GenBio1: Cell Membrane Structure - Enzymes

cell membrane

• Surrounds the cell and controls what goes in and out of the cell

• Cells need to take in nutrients and get rid of waste, and the membrane regulates this exchange

• It helps cells to do things like: 

  • Produce energy in mitochondria.

  • Synthesize (make) proteins in ribosomes.

  • Make carbohydrates in plastids.

selective permeability

the ability to control what passes through.

1. davson-danielli model

2. singer-nicolson model

two important model

davson-danielli model

• Proposed that the phospholipid bilayer was sandwiched between two layers of proteins

• Called the “lipo-protein sandwich”

• Assumed all membranes were the same in thickness, protein content, and symmetrical

• Could’nt explain how some molecules passed through 

• Didn’t match actual membrane behavior

singer-nicolson model

• Known as the fluid-mosaic model

• Shows the membrane as a phospholipid bilayer with proteins floating within it, not just on the surface.

• Called "fluid" because the components can move.

• Called "mosaic" because of the mix of lipids, proteins, and carbohydrates.

• Proteins are not fixed and move within the membrane

Proteins vary in size and shape

1. phospholipid bilayer

2. membrane proteins

3. cholesterol

4. carbohydrates

structures of the cell membrane

phospholipid bilayer

• Main structure of the membrane

• Each phospholipid molecule has:

• Head (hydrophilic) = loves water, faces outward.

• Tail (hydrophobic) = hates water, faces inward.

• Amphipathic - a chemical reaction in which…

• Non-polar (hydrophobic) substances pass through easily.

• Polar (hydrophilic) substances cannot pass easily.

• Allows the membrane to change shape easily, which is required in bulk transport such as endocytosis and exocytosis.

membrane proteins

• Made up of 50% of the membrane

• Parts like the chloroplast and mitochondria are as high as 75%

• Helps in cell signalling

pump proteins

channel proteins

carrier proteins

types of integral proteins (transmembrane proteins)

pump proteins

• actively moving substances in and out

channel proteins

•  form tubes or passageways that passively move substances from one side to the other.

carrier proteins

• change in shape when transferring molecules across the lipid bilayer.

peripheral proteins

• Attached temporarily to the membrane surface

• Cell Recognition - function as unique identity tags of the cell

• Anchoring (or Adhesion) Proteins - proteins fasten adjacent cells in animal tissue

• Receptor Proteins - proteins with specific binding sites for molecules

junction

Connect and join two cells

enzyme

Localize metabolic pathways

transport

Help in diffusion and active transport

recognition

Act as markers for cell identification

anchorage

Anchor cytoskeleton and extracellular matrix

transduction

Receptors for hormones

cholesterol

• A lipid found in the hydrophobic part of the bilayer but it’s also a steroid.

• Only in Animal cell membranes (not in plants)

• It has a:

• Hydrophobic (water-fearing) body

• Hydrophilic (water-loving) -OH group

• Stabilizes membrane fluidity:

• Prevents it from being too stiff at low temperatures.

• Prevents it from being too fluid at high temperatures

• Helps in vesicle formation  (during endocytosis/endocytosis).

carbohydrates

• Found on the outer surface of the plasma membrane

• Always attached to proteins or lipids:

1. Glycoproteins - proteins with carbohydrate chains

2. Glycolipids - phospholipids with carbohydrate chains

• Together, these form the Glycocalyx (“sugar coating” around the cell)

1. Cell recognition - helps cells recognize each other

2. Cell signalling - acts as a binding site for signalling molecules like hormones

3. Cell adhesion - helps cells stick to each other to form tissues

4. Protection - the glycocalyx cushions the cell and protects it from mechanical and chemical damage.

carbohydrates functions as:

1. active transport

2. passive transport

two types of transport

passive transport

transport does not require energy

concentration gradient

forms when there is a difference in concentration between two areas.

1. simple diffusion

2. osmosis

3. facilitated diffusion

types of passive transport

simple diffusion

1. Movement of oxygen and carbon dioxide molecules from a high concentration to a low concentration across membranes.

hypertonic

• Concentration is higher than inside the cell → water moves out of the cell.

• Shrivels (Animal Cell). Plasmolysis (Plant Cell).

isotonic

• Neither skrinks nor swells because the concentration of molecules outside the cell is the same as inside

• Normal shape (Animal cell). Flaccid (Plant cell)

hypotonic

• Concentration is lower than inside the cell → water moves into the cell.

• Cytolysis (Animal cell). Turgid  (Plant cell).

osmosis

1. water moves across membranes from low solute concentration to high solute concentration 

facilitated diffusion

1. molecules move down their concentration gradient

active transport

Movement against the concentration gradient with the requirement of cellular energy. Uses protein carriers

1. primary active transport

2. secondary active transport

3. bulk active transport

types of active transport:

primary active transport

1. uses ATP and sources of chemical energy to move molecules across a membrane against their gradient 

secondary active transport

1. describe the movement of material using the energy of the electrochemical gradient established by primary

symport

antiport

2 types of secondary active transport

symport

same direction

antiport

reverse transport

bulk active transport

transports large molecules in cell parts

exocytosis (out of the cell)

• the process by which a cell exports material using a vesicle.

• Plagocytosis - eats solids/large molecules

• Pinocytosis - drinks liquids/smaller molecules

• Receptor-Mediated Endocytosis - specific uptake

types of endocytosis (in the cell)

enzymes

• biological molecules, typically proteins, that act as catalysts, significantly speeding up chemical reactions within living organisms

1. lock and key model

2. induced fit model

two models of enzymes

lock and key model

• The substrate binds to the perfectly fit active site to create a new product

induced fit model

• An enzyme’s active site is not an exact fit for the substrate

• The active site will undergo conformational change to improve binding with the substrate (induced)

• Explains how enzymes exhibit broadened specificity.

1. environmental conditions

2. cofactors coenzymes

3. enzyme inhibitors

factors affecting enzyme activity

substrate concentration

increasing substrate concentration will increase the reaction rate, but it will plateau at a certain point

optimal temperature for two enzymes

• Increasing the temperature also increases the rate of reaction.

• However, as the temperature moves away from the optimal temperature, it decreases.

optimal pH for two enzymes

• Reaction rate increases as pH approaches the optimum pH

apoenzyme

• becomes active by binding of coenzyme or cofactor to the enzyme

holoenzyme

• formed when an associated cofactor or coenzyme binds to the enzyme’s active site

• Enzyme helpers 

• Cofactors - inorganic (metal in ionic form: Fe, Mg, Zn, etc.)

• Coenzyme - organic (vitamins: Thiamine, folic acid, VitC, etc.)

types of cofactors coenzymes

• Toxins, poisons, pesticides, antibiotics, medicines

• Normal Binding

• Competitive Binding 

• Noncompetitive Inhibition

types of enzyme inhibitors

1. digestion

2. dna replication

3. detoxification

4. help generate energy

5. regulate many cell activities

importance of enzymes

• Amylase

• Lactase 

• Penicillinase

• DNA Polymerase

• Protease 

• Lipase 

• Cellulase 

• Catalase

examples of enzymes