Cells as the basic unit and common structures

• Cells are the basic structural unit of all living organisms.

• Cell theory predicts that a newly discovered organism will consist of one or more cells.

• Typical cells contain DNA as their genetic material.

• Their cytoplasm is composed mainly of water, providing an aqueous internal environment.

• A lipid plasma membrane encloses the cytoplasm and separates the cell from its surroundings.

Developments in microscopy

• Electron microscopy allows cell ultrastructure to be investigated at substantially higher resolution than ordinary light microscopy.

• Freeze fracture exposes internal fracture surfaces of cells and membranes for structural investigation.

• Cryogenic electron microscopy examines rapidly frozen material while reducing structural alteration during preparation.

• Fluorescent stains make selected cell structures easier to locate using light microscopy.

• Immunofluorescence uses fluorescently labelled antibodies to reveal specific cellular molecules or structures.

Eukaryote cell structure

• A plasma membrane encloses a highly compartmentalized cytoplasm containing ribosomes of type $80S$.

• The nucleus contains chromosomes of DNA bound to histones and is surrounded by a double membrane with pores.

• Required membrane-bound organelles include mitochondria, endoplasmic reticulum and the Golgi apparatus.

• Know a variety of vesicles and vacuoles, including lysosomes.

• The cytoskeleton includes microtubules and microfilaments.

Use the image for active recall of typical eukaryotic cell structures. Identify structures corresponding to the nucleus, mitochondria, endoplasmic reticulum, Golgi apparatus, ribosomes and cytoskeleton. Source

Animal, fungal and plant cell differences

Micrograph identification and annotation

• From light and electron micrographs, distinguish prokaryotic, plant and animal cells.

• In electron micrographs recognize the nucleoid, prokaryotic cell wall, nucleus, mitochondrion, chloroplast and sap vacuole.

• Also recognize the Golgi apparatus, rough ER, smooth ER, chromosomes and ribosomes.

• Identify the cell wall, plasma membrane and microvilli where visible.

• Draw and annotate the required organelles and structures directly from electron micrographs.

• Annotations must include the functions of the labelled structures.

Microscopy practical skills

• Be able to prepare temporary mounts of cells and tissues and use appropriate staining.

• Use coarse and fine focus adjustments correctly when observing specimens.

• Measure cell dimensions using an eyepiece graticule and treat instrument measurement as quantitative observation.

• Calculate magnification using $M=\frac{\text{image size}}{\text{actual size}}$.

• Calculate actual size using $\text{actual size}=\frac{\text{image size}}{M}$.

• Be able to produce a scale bar and take photographs of microscopic specimens.

Prokaryote cell structure

• Know the required model as a Gram-positive eubacterium, such as Bacillus or Staphylococcus.

• A prokaryotic cell has a cell wall, plasma membrane and cytoplasm.

• Its genetic material is naked DNA arranged in a loop rather than enclosed in a nucleus.

• Protein synthesis involves ribosomes of type $70S$.

• Prokaryotic structures vary, but detailed variations such as wall-less phytoplasmas and mycoplasmas are not required.

Use the diagram to recognize the overall organization of a prokaryotic cell and locate the cell wall, plasma membrane, cytoplasm, ribosomes and nucleoid/DNA region. Source

Processes of life in unicellular organisms

• A unicellular organism must perform all essential processes of life within one cell.

• These include homeostasis, metabolism, nutrition and excretion.

• They also include movement, growth and response to stimuli.

• The cell must also carry out reproduction.

Atypical eukaryotic cell structure

• Aseptate fungal hyphae contain many nuclei within continuous cytoplasm rather than one nucleus per cell compartment.

• Skeletal muscle fibres are unusually long cells containing many nuclei.

• Mature red blood cells are atypical because they lack a nucleus.

• Phloem sieve tube elements also lack a nucleus at functional maturity.

• These examples show that the number of nuclei cannot always be used as a simple defining feature of a typical eukaryotic cell.