What is a cell? Cell theory
A brick is the smallest unit of a wall, and a cell is the smallest unit of a living body. Some living things, such as an Amoeba, are just one cell, while a human is made of billions of them.
A cell is the basic structural and functional unit of life. Cell theory: all living organisms are made of cells and their products; the cell is the unit of structure and function; and new cells arise from pre-existing cells.
In the figure : Three scientists who built the cell theory.
Rule 1: History of the cell theory — Robert Brown discovered the nucleus. Matthias Schleiden (1838) saw that plants are made of cells, and Theodor Schwann (1839) said that the animal body is also made of cells and found a thin cell membrane and a plant cell wall. Rudolf Virchow (1855) explained that new cells arise from pre-existing cells (omnis cellula-e cellula).
- Easy : Both a mango tree and a goat are made of cells.
- Medium : A new skin cell is made when an old skin cell divides.
- Exam-type : Q: Who said that new cells arise from pre-existing cells? A: Virchow, in 1855.
Rule 2: Size and shape — Cells differ in size, shape and activity. Mycoplasma, the smallest cell, is about 0.3 µm long; bacteria are 3–5 µm; the largest isolated single cell is the egg of an ostrich; human red blood cells are about 7 µm across. Nerve cells are some of the longest cells. Cell shapes can be disc-like, polygonal, columnar, cuboid, thread-like or irregular.
- Easy : A human RBC is about 7 µm in diameter.
- Medium : A nerve cell can be as long as a leg.
- Exam-type : Q: Which is the smallest cell known? A: Mycoplasma.
Cell theory has three points: made of cells; unit of structure and function; cells come from cells.
Thinking that Schleiden and Schwann discovered the cell. Hooke first saw cells, and Virchow completed the theory.
Memory clue: S-S-V — Schleiden plants, Schwann animals, Virchow division.
Spot the difference
| Scientist | Year | Contribution |
|---|---|---|
| Schleiden | 1838 | Plants are made of cells |
| Schwann | 1839 | Animals are made of cells |
| Virchow | 1855 | Cells arise from cells |
Practice questions
- 1Define a cell.
- 2Who gave the idea that new cells arise from old cells?
- 3Name the smallest cell.
- 4Which cell is the largest isolated single cell?
- 5Write the three points of the cell theory.
- 6Who discovered the nucleus?
- 7Why is the cell called the unit of life?
- 8Give two examples of how cells differ from one another.
Show answers (try them yourself first)
- 1The basic structural and functional unit of life.
- 2Rudolf Virchow.
- 3Mycoplasma (about 0.3 µm).
- 4The egg of an ostrich.
- 5All organisms are made of cells; the cell is the unit of structure and function; new cells come from pre-existing cells.
- 6Robert Brown.
- 7All functions of life such as respiration, growth and reproduction occur in the cell, and it can live independently in unicellular organisms.
- 8They differ in shape (disc, columnar, thread-like) and in size (Mycoplasma 0.3 µm; RBC 7 µm; nerve cells very long).
Prokaryotic cells: structure and features
Curd, a sore throat and cholera all involve tiny cells that have no true nucleus. These prokaryotic cells are the simplest living cells.
Prokaryotic cells (bacteria, blue-green algae, mycoplasma, PPLO) have no membrane-bound nucleus or organelles. The genetic material is a naked circular DNA in the nucleoid. They also have small circular plasmids.
In the figure : Main parts of a prokaryotic cell.
Rule 1: Cell envelope — It is made of the glycocalyx, the cell wall and the plasma membrane. The glycocalyx is a loose slime layer or a tough capsule. The cell wall gives shape and strength. Gram-positive bacteria take up the gram stain and Gram-negative ones do not. A mesosome is a membranous infolding of the plasma membrane that helps in cell wall formation, DNA replication and respiration.
- Easy : A bacterial capsule protects it from drying.
- Medium : Gram-positive bacteria look purple after staining.
- Exam-type : Q: What is a mesosome? A: A membranous structure formed by the infolding of the plasma membrane.
Rule 2: Motility, ribosomes and inclusions — Flagella help bacteria move. Pili and fimbriae are surface structures; pili help in attachment (and transfer of DNA), fimbriae help the cell stick to rocks and hosts. Ribosomes are 70S (50S + 30S subunits) and lie free in the cytoplasm. Inclusion bodies are stored food: phosphate granules, cyanophycean granules, glycogen granules and gas vacuoles (in blue-green and purple bacteria).
- Easy : A bacterium moves by rotating its flagellum.
- Medium : Gas vacuoles help aquatic bacteria float.
- Exam-type : Q: What is the size of the ribosome of a prokaryote? A: 70S, made of 50S and 30S subunits.
Prokaryote: no nucleus, no membrane-bound organelles, 70S ribosomes, naked circular DNA.
Thinking prokaryotes have no ribosomes. They have 70S ribosomes.
Memory clue: PNO — Prokaryotes: No nucleus, No organelles.
Spot the difference
| Point | Pili | Fimbriae | Flagella |
|---|---|---|---|
| Role | Attachment, DNA transfer | Attachment | Movement |
| Found in | Bacteria | Bacteria | Motile bacteria |
Practice questions
- 1What is a prokaryotic cell?
- 2Name the three layers of a bacterial cell envelope.
- 3What is a nucleoid?
- 4What is the size of ribosomes in prokaryotes?
- 5Name two inclusion bodies.
- 6What is a plasmid?
- 7How is a Gram-positive bacterium different from a Gram-negative one?
- 8State the role of mesosomes.
Show answers (try them yourself first)
- 1A cell without a membrane-bound nucleus and membrane-bound organelles.
- 2Glycocalyx, cell wall and plasma membrane.
- 3The region where the naked DNA lies in a prokaryotic cell.
- 470S.
- 5Phosphate granules and glycogen granules (or cyanophycean granules, gas vacuoles).
- 6A small circular DNA outside the main DNA of a bacterium.
- 7Gram-positive bacteria retain the gram stain; Gram-negative bacteria do not.
- 8They help in cell wall formation, DNA replication and distribution, and respiration.
The cell membrane and the cell wall
A school gate lets some people in and keeps others out. The cell membrane works as a gate, and the cell wall of a plant cell is like a boundary wall around the gate.
The plasma membrane is a thin, selectively permeable boundary around all cells. The fluid mosaic model (Singer and Nicolson, 1972) describes it as a quasi-fluid lipid bilayer with proteins floating in it. The cell wall is a rigid outer layer found in plants and fungi.
In the figure : A membrane is a lipid bilayer with proteins; plants have a wall outside it.
Rule 1: Membrane structure and transport — Lipids are arranged in two layers, with polar heads outside and hydrophobic tails inside. Proteins may be peripheral or integral. The fluid nature lets lateral movement and helps cell growth, endocytosis and cell division. Small non-polar molecules cross by simple diffusion, while polar molecules need carrier proteins (facilitated diffusion), and active transport needs energy (ATP) to move molecules against a gradient, as in the Na+/K+ pump.
- Easy : Oxygen passes easily through the membrane.
- Medium : Glucose needs a carrier protein to enter the cell.
- Exam-type : Q: Who proposed the fluid mosaic model? A: Singer and Nicolson (1972).
Rule 2: Cell wall — The wall gives shape, protects against mechanical damage and infection, helps cell-to-cell interaction and prevents bursting. In algae it is cellulose, galactans, mannans and minerals; in plants, cellulose, hemicellulose, pectins and proteins. The middle lamella (calcium pectate) glues neighbouring cells, and plasmodesmata connect the cytoplasm of adjacent cells. Secondary wall forms in some cells.
- Easy : A cell wall stops a plant cell from bursting in water.
- Medium : Plasmodesmata let materials pass from one cell to the next.
- Exam-type : Q: What holds two plant cells together? A: The middle lamella of calcium pectate.
Membrane: lipid bilayer + proteins; polar heads out, non-polar tails in. Passive transport needs no energy; active transport needs ATP.
Thinking that the cell wall is selectively permeable. The wall is permeable; the plasma membrane is selectively permeable.
Memory clue: W-P / M-S — Wall is Permeable; Membrane is Selective.
Spot the difference
| Point | Plasma membrane | Cell wall |
|---|---|---|
| Present in | All cells | Plants, fungi, bacteria |
| Nature | Selectively permeable | Freely permeable |
| Made of | Lipids and proteins | Cellulose, pectin and others |
Practice questions
- 1Who proposed the fluid mosaic model?
- 2Name the main chemical components of the membrane.
- 3What is the middle lamella made of?
- 4What are plasmodesmata?
- 5What is active transport?
- 6Which transport needs a carrier protein but no energy?
- 7Why are the lipid tails of the membrane inside and the heads outside?
- 8State three functions of the cell wall.
Show answers (try them yourself first)
- 1Singer and Nicolson in 1972.
- 2Lipids (phospholipids) and proteins.
- 3Calcium pectate.
- 4Fine cytoplasmic channels that connect neighbouring plant cells.
- 5Movement of molecules across the membrane against the gradient using energy (ATP).
- 6Facilitated diffusion.
- 7The tails are water-hating (hydrophobic) and the heads are water-loving, so the tails are shielded from water.
- 8It gives shape, protects the cell and prevents bursting; it also helps cell-to-cell interaction.
Endomembrane system: ER, Golgi, lysosomes and vacuoles
A factory has a production line, a packing section, a dispatch gate and a waste room. A cell has similar linked compartments that work together as the endomembrane system.
The endomembrane system includes the endoplasmic reticulum (ER), Golgi apparatus, lysosomes and vacuoles, whose functions are coordinated. Mitochondria, chloroplasts and peroxisomes are not part of it.
In the figure : Materials move from the ER through the Golgi apparatus to their destination.
Rule 1: ER and Golgi apparatus — The ER is a network of tubules, cisternae and vesicles. Rough ER (RER) has ribosomes on its outer surface and makes proteins; smooth ER (SER) has no ribosomes and makes lipids (and steroids in animal cells). Camillo Golgi (1898) first saw the Golgi apparatus: flat disc-like cisternae stacked in parallel, with a convex cis face and a concave trans face. It packages materials and sends them to the destination, and it makes glycoproteins and glycolipids.
- Easy : Cells that make a lot of protein have much RER.
- Medium : The trans face of the Golgi is where the packed material leaves.
- Exam-type : Q: Which ER has ribosomes? A: The rough ER.
Rule 2: Lysosomes and vacuoles — Lysosomes are membrane-bound vesicles with hydrolytic enzymes (lipases, proteases, carbohydrases) that work best at acidic pH and digest all organic matter. Vacuoles are membrane-bound spaces: the plant vacuole is bounded by a membrane called the tonoplast and may take up to 90 per cent of the cell volume; it holds water, sap, excretory products and other materials. In Amoeba the contractile vacuole helps in excretion, and in many protists food vacuoles are formed.
- Easy : A lysosome digests a bacterium that enters a white blood cell.
- Medium : The big central vacuole of a plant cell keeps the cell firm.
- Exam-type : Q: What is the membrane around a plant vacuole called? A: The tonoplast.
RER = protein; SER = lipid; Golgi = packing; lysosomes = digestion; vacuole = storage.
Including mitochondria and chloroplasts in the endomembrane system. Their functions are not coordinated with it.
Memory clue: ER-G-L-V — ER makes, Golgi packs, Lysosome digests, Vacuole stores.
Spot the difference
| Organelle | Main function | Feature |
|---|---|---|
| RER | Protein synthesis | Ribosomes on surface |
| SER | Lipid synthesis | No ribosomes |
| Golgi | Packing and dispatch | Cis and trans faces |
| Lysosome | Digestion | Hydrolytic enzymes |
Practice questions
- 1Which organelles form the endomembrane system?
- 2What do ribosomes on the RER do?
- 3Who first saw the Golgi apparatus?
- 4Name the two faces of the Golgi.
- 5What is the membrane around a vacuole called?
- 6What enzymes do lysosomes contain?
- 7Why is the SER well developed in cells that make steroid hormones?
- 8Why are lysosomes called suicide bags?
Show answers (try them yourself first)
- 1ER, Golgi apparatus, lysosomes and vacuoles.
- 2Make proteins.
- 3Camillo Golgi (1898).
- 4Cis (convex) and trans (concave).
- 5Tonoplast.
- 6Hydrolytic enzymes such as lipases, proteases and carbohydrases.
- 7SER makes lipids including steroids.
- 8If they burst, their enzymes can digest the cell's own materials.
Mitochondria and plastids
A power plant makes electricity for a town, and a solar farm captures sunlight. In a cell, mitochondria are the power plants, and chloroplasts are the solar panels of the plant cell.
Mitochondria are double-membrane organelles that make ATP by cellular respiration. Plastids are found in plants and some protists; they are chloroplasts (green), chromoplasts (coloured) and leucoplasts (colourless).
In the figure : Three kinds of plastids.
Rule 1: Mitochondria — The outer membrane is smooth and the inner membrane folds inward as cristae, which increase the surface for ATP-making enzymes. The matrix has enzymes, a single circular DNA, a few RNA molecules and 70S ribosomes. Mitochondria divide by fission and are the 'powerhouses' of the cell.
- Easy : Muscle cells have many mitochondria.
- Medium : Cristae give a large surface for respiration enzymes.
- Exam-type : Q: What are cristae? A: Infoldings of the inner membrane of a mitochondrion.
Rule 2: Plastids — Chloroplasts have a double membrane, a fluid stroma with enzymes, DNA and 70S ribosomes, and flat membranous thylakoids that are stacked as grana. Chlorophyll is in the thylakoids. Chromoplasts have fat-soluble carotenoid pigments (yellow, orange, red). Leucoplasts store food: amyloplasts (starch), elaioplasts (oils and fats) and aleuroplasts (proteins).
- Easy : Chlorophyll in the thylakoids traps the sunlight.
- Medium : A potato tuber has amyloplasts that store starch.
- Exam-type : Q: Which plastid stores oils? A: Elaioplast.
Both mitochondria and chloroplasts have double membranes, their own DNA and 70S ribosomes.
Saying mitochondria make energy. They convert the energy in food into ATP; they do not create it.
Memory clue: M-ATP, C-Sun — Mitochondria make ATP; Chloroplasts catch the Sun.
Spot the difference
| Point | Mitochondrion | Chloroplast |
|---|---|---|
| Inner structure | Cristae | Thylakoids in grana |
| Fluid | Matrix | Stroma |
| Function | ATP production | Photosynthesis |
Practice questions
- 1What is the function of mitochondria?
- 2What are the infoldings of the inner mitochondrial membrane called?
- 3Name the three types of plastids.
- 4Where is chlorophyll present in a chloroplast?
- 5What do amyloplasts store?
- 6What is the stroma?
- 7Why are mitochondria and chloroplasts called semi-autonomous organelles?
- 8Why are chromoplasts coloured?
Show answers (try them yourself first)
- 1They make ATP by cellular respiration.
- 2Cristae.
- 3Chloroplasts, chromoplasts and leucoplasts.
- 4In the thylakoids.
- 5Starch.
- 6The fluid matrix of the chloroplast containing enzymes, DNA and ribosomes.
- 7They have their own DNA and 70S ribosomes and can make some of their own proteins.
- 8They contain carotenoid pigments such as carotene and xanthophyll.
Ribosomes, cytoskeleton, cilia, flagella and centrosome
A building needs workers to build it, a frame to hold it up and some motors to move things. Ribosomes build proteins, the cytoskeleton gives the frame and cilia and flagella move.
Ribosomes are non-membranous granules that make proteins. The cytoskeleton is a network of protein filaments. Cilia and flagella are hair-like outgrowths for movement. The centrosome has two centrioles that help in cell division.
In the figure : Non-membranous cell structures.
Rule 1: Ribosomes and cytoskeleton — George Palade (1953) saw ribosomes as dense particles. Eukaryotic ribosomes are 80S (60S + 40S subunits); prokaryotic ribosomes are 70S (50S + 30S). The 'S' stands for the Svedberg unit and it is a measure of density and size. The cytoskeleton, made of microtubules, microfilaments and intermediate filaments, helps in shape, support, movement and mechanical strength.
- Easy : Ribosomes are found free in the cytoplasm and on the rough ER.
- Medium : The cytoskeleton keeps an animal cell in shape, even without a wall.
- Exam-type : Q: What is the size of eukaryotic ribosomes? A: 80S.
Rule 2: Cilia, flagella and centrosome — Cilia are small and work like oars; flagella are longer and cause cell movement. Both have a core, the axoneme, with a 9+2 arrangement of microtubules. Each is covered by the plasma membrane. The centrosome contains two cylindrical centrioles lying at right angles to each other; each has nine evenly spaced peripheral fibrils of tubulin, each a triplet, in a cartwheel pattern (9+0). Centrioles form the basal body of cilia and flagella and spindle fibres in animal cells.
- Easy : Cilia in the windpipe sweep dust out.
- Medium : A sperm moves with the help of a flagellum.
- Exam-type : Q: What is the arrangement of microtubules in a cilium? A: 9+2.
Cilia and flagella: 9+2. Centriole: 9+0 triplets.
Saying that a centriole has the 9+2 pattern. It has nine triplets and no central tubule (9+0).
Memory clue: 9+2 / 9+0 — Cilium 9+2; Centriole 9+0.
Spot the difference
| Point | Cilia | Flagella |
|---|---|---|
| Size | Short, many | Long, few |
| Structure | 9+2 | 9+2 |
| Action | Oar-like | Wave-like |
Practice questions
- 1Who discovered ribosomes?
- 2What does 'S' stand for in 70S?
- 3Name the three types of cytoskeletal filaments.
- 4What is an axoneme?
- 5How many centrioles are present in a centrosome?
- 6Which cells make a protein-rich secretion with many ribosomes?
- 7Differentiate between cilia and flagella.
- 8What is the structure of a centriole?
Show answers (try them yourself first)
- 1George Palade (1953).
- 2The Svedberg unit, a measure of density and size.
- 3Microtubules, microfilaments and intermediate filaments.
- 4The core of a cilium or flagellum made of microtubules in a 9+2 pattern.
- 5Two.
- 6Cells with a lot of rough ER and free ribosomes, such as gland cells.
- 7Cilia are short and numerous with an oar-like action; flagella are long and few, and move the cell with a wave-like action.
- 8A cylinder made of nine peripheral triplets of tubulin fibrils joined in a cartwheel pattern.
The nucleus and chromosomes
A school principal's office holds the records of every student and gives orders. The nucleus is the 'office' of a cell, with the DNA records that control the cell.
The nucleus (Robert Brown, 1831) is the control centre of a eukaryotic cell. It has a double-layered nuclear envelope with pores, a nucleoplasm, a nucleolus and chromatin.
In the figure : Main parts of the nucleus.
Rule 1: Parts of the nucleus — The nuclear envelope has two membranes with a gap (perinuclear space), and pores let RNA and proteins pass. The nucleolus is a spherical, membrane-less structure that makes ribosomal RNA, so cells that make a lot of protein have large nucleoli. Chromatin is a network of DNA, histone proteins, some non-histone proteins and RNA; it condenses into chromosomes at cell division.
- Easy : mRNA leaves the nucleus through the nuclear pores.
- Medium : A cell that makes much protein has a large nucleolus.
- Exam-type : Q: What is chromatin made of? A: DNA, histones, non-histone proteins and RNA.
Rule 2: Chromosomes — A chromosome has a primary constriction, the centromere, with a kinetochore on each side. Position of the centromere gives four types: metacentric (middle), sub-metacentric (slightly away from the middle), acrocentric (near one end) and telocentric (at the terminal end). Some have a secondary constriction and a satellite.
- Easy : A metacentric chromosome looks like a V or an X when it separates.
- Medium : A telocentric chromosome has its centromere at the very end.
- Exam-type : Q: What is a centromere? A: The primary constriction that holds two chromatids together and bears kinetochores.
Nucleolus makes rRNA; chromatin = DNA + histones; centromere position gives four chromosome types.
Saying that the nucleolus has a membrane. It is not membrane-bound.
Memory clue: M-S-A-T — Metacentric, Sub-metacentric, Acrocentric, Telocentric: centromere moves from the middle to the end.
Spot the difference
| Type | Centromere position | Shape at anaphase |
|---|---|---|
| Metacentric | Middle | V-shaped |
| Sub-metacentric | Slightly away from the middle | L-shaped |
| Acrocentric | Close to one end | J-shaped |
| Telocentric | At the end | Rod-like |
Practice questions
- 1Who discovered the nucleus?
- 2What is the function of nucleolus?
- 3What is the nuclear envelope?
- 4Name the four types of chromosomes based on the centromere.
- 5What is a kinetochore?
- 6What is a satellite chromosome?
- 7Why do nuclear pores matter?
- 8What is the difference between chromatin and chromosomes?
Show answers (try them yourself first)
- 1Robert Brown.
- 2It makes ribosomal RNA and helps to form ribosomes.
- 3A double membrane that surrounds the nucleus and has nuclear pores.
- 4Metacentric, sub-metacentric, acrocentric and telocentric.
- 5A disc-shaped structure on the centromere where the spindle fibres attach.
- 6A chromosome with a secondary constriction that forms a small fragment, the satellite.
- 7They allow movement of RNA and proteins between the nucleus and the cytoplasm.
- 8Chromatin is the loose network seen in the non-dividing cell, while chromosomes are the condensed form seen during division.
Microbodies and comparing prokaryotic and eukaryotic cells
A chemistry lab has small closed rooms where specific chemicals are handled. Microbodies are like those small rooms inside a cell, each holding certain enzymes.
Microbodies are small, membrane-bound vesicles containing various enzymes, found in both plant and animal cells. Eukaryotic cells have a true nucleus and membrane-bound organelles; prokaryotic cells do not.
In the figure : Two cell types.
Rule 1: Microbodies — Peroxisomes and glyoxysomes are examples. Peroxisomes handle hydrogen peroxide and fatty-acid breakdown; glyoxysomes of germinating oily seeds convert fat into carbohydrate.
- Easy : A germinating castor seed has glyoxysomes.
- Medium : Peroxisomes break down harmful hydrogen peroxide.
- Exam-type : Q: Where are glyoxysomes found? A: In germinating seeds that store oil.
Rule 2: Prokaryote versus eukaryote — Prokaryotes: nucleoid, naked circular DNA, 70S ribosomes, no membrane-bound organelles, cell wall of different chemistry, binary fission. Eukaryotes: true nucleus with nuclear envelope, linear DNA with histones, 80S ribosomes, many membrane-bound organelles, and mitosis and meiosis.
- Easy : A bacterium is a prokaryote; an Amoeba is a eukaryote.
- Medium : Eukaryotic cells have mitochondria; prokaryotic cells do not.
- Exam-type : Q: Which ribosomes occur in eukaryotes? A: 80S.
Prokaryote = nucleoid, 70S, no organelles. Eukaryote = nucleus, 80S, organelles.
Saying that all prokaryotes lack a cell wall. Only mycoplasma lacks it.
Memory clue: Pro-No / Eu-True — Prokaryote: No nucleus; Eukaryote: True nucleus.
Spot the difference
| Point | Prokaryotic | Eukaryotic |
|---|---|---|
| Nucleus | No true nucleus | True nucleus |
| Ribosome | 70S | 80S |
| Organelles | No membrane-bound | Present |
| Division | Binary fission | Mitosis / meiosis |
Practice questions
- 1What are microbodies?
- 2Name two kinds of microbodies.
- 3Which cell type has a true nucleus?
- 4State the size of the ribosomes in eukaryotic cells.
- 5Give one example each of a prokaryote and a eukaryote.
- 6What is the function of glyoxysomes?
- 7List three differences between prokaryotic and eukaryotic cells.
- 8Why is an oil-rich seed rich in glyoxysomes during germination?
Show answers (try them yourself first)
- 1Small membrane-bound vesicles containing enzymes.
- 2Peroxisomes and glyoxysomes.
- 3Eukaryotic cells.
- 480S.
- 5Prokaryote: a bacterium; eukaryote: Amoeba or a plant cell.
- 6They convert stored fat to carbohydrate in germinating seeds.
- 7Prokaryotes have no nucleus, 70S ribosomes and no membrane-bound organelles; eukaryotes have a nucleus, 80S ribosomes and membrane-bound organelles.
- 8They convert the stored fat into sugars that the seedling uses for growth.