1.1 Essential ideas

1.1.2 Ultrastructure of cells

Prokaryotes are single-celled organisms that have a simple structure without any compartmentalisation. Eukaryotes have a compartmentalised cell structure with membrane-bound organelles. Eukaryotes may be unicellular (protists) or multicellular (animals, plants, fungi).

typical prokaryote cellFigure 1.1.2a – Typical structure of a prokaryote cell 

Skill: Drawing prokaryotic cells from electron micrographs

Practise drawing a diagram of prokaryotic ultrastructure using the two micrographs below. Your drawing should include:

  • cell wall
  • pili
  • flagella
  • plasma membrane
  • cytoplasm
  • 70S ribosomes
  • nucleoid containing naked DNA.

Check your answers by scrolling over the labels. 

Figure 1.1.2b – Ultrastructure micrograph of Escherichia coli, a typical prokaryote: external Figure 1.1.2c – Ultrastructure micrograph of Escherichia coli, a typical prokaryote: internal

Asexual reproduction by binary fission

On Page 1.1.6, you will learn about cell division in eukaryotic cells. Prokaryotes divide through a simpler process called binary fission, which literally means ‘splitting in two’. Here’s how it works:

  1. The single strand of DNA replicates in the parent cell.
  2. The cell elongates.
  3. A septum forms in the middle of the cell which pinches the membrane on opposite sides together. This process is called cytokinesis.
  4. The result is two identical daughter cells.

Figure 1.1.1d - binary fissionFigure 1.1.2d – Binary fission
Binary fission is a form of asexual reproduction which results in one cell splitting into two. The daughter cells are genetic clones.

Skill: Drawing eukaryotic cells from micrographs

Practise drawing diagrams of the ultrastructure of the cells shown in Figure 1.1.2e. Your diagram should include:

  • plasma membrane
  • cell wall (when present)
  • cytoplasm
  • 80S ribosomes
  • nucleus
  • mitochondria
  • other membrane-bound organelles when present (Golgi apparatus, lysosomes, centrioles, vacuole, chloroplasts, endoplasmic reticulum, nucleolus).

Check your answers by scrolling over the labels.

Figure 1.1.2e – Micrographs of eukaryotic ultrastructure: typical plant cell (top), typical animal cell (bottom)

Application: Electron micrographs of specialised cells 

Exocrine glands are cells that are specialised to secrete substances through the plasma membrane via a duct. The pancreas contains exocrine glands, called acinus cells, which release substances necessary for protecting tissues from digestive enzymes.

Exocrine gland cell of the pancreasFigure 1.1.2f – Exocrine gland cell of the pancreas
The secretory vesicles are stained very dark. Also visible is the plasma membrane, nucleus (at the base of the cell), mitochondria, Golgi apparatus, lysosomes and endoplasmic reticulum.

Photosynthesis requires light and water. Cells in the mesophyll layer of a leaf are specialised for photosynthesis. A palisade mesophyll cell lies just under the epidermis at the top of a leaf. It contains a large food vacuole and a high density of chloroplasts (which contain the green photosynthetic pigment, chlorophyll).

A cell specialised for photosythesisFigure 1.1.2g – A cell specialised for photosynthesis
The cell is shaped like a cylinder (we are viewing a transerve section), and is densely packed with large chloroplasts.

Activity

  • Locate and identify the organelles shown in Figures 1.1.2f and 1.1.2g. View the answers by clicking the figures above.
  • Describe the relationship between structure and function in these two examples.

Essential ideas

  • Eukaryotes have a much more complex cell structure than prokaryotes.
  • Specialisation of cells can only occur when there is a division of life functions in multi-cellular organisms.

Language tools

  • The plasma membrane should not be called cell membrane.
  • The cell wall of a prokaryotic cell is often called a capsule.
chlorella

Figure 1.1.2h – Chlorella
Chlorella is a single-celled eukaryote with membrane-bound chloroplasts (green). In this micrograph, Chlorella is undergoing cell division. Learn more about cell division on Page 1.1.6.

Concept help: Organelles

  • Organelles are membrane bound and are present only in eukaryotic cells.
  • You should know the functions of the organelles that you identify in the micrographs.
  • Ribosomes are not considered organelles. Prokaryotic cells have 70S ribosomes, which are smaller than eukaryotic 80S ribosomes. The ‘S’ is a non-SI unit of sedimentation: how fast the ribosome sinks to the bottom of a centrifuge.
  • HL students will learn more about ribosomes on Page 7.1.3.

centrifugeFigure 1.1.2i – Cell components spun in a centrifuge settle at different rates

Concept help: Exocrine glands

Exocrine glands can also be found in the liver, kidneys and breasts of mammals. Where else might you find exocrine glands?

Nature of Science

Technological development: The invention of the microscope allowed for cells to be discovered. The more recent invention of the electron microscope led to greater understanding of cell structure.

TEMFigure 1.1.2j – TEM: Transmission Electron Microscope

All of the micrographs on this page were taken using a transmission electron microscope (TEM). It works by streaming a beam of electrons at a thin specimen. The way the electrons interact with the specimen provides a picture on a viewing screen, which can be observed through a lens, as shown below.

TEM diagramFigure 1.1.2h – TEM diagram

TOK

We are limited in perception by what we can see. Is there any distinction to be drawn between knowledge claims dependent on observations made by our own senses, and those that were made with the assistance of technologies like the microscope (or pH meter, or conductivity probe)? How does calibration of the tool or a trained ability to interpret results change the value of perception?