Cells

• If you can say what a structure is, what its shape or position causes, and what the cell therefore can do, you can answer almost every question in Topic 1.
• Three returns beat one long session. Redraw, rebuild, then test yourself cold.

• I can identify the cell membrane, cytoplasm, ribosomes, nucleus and mitochondria on a diagram, a photomicrograph or an electron micrograph of an animal cell.
• I can identify, in addition, the cellulose cell wall, chloroplasts and the permanent sap vacuole in a plant cell.
• I can describe a bacterial cell as having a cell membrane, cytoplasm, ribosomes, a cell wall that is not cellulose, circular DNA free in the cytoplasm and sometimes plasmids — and no nucleus, no mitochondria and no chloroplasts.
• I can state a correct function for every one of those structures in a single accurate sentence.
• I can explain why a plant cell needs both a cell membrane and a cell wall, and say which one controls entry.
• I can prepare a temporary stained slide of plant cells and, under safe school practice, of cheek cells.
• I can make a biological drawing that follows the marking standard: large, single clear lines, no shading, ruled horizontal labels, a title, and magnification or scale when asked.
• I can use magnification = image size ÷ actual size in all three rearrangements, after converting both lengths to the same unit.
• I can convert confidently between millimetres and micrometres and explain why magnification has no unit.
• I can explain what specialisation means and give the features of a root hair cell, a palisade mesophyll cell, a red blood cell, a ciliated cell, a sperm cell and a neurone.
• I can link each feature to the function it makes possible, rather than listing features alone.
• I can define tissue, organ and organ system precisely enough to separate them from each other.
• I can place organelle, cell, tissue, organ, organ system and organism in the correct order and give a complete worked example from one organism.

Why this chapter carries so much weight. Every later topic in Biology 5090 rests on it. Diffusion and osmosis happen across the cell membrane. Photosynthesis happens in chloroplasts. Respiration happens in mitochondria. Enzymes work in the cytoplasm. Transport, gas exchange and excretion are all organ-system stories that begin with a specialised cell. If Topic 1 is secure, the rest of the course has somewhere to attach itself.

Magnification

The number of times larger an image is than the object it represents, calculated as image size divided by actual size. Because it compares two lengths measured in the same unit, the units cancel and magnification has no unit: it is written as a number of times, such as times four hundred.

Cell

The smallest structural and functional unit of a living organism. A cell is enclosed by a cell membrane that controls the movement of substances into and out of it, contains cytoplasm in which many enzyme-controlled reactions take place, and carries genetic information as DNA which controls the cell's activities and the production of proteins.

Biological drawing

A record of what is actually observed through a microscope, drawn large with clear single continuous lines, without shading or colour, in realistic proportions and showing only structures that can genuinely be seen. Labels are written horizontally on straight ruled lines that do not cross, and the drawing carries a title, with magnification or scale information when it is requested.

Bacterial cell

A cell that has a cell membrane, cytoplasm, ribosomes and a cell wall which is not made of cellulose. Its genetic material is a circular molecule of DNA lying free in the cytoplasm, sometimes accompanied by small extra circles of DNA called plasmids. A bacterial cell has no nucleus, no mitochondria and no chloroplasts.

Plasmid

A small circular molecule of DNA found in the cytoplasm of some bacterial cells, separate from the main circular DNA. Plasmids carry additional genes and are not always present.

Tissue

A group of similar cells working together to perform a particular function. Defining a tissue simply as many cells is not enough: the cells must be similar to one another and must work together.

Chloroplast

A green organelle found in the photosynthetic cells of a plant. It contains the pigment chlorophyll, which absorbs light energy, and it is the site of photosynthesis. Chloroplasts are absent from plant cells that receive no light, such as root hair cells.

Cellulose cell wall

A rigid layer made of cellulose that lies outside the cell membrane of a plant cell. It supports the cell, resists bursting when water enters by osmosis, and helps the cell keep its shape. It is fully permeable, so it does not control which substances enter or leave; that is the job of the cell membrane inside it.

Cell membrane

The thin partially permeable boundary present in every cell, animal, plant and bacterial. It controls the movement of substances into and out of the cell. In a plant or bacterial cell it lies immediately inside the cell wall, which supports the cell but does not control what passes through it.

Cell specialisation

The development of a cell so that it has features suited to one particular function. A specialised cell keeps the structures common to all cells but adds, enlarges or loses others, so that it performs one job far better than an unspecialised cell could.

Temporary slide

A microscope slide prepared for immediate viewing, in which the specimen is mounted in a drop of liquid under a coverslip rather than being permanently fixed. A stain such as iodine solution or methylene blue is usually added so that structures which are otherwise transparent become visible.

Organ

A structure made of several different tissues working together to perform a particular function. The heart is an organ because it contains cardiac muscle tissue, nervous tissue, blood vessels and epithelium, all serving the single function of pumping blood.

• “\(48 \div 120 = 0.4\), so the magnification is 0.4.” Fix The two lengths were in different units. Convert first: \(48\ \mathrm{mm} = 48\,000\ \mu\mathrm{m}\), then \(48\,000 \div 120 = 400\).
• “The magnification is \(400\ \mu\mathrm{m}\).” Fix A magnification is a ratio of two lengths in the same unit, so it has no unit. Write \(\times 400\).
• “I multiplied by the magnification to get the actual size.” Fix The image is the enlarged one, so the actual size must come out smaller. Divide the image size by the magnification, and check that your answer is smaller than what you started with.
• “The cell wall controls what enters the cell.” Repair The cell membrane controls exchange; the wall supports. The wall is fully permeable, so it cannot select anything.
• “\(48 \div 120 = 0.4\)” — calculating before converting millimetres and micrometres. Repair Put both lengths in the same unit first: \(48\ \mathrm{mm} = 48\,000\ \mu\mathrm{m}\), then \(48\,000 \div 120 = \times 400\).
• Drawing every organelle you know into a photomicrograph. Repair Draw only what can actually be observed. A light microscope shows no ribosomes at all, and mitochondria only as specks.
• “A tissue is many cells.” Repair A tissue is a group of similar cells working together to perform a function. Both halves are needed.
• “Bacteria have no DNA because they have no nucleus.” Repair They have DNA. It is a circular molecule lying free in the cytoplasm, and there may also be plasmids. What is absent is the nucleus.
• “All plant cells have chloroplasts.” Repair Only photosynthetic cells do. A root hair cell has none, because no light reaches it. An onion epidermal cell has none either.
• “Mitochondria make energy for the cell.” Repair Mitochondria are the site of aerobic respiration, which releases energy from nutrient molecules. Energy is never made.
• “The magnification is \(400\ \mu\mathrm{m}\).” Repair Magnification is a ratio of two lengths in the same unit, so the units cancel and it has none. Write \(\times 400\).
• Listing features when the question said explain. Repair Use the FLE chain: feature, then what it physically causes, then what the cell can therefore do. “Many mitochondria” is a feature; “so energy is released for active transport” is the mark.
• Writing two separate lists when the question said compare. Repair Put both organisms in the same sentence: “A plant cell has a nucleus, whereas a bacterial cell does not.”

• Reveal buttons are a study tool, not decoration. Every hidden answer in this chapter is a chance to retrieve before you read. Say or write your answer first, then press the button. Reading the answer straight away feels faster and teaches you far less.
• The single most transferable habit in this topic: when a question gives you a structure, answer with a job; when it gives you a job, answer with a structure. Most Topic 1 marks are a one-to-one mapping, and the mapping runs in both directions.
• Read the “Plant” column carefully. Two of its entries are conditional, not absolute. Chloroplasts occur in photosynthetic cells, and the entry for the animal column is no large permanent vacuole rather than a flat “no vacuole”. Precision in these two cells of the table is worth real marks.
• Never share swabs. A swab that has been in one person's mouth must not go anywhere near another's. Follow your school's stated procedure for using and disposing of swabs, and treat used swabs and slides as your teacher directs. Any answer that describes sharing equipment between students is unsafe as well as unmarked.
• A quick sanity check that costs five seconds. Cells are of the order of micrometres and images on a page are of the order of millimetres, so a magnification should come out in the hundreds or thousands. If your magnification is 0.4, you divided before converting. If an “actual size” comes out larger than the image, you multiplied when you should have divided.
• Two cells, one idea. Whenever a cell has to absorb or exchange something, look for a shape that raises surface area: the long thin projection of a root hair cell, and the biconcave dish of a red blood cell. If you can spot which problem a shape is solving, you can usually work out the function of a specialised cell you have never seen before.

• Cells is examined in more than one style, and the style changes what a good answer looks like. The descriptions below are about question form, not about how often a topic appears: no one can promise you what will come up.
• Recognition under time pressure. You are shown a labelled diagram, a photomicrograph or a short comparison and asked which structure, which function or which cell type. Precision of the boundary cases decides these — wall versus membrane, DNA versus nucleus.
• Written answers: state a function, describe a bacterial cell, explain why a root hair cell contains many mitochondria, complete a comparison table, or calculate a magnification and show the working. Command words matter here.
• Slide preparation, use of the microscope, and biological drawing with a title and a magnification or scale. Drawing marks are awarded against a standard, so they are among the most reliably earned marks in the whole course.
• The single most transferable habit in this topic: when a question gives you a structure, answer with a job; when it gives you a job, answer with a structure. Most Topic 1 marks are a one-to-one mapping, and the mapping runs in both directions.