Proteins

• Transport proteins: Most transport of small molecules across the membrane take place through integral proteins. This transport includes facilitated diffusion and active transport.
• Receptor proteins: Receptor proteins must be on the outside surface of cell membranes and have a specific binding site where hormones or other chemicals can bind to form a hormone-receptor complex (like an enzyme substrate complex) This binding then triggers other events in the cell membrane or inside the cell.
• Enzymes: Enzyme proteins catalyse reactions in the cytoplasm or outside the cell, such as maltase in the small intestine. 
• Recognition proteins: some proteins are involved in cell recognition. These are often glycoproteins such as the A and B antigens on red blood cell membranes. 
• Structural proteins: Structural proteins on the inside surface of the cell membranes are attached to the cytoskeleton. They are involved in maintaining the cells shape, or in changing the cells shape for cell motility. Structural proteins on the outside surface can be used in cell adhesion - sticking cells together temporarily or permanently. 

Movement of substances

Factors affecting the rate of diffusion:

• Surface area: increasing the surface area increases the rate of diffusion, 
• Concentration gradient: Increasing the concentration gradient increases the rate of diffusion, 
• Temperature: Increasing the temperature increases the rate of diffusion. 
• Diffusion distance: Increasing the diffusion distance decreases the rate of diffusion. 

Rate of diffusion = Concentration gradient x surface area / diffusion distance 

• Osmosis: Water molecules are small enough to pass between phospholipid molecules, down a water potential gradient. The molecules move from an area of high water potential to an area of low water potential across the membrane. 
• Active Transport: Water-soluble substances that must be moved against a concentration gradient require carrier protein and ATP. 
• Facilitated diffusion: Polar, water soluble substances that cannot pass through the membrane can be transported bus specific channel proteins down a concentration gradient.
• Diffusion: Small lipid soluble substances can pass throughout the lipid bilayer - between the phospholipid molecules - down a concentration gradient. 
• Facilitated diffusion: Large water soluble substances cannot pass between the lipid molecules so are carried through the membranes by carrier proteins down a concentration gradient.

Water Potential

Osmosis is the movement of water molecules down a water potential gradient across a partially permeable membrane. Water molecules will move from an area of high water potential (e.g. pure water), to an area of low water potential (e.g. saline or sugar solutions) Water molecules move from a position of less negative to more negative water potential. 

Fluid mosaic model

The fluid mosaic model for membranes:

Membrane components and their functions: 

• Phospholipid: Hydrophilic head (phosphate), Glycerol, Hydrophobic fatty acid tail, forms a semipermeable phospholipid bilayer. 
• Carrier protein: protein with a specific shaw that complements the shape of the substance to be transported across the membrane, it is used in active transport and facilitated diffusion. 
• Cholesterol: Regulates membrane fluidity.
• Glycoprotein: Cell signalling and recognition and binding cells together.
• Glycolipid: Carbohydrate attached to lipid/phospholipid, cell signalling and cell recognition 
• Channel protein: Protein with a specific shape that complements the shape of the substance to be transported across the membrane, used in facilitated diffusion. 
• Cell signalling: Receptors on cells bind to hormones, drugs and other cells leading to a series of reactions within the cell

Temperature and permeability:

• A high temperature boosts the kinetic energy of the component molecules of the membrane and the transported substance, therefore the membrane becomes more permeable. 
• Very high temperatures will denature the protein molecules, changing their shape and making the membrane more permeable, eventually the membrane will be destroyed. 

Specialised membranes:

• Different cells’ membranes have varying properties, functions and capabilities. These depend on the glycoproteins, glycolipids, channel proteins and carrier proteins that are present. 
• Some membranes are folded to increase surface area for transport or absorption, e.g. microvilli.
• Membranes are fluid, so they can be folded by an organisms cytoskeleton to for vesicles. The active process (meaning it requires ATP) is part of endocytosis.
• Vesicles can fuse with the membrane as part of exocytosis. 

The structure of a cell membrane and phospholipids: 

Fluid mosaic model: Fluid - molecules within the membrane are free to move in relation to each other.

Mosaic - mixture of phospholipids and proteins.

Double layer of phospholipid molecules, phospholipid consists glycerol, to which are joined two fatty acids, and a phosphate, formed by a condensation reaction. The phosphate head is hydrophilic and the fatty acid tail is hydrophobic, meaning in the membrane the phospholipids are arranged as a bilayer, heads on the outside and tails on the inside. 

• Intrinsic proteins pass through the entire bilayer, some of the proteins have channels/pores, and some have binding sites and are carrier proteins. These proteins allow the transport of water soluble molecules.
• Extrinsic proteins are only in one layer, those on the outer side ofter act as receptors for hormones. 

Many of the proteins and phospholipids have carbohydrates attached forming glycolipids and glycoproteins that make up the glycocalyx. 

Movement: 

• Most molecules move across the membrane by diffusion down a concentration gradient, 
• Small molecules (water/gases) and lipid soluble molecules diffuse between the phospholipid, 
• Polar molecules require channel or carrier proteins to move them.
• Channels are water filled pores that can be open at all times or they can be gated. 
• Carrier proteins have a specific binding site for the molecules/ions, This cal me facilitated diffusion, a passive (no ATP required) process,
• Some molecules are actively transported across the membrane (against the concentrate gradient). This requires ATP (released in respiration). The ATP changes the shape of the protein to move the molecule across the membrane. 

Lactose intolerance

Lactose is a sugar found in milk, it is digested by an enzyme called Lactase, found in the intestines. So if you don't have enough lactase, you wont be able to fully digest the lactose. 

Undigested lactose is fermented by bacterial, which releases gasses, meaning you have symptoms such as, stomach cramps, wind and bloating. 

Diarrhoea is caused by having a high concentration of lactose in the intestines, this causes the water to move out of the blood and into the intestines (by osmosis). The increase in water means you get runny faeces. 

Milk can be artificially treated with purified lactase to make it suitable for lactose intolerant people. Its fairly common to be lactose intolerant.

Digestion and the small intestines

Absorption of Glucose in the intestines: 

• When Carbohydrates are first broken down there is a higher concentration of glucose in the small intestine than there is in the blood. Therefor glucose moves across the epithelial cells of the small intestine, into the blood, by a process called diffusion. When the concentration in the lumen becomes lower than in the blood, there is no longer a concentration gradient, therefore diffusion stops. 
• The remaining glucose is absorbed by active transport with sodium ions:
• Sodium ions are actively transported out of the epithelial cells into the blood, by the sodium potassium pump, creating a concentration gradient, theres a higher concentration of sodium ions in the lumen than the cells.
• This concentration gradient causes sodium ions to diffuse into the cell from the lumen, this is done by sodium-glucose co-transporter proteins. The co-transporter carries glucose into the cell with the sodium, the increase of glucose in the cell increases the concentration.
• The glucose then diffuses down its concentration gradient into the blood, through a protein channel, via facilitated diffusion.

How is the small intestine adapted to its digestive and absorptive functions:

• Large surface area provided by villi and microvilli, 
• Thin epithelium gives a short diffusion pathway, 
• The dense capillary network for absorbing amino acids and sugars and the lacteal for the absorption of digested fats; which ensures a steep concentration is maintained,
• Many mitochondria in the epithelial cells provide ATP/ energy for active transport,
• Carrier proteins (in membranes) provide a path for polar molecules to pass through the membrane,
• Enzymes built into the epithelial membrane make it more likely for enzyme substrate complexes to form and ensure products for absorption are released close to the channel and carrier proteins, 

Digestion of Carbohydrates and Disaccharides:

Food moves through the digestive system by peristalsis. 

Mouth = digestion of carbohydrates,

Stomach = digestion of protein,

Duodenum = most digestion, receives pancreatic juice from the pancreatic duct, and bile from the gall bladder (produced in the liver.)

Ileum = most absorption, 

Colon = absorption of water and minerals,

Enzymes

Enzymes break down substances by: 

• Lowering the activation energy, a substance with a complementary shape to the enzyme entries the active site, forming an enzyme substrate complex, the active site changes shape to mould around the substrate (induced fit), this weakens the bonds in the substrate (lowering the activation energy) by stretching and distorting them, the bonds are broken, the products then leave the active site, leaving the enzyme unchanged. 

The effect of pH on the rate of enzyme activity: 

• Changes in pH affect the charges on the R groups of the amino acids at the active site. Therefore the interactions between the enzyme and the substrate are disrupted, reducing the frequency of enzyme substrate complexes forming, 
• Mote extreme pH conditions can cause the bonds (hydrogen, ionic, disulphide) holding the tertiary structure, to break. 
• If the enzyme denatures then the active site is no longer complementary to the substrate, therefore no enzyme substrate complexes can form. 

How the shape of the enzyme/protein molecules is suited to its function:

• Enzyme has a specific primary structure (amino acid sequence), this sequence determines where the hydrogen bonds will form during development of the secondary structure.Proteins have a unique tertiary structure held together by ionic, hydrogen and disulphide bonds. Globular proteins have an active site with unique structure. 
• The shoe of the active site is complementary to the substrate, so it'll only fit that substrate, so that enzyme substrate complexes can form.

The effect of temperature on the rate of enzyme activity: 

• As temperature increases dodoes the rate of the reaction, as the substrates gan kinetic energy and therefore collide more frequently, therefore there are more enzyme substrate complexes forming.
• A further increase of temperature can cause bonds (ionic, disulphide and hydrogen) which are holding the tertiary structure of the enzyme inlace to begin to break, this means the enzyme denature. 
• The active site no longer complements the substrate, therefore no enzyme substrate complexes can form.

How does an enzyme catalyse a condensation reaction: 

• The enzyme has a complementary shape to the substrate, meaning an enzyme substrate complex can be formed, the reactive groups are brought close together, the change in the active site (induced fit) lowers activation energy, water is removed and a bond is formed (glycosidic, peptide or ester bond) The products leave the active site, the enzyme remains unchanged

How do inhibitors affect enzyme activity:

• There are two types of inhibitors, competitive and non-competitive:  

• Competitive inhibitors -  have a similar shape to the substrate they can enter and bind with the active site, preventing enzyme substrate complexes forming, this problem can be  overcome by increasing the substrate concentration, 

• Non-competitive inhibitors -  have a different shape and bind to the enzyme at any point other than the active site, causing a change in the shape of the active site meaning the enzyme is no longer complementary to the substrate, preventing the formation of enzyme substrate complexes.

Biochemical Tests

Biochemical Tests: