1.2.8 Skills: Drawing Cells

Drawing the ultrastructure of cells

• To record the observations seen under the microscope (or from photomicrographs taken) a labelled biological drawing is often made
• Biological drawings are line pictures that show specific features that have been observed when the specimen was viewed
• There are a number of rules/conventions that are followed when making a biological drawing

Drawing conventions

• The drawing must have a title
• The magnification under which the observations shown by the drawing are made must be recorded
• A sharp HB pencil should be used (and a good eraser!)
• Drawings should be on plain white paper
• Lines should be clear, single lines (no thick shading)
• No shading
• The drawing should take up as much of the space on the page as possible
• Well-defined structures should be drawn
• The drawing should be made with proper proportions
• Label lines should not cross or have arrowheads and should connect directly to the part of the drawing being labelled
• Label lines should be kept to one side of the drawing (in parallel to the top of the page) and drawn with a ruler
• Drawings of cells are typically made when visualizing cells at a higher magnification power, whereas plan drawings are typically made of tissues viewed under lower magnifications (individual cells are never drawn in a plan diagram)

• Due to the size of prokaryotes (0.1 to 5 µm) their ultrastructure can only be seen using an electron microscope
• Therefore drawings of prokaryotes are based on electron micrographs
• When viewing an electron micrograph of a prokaryote there is no distinct dark circular area within the cell, as there is no nucleus and no organelles are visible (apart from ribosomes, but as they are 70 S in size these are difficult to distinguish)

Biological drawings should show only visible structures, and should be labelled using the correct labelling conventions

• When viewing a eukaryotic cell under a light microscope it is possible to identify the nucleus and if it is a plant cell the cell wall and vacuole
• However, under an electron microscope, more detail of the ultrastructure of the eukaryotic cell can be seen
• The following organelles should be able to be identified, although it does depend on whether it is a plant or animal cell and the specialisation of the cell:
  • Rough endoplasmic reticulum
  • Golgi apparatus
  • Lysosomes
  • Vesicles
  • Ribosomes
  • Vacuole (plant)
  • Nucleus
  • Mitochondrion
  • Chloroplast

• The nucleus, mitochondrion and chloroplast all have double membranes
• The cell wall will be present in plant eukaryotic cells. This is an extra-cellular component

Cell structures under an electron microscope

• Electron microscopes can produce highly detailed images of animal and plant cells
• The key cellular structures within animal and plant cells are visible within the electron micrographs above

• The nucleus should be clearly identifiable as it is the largest structure in the eukaryotic cell

Electron micrograph of the nucleus

• To identify the mitochondrion look for the crista (the foldings of the inner membrane) which are often visible in electron micrographs

Electron micrograph of the mitochondrion

• The rough endoplasmic reticulum (rER) is located next to the nucleus and the attached ribosomes can be used to identify the rER as they make the membrane appear darker

Electron micrograph of the rough endoplasmic reticulum

• The chloroplast can be identified by the thylakoid stacks (grana), as they appear as dark lines within the organelle
• Chloroplasts are large

Electron micrograph of the chloroplast

• Golgi apparatus will be located near the endoplasmic reticulum and it:
  • Does not have long membrane sacs
  • The sacs are more curved than the endoplasmic reticulum
  • Does not have ribosomes attached
  • Has many vesicles close by

Electron micrograph of the Golgi apparatus

• Vesicles are spherical shapes

Electron micrograph of the vesicles

• Free ribosomes appear as dark granules (tiny dark dots) in the cytoplasm

Electron micrograph of the ribosomes

Plant cell electron micrographs

• Electron micrographs of plant cells, such as palisade mesophyll cells, may show:
  • The chloroplasts along the plasma membrane, as this is where the most light can be absorbed
  • A large vacuole in the centre
  • A cell wall

Electron micrograph of a plant cell

Animal cell electron micrographs

• An exocrine gland cell of the pancreas may show:
  • Many large secretory vesicles (carrying the digestive enzymes)
  • Many mitochondria
  • Rough endoplasmic reticulum

Electron micrograph of an exocrine gland cell of the pancreas