Cell and Tissues: Organelles, Plant and Animal Tissues

The correct answer is B, Robert Hooke. In 1665 he cut a thin slice of cork, examined it under a microscope of his own making and saw rows of empty boxes, which he named cells after the small rooms of a monastery; the drawing and description appeared in his book Micrographia. Option A is wrong because Robert Brown discovered the nucleus in 1831, more than a century and a half later, and is also remembered for Brownian motion. Option C is wrong because Theodor Schwann, with Matthias Schleiden, proposed the cell theory, that all plants and animals are made of cells; he did not discover the cell itself. Option D is wrong because Anton van Leeuwenhoek was the first to see living cells such as bacteria and protozoa in pond water, but Hooke's observation came first and gave the structure its name. Note that what Hooke actually saw were dead cell walls, since cork is dead tissue.

The correct answer is B, mitochondria and plastids. Both carry a small circular DNA of their own and their own 70S ribosomes, so they make a part of their protein themselves and divide independently of the cell; this semi-autonomous nature is the main evidence for the view that they descend from free-living bacteria engulfed by an early cell. Option A is wrong because the Golgi body and the endoplasmic reticulum are membrane systems that modify and transport material and carry no genetic matter of their own; the rough endoplasmic reticulum merely holds ribosomes on its outer surface. Option C is wrong because the lysosome is only a sac of digestive enzymes and the Golgi body, as stated, has no DNA. Option D is wrong because the vacuole is a storage sac bounded by the tonoplast and has no DNA, even though its partner in the option, the plastid, does. Remember the pair as the two power organelles of the cell.

The correct answer is B, lysosome. A lysosome is a membrane-bound sac of powerful digestive enzymes that breaks down worn-out organelles and any foreign particle the cell takes in; if the cell is damaged or starving, the sacs burst and the same enzymes digest the cell itself, which is why the name suicide bag is used. Option A is wrong because the ribosome is the site of protein synthesis and is called the protein factory of the cell. Option C is wrong because the centrosome, present in animal cells, organises the spindle fibres during cell division. Option D is wrong because the vacuole stores water, sap and waste and keeps a plant cell turgid, which is a storage function and not a destructive one. Two more nicknames belong with this set and are asked just as often: the mitochondrion is the powerhouse of the cell and the Golgi apparatus is its packaging and dispatch unit.

The correct answer is A, Schleiden and Schwann. Matthias Schleiden, a botanist, concluded that all plants are made of cells, and Theodor Schwann, a zoologist, reached the same conclusion for animals; together their statement became the cell theory. Option B is wrong because Robert Hooke only discovered and named the cell and Robert Brown discovered the nucleus, and neither framed a general theory. Option C is wrong because Charles Darwin is associated with natural selection and the origin of species while Gregor Mendel founded the science of heredity through his work on pea plants. Option D is wrong because James Watson and Francis Crick described the double helix structure of DNA in 1953, which belongs to molecular biology. One addition completes the theory and is often the second half of the question: Rudolf Virchow stated that every cell arises from a pre-existing cell, which ruled out the idea of cells forming on their own.

The correct answer is C, tendon. A tendon is made of dense white fibrous connective tissue with great tensile strength and almost no elasticity, so when a muscle contracts the whole pull is transmitted to the bone without being lost in stretching. Option A is wrong because a ligament joins one bone to another across a joint; it contains elastic fibre, stretches to a degree and holds the bones in position, and this is the pair examiners most often interchange. Option B is wrong because cartilage is a soft, flexible supporting tissue found at the ends of bones, in the nose, the ear and the windpipe, where it gives shape and cushions movement. Option D is wrong because areolar tissue is a loose packing tissue that fills the space between organs and under the skin and helps in repair. Fix the pair by the words themselves: tendon for muscle to bone, ligament for bone to bone.

The correct answer is B, xylem. Xylem is a complex permanent tissue of tracheids, vessels, xylem parenchyma and xylem fibres; the vessels and tracheids are dead, hollow and joined end to end, which makes them ideal pipes for carrying water and minerals from the root to the leaves in one direction. Option A is wrong because phloem conducts the food prepared in the leaves and moves it in both directions, upward and downward, to wherever it is stored or used. Option C is wrong because collenchyma is a simple permanent tissue that gives flexibility and mechanical support to young and climbing stems; it conducts nothing. Option D is wrong because cambium is a lateral meristem whose business is division, adding new xylem inward and new phloem outward and so increasing the girth of the stem. A useful memory aid is that only the parenchyma is alive in xylem, while in phloem only the fibres are dead.

The correct answer is C, nerve cell. A neuron carries a long process called an axon, which in a human being may run from the base of the spine to the toe, so a single nerve cell can be about a metre long; this length lets one cell carry an impulse over a great distance without any relay. Option A is wrong because the red blood cell is a small biconcave disc of a few micrometres, and in mature form it has no nucleus at all, which leaves more room for haemoglobin. Option B is wrong because a skeletal muscle fibre is indeed long and multinucleate, but shorter than the longest neuron, and it is the second choice examiners place as a distractor. Option D is wrong because a bone cell, the osteocyte, sits in a small cavity called a lacuna and is compact. Two companion facts are worth carrying: the smallest cell known is Mycoplasma and the largest single cell is the egg of an ostrich.

The correct answer is A, cardiac muscle. The muscle of the heart shows light and dark bands under the microscope, so it is striated, yet it is not under the control of the will and it contracts rhythmically without rest for the whole of life, so it is involuntary; this unusual combination is exactly why it is asked. Option B is wrong because skeletal muscle is striated but voluntary, moving when the person wills it. Option C is wrong because smooth or unstriated muscle, found in the walls of the alimentary canal, blood vessels and urinary bladder, has no bands and is involuntary, so it fails the striated half of the question. Option D is wrong because the biceps is a named skeletal muscle of the arm and is therefore voluntary. Remember the three-line table: skeletal, striated and voluntary; smooth, unstriated and involuntary; cardiac, striated and involuntary.

The correct answer is B, blood. Blood is classed as a connective tissue because its cells lie in a matrix, and because that matrix, the plasma, is liquid it is called a fluid connective tissue; the red cells, white cells and platelets float in it and it connects the organs of the body by carrying gases, food, wastes and hormones between them. Option A is wrong because cartilage is a connective tissue too, but its matrix is solid and flexible, not fluid. Option C is wrong because epithelium is a covering and lining tissue with its cells packed tightly together and almost no matrix between them. Option D is wrong because nervous tissue is made of neurons specialised to conduct impulses and is a separate one of the four animal tissue types. Lymph is the second fluid connective tissue and is the usual second option in a two-statement version of this question.

The correct answer is C, lateral meristem. The lateral meristem, better known as the cambium, lies as a ring along the length of the stem and root and divides to add new xylem on its inner side and new phloem on its outer side, so the stem grows thicker year after year; this is secondary growth, and it is what produces the wood of a tree. Option A is wrong because the apical meristem sits at the tip of the root and the shoot and adds to length, which is primary growth. Option B is wrong because the intercalary meristem lies at the base of leaves and at the internodes and lets grasses and bamboo grow again after cutting or grazing, again adding length. Option D is wrong because permanent tissue is made of cells that have lost the power to divide and have taken a fixed shape and function. Keep the three meristems as a set: apical for length, lateral for girth, intercalary for regrowth.