Biology Chapter: Cell Membrane Functions

Cell Membrane Structure & Transport

• The cell membrane maintains homeostasis and is selectively permeable.
• All cells have a cell membrane, also known as a plasma membrane.
• It provides protection and support.

Fluid Mosaic Model

• The cell membrane is fluid (flexible).
• Different molecules, including phospholipids and proteins, are embedded within the membrane.
• Phospholipids form a bilayer, with hydrophilic heads facing the watery environments (extracellular fluid and cytoplasm) and hydrophobic tails facing inward.

Mosaic Structure

• Scientists describe the cell membrane as a mosaic because it resembles a mosaic artwork, with diverse embedded components.
• Various components, such as proteins, carbohydrates, and lipids, are arranged in a complex pattern.

Structure of the Plasma Membrane

• Composed of a lipid bilayer, which are two layers of phospholipids.
• Embedded components include proteins and cholesterol.
• Hydrophilic regions (water-loving) face the watery environment inside and outside the cell.
• Hydrophobic regions (water-fearing) face each other and form the middle of the membrane layers.

Structure of the Cell Membrane

• The cell membrane is composed of a lipid bilayer, with proteins spanning and embedded within the layers.
• Carbohydrate chains are attached to proteins and lipids on the outer surface, for cell recognition.

Membrane Proteins

• Some proteins speed up reactions.
• Some proteins recognize other cells (receptors).
• Some proteins transport materials across the membrane.
• Proteins include transporters, enzymes, receptors, and anchors.

Glycoproteins

• Proteins with attached carbohydrates.
• Function in cell recognition (chemical identification cards).
• Important in the immune system.

Membrane Characteristics

• The plasma membrane has polar and nonpolar regions.
• The polar region is hydrophilic (water-soluble) and includes hydrophilic heads that face the inside and outside of the cell.
• The nonpolar region is hydrophobic (water-fearing) and consists of hydrophobic tails that form the middle of the membrane layers.

Polar and Nonpolar Reactivity

• Nonpolar molecules do not usually interact with polar molecules (e.g., oil and water).
• Nonpolar molecules interact with each other.

Questions

• What is the difference between hydrophilic and hydrophobic?
• Which part of the cell membrane is hydrophilic?
• Which part of the cell membrane is hydrophobic?
• What is the function of the proteins?

Movement Through the Cell Membrane

• Movement through the cell membrane can happen in two ways: passive transport and active transport.

Passive Transport

• Movement of substances across the cell membrane without the input of energy.
• Substances move from areas of high concentration to areas of low concentration (down the concentration gradient).
• Includes simple diffusion, facilitated diffusion, and osmosis.

Diffusion

• Process where molecules move from regions of high concentration to regions of low concentration.
• Diffusion occurs down, or with, the concentration gradient.
• Does not require energy (ATP).
• Driven by the kinetic energy in molecules, caused by the movement of particles due to their atoms' movement.
• Factors that affect diffusion include temperature, size of molecules and type of molecule.
• Diffusion stops when equilibrium is reached, meaning particles are evenly distributed, and there is no gradient.
• Small molecules, such as oxygen (O2) and carbon dioxide (CO2), can easily pass through the cell membrane via simple diffusion.

Facilitated Diffusion

• Membrane proteins facilitate (help) the movement of molecules or ions across the membrane.
• These proteins are specific for particular molecules.
• Substances move down the concentration gradient.
• This type of transport does not require energy (ATP).
• Example: Glucose movement using glucose channels.
• Substances that move via facilitated diffusion include glucose, sodium ions (Na+), chloride ions (Cl-), and potassium ions (K+).

Osmosis

• Diffusion of water across a selectively permeable membrane.
• Does not require energy (ATP).
• Water moves from areas of high water concentration to areas of low water concentration (or from areas of low solute concentration to high solute concentration).

Cytoplasm Composition

• Cytoplasm is mostly water (70-90%) with dissolved solutes.
• Other components include proteins (10-15%), carbohydrates (3%), lipids (2%), nucleic acids (5-7%), and ions (1%).

Different Solutions

• Hypertonic solutions: the solute concentration outside the cell is higher than inside the cell, water diffuses out, causing the cell to shrink (plasmolysis).
• Hypotonic solutions: the solute concentration outside the cell is lower than inside the cell, water diffuses into the cell causing the cell to swell (cytolysis). Plant cells are protected from bursting by their cell wall.
• Isotonic solutions: the solute concentration outside and inside the cell are equal, water diffuses into and out of the cell at equal rates; no net movement of water.

Active Transport

• Movement of substances across the cell membrane against the concentration gradient (low to high).
• Requires energy (ATP).
• Includes protein pumps, endocytosis, and exocytosis.

Protein Pumps

• Important for maintaining ion concentration in cells and between cells.
• Uses carrier proteins; cell membrane pumps use energy (ATP).
• Example: sodium-potassium pumps are important for muscle contractions and nerve impulses in animals, and enabling root absorption of nutrients in plants.

Endocytosis

• Taking large amounts of material into a cell (uses energy).
• Including phagocytosis ("cell eating"), and pinocytosis ("cell drinking").

Exocytosis

• Forces material out of a cell (uses energy).
• Examples: hormones and wastes being released from a cell.

Questions:

• What are the 3 types of passive transport?
• Which type of passive transport moves water across a semi-permeable membrane?
• In what direction do materials move in passive transport?
• Which type of transport uses energy?
• What is the source of energy in question #1?
• When do molecules stop diffusing?
• List two similarities and two differences between active and passive transport.
• Name the three types of active transport.
• Provide three examples of how animals use active transport.
• What is the difference between exocytosis and endocytosis?

Problems with Cell Transport

• Cystic Fibrosis (CF) is a genetic disorder that affects cell transport, specifically chloride ion transport.
• The CFTR channel becomes mutated, hindering the movement of chloride ions, which then causes sticky mucus buildup.
• The buildup affects various organs, leading to complications.

Test your knowledge of the cell membrane's structure and functions with this quiz. Learn about passive transport processes, including osmosis and facilitated diffusion, while matching components to their roles. Perfect for biology students studying cell biology concepts.