Cell Biology and Organisation, practice questions

B, The cell wall, chloroplast and large permanent vacuole are the three structures unique to plant cells; the other options list structures that both cell types share.

C, The mitochondrion is the site of respiration, which releases energy from glucose; the chloroplast carries out photosynthesis, not respiration.

B, The biconcave shape increases the surface area to volume ratio for diffusion, and the absence of a nucleus leaves more space for haemoglobin, so more oxygen is carried.

B, Cells form tissues, tissues form organs, organs form systems, and systems together form the organism; only option B lists this sequence correctly.

B, The contractile vacuole collects excess water and then contracts to expel it from the cell; a food vacuole is used for digestion, not water removal.

C, The plasma membrane is selectively permeable and controls what enters and leaves; the cell wall in plant cells is freely permeable and does not perform this control.

A, The long, thin projection increases the surface area in contact with soil water, speeding up the absorption of water and dissolved mineral salts.

C, A tissue is a group of similar cells that work together to carry out one function, such as muscle tissue, which contracts.

B, A small cell has a large surface area compared with its volume, so substances have only a short distance to travel and diffusion is fast enough without a transport system.

C, Chlorophyll is the green pigment inside the chloroplast that absorbs light energy; haemoglobin is found in red blood cells and carries oxygen instead.

• Shared structure 1: nucleus, controls the activities of the cell and contains the chromosomes.
• Shared structure 2: mitochondrion, the site of respiration, releasing energy from glucose (any shared structure with its function is accepted).
• Plant-only structure 1: cellulose cell wall, gives support and a fixed shape.
• Plant-only structure 2: chloroplast, traps light energy for photosynthesis (or large permanent vacuole, stores cell sap and keeps the cell turgid).
• The plant cell keeps a fixed shape because the rigid cellulose cell wall surrounds it, supported by the turgid vacuole pressing outwards.
• The animal cell has no cell wall, so its flexible plasma membrane cannot hold a fixed shape in the same way.

• In an Amoeba, oxygen diffuses into the cell and carbon dioxide diffuses out directly across the whole cell surface, with no special organ.
• In a human, oxygen enters the blood at the alveoli of the lungs and carbon dioxide leaves there; the blood of the circulatory system then transports the gases between the lungs and the body cells.
• A human is large, so its surface area to volume ratio is small and diffusion across the body surface alone cannot supply the deep-lying cells.
• Specialised organs such as the lungs provide a large gas-exchange surface and the blood provides transport, which a small Amoeba does not need because diffusion over a short distance is enough.

• Order from lowest to highest: muscle cell, blood, heart, circulatory system, human (blood is a tissue placed above the single cell).
• Blood is the tissue; the heart is the organ built from more than one tissue.
• Division of labour means different specialised cells, tissues, organs and systems each carry out a particular life process rather than one cell doing everything.
• This allows the whole organism to work efficiently, as no single cell has to perform every function on its own.

A generalised cell has a nucleus that controls its activities, cytoplasm where chemical reactions take place, mitochondria that release energy through respiration, and ribosomes that build proteins. A plant cell adds three structures an animal cell lacks: a rigid cellulose cell wall for support, chloroplasts that trap light energy, and one large permanent vacuole that keeps the cell turgid.

Plant cells have a cellulose cell wall outside the plasma membrane, chloroplasts for photosynthesis, and a large permanent vacuole for water storage and turgor; animal cells have none of these three structures. Both cell types still share a nucleus, cytoplasm, mitochondria, ribosomes and a plasma membrane that controls what enters and leaves.

Organisms such as Amoeba and Paramecium consist of a single cell that must carry out nutrition, respiration, excretion, growth, movement and response entirely on its own, without any other cell to share the work. Food, oxygen and waste all cross the same cell surface, and the cell has no separate organs to help, unlike the many cells of a multicellular animal.

In a multicellular organism, no single cell performs every life process alone. Specialised cells, tissues, organs and systems divide the work between them, nutrients absorbed by the digestive system are transported by the circulatory system to cells all over the body, which could not happen inside one isolated cell.

Multicellular organisms are built as a hierarchy: similar cells form a tissue, different tissues working together form an organ, related organs form a system, and every system together forms the organism. A fault at one level, such as damaged cells in a tissue, can disrupt the organ and system built from it. Examiners often test this hierarchy by asking you to place a named human example, such as a muscle cell, muscle tissue, the heart and the circulatory system, at the correct level.

Cell structure matches function: a red blood cell is biconcave and lacks a nucleus so it can carry more oxygen; a root hair cell is long and thin to increase the surface area for absorbing water; a nerve cell has a long fibre to carry electrical impulses quickly over a distance. A sperm cell has a tail for swimming towards an egg, and a guard cell changes shape to open or close a leaf's stomata, showing that specialisation applies equally to plant and animal cells.

A tissue is a group of similar cells working together for one function, such as muscle tissue that contracts or xylem tissue that transports water in a plant. Recognising a named tissue is the link between the cell level and the organ level in the hierarchy of organisation. Other examples include epithelial tissue that lines surfaces such as the gut, and phloem tissue that transports dissolved food in a plant.

Every reaction that keeps an organism alive, including respiration, protein synthesis and waste removal, happens inside cells. Because a cell is the smallest unit that can carry out these processes on its own, it is called the basic structural and functional unit of life, and damage at the cell level ultimately affects the whole organism.

The plasma membrane surrounds every cell and controls which substances enter and leave, letting in useful molecules such as oxygen and glucose while allowing waste products to pass out. This control keeps the cell's internal conditions suitable for the reactions it carries out, and the same basic membrane structure appears in every type of cell, from bacteria to human neurons.

The nucleus contains chromosomes made of DNA, carrying the genetic instructions that direct every activity inside the cell, including which proteins it builds. A cell without a working nucleus, such as a mature red blood cell, cannot divide or repair itself, and eventually breaks down and is replaced. Chromosome number is also characteristic of a species; human body cells, for instance, normally contain 23 pairs.