Fundamentals of Cells and Tissues

Cells are the building blocks that make up all living organisms, performing vital chemical activities necessary for life. The cell theory establishes that cells are the basic structural and functional units of living organisms, and an organism's activities depend on the collective functioning of its cells.

According to the principle of complementarity, a cell's biochemical activities are determined by the relative number of its specific subcellular structures. This principle helps explain why different cells can perform specialized functions throughout the body.

Cells join together to form tissues—groups of cells with similar structure and function. These tissues then combine to create organs and organ systems that work together to maintain life.

Did you know? The continuity of life has a cellular basis, meaning that all cells come from pre-existing cells through cell division, allowing organisms to grow, develop, and reproduce!

Chemical Composition and Basic Functions

Most cells are primarily composed of four key elements: carbon, hydrogen, oxygen, and nitrogen. These elements form the organic molecules that provide structure and function to cells, including proteins, carbohydrates, lipids, and nucleic acids.

Cells carry out all the chemical activities needed to sustain life. They process nutrients, produce energy, eliminate waste, and respond to their environment. The collective work of trillions of cells allows complex multicellular organisms like humans to function properly.

Tissues, which are groups of cells with similar structure and function, work together to perform specific roles in the body. For example, muscle tissue contracts to produce movement, while epithelial tissue forms protective barriers.

Understanding the chemical basis of cells helps explain how they can perform such diverse functions throughout the body, from generating electrical signals in nerve cells to producing mechanical force in muscle cells.

Cellular Anatomy

All cells share three main regions, though their specific structures may vary based on their functions. The nucleus serves as the control center containing genetic material, the cytoplasm fills the area between the nucleus and cell membrane, and the plasma membrane forms the outer boundary of the cell.

The nucleus contains the cell's DNA (genetic material) and has three main parts: the nuclear envelope, which is a double membrane with pores allowing material exchange; nucleoli, where ribosomes are assembled; and chromatin, which consists of DNA and proteins that condense to form chromosomes during cell division.

The cytoplasm contains organelles—specialized structures that perform specific functions within the cell. These "little organs" include mitochondria (which generate energy), ribosomes (which build proteins), and the endoplasmic reticulum (which transports materials).

Remember: While all cells share general structures, they are not identical. Their specific structures vary based on their functions in the body!

The Plasma Membrane

The plasma membrane forms a selective barrier around the cell, controlling what enters and exits. It consists of a double phospholipid layer with hydrophilic $water-loving$ heads facing outward and hydrophobic $water-repelling$ tails facing inward. This structure allows the membrane to maintain separation between the cell's internal environment and the surrounding fluid.

Proteins embedded in the membrane perform various functions, including transport of materials, cell recognition, and attachment to other cells. Cholesterol molecules give the membrane stability while allowing it to remain fluid. Glycoproteins on the cell surface serve as identification markers.

Cell membranes can form specialized junctions that connect adjacent cells. Tight junctions create leakproof seals between cells, desmosomes anchor cells together to prevent them from being pulled apart, and gap junctions allow direct communication between neighboring cells by creating channels for small molecules.

These membrane specializations are crucial for tissues to function properly. For example, tight junctions in intestinal cells prevent stomach acid from leaking between cells, while gap junctions in heart muscle cells allow electrical signals to pass directly from one cell to another.

Cytoplasmic Organelles: Power and Protein Production

The cytoplasm contains three major elements: cytosol (the fluid portion), organelles (specialized structures), and inclusions (stored substances). Within this busy cellular environment, various organelles perform crucial functions for cell survival.

Mitochondria serve as the cell's "powerhouses," continuously changing shape as they break down food molecules using oxygen to produce ATP—the energy currency of the cell. Cells with higher energy needs, like muscle cells, contain more mitochondria than less active cells.

Ribosomes function as protein factories, assembling amino acids into specific protein molecules based on genetic instructions. They can be found floating freely in the cytoplasm or attached to the rough endoplasmic reticulum (rough ER). These tiny structures are essential for cell growth, repair, and normal function.

The endoplasmic reticulum (ER) forms an extensive network of fluid-filled tubules throughout the cytoplasm. Rough ER, studded with ribosomes, specializes in protein synthesis and transport, while smooth ER functions in lipid metabolism and detoxification of drugs and other potentially harmful substances.

Fascinating fact: A single cell may contain hundreds or even thousands of mitochondria, depending on its energy requirements!

Cellular Transport Systems

The Golgi apparatus works like a cellular packaging and shipping department, modifying and sorting proteins into different types of vesicles. When proteins arrive from the rough ER in transport vesicles, the Golgi processes them and packages them for their final destinations.

These destinations include secretory vesicles (containing proteins to be released from the cell), lysosomes (containing digestive enzymes), or the cell membrane itself. This sophisticated sorting system ensures that proteins reach their correct locations to perform their functions.

Lysosomes contain powerful digestive enzymes that break down worn-out cell parts, bacteria, and other materials. They function as the cell's recycling centers, breaking down complex molecules into simpler ones that can be reused. Without proper lysosomal function, waste materials would accumulate in cells, potentially causing disease.

Peroxisomes are membranous sacs containing oxidase enzymes that detoxify harmful substances like alcohol and formaldehyde. They also neutralize free radicals—highly reactive chemicals that can damage cell structures. Unlike many other organelles, peroxisomes can replicate themselves by simply dividing in half.

The cytoskeleton provides an internal framework that supports the cell and allows for movement. It consists of three types of protein structures: microfilaments (for muscle contraction and cell division), intermediate filaments (for structural support), and microtubules (for moving materials within the cell).

Specialized Cell Structures

Centrioles are rod-shaped structures made of microtubules that play a crucial role in cell division. They direct the formation of the mitotic spindle, which helps separate chromosomes during cell division to ensure each new cell receives the correct genetic material.

Many cells possess specialized projections that extend from their surface. Cilia are short, hairlike extensions that move in a coordinated, wavelike motion to sweep materials across the cell surface. In your respiratory tract, ciliated cells move mucus and trapped particles up and away from your lungs.

Flagella are much longer than cilia and typically function to propel cells. Sperm cells are the only cells in the human body that have flagella, which enable them to swim through the female reproductive tract toward the egg.

Microvilli are tiny, fingerlike extensions of the plasma membrane that dramatically increase the surface area available for absorption. They're especially abundant in the small intestine, where they help maximize nutrient absorption from food.

Think about it: The specialized structures of different cell types reflect their specific functions in the body. For example, muscle cells are packed with mitochondria to provide energy for contraction, while intestinal cells have abundant microvilli for nutrient absorption.

Membrane Transport: Moving Materials

The plasma membrane is selectively permeable, allowing some substances to pass through while blocking others. This property is crucial for maintaining the cell's internal environment while allowing necessary exchanges with the surroundings.

Passive transport processes move substances across the membrane without requiring energy. Simple diffusion occurs when molecules move from areas of high concentration to low concentration until they're evenly distributed. Lipid-soluble molecules like oxygen and carbon dioxide can diffuse directly through the phospholipid bilayer.

Osmosis is the diffusion of water molecules across a membrane. Water moves through special protein channels called aquaporins or directly through the lipid bilayer. The direction of water movement depends on the relative concentration of dissolved substances on either side of the membrane.

Facilitated diffusion uses protein carriers to transport substances that cannot pass through the membrane on their own. This process doesn't require energy but still moves substances down their concentration gradient. Glucose enters most cells this way.

Filtration occurs when water and solutes are pushed through a membrane by hydrostatic pressure. This process is important in the kidneys, where blood pressure forces water and small solutes through capillary walls.

Active Transport Mechanisms

When cells need to move substances against their concentration gradient (from low to high concentration), they must use active transport processes that require energy from ATP. This allows cells to accumulate needed substances or remove waste products regardless of concentration differences.

In solute pumping, protein carriers called solute pumps use ATP energy to change their shape, moving specific molecules across the membrane. The sodium-potassium pump is a prime example, expelling sodium ions from the cell while bringing potassium ions in—both against their concentration gradients.

Vesicular transport involves the movement of larger materials using membrane-enclosed sacs called vesicles. In exocytosis, materials packaged in vesicles inside the cell are transported to the plasma membrane, where the vesicle fuses with the membrane and releases its contents to the outside.

Endocytosis is essentially the reverse of exocytosis—the cell membrane folds inward to engulf material from outside the cell, forming a vesicle that detaches and moves into the cytoplasm. There are two main types: phagocytosis ("cell eating"), which engulfs solid particles, and pinocytosis ("cell drinking"), which takes in droplets of fluid.

Clinical connection: Understanding cell transport is critical in medicine. Many drugs work by affecting transport processes, and diseases like cystic fibrosis result from defects in membrane transport proteins.

Cell Life Cycle and Division

Cells go through two major periods in their life cycle. During interphase, the cell grows, carries out normal metabolic processes, and prepares for division by replicating its DNA. This is followed by cell division, where the cell divides to produce two identical daughter cells.

DNA replication occurs toward the end of interphase, preparing the cell for division. The DNA double helix unwinds, and each strand serves as a template for building a complementary strand, resulting in two identical DNA molecules.

Cell division involves two main events: mitosis (division of the nucleus) and cytokinesis (division of the cytoplasm). Mitosis ensures that each daughter cell receives an identical set of chromosomes, while cytokinesis divides the cytoplasm and organelles.

Mitosis proceeds through four phases: prophase (chromosomes condense and the nuclear envelope breaks down), metaphase (chromosomes align at the cell's center), anaphase (chromosomes separate and move toward opposite poles), and telophase (nuclear envelopes reform around the separated chromosomes). Cytokinesis typically begins during late anaphase and completes during telophase, forming a cleavage furrow that pinches the cell in two.

Cell division is essential for growth, development, and tissue repair. However, abnormal cell division can lead to cancer, highlighting the importance of proper regulation of this process.