2.1.1 Cell structure

What are the main types of microscope?

• Light microscope (LM)

• Transmission electron microscope (TEM)

• Scanning electron microscope (SEM)

Compare light, TEM and SEM images

• LM: Colour, lower resolution, shows live cells.

• TEM: 2D, black-and-white, highest resolution, internal ultrastructure.

• SEM: 3D, black-and-white, surface detail, lower resolution than TEM.

How are microscope slides prepared for light microscopy?

• Thin sections or smears of tissue.

• Use of stains to improve contrast.

• Cover slip placed to avoid air bubbles.

What is an eyepiece graticule?

Transparent ruler in the eyepiece with arbitrary units.

What is a stage micrometer used for?

• A slide with a scale of known length (µm).

• Used to calibrate eyepiece graticule.

Unit conversions

• 1 mm = 1000 µm

• 1 µm = 1000 nm

Define magnification.

How many times larger an image is compared to the actual specimen.

Resolution

The ability to distinguish separate points on an image as two separate objects

Compare resolution of LM, TEM, SEM.

• LM: lowest resolution.

• TEM: highest resolution (internal detail).

• SEM: intermediate (surface detail).

Nucleus – structure & function?

• Double membrane with pores.

• Contains DNA stored as chromatin

• Controls cell activities.

Nucleolus – function?

Ribosome synthesis

Rough ER – structure & function?

• Cisternae with ribosomes.

• Protein synthesis & transport.

Smooth ER – function?

Lipid and carbohydrate synthesis.

Golgi apparatus – function?

• Modifies proteins (e.g. adds carbohydrates).

• Packages proteins into vesicles for secretion.

Ribosomes – structure & function?

• Small organelles of rRNA + protein.

• Site of protein synthesis.

Mitochondria – structure & function?

• Double membrane, cristae, matrix.

• Site of aerobic respiration → ATP production.

Lysosomes – function?

• Vesicles containing digestive enzymes.

• Breakdown of pathogens/old organelles.

Chloroplasts – structure & function?

• Double membrane, thylakoids, grana, stroma.

• Photosynthesis (light-dependent & -independent)

Plasma membrane – function?

• Partially permeable barrier.

• Controls entry/exit of substances.

Centrioles – function?

Spindle formation in cell division.

Cell wall (plants, fungi, prokaryotes)?

• Plants: cellulose

• Fungi: chitin

• Prokaryotes: peptidoglycan

Flagella & cilia – function?

Movement of cells or fluid.

How are TEM and SEM images interpreted?

• TEM: internal detail (2D).

• SEM: surface structures (3D).

Outline the steps in protein secretion.

• Ribosomes on RER → synthesise protein.

• RER → transports proteins in vesicles to Golgi.

• Golgi → modifies & packages into vesicles.

• Vesicles fuse with plasma membrane → exocytosis.

Functions of cytoskeleton?

• Provides mechanical strength.

• Maintains shape.

• Aids intracellular transport (e.g. vesicles).

• Enables whole-cell movement (cilia/flagella).

Similarities between prokaryotic and eukaryotic cells?

• Plasma membrane.

• Cytoplasm.

• Ribosomes.

• DNA as genetic material.

Differences between prokaryotic and eukaryotic cells?

• Prokaryotes: no nucleus, no membrane-bound organelles, smaller ribosomes (70S), circular DNA, peptidoglycan cell wall, plasmids, sometimes flagella (different structure).

• Eukaryotes: nucleus, membrane-bound organelles, larger ribosomes (80S), linear DNA with histones, cellulose/chitin cell walls.

Microfilaments

• gives structure and provides movement

• made of actin

Intermediate filaments

• maintains position of organelles in cytoplasm

• ‘guide wires’ to prevent collisions

Microtubules

• transportation of materials inside cell

• largest fibres in cytoskeleton

• made of tubulin

• tubulin dimers form long chains of alpha and beta tubulin