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Biology notes IGCSE Raida1- Characteristics and classification of living organisms
Characteristics of living organisms: all living organisms undergo these processes:
Movement: an action by an organism or part of an organism causing a change
of position or place
Respiration: the chemical reaction in cells that break down nutrient molecules
and release energy for metabolism
Sensitivity: The ability to detect or sense stimuli in the internal or external
environment and to make appropriate responses/reflexes
Growth: a permanent increase in size and dry mass caused by an increase in
cell number, cell size, or both
Reproduction: the process that makes more of the same kind of organism
Excretion: removal from organisms of the waste products of metabolism
(chemical reactions in cells including respiration), toxic materials, and
substances in excess of requirements
Nutrition: taking in of materials for growth, energy, and development; plants
require CO2, water, ions, and light, animals need organic compounds, ions,
and usually water
> Viruses aren’t classified as living organisms as they can't complete these processes
Viruses are non-cellular and consist of genetic material surrounded by a
protein coat
When viruses reach a target cell, they inject this genetic material into the cell,
which is then replicated to reproduce the virus and harm the target cell,
which isn't able to function normally
Classification:
Used to group different species. A group of organisms belongs to the same
species if they can reproduce fertile offspring
Species: a group of organisms that can reproduce to produce fertile offspring
> Organisms are classified by evolutionary relationships; these relationships can be
found by studying physical characteristics and DNA base sequences
Organisms were categorised based on Morphology (study of
biological/external structure of organisms) and Anatomy (analysis of the
structure of organisms by dissection); the structure of bones and organs were
through dissections, and organisms were grouped based on similarities
> DNA base sequencing (ex. Human genome project) allows the DNA for each
species to be mapped and compared, and by that, species with similar ancestors are
more easily found as they have a close DNA base sequence
This is more accurate than morphology and anatomy/ comparing physical
characteristics since different species living in the same habitat often evolve
similar traits and resemble one another
Organisms that share a more recent ancestor (are more closely related) have
DNA base sequences that are more similar to those that share only a distant
ancestor
,> Organisms are named based on the Binomial system; naming system in which the
scientific name of an organism is made up of two parts showing the genus and
species
The five kingdoms:
Plants, Animals, Fungi, Protista, and Prokaryotes,
> features of cells can help classify living organisms into kingdoms.
Ex. Animal cells do not contain cell walls or chloroplasts, while Plant cells do
Features of all cells:
Cytoplasm: a jelly-like material within the cell in which reactions occur; it
contains organelles such as the nucleus and ribosomes
Cell membrane: a thin membrane that surrounds the cell and controls the
entry and exit of substances
DNA: genetic material contained in the nucleus that codes for proteins
Ribosomes: site of protein synthesis
Enzymes: catalyse reactions such as respiration in the cell
ANIMAL KINGDOM:
-> vertebrates: contain backbones, ex. Mammals, birds, fish, reptiles, and amphibians
-> arthropods: do not contain backbones, ex. Myriapods, crustaceans, arachnids, and
insects
PLANT KINGDOM:
-> flowering plants: 1. Monocotyledons, 2. Dicotyledons
-> non-flowering plants: ferns (reproduce through spores rather than seeds
Difference between Monocots and Dicots:
Monocots: 3 petals, leaves have parallel veins
Dicots: petals in 4s or 5s, leaves have branched/reticulated veins
2- Organization of the organism
Cells can be viewed through a microscope to study their structure
Plant and animal cells are known as eukaryotic cells as they contain a nucleus
and membrane-bound organelles such as ribosomes and mitochondria.
Bacteria are prokaryotes since they don’t contain the above or rough
endoplasmic reticulum.
Features of eukaryotic cell structure:
Cytoplasm: jelly-like material found within the cell in which reactions occur
and contains organelles such as ribosomes and vesicles
Cell membrane: a thin membrane that surrounds the cell and controls the
entry and exit of substances
DNA and Nucleus: the nucleus contains genetic material in the form of DNA,
which codes for proteins; DNA replication occurs in the nucleus
RER (rough endoplasmic reticulum): the RER surrounds the nucleus and
ribosomes are attached to it.
Ribosomes: the site of protein synthesis
, Mitochondria: site of respiration, it provides energy for the cell to function
Vesicles: used to transport materials in the cell
> Eukaryotic plant cells additionally contain:
Large vacuole: a fluid-filled sac containing mineral salts, sugars, amino acids,
waste substances, and pigments that colour the cell and attract pollinating
insects
Chloroplasts: the site of photosynthesis which allows the plant to convert light
energy into glucose
Cell wall: gives the cell structure and prevents bursting and controls what
enters and exits the cell
> Levels of organisation:
Tissue: a group of similar cells working together to carry out a particular
process
Organ: a group of tissues working together to carry out a specific function
Organ system: a group of related organs working together to carry out
functions in the body
> Specialised cells:
Cells and tissues are specialised to carry out their particular function, examples:
Ciliated cells: found in the trachea, they have hair-like projections called cilia
which move together to transport mucus, dust, and bacteria upwards to the
throat
Root hair cells: are adapted to have a large surface area, this speeds up the
rate of osmosis and mineral ion uptake in plants
Xylem vessels: used to transport water through plants in transpiration. The
xylem is made from hollowed-out dead cells that have the ends removed to
make a tube for water to pass through. They have a thick cell wall to provide
structural strength and are thin to allow capillary action
Palisade mesophyll cells: where photosynthesis occurs, they’re tall and closely
packed to efficiently absorb light and contain lots of chloroplasts for
photosynthesis, they're also placed at the top of the leaf where most of the
light hits, enabling them to absorb as much light energy as possible
Nerve cells: adapted to conduct/rapidly transmit electrical impulses. Nerve
cells are myelinated, which insulates the cell and prevents the impulse from
weakening and slowing down, they contain lots of mitochondria to provide
energy. Dendrites have a large surface area and are branched to receive
impulses from many other neurons
Red blood cells (RBC’s): contain haemoglobin which allows them to carry
oxygen around the body, they have a biconcave shape which increases their
surface area for carrying oxygen and allowing for rapid diffusion, a thin cell
membrane to decrease the diffusion distance, and do not contain a nucleus
thus having more space for oxygen transport
Sperm cell: contain lots of mitochondria so the cell has enough energy to
reach the egg cell, a tail to allow movement, and digestive enzymes to
penetrate the egg cell membrane
> Magnification:
Actual size = image size/magnification
3- Movement in and out of cells
, > Diffusion:
The net movement of particles from an area of high concentration to an area
of low concentration down the concentration gradient as a result of their
random movement, the energy for diffusion comes from the kinetic energy of
the molecules
Solutes and gases such as CO2 and O2, are able to diffuse in and out of cells
across the membrane, this is important as these substances are crucial to
metabolic reactions which occur within the cell, i.e respiration, and
photosynthesis, without them the process wouldn't occur and the cell would
die
Factors affecting the rate of diffusion:
Surface area: as the surface area increases, the rate of diffusion increases, this
is because there is more space/area available for the substances to diffuse
through
Temperature: as the temperature increases, so does the rate of diffusion,
because the molecules gain kinetic energy and thus move faster
Concentration gradient: as the concentration gradient increases, so does the
rate of diffusion
Diffusion distance: a greater diffusion distance slows the rate of diffusion as
molecules must travel further
> Osmosis:
The net movement of water molecules from a region of higher water potential
(dilute) to a region of low water potential (concentrated) through a partially
permeable membrane
Turgid: when cells are swollen due to a high-water content
Turgor pressure: the pressure on the cell wall from the cell membrane
pushing upon it, as a result, the cell becomes turgid as water moves into the
cell via osmosis
Flaccid: when water moves out of the cell via osmosis the cell shrinks but the
cell membrane doesn't peel away from the cell wall, if more water leaves the
cell it becomes plasmolyzed
Plasmolysis: occurs when there is too little water in cells. For plant cells, the
cell membrane peels away from the cell wall
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