# 1.4 Ultrastructure of Animal and Plant Cells This section details the fine structure of cells, known as ultrastructure, as revealed by electron microscopy. It compares the components of animal and plant cells and introduces the technology and units used to study them. **Ultrastructure**: The detailed structure of a biological specimen, such as a cell, that is only visible with an electron microscope. ### Cellular Components Below are diagrams illustrating the ultrastructure of typical animal and plant cells, followed by a comparison of their organelles. ### Comparison of Animal and Plant Cells | Organelle / Structure | Animal Cell | Plant Cell | | ------------------------- | --------------------------------------------- | ---------------------------------------------------------------- | | **Cell Wall** | Absent | Present (made of cellulose); provides rigid support and shape. | | **Plasma Membrane** | Present | Present (located inside the cell wall). | | **Cytoplasm** | Present | Present | | **Nucleus** | Present | Present (often pushed to the side by the vacuole). | | **Mitochondrion** | Present | Present | | **Ribosomes** | Present (e.g., 80S) | Present (e.g., 80S) | | **Endoplasmic Reticulum** | Present (Smooth and Rough) | Present (Smooth and Rough) | | **Golgi Apparatus** | Present | Present | | **Chloroplasts** | Absent | Present (site of photosynthesis). | | **Vacuole** | Small, temporary vacuoles (if present). | Large, permanent central vacuole; stores water, ions, and waste. | | **Centrioles** | Present (involved in cell division). | Absent in higher plants. | | **Lysosomes** | Present | Generally absent; their function is performed by the vacuole. | ### Tools and Units in Ultrastructure Studies **Electron Microscopes**: Powerful microscopes required to view cellular ultrastructure. - **Transmission Electron Microscope (TEM)**: Forms an image by passing a beam of electrons *through* a very thin specimen. It is used to view the internal structure of organelles. - **Scanning Electron Microscope (SEM)**: Forms an image by scanning the surface of a specimen with a focused beam of electrons, detecting *secondary or backscattered electrons*. It produces detailed 3D images of the specimen's surface. **Svedberg Unit (S)**: A non-metric unit used to measure the sedimentation rate of a particle (like a ribosome) during centrifugation. - The rate depends on the particle's *molecular mass, volume, and shape*. - Larger, more compact particles have a higher Svedberg value. For example, eukaryotic ribosomes are 80S, while prokaryotic ribosomes are 70S. ### Possible Questions and Answers **Q:** What is cellular ultrastructure? **A:** It is the fine, detailed structure of a cell and its organelles that can only be observed using an electron microscope. **Q:** What are the three main structural differences between an animal cell and a plant cell? **A:** Plant cells have a cell wall, chloroplasts, and a large central vacuole, all of which are absent in animal cells. Animal cells have centrioles, which are absent in higher plant cells. **Q:** What is the difference between a TEM and an SEM? **A:** A TEM (Transmission Electron Microscope) transmits electrons *through* a specimen to show internal structures, while an SEM (Scanning Electron Microscope) scans the *surface* of a specimen to create a 3D image.