Characteristics of living organisms
The key features that define all living things
- Biology
- April 6, 2025
Table of Contents
Why are the Characteristics of Living Organisms Important?
All living organisms share 8 fundamental characteristics that allow them to survive, adapt, and interact with their environment. Understanding them helps us identify what makes something truly alive.
Biology is the study of living organisms, so to classify any organism as LIVING, they MUST perform these 8 processes.
We use the acronym MRS H GREN to represent these processes:
Movement
What is Movement?
Movement is the ability of living organism to change position.
Why Move?
Moving helps organisms find food, run from danger, and search for mates.
How Do Organisms Move?
Movement can take different forms across organisms. In animals, movement often involves muscle contractions, whilst plants change their orientation and grow towards sunlight (tropism) as a form of movement.
EXTRA: When organisms to move from one place to another this is called locomotion (eg. walking to to the kitchen)
Respiration
What is Respiration?
Respiration is a chemical reaction where glucose is broken down to release energy. This reaction mostly takes place in the mitochondria of cells.
Why Respire?
Respiration gives organisms the energy to live, grow, and move.
How Do Organisms Respire?
We can breakdown glucose to release energy either with oxygen (aerobic respiration) or without oxygen (anaerobic respiration).
“We release energy” means we produce a little molecule called ATP (adenosine triphosphate) which is the currency for energy (just like you pay your food with money, when we spend energy, we use ATP molecules).
Sensitivity
What is Sensitivity?
Sensitivity is the ability of living organisms to detect and respond to stimuli (changes in their environment).
Why Have Sensitivity?
Being able to respond to changes in your environment allows organisms to find food, and avoid danger.
How Do Organisms respond to stimuli?
The detection and response mechanism requires different parts of the body to communicate between each other by sending messages.
In plants, the response to a stimulus is usually much slower.
For example, when a plant can respond to light by growing towards sunlight. This is called phototropism (more on this and other “-tropisms” in 2.84)
In Animals there are 2 ways for messages to get sent around the body:
1. The nervous system (receptors, neurones and effectors which detect and respond to different stimuli using electrical impulses)
2. The endocrine system (chemical messengers, which travel in the blood, called hormones)
Homeostasis / Control
Some people call it “homeostasis” and some call it “control of internal conditions”. They mean the SAME thing.
What is Homeostasis?
Homeostasis is the maintenance of a constant internal environment. This “internal environment” being vitals like temperature, water levels, glucose levels, etc.
Why have Homeostasis?
By having the same internal environment, organisms can resist changes in their environment and survive under different conditions.
Example: We keep temperature the same in our bodies, around 37 degrees Celsius. When we get too hot, we sweat to cool down and when we’re too cold we shiver to warm up.
How Do Organisms Control Their Internal Environment?
There are many different mechanisms for controlling an organisms body temperature, water levels, glucose levels, etc., and these mechanisms even vary from species to species. We see some of these mechanisms in the “Coordination and Response” chapter.
Growth
What is Growth?
Growth is a permanent increase in size and complexity.
Why Grow?
Growing allows an organism to not only protect themselves from physical threats but also enables them to also reach maturity and reproduce to ensure the survival of its species.
How Do Organisms Grow?
Organisms grow through a combination of cell division and cell enlargement. Most growth typically occurs through a cell division called mitosis (covered in 3.28).
Reproduction
What is Reproduction?
Reproduction is the process by which living organisms produce offspring.
Why Reproduce?
Reproducing ensures that the species doesn’t go extinct. It also causes genetic variation, which allows organisms to adapt and evolve.
How do Organism Reproduce?
There are 2 types of reproduction: sexual and asexual. They both have different outcomes, and we cover it in much more detail in the “Reproduction” chapter.
Sexual reproduction: 2 individuals combine their DNA to produce offspring. The offspring is a combination of the maternal and paternal DNA, so the offspring is genetically different to both parents.
Asexual reproduction: 1 cell gives rise to 2 cells that are genetically identical to itself (clone).
Nutrition
What is Nutrition?
Nutrition is the process by which living organisms obtain food.
How Do Organism Nourish Themselves?
Animals consume other living organisms and digest them for energy. We call this type of nutrition heterotrophic nutrition.
Why get Nutrition?
The food that organisms obtain can be used in respiration and contains the energy they need to move, grow and perform any living process.
Plants make their own food from sunlight and carbon dioxide (photosynthesis) to get their energy. We call this type of nutrition autotrophic nutrition.
Autotrophs
Heterotrophs
Excretion
What is Excretion?
Excretion is the process by which living organisms remove waste products from their bodies.
Why Excrete?
There are many chemical reactions that are happening in our bodies and many of these reactions create waste products (eg. carbon dioxide, urea). Waste products can have toxic and even lethal effect on an organism if not removed. So excretion keeps the organism healthy by removing these harmful substances.
How Do Organisms Excrete?
Because not all waste products are alike, we have different ways to remove them from our bodies (eg. breathe out CO2, urinate out urea).
Plants on the other hand different waste products so they have different ways of excretion.
Classification
Let’s try to picture every single organism that ever existed. They’re all so different, right? So, for us to study each one properly we have to group them by their similarities. But we’re not going to group all red or all yellow animals together, rather group organisms with similar morphology and genetics.
The first broad groups in biology are the five kingdoms: animals, plants, fungi, protoctists, and bacteria.
What Are Eukaryotes and Prokaryotes?
Eukaryote comes from Greek, meaning “true nucleus” and Prokaryote “before nucleus”.
So, we call organisms that have a nucleus (animals, plants, etc.) “Eukaryotes” and ones that don’t (bacteria) “Prokaryotes”.
Eukaryotes
Plants
When we think about plants, we tend to think of flowering plants but there are a huge variety of other types, such as ferns, mosses, and conifers (like pine trees), each with their own unique features. Here are some characteristics that nearly all plants have:
- Multicellular organisms: Made of many cells working to perform various specialised functions (roots, stems, leaves, flowers etc.).
- Autotrophic: Plants photosynthesise by using their chlorophyll (the green pigment in their cells) to capture sunlight and convert it into food, which they store as starch. This makes them autotrophs (self- feeder in Greek). (more on photosynthesis on 2.18)
- No nervous system: Unlike animals, plants do not have nerve cells and do not have a nervous system. However, they can respond to their environment using chemical signals such as hormones like auxins (more on this on 2.84) to control growth and movement.
When we view a plant cell under a microscope, this is what it generally looks like:
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Organelle
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Nucleus
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Cytoplasm
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Cell Membrane
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Mitochondria
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Ribosomes
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Permanent Vacuole
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Chloroplast
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Cell Wall
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Flagella
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Plasmid
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Present
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Made of Cellulose
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Absent
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Animals
When we think of animals, we might picture creatures that move around, hunting for food, and using their senses to explore their surroundings. From mammals and birds to insects and fish, animals come in a huge range of shapes and sizes but often share some key characteristics:
- Multicellular organisms: Many cells working to form organs that perform various specialised functions (heart, liver, skin, brain etc.)
- Heterotrophic: Animals are heterotrophs (other-feeder in Greek) which means they rely on eating other organisms (plants, animals, or both) for energy because they can’t make their own food.
- Energy storage: Instead of storing energy as starch (like plants), animals store energy as glycogen, a carbohydrate they can break down for quick energy when needed.
- Nervous coordination: Animals have a nervous system that helps them sense and respond to their surroundings. They use special cells called neurons to send messages throughout the body, which enables movement, reaction to stimuli, and complex behaviours.
When we view animal cells under a microscope, this is what they generally look like:
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Organelle
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Nucleus
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Cytoplasm
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Cell Membrane
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Mitochondria
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Ribosomes
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Permanent Vacuole
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Chloroplast
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Cell Wall
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Flagella
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Plasmid
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|---|---|---|---|---|---|---|---|---|---|---|
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Present
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Only in Sperm Cells
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Absent
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Fungi
Fungi are a very diverse group of organisms that range from moulds and mushrooms to yeast. They differ significantly from plants and animals in their structure and way of obtaining food. Here are some characteristics that nearly all fungi have:
- Multicellular or unicellular: Fungi are usually multicellular (made up of many cells), but some, like yeast, are unicellular (single-celled).
- No photosynthesis: Fungi cannot carry out photosynthesis and make their own food.
- External digestion: Fungi are saprotrophs (rotten-feeder in Greek) which means that they release enzymes to break down dead or decaying food outside their bodies and then absorb the nutrients.
- Energy storage: Like animals, fungi store carbohydrates as glycogen, which they can break down when they need energy.
- Structure: The main body of most fungi is a network called mycelium, which is made up of thread-like structures called hyphae. These hyphae contain multiple nuclei and help fungi absorb nutrients from their surroundings.
- No nervous system: Fungi lack nerve cells and have no nervous system, so they cannot respond to their surroundings in the same way animals do.
- Parasitic behaviour: Some fungi are parasitic, meaning they feed on living material, such as plants or animals, often causing disease in their hosts.
When we look at fungal cells under a microscope, this is what they generally look like:
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Organelle
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Nucleus
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Cytoplasm
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Cell Membrane
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Mitochondria
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Ribosomes
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Permanent Vacuole
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Chloroplast
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Cell Wall
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Flagella
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Plasmid
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|---|---|---|---|---|---|---|---|---|---|---|
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Present
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Made of Chitin
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Absent
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Protoctists
Protoctists are a group of tiny organisms you might not think about often, but they’re all around us. You can find them in ponds, soil, or even inside other living organisms. Some look like tiny animals, others like plants, and some even switch between the two. Here are some characteristics that protoctists have:
- Unicellular organisms: Primarily single-celled organisms.
- Aggregation: While they are usually unicellular, some protoctists can group together to form larger structures, such as colonies or chains of cells, which may form structures like filaments.
- Diversity: Some protoctists resemble animal cells (e.g., Amoeba), while others are more similar to plant cells (e.g., Chlorella, a type of algae).
- Nutrition: Their diversity means that some protoctists are photosynthetic (like plant-like algae), while others feed on organic substances, similar to animals (e.g., Amoeba).
- No nervous system: Protoctists do not have a nervous system or nerve cells and therefore lack nervous coordination.
When we view protoctist cells under a microscope, this is what they can look like:
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Organelle
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Nucleus
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Cytoplasm
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Cell Membrane
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Mitochondria
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Ribosomes
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Permanent Vacuole
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Chloroplast
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Cell Wall
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Flagella
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Plasmid
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|---|---|---|---|---|---|---|---|---|---|---|
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Present
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Variable (may be present or absent)
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Variable
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Variable
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Variable
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Absent
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Prokaryotes
Bacteria
Bacteria are some of the smallest living organisms on Earth, and they’re everywhere. While they might be too tiny to see with the naked eye, bacteria are essential for processes like breaking down waste, recycling nutrients, and even making food like yogurt. Despite their size, bacteria come in many shapes and forms. Here’s a quick look at what all bacteria have in common:
- Unicellular: Single-celled organisms, often too small to see without a microscope.
- Nutrition: While some bacteria are photosynthetic (like certain types of bacteria found in ponds), most bacteria get their nutrients by feeding on other living or dead organisms, which can make them decomposers or pathogens.
- Simple structure: Bacteria don’t have a nucleus like plant or animal cells. Instead, their DNA floats freely in the cytoplasm in a region called the nucleoid. They also have a cell wall and a cell membrane to protect and support them.
- Circular DNA: Instead of having linear chromosomes like humans, bacteria have a single, circular strand of DNA that holds all the instructions for the bacterium’s life processes.
- Plasmids: Many bacteria have extra bits of DNA called plasmids, which can carry important genes, like those that help bacteria survive in tough environments or become resistant to antibiotics.
When we view bacteria cells under a microscope, this is what generally look like:
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Organelle
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Nucleus
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Cytoplasm
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Cell Membrane
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Mitochondria
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Ribosomes
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Vacuole
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Chloroplast
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Cell Wall
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Flagella
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Plasmid
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|---|---|---|---|---|---|---|---|---|---|---|
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Present
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Not Present
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Importance of Bacteria
Beneficial: Many bacteria play essential roles in nature and human life. They contribute to nutrient cycling in ecosystems, aid in food production (such as Lactobacillus bulgaricus in the fermentation of yogurt), and even help us digest food in our bodies.
Pathogenic: Some bacteria can cause diseases in humans, animals, and plants. Pathogens like Pneumococcus are responsible for illnesses such as pneumonia, making it crucial to understand and monitor these organisms.
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Organelle
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Nucleus
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Cytoplasm
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Cell Membrane
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Mitochondria
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Ribosomes
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Permanent Vacuole
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Chloroplast
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Cell Wall
|
Flagella
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Plasmid
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|---|---|---|---|---|---|---|---|---|---|---|
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Present
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Variable (may be present or absent)
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Variable
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Variable
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Variable
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Absent
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Eukaryotes vs Prokaryotes
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Characteristic
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Eukaryotes
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Prokaryotes
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Nucleus
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True nucleus with membrane-bound organelle
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No distinct nucleus; genetic material in nucleoid region
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Cell Size
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Generally larger and more complex
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Generally smaller and simpler
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Membrane-bound Organelles
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Present (e.g., mitochondria, chloroplasts)
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Absent
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Cell Wall
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Cellulose (Plants)
Chitin (Fungi)
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Present (peptidoglycan in bacteria)
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Chromosomes
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Linear chromosomes
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Circular chromosome
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Examples
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Plants, animals, fungi, protists
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Bacteria
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Nutritional Strategies
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Heterotrophic and autotrophic
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Heterotrophic and autotrophic
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Viruses
Viruses are a special case in biology because they’re not actually alive. We know that to be classified as “living” an organism has to exhibit the 8 processes in MRS H GREN, however viruses lack nearly all of them. Here are some of their key characteristics:
- Diverse Shapes and Sizes: Viruses vary greatly in shape and size, from rod-shaped to spherical and more complex forms, depending on the specific type of virus.
- Simple Structure: Viruses are essentially a protein coat wrapped around genetic material (either DNA or RNA). They lack cellular components like organelles, a nucleus, or a cell membrane, making them structurally very simple.
- Pathogen: Viruses act much like pirates. They invade a host cell, take it over and use the cell’s resources to make copies of themselves. They can only do this while inside a living cell, outside of a host, they’re completely inactive.
Examples of Viruses:
- Tobacco Mosaic Virus: Infects tobacco plants, causing discoloration in the leaves by interfering with chloroplast production.
- Influenza Virus: Causes the flu in humans, affecting the respiratory system.
- HIV: Attacks the human immune system, leading to AIDS by weakening the body’s ability to defend itself.
Pathogens
Viruses are what we call pathogens. Among viruses, other pathogens include some bacteria, fungi, and protoctists.
Pathogens are any organisms or particles that cause disease in other living beings.
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