Cell structure

• Cell Theory:

  • Before microscopes, the composition of living organisms was unknown
  • Robert Hooke first viewed cells, including single-celled microorganisms, and termed them "cells"
  • Matthias Schleiden and Theodor Schwann proposed in 1837 that all living organisms are made of cells, known as the cell theory
  • The cell theory: all living organisms are made up of one or more cells, cells are the basic functional unit in living organisms, new cells are produced from pre-existing cells
• Structure of Eukaryotic Cells:

  • Cell surface membrane controls material exchange, described as 'partially permeable'
  • Cell wall provides structural support with cellulose in plants and peptidoglycan in bacterial cells
  • Nucleus contains chromatin, separated from cytoplasm by a double membrane with nuclear pores
  • Mitochondria are the site of aerobic respiration, with a double membrane and inner membrane folded into cristae
  • Chloroplasts, found in green plant parts, are the site of photosynthesis with thylakoids and grana
  • Ribosomes are the site of translation, composed of RNA and protein
  • Endoplasmic reticulum includes rough (RER) and smooth (ER), with RER processing proteins made by ribosomes
  • Golgi apparatus modifies proteins and lipids before packaging them into vesicles for transport
• Vesicles are membrane-bound sacs used for transport and storage
• Lysosomes are specialist forms of vesicles containing hydrolytic enzymes that break down waste materials
• Lysosomes are used extensively by cells of the immune system and in apoptosis (programmed cell death)
• Centrioles are hollow fibers made of microtubules, two centrioles at right angles to each other form a centrosome, which organizes the spindle fibers during cell division
• Centrioles are not found in flowering plants and fungi
• Microtubules make up the cytoskeleton of the cell, about 25 nm in diameter
• Microtubules are made of α and β tubulin combined to form dimers, which are then joined into protofilaments
• Thirteen protofilaments in a cylinder make a microtubule
• The cytoskeleton, including microtubules, provides support and movement for the cell
• Microvilli are cell membrane projections that increase the surface area for absorption
• Cilia are hair-like projections made from microtubules, allowing the movement of substances over the cell surface
• Flagella are similar in structure to cilia, made of longer microtubules, and contract to provide cell movement, for example in sperm cells
• Specialized eukaryotic cells have specific adaptations to help them carry out their functions
• Structural adaptations of specialized eukaryotic cells include the shape of the cell and the organelles it contains
• Examples of specialized eukaryotic cells and their adaptations:

  • Neurones: elongated structure for coordinating information, with dendrites and axons for communication
  • Muscle cells: contain layers of fibers for contraction, high density of mitochondria for energy
  • Sperm cells: motile with a tail for propulsion, head contains nucleus and acrosome with digestive enzymes
  • Root hair cells: have extensions to increase surface area for absorption of water and minerals
  • Xylem vessel cells: form continuous tubes for water transport, with thickened lignin walls for support
  • Phloem vessel cells: transport dissolved sugars and amino acids, made of living cells supported by companion cells
• In eukaryotic cells, cells are joined end-to-end and contain holes in the end cell walls (sieve plates) forming tubes which allow sugars and amino acids to flow easily through (by translocation)
• Eukaryotic cells have very few subcellular structures to aid the flow of materials
• Specialised cells in complex multicellular organisms become adapted for specific functions, like epithelial cells in the small intestine being specialised to absorb food efficiently
• Red blood cells are specialised to transport oxygen
• In multicellular organisms, specialised cells of the same type group together to form tissues, which work together to form organs and then organ systems
• Prokaryotic cells are much smaller than eukaryotic cells (between 100 - 1000 times smaller) and lack membrane-bound organelles
• Prokaryotic cells have a cytoplasm that lacks membrane-bound organelles and their ribosomes are structurally smaller (70 S) compared to eukaryotic cells (80 S)
• Prokaryotic cells have no nucleus, instead, they have a single circular DNA molecule free in the cytoplasm and a cell wall containing murein
• Plasmids in prokaryotic cells are small loops of DNA separate from the main circular DNA molecule, containing genes that can be passed between prokaryotes
• Some prokaryotes are surrounded by a final outer layer known as a capsule, which helps protect them from drying out and immune system attacks
• Flagella in prokaryotic cells are long, tail-like structures that rotate, enabling movement
• Prokaryotic cells are often described as being 'simpler' than eukaryotic cells and are believed to have emerged as the first living organisms on Earth
• Viruses are non-cellular infectious particles that straddle the boundary between 'living' and 'non-living', with relatively simple structures and a parasitic nature