Cell Structure and Membrane Transport: Study Notes for Anatomy & Physiology

The Cell

Generalized Cell Structure

All human cells share certain structural features and functions, though they may differ in specialized roles. Understanding the basic parts of a cell is foundational for studying anatomy and physiology.

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

• Cytoplasm: The internal fluid containing organelles and cytosol.

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

Example: Neurons, muscle cells, and epithelial cells all possess these basic structures but differ in shape and function.

Types of Human Cells

There are over 200 different types of human cells, each specialized for particular functions.

• Muscle cells: Specialized for contraction and movement.

• Nerve cells (neurons): Specialized for communication and signal transmission.

• Epithelial cells: Form protective barriers and are involved in absorption and secretion.

• Connective tissue cells: Provide structural support and connect tissues.

Additional info: Blood cells (erythrocytes, leukocytes) and reproductive cells (sperm, ova) are also major cell types.

Cell Membrane Structure

Plasma Membrane Components

The plasma membrane is a selectively permeable barrier composed mainly of lipids and proteins. It maintains cellular integrity and mediates communication with the environment.

• Phospholipid bilayer: Forms the basic structure; hydrophilic heads face outward, hydrophobic tails face inward.

• Proteins: Integral and peripheral proteins serve various functions (transport, signaling, etc.).

• Cholesterol: Stabilizes membrane fluidity.

• Glycolipids: Contribute to cell recognition and interaction.

Membrane Lipids

Membrane lipids are crucial for the structure and function of the plasma membrane.

• 75% phospholipids: Main component, forming the lipid bilayer.

• 5% glycolipids: Lipids with attached sugar groups, found on the outer membrane surface.

• 20% cholesterol: Increases membrane stability and fluidity.

Key Terms: Hydrophilic (water-loving), Hydrophobic (water-fearing).

Membrane Proteins

Membrane proteins are essential for various cellular functions. They can be classified as integral (spanning the membrane) or peripheral (attached to the surface).

• Transport: Move substances across the membrane.

• Receptors for signal transduction: Bind chemical messengers and initiate cellular responses.

• Attachment to cytoskeleton and extracellular matrix: Maintain cell shape and stabilize location.

• Enzymatic activity: Catalyze chemical reactions at the membrane surface.

• Intercellular joining: Facilitate cell-cell adhesion and communication.

• Cell-cell recognition: Allow cells to identify each other via glycoproteins.

Functions of Membrane Proteins (Table)

Function	Description	Example
Transport	Move molecules across membrane	Ion channels, carrier proteins
Signal Transduction	Receive and transmit signals	Hormone receptors
Attachment	Bind cytoskeleton/ECM	Integrins
Enzymatic Activity	Catalyze reactions	ATPase
Intercellular Joining	Cell adhesion	Desmosomes
Cell Recognition	Identify cells	Glycoproteins

Cell Junctions

Types of Cell Junctions

Cell junctions are specialized structures that connect adjacent cells, providing communication and structural integrity.

• Tight Junctions: Prevent passage of substances between cells; found in epithelial tissues.

• Desmosomes: Anchor cells together, providing mechanical strength.

• Gap Junctions: Allow direct communication between cells via channels.

Example: Tight junctions in the intestinal lining prevent leakage of digestive enzymes.

Membrane Transport

Types of Membrane Transport

Substances move across the cell membrane via passive or active processes.

• Passive Processes: Do not require cellular energy (ATP); substances move down their concentration gradient.

• Active Processes: Require energy (ATP); substances move against their concentration gradient.

Additional info: Only living cell membranes can perform active transport.

Passive Processes

Passive transport includes diffusion and filtration.

• Diffusion: Movement of molecules from high to low concentration.

• Filtration: Movement of water and solutes due to hydrostatic pressure.

Diffusion Across Membranes

Simple diffusion allows nonpolar and lipid-soluble substances to pass directly through the lipid bilayer.

• Facilitated diffusion: Uses carrier or channel proteins for polar molecules.

Equation:

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

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

Osmosis

Osmosis is the diffusion of water across a selectively permeable membrane.

• Osmolarity: Measure of total concentration of solute particles.

• Both solutes and water cross the membrane until equilibrium is reached.

• Water moves by osmosis from areas of low solute concentration to high solute concentration.

Equation:

$\Pi = iMRT$

Where $\Pi$ is osmotic pressure, i is the van 't Hoff factor, M is molarity, R is the gas constant, and T is temperature.

Membrane Permeability and Osmosis (Table)

Membrane Type	Solute Movement	Water Movement	Result
Permeable to both solute and water	Solute and water move freely	Water moves to balance solute	Equilibrium reached
Impermeable to solute, permeable to water	Solute cannot move	Water moves to dilute solute	Cell swells or shrinks

Importance of Osmosis

Osmosis is vital for maintaining cell volume and function. Changes in cell volume can disrupt cellular activities, especially in excitable cells like neurons.

• Osmosis causes cells to swell (in hypotonic solutions) or shrink (in hypertonic solutions).

• Altered cell volume affects cell function, particularly in nerve and muscle cells.

Example: Rapid changes in extracellular osmolarity can lead to neurological symptoms due to neuron swelling or shrinkage.