Transport Across Cell Membranes

• All membranes and within all cells have plasma membranes.
• The cell-surface membrane is the name given to the plasma membrane that surrounds cells and forms the boundary between the cell cytoplasm and the enviornment.
• The plasma membrane allows different conditions to be established inside and outside a cell.
• The plasma membrane controls the movement of substances in and out of the cell.
• Phosopholipids form a bilayer or the plasma membrane.
• Phospholipids are important components of the cell-surface membrane because the hydrophillic heads of both phospholipid layers point to the outside of the cell-surface membrane attracted water on both sides.
• Phospholipids are important components of the cell-surface membrane because the hydrophobic tails of both phospholipid layers point into the centre of the cell membrane, repelled by the water on both sides.
• Lipid-soluble material moves through the membrane via the phospholipid portion.
• Function of the phospholipid layer: allow lipid-soluble substances to enter and leave the cell
• Function of the phospholipid layer: prevent water-soluble substances entering and leaving the cell
• Function of the phospholipid layer: makes the membrane flexible and self-sealing
• Proteins are interspersed throughout the cell-surface membrane.
• Some proteins occur in the surface of the phospholipid bilayer and never extend completely across it. They act either to give mechanical support to the membrane or as cell receptors for molecules like hormones, with glycolipids.
• Some proteins completely span the phospholipid bilayer. Some are protein channels, which form water-filled tube to allow water-soluble ions to diffuse across the membrane.
• Some proteins completely span the phospholipid bilayer. Some are carrier proteins that bind to ions or molecules, like glucose and amino acids, then change shape in order to move these molecules across the membrane.
• Functions of protiens in the membrane: provide strucutral support
• Functions of protiens in the membrane: act as channels transporting water-soluble substances across the membrane
• Functions of protiens in the membrane: allow active transport across the membrane through carrier proteins
• Functions of protiens in the membrane: form cell-surface receptors for identifying cells
• Functions of protiens in the membrane: help cells adhere together
• Functions of protiens in the membrane: act as receptors, like for hormones
• Cholestrol molecules occur within the phospholipid bilayer of the cell-surface membrane.
• Cholestrol molecules add stregnth to the membranes.
• Cholestrol molecules are very hydrophobic and therefore play an important role in preventing the loss of water and dissolved ions from the cell.
• Cholestrol pull together the fatty acid tails of the phospholipid molecules, limiting their movement and that of other molecules but without making the membrane to rigid
• Functions of cholestrol: reduce lateral movement of other molecules including phospholipids
• Functions of cholestrol: makes the membrane less fluid at high temperatures
• Functions of cholestrol: prevent leakage of water and dissolved ions from the cell
• Glycolipids are made up of a carbohydrate covalently bonded with a lipid.
• Glycolipids - The carbohydrate portion extends from the phospholipid bilayers into the watery environment outside the cell where it acts as a cell-surface receptor for specific chemicals.
• Functions of glycolipids: cell recognition sites
• Functions of glycolipids: helps maintain the stability of the membrane
• Functions of glycolipids: help cells to attach to one another and so forms tissues.
• Carbohydrate chains are attached to many extrinsic proteins on the outer surface of the cell membrane. - glycoprotein
• Glycoproteins act as cell-surface receptors, more specifically hormones and neurotransmitters.
• Functions of glycoproteins: recognition sites
• Functions of glycoproteins: help cells to attach to one another, so form tissues
• Functions of glycoproteins: allows cells to recognise one another
• Cell membrane:

  A) glycoprotein

  B) protein

  C) glycolipid

  D) protein channel