topic 2: cells

• cells need cell surface membranes to

  • control the movements of substances in and out of cells
  • forms the barrier between the cell cytoplasm and the environment
  • allows different conditions to be established inside and outside of the cell
• cell membranes are made of phospholipids, proteins, cholesterol, glycolipid and glycoproteins
• in water, phospholipid form a bilyaer with hydrophilic polar heads (water loving) on the surface and hydrophobic chains (water fearing) inside
• you cant see membrane proteins in the light microscope because they dont have a lipid bilayer
• functions of the phospholipid in the mebrane:

  allow lipid souble substance to enter and leave the cell

  prevent water soluble substances entering and leaving the cell "makes the membrane and self sealing
• the bilyaer of phospholipid form the basic structure of the membranes but there are several other components

  extrinisic (peripheral) proteins- occurs in the surface of the bilayer and never extend completely across it

  functions:

  give mechanical support to the membrane

  in conjugation with glycolipids

  cell receptors for molecules such as hormones
• intrinsic(intergral) proteins- completely embedded in the membrane, no lipid bilayer
• some are protein channels- which form water filled tubes that allow ions to pass through
• some are carrier proteins- that binds to ions or molecules like glucose and amino acids, then changes shape in order to move these molecules across the membrane
• intergral proteins have hydrophobiv areas inside the membranes and hydrophilic parts which sticks out into the cytoplasm
• the functions of the proteins in the membrance are to

  • provide structural support
  • act as a channels transporting water soluble molecules across the membrane
  • allows active transport across the membrane through carrier proteins
  • forms cells surface receptors for identifying cells
  • helps cells adhere together
  • act as a receptors for example hormones
• the cell surface membrane is referred to as the fluid mosaic model- as the proteins which are embedded in the bilayer vary in shape, size, pattern, like the tiles of a mosaic
• cholesterol- a type of lipid that is found in the cell membrane

  at high temperature it will stabilies the membrane and raise the melting point

  at low temperature, it intercalates between the phospholipids and prevents clustering( packing too tightly)
• cholesterol molecules are very hydrophobic so prevents the loss a water and dissolved ions from the cell. they pull the fatty acid tails of the phospholipid together, limiting their movement but not making the membrane too rigid
• functions of cholesterol: reduces cateral movements movement of other molecules including phospholipids

  makes the membrane less fluid at high temps

  prevents leakage of water and dissolved ions from the cells
• glycolipids are made of a carbohydrate covalently attached to a lipid. The carbohydrate portion extends from the bilayer into the environment outside the cell, where it acts as a cell surface receptor for specific chemicals, eg ABO blood system in humans
• glycolipids functions: functions to immobolise the outer surface of the membrane, reudcing fluidity

  it makes the membrane less permeable to very small water soluble molecules that would otherwise freely cross

  it functions to seperate phospholipid tails and so prevents crystallisation of the membrane

  it helps secure peripheral proteins by forming high density lipid rafts capable of anchoring the protein.
• glycoproteins: proteins on the cell membrane's outer surface are often modifified by carbohydrates, forming glycoproteins. modification is thought to have a part in the cell's recognition by other cells especially for hormones and neurotransmittter
• glycoproteins functions:

  acts as a recognition sites

  helps cells to attach to one attach to one another and so forms tissues

  allows cells to recognise one another e.g. lymphocytes can recognise an organism's own cell
• organelles such as mitochondria and chloroplasts are surrouded by two plasma membranes. These membranes also have important functions in ensuring the organelles are functioning properly:

  1. control the entry and exit of material in discrete organelles
  2. seperate organelles from cytoplasm, ensring metabolic reaction can take place within them.
  3. provide an internal transport system eg endoplasmic reticulum
  4. isolate enzymes that might damage the cell eg. lysosomes
  5. provides surfaces on which reactions can occur eg protein synthesis and ribosomes on endoplasmic reticulum.
• the extracellular matrix is a mesh of protein fibres and glycoproteins that exist externally of the cell in multicellular organisms
• the ECM typically provides structural and biochemical support to surrounding cells including:

  providing sites for anchorage by cells within a tissue and segregating seperate tissures from one another

  sequestering and storing growth factors until receipt of a chemical signal
• in plant cells the extracellular matric includes cell wall components (like cellulose) and hence plays an important roles in:

  regulating water uptake ( maintenance of cell turgor)

  providing mechanical strength and rigidity to the cell (maintains cell shapes)
• cellular membranes are semi permeable ( only certain substances can pass through )

  and they are selective( membrane proteins may regulate the passage of material that cannot freely cross)
• Movement of materials across a biological membrane may occur actively or passively
• Passive transport involves the movement of material along or down a concentration gradient (high concentration ⇒ low concentration)
• Passive transport does not require the expenditure of energy (ATP hydrolysis)
• Three main types of passive transport:

  • Simple diffusion: movement of small or lipophilic molecules (e.g. O2, CO2, etc.)
  • Osmosis: movement of water molecules dependent on solute concentrations
  • Facilitated diffusion: movement of large or charged molecules via membrane proteins (e.g. ions, sucrose, etc.)
• Active transport involves the movement of materials against a concentration gradient (low concentration ⇒ high concentration)
• Active transport requires the expenditure of energy (e.g. ATP hydrolysis)
• Two main types of active transport:

  • Primary (direct) active transport: involves the direct use of metabolic energy (e.g. ATP hydrolysis) to mediate transport
  • Co-transport or Secondary (indirect) active transport: involves coupling the molecule with another moving along an electrochemical gradient
• Permeability:

  • Some substances are not soluble in lipids and cannot pass through the lipid layer
  • Some substances are too large to pass through protein channels
  • Some substances have the same charge as the protein channels and are repelled
  • Some substances are electrically charged and cannot pass through the non-polar hydrophobic tails in the phospholipid bilayer
• Simple Diffusion:

  • Kinetic energy in chemistry explains the constant motion of particles
  • Kinetic energy is the reason why substances move from one place to another, distributing themselves evenly
  • Examples of substances that diffuse across membranes are oxygen and carbon dioxide
• Factors affecting diffusion:

  • Temperature: increased temperature = increased kinetic energy = increased diffusion
  • Concentration gradient: increased molecules = increased concentration gradient = increased diffusion
  • Surface area: increased surface area = increased diffusion
  • Distance/thickness: increased thickness = decreased diffusion
  • Size of molecules: generally, the smaller the molecule, the faster the diffusion
• Facilitated diffusion:

  • A passive process that does not require ATP, only the kinetic energy of the diffusing molecules
  • Requires special protein molecules to ensure molecules can cross the cell surface membrane
  • Involves two types of proteins: protein channels and carrier proteins
• Protein channels:

  • Form water-filled channels or pores through the membrane
  • Allow water-soluble ions to pass through but are specific to certain ions
  • Have a much faster rate of transport than carrier proteins
• Carrier proteins:

  • Span the plasma membrane and change shape when a specific chemical binds with it
  • Change in shape deposits the chemical on the other side of the membrane with no need for additional energy
  • Have a much slower rate of transport than channel proteins
• Carrier proteins may move molecules against concentration gradients in the presence of ATP (used in active transport)
• Osmosis:

  • Net movement of water molecules across a semi-permeable membrane from low solute concentration to high solute concentration until equilibrium is reached
  • Water potential is the pressure created by water molecules and is important in biology
• Osmolarity:

  • Measure of solute concentration
  • Solutions categorized as hypertonic, hypotonic, or isotonic based on relative osmolarity