Cell Membrane Structure and Function

• Cell membranes, also known as Plasma membranes, regulate the movement of substances into and out of the cell
• Cells require the import of food and the export of waste
• Communication also occurs at the cellular level

Cell Membrane Structure

• The cell membrane consists of a bi-layer of phospholipids
• The phospholipid has two components:
• A hydrophillic head that attracts water and is water soluble
• Hydrophobic tails that repel water and are insoluble in water

3 Main Components of Cell Membranes

• Lipids is a main component
• Proteins facilitate transport
• Carbohydrates facilitate communication

Fluid Mosaic Model

• The cell membrane is considered a dynamic model wherein components move freely

Phospholipids

• Hydrophilic head: Is negatively charged, contains a polar phosphate group, and faces outwards
• Hydrophobic tail: Consists of fatty acids that tuck into the membrane's interior to avoid water

Reasons for Cell Membrane Structure

• Water exists both outside the cell and inside the cell
• Hydrophilic heads are in contact only with water
• Hydrophobic lipid tails never contact water
• Cell membrane structure blocks substances from passing through

Membrane Fluidity

• Saturated fatty acid tails pack together while unsaturated fatty acid tails cannot pack together
• Saturated fatty acid tails allows molecules to pack together more densely at cooler temperatures
• Unsaturated fatty acid tails cannot pack together due to double bonds, thus remaining fluid at lower temperatures

Cholesterol

• Found alongside phospholipids in the membrane
• Acts as a membrane stabilizer
• Minimizes temperature effects
• At low temperatures it keeps the phospholipids from packing together

Proteins

• Function as transport
• Function as enzymes
• Facilitate signaling by binding to chemicals to trigger changes on the inner surface
• Attachment points for cytoskeleton support
• Recognize microbes for immune response.

Integral Membrane Proteins

• Embedded within the membrane
• One region is anchored to the hydrophobic core
• Span part of- or all the way through the membrane
• Transmembrane Proteins span the entire membrane

Peripheral Membrane Structure

• Found on the outer or inner surface
• Does not touch the hydrophobic core
• Located mostly on the cytosol side
• Some part of cytoskeleton

Carbohydrates

• Found on the outer surface (extracellular)
• Glycoprotein is bound to a protein
• Glycolipid is bound to a lipid
• Cellular markers enable functions such as:
• Cell-to-cell recognition
• Immune response

Membrane Permeability

• Membranes can be semi-permeable or selectively-permeable
• Membranes control what comes in and goes out
• Some substances can pass in and out easily
• Some require special structures
• Some require energy

Types of Transport

• Passive transport: movement without energy use
• Active: movement with energy use

Transport Across the Membrane

• Passive Transport
• Facilitated Diffusion
• Active Transport
• Endocytosis
• Exocytosis

Transport Summary

• Simple diffusion: Driven by a concentration gradient without the need of ATP
• Common substances include nonpolar compounds, lipids, oxygen, and carbon dioxide
• Facilitated diffusion: Driven by a concentration gradient without the need of ATP
• Common substances include polar compounds, water, ions such as Na+ and Cl-, and glucose
• Active Transport: Requires ATP
• Vesicle Transport: Moves larger quantities of small molecules

Passive: Diffusion

• Molecules move from areas of high concentration to areas of low concentration until equilibrium is reached
• Two types:
• Simple Diffusion
• Facilitated Diffusion

Passive Transport

• Molecules move from areas of high concentration to areas of low concentration without energy investment
• In simple diffusion, the concentration gradient moves from high to low concentration
• Lipid bilayers allow small uncharged polar molecules to pass through

Facilitated Diffusion

• Transport proteins protect charged/polar molecules from hydrophobic core by providing a route through the membrane
• Diffusion "helped" by membrane proteins
• Transport proteins are used for charged and polar molecules
• Transport proteins protect charged/polar molecules from hydrophobic core by providing a route through the membrane.

Types of Proteins used in Facilitated Diffusion

• Channel
• Aquaporins
• Carrier proteins

Types of Passive Transport

• Osmosis is the passive diffusion of water across a membrane
• Requires water movement from a high concentration to a low concentration
• Moves water from a lower solute concentration to a higher solute concentration

Passive Transport: Osmosis

• Three Solution descriptions:
• Hypotonic Solution
• Isotonic Solution
• Hypertonic Solution

Isotonic

• Solute concentration is equal inside and outside the cell

Hypertonic

• The concentration of solute molecules outside the cell is higher than the concentration in the cytoplasm

Hypotonic

• The concentration of a solute molecules inside the cell is higher than the concentration outside the cell

Active Transport

• The cell expends energy (ATP)
• Moves materials against the concentration gradient by using a protein pump

Pumps in the Nervous system

• Pumps 3 sodium out and 2 potassium in
• Protein goes back and forth between 2 shapes
• Inward facing has a high affinity for Na+
• Outward facing has a high affinity for K+
• Plays a major role in establishing membrane voltage

Bulk Transport

• Large particles (or large quantities of small particles) are transported across the membrane
• Substances enclose in their own small globes of membrane

Endocytosis - Bringing IN

• Phagocytosis: Fuses solids with a lysosome for digestion
• Pinocytosis: Brings in liquids; both are non-specific processes
• Receptor-mediated endocytosis: bringing in cholesterol; SPECIFIC

Exocytosis - Bringing OUT

• Vesicles fuse with the plasma membrane releasing contents to exterior of cell
• Includes hormones released from endocrine glands or digestive juice from pancreas
• Active Transport to export substances out of cell

Endocytosis

• Moves particles in by enclosing them in a vesicle made out of plasma membrane
• Substances are trapped in a pit/depression which pinches off, trapping the particles
• This includes phagocytosis and pinocytosis

Receptor-Mediated Endocytosis

• Used to capture a specific target molecule using receptor proteins on the cell surface
• Receptors act as proteins clustered in “coated pits", such as LDL cholesterol

Phagocytosis

• Used in cells to engulf bacteria, viruses, parts of dead cells, etc
• Ex: macrophages (white blood cells)