Cells and cell transport

• Cellular organisms can be divided into two main groups

  • Prokaryotes
  • Eukaryotes
• Prokaryotes


  • No nucleus
  • No membrane-bound organelles in cytoplasm
• Structures always present in a prokaryotic cell

  • Cell wall (made of murein)
  • Cell surface membrane
  • Circular DNA molecule
  • Ribosomes (smaller than eukaryotes)
  • Cytoplasm
• Structures that may be present in a prokaryotic cell

  • Capsule surrounding cell wall
  • Plasmids
  • Flagella
• Eukaryotes


  • Presence of nucleus and cytoplasm containing many cell organelles (e.g. mitochondria, endoplasmic reticulum)
• Binary fission


  1. Replication of circular DNA and plasmids

  2. Division of cytoplasm to produce two daughter cells
• The time taken for a bacterial population to double in number is called the generation time
• Cell organelles found in eukaryotic cells

  • Nucleus
  • Ribosomes
  • Rough and smooth endoplasmic reticulum
  • Golgi apparatus
  • Lysosomes
  • Mitochondria
  • Chloroplasts
• Nucleus


  Contains the genetic material, DNA, determining the development, structure and function of the cell
• Ribosomes


  Very small organelles, made up of protein and ribosomal RNA (rRNA), used in protein synthesis
• Endoplasmic reticulum


  • Flattened membrane sacs forming an internal transport system
  • Rough ER has ribosomes that produce secretory proteins
  • Smooth ER lacks ribosomes and is involved in production and transport of lipids
• Golgi apparatus and Golgi vesicles

  • Adds carbohydrates to proteins received from RER to form glycoproteins
  • Packages proteins/glycoproteins into Golgi vesicles for secretion
  • Produces lysosomes
• Lysosomes


  Simple sacs containing hydrolytic enzymes surrounded by a single membrane, formed by the Golgi apparatus
• Mitochondria


  • Involved in aerobic respiration which produces ATP
  • Bounded by two membranes forming an envelope around an inner matrix
  • Contain enzymes for respiration, DNA and ribosomes
• Chloroplasts


  • Only found in photosynthetic plant cells and algae
  • Flattened biconvex discs surrounded by an envelope
  • Contain a membrane system of flattened sacs called thylakoids which form stacks called grana
  • Provide a large surface area for chlorophyll molecules which absorb light for photosynthesis
• Additional structures in eukaryotic plant cells

  • Cell wall
  • Chloroplasts
  • Large vacuole
• Cell wall


  Provides support, strength and shape to the cell, consisting of cellulose microfibrils embedded in a matrix
• Cell differentiation


  The development of cells into specialized types
• Tissue


  Groups of similar cells that perform a specific function and have a common origin
• Organ


  A structure consisting of different tissues, which has a specific function
• System


  Consists of two or more organs
• Epithelial cell from small intestine

  • Cell surface membrane folded into microvilli providing large surface area for absorption
  • Numerous mitochondria providing energy for active uptake of digested food molecules
• Cell fractionation and ultracentrifugation

  1. Cells are broken open by grinding in an isotonic, buffer solution

  2. Centrifugation at different speeds separates organelles of different densities
• Order of organelle isolation

  • Nuclei
  • Chloroplasts (if plant cell)
  • Mitochondria
  • Endoplasmic reticulum
  • Ribosomes
• Light (optical) microscopy


  Uses beam of light, relatively low resolution, specimens can be living
• Electron microscopy


  Uses beam of electrons, much greater resolution, specimens are dead and dehydrated
• Electron microscope


  Uses beam of electrons
• Electron microscope


  • Much greater resolution than light microscope
  • Focused using magnets
  • Much greater detail/smaller structures visible
  • Specimens are dead and dehydrated
  • Image not in colour
  • Preparation of specimen very complex and time-consuming
• Light (optical) microscope


  • Uses beam of light
  • Relatively low resolution
  • Focused using glass lenses
  • Much lower detail/smaller structures not visible
  • Specimens can be living (movement can be observed)
  • Image in colour
  • Preparation of specimen relatively easy and quick
• Magnification


  Ratio of image size to actual size
• Calculating size of cellular structures

  Actual size = Image size / Magnification
• Cell-surface membrane


  • Consists mainly of a phospholipid bilayer and protein
  • Described as the fluid-mosaic model
• Phospholipid bilayer


  • Hydrophilic heads oriented inwards or outwards
  • Hydrophobic tails oriented towards each other
• Permeability of cell membrane

  • Very small molecules can diffuse through gaps between phospholipids
  • Water passes through water protein channels (aquaporins)
  • Lipid soluble molecules can pass quickly
  • Water soluble ions and polar molecules require transport proteins
• Cholesterol


  • Provides strength to the membrane and restricts the movement of phospholipids
• Membrane proteins


  • Act as carriers or channels which aid the passage of water soluble ions and polar molecules across the membrane
  • Carrier proteins have a specific tertiary structure complementary to polar molecules
  • Channel proteins have a specific tertiary structure, charge and size that determines which molecules can be transported
• Other membrane proteins


  • Important in 'cell signalling' and act as specific receptors for hormones
  • Branched carbohydrate (sugar) chains stick out from the outer surface and can act as receptors or be involved in cell-cell recognition
• Diffusion


  • Net movement of molecules from a high concentration to a low concentration until equally distributed
  • A passive process that does not require energy
• Facilitated diffusion


  • Allows transport of polar molecules down a concentration gradient through channel and carrier proteins
  • A passive process that does not require energy
• Active transport


  Movement of molecules or ions against a concentration gradient using carrier proteins and energy from ATP hydrolysis