cell structure

• light microscopes

  • Resolution much lower than electron microscopes

  • Magnification → overall magnification calculated by multiplying the eyepiece lens magnification by the objective lens magnification

  • Use light to create colour images

  • Used to view whole cells and tissues (most organelles not visible)

  • Can be used to view live specimens

  • The eyepiece graticule must be calibrated against a stage micrometer
• Scanning electron microscopes (SEM)

  • Resolution and magnification → lower than TEM but much higher than optical microscopes

  • Use electrons to scan the surface of cells (cannot see internal structures)

  • Create a 3D image in black and white
• Transmission electron microscopes (TEM)

  • Highest resolution

  • Highest magnification

  • Use an electron beam to view internal structure

  of cells and organelles

  • Create a 2D image in black and white

  • Can only view dead specimens

  • Preparation is more complex than for optical

  microscopes

  • Specimens must be very thin
• Laser scanning confocal microscope (LSCM)

  • Lower magnification and resolution than electron microscopes

  • Use a laser beam to view objects at a certain depth within a cell → can view different depths

  • Can use fluorescent tags to stain and view cells → they fluoresce when hit with the laser beam

  • Can use live specimens and see movement → but laser beam could damage specimen

  • Specimens can be thick

  • Creates 2D or 3D images in colour

  • Time consuming to create a stack of images at different depths
• Magnification and resolution

  • Resolution → the ability to separate two objects

  → a high resolution allows the microscope to distinguish between structures that are very close

  • Magnification → how many times larger the image is than the actual object
• • Eyepiece graticule → fixed onto the eyepiece, has divisions with no scale

  • Stage micrometer → placed onto the stage, has a scale
• • To measure something with the eyepiece graticule

  → calibrate the eyepiece graticule with the stage micrometer to find the length of one division on the graticule

  → measure the object with the graticule (take repeat measurements and calculate a mean)

  → convert the number of graticule divisions into a length e.g. in millimetres or micrometres
• Differential staining in light microscopy

  • Allows identification of different cell types and different organelles

  • Allows white cells to be visualised and counted, and gives contrast to images

  • Many different stains available → Sudan red stains cell membranes

  → Nile blue, methylene blue and haematoxylin all stain nuclei (DNA)

  → eosin stains proteins pink (including the cytoskeleton proteins)

  → iodine in potassium iodide solution stains starch blue-black
• Preparing a microscope slide

  • Add a drop of water to a glass microscope slide

  • Put a thin section of your sample onto the water drop using tweezers

  • Pipette a drop of stain at the edge of the sample

  • Carefully lower the cover slip at an angle using a mounted needle

  • Use blotting paper to remove excess stain
• When taking measurements using a microscope, take multiple measurements from different fields of view and calculate a mean.
• Representing cell structure using annotated diagrams

  • Use continuous lines with no shading or cross hatching

  • Add a scale bar and title to the diagram

  • Draw label lines using a ruler

  → do not cross the lines

  → do not add arrowheads to the line

  • Use two lines to represent a cell wall

  • Annotate colours where appropriate
• Nucleus

  • Contains genetic material (DNA)

  • Surrounded by the nuclear envelope (a double membrane)

  • Nuclear pores allow exchange with the cytoplasm

  • Contains one or more nucleoli → make ribosomes

  • Eukaryotic DNA → associated with histone proteins to form chromatin

  → linear and arranged into chromosomes
• Mitochondria

  • Site of aerobic respiration

  • Surrounded by a double membrane → the space between the membranes is the intermembrane space

  • Inner membrane is folded → folds are called cristae

  • Contain circular DNA and 70S ribosomes

  • Inner fluid is called the matrix
• Chloroplasts

  • Site of photosynthesis

  • Surrounded by the chloroplast envelope

  • Thylakoid membranes are stacked into grana

  • Contain circular DNA and 70S ribosomes

  • Contain starch grains

  → stores glucose made in photosynthesis

  • Inner fluid is called the stroma
• mooth endoplasmic reticulum (SER)

  • Synthesises and processes lipids

  • Network of membranes containing fluid
• Plant cell vacuole

  • Surrounded by a membrane called the tonoplast

  • Contains cell sap

  • Helps to keep plant cells rigid
• Lysosomes

  • A type of Golgi vesicle produced by the Golgi apparatus

  • A ball of membrane containing lysozymes
• Ribosomes

  • Consist of ribosomal RNA (rRNA) and protein

  • Made in the nucleolus

  • Site of translation

  • Found in the cytoplasm or on the RER

  • Eukaryotic cells have larger 80S ribosomes,

  prokaryotic cells have smaller 70S ribosomes
• Rough endoplasmic reticulum (RER)

  • Network of membranes containing fluid

  • Membrane joins to the nuclear envelope

  • Surrounded by ribosomes

  • Produces and folds proteins
• Golgi apparatus

  • Network of membranes containing fluid

  • Modifies proteins → e.g. adds carbohydrates to proteins to produce glycoproteins

  • Adds triglycerides to proteins in ileum epithelial cells and packages them into vesicles

  • Packages proteins into secretory vesicles for transport to the plasma membrane and secretion by exocytosis

  • Makes lysosomes (a type of vesicle)
• Centrioles

  • Made of protein microtubules

  • Involved in separation of chromosomes during mitosis

  • Not found in all plant cells
• Cilla and eukaryotic flagella

  • Made of protein microtubules in the 9+2 formation

  • Microtubules can contract to allow movement
• Production and secretion of proteins

  1)Ribosomes associated with the RER synthesise a polypeptide chain in translation.

  2) The polypeptide chain is folded and processed in the RER.

  3) The folded protein is transported to the Golgi apparatus in a transport vesicle.

  4) The protein is modified in the Golgi apparatus and packaged into a secretory vesicle.

  5) The secretory vesicle is transported to the plasma (cell-surface) membrane by moving along the cytoskeleton. This requires ATP.

  6) The secretory vesicle fuses with the plasma membrane and the protein is secreted by exocytosis
• Cytoskeleton

  • A network of protein microtubules throughout the cytoplasm

  • Maintains the shape of the cell

  • Helps transport within cells e.g. moves organelles and vesicles around the cell

  • Enables cell movement e.g. moves cilia and flagella

  • Provides mechanical strength to cells
• Prokaryotic cell structure

  • Simple single-celled organisms

  • Cytoplasm surrounded by three layers → plasma (cell-surface) membrane

  → cell wall made of murein (also called peptidoglycan)

  → capsule

  • Single circular DNA loop → not associated with histone proteins

  → free in the cytoplasm

  • Do not contain membrane-bound organelles

  • Contain one or more plasmids → small loops of DNA

  • Can have one or more flagella for propulsion

  • Pili help the cell to attach to other cells or surfaces