Cell Biology Basics: Structure, Function, and Types

Understanding Cells: The Building Blocks of Life

Cells are the fundamental building blocks of all living organisms.

Cell Structure Essentials

A typical cell consists of several key components:

• Plasma membrane: A very thin, stretchy cover composed of lipids, proteins, and carbohydrates that separates the cell from its external environment.
• Cytoplasm: The jelly-like substance filling the cell, enclosing the organelles.
• Genetic material (DNA): A complex chemical substance containing the genetic information necessary to regulate the cell's functions.

Prokaryotic vs. Eukaryotic Cells

Cells are broadly classified into two types based on the location and organization of their DNA:

Prokaryotic Cell Characteristics

• Kingdom: Monera (bacteria).
• Cellularity: Typically unicellular.
• Size: Small, generally 1-10 µm (micrometers).
• DNA Location: Found in the cytoplasm within a region called the nucleoid (no true nucleus).
• Plasmids: May contain small, circular DNA fragments called plasmids.
• Organelles: Cytoplasm contains ribosomes but lacks membrane-bound organelles.
• Mesosomes: The plasma membrane may fold inward, forming mesosomes, which increase surface area for metabolic activity.
• Cell Wall: Possess a rigid outer cell wall that provides shape and protection.
• Appendages: May have protrusions like fimbriae (for attachment to surfaces) and flagella (a tail-like structure for locomotion).
• Chloroplasts: Absent.

Eukaryotic Cell Characteristics

• Kingdoms: Protoctista, Fungi, Plantae, Animalia.
• Cellularity: Can be unicellular or multicellular.
• Size: Larger than prokaryotic cells, typically 10-100 µm.
• Nucleus: Possess a true nucleus, where DNA is enclosed within a membrane.
• Organelles: Contain various membrane-bound organelles, including mitochondria and, in some types, chloroplasts.
• Cell Wall: Present in plant and fungi cells, but absent in animal cells.
• Cytoskeleton: An internal network of protein filaments and tubules that provides structural support, shape, and enables movement.

Plant Cells

Plant cells typically have a rigid cell wall, a large central vacuole, a nucleus, and chloroplasts for photosynthesis.

Animal Cells

Animal cells lack a cell wall and chloroplasts but possess a nucleus and various other organelles. They have a flexible plasma membrane.

The Structure of the Nucleus

When a cell is not dividing (during interphase), the nucleus is typically visible and has the following structure:

• Nuclear membrane (Envelope): A double membrane surrounding the nucleus, perforated by nuclear pores that regulate passage. The outer membrane may be studded with ribosomes.
• Nucleoplasm: The fluid substance within the nucleus.
• Nucleolus: A dense structure within the nucleus responsible for synthesizing ribosomal components.
• Chromatin: A complex of DNA filaments tightly wound around proteins. During cell division, chromatin condenses to form visible chromosomes.

The Function of the Nucleus

The nucleus serves two primary functions: it contains the cell's hereditary information (DNA) and controls all cellular activities by regulating gene expression.

Cellular Functions: Nutrition

Cellular nutrition is the process by which cells obtain the necessary matter and energy to grow, replace structures, divide, and interact with their environment.

The Process of Cellular Nutrition

Nutrition involves several key steps:

1. Entry of substances: Nutrients cross the plasma membrane into the cell.
2. Transformation of substances: Internal chemical processes collectively known as metabolism modify these substances.
3. Excretion: Waste products are expelled from the cell into the extracellular environment.

Substance Entry Across the Plasma Membrane

The plasma membrane acts as a selective barrier, controlling what enters and leaves the cell.

• Small molecules: Can often pass freely through the membrane via diffusion, moving from an area of high concentration to low concentration (down the concentration gradient).
• Medium-sized molecules and ions: Often require the help of transport proteins embedded in the membrane.
  • Channels: Proteins that form pores, allowing specific substances to pass through, usually down their concentration gradient.
  • Pumps: Proteins that actively transport substances across the membrane, often against their concentration gradient, requiring energy (e.g., ATP).
• Large molecules: Cannot easily pass directly through the membrane and are transported via bulk transport mechanisms:
  • Endocytosis: The plasma membrane engulfs a particle, forming a vesicle that moves into the cytoplasm. This vesicle may fuse with a lysosome, which contains digestive enzymes to break down the vesicle's contents into smaller molecules.
  • Exocytosis: Vesicles containing waste products or substances for secretion fuse with the plasma membrane, releasing their contents outside the cell.

Cellular Metabolism Explained

Metabolism encompasses all the chemical reactions occurring within a cell. There are two main types:

Catabolism: Breaking Down Molecules

• Complex molecules entering the cell are broken down into simpler ones.
• This process typically releases energy.
• Some energy is lost as heat, while some is captured (e.g., as ATP).
• Cellular respiration is a key example of catabolism.

Anabolism: Building Complex Molecules

• Simple molecules are used to synthesize more complex ones.
• This process generally requires an input of energy.
• Energy can come from catabolism or, in some cells (like plants and algae), from light energy via photosynthesis.

The Role of Enzymes in Metabolism

• Enzymes are biological catalysts (usually proteins) that control the chemical reactions within cells.
• They significantly increase the speed of these reactions without being consumed in the process.

Types of Cellular Nutrition

Based on how they obtain organic matter, cells exhibit different nutritional strategies:

Heterotrophic Nutrition

• Cells obtain organic matter by consuming substances produced by other organisms.
• Found in animals, fungi, and many protoctists and bacteria.

Autotrophic Nutrition

• Cells synthesize their own organic matter (food) from simple inorganic substances (like CO2 and water).
• Energy for this process often comes from sunlight (photosynthesis) or chemical reactions (chemosynthesis).
• Photosynthesis occurs in chloroplasts.
• Found in plants, algae, and some bacteria.

Cellular Functions: Interaction & Reproduction

Cellular Interaction and Response

Cells can detect changes (stimuli) in their internal or external environment and respond appropriately. This is cellular interaction.

A stimulus is any change that elicits a response. Stimuli can be:

• Chemical: Changes in pH, nutrient concentration, presence of signaling molecules, etc.
• Physical: Changes in temperature, pressure, light, touch, etc.

A cellular response is the reaction of the cell to a stimulus. Responses can be:

• Static: A response that does not involve movement (e.g., secreting a substance, changing metabolic activity).
• Dynamic: A response involving movement (e.g., movement of the entire cell, movement of internal components).

Cellular Reproduction Fundamentals

Cellular reproduction is the process by which a parent cell divides to form two or more new cells, called daughter cells, which are typically identical to the parent cell.

• Unicellular organisms: Cell division is the primary means of reproduction for the entire organism.
• Multicellular organisms: Cell division is essential for growth (increasing the number of cells), repair (replacing damaged or worn-out cells), and sometimes asexual reproduction.

The Cell Theory: A Foundation of Biology

The development of higher-quality microscopes in the 19th century allowed scientists to observe cells in detail and recognize their universal importance.

Three German scientists – the botanist Matthias Schleiden, the zoologist Theodor Schwann, and the physician Rudolf Virchow – synthesized observations into the foundational Cell Theory. Its main principles are:

• Cells are the structural units of living things: All living organisms are composed of one or more cells.
• Cells are the functional units of living things: The cell is the smallest unit capable of carrying out all essential life processes.
• Cells originate from pre-existing cells: All cells arise from the division of other cells.

Further advancements, particularly the invention of the optical and electron microscopes (which can magnify images hundreds of thousands or even a million times), have continued to refine our understanding of cell structure and function.