Cell Membrane and Transport Quiz

Cell Membrane and Transport

• The cell membrane is the outer layer of the cell, selectively permeable.
• The cell membrane structure is described as a fluid-mosaic model.
• The cell membrane is 7.5-10 nanometers thick.
• It contains 55% protein, 25% phospholipids, 13% cholesterol, 4% other lipids, and 3% carbohydrate.
• The main component is a double-layered lipid bilayer.

Membrane Lipids

• The lipid bilayer is made of phospholipid molecules.
• The head of phospholipids are hydrophilic (water-loving), phosphate-containing.
• The tails of phospholipids are hydrophobic (water-fearing), fatty acid-based.
• Hydrophobic tails are positioned inside the bilayer.
• Cholesterol is also present in the membrane, acting as a dissolved form.
• Cholesterol impacts membrane fluidity and elasticity. Increasing cholesterol makes the membrane more rigid.

Membrane Proteins

• Two types exist: integral (spanning the membrane) and peripheral (on one side).
• Integral proteins perform various roles, including transport, recognition, receptors and enzymes.
• Some integral proteins form channels or carriers for molecule transport.
• Peripheral proteins are involved in cell communication, connections to a cytoskeleton or extracellular matrix, and recognition.

Membrane Carbohydrates

• Found only on the outer surface of the cell membrane.
• Attaching carbohydrates to proteins forms glycoproteins.
• Attaching carbohydrates to lipids forms glycolipids.
• The combination of glycolipids and glycoproteins forms the glycocalyx (cell coat).
• The glycocalyx has functions like providing a negative charge. promoting cell-to-cell adhesion and recognizing foreign substances.

Membrane Permeability

• Factors affect membrane permeability.
• Small, nonpolar, fat-soluble molecules easily pass through the lipid bilayer (e.g., oxygen, carbon dioxide, fat-soluble vitamins).
• Water can pass through the bilayer in limited amounts, often through aquaporins.
• Charged particles (ions) cannot pass directly through the hydrophobic core.
• Large, polar molecules (e.g., glucose, amino acids) need transport proteins to cross the membrane.

Membrane Transport

• Substances move across the membrane based on their size, concentration, and solubility.
• Small substances can move through the membrane actively (requiring energy) or passively (no energy).
• Large substances are transported in or out of the cell through endocytosis or exocytosis.

Passive Transport

• Passive transport is a movement of substances across a cell membrane without energy input.
• It involves a concentration gradient; substances move from high concentration to low.
• Three types: simple diffusion, facilitated diffusion and osmosis.
• Simple diffusion involves small, nonpolar molecules passing directly through lipid bilayer.
• Facilitated diffusion uses specialized membrane proteins to transport larger molecules.
• Osmosis is the movement of water across a semipermeable membrane, from low to high solute concentration.

Facilitated Diffusion

• Molecules move across the membrane with specific carrier or channel proteins.
• Facilitated diffusion uses membrane proteins and does not need ATP unlike active transport.
• The speed of this transport is Vmax, which is also affected by the quantity of membrane proteins.

Active Transport

• Active transport moves molecules against their concentration gradient, from low to high, which requires energy.
• Two types: primary (directly uses ATP) and secondary (indirectly uses ATP).

Primary Active Transport

• Directly uses ATP to move ions or molecules against their concentration gradient.
• Examples include sodium-potassium pump, calcium pump, and proton pump.

Secondary Active Transport

• Uses the ion gradient established by primary active transport to move other molecules against their concentration gradient.
• Examples include sodium-glucose transporter (SGLT).
• The sodium is transported down the concentration gradient (from high to low) and drives other molecules across the gradient (e.g. glucose) from low to high concentration.

Endocytosis

• Cells take in large molecules or liquids from the outside by folding its membrane inward.
• Three main types of endocytosis include phagocytosis (bulk uptake of solid particles), pinocytosis (bulk uptake of fluids), and receptor-mediated endocytosis (engulfing specific molecules).

Exocytosis

• The process of expelling substances outside the cell.
• Substance are transported out of the cell by packaging it in vesicles that fuse with the plasma membrane.