topic 1 — cell structure and organisation — organelles

• cell surface membrane:

  • controls the movement of substances into and out of a cell
  • helps the cell to recognise other cells
  • encloses the contents of a cell
  • partially/selectively permeable membrane
  • if the cell membrane is damaged by heat/pH/chemicals it becomes fully permeable 
• cytoplasm:

  • found in sol (liquid) or gel (semi-solid) states
  • contains substances such as enzymes, oil droplets, glycogen granules/starch grains
  • the part of the cell where most cellular functions and reactions take place
  • stores a variety of organelles and provides a stable medium for the organelles to function optimally 
• nucleus:

  • the nucleus consists of the nucleolus, nuclear membrane, nuclear pores, chromatin, and nucleoplasm 
  • acts as a control center for cellular activities such as cell growth and repair
  • plays an essential role in cell division and the production of ribosomes
  • the nucleolus synthesises a type of RNA which is part of ribosomes
  • chromosomes are made of deoxyribonucleic acid, the molecule of inheritance 
• components of the nucleus:

  • nucleoplasm contains enzymes necessary for the replication of DNA and the synthesis of RNA and contains chromatin, which condenses to form chromosomes during cell division
  • nuclear envelope consists of 2 membrane which act as a boundary between the nucleus and the rest of the cell and regulates exchange of substances between nucleus and cytoplasm
  • nuclear pores regulate and assist in the transfer of large substances (RNA and ribosomes) across the nuclear membrane 
• mitochondria:

  • releases energy in the form of adenosine triphosphate (ATP) through the oxidation of glucose in a process called aerobic respiration 
  • has an inner membrane which is highly folded, resulting in partitions called cristae to increase the SA:V for enzymes to attach 
  • rod like structure
• golgi apparatus:

  • consists of a series of 3-7 parallel membranes called cisternae, which is a flattened membrane disk with a fluid filled space
  • trans face (leaving) and cis face (receiving)
  • secretory vesicles and lysosomes are formed by budding off from the trans (concave) face
  • collects, modifies, and repackages substances from the ER into secretory vesicles for export out of the cell
  • produces glycoproteins, polysaccharides, and secretory enzymes
• rough endoplasmic reticulum:

  • rough endoplasmic reticulum is an extensive network of membrane continuous with the nuclear membrane. it has ribosomes attached to its surface, making it appear rough
  • rER is involved in the synthesis of extracellular proteins (out of the cell) and provide a pathway of transport for materials in the cell
• smooth endoplasmic reticulum:

  • smooth endoplasmic reticulum is an extensive network of tubule continuous with the rER 
  • sER produces lipids and contains enzymes which help detoxify drugs and poisons and stores calcium ions used in muscle contraction 
• ribosomes:

  • membrane bound ribosomes are attached to the rER and free ribosomes are suspended throughout the cytoplasm
  • attached ribosomes synthesise extracellular proteins to be released out of the cell and enzymes for export out of the cell
  • free ribosomes synthesise intracellular proteins and enzymes for the cell's internal use
• centrioles:

  • consists of 1 pair of cylindrical structures made of 9 groups of microtubules with 3 in each group
  • the pair replicates and moves to opposite poles during cell division
  • the centriole forms a spindle during cell division of animal cells
  • only found in animal cells and required for cell division
  • forms cilia and flagella of cells
• chloroplasts:

  • a tiny disc like structure only found in plant cells which is a type of plastid
  • chlorophyll in chloroplasts trap light energy and convert it into chemical energy during photosynthesis 
  • contains enzymes for light dependent and light independent reactions
  • stores photosynthetic products such as starch grains
  • contain stacks of thylakoid membrane called the granum
• vesicles:

  • vacuoles temporarily store water and food substances
  • lysosomes contain enzymes which break down worn-out organelles, food particles, and engulfed viruses or bacteria
  • transport vesicles move molecules between locations within the cell
• vacuole:

  • in plant cells, there is a large central vacuole which stores sap consisting of anthocyanins (provide various colours to fruit/leaves), water, soluble sugars, amino acids, and organic wastes (in leaf cells)
  • it is enclosed in tonoplasts (which are partially permeable) and supports plants by storing water to provide turgor pressure 
  • in animal cells, there can be many small, temporary vacuoles which temporarily store water, mineral salts, soluble sugars, and amino acids and contain digestive enzymes to destroy bacteria 
• cell wall

  • the outermost part of a plant cell
  • made of cellulose (complex carbohydrate)
  • fully permeable to small molecules
  • protects the plant cell from injury
  • gives the plant cell a fixed shape
  • prevents the cell from bursting when water enters by osmosis, providing a state of turgidity 
  • gies support to the cell and the plant by providing mechanical strength