Cellular Structure: Anatomy and Physiology of the Cell

Cells: Structure and Function

Introduction to Cells

• Cells are the basic structural and functional units of all living organisms.

• They carry out all chemical activities necessary to sustain life.

Example: Red blood cells transport oxygen, while nerve cells transmit electrical signals.

Anatomy of the Cell

General Features of Cells

• Cells are not all the same, but they share most general structures.

• Common features of all cells include:

  • Enclosed by a plasma membrane

  • Contain hereditary material (DNA and RNA)

  • Contain cytoplasm

  • Obtain nutrients and energy from their environment

Example: Skin cells, nerve cells, and plant cells all have these features, despite their differences in shape and function.

Common Features of All Cells

Plasma Membrane

• The plasma membrane surrounds the entire cell and controls what enters and exits the cell.

• It separates the cell's interior from the external environment, both of which are watery.

• The plasma membrane is described by the fluid mosaic model:

  • Fluid: Substances within the membrane can move and flow.

  • Mosaic: Composed of diverse molecules, including lipids, proteins, and carbohydrates.

• Structured as a phospholipid bilayer:

  • Phospholipid heads are hydrophilic ("water-loving") and face outward toward water.

  • Phospholipid tails are hydrophobic ("water-fearing") and face inward, away from water.

• Proteins are embedded within the bilayer, some floating and others fixed, serving various functions such as transport and communication.

• Other compounds, such as cholesterol (maintains flexibility) and glycoproteins/glycolipids (cell recognition), are scattered throughout the membrane.

Example: The plasma membrane allows nutrients to enter and waste products to exit the cell.

Transport Across the Membrane

• Proteins in the membrane facilitate the movement of substances that are not soluble in lipids.

• Lipid-soluble molecules can pass directly through the bilayer, while others require protein channels or carriers.

Additional info: Transport mechanisms include diffusion, osmosis, facilitated diffusion, and active transport.

Cellular Junctions

Types of Cellular Junctions in Animal Cells

• Animal cells use three main types of junctions:

  • Tight junctions: Bind cells tightly together, forming a watertight seal. Located in tissues where fluid must not leak, such as the bladder and kidneys.

  • Desmosomes: Hold cells together with small gaps, allowing fluid and small molecules (e.g., ions) to pass. Found in tissues subject to stress, such as skin and intestines.

  • Gap junctions: Channels between cells that allow ions and small molecules to pass easily. Important in tissues that require rapid communication, such as neurons and cardiac muscle.

Example: Gap junctions in cardiac muscle allow for synchronized contraction of the heart.

Cell Organization: Three Main Regions

• Cells are organized into three main regions:

  1. Plasma membrane

  2. Nucleus

  3. Cytoplasm

The Nucleus

• The nucleus is the control center of the cell.

• It has three main regions:

  • Nuclear membrane: Double-layered structure with pores for exchange of materials with the cytoplasm.

  • Nucleolus: Site of ribosome synthesis; cells may have one or more nucleoli.

  • Chromatin: Loosely organized DNA and protein, scattered throughout the nucleus; condenses to form chromosomes during cell division.

Example: The nucleolus produces ribosomes, which are essential for protein synthesis.

Cytoplasm

• The cytoplasm is the material inside the plasma membrane but outside the nucleus.

• It includes:

  • Cytosol: Fluid that suspends other elements.

  • Inclusions: Non-functioning units, such as stored nutrients or pigments.

  • Organelles: Specialized structures that perform specific cellular functions (e.g., mitochondria, endoplasmic reticulum, Golgi apparatus).

Additional info: The cytoplasm is the site of most cellular metabolic reactions.

Summary Table: Main Features of the Cell

Feature	Description	Example/Function
Plasma Membrane	Phospholipid bilayer with proteins, cholesterol, and carbohydrates	Controls entry/exit of substances
Nucleus	Control center; contains DNA	Regulates cell activities
Cytoplasm	Fluid and organelles between membrane and nucleus	Site of metabolic reactions
Cellular Junctions	Tight, desmosomes, gap junctions	Cell adhesion and communication

Types of Cells

Prokaryotic vs. Eukaryotic Cells

• All organisms are classified based on their cell type:

  • Prokaryotes: Do not have a nucleus or membrane-bound organelles (e.g., bacteria).

  • Eukaryotes: Have a nucleus and membrane-bound organelles (e.g., plant, animal, fungal, and protist cells).

Plant Cells vs. Animal Cells

• Both are eukaryotic but have differences:

  • Plant cells: Have cell walls, chloroplasts, and large central vacuoles.

  • Animal cells: Lack cell walls and chloroplasts, have smaller vacuoles.

Example: Plant cells can perform photosynthesis due to chloroplasts; animal cells cannot.

Review

• Common features of all cells: plasma membrane, hereditary material, cytoplasm, nutrient/energy acquisition

• Cell anatomy: three main regions (plasma membrane, nucleus, cytoplasm)

• Plasma membrane: fluid mosaic model, phospholipid bilayer, embedded proteins and other molecules

• Cellular junctions: tight, desmosomes, gap junctions

• Types of cells: prokaryotic vs. eukaryotic, plant vs. animal cells