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

Microscopy in Cell Biology

Types of Microscopy

Microscopy is essential for visualizing cells and their components. Different types of microscopes are used based on the level of detail required.

• Light Microscopy: Uses visible light to illuminate specimens. Suitable for viewing live cells, tissues, and general cell structure. Limited by resolution (~200 nm).

• Transmission Electron Microscopy (TEM): Passes electrons through thin sections of specimens. Provides high-resolution images of internal cell structures (organelles, membranes).

• Scanning Electron Microscopy (SEM): Scans the surface of specimens with electrons. Produces detailed 3D images of cell surfaces and external features.

Example: TEM is used to study mitochondria structure, while SEM is used to examine the surface of red blood cells.

Cell Structures and Their Functions

Major Organelles and Components

Cells contain specialized structures called organelles, each with distinct functions necessary for cell survival and activity.

• Nucleus: Contains genetic material (DNA); controls cell activities and gene expression.

• Rough Endoplasmic Reticulum (RER): Studded with ribosomes; synthesizes and processes proteins for secretion or membrane insertion.

• Smooth Endoplasmic Reticulum (SER): Lacks ribosomes; involved in lipid synthesis, detoxification, and calcium storage.

• Golgi Apparatus: Modifies, sorts, and packages proteins and lipids for delivery to different destinations.

• Lysosomes: Contain digestive enzymes; break down waste materials and cellular debris.

• Peroxisomes: Break down fatty acids and detoxify harmful substances.

• Mitochondria: Produce ATP through cellular respiration; known as the "powerhouse" of the cell.

• Cytosol: Fluid portion of the cytoplasm; site of many metabolic reactions.

• Ribosomes: Synthesize proteins; can be free in cytosol or attached to RER.

• Cytoskeleton: Network of protein filaments (microtubules, microfilaments, intermediate filaments); provides structural support, shape, and aids in movement.

Example: Muscle cells have abundant mitochondria to meet high energy demands.

Plasma Membrane Structure and Function

Components and Roles

The plasma membrane is a selectively permeable barrier that surrounds the cell, maintaining the internal environment and mediating communication with the external environment.

• Lipid Bilayer: Composed mainly of phospholipids; forms the fundamental structure, providing a hydrophobic barrier.

• Proteins: Embedded or attached to the membrane; serve as channels, carriers, receptors, enzymes, and structural anchors.

• Carbohydrates: Attached to proteins (glycoproteins) or lipids (glycolipids); involved in cell recognition and glycosylation.

Example: The sodium-potassium pump is a membrane protein that helps maintain ion gradients across the plasma membrane.

Passive Membrane Transport

Mechanisms and Definitions

Passive transport is the movement of substances across the cell membrane without the use of cellular energy (ATP). It relies on concentration gradients.

• Concentration Gradient: The difference in the concentration of a substance between two regions. Substances move from areas of higher to lower concentration.

• Simple Diffusion: Movement of small, nonpolar molecules (e.g., O2, CO2) directly through the lipid bilayer.

• Facilitated Diffusion: Movement of larger or polar molecules (e.g., glucose, ions) via specific transport proteins (channels or carriers).

• Osmosis: Diffusion of water molecules across a selectively permeable membrane from an area of low solute concentration to high solute concentration.

Key Equation:

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

Where: J = flux (rate of diffusion) D = diffusion coefficient \frac{dC}{dx} = concentration gradient

Example: Oxygen enters cells by simple diffusion, while glucose enters via facilitated diffusion.