Intro to cells, cell structure

• Cells are the fundamental unit of life - all life is composed of one or more cells
• Cells themselves are composed of common building blocks such as proteins, lipids, nucleic acids and carbohydrates
• Some organisms are made up of just a single cell, while larger organisms are made up of large numbers of cells that are organised into tissues and organs and can take on specialised functions
• All life is made up of cells
• Cells are the target for most medicines, with viruses being the exception as they need cells to replicate
• Cells

  • Many different types of specialised cell
  • In single-celled organisms, cell must carry out all functions necessary for life – finding food, producing energy, reproducing, repairing damage etc.
  • In multi-cellular organisms cells can take on specialist roles e.g. defence, sensing the environment, information processing, digestion, generation of heat, gas exchange etc.
  • Cells are extremely flexible and adaptable to allow all these different functions to take place
• Eukaryotic cell

  (animal, plants, fungi, protists)
• Prokaryotic cell

  (Bacteria, Archaea)
• Eukaryotic cells have a nucleus and other membrane-bound compartments
• Cell size

  • Prokaryote cell ~ 1 μm³
  • Eukaryote cell ~1000 μm³
  • 1000x larger volume for even the smallest eukaryotes
  • Diffusion of molecules takes longer, chemical reactions occur more slowly (eukaryotes)
  • The solution is to organize the cell into smaller compartments – the organelles
• Cell structure

  • Plasma membrane
  • Nucleus
  • Ribosomes & ER
  • Golgi
  • Lysosomes, endosomes & autophagosomes
  • Mitochondria
  • Cytoskeleton
  • Vacuoles, chloroplasts & cell walls
• Plasma membrane

  • Surrounds contents of cell and defines its outer boundary
  • Acts as a barrier to external environment but also has many other roles
  • Controls transport into and out of cell, controls diffusion
  • Passes signals from environment into cell
  • Connects to other cells
  • Concentrates enzyme activity
• Plasma membrane

  • Composed of lipids and proteins
  • Lipid molecules are much smaller than proteins so approximately 50 lipid molecules per protein in a plasma membrane
  • Membrane proteins vary in structure and function
• Mitochondria

  • Large organelles, easily visible using light microscopes and first identified in the 19th century
  • Mainly responsible for producing energy for the cell
  • Can be thousands in energy-demanding cells or just two or three in low energy cells
  • Usually depicted in textbooks as cylinders around 0.5 to 1 μm in diameter and up to 7 μm long, remarkably plastic and can change shape and move around cell (flexible)
  • They can even fuse with one another and then move apart again
• Mitochondria structure

  • Two membranes – inner one highly folded to increase surface area
  • Inner membrane contains the electron transport chain and ATP synthase molecules that make ATP (the energy currency for cells) during respiration
  • Mitochondria also contain their own DNA
  • In the centre is the matrix – the site of many biochemical reactions
• Nucleus

  • Home to DNA, control centre of the cell
  • DNA is organised into chromosomes, although most of the time, chromosomes adopt an open structure called chromatin
  • Chromatin is DNA bound to proteins that make RNA, repair DNA, copy DNA etc.
  • Nuclear pores allow signals to enter nucleus and mRNA to leave nucleus
• Chromosomes and chromatin

  • DNA is tightly packaged by histone proteins and bundled into chromosomes
  • Chromosomes are only visible during cell division
  • Most of the time DNA is unwound, allowing it to be read and transcribed
  • Not all regions of DNA are able to be read – some are silenced by epigenetic modifications which mean those genes are silenced in cell
• Nucleolus

  • Region within nucleus that is the site of ribosome biogenesis
  • Some cells can have a single, large nucleolus while other cells can have several, smaller nucleoli
  • Highly dynamic and can change over time
• Protein synthesis

  1. Find the gene that codes for protein in the nucleus

  2. Copy the gene into mRNA (transcription)

  3. Send the mRNA out of nucleus

  4. mRNA binds to ribosomes

  5. Ribosomes "read" sequence of mRNA and link together the appropriate individual amino acids to make protein (translation)
• Ribosomes

  • Generally located in endoplasmic reticulum (ER) – which is another type of membrane, similar to plasma membrane but internal
  • There are two types of ER – rough and smooth
  • Rough ER contains ribosomes and is site of protein synthesis. Folds & processes proteins that's made from ribosomes
  • Smooth ER is the site of a variety of metabolic processes such as steroid metabolism. Synthesises and processes lipids
• Golgi apparatus

  • Made up of membranes
  • Proteins are made by ribosomes but often need to function in other parts of the cell or in other parts of the body
  • The Golgi packages, processes proteins (lipids too) into membrane-bound parcels (called vesicles) which are then transported to other parts of cell or for secretion
  • Makes vesicles that stores lipids and proteins for transportation out CM via diffusion
• Secretion of proteins

  Secreted proteins are synthesised in the rough ER (signal peptide) and packaged by the Golgi body
• Transporting things into the cell

  1. Solutes

  2. Pseudopodium

  3. Food or other particle

  4. Food vacuole

  5. Plasma membrane

  6. Vesicle

  7. Receptor

  8. Ligand

  9. Coat proteins

  10. Coated pit

  11. Coated vesicle

  12. Phagocytosis

  13. Pinocytosis

  14. Receptor-Mediated Endocytosis

  15. Endosomes
• Endosomes and lysosomes

  • Endosomes: membrane enclosed vesicles, are created when plasma membrane pulls large particles into cell
  • Endosomes are transported into cell where they fuse with lysosomes
  • Lysosomes contain a variety of powerful degradative/ hydrolytic enzymes that breakdown proteins and other biological molecules into their constituent parts (amino acids etc)
  • These can then be re-used
  • Lysosomes are therefore part of the cell's recycling system
• Cytoskeleton

  • Series of fibres that runs throughout the cell
  • Provides mechanical strength and support for cells
  • Allows cells to change shape and move
  • Provides the cell's transport system to move organelles and vesicles around
  • Separates chromosomes and splits cells in half during cell division- cytokinesis
• Components of cytoskeleton

  • Microtubules
  • Actin filaments
  • Intermediate filaments
• Vacuoles

  • In plants can take up a large fraction of cell (typically 30% but can be up to 80% of cell volume)
  • Functions as storage for water and nutrients but also helps maintain pressure on cell wall
• Cell walls

  • Provide more support and strength than plasma membrane
  • Helps prevent cell bursting when water enters cell- cytolysis
• Chloroplasts

  • Mitochondria generate energy in form of ATP in both plant and animal cells
  • Mitochondria convert carbohydrate into ATP. Animals get this carbohydrate from eating plants. Plants make their carbohydrate in chloroplasts from light and carbon dioxide (photosynthesis)
  • Like mitochondria, chloroplasts have a double membrane and their own DNA
• It is thought that both mitochondria and chloroplasts were originally free-living prokaryote cells that were phagocytosed by a cell and kept rather than broken down for parts. This symbiosis between the two cells eventually evolved into organelles we see today. Known as endosymbiotic theory
• Cell structure - summary

  • Cells have incredibly diverse roles, shapes and sizes
  • The diversity of organelles and sub-cellular features allows cells to take on their vast range of functions
  • Cells have: Transport networks (cytoskeleton), Factories to make proteins (ribosomes), Power plants (mitochondria and chloroplasts), Recycling plants (lysosomes), Control centres (nucleus), and a barrier to keep it all contained (plasma membrane)