Cell structures

• Eukaryotic cell


  Cells that make up multicellular organisms like animals, plants, and fungi. They have a much more complicated internal structure, containing a membrane-bound nucleus (nucleoplasm) and cytoplasm, which contains many membrane-bound cellular components.
• Eukaryotic cell structure


  • Membrane-bound nucleus (nucleoplasm)
  • Cytoplasm containing membrane-bound cellular components
• Using different types of microscopes you can discover how cells are organised and investigate the ways in which the structures you can see relate to their function
• Microscopy allows you to see what goes on in a healthy cell, and to observe some of the changes which take place if the cell is attacked or diseased
• The increased knowledge of structure has led to a better understanding of cell function
• Prokaryotic cells


  Single-celled organisms with a simple structure of just a single undivided internal area called the cytoplasm (composed of cytosol, which is made up of water, salts and organic molecules)
• Chemical reactions are the fundamental processes of life and in cells they require both enzymes and specific reaction conditions
• Metabolism involves both the synthesis (building up) and the breaking down of molecules
• Different sets of reactions take place in different regions of the ultrastructure of the cell
• Cell cytoplasm


  The region of the cell that is separated from the external environment by a cell-surface membrane
• Organelles


  Membrane-bound compartments in the cytoplasm that provide distinct environments and conditions for different cellular reactions
• Membranes are selectively permeable and control the movement of substances into and out of the cell and organelles
• Membranes are effective barriers in controlling which substances enter and exit cells but they are fragile
• There are a number of organelles that are common to all eukaryotic cells, each with a distinct structure and function
• DNA contains genetic information that controls how cells function.
• Chromosomes are made up of DNA coiled around proteins called histones.
• Eukaryotic cells have a nucleus, mitochondria, endoplasmic reticulum, Golgi apparatus, and other organelles
• Nucleus


  • Contains coded genetic information in the form of DNA molecules
  • DNA directs the synthesis of all proteins required by the cell
  • DNA is contained within a double membrane called a nuclear envelope
• Chromatin


  • DNA associates with proteins called histones to form a complex called chromatin
  • Chromatin coils and condenses to form structures known as chromosomes
• Nucleolus


  • Responsible for producing ribosomes
  • Composed of proteins and RNA
• Mitochondria


  • Site of the final stages of cellular respiration
  • Have a double membrane with the inner membrane highly folded to form cristae
  • Contain a small amount of DNA called mitochondrial (mt)DNA
  • Can produce their own enzymes and reproduce themselves
• Vesicles


  • Membranous sacs with storage and transport roles
  • Consist of a single membrane with fluid inside
• Lysosomes


  • Specialised vesicles that contain hydrolytic enzymes
  • Responsible for breaking down waste material in cells, including old organelles
  • Play an important role in the immune system and programmed cell death
• Cytoskeleton

  • Network of fibres necessary for the shape and stability of a cell
  • Organelles are held in place by the cytoskeleton and it controls cell movement and the movement of organelles within cells
  • Components: microfilaments, microtubules, intermediate fibres
• Cell movement


  1. Actin filaments in the cytoskeleton change length by addition and removal of subunits

  2. Subunits added faster at plus end, leading to filament growth in one direction

  3. This causes cells like phagocytes to move in a particular direction
• Centrioles


  • Component of the cytoskeleton present in most eukaryotic cells
  • Two associated centrioles form the centrosome, involved in spindle fibre assembly during cell division
  • Play a role in positioning flagella and cilia
• Flagella and cilia


  • Extensions that protrude from some cell types
  • Flagella are used for cell motility and as sensory organelles
  • Cilia can be mobile or stationary, with mobile cilia creating currents to move fluids or objects
  • Have a 9+2 arrangement of microtubules
• Endoplasmic reticulum (ER)


  • Network of membranes enclosing flattened sacs called cisternae
  • Smooth ER responsible for lipid and carbohydrate synthesis and storage
  • Rough ER has ribosomes bound to the surface and is responsible for protein synthesis and transport
• Ribosomes


  • Site of protein synthesis
  • Can be free-floating in the cytoplasm or attached to ER (rough ER)
  • Constructed of RNA molecules made in the nucleolus
• Golgi apparatus


  • Compact structure formed of cisternae, without ribosomes
  • Modifies proteins and packages them into vesicles for secretion or lysosome formation
• Protein production


  1. Proteins synthesised on ribosomes of rough ER

  2. Transported in vesicles to Golgi apparatus for modification

  3. Secretory vesicles carry proteins to be released from the cell by exocytosis

  4. Some vesicles form lysosomes containing enzymes for use in the cell
• Plant cells have a cellulose cell wall surrounding the cell-surface membrane, unlike animal cells
• Cellulose cell wall


  • Gives plant cells rigidity and shape
  • Freely permeable to allow substances to pass in and out of the cell
• Ribosomes, endoplasmic reticulum, and Golgi apparatus


  • Work together to synthesise, modify and then transport proteins, including enzymes and hormones, out of the cell
• Plant cells have all of the cellular components seen in animal cells. However, there are some structures that are only seen in plant cells, that carry out photosynthesis
• Cellulose cell wall


  Plant cells, unlike animal cells, are rigid structures. They have a cell wall surrounding the cell-surface membrane
• Plant cell walls


  • Made of cellulose, a complex carbohydrate
  • Freely permeable so substances can pass into and out of the cell
  • Give the cell shape and support the individual cell and the plant as a whole
  • Act as a defence mechanism, protecting the contents of the cell against invading pathogens
• Vacuoles


  • Membrane lined sacs in the cytoplasm containing cell sap
  • Many plant cells have large permanent vacuoles which are very important in the maintenance of turgor, so that the contents of the cell push against the cell wall and maintain a rigid framework for the cell
  • The membrane of a vacuole in a plant cell is called the tonoplast and is selectively permeable
• Chloroplasts


  • Organelles responsible for photosynthesis in plant cells
  • Found in the cells in the green parts of plants such as the leaves and the stems but not in the roots
  • Have a double membrane structure, similar to mitochondria
  • The fluid enclosed in the chloroplast is called the stroma
  • Have an internal network of membranes, which form flattened sacs called thylakoids
  • Several thylakoids stacked together are called a granum, joined by membranes called lamellae
  • The grana contain the chlorophyll pigments, where light-dependent reactions occur during photosynthesis
  • Starch produced by photosynthesis is present as starch grains
  • Contain DNA and ribosomes, so can make their own proteins
  • The internal membranes provide the large surface area needed for the enzymes, proteins and pigment molecules necessary in the process of photosynthesis
• Animals, plants, and fungi are all complex multicellular organisms. The cells making up these organisms are eukaryotic. There is a lot of evidence that suggests that eukaryotic cells evolved from less complex prokaryotic cells