3.2.1 Cell Structure

• Distingusing features of eukaroytic cells are that the cytoplasm contains membrane bound organelles, so DNA is enclosed in a nucleus
• The cell surface membrane is selectively permeable so it enables control of passage of substances in and out of cells. Molecules, receptors and antigens are on the surface so it allows recognition and signalling.
• The nucleus contains:

  • Nuclear envelope - Double membrane and has nuclear pores
  • Nucleoplasm
  • Nucleolus
  • Proteins or histone bounds linear DNA which has chromatin which is condensed and chromosomes which are highly condensed
• The nucleus stores geentic information which codes for polypeptides (proteins). Site of DNA replication. SIte of transcription which produces mRNA. Nucleolus also makes ribosomes.
• Ribosomes are made of ribosomal RNA and protein and it is not a membrane bound organelle
• Ribosomes are the site of protein synthesis
• The Rought Endoplasmic Reticulum (rER) have

  • Ribosomes on the surface synthesis proteins.
  • Proteins are processed, folded and transported inside rER
  • Proteins are also packaged into vesciles for transport e.g, to Golgi apparatus
• The Smooth Endoplasmic Reticulum (sER)

  • Synthesises and processes lipids
  • E.g cholesterol and steroid hormones
• The Golgi apparatus

  • Modifies proteins e.g, adds carbohydrates to produce glycoproteins
  • Modifies lipids e.g, adds carbohydrates to make glycolipids
  • Packages proteins and lipids into golgi vesicles
  • Produces lysosomes - type of golgi vesicle
• Golgi Vesicles

  • Transpports proteins and lipids to their required destination
  • E.g, moves to and fuses with cell surface membranes
• Lysosomes

  • Release hydrolytic enzymes - lysozymes
  • To hydrolyse pathogens or worn out cell components
• Mitochondria

  • Its the site of aerobic respiration
  • Produces ATP for energy release
  • E.g, for protein synthesis, vesicle movement and active transport
• Animal and Plant Cells contain:

  • Cell surface membrane
  • Mitochondrion
  • Nucleus
  • Ribosomes
  • Rough Endoplasmic Reticulum
  • Smooth ER
  • Golgi apparatus
  • Lysosome

  ONLY plant cells contain:

  • Chloroplast - plants and algae
  • Cell Wall - plants, algae and fungi
  • Cell vacuole - plants
• Chloroplasts

  • Double membrane
  • Stroma containing:
  • Thylakoid membrane
  • Small/ 70S ribsomes
  • Circular DNA
  • Starch granules and lipid droplets
  • Lamella - thylakoid linking grana
  • Grana - stacks of thylakoid
• Chloroplasts

  • Absorbs light energy for photosynthesis - produce organic substances e.g, carbohydrtaes and lipids
• Cell wall in plants and algae is made of cellulose (a polysaccharide)

  Cell wall in fungi made of chitin (nitrogen containing polysaccharide)
• Cell walls provides mechanical strength to cell, so it prevents cell changing shape or bursting under pressure due to osmosis
• Cell vacuole maintains turgor pressure in cell

  Also contains cell sap -> stores sugars, amino acids, pigments and any waste chemicals
• In complex multicellular organisms, eukaroytic cells become specialised for specific functions
• Tissues are a group of specialised cells with a similar structure working together to perform a specific function, often with the same antigen.
• Organ is an aggregations of tissues performing specific functions
• Organ system is a group of organs working together to perform specific functions
• Porkaryotes have no cystoplasm, lacking membrane bound organelles. This means that genetic material is not enclosed in a nucleus.
• Prokaryotes always contain:

  • Cell surface membrane
  • Cell wall which contains murein which is a glycoprotein
  • Cytoplasm
  • Small ribosomes
  • Circular DNA which is free in the cytoplasm and not associated with proteins

  Prokaryotes sometimes contain:

  • Capsule
  • Plasmids which are small rings of DNA
  • Flagella
• Eukaryotic cells

  • Have membrane bound organelles such as mitochondria
  • Have no nucleus containing DNA, DNA is free in the cytoplasm
  • Have DNA which is long and linear and associated with histone proteins
  • Have larger ribosomes (80s) in the cytoplasm
  • Have a cell wall only in plants, algae and fungi which contains either chitin or cellulose
  • Don't contain plasmids and capsules
• Prokaryotic cells

  • Have no membrane bound organelles
  • Have a nucleus so DNA is free in the cytoplasm
  • Have DNA which is short and circular and not associated with proteins
  • Have smaller (70s) ribosomes
  • Contain a cell wall made of murein which is a glycoprotein
  • Contain plasmids, flagella and capsule - sometimes present
• Eukaryotic cells

  Are larger overall in size
• Prokaryotic cells

  Are much smaller overall in size
• Viruses are

  • Acellular - not made of cells, no cell membrane, cytoplasm or organelles
  • Non-living - have no metabolism and cannot independently move, respire, replicate or excrete
• Viruses contain:

  1. Nucleic acids which are surrouned by a capsid - a protein coat
  2. Attachment proteins which allow attachment to a specifc host cells
  3. No cytoplasm, ribosomes, cell wall, cell surface membrane + etc
  4. Some also surrounded by a lipid envelope such as HIV