Cell Structure & Microscopy Revision

A comprehensive set of question-and-answer flashcards covering cell theory, universal cell structures, microscopy techniques and calculations, developments in light and electron microscopy, staining methods, detailed prokaryotic and eukaryotic cell structures (including atypical examples), functions of life, and key size & resolution concepts.

What are the three main statements of modern cell theory?

1) All living organisms are composed of one or more cells. 2) The cell is the smallest unit of life. 3) All cells arise from pre-existing cells by division.

Which three structures are found in every known cell?

A plasma (cell) membrane made of lipids, cytoplasm that is mostly water, and DNA as genetic material.

Why is the plasma membrane essential for life?

Its lipid bilayer forms a selectively permeable barrier that maintains a distinct internal environment and controls exchange with the surroundings.

What is the function of cytoplasm in all cells?

It provides an aqueous medium for metabolic reactions and suspends cellular components.

Define magnification of a microscope image.

Magnification = objective lens power × eyepiece lens power; it is how many times larger the image appears compared to the specimen.

What tool is used to calibrate an eyepiece graticule?

A stage micrometer – a slide with a known scale (usually 0.01 mm per division).

How do you calculate actual size from an image?

Actual size = measured image size ÷ magnification.

State the formula for magnification using a scale bar.

Magnification = image length of scale bar ÷ actual length indicated by the scale bar.

What limits the resolving power of a light microscope?

The wavelength of visible light (≈400–700 nm); objects closer than about 200 nm cannot be distinguished.

Why can electron microscopes resolve smaller structures than light microscopes?

They use electron beams with much shorter wavelengths than light, giving a resolution up to about 2 nm.

Give two advantages of light microscopes over electron microscopes.

1) Can view living, naturally coloured specimens. 2) Equipment is cheaper and sample preparation is simpler.

Give two advantages of electron microscopes over light microscopes.

1) Much higher resolution and magnification (up to ~1,000,000×). 2) Ability to visualise ultrastructure such as ribosomes and membranes.

What is the key difference between TEM and SEM imaging?

TEM transmits electrons through thin sections to show internal structures; SEM reflects electrons off surfaces to produce 3-D surface images.

Why are electron-microscope images usually black and white?

Electrons have no colour; contrast is produced by heavy-metal stains, and colours are added artificially later if desired.

What is cryo-electron microscopy used for?

Imaging rapidly frozen biological molecules or cells to determine high-resolution 3-D structures, especially of proteins.

Explain freeze-fracture microscopy.

Cells are frozen and cracked along membrane interiors; replicas are coated with metal and imaged by EM to reveal membrane surfaces and embedded proteins.

Which dye is commonly used to stain nuclei in light microscopy and why?

Methylene blue – it is positively charged and binds to negatively charged nucleic acids (DNA/RNA).

How does immunofluorescence locate specific proteins?

Antibodies tagged with fluorescent molecules bind to target proteins, which then emit coloured light when excited by specific wavelengths.

List the five mandatory components of a typical prokaryotic (Gram-positive) cell.

1) Cell wall (peptidoglycan), 2) Plasma membrane, 3) Cytoplasm, 4) Naked circular DNA in a nucleoid, 5) 70S ribosomes.

How do prokaryotic ribosomes differ from eukaryotic ribosomes?

Prokaryotes have smaller 70S ribosomes; eukaryotes have larger 80S ribosomes.

What is the composition and role of the prokaryotic cell wall?

Made of peptidoglycan; it maintains shape, protects the cell, and prevents osmotic lysis.

State four structures common to all eukaryotic cells.

Plasma membrane, nucleus with DNA bound to histones, 80S ribosomes, and membrane-bound organelles (e.g., mitochondria, ER, Golgi).

Describe the structure and function of the nucleus.

Double membrane with nuclear pores; contains chromatin and nucleolus; stores genetic information and produces rRNA for ribosome assembly.

What is the role of free cytoplasmic ribosomes?

They synthesise proteins that function within the cytosol of the cell.

Why does the rough ER appear ‘rough’?

Because ribosomes are bound to its cytoplasmic surface, giving a studded appearance.

Give two functions of the smooth ER.

Synthesis of lipids and phospholipids; storage of Ca²⁺ ions (in muscle) or detoxification of drugs (in liver).

What is the main function of the Golgi apparatus?

Modification, sorting, and packaging of proteins and lipids for secretion or delivery to other organelles.

Explain how lysosomes are formed and what they do.

Formed from Golgi vesicles; contain hydrolytic enzymes that digest worn-out organelles, food particles, or pathogens.

State two features unique to plant cells compared with animal cells.

Presence of a cellulose cell wall and chloroplasts; also typically a large central vacuole.

What are centrioles and where are they found?

Pairs of 9×3 microtubule bundles located near the nucleus in animal cells; organise spindle microtubules during cell division.

Describe the 9+2 arrangement.

Eukaryotic cilia and flagella contain a ring of nine double microtubules surrounding two central single microtubules.

How do fungal cells differ from plant cells?

Fungi have cell walls made of chitin, lack chloroplasts, and often have large vacuoles but no plastids.

List the seven functions of life exhibited by unicellular organisms.

Metabolism, Response to stimuli, Homeostasis, Growth, Reproduction, Nutrition, Excretion (often remembered as MR H GREN).

Give an example of a unicellular organism and one life function it performs.

Paramecium – its contractile vacuoles expel water, demonstrating homeostasis.

Why are skeletal muscle fibres considered ‘atypical’ cells?

They are very long (up to 30 mm) and multinucleated, challenging the idea of small, single-nucleus cells.

What makes red blood cells atypical among eukaryotic cells?

Mature mammalian RBCs lack a nucleus and most organelles to maximise space for haemoglobin.

Explain why aseptate fungal hyphae challenge traditional cell theory.

They consist of long tubes of cytoplasm with many nuclei and no internal cross-walls, so they are not compartmentalised into separate cells.

What are phloem sieve tube elements missing and how is their function sustained?

They lack a nucleus and many organelles; companion cells maintain their metabolism and control.

Compare average sizes of prokaryotic and eukaryotic cells.

Prokaryotes: ~0.2–3 µm; Eukaryotes: ~10–100 µm.

Which unit would you convert 250 mm into when calculating magnification with µm-sized specimens?

Millimetres must be converted to micrometres (1 mm = 1000 µm) to match specimen units.

What does a higher magnification without increased resolution result in?

A larger but not sharper image; details closer than the resolution limit remain blurred or fused.

Why must specimens for TEM be extremely thin?

Electrons must pass through the specimen; thick sections scatter electrons excessively and reduce image clarity.

What is the purpose of a scale bar on a micrograph?

It provides a visual reference for actual distances, allowing calculation of magnification or specimen size directly from the image.

How are fluorescent dyes excited in a fluorescence microscope?

A first filter lets through only light of a specific (short) wavelength that excites the dye; emitted longer-wavelength light is then collected through a second filter.

Name two specialised light-microscopy stains and the structure each highlights.

Methylene blue – nuclei; Iodine – starch granules in plant cells.

What property of peptidoglycan makes Gram-positive bacteria stain purple?

Their thick peptidoglycan layer retains the crystal violet–iodine complex during Gram staining.

State one variation found among prokaryotes that students are NOT required to know in detail.

Some prokaryotes (e.g., phytoplasmas, mycoplasmas) lack a cell wall.

Which organelle carries out aerobic respiration and what structure inside it increases surface area?

Mitochondrion; the inner membrane folds called cristae.

How does the central vacuole help maintain turgor in plant cells?

By accumulating water and solutes, it generates internal pressure that pushes the plasma membrane against the cell wall, keeping the cell rigid.

What is the cytoskeleton and name two of its components.

A network of protein fibres that supports cell shape and movement; components include microtubules (tubulin) and microfilaments (actin).

During cell division, what moves chromosomes apart in eukaryotic cells?

Spindle microtubules originating from centrosomes (centrioles in animal cells).

Why can living cells be observed with light microscopy but not with TEM?

TEM requires vacuum conditions and heavy-metal staining that kill or remove water from the specimen.

Define 70S and 80S in the context of ribosomes.

They refer to sedimentation coefficients (Svedberg units) indicating size and density: 70S (smaller) in prokaryotes, 80S (larger) in eukaryotes.

What feature allows you to identify a chloroplast in an electron micrograph?

Double membrane enclosing stacks of thylakoid discs (grana) and often an internal starch grain.

How can you distinguish rough ER from smooth ER in EM images?

Rough ER has ribosomes attached to its surface and flattened cisternae; smooth ER lacks ribosomes and appears more tubular.

Which two micrograph clues indicate a cell is a prokaryote?

Absence of membrane-bound nucleus and presence of a cell wall with a diffuse nucleoid region.

Give one reason why lipid-rich areas appear light in TEM images.

Lipids are less electron-dense than protein; they scatter fewer electrons, producing lighter regions.

What size particle can enter the human respiratory tract and why is this relevant to pathogen spread?

Particles ≤10 µm can be inhaled deeply; respiratory droplets in this range can carry viruses such as SARS-CoV-2.

Convert 0.2 mm to micrometres.

0.2 mm × 1000 µm/mm = 200 µm.

If an image of a bacterium measures 40 mm and the magnification is ×10 000, what is the actual length?

Actual size = 40 mm ÷ 10 000 = 0.004 mm = 4 µm.

What does the abbreviation ‘TEM ×50 000’ mean on a micrograph?

The image is magnified fifty-thousand times relative to the specimen's actual size, as captured with a transmission electron microscope.

Why do plant cells lack centrioles but still form spindles?

Plant cells organise spindle microtubules from other microtubule-organising centres without distinct centriole structures.

Explain the role of companion cells in phloem.

They provide metabolic support and ATP to sieve tube elements, maintaining translocation of sugars.

What measurement tool is embedded in a microscope eyepiece?

Eyepiece graticule – a transparent scale for measuring specimens once calibrated.

To enhance contrast and visibility of specific cellular structures that are otherwise transparent or difficult to discern under a microscope.

What is the general function of staining in microscopy?

Hair-like appendages on the surface of some prokaryotes; primarily involved in attachment to surfaces or other cells, and conjugation.

What are pili (fimbriae) in prokaryotic cells and what is their function?

Contain enzymes that produce and break down hydrogen peroxide (H2O2); involved in various metabolic processes like fatty acid breakdown and detoxification.

What is the primary function of peroxisomes?

As a cell grows, its volume increases faster than its surface area, reducing the surface area-

How does the surface area-to-volume ratio affect cell size?