Cell Structures

Overview of Cells

Cells are the fundamental units of life, forming the basis of structure and function in all living organisms. The study of cells is central to understanding biology at all levels.

• Cell Theory:

  • All organisms are composed of one or more cells.

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

  • Cells arise only from the division of preexisting cells.

• Examples: Bacteria, Archaea, Protists, Algae, Fungi, Animal cells, and Plant cells all demonstrate the diversity of cell types.

Microscopy

Light Microscopy

Microscopy allows scientists to observe structures not visible to the naked eye. The light microscope (LM) uses visible light passed through a specimen and glass lenses to magnify images of cells.

• Refraction: The bending of light as it passes through lenses, enabling magnification and focus of the image.

• Magnification: The ratio of an object's image size to its real size.

• Resolution: The clarity of an image, defined as the minimum distance two points can be distinguished as separate.

• Contrast: The difference in brightness between parts of the sample, which can be enhanced by staining or labeling cell components.

Light microscopes are limited in resolution, making it difficult to see many organelles. Techniques such as fluorescent labeling, confocal microscopy, and super-resolution microscopy can improve contrast and resolution.

Electron Microscopy

Electron microscopes use beams of electrons for much higher resolution imaging, allowing visualization of subcellular structures.

• Scanning Electron Microscope (SEM): Focuses electrons onto the surface of a specimen, producing detailed 3D images.

• Transmission Electron Microscope (TEM): Passes electrons through a specimen to study internal cell structures.

• Cryo-Electron Microscopy (Cryo-EM): Involves rapid freezing of samples to view molecular structures in a near-native state.

Micron (μm): A micrometer is one-millionth of a meter ().

Types of Cells

Prokaryotic vs. Eukaryotic Cells

Cells are classified into two major categories based on their structural characteristics: prokaryotic and eukaryotic.

• Common Features of All Cells:

  • Plasma membrane

  • Cytoplasm (cytosol and cellular contents)

  • Chromosomes (carry genetic information)

  • Ribosomes (synthesize proteins)

Plasma Membrane

Structure and Function

The plasma membrane is a selectively permeable barrier composed of a phospholipid bilayer with embedded proteins. It separates the internal environment of the cell from the external environment.

• Hydrophilic regions: Face outward toward aqueous environments.

• Hydrophobic regions: Face inward, away from water.

• Function: Regulates the passage of substances in and out of the cell.

Surface Area to Volume Ratio: As a cell increases in size, its volume grows faster than its surface area, limiting the efficiency of material exchange. This sets an upper limit on cell size.

Eukaryotic Cell Structure and Function

Nucleus

The nucleus is the information center of the cell, containing most of the cell's genetic material.

• Nuclear Envelope: Double membrane enclosing the nucleus, with nuclear pores for transport of molecules.

• Nuclear Lamina: Protein network that maintains nuclear shape.

• Chromatin: DNA and associated proteins (histones) organized into chromosomes.

• Nucleolus: Site of ribosomal RNA (rRNA) synthesis and ribosome assembly.

Ribosomes

Ribosomes are complexes of rRNA and protein that carry out protein synthesis.

• Free ribosomes: Suspended in cytosol; synthesize proteins for use within the cell.

• Bound ribosomes: Attached to the endoplasmic reticulum or nuclear envelope; synthesize proteins for export or for membranes.

Endomembrane System

The endomembrane system is a group of membranes and organelles in eukaryotic cells that work together to modify, package, and transport lipids and proteins.

• Components: Nuclear envelope, endoplasmic reticulum (ER), Golgi apparatus, lysosomes, vacuoles, and plasma membrane.

• Components are either continuous or connected via vesicles.

Endoplasmic Reticulum (ER)

• Smooth ER:

  • Synthesizes lipids

  • Metabolizes carbohydrates

  • Detoxifies drugs and poisons

  • Stores calcium ions

• Rough ER:

  • Studded with ribosomes

  • Synthesizes proteins (especially those for secretion or membranes)

  • Modifies proteins (e.g., glycosylation)

  • Distributes transport vesicles

  • Produces membranes

Golgi Apparatus

• Consists of flattened membranous sacs (cisternae)

• Modifies, sorts, and packages products from the ER

• Manufactures some macromolecules

• Ships materials in transport vesicles

Lysosomes

• Membranous sacs containing hydrolytic enzymes

• Digest macromolecules and cellular debris

• Enzymes function best at acidic pH (~5)

• Involved in phagocytosis (engulfing food particles) and autophagy (recycling cell components)

Vacuoles

• Large vesicles with diverse functions

• Food vacuoles: Formed by phagocytosis

• Contractile vacuoles: Pump excess water out of cells (in protists)

• Central vacuole: Found in plant cells; stores water and organic compounds

Mitochondria and Chloroplasts

These organelles are involved in energy conversion and have double membranes.

• Mitochondria: Sites of cellular respiration; convert chemical energy in food to ATP. Contain an outer membrane and a highly folded inner membrane (cristae).

• Chloroplasts: Sites of photosynthesis in plants and algae. Contain thylakoids (stacked into grana), stroma (internal fluid), and chlorophyll (light-absorbing pigment).

Peroxisomes

• Single-membrane organelles

• Carry out oxidation reactions, producing hydrogen peroxide ()

• Detoxify harmful substances and break down fatty acids

Cytoskeleton

Structure and Function

The cytoskeleton is a network of protein fibers that organizes cell structure, anchors organelles, and facilitates movement.

• Microtubules: Hollow rods made of tubulin; maintain cell shape, guide organelle movement, and separate chromosomes during cell division.

• Microfilaments (Actin Filaments): Thin rods of actin; support cell shape, enable cell movement, and drive cytoplasmic streaming.

• Intermediate Filaments: Fibers of intermediate thickness; provide structural support and anchor organelles (e.g., keratin).

Cell Motility

• Motor Proteins: Interact with cytoskeletal elements to produce movement.

• Cilia and Flagella: Microtubule-containing extensions that move cells or fluids; powered by the motor protein dynein.

• Pseudopodia: Temporary actin-based projections for movement in amoeboid cells.

Extracellular Components and Connections

Cell Walls

Plant cells, fungi, and some protists have cell walls external to the plasma membrane, providing protection, shape, and preventing excessive water uptake.

• Plant Cell Wall Structure:

  • Primary cell wall: Thin and flexible

  • Secondary cell wall: Thicker, between plasma membrane and primary wall (in some cells)

  • Middle lamella: Pectin-rich layer between adjacent cells

Extracellular Matrix (ECM) in Animal Cells

• Composed of glycoproteins (collagen, proteoglycans, fibronectin)

• ECM proteins bind to integrins (receptor proteins) in the plasma membrane

• Regulates cell behavior and gene activity

Cell Junctions

• Plasmodesmata (plants): Channels that connect plant cells, allowing passage of water, solutes, and some macromolecules.

• Tight Junctions (animals): Seal neighboring cells to prevent leakage of extracellular fluid.

• Desmosomes (animals): Anchor cells together into strong sheets.

• Gap Junctions (animals): Provide cytoplasmic channels for communication between adjacent cells.

Key Terms and Concepts

• Microscopy: Study of using microscopes to view small structures

• Phagocytosis: Cellular process of engulfing particles

• Autophagy: Cellular process of recycling components

• Refraction, Magnification, Resolution, Contrast: Key terms in microscopy

Summary Table: Major Eukaryotic Cell Organelles