Cell Biology: Cell Membrane

Cell Membrane

• The cell membrane is also known as the plasma membrane or plasmalemma, forming the outer protective layer of the cell.
• It maintains the cell's shape and size, and regulates exchange between the cell's interior and exterior.

Cell Membrane Structure

• The cell membrane is very thin (about 10 nm) and pliable.
• Its main constituents are lipids (40%), proteins (55%), and carbohydrates (5%).
• Lipids are arranged in a lipid bilayer, with two types: phospholipids and cholesterol.
• Phospholipids have a phosphate head (hydrophilic) and a lipid tail (hydrophobic), and are freely moving along the plane of the layer, making the membrane fluid.

Lipid Components

• Cholesterol molecules are dissolved in the lipid bilayer, controlling the membrane's fluidity.
• At modest concentrations, cholesterol decreases fluidity, while at high concentrations, it increases fluidity.

Permeability of Lipid Bilayer

• The lipid bilayer is semi-permeable, allowing lipid-soluble substances to pass through.
• Water, water-soluble substances, and large molecules cannot pass through.

Protein Components

• Proteins are mostly in the form of glycoproteins.
• There are two types of membrane proteins: integral membrane proteins (tightly integrated with lipid bilayer) and peripheral membrane proteins (loosely attached to lipids).
• Membrane proteins serve as transport proteins, allowing passage of water and water-soluble substances.
• Other functions of membrane proteins include:
  • Receiving signals (receptor proteins)
  • Intracellular signaling (second messenger)
  • Enzyme functions
  • Adhesion molecules for cell attachment or cell-to-cell contact
  • Submembrane cytoskeleton for strength and resilience
  • Antigen functions

Carbohydrate Components

• Most carbohydrates are in combination with proteins and lipids in the form of glycoproteins and glycolipids.
• Carbohydrates are usually found on the outer side of the cell, forming a glycocalyx.
• Functions of carbohydrates include:
  • Making the outer surface of the cell electronegative
  • Allowing neighbor cells to attach to each other through glycocalyx
  • Involvement in immune reactions

Cell Membrane Transport

Passive Transport / Diffusion

• Diffusion is the movement of molecules from an area of higher concentration to an area of lower concentration.
• Examples: amino acid movement from intestine to blood, and CO₂ into plant leaves.
• There are two types of diffusion:
  • Simple diffusion: molecules pass through the concentration gradient in a solution across a semipermeable membrane.
  • Facilitated diffusion: transport of substances with the help of a transport protein molecule.

Passive Transport / Osmosis

• Osmosis is the net movement of water molecules through the cell membrane from an area of higher water potential to an area of lower water potential.
• Examples: absorption in plant roots and small intestine in animals.

Active Transport / Carrier Protein

• Active transport is the movement of molecules from low concentration to high concentration, requiring energy.
• It is carried out by selective protein carriers, such as the sodium-potassium ion pump.

Active Transport / Vesicular Transport

• There are two types of vesicular transport:
  • Endocytosis: engulfing foreign substances from outside the cell to inside the cell, requiring energy.
    • Pinocytosis: engulfing small particles or molecules (e.g. protein, Vitamin B12, glycoprotein).
    • Phagocytosis: engulfing large particles or cells (e.g. bacteria, degenerating tissue).
    • Receptor-mediated endocytosis (clathrin-mediated).
  • Exocytosis: expelling material into the extracellular space, such as hormonal secretion by endocrine glands.

Endoplasmic Reticulum

• Involved in protein synthesis, folding, and modification
• Rough ER has ribosomes, involved in protein synthesis
• Smooth ER lacks ribosomes, involved in lipid synthesis, detoxification, and calcium ion storage
• ER dysfunction linked to various neurological disorders (e.g., Alzheimer's, multiple sclerosis)

Golgi Apparatus

• A stack of flattened membrane sacs called cisternae
• Involved in processing, sorting, and packaging of proteins and lipids from the ER
• Has three main regions: cis-Golgi network, medial Golgi, and trans-Golgi network

Mitochondria

• Powerhouses of the cell, generating ATP through cellular respiration
• Double membrane structure with inner membrane folded into cristae, increasing surface area for ATP production
• Contain their own DNA (mitochondrial DNA)
• Matrix contains enzymes involved in citric acid cycle and fatty acid oxidation

Lysosomes

• Membrane-bound organelles containing digestive enzymes
• Involved in intracellular digestion and recycling of cellular components
• Acidic interior optimal for enzyme activity
• Break down macromolecules into smaller components

Peroxisomes

• Involved in fatty acid breakdown, detoxification, and photorespiration in plants
• Generate hydrogen peroxide, maintaining a delicate balance to prevent cellular damage
• Dysfunction linked to genetic disorders like Zellweger syndrome

Eukaryotic Characteristics

• Possess a true nucleus and membrane-bound organelles
• Found in plants, animals, fungi, and protists (e.g., paramecium, chlamydia, red algae)
• More complex than prokaryotic cells

Nucleus

• Well-defined nucleus housing genetic material
• Enclosed by a double membrane (nuclear envelope) with nuclear pores
• Genetic material organized into linear chromosomes made up of DNA and proteins

Nucleus Components

• Nuclear envelope separates nucleus from cytoplasm
• Nucleoplasm contains chromatin, nucleolus, and proteins involved in DNA replication and RNA processing
• Chromatin is a complex of DNA, histone proteins, and other proteins that make up chromosomes
• Nucleolus is a region for rRNA synthesis and ribosome assembly