CDS Biology · Formula sheet
Cell Biology formulas
1 formula, 10 reference tables and 23 common traps for CDS Biology Cell Biology, grouped by subtopic.
Kinds of Cells and the Microscope
Learn this subtopic in the notesThe compound microscope and magnification
Magnification
- Size of the imagehow big the specimen looks through the microscope
- Actual size of the objectthe real size of the specimen
Prokaryotic and eukaryotic cells
| Feature | Prokaryotic cell | Eukaryotic cell |
|---|---|---|
| Nucleus | No nuclear membrane; DNA in the nucleoid | True nucleus inside a nuclear membraneQ Asked twice, CDS 2023 (II) and 2026 (I): the region holding a bacterium's DNA is the nucleoid. |
| Mitochondria, Golgi, ER | Absent | Present |
| Ribosomes | Present (70S) | Present (80S)Q CDS 2025 (II): of mitochondria, a nucleus, ribosomes and Golgi bodies, only ribosomes are in a prokaryotic cell. |
| Photosynthetic pigment | On membranous vesicles of the cell membrane | In chloroplastsQ CDS 2019 (II): chlorophyll in photosynthetic bacteria is held on membranous vesicles. |
| Examples | Bacteria, cyanobacteria | Plants, animals, fungi, protists |
Common traps
Image size goes on top
Magnification is image ÷ object, so it is bigger than 1 for an enlarged image. Object ÷ image, or the difference or product of the two sizes, are the wrong options.
The condenser comes before the objective
Light meets the condenser first (below the stage), then the specimen and the objective, then the body tube, and the eyepiece last. An order that puts the eyepiece before the body tube is wrong.
Nucleoid, not nucleolus or nucleosome
The nucleoid is the bacterial DNA region with no membrane. The nucleolus is a body inside a true nucleus, and a nucleosome is DNA wound on proteins in eukaryotes.
Prokaryotes do have ribosomes and a wall
'No organelles' means no membrane-bound ones. Ribosomes, the cell wall, the cell membrane and plasmids are all found in bacteria; the nuclear membrane is not.
Cell Wall, Membrane and Transport
Learn this subtopic in the notesThe cell wall: who has one and what it is made of
| Living thing | Cell wall made of |
|---|---|
| Plants | Cellulose |
| Fungi, such as mushrooms | Chitin |
| Bacteria | PeptidoglycanQ CDS 2026 (I): penicillin spares human cells because it targets the bacterial cell wall. |
| Diatoms | Silica |
| Animals, including humans | No cell wallQ Asked three times: CDS 2019 (II), 2021 (I) and 2023 (II). The organism without a wall is always the animal. |
The cell membrane
| Statement about the cell membrane | True or false |
|---|---|
| It is a phospholipid bilayer with proteins and cholesterol in it | TrueQ CDS 2022 (II): the bilayer is of phospholipid, not of protein or neutral lipid. |
| It is flexible and lets the cell engulf material | TrueQ CDS 2022 (I): this is the correct statement; the other three are false. |
| It is made of cellulose | False: that is the plant cell wall |
| It lets every substance through | False: it is selectively permeable |
| It stops oxygen and carbon dioxide | False: these gases diffuse through it |
Diffusion, osmosis and active transport
| Process | Direction | Energy used | Example |
|---|---|---|---|
| Diffusion | High to low concentration | None | Oxygen entering a cell |
| Osmosis | Water, through a semi-permeable membrane, from dilute to strong solution | None | Root hairs taking in waterQ CDS 2021 (I): the membrane is semi-permeable, not fully permeable and not the cell wall. |
| Facilitated diffusion | High to low, through a protein channel | None | Na⁺ and Cl⁻ through protein channelsQ CDS 2026 (II): this is the one process in the list that is not active transport. |
| Active transport | Low to high concentration | ATP from respiration | Glucose reabsorbed in the kidney |
| Reverse osmosis | Water forced out of a strong solution | Pressure above the osmotic pressure | Desalination of sea waterQ CDS 2017 (II): the pressure applied is larger than the osmotic pressure. |
Common traps
Fungi and bacteria do have walls
A mushroom (a fungus) has a chitin wall and a bacterium a peptidoglycan wall. Only animal cells, human or tapeworm, have none.
Not every wall is cellulose
Cellulose is the plant wall. Fungi use chitin and bacteria peptidoglycan, which is why penicillin harms bacteria and not plants or people.
A bilayer of lipid, not of protein
The two layers are phospholipid. Proteins sit in the bilayer; they do not form it. An option calling it a protein bilayer, or a neutral-lipid bilayer without proteins, is wrong.
Membrane and wall are different layers
The membrane is in every cell and is made of lipid and protein. The cell wall is outside it, only in plants, fungi and bacteria, and is the part made of cellulose (in plants).
Active transport runs on respiration
Active transport moves a substance against the gradient using ATP from respiration. 'Without energy', 'energy from photosynthesis' or 'along the gradient' are the wrong versions.
A protein channel is not active transport
Ions flowing down their gradient through a protein channel is facilitated diffusion and uses no energy. Kidney reabsorption, loading phloem and taking glucose from the gut all pump against the gradient.
Isotonic means no change
In an isotonic solution such as normal saline the water moving in equals the water moving out, so red cells neither swell nor shrink.
Cell Organelles
Learn this subtopic in the notesEndoplasmic reticulum, ribosomes and the Golgi body
| Organelle | Main work |
|---|---|
| Rough ER | Carries ribosomes on its surface; makes and transports proteinsQ CDS 2025 (I): it looks rough because of the ribosomes on its surface. |
| Smooth ER | Makes lipids and steroid hormones; detoxifies drugs in the liverQ CDS 2019 (I): protein synthesis is the one job smooth ER does not do. |
| Ribosome | Joins amino acids into proteins |
| Golgi body | Packs and dispatches products; forms lysosomesQ CDS 2018 (I): lysosomes are formed from the Golgi bodies. |
Lysosomes and microbodies
| Question about lysosomes | Answer |
|---|---|
| What do they store? | Digestive (hydrolytic) enzymesQ CDS 2019 (I): the lysosome is the storehouse of digestive enzymes. |
| What can they digest? | Food, germs, foreign particles and worn-out cell partsQ CDS 2024 (II): the organelle that digests foreign material in animal cells is the lysosome. |
| Which cell has the most? | White blood cells that engulf germsQ CDS 2026 (I): a white blood cell, not a pancreatic, ovarian or muscle cell. |
| Which organelle makes them? | The Golgi body |
| What do microbodies contain? | EnzymesQ CDS 2024 (I): microbodies in plant and animal cells contain enzymes. |
Mitochondria and plastids: energy and their own DNA
| Question | Answer |
|---|---|
| Which organelles have their own DNA and ribosomes? | Mitochondria and plastidsQ Asked twice: CDS 2020 (I) (mitochondrion) and 2021 (I) (mitochondria and plastids). |
| Which molecule is the cell's energy currency? | ATP |
| How many carbon atoms does pyruvate have? | ThreeQ CDS 2025 (I): pyruvate is a three-carbon molecule. |
| How many membranes surround a mitochondrion or chloroplast? | Two |
| Which plastids store starch? | Leucoplasts |
Matching organelles to their jobs
| Organelle | Job |
|---|---|
| Mitochondria | Respiration and ATP formationQ In all three CDS match items (2017 (II), 2023 (II), 2026 (I)) mitochondria go with respiration or ATP. |
| Chloroplast | Photosynthesis |
| Ribosome | Protein synthesis |
| Lysosome | Intracellular digestion |
| Cytoskeleton | Keeps the shape of the cellQ CDS 2026 (I): cytoskeleton with cell shape, smooth ER with detoxification in liver cells, vacuoles with storage and cell enlargement. |
| Vacuole | Storage of sap and chemicals; enlarges the plant cell |
Common traps
Smooth ER makes fats, not proteins
Proteins need ribosomes, and ribosomes sit on the rough ER. The smooth ER makes lipids and steroids, detoxifies and transports, but never synthesises protein.
Lysosomes come from the Golgi
The digestive enzymes are made on the rough ER, but they are packed and budded off as lysosomes by the Golgi body. 'Formed from the ER' or 'from the nucleus' is the trap.
Suicide bags are lysosomes, not vacuoles
A vacuole stores sap and water. The organelle full of digestive enzymes, which can destroy the cell, is the lysosome. Ribosomes and desmosomes are distractors too.
Phagocytes, not busy secretory cells
A pancreatic cell makes and exports enzymes (lots of ER and Golgi) and a running muscle needs energy (lots of mitochondria). The cell richest in lysosomes is the one that eats: the white blood cell.
Golgi, ER and lysosomes have no DNA
Apart from the nucleus, only mitochondria and plastids (chloroplasts) carry DNA and ribosomes of their own. A vacuole paired with plastids is half wrong, so the option fails.
ATP is the currency, glucose is the fuel
Glucose is broken down to make energy available, but the molecule the cell actually spends is ATP. ADP is what is left after spending it, and pyruvic acid is a step in between.
Ribosome makes; rough ER carries
Options love to swap these two. Ribosomes synthesise protein; the rough ER they sit on transports it. Likewise mitochondria (respiration) and chloroplasts (photosynthesis) are swapped in the wrong codes.
Fix one sure pair first
Do not try to match all four at once. Mitochondria with respiration usually removes two of the four codes immediately; one more sure pair leaves only the answer.
The Nucleus and Cell Division
Learn this subtopic in the notesThe nucleus, and the cells that have none
| Cell or part | Nucleus |
|---|---|
| Mature red blood cell (mammal) | None: it is lost as the cell maturesQ CDS 2016 (II): red blood cells and platelets are the two without a nucleus; root hair cells and monocytes have one. |
| Platelet | None: it is a cell fragment |
| White blood cell, muscle cell | Present |
| Plant cell | Present, in its own double membrane, as each chloroplast isQ CDS 2022 (II): in plant cells the nucleus and the chloroplasts are each surrounded by their own membrane. |
Mitosis, meiosis and cancer
| Feature | Mitosis | Meiosis |
|---|---|---|
| Daughter cells | Two | Four |
| Chromosome number | Same as the parent | Half of the parent (haploid)Q CDS 2026 (II): 'the cells become haploid' is the one statement that is not true of mitosis. |
| Where it happens | Body cells everywhere | Cells that form eggs and sperm |
| Used for | Growth, repair, asexual reproduction | Making gametes |
Common traps
Platelets have no nucleus either
A platelet is a fragment broken off a large bone-marrow cell, so it has no nucleus, just like a mature red blood cell. Monocytes and other white blood cells keep theirs.
RNA is not only in the nucleus
RNA is made in the nucleus but works in the cytoplasm and ribosomes, and mitochondria and chloroplasts have their own. The endoplasmic reticulum membranes are not where RNA lives.
Mitosis never halves the number
Halving to the haploid number is meiosis. Mitosis is equational: each daughter keeps the parent's full set, which is why it can repair tissue.
Cancer spreads by blood and lymph
In metastasis the cells travel through both the blood and the lymphatic system. And cancer can have outside causes, including some viruses, as well as changed genes.