AQA biology AS topic

Transport across cell membranes

Structure of the cell-surface membrane

• All membranes around and within cells have same basic structure = plasma membranes 
• Cell-surface membrane = name given to plasma membrane that surrounds cells and forms boundary between cell cytoplasm and environment 
• Allows different conditions to be established inside and outside cell 
• Controls movement of substances in and out of cell 

Phospholipids

• Important components of cell-surface membrane
• Hydrophilic head = points to the outside of the membrane attracted by water on both sides
• Hydrophobic head = points into the centre of the cell membrane, repelled by water on both sides
• Lipid-soluble material moves through the membrane through the phospholipid. 
• Function of the phospholipids in the membrane are:
• Allow lipid-soluble substances to enter and leave the cell
• Prevent water-soluble substances entering and leaving the cell
• Make the membrane flexible and self-sealing

Protein

• Embedded into the phospholipid bilayer in two ways;
• Some occur in surface and never extend across it. They act to give either mechanical support to membrane or with glycolipids as cell receptors for molecules such as hormones
• Some span from one side to the other. 
• Some are protein channels which from water-filled tubes to allow water-soluble ions to diffuse across the membrane. 
• Others are carrier proteins that bind to ions or molecules (e.g glucose and amino acids) and change shape in order to move them across the membrane
• Functions of proteins in the membrane:
• Provide structural support
• Act as channels transporting water-soluble substances across the membrane
• Allow active transport across the membrane through carrier proteins
• Form cell-surface receptors from identifying cells
• Help cells adhere together
• Act as receptors E.g for hormones

Cholesterol

• Occur within phospholipid bilayer for cell surface membrane
• They add strength to the membranes. 
• They are very hydrophobic and play important role in preventing loss of water and dissolved ions from cell. 
• Pull together fatty acid tails of phospholipid molecules limiting their movement and other molecules without making membrane too rigid
• Functions of cholesterol:
• Reduce lateral movement of other molecules including phospholipids
• Make the membrane less fluid at high temperatures
• Prevent leakage of water and dissolved ions from the cell

Glycolipids

• Made up of carbs covalently bonded with lipids. 
• Carb portions extends from bilayer into outside watery environment where acts as cell-surface receptor for specific chemicals
• Functions of glycolipids:
• Act as recognition sites
• Help maintain the stability of the membrane
• Help cells…