Cell Membrane: Components and Functions

• Homeostasis, also known as equilibrium, is maintained by the plasma membrane.

Membrane Components

• The plasma membrane regulates what enters and leaves the cell.
• The key components of the cell membrane are phospholipids, proteins, cholesterol, cytoskeleton, and glycocalyx.

Phospholipids

• Phospholipids form the basic structure of the cell membrane.
• They have two nonpolar fatty acid chains.
• The polar heads of phospholipids are hydrophilic, meaning they are water-loving.
• The nonpolar tails are hydrophobic, meaning they are water-hating.
• Cytosol is the fluid inside the cell.

Plasma Membrane Functions

• Cell recognition.
• Protective barrier.
• Regulation of substance movement in and out of the cell.

Proteins

• Integral proteins are inserted into the membrane.
• Peripheral proteins are attached to the surface of the membrane.
• Integral and peripheral proteins regulate, act as identity markers and enzymes, or anchor cells.

Cholesterol

• Cholesterol strengthens the cell membrane.

Functions of Proteins in the Cell Membrane

• Structural support.
• Recognition
• Communication.
• Transport.
• Carbohydrates facilitate cell to cell recognition.

Types of Membrane Transportation

• Diffusion doesn't require energy and moves from high to low concentration.
• It relies on kinetic energy and is a passive process.
• Diffusion does not require cell membranes.

Factors Affecting Diffusion Rate

• Barriers slow down diffusion
• Low temperature slows down diffusion
• Thick membranes slow down diffusion
• Heavy molecules slow down diffusion
• Small concentration differences slow down diffusion
• High temperature speeds up diffusion
• Lighter weight speeds up diffusion
• Thinner membranes speed up diffusion
• Higher concentration difference speeds up diffusion
• Carbon dioxide, hydrophobic molecules, and small hydrophilic molecules use diffusion.
• Facilitated diffusion involves carrier proteins to help substances cross the cell membrane.

Osmosis

• Osmosis involves water movement from high to low concentration.
• Water moves to the side with more solutes.
• Solutes are dissolved substances.

Tonicity

• Hypertonic solutions have too much salt.
• Hypotonic solutions have too much water.
• Isotonic solutions have an equal amount of salt and water, representing equilibrium.

Effects of Hypertonic Environments

• Cause plasmolysis
• Water moves out of the cell, causing it to shrink
• Cytoplasm loses water.

Effects of Hypotonic Environments

• Cause cytolysis
• Water moves into the cell, causing it to swell and burst.
• Plants prefer hypotonic environments due to high turgor pressure.
• The cell wall prevents bursting.

Red Blood Cells in Different Environments

• In salt water, red blood cells undergo crenation and shrivel.
• In distilled water, red blood cells undergo hemolysis, swelling and bursting.
• In isotonic solutions, there is no net movement, and the cell shape is maintained.

Types of Movement Across Cell Membranes

• Simple diffusion moves substances from high to low concentration and is a passive transport.
• Facilitated diffusion requires a membrane and protein (channel or carrier) to move materials like glucose, amino acids, and ions from high to low concentration, and it is passive.
• Active transport requires extra ATP and carrier proteins (pumps) to move substances against the concentration gradient, from low to high.

Energy

• Energy is not needed for passive transport, including osmosis.

Filtration

• Filtration is movement from high to low pressure and is a passive process.
• One way the kidneys process and filter blood.

Channel and Carrier Proteins

• Channel and carrier proteins are types of transport proteins in cell membranes.

Channel Proteins

• Channel proteins are embedded in the cell membrane and have pores for materials to cross.
• They move passively by facilitated diffusion.
• Some are always open, while others are gated and require a stimulus to open.
• Integral and specific proteins.
• Aquaporins are channel proteins involved in osmosis. Move ions like Na+, Cl-, and K+

Carrier Proteins

• Carrier proteins help move substances by binding with them and dragging them across the membrane and can change shape.
• Some only go halfway through the membrane and are specific when used in facilitated diffusion.
• Move materials across the cell membrane

Cystic Fibrosis

• Cystic fibrosis occurs when the right channel protein isn’t made, preventing chlorine from being pumped out of the cell to the lumen.
• Water cannot move, causing mucus to harden and stick to the respiratory system.
• The thick, sticky mucus can clog the airways in the lungs, leading to difficulty breathing and digestion.
• In normal cells, the protein helps transport CI- ions (chloride) across the cell membrane, but cystic fibrosis disrupts this normal ion transport.

Sodium-Potassium Pump

• The sodium-potassium pump and proton pumps are examples of active transport.
• The sodium-potassium pump moves 3 sodium ions out of the cell and 2 potassium ions into the cell, against their concentration gradients, requiring ATP.
• This creates voltage and maintains resting membrane potential (RMP), which is typically -70mV.
• Stimulus opens Na+ channel and Na+ enters inside of membrane causing depolarization.
• The channel has to reach -55mV to open and closes at +30mV.
• Potassium gates open at +30mV and close at-70mV

Exocytosis vs. Endocytosis

• Exocytosis and endocytosis are types of active transport used for moving large molecules into or out of the cell.

Exocytosis

• Exocytosis moves large molecules out of the cell through vesicles that fuse with the cell membrane.
• Golgi apparatus ships proteins made in the Rough Endoplasmic Reticulum (RER) outside the cell

Endocytosis

• Endocytosis includes pinocytosis, cell mediated endocytosis, and phagocytosis.

Pinocytosis

• Pinocytosis brings large particles into the cell and is referred to as "cell drinking”.
• Vesicles are used

Cell Mediated Endocytosis

• Cell mediated endocytosis involves integral (channel) proteins that have receptors and recognize specific substances.
• Cholesterol and hormones use this to enter the cell.

Phagocytosis

• Phagocytosis involves the extension of the cell membrane to form pseudopods.
• Pseudopods are extensions of a cell that creates a temporary "leg"
• It is referred to as "cell eating”.
• White blood cells (leukocytes) use phagocytosis to engulf bacteria.

Bacteria and Plants

• Bacteria and plants have cell walls to prevent over-expansion in hypotonic environments.
• In plants, the pressure exerted on the wall is called turgor pressure or osmotic pressure.

Protists

• Protists (e.g., paramecia) have contractile vacuoles that collect and pump out extra water to prevent over-expanding in hypotonic environments.

Salt Water Fish

• Salt water fish pump salt out of special gills to prevent dehydration in hypertonic environments.

Kidneys in Animals

• Kidneys in animals keep blood isotonic by removing excess salt and water.

Ion Concentration

• Outside the cell membrane: high [Na+], low [K+], low [Cl-].
• Inside the cell membrane: low [Na+], high [Cl-], high [K+], high [anions-].

Pinocytosis vs. Phagocytosis

Both

• Use ATP
• Moves large molecules
• Use vesicles
• Bring large molecules into cell
• Type of endocytosis

Pinocytosis

• “Cell drinking”
• More than one molecule dissolved in extracellular fluid
• ECF is mostly water
• Invagination of cell membrane to form vesicles
• Most common type
• Least specific

Phagocytosis

• “Cell eating”
• Evagination of cell membrane forming pseudopods
• Cytoplasmic streaming
• Usually one large molecule/cell
• Immune system
• WBC use phagocytosis
• NOT in plants

Evagination and Invagination

• Evagination is the process of turning outward/inside out
• Invagination is the process of turning inside

Endocytosis vs. Exocytosis

• Both involve large molecules, use vesicles, and require ATP.

Endocytosis

• Includes three types: pinocytosis, phagocytosis, and receptor-mediated endocytosis (all use vesicles) and substances coming into the cell.

Exocytosis

• Involves substances leaving the cell and uses the Golgi apparatus, where vesicles merge with the cell membrane.

Channel vs. Carrier Proteins

Both

• Proteins
• Involved in transport
• Integral
• Specific to what it moves
• Facilitated diffusion

Channel

• Have channels/pores
• Don’t change shape
• Can be opened always or gated
• Only in facilitated diffusion
• Never ATP
• Move ions
• Contain aquaporins

Carrier

• Can go partially through the membrane
• Can change shape
• Can be used in active transport
• If used in active transport, they are called pumps