could reach up to 100mm in length with only one nucleus.
Unicellular Organisms:
The one cell making up unicellular organisms must be able to carry out all the functions of life.
Unicellular organisms are able to carry out at least 7 functions of life:
Nutrition: obtaining food to provide the needed energy and materials for growth.
Metabolism: chemical reactions inside the living cells such as respiration.
Growth: an irreversible increase in size.
Response: the ability to react to changes in the environment.
Excretion: getting rid of the waste products of metabolism.
Homeostasis: keeping conditions inside the organism within tolerable limits.
Reproduction: producing an offspring, either sexually or asexually.
Many unicellular organisms also have the ability to move.
Limitations on Cell Size:
The metabolic rate of the cell is proportional to its volume.
For metabolism to continue, the cell must absorb reactants and remove waste. The rate at which substances
move through the cell membrane depends on its surface area.
, A small volume-to-surface area ratio may prevent cells from efficiently letting substances in and out of their
membranes, and might cause them to overheat.
As the cell grows, volume (cytoplasm) increases in ratio to the surface area (plasma membrane), which causes:
Many organelles forming that require more nutrients and produce more waste.
Lack of energy as respiration does not occur fast enough.
Excretion to be too slow.
Diffusion to be too slow.
Overheating.
Functions of Life in Unicellular Organisms:
Paramecium is an organism that can be easily cultured in a laboratory.
As a unicellular organism, the paramecium contains:
A nucleus, which can divide for the often asexual reproduction in which one parent cell divides into two
daughter cells.
Food vacuoles that contain organisms ingested by the paramecium.
A cell membrane to control chemicals entering and leaving the cell.
Contractile vacuoles that fill up with water and expel it through the cell plasma to keep the cell within
tolerable conditions (homeostasis and excretion).
A cytoplasm in which chemical reactions take place.
Cilia around the cell plasma to move the cell around (response).
o Learn to draw.
Chlamydomonas is a unicellular organism that lives in soil and freshwater habitats. It carries out
photosynthesis but is not a plant, and its cell wall is not composed from cellulose.
As a unicellular organism, the Chlamydomonas contains:
A nucleus which can divide into another with identical genetic information as a form of asexual reproduction
or fuse and divide with another nucleus as a form of sexual reproduction.
A cytoplasm in which chemical reactions take place.
A freely permeable cell wall, not made of cellulose.
A cell plasma to control what enters and leaves the cell.
Contractile vacuoles at the base of the flagella to fill up with water and expel it through the cell plasma to
keep the cell within tolerable conditions (homeostasis and excretion).
Chloroplasts used for photosynthesis (growth and nutrition).
Two flagella that move the cell around, often to the areas with the most light according to the light-sensitive
"eyespot" (response).
o Learn to draw.
Multicellular Organisms:
Multicellular organisms consist of a single mass of cells fused together.
Undetailed.
Cell Differentiation in Multicellular Organisms:
Differentiation: the development of cells in different ways to carry out specific functions.
Cell differentiation can develop tissues in multicellular organisms.
Through differentiation, cells become specialized and specific to a certain function.
Specialized cells can develop ideal structures for their functions and make the necessary enzymes, making
them more efficient (e.g.: RBC's do not have a nucleus for more space).
Gene Expression and Cell Differentiation:
Differentiation involves the expression of some genes and not others in a cell's genome.
The human genome contains over 2,500 genes, but less than half of those are needed or used in a single
specialized cell.
The genes used in a cell are said to be "expressed"
The control of gene expression is the key to development.
Stem Cells:
Early embryonic cells, also called stem cells, are capable of developing and differentiating into any type of
adult cells, granting stem cells therapeutic uses.
, Stem cells have two key properties:
They can divide repeatedly to produce copious quantities of new cells. Therefore, they can be used to replace
or repair damaged organs.
They are not fully differentiated, meaning that they can differentiate into different types of cells and produce
them.
Stem cells can be used for therapeutic purposes and regenerate tissues such as those in the skin, kidneys and
hearts.
Stem cells can be used for non-therapeutic purposes and divide into large quantities of striated muscle fibers,
or meat, consumable by humans (beef).
At their early stages, stem cells are the most versatile as they are not differentiated and can specialize into any
type of cell. When committed, a cell can divide only into its type and is no longer a stem cell.
Stem cells are present in adult bodies, but in smaller quantities and more specific places such as the bone
marrow, skin and liver. They assist in reparations of various tissues.
Stargardt's Disease:
The full name of the disease is Stargardt's macular dystrophy.
It s a genetic disorder that develops for children between 6 and 12 years old.
The disorder causes a membrane protein used for active transport in the retina cells to malfunction.
The loss of vision can be so severe as to register a person as blind.
Experimenting with mice stem cells showed that injecting them into eyes caused no rejection, tumors or
problems.
The stem cells caused no side-effects and improved vision when tried on humans.
Heart Disease:
Ethics of Stem Cells:
Stem cells can be obtained form a variety of sources:
Deliberately fertilizing egg cells with sperm and allowing the zygote to develop into between 4 and 16 cells.
Blood from the umbilical cord can be taken and the cells frozen for possible use later in the baby's life.
Some adult tissues such as bone marrow have stem cells.
Stem cells are extremely helpful but have been regarded as unethical.
Stem cells kill an embryo when taken from the embryonic cells. People argue that embryos only become
human when they develop more - after several weeks.
In vitro fertilization (IVF) is creating humans for the sole purpose of taking stem cells, which is said to be
unethical.
IVF involves hormone treatment for women as well surgical risks in extracting the stem cells. If women are
paid for this it could exploit vulnerable groups such as college girls.
At the same time, they are very contributive to curing diseases.
Ultrastructure of Cells (1.2):
The Resolution of Electron Microscopes:
Electron microscopes have much higher resolutions than light microscopes.
The light microscope can have a maximum resolution of 0.2um (200nm), but not higher since It is limited by
the wavelength of light (400nm-700nm). This is why the maximum magnification of a light microscope is often
x400.
Electron microscopes have a much higher resolution due to the shorter wavelength of an electron beam. They
have the resolution of 0.001um (1nm) which is 200 times higher than that of the light microscope, allowing
the to reveal the ultrastructure of cells.
Resolution in Millimeters Resolution in Resolution in
(mm) Micrometers(um) Nanometers(nm)
Unaided Eyes 0.1 100 100,000
Light Microscope 0.0002 0.2 200
Electron 0.000001 0.001 1
, Microscope
Prokaryotic Cell Structure:
Prokaryotes have a simple cell structure without compartments.
Prokaryotes do not have a nucleus, making them the first organisms to evolve on Earth and still the simplest.
They are everywhere.
All cells have a cell membrane, but some, including prokaryotes, have a cell wall, which is much stronger and
gives the cell its structure and prevents it from bursting.
The cell wall of prokaryotes contains peptidoglycan and is often said to be extracellular.
Prokaryotes have an uninterrupted cytoplasm with no compartments or nucleus, but is still complex with its
enzymes and chemicals; less than eukaryotic cells, though.
Prokaryotic cells only have ribosomes, smaller than those of eukaryotic cells, and a nucleoid in their
cytoplasm.
The nucleoid (nucleus-like but not a true nucleus). appears lighter than other organelles in many electron
micrographs, and contains the naked DNA, which is not associated with proteins (that's why is appears lighter
in micrographs).
Cell Division In Prokaryotes:
Prokaryotes divide by binary fission.
Binary fission is used by prokaryotes as a form of asexual reproduction.
In binary fission, prokaryotic cells' chromosomes are replicated, and the two copies move to opposite ends of
the cell, and the cytoplasm quickly divides afterwards, forming two genetically identical cells.
Learn to draw.
Eukaryotic Cell Structure:
Eukaryotes have a compartmentalized cell structure.
Eukaryotic cells have structures inside them that are compartmentalized, and the partitions are either single
or double membranes.
The internal structure of a eukaryotic cell is much more complex than that of a prokaryotic cell.
The compartments in a eukaryotic cytoplasm are known as organelles.
There are several advantages in being compartmentalized:
Enzymes and substrates for a particular process can be much more concentrated than if they were spread
across the cytoplasm.
Substances that can cause damage to the cell can be safely stored inside the membrane of an organelle.
Conditions such as PH can be maintained at an ideal level for a certain process.
Organelles with their contents can be moved around within the cell.
Learn to draw.
The cell contains:
Nucleus:
o The nuclear membrane is double and has pores through it. It contains chromosomes, which contain
DNA associated with histone proteins.
o Uncoiled proteins are spread through the nucleus and called chromatin. There are often densely
staining areas of chromatin around the edge of the nucleus.
o The nucleus is where DNA is replicated and transcribed to form mRNA, which is exported via the nuclear
pores to the cytoplasm.
Rough Endoplasmic Reticulum (rER):
o The rER consists of flattened membrane sacs called cisternae which have ribosomes attached to their
outside.
o Ribosomes are larger than in prokaryotes and classified as 80S.
o The main function of the rER is to synthesize protein for secretion.
o Proteins synthesized by the rER are exported into the Golgi apparatus then outside the cell by
exocytosis.
Golgi apparatus:
o The organelle consists of flattened membrane sacs called cisternae, like rER. However, these are
shorter, curved, do not have as many vesicles nearby and do not have ribosomes.
o The Golgi apparatus processes proteins brought in by the rER.
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