Chapter 1: Living Organisms

Lesson 1: Cells Lesson 2: Plants Lesson 3: Animals

Lesson 1: Cells

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All living things are made up of cells. A cell is the smallest unit of living matter that can carry out the basic functions of life.

Cells reproduce through cell division, Cell division is where one cell multiplies into two cells and the process continues until a different (regulatory) process reduces the rate of cell division. This way the number of cells increases. Some cells also die in the process. Each cell that you observe came from another cell.

A Unicellular organism is made up of only one cell that can independently carry out life functions.

A Multicellular organism is made up of multiple cells. Humans, frogs, insects are all multicellular organisms.

Cells in multicellular organisms are capable of specializing. Specialzing is a simple way of indicating that cells can develop the ability to perform specific functions.

Cells of the same type (and function) are grouped together to form tissues. Tissues can also group together to form an organ. An organ is a group of tissues that perform a function. For example, the heart is an organ and its function is to pump blood. Many organs work together to form an organ system. The heart, together with blood, and blood vessels make up the circulatory system whose function is to pump and distribute blood to all parts of the body.

Scientsists estimate that there are more than 1 billion kinds of organisms, most of which not yet even been identified.

Parts of a Cell: The human body has more than 200 different kinds of cells. Plant and animal cells have several basic structures in the cell, called organelles.

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Animal Cells

• Nucleus: The nucleus is the cell’s control center. It is a large, round organelle usually found in the center of the cell. It has a membrane with pores, or openings, that allow certain materials to pass in and out. The nucleus contains DNA, which contains the master plans for all the cell’s activities. It sends signals to all other parts of the cell and results in processes like cell growth, cell movement, and cell division.
• Cell Membrane: The cell membrane is a layer around the outside of the cell. It wraps around the cell in somewhat the same way your skin wraps around you. It gives the cell its shape. The cell membrane controls what materials move into and out of the cell. Only certain substances are able to enter and leave the cell.
• Cytoplasm: The cytoplasm is a gel-like liquid that occupies the region inside the cell from the nucleus to the cell membrane. The cytoplasm is made mostly of water. A variety of organelles float in the cytoplasm. The cytoplasm supports all the cell’s structures. It is constantly moving through the cell in a stream-like motion. Some of the cell’s functions take place in the cytoplasm.
• Mitochondria: These are oval organelles inside the cytoplasm. They are surrounded by a plasma membrane (similar to cell membrane). Mitochondria are the cell's powerhouse. They break down food, which releases energy for the cell to use. Some cells are more active than others and require more energy. Cells that require a lot of energy, such as muscle cells, usually have more mitochondria than cells that are less active, such as fat cells.
• Vacuoles: A vacuole is a membrane-covered structure used for storage. It can store water, food, or waste material. The nucleus can signal a vacuole to release whatever it is storing. Some animal cells have many small vacuoles and some may not have any vacuoles.

Plant Cells

Cell Wall: Plant cells have an additional outer covering around the outside of the cell. This layer is called the cell wall. The cell wall is a stiff structure outside the cell membrane. It provides the plant cell with strength and extra support and gives the plant sell a regular shape.

Chloroplast: Plants make their own food using sunlight in a process called Photosynthesis. This process requires structures inside the cells called chloroplasts. A chloroplast is a green structure where the energy from sunlight is used to produce food for the plant. Chloroplasts are green because they contain a chemical called chlorophyll. Chlorophyll is able to use the energy in sunlight. Many plant cells are green because of the chlorophyll in their chloroplasts. Plant cells that lack chloroplasts are not green. Chloroplasts are mainly found in the leaves, and in some cases in the stem.

Vacuole: Plant cells also contain a vacuole, but unlike animal cells, plant cells usually have one large, central vacuole. In plant cells this vacuole stores excess water and provides extra support. The extra water in the vacuoles of plant cells keeps the plant from drying out. When a plant needs extra water the vacuoles release the water they have stored into the cells.

From a cell to an Organism

Multicellular organisms are made up of multiple cells, that perform different functions, intially forming tissues. Several tissues come together to form organs. For example, the heart is an organ. Several organs can function together to form an organ system. For example, the circulatory system is an organ system. Its made up of the heart, blood vessels and blood. Many systems work together to make an organism.

Lesson 2: Plants

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All plants need air, water and sunlight. Plants can obtain air and sunlight directly from their environments. Water transport within the plant requires more complex processes.

Plants can be classified into two types, depending on how they transport water. Non-vascular plants are small and survive without a water transport system. Mosses, for wxample, reach heights of a centimeter or less, their parts are so close to the ground and they can absorb water directly. Vascular plants are larger can grow to heights over 60 meters. Larger trees have a vascular system, which is a series of hollow tubes. These tubes can transport water and nutrients to the top of the tallest trees.

Vascular plans are further divided into seed and seedless plants. seed plants indicate that they are flowering plants and produce seeds. A seed can develop into a plant when the environmental conditions are favorable. Seeds have a a protective coating that prevents the seed from drying out or getting damaged. Seedless plants, like ferns, produce spores for reproduction. A spore is a single cell that can develop into a new plant exactly like the plant that produced it.

There are two main types of seed plants: gymnosperms and angiosperms. A Gymnosperm is a seed plant that does not produce a flower. These include pines, firs and other cone- bearing trees. An Angiosperm is a seed plant that produces flowers. All angiosperms produce seeds that are covered by some kind of a fruit. In some angiosperms, the fruits a more obvious, such as apples and peaches et cetera. In other angiosperms, the fruits are less distinct, such as grasses. There are over 250,000 different kinds of angiosperms making this group one of the largest of its type.

Roots: A root is the part of the plant that absorbs water and minerals, stores food and anchor the plants to the ground. Roots absorb water using fuzzy hairs called root hairs. A root hair is a threadlike projectiong from a plant root. Each root hair is less than 1mm in length but together they soak up moisture like a sponge.

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The outer layer of a root and the whole plant is the epidermis. The epidermis is where the root hairs are located. The cortex layer is located just under the epidermis. It is used to store food and nutrients. The vascular system is located in the center of the root. This system transports water and minerals absorbed by the root hairs.

Different plants have specialized roots for their environment.

Aerial roots are roots that never touch the ground. They anchor the plant to surfaces or to other trees and they absorb water from the air and from rain, not from the soil.

Fibrous roots are thin, branching roots that do not grow too deep into the ground but they cover a large area. A single clump of grass has over 600km of fibrous root.

Taproots have a single stalk-like root that plunges deep into the ground. Several smaller roots branch off of the main main taproot. Pine trees and trees that grow in dry areas often have taproots.

Prop roots usually grow at the bottom of a plant's stem. They prop up so as to prevent the plant from being knocked over. Corn/maize plants and mangrove trees have prop roots.

Stems: A plant's stem has two functions, as a support structure and as a transport system for the plant. The stem supports the tree's leaves, flowers, fruits and other parts. Stems can either be soft or woody. Soft stems, like those found in grasses, can easily bend. They are usually green because they contain chlorophyll and can produce food for the plant. Shrubs and larger trees have woody stems, which are covered with bark. The bark is a tough outer covering that serves as a protective layer. Woody stems do not contain chlorophyll.

The transport system in the stem actually begins in the root. There are two kinds of cells that make up the system. Xylem is a series of tubes that move water and minerals up the stem. Transport through the xylem only happens in one direction, away from the roots to the leaves. Phloem moves sugars that are made in the leaves to other parts of the plant. Phloem tissue is a two way transport system. For example, in carrots, sugars are transported down to the root. The phloem tissue also transport sugars up from one part of the plant to another. The layer that separates the xylem and phloem tissue is called the cambium.

Leaves : The most important function of the leaves is to carry out photosynthesis.Cells within the epidermis make up the plant's main food factory.

To perform photosynthesis, plants need sunlight, water and carbon. Many leaves are flat and wide to enable them to collect the most sunlight.

Water enters the plant through the roots. The top part of the leaf has a shiny/waxy cuticle which prevents water loss through evaporation.

Leaves get carbon dioxide from the air. Air enters the leaf through areas of the leaf called Stomata (one is called stoma). Each stoma is surrounded by Guard cells. When the guard cells swell, they open the stomata and allow water to leave the leaf. When the plant is low on water, the guard cells shrink and close the stomata and prevents the loss of water.

The loss of water through the leaves is called Transpiration.

As water evaporates through the leaves, more water is carried by the tree from the roorts and moves into the leaf, replacing what was lost.

Carbon dioxide enters the leaf and through a series of reactions with water, in the presence of sunlight energy, produces sugars (C H O). The sugars are transported to other parts of the plant to support life processes. Excess sugar is converted and stored as starch. This photosynthesis process produces oxygen as a by-product, which is why we say that plants are important to remove carbon dioxide from the atmosphere and add oxygen.

The sugars produced during photosynthesis are used by most organisms for energy. The energy is released when the cells of organisms use oxygen to break down the sugars stored as starch in the process of cellular respiration.

During cellular respiration, plant and animal cells produce carbon dioxide and water as waste products, which are then released back into the air. Plants use the released carbon dioxide and water to produce sugars during photosynthesis.

Lesson 3: Animals

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We begin to study Animals in more details in Grade 5. We will start with the group of animals called Invertebrates.

Invertebrates

Invertebrates are animals that do not have a backbone (vertebrae). They can live on land and in water. Most lower invertebrates live in aquatic environments (in water). Lower invertebrates include sponges, cnidarians and worms.

Sponges

Sponges are the most simple of all animals. They have no true organization. They do not have real tissues or organs. They are also asymmetrical, which means, their body cannot be dividied into mirror images. Their body is organized around a single tunnel like cannal and the tissue surrounding the canal has many pores (holes). All sponges live in water. Sponges filter the food out of the water that goes in their pores. The 'filtered' water then exits through the osculum.

Cnidarians

Jelly fish, sea anemones, hydras and corals are all cnidarians. Unlike sponges, cnidarians have radial symmetry, which means all the body parts are arranged around a central point. Organisms with radial symmetry have more than one line that divides the organism into two mirror images. Cnidarians have a mouth, tentacles, muscle tissues, and stinger cells. When they hunt, their stingers shoot out like tiny harpoons. The poison inside these cells helps them capture other animals.

Worms

Worms can be divided into three main groups:

• Flatworms
• Roundworms
• Segmented worms

All worms have bilateral symmetry, which means their body can be divided along only one plane to produce two mirror images.

Flatworms are calso called Platyhelminthes. They have a flat body and a head with simple eyes and mouth. They have one opening which acts as a mouth, but also undigested food waste also leaves the body through the same opening.

Roundworms are also called Nematodes. They have a simple digestive and nervous systems. They are some of the most abundant animals on the earth. They often live inside the body of other animals.

Segmented worms are also called Annelids. They have a body plan that is divided into segments. Unlike flatworms, segmented worms have two openings, and they also have organs including a stomach, heart and brain.

Complex Invertebrates

Invertebrates are a very large diverse group of animals and live in many diverse environments. As you already read that some are developed only to a rudimentary degree, while some are more advanced, with multiple body openings, some organs etc. Some invertebrates have specialized organs and complex body structures. These include Mollusks, Echinoderms and Arthropods.

Mollusks

All mollusks share the same body plan. They have a muscular foot or tentacles, a fold of tissue called the mantle and a mass of internal organs. They are all bilaterally symmetrical. They include snails, clams, and squids. Almost all mollusks have a shell, which is secreted by the mantle. Mollusks have several specialized organs including a heart, gills for breathing and a well developed nervous syetm.

Echinoderms

Echinoderms include sea stars (star fish), sea cucumbers and sea urchins. They have a hardened skeleton located inside the body called Endoskeleton. Echinoderms use water pressure to feed, breath and move. Sea water enters the system and moves to different parts of the body under pressure. The system ends in the tube feet which cling to surfaces like sunction pumps.

Arthropods

Arthropods are the most numerous animal group on earth. More than half of the world's animal spcies are arthropods. They are small and have a skeleton on the outside (exoskeleton). All arthropods have bilateral symmetry. Arthropods have a segmented body with paired limbs on either side of the body. These limbs are used as wings or claws. Arthropods have a simple but very efficient nervous system.

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Vertebrates

Vertebrates are animals that have a backbone (a vertebrae). They all have an endoskeleton and are bilaterally symmetrical.

Fish

There are three classes of fish:

• Jawless fish
• Cartilaginous fish
• Bony fish

Jawless fish include Lamprey and hagfish. They have a flexible nerve cord and because they do not have jaws, they obtain food by sucking.

Sharks, skates and rays are cartilaginous fish. Their skeleton is made of cartilage rather than bones. Cartilage is the material that make your ear stiff, its not as hard as bone. These fishes also have paired fins amd jaws.

Bony fish have a nerve cord covered by bone, not cartilage. They have jaws and paired fins. They have ballonlike swim bladders that allow them to easily go up and down in the water. They also have moving flaps that push water into their gills where they obtain oxygen while not moving in water.

Amphibians

Frogs and salamanders are amphibians. They spend parts of their life in water and part in land. A frog begins its life in water as a tadpole with gills. As it matures, the frog develops four legs and lungs for breathing air. Most adult amphibians do not leave the water for too long. For example, most frogs can breath through their lungs, and also through their skin so they stay moist at all times. They also lay their eggs in water.

Reptiles

A reptile is a true land animal with one or two lungs. They have a thick scaly waterproof skin. They include lizards, snakes, turtles, alligators and crocodiles. Reptiles are cold blooded animals, which means they cannot generate much body heat. They stay warm by exposing themselves to the sun and making use of the heat in their environment. This means they cannot keep their body temperature stable.

Birds

Birds have several features that make them different from other vertebrates. They have two legs and two wings. Birds have hollow bones to reduce their weight and enhance flight.

The unique feature of birds is the presence of feathers. They are strong but also light. Feathers help keep the bird warm and enable the birds to keep their body temperature steady. Feathers also assist in flight because they are smooth and point backwards reducing the resistance and drag.

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Mammals

The word mammals is derived from the word 'mamma', which means teat/pap. Mammals are characterized by the presence of milk-producing mammary glands for feeding their young. Mammals also have a big brain and hair or fur. Tehy generate their own body heat and are able to maintain a constant body temperature within a range of environmental temperature.

Mammals are divided into three groups (subclasses):

• Monotremes: They lay eggs. After the eggs hatch, the young are fed on milk from the mothers. Examples include the duck-billed platypus and the anteater.

• Marsupials: These are puoched mammals. They give birth to partially developed offspring. They carry these offspring in a pouch on the front of their bodies. Kangaroos and Koala bears are examples of marsupials.

• Placental mammals: Placental mammals are a type of mammals where the young develops within the mother and are born more mature that the offspring of marsupials. Humans, dogs, cattle etc are placental mammals.

Lesson 1: Energy Flow in Ecosystems

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Living things depend on each other. They also depend on nonliving things like sunlight. Living and nonliving things that interact in an environment make up an ecosystem. An ecosystem may be a pond, a swamp, or a field, maybe large or small.

Different organisms live in different parts of an ecosystem. Fish live in the water, so the water is the fish's habitat.

A food chain shows how energy passes from one organism to another in an ecosystem. When a buffalo feeds on grass, they obtain energy from the grass, and when a lion feed on the buffalo, they obtain energy from the buffalo. energy flows from grass to buffalo to lion.

The first organism in a food chain is called a producer, these are organisms that make their own food. Green plants are examples of producers. Most producers use energy from the Sun to make their own food. This means that the energy in most food chains starts with the Sun.

A consumer is the organism that eats other organisms. All animals are consumers. A food chain may have many consumers.

Organisms that eat mostly plants are herbivores. Some animals, such as herons, eat mostly other animals. These organisms are carnivores. Animals that eat both plants and animals are omnivores.

Predators hunt other organisms for food. The organisms they hunt are prey.

A decomposer is an organism that breaks down dead plant and animal material. Decomposers put nutrients back into the soil. Some worms and bacteria are decomposers.

Since consumers can eat many types of organisms, many food chains can join to form a food web.

Plants produce their own food.
Herbivores eat plants.
Carnivores eat other animals.
Omnivores can consume both plants and animals.
Decomposers utilize dead and decaying matter into waste and simpler substances.

Plants are called producers because they produce their own food. Animals are called consumers because they eat, or consume, other living things. Plants are essential to ecosystems because they produce the food which all other living things need. To represent how living things feed off other living things in an ecosystem, we can use a food chain.

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Competition: Sometimes living things have to compete to get what they need. This is called competition. Predators compete with each other. For example, lions and cheetahs hunt the same herbivores. Plants in a forest compete for sunlight.

Cooperation: When living things help each other to survive in an ecosystem, this relationship is called cooperation. For example, a tree may provide a home for a bird's nest. Bees can pollinate flowers.

Energy Pyramid: Whenever we consume food, we do not use up all the energy available in that food. Some of it is wasted. For example, plants use the energy from the sun to produce their food. However, when herbivores and omnivores eat the plants, they cannot be able to extract all the energy. In fact, only about 10% of energy is transferred from one level of the food chain/web to the next. An energy pyramid is a representation of the energy transferred from one level of the food chain to the next, with producers at the base of the pyramid.

Lesson 2: Interactions in Ecosystems

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All important resources in an ecosystem are scarce, as in there is less than the ecosystem needs to survive. This includes resources like water, food, space and other resources. The fight for these resources is called competition. For example, a fox will compete with other foxes to catch rabbits. Competition can also occur across different kinds of animals. For example, foxes also compete with hawks for rabbits. The rabbits compete with other herbivores for the food.

A limiting factor is any resource that restricts the growth of populations. A forest gets more rainfall and much warmer in summer than in winter. In summer, the forest can support many more organisms than in winter. Limiting factors may also include abiotic factors. Abiotic factors are non-living factors, such as water, temperature, soil type, shelter, sunlight and others.

Carrying capacity is the largest number of individuals within a population that an ecosystem can support. As the population increases, the food becomes harder to find and some of the individuals die to the level where there is an equilibrium where there is just enough individuals to survive successfully in the ecosystem.

Overcrowding (limited space) can also limit growth. A population of algae in a pond or bacteria in a petri dish will eventually become too thick that they exhaust the resources in the space such as oxygen. Without sufficient resources, the algae begin to die off.

We can define a habitat as the physical place where an organism lives and finds its food. Some individuals have small habitats like some bugs may spend almost their entire life under a rock. In some cases, an individual's habitat can be large. For example, a bee may occupy a large habitat where it goes around obtaining nectar.

A niche is the special role that an organism plays in a community. Two birds might live in the same location and eat the same food, but one bird is active at night, the other is active during the day. Or two birds might be in the same habitat but eating different food. Multiple animals can live in the same tree but some may be at the tallest tip of the tree and others may live closer to the ground.

Living things depend on each other for survival. They establish interlocking relationships. This relationship is called Interdependence. Symbiosis is the terminology used to define a relationship between two or more kinds of organisms that lasts over a period of time.

Mutualism

Mutualism is a symbiotic relationship where both organisms benefit and neither is harmed. The relationship between pollinators and flowers is a good example. Pollinators such as bees obtain food (nectar) from the flowers while the flowers obtain pollination.

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Commensalism

Commensalism is an interelationship between two organisms where one organism benefits from the other, but it does not cause harm to the other.

Remora are fish that attach themselves to the bodies of rays and sharks. The remora gets food scraps, transportation, and protection from the ray. What does the ray get from the remora? While the remora does not hurt the ray in any way, it does not help the ray either.

Sometimes its difficult to be sure whether an organisms is benefiting from a relationship or getting harmed by the relationship.

Parasitism

Parasitism is a symbiotic relationship where one organism benefits and the other is harmed. The individual that benefits is called a parasite. The organism that gets harmed is called the host. A parasite may live ON or IN the host. For example, ticks are parasites to dogs, cattle, goats and many other animals. A tick attaches on the host, feeds on on the host's blood and may sometimes transmit diseases to the host. Tapeworms are parasites in the digestive system of humans and some other individuals.

Lesson 3: Adaptations and Survival

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We have mentioned that resources in an ecosystem are usually scarce and individuals within that ecosystem have to compete for those resources. An adaptation is any characteristic that helps an organism survive in its ecosystem. The offspring inherit these adaptations and so they are also able to better survive in the ecosystem.

Adaptations can be structural or behavioral.

Structural Adaptations

Structural adaptations are adjustments to physcial structures on the organism. For example, it could be fur color, strong jaws, the ability to run very fast, having a long neck etc. Cactuses have a thick waxy cuticle that prevents water loss in their dry environment.

Behavioral Adaptations

Some individuals change their behavior to be able to survive in the ecosystem. For example, wolves travel in packs, this way they can hunt more successfully. Some fish swim in groups (called schools), which protects them from predators. Some behaviors help animals survive different climatic conditions in their ecosystem. Some animals move (migrate) to find food, water and a less severe climate. Other animals such as snakes, turtles, frogs etc hibernate to escape the cold. Hibernation is when an animal drastically reduces its activity and as a result, its metabolism, as a way to conserve energy during cold winters, and resumes normal activity when temperatures improve in Spring.

Plants also have many adaptations to survive in their environments. Some have scented flowers to attract pollinators. Some aquatic plants, such as water lilies, have stomata on the top surface of the leaf instead of the bottom. This enables the stomata to take in and release carbon dioxide and oxygen.

Desert animals are often nocturnal, which means they are active at night when the temperature is cooler. During the day, these animals stay in underground burrows to avoid the heat.

Camouflage

Camouflage is any coloring, shape, or pattern that allows an organism to blend in with its environment. Predators with camouflage can sneak up on prey. Camouflage also helps prey animals hide from predators.

Protective coloration is a type of camouflage where the color of an animal helps it blend in with its background. In winter, the arctic fox has a white coat that blends in with the snow. In summer, the foz's coat changes color to blend in with plants that grow in the warm weather.

Protective resemblance is when the organism combines multiple mechanisms such as protective coloration, shape, texture etc to resemble the environment.

Mimicry

Some animals have adapted to their environment by copying other well-adapted organisms. An adaptation in which an animal is protected against predators by its resemblance to an unpleasant animal is called mimicry. The example below shows a coral snake that is poisonous and a milk snake that is harmless. The milk snake mimics the coloration of the coral snake so it will also appear to be poisonous.

Lesson 1: Cycles in Ecosystems

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The Water Cycle

The water cycle is made up of three manin processes.

• Evaporation
• Condensation
• Precipitation

Evaporation

When water heats up, some of it changes into a gas called water vapor. This process is called evaporation.

Water evaporates from lakes, oceans, rivers, ponds and other water bodies.

Water can also evaporate from the surface of leaves in a process called transpiration.

Condensation

The water vapor travels in the air. As it rises into the air, it cools down and turns back into a liquid. The change from gas to liquid is called Condensation.. If many water droplets in the sky come together they form clouds. A cloud is a group of water droplets in the atmosphere.

Precipitation

The water in the clouds and the water vapor in the air will then fall down to the ground as rain or other kids of precipitation.

Precipitation refers to any liquid or frozen water that forms in the atmosphere and falls back to the earth. It comes in many forms, like rain, sleet, and snow.

If its too cold, the water droplets in clouds will freeze into ice. Freezing refers to the change from liquid to solid.

Some of the water that falls as precipitation collects on land and flows downhill. A watershed is an area from which water is drained. Precipitation that flows across the land’s surface and is not absorbed will flow into rivers, lakes, and streams as runoff. Most of the water will flow from rivers to the ocean. Some of the water will settle underground and become groundwater.

Plants and animals also play a role in the water cycle. Plants absorb water from the ground through their roots. Excess water in the plant is lost through transpiration. Animals drink water and then release the excess as waste and sweat.

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The Carbon Cycle

Carbon is one of the most important elements in every living thing. About 18% of the body is Carbon. Carbon is also plentiful in the atmosphere in form or carbon dioxide CO₂ gas. Carbon is also present in rocks (as limestone).

Carbon is exchaned continously between the different sites and chemical forms. Plants take up carbon dioxide from the air to fulfill their photosynthetic needs and produce sugars (remember sugars are made of C H and O). These carbon rich compounds are then eaten by herbivores (like cattle, sheep, deer, rabbits) and omnivores (such as humans). The herbivores and omnivores use the carbon rich compounds from plants to make their own carbon rich compounds such as proteins, sugars and fats. Herbivores and omnivores are then consumed by carnivores enabling the transfer of these carbon rich compounds to all different levels of the food chain. Living organisms can also transfer their carbon when they die and decompose. In such cases, the carbon may be transferred to the soil, or maybe used by decomposers such as bacteria and fungi. The breakdown of carbon compounds by decomposers releases carbon to the environment as methane, carbon dioxide, or other carbon compounds. After millions of years, the carbon that remained in the ground will turn into fossil fuels such as coal, oil and natural gas. These will be harvested/mined by humans and brought back into the atmosphere through their use in cars, heating homes or cooking. All these processes result in the formation of carbon dioxide which goes back into the atmosphere. Plants remain the most significant users of atmospheric carbon dioxide so it is important to ensure there are many trees to avoid too much increase in the levels of atmospheric carbon dioxide. Ofcourse, it is also important to reduce the release of carbon dioxide into the atmosphere.

The Nitrogen Cycle

The air is 78% nitrogen gas. Nitrogen cycles through both the abiotic and biotic parts of the Earth system. The largest reservoir of nitrogen is found in the atmosphere, mostly as nitrogen gas (N₂). Nitrogen gas makes up 78% of the air we breathe. Most nitrogen enters ecosystems via certain kinds of bacteria in soil and plant roots that convert nitrogen gas into ammonia (NH₃). This process is called nitrogen fixation. A very small amount of nitrogen is fixed via lightning interacting with the air. Once nitrogen is fixed, other types of bacteria convert ammonia to nitrate and nitrite, which can then be used by other bacteria and plants. Consumers (herbivores and predators) get nitrogen compounds from the plants and animals they eat. Nitrogen returns to the soil when organisms release waste or die and are decomposed by bacteria and fungi. Nitrogen is released back to the atmosphere by bacteria get their energy by breaking down nitrate and nitrite into nitrogen gas (also called denitrification).

Lesson 2: Changes in Ecosystems

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Natural events include natural disasters and changes caused by organisms. Earthquakes, floods, storms, volcanoes, droughts, and other natural disasters can drastically alter ecosystems. People can try to repair the damage from these disasters, but there is little or nothing anyone can do to prevent such events from occurring. The second type of natural change is caused by organisms. Large animals, like elephants, can cause changes by trampling trees and seedlings. Humans cause ecosystem changes by shaping the environment to meet their needs. These changes often destroy or alter habitats and affect the organisms that live in those habitats. Humans also change ecosystems by introducing new species or removing species. Introduced plant and animal species can threaten native species.

Some ecosystem changes are permanent. Organisms must respond to changes in order to survive. Organisms that cannot respond to ecosystem changes begin to die. When the last member of a species dies, the species becomes an extinct species. Some extinct organisms include all species of dinosaurs, mammoths, the saber-toothed cat, and many others.

The Tasmanian wolf, for example, became extinct about 65 years ago as a result of human actions. These wolves once lived in Australia. Farmers saw the Tasmanian wolf as a threat to their livestock and hunted the animal to extinction.

Pollution, global warming, habitat destruction, and hunting can also threaten the survival of organisms.

Below are examples of extinct animals, the first is the Tasmanian wolf and the second is the Saber toothed cat.

When a species is in danger of becoming extinct, it is called an endangered species. The flying squirrel is an example of an endangered species. Usually, only a few hundred individuals of the species exist.

Species with low numbers that could become endangered are called threatened species. The gray wolf, the manatee, and many others are threatened species.

Over time, an ecosystem can change to a new type of ecosystem, this change is called Succession. There are two kinds of succession:

Primary succession occurs where there are few living things that exist, or where the earlier community was wiped out. Primary succession occurs in barren, lifeless areas that have little or no soil. Particles of soil and seed blow from neighboring environments and lichens and mosses start to grow. In this case, the first organisms that begin to grow in the area are called Pioneer species. If there are multiple species that grow first in an area, this can be described as a pioneer community. As more plants grow, the soil quality and nutrients improve, the soil becomes more suited for even more plants and some animals. grasses, ferns, shrubs begin to sprout. Flowering plants attract pollinators to the area, such as insects, birds, and small mammals. These animals attract larger predators to the community. After many years, this community may become a grassland or prairie. A climax community is the final stage of succession. Unless the community is disturbed by some natural disaster or human activity, the climax community will remain.

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Secondary succession is where a new community develops in a place where another community already exists. Secondary succession can occur in a forest after a fire has occured. Secondary succession utilizes soil that already has the nutrients and factors needed for good plant growth. For example, when a farm is abandoned, weeds begin to grow and after a couple of seasons, shrubs also begin to grow.

Lesson 3: Biomes

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A biome is one of Earth’s major land ecosystems with its own characteristic animals, plants, soil, and climate. A climate is an average weather pattern for a region. How is a biome different from other habitats? You can think of a biome as a set of habitats or ecosystems all grouped together into a kind of 'super-ecosystem.'

There are 5 major land biomes:

• Desert
• Tundra
• Aquatic
• Forest
• Grassland

Aquatic biomes include both freshwater and marine biomes. Freshwater biomes are bodies of water surrounded by land—such as ponds, rivers, and lakes - that have a salt content of less than one percent. Marine biomes cover close to three-quarters of Earth’s surface. Marine biomes include the ocean, coral reefs, and estuaries.

Grasslands are open regions that are dominated by grass and have a warm, dry climate. There are two types of grasslands: tropical grasslands (sometimes called savannas) and temperate grasslands. Savannas are found closer to the equator and can have a few scattered trees. They cover almost half of the continent of Africa, as well as areas of Australia, India, and South America. Temperate grasslands are found further away from the equator, in South Africa, Hungary, Argentina, Uruguay, North America, and Russia. Prairies are types of temperate grasslands; prairies are characterized as having taller grasses.

Forests are dominated by trees, and cover about one-third of the Earth. The three major forest biomes are temperate forests, tropical forests, and boreal forests (also known as the taiga). Tropical forests are warm, humid, and found close to the equator. Temperate forests are found at higher latitudes and experience all four seasons. Boreal forests are found at even higher latitudes, and have the coldest and driest climate, where precipitation occurs primarily in the form of snow.

Deserts are dry areas where rainfall is less than 50 centimeters (20 inches) per year. They cover around 20 percent of Earth’s surface. Deserts can be either cold or hot, although most of them are found in subtropical areas. Because of their extreme conditions, there is not as much biodiversity found in deserts as in other biomes.

A tundra has extremely inhospitable conditions, with the lowest measured temperatures of any of the five major biomes with average yearly temperatures ranging from -34 to 12 degrees Celsius. They also have a low amount of precipitation, just 15–25 centimeters per year, as well as poor quality soil nutrients and short summers. There are two types of tundra: arctic and alpine. The tundra does not have much biodiversity and vegetation is simple, including shrubs, grasses, mosses, and lichens. This is partly due to a frozen layer under the soil surface, called permafrost.

Lesson 4: Water Ecosystems

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An aquatic ecosystem is an ecosystem formed by surrounding a body of water, in contrast to land-based terrestrial ecosystems. Aquatic ecosystems contain communities of organisms-aquatic life-that are dependent on each other and on their environment. The two main types of aquatic ecosystems are marine ecosystems and freshwater ecosystems

The organisms in water ecosystems are divided into three main categories. Plankton are creatures that drift freely in the water. They are not able to swim. Some plankton, such as diatoms, are producers, and others are consumers, such as some animal larvae.
The second group includes the larger, active swimmers in a body of water called nekton. Fish, turtles, and whales are all nekton. The third group, organisms that live on the bottom of a body of water, are called benthos. Many benthos are scavengers or decomposers because they feed on material that floats down from shallower water.

Unlike land ecosystems, water is never a limiting factor. However, the amount of light, dissolved salt, and dissolved oxygen are important. They can all affect the types of organisms that can live in bodies of water.

Running-Water Ecosystems: Faster-moving bodies of water tend to have more oxygen, because air mixes in as the water flows. Other nutrients are washed into the water from the land. Organisms that live in fast-moving streams or rivers have adaptations to prevent them from being swept away. Slower-moving waters have less oxygen and are less dependent on the land for nutrients. More producers, such as algae, are able to survive in slow-moving water.

Standing-Water Ecosystems: The typical freshwater lake or pond is divided into three zones. The shallow-water zone along the shore is where most of the organisms live. Cattails, sedges, arrowgrass, and other rooted plants grow here. The open-water zone includes the water away from the shore. This zone may be too deep for rooted plants to survive. Algae and plankton float near the surface. Nekton, such as trout, whitefish, and pike are found here. The third zone is below the openwater zone and includes the bottom. Very little light reaches the bottom, so producers cannot grow here. Benthos, including worms and mollusks, are found in this zone.

Freshwater Wetlands: Wetlands, such as marshes, swamps, and bogs, are regions that are wet for most of the year. Grasslike plants, moss, and some shrubs are found in wetlands. Beavers, muskrats, otters, birds, and fish live in wetlands.

Marine Ecosystems: The shallow part of the ocean ecosystem is called the intertidal zone. Every day, the pull of the Moon’s gravity causes ocean tides to rise and fall over the intertidal zone. Beyond the intertidal zone is the neritic zone. The key resource in this zone is sunlight. Algae, kelp, and other producers grow in huge numbers near the surface water where sunlight can penetrate. The third zone of the ocean is the oceanic zone. It is divided into the bathyal zone and the abyssal zone. The bathyal zone is home to many large consumers, such as sharks, but few producers. Further down is the abyssal zone, where it gets darker and colder because the sunlight is completely blocked. Organisms in this zone tend to be scavengers or decomposers. They live on nutrients that float down from other zones.

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The boundary where fresh water feeds into salt water is called an estuary. Estuaries are unique ecosystems that are part salt water and part fresh water. Like intertidal zones, estuaries change with the tides. When the tide comes in, estuary water becomes more salty. The tide also brings in nutrients from the land. Many ocean fish return to estuaries to lay their eggs. Countless insect larvae, young fish, and tiny crustaceans begin their lives in the calm, protected waters within an estuary. Larger organisms, including egrets, herons, frogs, turtles, muskrats, raccoons, otters, and bobcats feed on these smaller consumers.