Cell Membrane: Structure and Function

Primary Cell Membrane Function

• The cell's internal conditions must remain relatively constant despite external changes to carry out life functions.
• Homeostasis defines the constant state a cell maintains.

Cell Membrane Functions

• Cell membranes transport materials in and out.
• Cell membranes prevent unwanted matter from entering the cell.
• Cell membranes prevent the escape of materials needed to perform cellular functions.

Cell Membrane Structure

• Lipids, proteins, and the phospholipid bilayer make up the cell membrane.

Lipids Component

• Phospholipids are composed of a hydrophilic head and two hydrophobic tails.
• Cholesterol regulates membrane rigidity across different temperatures.

Proteins Component

• Channel or Gate proteins facilitate the movement of materials in and out of the cell, and are often molecule-specific.
• Glycoproteins are proteins with a carbohydrate chain that often help cells identify one another, and are involved with immune response.

Fluid Mosaic Model

• Describes how the cell membrane has a fluid-like consistency, allowing individual phospholipids to move independently. Lipids can move laterally or flip-flop.
• All membrane components move freely as if floating, and the membrane can change shape without damage.

Cholesterol's Role

• Cholesterol allows animal cell membranes to exist across a wide temperature range.
• At warmer temperatures, it maintains bilayer rigidity, preventing melting.
• At colder temperatures, it keeps the membrane fluid, flexible, and functional, preventing cell death.
• Plant cells use a different lipid with a similar function.

Membrane Proteins Functions

• Cell membranes have diverse functions and may contain multiple protein types.

Some Membrane Proteins Functions

• Cell-cell recognition
• Transport
• Receptor sites
• Structural support to the cytoskeleton

Maintaining Homeostasis

• Cell membranes are selectively permeable, that allows certain molecules to pass through while blocking others.
• Water acts as a solvent inside and outside the cell, that allows materials can dissolve easily.
• In multicellular organisms, every cell is covered in extracellular fluid composed mostly of water and dissolved materials.
• Wastes (carbon dioxide or urea) get eliminated.
• Substances (oxygen or water) get ingested.

Diffusion

• Passive transport requires no cellular energy.
• Diffusion is the movement of molecules from a region of higher concentration to lower concentration.
• Diffusion occurs along a concentration gradient - high to low.
• Equilibrium is reached when there is no change in concentration of the molecule.
• The rate of diffusion is temperature dependent, increasing with temperature due to increased Brownian motion of vibrating molecules.

Osmosis

• Osmosis specifically references the diffusion of water across a semi-permeable membrane.
• Water can cross from high to low concentration even when the membrane prevents the diffusion of other materials.

Cellular Tonicity

• Isotonic: The concentration outside and inside the cell are equal.
• Hypotonic: A higher concentration inside the cell causes water to rush in, increasing the cell's size.
• Hypertonic: A higher concentration outside the cell causes water to rush out, decreasing the cell's size.
• Water movement occurs down the concentration gradient, in an attempt to reach equilibrium.

Facilitated Diffusion

• Facilitated diffusion needs help from a protein because some materials is too large or insoluble in lipids to diffuse across the membrane alone.
• Specific carrier and channel proteins facilitate movement of molecules by size, shape, and electrical charge.
• While the movement of molecules still follows the concentration gradient, it requires no cellular energy for the carrier protein's function i.e passive.

Carrier vs. Channel Proteins

• Channel proteins have a tunnel that allows the movement of specific charged ions in or out of the cell, with sodium and chloride as examples.
• Carrier proteins change shape to transport specific molecules like glucose in or out of the cell.

Active Transport

• Active transport is the process to move materials against their concentration gradient (from low to high), and requires energy.
• ATP activates the transport protein, and pumps the material out of the cell.
• Binding of a specific shape to the protein is necessary for ATP to release energy and open the pump.
• The process is similar to pushing an object uphill.

Examples of Active Transport

• Active transport is used in kidney cells to pump glucose and amino acids from urine back into the blood.
• Intestinal cells use active transport to pump nutrients from the gut.
• Nutrients are pumped from the soil by plant root cells via active transport.
• Fish gill cells pump out sodium ions using active transport.

Sodium-Potassium Pump

• The carrier protein allows binding of 3 Na+ ions.
• An ATP molecule splits, releasing the energy and binding ADP + phosphate to the protein.
• Energy released changes the carrier protein's shape, releasing sodium out of the cell.
• The protein contains attachment points for two K+ ions.
• Phosphate is released, reverting the protein to its original shape.
• Shape change transports 2 K+ ions into the cell.

Bulk Membrane Transport

• Bulk transport becomes necessary when molecules are too large or polar to cross the cell membrane.
• Cells use specialized methods to move such materials in or out.
• The cell membrane can fold in on itself to engulf material into a membrane bubble, known as a vesicle.
• Vesicles can be newly formed or fuse with the membrane to release their contents.

Endocytosis

• The membrane folds inward to trap matter from the extracellular fluid.
• Endocytosis has three types that depends on what the cell is engulfing: pinocytosis, phagocytosis, and receptor-assisted endocytosis.

Pinocytosis

• Cell "drinking" refers to engulfing extracellular fluid with dissolved nutrients.

Phagocytosis

• Cell "eating" refers to engulfing extracellular fluid containing matter or bacteria.
• Macrophages in the immune system use this process for bacteria.

Receptor-Assisted Endocytosis

• The membrane receptors selectively bind to specific molecules, triggering endocytosis; similar to a key in a lock.

Receptor-Assisted Endocytosis & Cholesterol

• Animal cells uptake cholesterol via receptor-assisted endocytosis when levels are low.
• The liver produces cholesterol, which is a lipid insoluble in extracellular, watery fluid.
• Each cholesterol molecule is surrounded by a phospholipid layer with a protein tag.
• Binding of the protein tag to the cell surface receptor initiates endocytosis.

Exocytosis

• Exocytosis is the reverse of endocytosis.
• It involves a vesicle fusing with the cell membrane.
• The vesicle's contents are then expelled into the extracellular fluid.
• The pancreas secreting insulin is a crucial process for hormone-secreting organs.