BIO SEM 1 UNIT 2

What is magnification, and how is it calculated?

The number of times larger an image is compared to the actual sample; magnification = image size ÷ actual size of object

What is resolution in microscopy?

The minimum distance apart two points can be while still being distinguished as separate; resolution depends on the wavelength used — shorter wavelength = higher resolution

What is the maximum magnification and resolving power of a light microscope?

Maximum magnification ~x1500; resolving power ~200 nm (limited by the wavelength of visible light, 500-650 nm)

Why are electron microscopes capable of much higher magnification/resolution than light microscopes?

Electrons have a much shorter wavelength than light, giving electron microscopes far greater resolving power and magnification

Why must samples for electron microscopy be viewed in a vacuum?

Electrons are scattered by air, so the electron beam must travel through a vacuum — meaning only dead/non-living specimens can be used

How does a Transmission Electron Microscope (TEM) produce an image, and what type of image is it?

A beam of electrons is transmitted through a thin specimen and projected onto a fluorescent screen; produces a 2D image showing internal structures (light areas = more electrons passed through, dark = fewer)

How does a Scanning Electron Microscope (SEM) produce an image, and what type of image is it?

A beam of electrons scans the specimen's surface, knocking off secondary electrons that are detected; produces a 3D image of the specimen's surface

What stains are used for light microscopy, TEM, and SEM respectively?

Light microscopy: coloured dyes (e.g. methylene blue, eosin); TEM: solutions of heavy metals (absorb/scatter electrons); SEM: a thin coating of gold/heavy metal (protects sample from the electron beam)

What is the purpose of cell fractionation?

To isolate a particular organelle from the rest of the cell for study

Outline the steps of cell fractionation.

1) Cells are placed in an ice-cold isotonic buffer and homogenised to release organelles; 2) the homogenate is filtered through gauze to remove debris; 3) the filtrate is centrifuged at increasing speeds, with each spin pelleting the next most dense organelles, leaving the supernatant to be re-spun

Why is the fractionation solution ice-cold, isotonic, and buffered?

Ice-cold to reduce enzyme activity (slows organelle breakdown); isotonic to prevent organelles gaining/losing water by osmosis; buffered to maintain a stable pH

In cell fractionation, what determines the order in which organelles separate out, and what come out first?

Organelles separate based on density; the densest organelles separate at the lowest centrifuge speeds (nuclei first, then chloroplasts/mitochondria/lysosomes, then ER/membranes, then free ribosomes last at the highest speed)

List the functions of the cell surface (plasma) membrane.

Acts as a barrier controlling which materials enter/leave the cell, involved in cell recognition, and involved in cell signalling

List the functions of membranes within cells (organelle membranes).

Organise the cell into compartments, control which materials enter/leave organelles, form transport vesicles, form internal barriers (e.g. thylakoid membranes), and may be sites of metabolic reactions (e.g. final stage of respiration)

What is the fluid mosaic model?

A model of membrane structure describing a phospholipid bilayer in which the phospholipids are constantly moving (fluid) with proteins and other molecules scattered throughout (mosaic)

Describe the structure of a phospholipid, and what "hydrophobic" and "hydrophilic" mean.

A phospholipid has a hydrophilic (water-attracting) head and a hydrophobic (water-repelling) tail; hydrophobic tails face inward, heads face outward, forming a bilayer impermeable to water-soluble substances

What is cholesterol, where is it found, and what is its function in membranes?

A lipid found in eukaryotic (not most prokaryotic) cell membranes; it increases the packing of phospholipids, making the membrane less fluid/more stable, while also preventing it from becoming too rigid at low temperatures

Describe peripheral (extrinsic) membrane proteins.

Proteins attached to the surface of the membrane; can act as enzymes or receptors

Describe integral (intrinsic) membrane proteins.

Proteins that penetrate part or all of the way through the membrane; can aid transport of substances, act as enzymes, receptors, or play a role in cell adhesion

What are glycoproteins and glycolipids, and what are their functions?

Glycoproteins = proteins with carbohydrate attached; glycolipids = lipids with carbohydrate attached; both can act as receptors or antigens for cell recognition

Why is cell signalling important?

Allows cells in multicellular organisms to communicate to control/regulate body conditions; allows unicellular organisms to detect changes in their environment

How do membrane receptors achieve specificity in cell signalling?

Receptor proteins have specific shapes, so only complementary messenger molecules can bind to them — different receptors bind different molecules

How does temperature affect membrane permeability?

Below 0°C: membrane proteins deform and permeability decreases (may increase later if ice crystals pierce the membrane); 0-45°C: permeability gradually increases as phospholipids move more and the membrane becomes more fluid; above 45°C: the bilayer breaks down and proteins denature, sharply increasing permeability

What are microvilli, and what is their function?

Tiny folds in the cell membrane that increase surface area, allowing greater exchange of substances between the cell and its environment (e.g. absorption in the small intestine)

What is diffusion, and is it active or passive?

The net movement of particles from a region of high concentration to low concentration until evenly distributed; it is passive (does not require energy)

What is simple diffusion, and what types of molecules can cross via this route?

Diffusion of molecules directly across the phospholipid bilayer; only small and/or fat-soluble (non-polar) molecules can cross this way (e.g. oxygen, carbon dioxide, steroid hormones)

List the factors affecting the rate of simple diffusion.

Surface area of membrane (larger = faster), length of diffusion pathway (shorter = faster), concentration gradient (steeper = faster), temperature (higher = faster, more kinetic energy), particle size (smaller = faster)

What is facilitated diffusion, and what two types of protein carry it out?

Passive movement of larger/polar molecules or ions across a membrane via membrane proteins; carried out by carrier proteins (which change shape to ferry molecules across) and channel proteins (which form pores for ions to pass through)

What factors affect the rate of facilitated diffusion?

The concentration gradient across the membrane (as with simple diffusion) and the number of channel/carrier proteins available — more proteins = faster rate

What is osmosis, and what is water potential?

Osmosis = movement of water from a region of high water potential to low water potential across a partially permeable membrane; water potential (Ψ) = the tendency of a solution to lose water, measured in kPa, with pure water having a Ψ of 0 (more solute = more negative Ψ)

What is osmotic pressure, and how does it relate to water potential and tonicity?

The pressure needed to stop net water movement into a solution; a strong/concentrated solution has high osmotic pressure and low (more negative) water potential; isotonic = equal strength, hypertonic = stronger solution (lower Ψ), hypotonic = weaker solution (higher Ψ)

What happens to an animal cell in a hypotonic solution, and why does this differ in a plant cell?

Animal cell: water moves in by osmosis and the cell may burst (cytolysis), since there's no cell wall; plant cell: water moves in too, but the cell wall prevents bursting — the cell becomes turgid as the vacuole/cytoplasm swell against the wall

What happens to an animal cell vs a plant cell in a hypertonic solution?

Animal cell: water moves out by osmosis and the cell shrivels; plant cell: water moves out, the vacuole shrinks, and the protoplast pulls away from the cell wall — the cell becomes plasmolysed

How is the water potential of a cell calculated?

Water potential (Ψ) = solute potential (Ψs, always negative) + pressure potential (Ψp, usually positive); this determines the direction of net water movement into or out of the cell