cells

• Prokaryotes


  Smaller and simpler like bacteria
• Eukaryotes


  • Multicellular organisms that include all animals, plants, algae, and fungi
  • More complex than prokaryotic cells

  
• Organelles


  Parts of cells that have a specific function
• Cell wall


  • A rigid structure that surrounds cells in plants, algae, and fungi
  • Made of cellulose in plants and algae, made of chitin in fungi
  • Supports cells and stops them from changing shape
• Cell-surface membrane


  • The membrane found on the surface of animal cells and under the cell wall in plants, algae, and fungi
  • Regulates the movement of substances in and out of a cell
  • Has receptors which allow it to respond to chemicals eg hormones
• Nucleus


  • A large organelle surrounded by a double membrane called a nuclear envelope which has openings called nuclear pores
  • Contains chromosomes which are made from linear DNA bound to proteins
  • Contains one or more nucleoli
  • Controls the cell's activities by controlling the transcription of DNA
  • The nuclear pores allow RNA to move out of the nucleus and into the cytoplasm
  • The nucleolus is the site of ribosome production
• Mitochondrion


  • An oval shaped organelle with a double membrane
  • Inner membrane is folded to form cristae
  • Space inside is called the matrix
  • Mitochondria are the site of aerobic respiration
• Chloroplast


  • Only found in plant and algal cells
  • Surrounded by a double membrane and contains structures called thylakoid membranes
  • A stack of thylakoid membranes is called a granum, and grana can be joined by thin membranes called lamellae
  • The fluid around the grana and lamellae is called the stroma
  • Chloroplasts are the site of photosynthesis

  
• Golgi apparatus


  • An organelle made of a stack of flattened sacs (called cisternae)
  • Processes and packages lipids and proteins by adding carbohydrate groups to them
  • Produces secretory enzymes
  • Forms lysosomes
  • Surrounded by golgi vesicles that transport and store lipids

  
• Lysosome


  • A Golgi vesicle that contains digestive enzymes (lysozymes) eg proteases and lipases
  • Can be used to break down material ingested by the cell or break down worn-out cell components
  • Also can release enzymes to the outside of the cell (exocytosis)
• Rough Endoplasmic Reticulum (RER)


  • A system of sheet-like membranes that enclose a fluid filled space and covered with ribosomes
  • The site of protein synthesis by ribosomes
  • Provides a pathway for protein transport
• Smooth Endoplasmic Reticulum (SER)

  • Similar to the RER but doesn't have ribosomes
  • Site of lipid and carbohydrate synthesis
• Ribosome


  • A very small organelle that either are free in the cytoplasm or bound to the RER
  • Made of two parts: larger and smaller subunits
  • Come in two sizes: 80S (25nm in diameter - found in eukaryotic cells) and 70S (slightly smaller - found in prokaryotic cells)
  • Synthesise proteins from mRNA and amino acids
• Vacuole


  • A large fluid-filled sac surrounded by a membrane called a tonoplast
  • Found in the cytoplasm of plant cells
  • Contains cell sap (a solution made of sugar and salts)
  • Helps to maintain pressure inside the cell and stop plants wilting
  • Also can be used to isolate unwanted chemicals in the cell
• Cell specialisation and organisation

  • In multicellular organisms, cells become specialised to carry out specific functions
  • Different types of cells have different structures to help them carry out their functions
  • Cells are organised into tissues, tissues into organs, and organs into organ systems
  • Specialised cells of the same function group together into tissues
  • Tissues of the same function form organs
  • Organs group with other organs to form organ systems eg the circulatory system
• Cell wall (prokaryotic)


  • Rigid structure that provides support to the cell
  • Made of murein (polymer of polysaccharides and polypeptides)

  
• Capsule (prokaryotic)


  • Some bacteria have a capsule made of slime
  • Helps protect bacteria from attack and allows groups of bacteria to stick together
• Cell-surface membrane (prokaryotic)


  Phospholipid bilayer similar to a eukaryotic cell

  
• Circular DNA (prokaryotic)


  Prokaryotes don't have nuclei or proteins associated with DNA - just a loop of circular DNA free in the cytoplasm
• Plasmids (prokaryotic)


  • Small extra loops of DNA that carry genes that can help with survival eg antibiotic resistance
  • Not always present in prokaryotes and some prokaryotes can have several
  • Plasmids can be transferred between prokaryotes
• Flagellum (prokaryotic)


  • A long hair-like structure found in some species (some prokaryotes can have more than one)
  • Used to make the cell move
• Cytoplasm (prokaryotic)


  • Has no membrane-bound organelles
  • Contains free 70S ribosomes (smaller than ribosomes in eukaryotic cells)
• Virus


  • Non-living and acellular (not cells)
  • Made up of nucleic acids (DNA or RNA) surrounded by a protein coat called a capsid
  • Attachment proteins are found on the surface of the virus to allow it to attach to and enter a host cell
• Mitosis


  • Eukaryotes replicate via mitosis
  • Produces two identical 'daughter' cells for growth or repair of tissue

  
• Meiosis


  Produces four genetically different daughter cells for reproduction
• Cell cycle


  • Consists of interphase and mitosis
  • Interphase is a period of growth and DNA replication that is divided into three stages: G1, S, and G2
  • Mitosis is a period of cell division that has four stages: prophase, metaphase, anaphase, and telophase and cytokinesis

  
• Binary fission (prokaryotic)


  1. The circular DNA replicates once and plasmids replicate multiple times

  2. The cell gets bigger and the loops of DNA move to opposite poles (ends) of the cells

  3. The cytoplasm divides and a new cell membrane and cell wall begins to form

  4. Two 'daughter' cells are formed, each with one loop of circular DNA but a variable number of plasmids

  
• Viruses and host cell replication

  Viruses are non-living so they cannot replicate by themselves

  They use a host cell to replicate

  The attachment proteins bind to complementary receptors on the cell-surface membrane of the host cell

  They then inject their DNA or RNA into the host cell and it begins producing new viral components which are assembled into new viruses
• Magnification


  Difference in size between the image and the real object

  Magnification = size of image/size of real object
• Resolution


  A measure of how detailed the image is

  Calculated as the minimum distance apart that two points have to be to be able to be distinguished by the microscope

  Increasing the magnification will not increase the resolution
• Light (optical) microscopes


  Use light to form an image

  Maximum resolution of 0.2 micrometers

  Maximum magnification of x1500
• Electron microscopes


  Use electrons to form an image

  Maximum resolution of 0.0002 micrometers

  Maximum magnification of x1,500,000
• Transmission electron microscopes (TEM)


  Uses electromagnets to focus electrons into a beam that passes through a thin section of the specimen and onto a screen behind to form a photomicrograph

  Denser areas of the specimen absorb more electrons and so appear darker
• Scanning electron microscopes (SEM)


  Uses electromagnets to scan the beam of electrons over the surface of the specimen

  Electrons are scattered and with computer analysis, a 3D image can be produced that show the surface of the specimen
• Cell fractionation


  A method of breaking cells up and separating the components

  Step 1: Homogenisation (breaking up the cells)

  Step 2: Filtration (removing complete cells and debris)

  Step 3: Ultracentrifugation (separating the organelles)
• Cell-surface membrane structure


  Made up of a phospholipid bilayer with other molecules embedded in it such as proteins, glycolipids, glycoproteins, and cholesterol

  Has a 'fluid mosaic' structure

  Phospholipid molecules form a continuous bilayer which is called 'fluid' as the phospholipids are constantly moving

  Proteins can either be embedded in one phospholipid layer or span the whole bilayer

  Glycolipids extend out of the phospholipid bilayer into the environment outside of the cell

  Glycoproteins have a carbohydrate group attached to them and act as receptors/recognition sites

  Cholesterol molecules increase the strength of the membrane

  
• Cell-surface membrane permeability


  Partially permeable - lets some things through but not others

  Small, lipid-soluble (non-polar) molecules can pass through the lipid bilayer

  Other molecules can only enter the cell if there is a specific channel or carrier protein for them

  
• Substances can move across cell membranes by diffusion, osmosis, facilitated diffusion, active transport, and vesicle transport
• Glycoproteins


  Proteins embedded in the membrane with a carbohydrate group attached to them

  
• Glycoproteins


  • Act as receptors/recognition sites and help cells bind together to form tissues
  • Allow immune cells to recognise self cells so they don't get attacked