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Plasma Membrane
Acts as a semi-permeable barrier, controlling movement of substances in and out of the cell.
Made of the Fluid Mosaic Model
"Fluid" = The membrane is flexible, not rigid
"Mosaic" = Made of many different molecules working together
Which includes four major components:
Phospholipid Bilayer
Cholesterol
Membrane Proteins
Carbohydrates
Role of Phospholipid Bilayer in the Membrane
The “foundation?”
Made up of phospholipids with:
Hydrophobic fatty acid chains (face inward)
Hydrophilic phosphate groups (face outward towards water)
Acts as a barrier – prevents water-soluble molecules from easily passing through. Keeps the membrane flexible and self-repairing.
Role of Cholesterol in the Membrane
The “stability factor”.
Acts as a patching substance that prevents unwanted small molecules from entering/ passing through,
Helps maintain membrane fluidity, preventing it from becoming too rigid or too fluid.
Membrane Proteins
The Functional Units
Integral vs. Peripheral
Role of Integral Proteins in the Membrane
(Embedded in the Membrane)
Also called transmembrane proteins.
Pass through the entire bilayer.
Functions:
Transport Proteins: Move substances across the membrane (like doors).
Receptor Proteins: Detect signals from the environment (like an antenna).
Enzymes: Speed up chemical reactions.
Role of Peripheral Proteins in the Membrane
(On the Surface) Attached to the inner or outer surface of the membrane.
Functions:
Cell signaling – Help in communication.
Cell structure – Connect to the cytoskeleton (inner framework of the cell).
Recognition – Help immune cells identify "self" vs. "non-self".
Role of Carbohydrates in the Membrane
The Identification Tags
Carbohydrates in the membrane attach to lipids (glycolipids) or proteins (glycoproteins).
Act as "name tags" for cells.
Allow cells to recognize and interact with each other.
Help immune cells distinguish "self" from invaders.
Transport Across the Membrane
The membrane is selectively permeable, meaning some molecules can pass freely while others need help. Passive Transport (No Energy Needed) vs Active Transport (ATP needed)
Diffusion
Movement of molecules from high concentration to low concentration until equilibrium is reached.
No energy required. Passive Transport.
Example: Oxygen & carbon dioxide diffuse across the membrane.
Osmosis
The diffusion of water across a semi-permeable membrane.
Water moves from low solute concentration (high water) to high solute concentration (low water).
Uses aquaporins (specialized protein channels for water transport).
Facilitated Transport
Molecules move from high to low concentration but require transport proteins.
Example: Glucose entering the cell.
Active Transport
Molecules move against the concentration gradient (from low to high concentration), using energy.
Protein Pumps
Active Transport. ATP-powered proteins pump molecules against the gradient.
Example: Sodium-Potassium Pump (important for nerve signals).
Vesicular Transport (Bulk Transport)
Cells use vesicles (small membrane sacs) to move large molecules.
Exocytosis (Exporting Substances)
Endocytosis (Importing Substances)
Exocytosis
Vesicle inside the cell fuses with the membrane and releases contents outside.
or vesicles fuse with the membrane and release materials outside the cell.
Used for secreting proteins, hormones, and metabolic waste.
Endocytosis
The membrane folds around a substance and brings it inside.
Types of Endocytosis:
Phagocytosis ("Cell Eating") – Engulfs large particles (like bacteria).
Pinocytosis ("Cell Drinking") – Takes in liquids with dissolved substances.
Receptor-Mediated Endocytosis – Uses receptors to bring in specific molecules (e.g., cholesterol uptake).
White blood cells use ___________ to get rid of foreign particles in the blood.
Phagocytosis
Tonicity
Water Balance in Cells. Isotonic, Hypotonic, Hypertonic.
Isotonic Solution
Equal solute concentration inside and outside the cell.
No net movement of water.
Cell remains the same size.
Hypotonic Solution
Lower solute concentration outside the cell.
Water enters the cell, causing it to expand and possibly lyse (burst).
Plant cells prefer this state (turgid).
Hypertonic Solution
Higher solute concentration outside the cell.
Water moves out of the cell, causing it to shrink (crenation in animal cells, plasmolysis in plant cells) Shrink in both.
Causes dehydration in animal cells.
Laws of Thermodynamics
First Law
Second Law
First Law
Conservation of Energy):
Energy cannot be created or destroyed, only transformed.
Example: Glucose energy is converted into ATP
Second Law
(Entropy):
Every energy transfer increases disorder (entropy) in the universe.
Some energy is lost as heat in every transfer.
Types of Reactions
Exergonic Reactions (Energy-Releasing) vs Endergonic Reactions (Energy-Absorbing)
Exergonic Reactions
(Energy-Releasing)
Release energy spontaneously.
Example: Cellular respiration.
Endergonic Reactions
Energy-Absorbing)
Require energy input to proceed.
Example: Photosynthesis.
Role of Enzymes in Cellular Reactions
Enzymes are proteins that speed up reactions.
They work by lowering activation energy (the energy needed to start a reaction).
How Enzymes Work:
Substrate binds to enzyme's active site.
Enzyme changes shape (induced fit).
Reaction happens, and products are released.
Example: Digestive enzymes breaking down food.
ATP
ATP (Adenosine Triphosphate) stores and releases energy for cellular activities.
Structure:Ribose sugar, Adenine nitrogen base, Three phosphate groups (high-energy bonds).
ATP Hydrolysis: Breaking a phosphate bond releases energy, converting ATP to ADP.
Kinetic energy vs Potential energy
Kinetic is the energy of motion.
Potential is energy stored in the location or structure of matter and includes chemical energy.
