Exam practice

Ribosomes join amino acids together, in the order set by the instructions from the DNA, to make all the cell’s proteins. Since they produce the cell’s proteins, as a factory produces goods, they are called its protein factories.

The cell wall is a thick, rigid layer of cellulose around the cell. It doesn’t change shape easily, and the water-filled vacuole presses the cell against it from inside. So the cell keeps the wall’s shape.

The cytoplasm holds the water, dissolved substances and enzymes that the cell’s chemical reactions need, and the organelles (mitochondria, ribosomes, ER and others) float in it. So most of the cell’s chemical reactions, its metabolism, take place in the cytoplasm.

Every living thing is made of one or more cells, so the cell is the structural unit of life. All of life’s processes (respiration, digestion, reproduction and others) happen in and by cells, so it is also the functional unit. And new cells come only from existing cells. The cell is the smallest thing that is alive by itself, so it is the basic unit of life.

No single cell or tissue can carry out a life process alone. In digestion, for example, muscle tissue in the stomach and intestine pushes food along while gland tissue makes the enzymes that digest it, and both must work at the right time and place. Cells make tissues, tissues make organs and organs make systems, each depending on the others. If they are not coordinated, the process fails.

A cell is the basic structural and functional unit of life: the smallest unit of a living thing that is itself alive. It has a cell membrane, cytoplasm and (in eukaryotes) a nucleus, and all the processes of life happen in it. Every organism is made of one or more cells.

Draw the two cells side by side, as in Figure 3.4.

Plant cell: a box-like shape with a thick cell wall, the cell membrane just inside it, cytoplasm, a large central vacuole, a nucleus pushed to one side, chloroplasts, mitochondria and a Golgi body.

Animal cell: a rounded, irregular shape with only a cell membrane outside, cytoplasm, a central nucleus, mitochondria, a Golgi body, lysosomes and small vacuoles.

Use a sharp pencil, label with straight lines that don’t cross, and give each drawing a title.

Mitochondria are found in the cytoplasm of the cells of all eukaryotes, plants and animals alike: many of them, especially in busy cells such as muscle cells.
Functions:

1. They carry out cellular respiration: they break down glucose with oxygen and release energy.
2. They store this energy in ATP, which supplies the rest of the cell, so they are called the power-houses of the cell.
3. They have a little DNA of their own, which is passed to the offspring.

The lysosome. It is a bag full of strong digestive enzymes. When a cell is old or badly damaged, its lysosomes burst, and their enzymes digest the cell itself. This clears away dead and useless cells so their materials can be reused. Because of this self-digestion, lysosomes are called suicide bags.

Cell organelles: mitochondria, plastids (chloroplasts, chromoplasts, leucoplasts), ribosomes, endoplasmic reticulum, Golgi body, lysosomes (and the nucleus, the largest organelle). Vacuoles are often listed with them.

Cell inclusions: droplets of water, crystals of salts and minerals, starch grains, fat droplets, pigments, and waste products.

1. Chloroplasts make the plant green and make its food by photosynthesis.
2. Chromoplasts color flowers, which attract insects and birds for pollination, and ripe fruits, which attract animals that disperse the seeds.
3. Leucoplasts store food such as starch, proteins and oils in roots, tubers and seeds.

Draw two overlapping circles: Plant on the left, Animal on the right, and Both where they overlap.

Plant only: cell wall, chloroplasts, one large central vacuole, a regular shape, stores starch.

Both: cell membrane, cytoplasm, nucleus, mitochondria, Golgi body, ER, ribosomes.

Animal only: no cell wall, lysosomes, centrioles, small vacuoles (or none), an irregular shape, stores glycogen.

Golgi body: sorts, packs and sends out the cell’s products (mucus, saliva, enzymes, hormones, gums); helps make the new cell membrane or cell plate during cell division; helps in transport inside the cell.

Vacuole: stores cell sap (water, salts, sugars, pigments); keeps the balance of water and salts, and keeps a plant cell firm; collects wastes before they are passed out of the cell.

• i.The figure shows a nucleus. A: nucleolus. B: nuclear membrane. C: nuclear pore. D: chromatin fibers (lying in the nucleoplasm).
• ii.The nuclear membrane protects the nucleus and separates it from the cytoplasm, and through its pores it controls what passes into and out of the nucleus.
• iii.Without a nucleus, a cell can’t control its activities, can’t make new proteins for long, and can’t divide, so it soon dies. (Mature red blood cells in humans have no nucleus, and that is why they live only about 120 days and can’t divide.)

There is no single correct answer; a good answer gives a reason. For example:
The nucleus, because it holds the DNA, the instructions for everything the cell does; it controls all the other parts and is needed for the cell to divide. Without it, the cell soon dies.
Or the mitochondria, because every activity of the cell needs energy, and without them most cells can’t release enough energy to live.

Without chloroplasts the plant couldn’t carry out photosynthesis, so it couldn’t make its own food. Its respiration, growth and reproduction would soon stop for lack of food and energy, and the plant would die once its stored food ran out. Animals that eat plants would lose their food too.

Just as the brain controls the body, the nucleus controls the cell. It holds the DNA, the cell’s instructions, and decides which proteins are made, and so controls the cell’s metabolism, growth and division. It also passes on the characters to the offspring. So it is called the brain, or control center, of the cell.

Metabolism is all the chemical reactions in a cell: releasing energy from food, making new materials and breaking down wastes. Without it, no energy would be released, so the cell couldn’t move, grow, take in materials or divide; no new substances would be made; and wastes would not be dealt with. The cell would die, and if this happened in all the cells, the organism’s life processes (digestion, respiration, excretion, growth, reproduction) would stop and the organism would die.

A virus has no cell, no cytoplasm and no metabolism: it is only genetic material (DNA or RNA) in a protein coat. It can’t feed, grow or multiply by itself, only inside a living host cell, where it causes infection. Since it is not made of cells and can’t live on its own, it is called an infectious particle rather than an organism.

A bad smell shows that microorganisms have grown in the food in large numbers and are breaking it down. Many of them make poisons (toxins). Eating such food can cause food poisoning, with vomiting, diarrhea, fever and dehydration. (Food can be unsafe even before it smells, so leftovers should be kept cold and eaten soon.)

It sorts organisms by many features together: whether the cell has a nucleus, whether the body has one cell or many, how the organism gets food, and its role in the ecosystem. So prokaryotes (Monera), unicellular eukaryotes (Protista) and fungi, which didn’t fit the two-kingdom system, each get their own kingdom, and organisms like Euglena and bacteria are no longer forced into “plants” or “animals”.

Microorganisms need water to grow and multiply. Well-dried fruit has very little water left, so bacteria and molds can’t grow on it, and it doesn’t rot. That is why dried fruits, such as raisins, dates and dried apricots, keep for months.

Living things too small to be seen with the naked eye, which can be seen only with a microscope, are called microorganisms (microbes or germs).
Examples: bacteria, Lactobacillus; fungi, yeast; algae, Chlamydomonas; protozoa, Amoeba; viruses (not true organisms), the coronavirus.

The main methods are dry preservation, wet preservation and cold preservation.

Dry preservation: food is dried in the sun or in a dryer, at a moderate warmth (about 20–30 °C), until very little moisture is left. Microbes can’t grow without water, so the food keeps for months at room temperature. Grain, sukuti, dried fish, gundruk and dried fruit are preserved this way.

Importance: preservation keeps food safe from microbes and food poisoning, keeps its nutrients, lets us store food for times when it isn’t grown (for winter, or for a bad harvest), reduces waste and loss to farmers, and lets food be carried to faraway places.

Illness caused by eating food or drinking water contaminated with harmful microorganisms or the poisons they make is called food poisoning. Its signs are vomiting, diarrhea, stomach pain, fever, dehydration, a dry mouth and headache.

Causes: microbes such as Clostridium, Salmonella, norovirus and Giardia, which get into food left uncovered, kept warm for hours, cooked with dirty hands or utensils, or made with dirty water.

To be safe: wash your hands before cooking and eating; keep food covered from flies and dust; eat food fresh and hot, or keep leftovers in a fridge; don’t eat stale, smelly or moldy food; drink boiled or filtered water; wash fruit and vegetables well.

The five-kingdom system uses more and deeper features: the type of cell, the number of cells, the mode of nutrition, the role in the ecosystem and evolutionary relationships. So it separates prokaryotes from eukaryotes and unicellular from multicellular organisms, gives fungi their own kingdom apart from green plants, and arranges the kingdoms in the order of evolution. Organisms that had no proper place in the two-kingdom system now have one.

Germs and microbes.

Sorting living things into groups and smaller groups according to their likenesses and differences is called classification.
It makes study easier because instead of studying millions of organisms one by one, we study groups: the features of a group belong to all its members, so knowing one member tells us a lot about the others. It puts organisms in order, makes them easy to identify and name, and shows how they are related and how they evolved.

1. Bacteria and fungi decompose dead bodies and wastes and return minerals to the soil, keeping it fertile.
2. Rhizobium in the root nodules of legumes fixes nitrogen from the air, increasing crop yields.
3. Lactobacillus turns milk into curd; yeast makes bread rise and is used to make alcohol; lactic acid bacteria ferment gundruk.
4. Antibiotics, vaccines, vitamins and insulin are made with the help of bacteria and fungi.

Merits: it was the first scientific system of classification; it is simple; it divides living things into two clear groups, plants and animals, which are easy to tell apart.

Drawbacks: it puts prokaryotes (bacteria) with eukaryotes; it doesn’t separate unicellular from multicellular organisms; it puts fungi, which have no chlorophyll, with green plants; and organisms such as Euglena and lichens fit in neither kingdom.

Bases: the type of cell (prokaryotic or eukaryotic), the body structure (unicellular or multicellular), the mode of nutrition (autotrophic or heterotrophic), the role in the ecosystem (producer, consumer or decomposer), and evolutionary relationships.

Features: unicellular and multicellular organisms are separated; prokaryotes and eukaryotes are separated; autotrophs (Plantae) are separated from heterotrophs (Animalia and Fungi); and the kingdoms are arranged from the simplest to the most developed, showing the path of evolution.

• i.A coronavirus (such as SARS-CoV-2): a round virus with an envelope covered in spike proteins, and RNA inside.
• ii.It causes COVID-19. Prevention: get vaccinated; wash hands often with soap and water; wear a mask in crowded places when there is an outbreak; keep sick people apart and cover your mouth and nose when coughing or sneezing.

The five-kingdom system first asks whether the cell has a proper nucleus. All prokaryotes (bacteria, cyanobacteria, mycoplasma, archaea), which have no nuclear membrane, are put in a kingdom of their own, Monera. All eukaryotes go into the other four kingdoms: Protista (unicellular), Fungi, Plantae and Animalia (multicellular). So blue-green algae, which are prokaryotes, are no longer kept with the true algae and plants.

Amoebae and other protozoa feed on bacteria. In soil and water, and in the tanks of sewage treatment plants, they eat large numbers of bacteria, which helps clean the water and keeps bacteria in check. Free-living amoebae are also studied in laboratories to learn how cells move, feed and divide.

Why: mold spores get in when the jar is opened with a wet or dirty spoon, or when it isn’t closed airtight, or if the vegetables weren’t dried well, or there isn’t enough salt, oil or acid to cover them. Where there is moisture and air, the spores grow.

Protection: wash and dry the vegetables (and the jar) well before making the pickle; use enough salt, acid and oil, with the pickle covered by oil; close the jar airtight; always take pickle out with a clean, dry spoon; keep the jar in the sun from time to time, or in a cool place.