The Cell: Structure, Membrane, and Transport Processes

3.1 Basic Processes of Cells

Cell Metabolism

Cell metabolism refers to the chemical reactions that occur within cells to maintain life. These reactions include:

• Catabolic reactions: Breakdown of molecules to release energy.

• Anabolic reactions: Synthesis of complex molecules from simpler ones.

• Oxidation-Reduction reactions: Transfer of electrons between molecules, crucial for energy production.

Substance Transport

Cells transport substances across their membranes to maintain homeostasis and communicate with their environment.

• Transport mechanisms include passive (no energy required) and active (energy required) processes.

• Transport is essential for nutrient uptake, waste removal, and signal transduction.

Communication

Cells communicate with each other via chemical signals, which can be received and interpreted to coordinate cellular activities.

Cell Reproduction

Many cells undergo cell division to reproduce, repair tissues, and maintain organismal health.

3.1 Overview of Cell Structure

Basic Components of Most Animal Cells

• Plasma Membrane: The outer boundary of the cell, separating the internal environment from the external.

• Cytoplasm: The region between the plasma membrane and the nucleus, containing organelles and cytosol.

• Organelles: Specialized structures within the cytoplasm that perform distinct cellular functions.

• Nucleus: The control center of the cell, containing genetic material (DNA).

Functions of the Plasma Membrane

• Physically isolates the cell from its surroundings.

• Regulates the movement of substances into and out of the cell.

• Defines and separates fluid compartments:

  • Intracellular Space: Fluid within cells.

  • Extracellular Space: Fluid outside cells, including extracellular fluid (ECF).

Components of the Cytoplasm

• Cytosol (Intracellular Fluid, ICF): Water with dissolved solutes, site of many cellular processes.

• Organelles: Membrane-bound and non-membrane-bound structures with specialized functions.

• Cytoskeleton: Network of protein filaments providing structural support and facilitating transport within the cell.

Nucleus

• Surrounded by a double membrane (nuclear envelope).

• Contains DNA, which codes for proteins and regulates cellular activities.

3.1 Cell Size and Diversity

Cells vary greatly in size, shape, and function, reflecting their specialized roles in the human body.

• Examples include red blood cells (small, biconcave) and neurons (large, with long extensions).

3.2 The Phospholipid Bilayer

Structure and Function

• The phospholipid bilayer forms the basic structure of the plasma membrane.

• Each phospholipid molecule has a hydrophilic (water-loving) head and two hydrophobic (water-fearing) fatty acid tails.

• This arrangement creates a semi-permeable barrier between the cell and its environment.

Phospholipid Bilayer Properties

• Hydrophilic heads face outward toward water (ECF and cytosol).

• Hydrophobic tails face inward, away from water.

• Allows selective passage of substances.

3.2 The Fluid Mosaic Model of the Plasma Membrane

Plasma Membrane Composition

• Composed of phospholipids, proteins, cholesterol, and carbohydrates.

• Proteins are either integral (span the membrane) or peripheral (attached to the surface).

• The Fluid Mosaic Model describes the dynamic and flexible nature of the membrane.

Membrane Proteins

• Integral proteins: Embedded within the membrane, often function as channels or carriers.

• Peripheral proteins: Attached to the membrane surface, involved in signaling and structural support.

Functions of Membrane Proteins

• Channels: Allow passage of ions and molecules.

• Carriers: Transport substances across the membrane.

• Receptors: Bind to specific molecules to trigger cellular responses.

• Enzymes: Catalyze chemical reactions.

• Structural support: Maintain cell shape and integrity.

Other Membrane Components

• Cholesterol: Stabilizes the membrane and affects fluidity.

• Glycolipids and Glycoproteins: Involved in cell recognition and communication.

3.3 Transport Across the Plasma Membrane

Types of Transport

• Passive Transport: Does not require energy; substances move down their concentration gradient. Includes diffusion, osmosis, and facilitated diffusion.

• Active Transport: Requires energy (usually ATP); substances move against their concentration gradient.

Passive Transport Processes

• Diffusion: Movement of solute molecules from an area of higher concentration to lower concentration. $J = -D \frac{dC}{dx}$ where $J$ is the flux, $D$ is the diffusion coefficient, and $\frac{dC}{dx}$ is the concentration gradient.

• Osmosis: Movement of water across a selectively permeable membrane from an area of lower solute concentration to higher solute concentration.

• Facilitated Diffusion: Movement of substances via membrane proteins (channels or carriers).

Osmosis and Tonicity

• Osmotic Pressure: The pressure required to prevent water movement by osmosis.

• Tonicity: The ability of a solution to change the shape of cells by altering their internal water volume.

  • Isotonic solution: Same solute concentration as the cell; no net water movement.

  • Hypertonic solution: Higher solute concentration than the cell; water moves out, cell shrinks.

  • Hypotonic solution: Lower solute concentration than the cell; water moves in, cell swells.

Dehydration and Water Balance

• Dehydration results from loss of cellular water, affecting cell function and health.

• Sports drinks can help restore electrolyte and water balance.

Summary Table: Diffusion and Osmosis

Process	Direction of Movement	Energy Required	Example
Diffusion	High to Low Concentration	No	Oxygen entering cells
Osmosis	Low to High Solute Concentration (water moves)	No	Water absorption in kidneys
Facilitated Diffusion	High to Low Concentration	No	Glucose transport into cells
Active Transport	Low to High Concentration	Yes (ATP)	Sodium-potassium pump

Active Transport via Membrane Proteins

• Requires energy (ATP) to move substances against their concentration gradient.

• Primary Active Transport: Direct use of ATP (e.g., sodium-potassium pump).

• Secondary Active Transport: Uses energy from the movement of another substance down its gradient.

Sodium-Potassium Pump

• Pumps 3 sodium ions out and 2 potassium ions into the cell per ATP molecule hydrolyzed.

• Maintains electrochemical gradients essential for nerve impulse transmission and muscle contraction.

• Equation: $3 \text{Na}^+_{\text{out}} + 2 \text{K}^+_{\text{in}} + \text{ATP} \rightarrow 3 \text{Na}^+_{\text{in}} + 2 \text{K}^+_{\text{out}} + \text{ADP} + \text{P}_i$

Consequences of Ion Transport: Electrophysiology

• Ion transport across the plasma membrane creates electrical potential differences (membrane potential).

• Resting membrane potential is essential for nerve and muscle cell function.

Active Transport via Vesicles

• Endocytosis: Movement of substances into the cell via vesicles.

• Phagocytosis: 'Cell eating'; uptake of large particles.

• Pinocytosis: 'Cell drinking'; uptake of fluids.

• Exocytosis: Movement of substances out of the cell via vesicles.

Example: Exocytosis

Release of neurotransmitters from nerve cells is an example of exocytosis.

Additional info: Academic context and equations have been added to clarify and expand upon the original slide content.