Chapter 5 // Pt2: Enzymes and Membranes 

Metabolism: Enzymes and Membranes

Enzymes

• Required for metabolism
• Reactions that would otherwise take decades can be done in the cell in seconds
• Speeds up reactions - catalysis
• Not changed during reaction - reusable
• Most enzymes are proteins

Enzyme interaction w/ molecules

• Active Site - on enzymes where catalysis occurs
• Substrates - on molecules that match active site
• Enzymes target specific molecules
• Induced fit model - Active Site and Substrate don’t fit perfectly until reaction starts

Enzymes can be influenced by environment

• pH, temperature, salt concentration, can alter enzyme’s shape
• Work best in specific conditions
• denaturation

Metabolic Pathways

• Linked series of enzymatic reactions
• One triggers a reaction, triggers the next reaction, so forth.
• Millions of molecules

Linear Metabolic Pathways

Ex: Glycolysis in cellular respiration

Cyclic Metabolic Pathways

Ex: Krebs cycle in cellular respiration

Pathways controlled by enzyme inhibitors

• Feedback inhibition - molecules block enzyme active sites

• Stops enzymes from rapidly making products in reactions

  

How do cells safely break bonds to get energy?

• Glucose bonds can combust if all energy is released at once
• Electron transfer chain: series of enzymes and molecules that transfer electrons as glucose bonds are broken one by one
• Lose energy as electron is transferred = more stability

Cofactors

Small molecules that help enzymes

• Bonds to active site or allosteric site

• Can denature enzymes if removed

• Ex: vitamins, metals, and minerals

  

Coenzymes

Organic cofactors

• Carry chemical groups, atoms, and elements to different reactions throughout the cell
• Unlike enzymes these don’t get modified in reactions so they must be regenerated

Coenzyme ATP

• Coenzymes can be multifunctional = ATP
• Phosphorylation - donates phosphate group to enzyme which donates to reaction
• Phosphate bonds hold lots of energy (triphosphate)

ATP/ADP Cycle (phosphorylation)

• Phosphorylation forms ADP (diphosphate) when enzyme takes a phosphate group from an ATP coenzyme

• Reactions constantly need ATP to run

• Get ATP from glucose in food (charged battery)

• Lose phosphate to phosphorylation (dead battery)

• Need more glucose from food to charge battery again

  

Membranes

• Cell membranes made from lipid bilayer
• Other molecules embedded in membrane
• Fluid mosaic model - move around freely within membrane bilayer

Proteins in Membranes

• Proteins provide different functions to membranes
• Integral proteins - permanent proteins found in membranes
• Peripheral proteins - temporary proteins found in membranes
• Four different types of proteins found in membranes with different functions

Adhesion Proteins

• Fasten cell’s membranes together
• Helps adhering and tight junctions
• Signals info about cell’s position to other cells
• Ex: animal tissues

Receptor Proteins

• Trigger change in cellular activity in response to stimuli
• Hormone binds to it to tell info and receptor proteins tell the cell how to respond (metabolize, move, divide, or cell death)
• Ex: immune system

Enzymes in Membranes

• Catalyzes reactions at membrane

Transport Proteins

• Transports substances across lipid bilayer
• Molecules bind to one side of transport protein and releases it on other side of membrane (could be in or out of cell)

Moving Substances across membranes

• Metabolic pathways need molecules to participate across cell membranes and other cells
• Diffusion - spontaneous spreading of molecules or atoms through a fluid or gas
• A difference in gradients triggers diffusion
• 5 factors that influence diffusion

1. Concentration

   1. Substances travel from areas with a high concentration to areas with a low concentration
   2. Helps with overcrowding of molecules

2. Temperature

   1. Atoms and molecules vibrate faster at higher temperatures
   2. Move quicker = diffuse quicker

3. Charge

   1. Opposite charges attract
   2. Positively charged ions will move to areas with a negative charge, vice versa

4. Molecular Size

   1. Larger molecules harder to diffuse
   2. Smaller molecules diffuse quicker

5. Pressure

   1. High pressure squeezes atoms and molecules together
   2. More crowded
   3. More likely to go somewhere less crowded and with less pressure
   4. Faster diffusion at higher pressures

Moving Water Across Membranes

Osmosis - the movement of fluid across membranes

Hypertonic - when water moves out of the cell, shrivels

Hypotonic - when water moves in the cell, expands

Isotonic - equal sharing of water inside and out, ideal

Moving Substances Across Cell Walls

Turgor - the water pressure that gives plant cell their structure

• Keeps cell membrane pushed against cell wall
• Moves high to low

Transport proteins used when diffusion is not possible

• Some molecules can’t diffuse on their own (ex. glucose)
• 2 types of transport protein used

Passive Transport

Using transport proteins

• Requires no input, happens naturally
• Ex: osmosis, facilitated diffusion (gradient differences in concentrations)

Active Transport

Using transport proteins

• Requires energy to move across membrane (ex. ATP)
• Goes against gradient differences
• Ex. sodium-potassium pump
  • (Moves sodium ions out of cell and potassium ions into the cell)

Membrane Trafficking using vesicles

Vesicles = cell taxis

• Carry materials to and from cell membrane
• 2 types of vesicle movements

Exocytosis

• Vesicle moves to membrane and fuses with it
• Contents of vesicle released outside the cell during fusion

Endocytosis

• Membrane encloses group of materials and takes them in
• Releases content inside the cell
• Pinocytosis - endocytosis of liquids and sometimes small molecules
• Phagocytosis - endocytosis of large molecules

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