The Invisible Living World: Beyond Our Naked Eye | CBSE Class 8 Science Notes

This note covers magnification, cells and their parts, differences between plant and animal cells, levels of organisation, microorganisms and their habitats, decomposition, biogas, food fermentation, root nodules, and the importance of microalgae.

How can we explore living things beyond the naked eye?

An organism is a living being. Organisms differ in shape, size, colour and structure. The naked eye, or unaided eye, can see objects only above a certain size. Many tiny organisms therefore remain invisible unless we use a tool that makes them appear larger.

What does magnification do?

Magnification makes an object appear bigger. A lens is a shaped piece of glass used to enlarge the appearance of small things. A magnifying glass reveals details of an ant, while a microscope helps us observe much smaller structures and organisms.

A water-filled, round-bottom glass flask can act like a magnifying glass. Fill the flask with water, close its mouth with a cork and place it on an open book. Letters viewed through the flask appear larger.

Improved lenses made it possible to investigate a previously hidden living world. A low-cost, foldable paper microscope makes this world accessible to many people. It may not provide the same level of detail as a high-powered laboratory microscope.

How did the word cell enter science?

In 1665, Robert Hooke published Micrographia, containing drawings of objects observed through his microscope. In a thin slice of cork, he saw small, empty compartments resembling a honeycomb. He called each compartment a cell, the term used for the basic unit of life.

During the 1660s, Antonie van Leeuwenhoek developed better lenses and more useful microscopes. He clearly observed and described tiny living things such as bacteria, single-celled organisms. These observations helped open the study of organisms too small to be seen unaided.

How can onion peel and cheek cells be observed?

Living bodies are made of cells. A thin sample placed on a glass slide allows cells to be examined under a microscope. A stain adds colour and improves visibility. A coverslip is the small cover placed over the material on the slide.

How is an onion-peel slide prepared?

The onion-peel preparation is a demonstration activity. The onion peel is the thin, transparent layer removed from the inner surface of an onion piece. Handle it carefully so that it does not fold or break during mounting.

1. Wash an onion bulb, cut it vertically into pieces and use forceps, a gripping tool, to remove a thin peel from an inner surface.
2. Place the peel in a petri dish, a shallow laboratory dish, containing a few drops of safranin, a red-coloured stain, for 30 seconds.
3. Transfer it with a thin brush to water in another dish. Rinse away the extra stain, then place the peel flat on a glass slide.
4. Add a drop of glycerin, the liquid used here to prevent drying and improve clarity. Lower a coverslip with a needle without trapping air bubbles.
5. Use blotting paper to remove extra glycerin from the edges. Observe the slide through a microscope.

The cells appear nearly rectangular and closely arranged without spaces between them. The outer supporting layer is the cell wall. Inside it lies the cell membrane, which encloses the cell contents. The nucleus regulates cell activities; cytoplasm fills the space around it.

The drawing shows rows of closely packed, nearly rectangular pink cells. Labels identify the cell wall, cell membrane, nucleus and cytoplasm. The adjacent brick-wall drawing provides a comparison with repeated building units.

See Fig. 2.3c in your NCERT textbook

How are cheek cells different in appearance?

For a cheek-cell preparation, rinse the mouth with clean water. Gently scrape the inner cheek with the blunt end of a clean toothpick. Spread the material in a drop of water on a clean slide.

Add methylene blue, a blue-coloured stain that increases contrast. After one minute, add glycerin, place a clean coverslip and remove excess glycerin. The cells appear polygon-shaped, meaning that their outlines have several sides. They form the inner lining of the mouth.

The drawing shows blue cells with differently shaped outlines. Arrows identify the cell membrane, cytoplasm and darker nucleus. A cell wall is not labelled around these animal cells.

See Fig. 2.4 in your NCERT textbook

What do the main parts of plant and animal cells do?

A cell contains different components with different functions. Its membrane encloses the contents, its cytoplasm supports most life processes, and its nucleus regulates activities. A plant cell also has a supporting wall outside the membrane.

How do the basic cell parts function?

The membrane is porous, meaning that it allows materials to pass through. Its role includes both the entry of materials essential for life processes and the removal of wastes. The membrane and wall are separate structures with different functions.

What are plastids and vacuoles?

Plastids are tiny rod-shaped structures in plant cells. Some plastids, called chloroplasts, contain chlorophyll, the green pigment, or colouring substance, that helps in photosynthesis, the process of making food using sunlight. Plastids in non-green parts help store substances.

A vacuole is an empty-looking space in the cell. The large vacuole in a plant cell stores important substances, helps remove waste and maintains cell shape. These functions contribute to the plant's strength and support.

In animal cells, vacuoles are usually not present. If present, they are usually small and store certain substances dissolved in water. This qualification matters: saying that an animal cell can never contain a vacuole would be incorrect.

Note: Most life processes take place within the cytoplasm. The nucleus regulates cell activities and growth. Do not replace “most” with “all”, or confuse regulation with the place where processes occur.

How do cell shapes and levels of organisation support life?

Cells vary in shape, size and structure. These differences help them perform specific functions. Even within one human body, a cheek cell, a muscle cell and a nerve cell have different forms and different roles.

How is shape related to function?

A nerve cell is also called a neuron. Muscle cells in the food pipe contract and relax in a wave-like manner, pushing food towards the stomach. Muscle cells in the stomach wall help churn food.

Other cells in the stomach's inner lining produce digestive juices and acid that help break down food. Thus, different types of cells work together during digestion. A cell's structure and its function should be studied together.

The muscle-cell drawing is narrow and spindle-shaped. The nerve-cell drawing has branching extensions and a long extension leading to further branches. The two labelled drawings illustrate contrasting cell shapes.

See Fig. 2.6 in your NCERT textbook

How are cells organised into a complete organism?

1. Cell: the basic unit of life.
2. Tissue: a group of similar cells.
3. Organ: a structure formed from different tissues.
4. Organ system: several organs working together to perform a major body function.
5. Organism: the complete living being, whose organ systems work together.

The order is cell → tissue → organ → organ system → organism. Each level builds on the preceding level. A cell is therefore a basic building unit, while an organ system represents the coordinated work of several organs.

Multicellular means made of many cells. Plants and animals, including humans, are multicellular organisms. Their cells perform specialised functions individually and cooperate with one another, increasing the organism's chance of survival.

What are microorganisms, and where do they live?

Definition: Microorganisms, also called microbes, are small living organisms that cannot be seen with the unaided eye. “Micro” means very small, while “organisms” means living beings.

Some microorganisms consist of a single cell. Unicellular means single-celled. Bacteria and Amoeba, a single-celled organism with an irregular shape, are examples. Other microorganisms contain many cells.

Which groups are included?

Microorganisms include bacteria, single-celled organisms; protozoa, single-celled microorganisms such as Amoeba and Paramecium; fungi, a group including yeast, a single-celled fungus, and moulds, fungi with thread-like structures; and some algae, organisms that include green, food-making forms. These groups differ in their cell structures and appearance.

Small size does not mean that all microbes look alike. They can have spherical, rod-like or irregular shapes. Some are made of one cell and others of more than one cell. Their structures and activities also vary.

Which habitats support microbes?

A habitat is a place where an organism lives. Microorganisms occur in water, soil, air, some foods, and the bodies of plants and animals. They can also be found on plant parts such as leaves, stems and roots.

Some microorganisms live in extreme conditions, including hot-water springs and snow-cold zones. Others live at moderate temperatures. Many bacteria live in the human intestine and help with digestion.

A powdery or cotton-like growth may appear on a lemon, tomato or orange left outside for some time. This growth results from infection by microbes. Microorganisms therefore connect the unseen living world with familiar changes in food and surroundings.

How can pond water and soil reveal microbial diversity?

A microscope makes it possible to observe organisms in pond water and soil. A drop of the sample can contain organisms that are invisible to the unaided eye. Their movement, shape and colour provide features to record during observation.

How is a pond-water sample examined?

1. Collect pond water or stagnant water with the help of an adult.
2. Use a dropper to place a drop on a microscope slide.
3. Place a coverslip over the sample.
4. Observe the tiny organisms through a microscope and record their features.

Stagnant water is water that remains standing. In a pond-water observation, Amoeba can appear as a moving, irregular, single cell. Paramecium is a single-celled protozoan that moves from place to place using specialised structures.

Green algae can also be observed. Green colour indicates the presence of a green pigment, a colouring substance. Record what is actually visible in the sample rather than assuming that every drop contains the same organisms.

How is a soil suspension prepared?

A soil suspension is water containing very fine soil particles. Collect moist soil from a field or garden with a spoon or gloves, avoiding direct contact with bare hands. Add water in a beaker and stir with a glass rod.

Let the mixture settle. Use a dropper to take water from the top layer, place a drop on a slide and cover it gently with a coverslip. Under the microscope, you may observe small moving organisms similar to those in pond water.

Soil observations can reveal fungi, algae and bacteria. Moulds, fungi with thread-like structures, include forms with branched threads and sac-like or brush-like structures. Bacterial forms include spherical, comma-shaped, spiral and rod-shaped cells.

The observation compares diversity, meaning variety, rather than just counting cells. A useful record separates the name or group, visible shape, number of cells and observed movement. These are distinct features of the organisms being examined.

How do microbial cells differ, and why are viruses distinct?

The cell is the basic unit of life because its components enable an organism to carry out functions needed for survival. In a unicellular organism, the one cell performs all necessary functions. In multicellular organisms, specialised cells also cooperate.

Which microbes are unicellular or multicellular?

Microbial cells have a cell membrane. Fungal cells additionally have a cell wall, but they lack chloroplasts and cannot make their own food through photosynthesis. Bacterial cells also have a wall outside the membrane.

Bacteria lack a well-defined nucleus and a nuclear membrane, the membrane surrounding a nucleus. Instead, they have a nucleoid, the nuclear region in a bacterial cell that is not enclosed by a nuclear membrane.

Why are viruses discussed separately?

Viruses are microscopic and acellular, meaning that they are not made of cells. They multiply when they enter a living cell. The organism in which they reproduce is called the host.

Viruses may infect plants, animals or bacterial cells and may cause a disease. Their reproduction inside a host distinguishes them from other microorganisms. The words “may cause” also matter: the statement should not become a claim that every infection necessarily causes disease.

How do microbes recycle waste and produce biogas?

Decomposition is the breakdown of complex substances in dead material into simpler substances. Some bacteria and fungi break down plant waste, returning nutrients to the soil. Nutrients are substances that support growth and other needs of living organisms.

How can manure formation be observed?

1. Take an empty container and fill it halfway with garden soil.
2. Add fruit and vegetable peels.
3. Cover the peels with a layer of soil and leave the container aside.
4. Observe changes after 2 to 3 weeks.

You may find that the peels have become a dark-coloured material. This is manure, a nutrient-rich material that increases soil fertility, or the soil's ability to support plant growth. Microorganisms slowly break down the plant waste to form it.

Manure formation occurs at an optimal temperature, a temperature suitable for the process, and an appropriate moisture level. The observation is not a promise that every container will form manure in exactly the same time.

Microbes also decompose dead animals and animal waste such as dung. By breaking down waste, they help clean the environment. Nutrients returned to the soil help plants grow, linking waste removal with the recycling of useful substances.

What is biogas?

Biogas is a mixture of gases released when certain bacteria decompose plant and animal waste or waste in household wastewater in an oxygen-free environment. It contains carbon dioxide and a high proportion of methane, both gases.

Methane is the gas present in a high proportion in biogas; carbon dioxide is another gas in the mixture and is also released by yeast. Biogas is used as fuel for cooking, heating, generating electricity and running vehicles.

Many microorganisms, including bacteria and fungi, live in environments without oxygen. Some of these bacteria carry out the waste decomposition associated with biogas production. This is another useful connection between microbial activity and everyday life.

Why does yeast make dough rise?

Yeast is a unicellular fungus that grows well in warm conditions. Like other organisms, it respires, meaning that it breaks down food to release energy for growth and life processes. Carbon dioxide is released during this process.

How can two bowls show the effect of yeast?

1. Label two bowls A and B. Put 200 grams of flour, either atta or maida, in each and add a pinch of sugar. A gram is a unit of mass.
2. Add a small amount of yeast powder to bowl A and mix it with the flour. Add no yeast to bowl B.
3. Knead each mixture with warm water to form soft dough.
4. Cover the dough with a damp cloth and keep both bowls in a warm place.
5. Observe after 4 to 5 hours. If there is no visible change, leave the dough for some more time.

The equal flour quantities and similar preparation make the bowls useful for comparison. The deliberate difference is the addition of yeast to bowl A.

What explains the change?

After some time, you may notice that the dough in bowl A has risen slightly, become fluffy and developed a different smell. Carbon dioxide forms bubbles in the dough, making it soft and fluffy.

Yeast also produces a small amount of alcohol, a substance formed during this process that gives the dough a slightly different smell. The bubbles explain the change in texture, while the alcohol explains the smell.

This property of yeast is used in making breads and cakes. Fermentation is the microbial conversion of sugars, as in these food-making processes. Some bacteria, including Lactobacillus, a bacterium also involved in curd formation, help ferment batter for idli and dosa and dough for bhatura.

Note: The dough may rise slightly after some time. The amount of rise and the quantities of gas and alcohol produced depend on the conditions.

How does Lactobacillus turn milk into curd?

Lactobacillus is a bacterium found among the several kinds of bacteria in curd. It feeds on lactose, the sugar in milk, multiplies and ferments milk to form curd. Warm conditions help these bacteria grow well.

How are warm and cool conditions compared?

1. Take two small glass bowls and label them A and B.
2. Pour lukewarm milk into A and cold milk into B.
3. Add a small spoonful of curd to each and mix well.
4. Cover both bowls. Keep A in a warm place and B in a cool place, such as a refrigerator.
5. Observe after a few hours or overnight, recording predictions separately from observations.

In bowl A, the milk turns into curd after a few hours and becomes a little sour. In bowl B, the milk has not curdled, but it might be a little sour. The two observations should not be treated as identical.

What causes the sour taste?

The bacteria produce lactic acid, the acid responsible for curd's sour taste. This differs from yeast producing alcohol. Both processes involve microbial activity, but their products and visible effects differ.

The spoonful of curd supplies bacteria to the milk. Temperature affects how well these bacteria grow. Keeping the cool bowl in a refrigerator is therefore a meaningful part of the comparison, rather than an incidental detail.

How do root-nodule bacteria help plants and soil?

Legumes include plants such as beans, peas and lentils. Their roots can contain nodules, swollen regions in which certain bacteria live. Rhizobium is a bacterium associated with these root nodules.

What is the role of Rhizobium?

Rhizobium traps nitrogen, a substance present in air, and makes it useful to plants. Nitrogen supplied in this useful form supports plant growth. Thus, a microscopic organism living in a root swelling can benefit the whole plant.

This association helps plants grow better without chemical fertilisers, substances added to supply nutrients. The visible nodule and the invisible bacteria inside it should be distinguished: the nodule is a region of the root, while Rhizobium is the organism living there.

The illustration shows a leafy cowpea plant above a branching root system. Small swellings appear along the roots. These root nodules contain Rhizobium bacteria.

See Fig. 2.12 in your NCERT textbook

Why are legumes grown in rotation?

Crop rotation means growing different crops in succession on the same land. Legumes can be included in the rotation. Farmers grow legumes in rotation because their association with Rhizobium naturally increases nitrogen in the soil. This helps keep the soil healthy for the next crop.

Two different microbial activities can therefore support soil fertility. Decomposition returns nutrients from dead material and waste, while Rhizobium makes nitrogen from air useful to plants. Both benefit plant growth, but they act on different materials and should not be described as the same process.

Why are microalgae important to life and the environment?

Microalgae are microscopic, plant-like organisms. They live in water, soil, air and even on trees. They make their own food using sunlight and release oxygen, the gas whose supply they help maintain on Earth.

What useful roles do microalgae play?

Microalgae produce more than half of Earth's oxygen supply. They are rich in nutrients and provide food for many aquatic animals, meaning animals that live in water. Their small size does not limit their importance to other living things.

Some microalgae, including Spirulina, Chlorella and diatoms, are used by humans as health supplements and medicines. Health supplements are products used to add nutrients to the diet. Microalgae also help clean water and are used to make biofuel, fuel obtained from biological material.

Why should their diversity be conserved?

Pollution, climate change and habitat destruction threaten the diversity and abundance of microalgae. Abundance means the amount present. Conserving these organisms helps protect the environment and maintain Earth's oxygen balance.

Microorganisms have many beneficial roles: supporting food production, recycling waste, supplying useful substances to plants and helping maintain oxygen. Some microbes also cause disease in plants and animals, including humans. Their effects must therefore be considered by the kind of organism and its activity.

Glossary

• Cell — The basic unit of life, containing components that support an organism's functions and survival.
• Cell membrane — The boundary that encloses cell contents and allows essential materials in and waste out.
• Cytoplasm — The region containing cell components where most of the cell's life processes take place.
• Nucleus — The cell structure that regulates activities within the cell and also regulates its growth.
• Cell wall — An extra covering outside the cell membrane, providing rigidity and strength in plant cells.
• Chloroplast — A plastid containing the green pigment chlorophyll, which helps the plant carry out photosynthesis.
• Vacuole — An empty-looking space that helps plant cells store substances, remove waste and maintain shape.
• Tissue — A group of similar cells forming a level of organisation between cell and organ.
• Microorganism — A tiny living organism that cannot be seen with the unaided eye.
• Unicellular — Describes an organism made of one cell that performs all necessary survival functions.
• Multicellular — Describes an organism made of many cells, with cells performing functions and cooperating.
• Nucleoid — The nuclear region in a bacterial cell, without a surrounding nuclear membrane.
• Decomposition — Breakdown of complex substances in dead material into simpler substances through microbial activity.
• Biogas — A fuel gas mixture produced during microbial waste decomposition, containing carbon dioxide and much methane.
• Root nodule — A swollen region on a legume root containing bacteria such as Rhizobium.

Common errors and misconceptions

• Misconception: Every cell has a cell wall. Correct: Plant, fungal and bacterial cells have cell walls; animal cells do not.
• Misconception: All life processes occur in the cytoplasm. Correct: Most life processes occur there; the nucleus regulates cell activities and growth.
• Misconception: Animal cells never have vacuoles. Correct: Vacuoles are usually absent in animal cells; if present, they are usually small.
• Misconception: Every microorganism is unicellular. Correct: Some fungi and algae are multicellular. Yeast is unicellular, while mould is multicellular.
• Misconception: Bacteria have a well-defined nucleus like a plant cell. Correct: They lack a well-defined nucleus and nuclear membrane, and have a nucleoid instead.
• Misconception: Yeast and Lactobacillus produce the same substances in food. Correct: Yeast releases carbon dioxide and a little alcohol; Lactobacillus produces lactic acid during curd formation.
• Misconception: All microbes are harmful. Correct: Many help with decomposition, food preparation, plant growth or digestion, although some cause disease.
• Misconception: Viruses multiply independently of living cells. Correct: Viruses multiply when they enter a living cell and reproduce only inside a host organism.

Exam-style questions with model answers

Q1. Distinguish between unicellular and multicellular organisms, giving one example of each. [2 marks]

1. A unicellular organism consists of one cell that carries out all functions needed for survival; a bacterium is an example.
2. A multicellular organism consists of many cells; a human is an example, with specialised cells cooperating to support the organism.

Q2. State one function each of the cell membrane, cytoplasm and nucleus. [3 marks]

1. The cell membrane controls exchange by allowing materials essential for life processes to enter the cell and waste materials to leave.
2. The cytoplasm contains other cell components and is the region where most life processes take place, rather than necessarily all processes.
3. The nucleus regulates activities occurring within the cell. It also regulates growth, helping coordinate the cell's functioning.

Q3. Explain how the shapes of inner cheek cells, nerve cells, muscle cells in the food pipe and water-carrying plant cells support their functions. [4 marks]

1. Inner cheek cells are thin and flat. Their form helps them make a protective lining along the inner surface of the cheek.
2. Nerve cells are elongated and branched. This structure helps them reach different body parts and pass messages quickly.
3. Food-pipe muscle cells are thin, flexible and spindle-shaped. Their contraction and relaxation produce wave-like movement that pushes food towards the stomach.
4. Some plant cells form long tubes. These tubes help carry water throughout the plant, linking their elongated structure with transport.

Q4. Two bowls each contain 200 grams of flour and a pinch of sugar. Yeast is added only to bowl A. Both mixtures are kneaded with warm water, covered with damp cloths and kept warm. After 4 to 5 hours, A has risen slightly, become fluffy and developed a slightly different smell. Explain the comparison, the warm conditions and the three changes. [5 marks]

1. Bowl B provides the comparison without added yeast. Both bowls contain the same flour quantity and sugar, and receive similar preparation, so the deliberate difference is yeast.
2. Warm conditions help yeast grow well. Yeast is a microorganism belonging to the fungi, and it breaks down food to release energy.
3. The yeast releases carbon dioxide during this process. The gas forms bubbles in the dough, explaining why the dough in A rises slightly.
4. The bubbles also make the dough soft and fluffy. The observed texture is therefore linked to gas production by the yeast.
5. Yeast produces a small amount of alcohol during the process. This gives the dough its slightly different smell compared with dough prepared without yeast.

Q5. Bowl A contains lukewarm milk and bowl B contains cold milk. A small spoonful of curd is mixed into each. Both are covered; A is kept warm and B in a refrigerator for a few hours or overnight. A forms slightly sour curd, while B has not curdled but might be a little sour. Explain these observations in five points. [5 marks]

1. The spoonful of curd introduces bacteria into each bowl. Curd contains several types of bacteria, including Lactobacillus, which is involved in making curd.
2. Lactobacillus feeds on lactose, the sugar in milk. It multiplies and ferments the milk, leading to the formation of curd.
3. These bacteria grow well in warm conditions. The lukewarm milk and warm surroundings of bowl A therefore support curd formation after a few hours.
4. During the process, the bacteria produce lactic acid. This acid accounts for the sour taste of the curd formed in bowl A.
5. Bowl B is kept under cool conditions rather than the warmth in which these bacteria grow well. Its milk has not curdled, although it might be a little sour.

Q6. A container is filled halfway with garden soil. Fruit and vegetable peels are added and covered with soil. After 2 to 3 weeks, the peels may form dark material. Identify this material, explain its formation and state its benefit. [3 marks]

1. The dark material is manure, a nutrient-rich material formed from the plant waste. The observation is a possible outcome after the stated period.
2. Some bacteria and fungi in the soil slowly break down complex plant waste into simpler substances. This process is called decomposition.
3. The manure increases soil fertility and supports plant growth by returning nutrients to the soil. Microbial breakdown also helps clean the environment.

Q7. Explain how Rhizobium in the root nodules of legumes such as beans, peas and lentils benefits plants, and why farmers grow legumes in rotation with other crops. [4 marks]

1. Rhizobium bacteria live in root nodules, the swollen regions found on the roots of certain legumes, including beans, peas and lentils.
2. These bacteria trap nitrogen from the air and make it useful to plants, supporting the plants' growth.
3. This association helps legumes grow better without chemical fertilisers because the bacteria help provide useful nitrogen to the plants.
4. Growing legumes in rotation naturally increases nitrogen in the soil and keeps it healthy for the next crop, explaining the value of this farming practice.

Q8. Give two features that distinguish viruses from cellular microorganisms. [2 marks]

1. Viruses are acellular: they are not made of cells, unlike cellular microorganisms such as bacteria and yeast.
2. Viruses multiply when they enter living cells and reproduce only inside a host organism, which may be a plant, animal or bacterium.

Key takeaways

• Cells are the basic units of life, and their different components support the functions needed for survival.
• Cell membranes regulate material entry and exit, cytoplasm supports most life processes, and nuclei regulate cell activities and growth.
• Cell shapes support their functions, while cells, tissues, organs and organ systems form successive levels of organisation.
• Microorganisms occupy many habitats and include unicellular organisms as well as some multicellular fungi and algae.
• Decomposition by some bacteria and fungi returns nutrients to soil and helps remove plant and animal waste.
• Yeast produces carbon dioxide that makes dough fluffy; Lactobacillus produces lactic acid during curd formation.
• Rhizobium in legume root nodules makes nitrogen useful to plants and supports the benefits of crop rotation.
• Microalgae support food supplies and oxygen production, while pollution, climate change and habitat destruction threaten their diversity.

Test yourself

Why is a stain added when preparing a cell slide?

A stain improves visibility under the microscope by adding colour or increasing contrast, making cell structures easier to observe.

What does glycerin do in onion-peel and cheek-cell preparations?

Glycerin prevents cells from drying. In the onion-peel preparation, it also improves clarity for viewing the cells.

What is the correct order of the five levels of organisation?

The order is cell, tissue, organ, organ system and organism, progressing from the basic unit to the complete living being.

How does a bacterial cell differ from a cell with a well-defined nucleus?

A bacterial cell lacks a well-defined nucleus and a nuclear membrane. It has a nucleoid instead.

Why can fungi not make food by photosynthesis?

Fungal cells lack chloroplasts, so they cannot make their own food through photosynthesis, although they have cell walls.

Which gas makes yeast dough fluffy, and which substance changes its smell?

Carbon dioxide forms bubbles that make the dough fluffy. A small amount of alcohol gives it a slightly different smell.

What is the difference between a root nodule and Rhizobium?

A root nodule is a swollen region of the root. Rhizobium is the bacterium that lives within such nodules.

Why does conserving microalgae matter?

Microalgae provide nutrients to aquatic animals and produce more than half of Earth's oxygen supply, so their conservation supports the environment.