Cell: The Unit of Life: common doubts, answered
The questions students ask most often about Cell: The Unit of Life, each with a short answer. For the full chapter, read the Cell: The Unit of Life notes.
About the chapter
Do all cells have a nucleus?
No. Mature red blood cells of most mammals, including humans, and the sieve tube cells of plants lack a nucleus. Prokaryotic cells never have a true nucleus either. This shows that the general rule is not absolute, and Mycoplasma likewise is an exception to the rule that prokaryotes have a cell wall.
What is a cell, and the cell theory
Read this section in the notes →Who proposed the cell theory and what did Virchow add?
Schleiden and Schwann proposed that all plants and animals are composed of cells and their products. Rudolf Virchow later added in 1855 that new cells arise from pre-existing cells, expressed as omnis cellula-e cellula. Without this addition the original theory could not explain how new cells are formed.
An overview of the cell
Read this section in the notes →Which is the smallest cell and which is the largest isolated single cell?
Mycoplasma, a prokaryote without a cell wall, forms the smallest cells known. Among isolated single cells, the ostrich egg is the largest. For comparison, a human red blood cell is about 7 micrometres across, while bacteria are about 3 to 5 micrometres, so cell sizes vary enormously.
Prokaryotic cells: general features
Read this section in the notes →What is the difference between prokaryotic and eukaryotic cells?
Prokaryotic cells have no membrane-bound nucleus and no membrane-bound organelles, and their genetic material lies in a nucleoid. Eukaryotic cells have a true nucleus enclosed by a nuclear envelope and several membrane-bound organelles. Prokaryotes also have 70S ribosomes, while eukaryotic cytoplasmic ribosomes are 80S.
Cell envelope, its modifications, ribosomes and inclusion bodies
Read this section in the notes →What is the difference between a capsule and a slime layer in bacteria?
Both are parts of the glycocalyx, the outermost covering of the bacterial cell. If the glycocalyx is a loose sheath it is called a slime layer, while if it is thick and tough it is called a capsule. Together with the cell wall and plasma membrane they form the cell envelope.
What is a mesosome and what does it do?
A mesosome is an infolding of the plasma membrane in a bacterial cell, seen as vesicles, tubules or lamellae. It helps in cell wall formation, DNA replication and its distribution to daughter cells, and in respiration and secretion. It also increases the surface area of the plasma membrane and the amount of enzymes it carries.
Why is 50S plus 30S equal to 70S and not 80S?
The S value is the Svedberg unit of sedimentation rate, not a measure of mass, so the values do not add up simply. In prokaryotes the 50S and 30S subunits make a 70S ribosome, and in eukaryotes the 60S and 40S subunits form an 80S ribosome. Mitochondria and chloroplasts also have 70S ribosomes.
Eukaryotic cells and the cell membrane
Read this section in the notes →What is the fluid mosaic model?
The fluid mosaic model, proposed by Singer and Nicolson in 1972, describes the plasma membrane as a quasi-fluid lipid bilayer with proteins embedded in it or lying on the surface. The lipids can move sideways, which gives fluidity, and this lateral movement is important for functions like cell growth and cell division. Human red cell membrane is about 52 per cent protein.
What is the difference between passive transport and active transport?
Passive transport moves molecules along their concentration gradient without using energy, as in simple diffusion and facilitated diffusion. Active transport moves them against the gradient and needs energy in the form of ATP. The sodium-potassium pump is an example of active transport.
Cell wall
Read this section in the notes →What is the order of layers in a plant cell wall?
From the outside inwards, the layers are the middle lamella, the primary wall and then the secondary wall, which is laid down last on the inner side. The middle lamella is made of calcium pectate and glues neighbouring cells together. Plasmodesmata connect the cytoplasm of adjacent cells through the wall.
Endomembrane system: ER and Golgi apparatus
Read this section in the notes →Which organelles are part of the endomembrane system?
The endoplasmic reticulum, Golgi apparatus, lysosomes and vacuoles make up the endomembrane system, because their functions are coordinated. Mitochondria, chloroplasts and peroxisomes are not included, since their functions are not coordinated with these. Having a membrane alone is not the test of membership.
What is the difference between smooth and rough endoplasmic reticulum?
Rough ER has ribosomes attached to its surface and is active in protein synthesis and secretion, and it is continuous with the outer nuclear membrane. Smooth ER has no ribosomes and is the major site of lipid synthesis, and in animal cells it makes steroidal hormones.
What is the difference between the cis and trans faces of the Golgi apparatus?
The cis face, also called the forming face, is convex and lies close to the ER, receiving its vesicles. The trans face, also called the maturing face, is concave and releases the finished products. The Golgi also helps make glycoproteins and glycolipids. Camillo Golgi first observed it in 1898.
Endomembrane system: lysosomes and vacuoles
Read this section in the notes →Why are lysosomes called suicide bags?
Lysosomes contain hydrolytic enzymes such as lipases, proteases and carbohydrases that can digest almost every type of biomolecule, and these work best at acidic pH. If a lysosome bursts, the enzymes can digest the cell itself, hence the name. Their membranes are formed by the Golgi, which packages the enzymes.
What is the tonoplast?
The tonoplast is the single membrane that encloses the vacuole in a plant cell. The vacuole contains water, sap, excretory products and other materials not useful to the cell, and in a mature plant cell it may fill as much as 90 per cent of the cell. The tonoplast helps transport ions into the vacuole against a gradient.
Mitochondria
Read this section in the notes →Why are mitochondria called the powerhouses of the cell?
Mitochondria produce cellular energy in the form of ATP through aerobic respiration, so they are called the powerhouses. A mitochondrion has a double membrane, the inner one folded into cristae that increase surface area for ATP production. The matrix contains a circular DNA molecule, some RNA, 70S ribosomes and the enzymes of the Krebs cycle.
Plastids
Read this section in the notes →What is the difference between amyloplasts, elaioplasts and aleuroplasts?
These are all leucoplasts, colourless plastids that store food. Amyloplasts store carbohydrates such as starch, elaioplasts store oils and fats, and aleuroplasts store proteins. Chloroplasts and chromoplasts are the other two kinds of plastid. The prefixes help recall them: amylo for starch, elaio for oil, and aleuro for protein.
Ribosomes, cytoskeleton, cilia, flagella and centrioles
Read this section in the notes →What is the difference between the structure of cilia or flagella and a centriole?
The axoneme of a cilium or flagellum has nine pairs of peripheral microtubules plus two central microtubules, a 9+2 arrangement. A centriole has nine peripheral triplets around a central hub of protein and no central microtubules. It is the base of cilia and flagella, and it forms the spindle apparatus during division of animal cells.
Nucleus and microbodies
Read this section in the notes →Is the nucleolus surrounded by a membrane?
No, the nucleolus is not bound by a membrane. It is a spherical structure within the nucleoplasm and is the site of active ribosomal RNA synthesis, so cells making many proteins have large nucleoli. The nucleus itself has a double membrane, the nuclear envelope, with a perinuclear space of 10 to 50 nanometres between the layers.
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