Cell Structure and Membrane unit 2

Generalized Cell

Basic Structure and Functions

All human cells share common structures and functions, though they may specialize for different roles. The generalized cell model helps us understand the essential components found in most cells.

• Plasma Membrane: The outer boundary of the cell, controlling entry and exit of substances.

• Cytoplasm: The internal fluid containing organelles and cytosol.

• Nucleus: The control center containing genetic material (DNA).

Example: Red blood cells lack a nucleus, but most human cells contain all three basic parts.

Extracellular Materials

Substances Found Outside Cells

Extracellular materials are substances located outside the cell membrane, playing roles in support, protection, and communication.

• Extracellular Fluid: Includes interstitial fluid, blood plasma, and cerebrospinal fluid.

• Cellular Secretions: Such as saliva, mucus, and hormones.

• Extracellular Matrix: A network of proteins and polysaccharides that provide structural support to tissues.

Example: Collagen fibers in the extracellular matrix give strength to connective tissues.

Plasma Membrane

Structure and Function

The plasma membrane is a selectively permeable barrier that separates the cell's internal environment from the external environment. It is composed of lipids, proteins, and carbohydrates.

• Phospholipid Bilayer: Forms the basic structure, with hydrophilic (water-loving) heads facing outward and hydrophobic (water-hating) tails facing inward.

• Cholesterol: Stabilizes membrane fluidity.

• Proteins: Integral and peripheral proteins serve as channels, carriers, receptors, and enzymes.

• Carbohydrates: Attached to proteins and lipids, forming glycoproteins and glycolipids for cell recognition.

Example: The sodium-potassium pump is an integral membrane protein that maintains cellular ion balance.

Membrane Proteins

Types and Functions

Membrane proteins are essential for communication, transport, and structural support. They are classified as integral (embedded in the membrane) or peripheral (attached to the surface).

• Integral Proteins: Span the membrane and function as channels, carriers, or receptors.

• Peripheral Proteins: Attach to the membrane surface, providing support and facilitating cell shape changes.

Six Functions of Membrane Proteins:

1. Transport: Move substances across the membrane (e.g., ion channels).

2. Receptors for Signal Transduction: Bind chemical messengers and initiate cellular responses.

3. Attachment to Cytoskeleton and Extracellular Matrix: Maintain cell shape and stabilize membrane position.

4. Enzymatic Activity: Catalyze chemical reactions at the membrane surface.

5. Intercellular Joining: Form cell junctions for tissue integrity.

6. Cell-Cell Recognition: Allow cells to identify each other (e.g., immune response).

Cell Junctions

Types and Functions

Cell junctions connect cells together, forming tissues and organs. They regulate communication, adhesion, and permeability between cells.

1. Tight Junctions: Integral proteins fuse adjacent cells, creating an impermeable barrier. Function: Prevent leakage of extracellular fluid. Example: Tight junctions in the lining of the intestines.

2. Desmosomes: Link proteins form plaques that anchor cells together, providing mechanical strength. Function: Resist tension and abrasion. Example: Desmosomes in skin and cardiac muscle.

3. Gap Junctions: Transmembrane proteins form channels for communication and passage of ions. Function: Allow electrical and chemical signals to pass between cells. Example: Gap junctions in cardiac muscle for synchronized contraction.

Membrane Transport

Overview

Membrane transport refers to the movement of substances across the plasma membrane. It is essential for maintaining cellular homeostasis.

• Passive Transport: Does not require energy (ATP). Includes diffusion, osmosis, and filtration.

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

Passive Transport

• Diffusion: Movement of molecules from high to low concentration. Types: Simple diffusion, facilitated diffusion, osmosis.

• Filtration: Movement of water and solutes through a membrane by hydrostatic pressure.

Equation for Rate of Diffusion:

$J = -D \frac{dC}{dx}$

Where $J$ is the flux, $D$ is the diffusion coefficient, and $\frac{dC}{dx}$ is the concentration gradient.

Osmosis

Osmosis is the movement of water across a selectively permeable membrane from an area of low solute concentration to high solute concentration.

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

• Tonicity: Ability of a solution to change cell shape by altering internal volume.

Solution Type	Effect on Cell
Isotonic	No net water movement; cell shape unchanged
Hypertonic	Water leaves cell; cell shrinks (crenation)
Hypotonic	Water enters cell; cell swells (lysis)

Facilitated Diffusion

Certain molecules (e.g., glucose, amino acids) are transported passively down their concentration gradient via carrier proteins or channels.

• Carrier-mediated: Specific for certain molecules.

• Channel-mediated: Allow passage of ions and water.

Active Transport

Active transport moves substances against their concentration gradient using energy (ATP).

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

• Secondary Active Transport: Uses energy from ion gradients created by primary transport.

Equation for Sodium-Potassium Pump:

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

Summary Table: Membrane Transport Mechanisms

Transport Type	Energy Required	Direction	Example
Simple Diffusion	No	High to Low	Oxygen across membrane
Facilitated Diffusion	No	High to Low	Glucose via carrier protein
Osmosis	No	Water: Low to High solute	Water through aquaporins
Filtration	No	High to Low pressure	Kidney filtration
Active Transport	Yes (ATP)	Low to High	Sodium-potassium pump

Additional info:

• Cell junctions are critical for tissue integrity and function, especially in organs subject to mechanical stress.

• Membrane transport mechanisms are fundamental for nutrient uptake, waste removal, and signal transduction.