cell structure & function

• Prokaryotes


  Cells that have no defined shape and do not possess membrane-bound organelles
• Eukaryotes


  Cells that have a defined cell-membrane shape and consist of open-ended chromosomes
• Cells are the "building blocks" of all organisms (living things)
• Cell types


  • Prokaryotes
  • Eukaryotes
• Prokaryotes


  • Found mainly in prokaryotes and cyanobacteria
  • Chromosomes are looped shaped, with circular DNA plasmids
  • Do not have specific membrane-bound organelles
• Eukaryotes


  • Have a defined cell membrane shape
  • Chromosomes are not looped shaped (open-ended)
  • Variety of organelles (cytoplasm, mitochondria, ribosomes)
• Cell Membrane (Plasma Membrane)

  • Very thin and semi-permeable to aid with maintaining concentrations of substances inside and outside cell
  • Membrane layer is called the Phospholipid layer
  • Many different transport and channel proteins are bound inside
  • Round hydrophilic ("water loving") "heads" are glycerol phosphate
  • Long hydrophobic ("water hating") "tails" are fatty acids
  • This hydrophobic/hydrophilic arrangement is called amphipathic and allows the membrane to "fix" and seal itself if it gets damaged
• Passive and active transport across cell membrane

  1. Small molecules (O2, CO2) able to passively diffuse in and out

  2. Larger molecules (starch) requires specific transport/channel proteins to be actively transported across
• Cell Membrane


  • Some membranes contain folds known as microvilli to help increase surface area (found in secretory and absorptive cells such as pancreatic cells and intestine cells)
• Mitochondria


  • Site of aerobic (using oxygen) respiration
  • Where Glucose is broken down into CO2 and H2O, and ATP is produced
  • Inner membrane contains folds called cristae, which provide surface area for the chemical reactions
  • Known as Hydrogen transfer chain
  • Cells with high energy demand will contain more mitochondria (e.g. muscle, sperm, liver cells)
  • Kerbs Cycle occurs here
  • More present in animal cells than in plant cells
  • Contains their own specific DNA (mitochondrial DNA) and protein making machinery
  • Able to reproduce by themselves
  • The cell's "Energy Production Centre" or "Powerhouse"
• Chromosomes


  • At the start of cell division, chromatin condenses to form chromosomes
  • These contain double-stranded material (DNA) coding for characteristics in the organism
  • Found in the nucleus of the cell
  • The cell's "Blue-Prints"
• Cytoplasm


  • Made mainly of a fluid called cytosol, which consists of water and dissolved substances such as sugar, minerals and amino acids
  • Acts as a jelly holding all the other organelles in place
  • It is where most metabolic reactions such as glycolysis take place
  • The cell's "Glue" holding everything together
• Vacuoles


  • Membrane-bound sacs filled with sap or cell sap
  • Very large in plant cells with extra purposes: acts as a liquid skeleton, can store or release fluid that enters the cell by osmosis and allows the cell to swell or crenate (shrink), aids cells in being rigid or turgid
  • Vacuoles also store other substances such as sugars, amino acids, inorganic ions, toxic wastes (lactic acid)
  • The cell's "Storage Centre"
• Cell Nucleus


  • A defined shape in eukaryotic cells
  • The control centre of the cell that contains the genetic material (DNA/chromatin)
  • During cell division chromatin condenses to become chromosomes and doubles up to form chromatids
  • Surrounding the nucleus is a thin layered "nuclear envelope" which allows materials to move in and out from the nucleus to cytoplasm
  • Inside the nucleus is the nucleolus which produces the RNA component of ribosomes
  • The cell's "Brain or Control Centre"
• Endoplasmic Reticulum (ER)


  • Found throughout the cytoplasm with interconnecting tubules, vesicles and cisternae
  • Rough ER has ribosomes attached and is involved in protein synthesis, more common in cells that make and secrete proteins
  • Smooth ER contains no ribosomes and is involved with lipid production, more common in cells that produce steroid hormones (e.g. testosterone, oestrogen, progesterone)
  • The cell's "Specialised Protein factory"
• Golgi Body


  • Looks similar to ER but consists of a stack of membrane sacs (semi-circular black rings) called cisternae
  • Stuff synthesised from the ER passes through the Golgi body and gets carried by transport proteins to the nearest cisterna
  • As the protein passes through each cisterna, they get modified and when it reaches the cell membrane, they get "pinched" off and discharged out of the cell
  • Common in secretory cells that produce hormones and/or enzymes
  • The cell's "Factory Line"
• Ribosome


  • Made in the nucleolus, this is the main site of protein synthesis
  • If attached to ER, they produce proteins for use inside the cell
  • If floating freely in cytoplasm, they make proteins for use outside the cell (i.e. gets secreted)
  • They are usually made of two subunits, a larger and smaller unit
  • The "machinery" in the Protein Factory
• Lysosomes


  • Smaller vacuoles that contain enzymes
  • Partly formed by the Golgi body
  • Variety of functions such as breaking down worn out organelles, breaking down of cells/tissues (during metamorphosis), digest materials that get phagocytised by white blood cells (WBC)
  • The cell's "Recycle Bin"
• Plant Cell Wall


  • Found outside the cell membrane of plant cells
  • Primary cell walls are formed in young plant cells and consist of cellulose bundled together into microfibrils
  • Water and ions are able to move freely through the mesh of microfibrils
  • Secondary cell walls form as the plant matures and contain lignin which help stiffen cells
  • Prevents the plant cell from taking on too much water through osmosis, and exploding, when in a hypotonic environment
• Chloroplast


  • The sites of photosynthesis in the plant
  • Large organelles found in leaf cells or at the green parts of a stem
  • They absorb red and blue wavelengths of light and reflect the green light, which is why we observe them as green pigments
  • Inside the chloroplast is the thylakoid, which are arranged in stacks known as granum
  • These chlorophyll membranes have a large surface area that "catches" the solar energy required for photosynthesis
  • Inside the fluid filled matrix is the stroma, where CO2 and H2 join to form glucose in the light independent phase (Calvin Cycle) of photosynthesis
  • Similar to mitochondria, chloroplasts have their own DNA and ribosomes, and are able to reproduce themselves, indicating some evolutionary origin to the development of plant cells
• Centrioles


  • Present in all animal cells and not in plant cells
  • Made up of microtubules
  • Responsible for forming the spindles during cell division
  • Plant cells have microtubule organizing centres (MTOC) that produce their microtubules during cell division and cytokinesis
• Plant cells have bigger vacuoles than animal cells because plants do not need to store food and water, as they do not possess the ability to move around freely like animals. The larger vacuole acts as a storage for unfavourable conditions.
• Prokaryotes have no defined shape and do not possess membrane-bound organelles, while eukaryotes have a defined cell-membrane shape and consist of open-ended chromosomes.
• Plants have a uniform rigid cell wall, whereas animal cells do not have a cell wall, because plants need a rigid structure to grow up and out, while animal cells can have various flexible shapes due to the cell membrane.