Cell structure

• Features of prokaryotic cells:

  • DNA is circular and free in the cytoplasm, not associated with proteins
  • No membrane-bound organelles
  • Small, 70s ribosomes
  • Cell wall containing murein
  • Smaller than eukaryotic cells
  • Plasmids
  • Flagella
• Basic features of eukaryotic cells:

  • DNA is linear and surrounded by a membrane-bound nucleus
  • DNA is associated with proteins called histones
  • Have membrane-bound organelles
  • Larger, 80s ribosomes
• What is the ultrastructure of a cell?

  The internal structure of a cell that suits it to perform a particular function.
• Describe the function of the cell wall(plants):

  Cell wall: Made of cellulose, has a middle lamella (boundary between adjacent cell walls)

  -> Maintains the shape of the cell, making it rigid

  -> Fully permeable to all molecules so stops cell bursting after osmosis
• Describe the function of chloroplasts and how they are adapted(plants):

  Contain photosynthetic pigments for absorption of light energy and a stroma(cytoplasm).

  -> have a double membrane

  -> have thylakoids containing chlorophyll which are stacked to form granum- increases surface area for light energy absorption
• Similarities between chloroplast and mitochondria structure:

  • Short circular DNA
  • 70s ribosomes
• Describe the function of the vacuole and how it is adapted(plants):

  Contains cell sap- sugars, amino acids, wastes and sometimes pigments.

  -> helps the plant to contain water entry into the cell making it turgid

  -> stored sugars and amino acids act as a temporary food store
• Structure and function of the nucleus:

  S- has a double membrane, nuclear pores and a dark staining region called the nucleolus

  F- nucleus stores cell's genetic material and controls the production of proteins, nucleolus makes ribosomal RNA
• Structure and function of mitochondria:

  S- highly folded inner membrane called the cristae, matrix(cytoplasm)

  F- the site of aerobic respiration so generates ATP for e.g. active transport, inner membrane provides high surface area for enzymes involved in respiration
• Function of Golgi apparatus:

  • Receives proteins from the RER and lipids from the SER
  • Modifies and processes proteins e.g. adding a carbohydrate to make a glycoprotein
  • Packages proteins and lipids into vesicles (membrane-bound sacs) for export from the cell
• Structure and function of rough endoplasmic reticulum:

  S- has 80s ribosomes attached

  F- acts as a channelling system in the cell in transporting materials, site of protein synthesis
• Structure and function of lysosomes:

  S- vesicles produced by Golgi apparatus

  F- contain lysozymes which hydrolyse cell walls of bacteria, contain proteases and lipases to hydrolyse worn-out organelles
• What are ribosomes?

  The sites for protein synthesis.
• Structure and function of the smooth endoplasmic reticulum:

  S- no ribosomes attached

  F- synthesises lipids and steroids and transports them
• How do bacterial cells divide?

  By binary fission.
• Function of murein cell wall in bacterial cells:

  Protects against osmotic lysis and mechanical damage.
• Function of slime capsule in bacterial cells:

  1. Allows bacteria to stick to surfaces.
  2. Prevents desiccation (drying out).
• Function of circular DNA in bacterial cells:

  Contains genetic material needed for replication of bacterial cells.
• Function of plasmids in bacterial cells:

  Contain genes that code for enzymes allowing bacteria to break down antibiotics.
• Function of cell surface membrane in bacterial cells:

  Selectively permeable barrier that controls the entry and exit of substances from the cell.
• Function of flagellum:


  To help the prokaryotic cell move more quickly.
• How to calculate number of bacteria after 'n' divisions:


  2^n where n is the number of cell divisions.
• 3 key features of viruses:

  1. Acellular: not made up of cells or contain a cell surface membrane.
  2. Non-living: no metabolism, cannot respire, excrete or replicate independently.
  3. Very small.
• 4 main structural features of viruses:

  1. Attachment proteins- allow a virus to bind to receptors on cells.
  2. Genetic material (DNA/RNA)- codes for the synthesis of viral proteins.
  3. Capsid protein coat- protects genetic material.
  4. Lipid envelope- derived from host cell.
  5. Matrix- structural proteins which link lipid envelope to virus core.
• Specific feature of HIV virus:

  The enzyme reverse transcriptase- this converts RNA to DNA.