Biology Cell Structure

Cell Structure and Function

• Living cytoplasm demands nutrients and produces wastes.
• Cells specialized in absorption utilize membrane modifications like microvilli to increase surface area per unit volume.

Cell Types

• There are two main types of cells: prokaryotic and eukaryotic cells.
• Prokaryotic cells lack a membrane-bound nucleus, are structurally smaller and simpler than eukaryotic cells, and are classified into two taxonomic domains: Bacteria and Archaea.

Cell Envelope

• The cell envelope includes:
• Plasma membrane: a lipid bilayer with embedded and peripheral proteins, which can form internal pouches (mesosomes).
• Cell wall: maintains the shape of the cell and is strengthened by peptidoglycan.
• Glycocalyx: a layer of polysaccharides on the outside of the cell wall, which protects the bacteria cell from drying out and resists the host's immune system.

Cytoplasm

• Cytoplasm is a semifluid solution containing water, inorganic and organic molecules, and enzymes.
• Ribosomes are particles with RNA- and protein-containing subunits that synthesize proteins.
• Mesosomes are plasma membrane folds into the cytoplasm.
• Inclusion bodies store nutrients for later use.
• The nucleoid is the location of a single, circular DNA molecule.

Endomembrane System

• The endomembrane system is a series of intracellular membranes that compartmentalize the cell, restricting enzymatic reactions to specific regions.
• It consists of:
• Nuclear envelope
• Membranes of endoplasmic reticulum
• Golgi apparatus
• Vesicles

Endoplasmic Reticulum

• The endoplasmic reticulum (ER) is a system of membrane channels and saccules (flattened vesicles) continuous with the outer membrane of the nuclear envelope.
• Rough ER:
• Is studded with ribosomes on the cytoplasmic side, synthesizing and modifying proteins.
• Forms vesicles that transport molecules to other parts of the cell, notably the Golgi apparatus.
• Smooth ER:
• Has no ribosomes.
• Synthesizes lipids.
• Is the site of various synthetic processes, detoxification, and storage.
• Forms vesicles that transport molecules to other parts of the cell, notably the Golgi apparatus.

Golgi Apparatus

• The Golgi apparatus consists of a stack of slightly curved saccules.
• It receives protein-filled vesicles from the rough ER and lipid-filled vesicles from the smooth ER.
• Enzymes within the Golgi apparatus modify carbohydrates on proteins.
• Proteins and lipids are sorted and packaged within the Golgi apparatus.
• Vesicles formed from the membrane of the outer face of the Golgi apparatus move to different locations in the cell, discharging their contents as secretions.

Lysosomes

• Lysosomes are membrane-bound vesicles produced by the Golgi apparatus.
• They contain powerful digestive enzymes and are highly acidic.
• Lysosomes digest large molecules that enter a cell by vesicle formation.
• They are involved in autodigestion, apoptosis (programmed cell death), and the digestion of engulfed bacteria in white blood cells.

Cilia and Flagella

• Cilia are short, move in coordinated waves, and are found in eukaryotes.
• Flagella are long, move like a propeller, and are found in both prokaryotes and eukaryotes.
• The structure of a flagellum includes a shaft with a ring of nine microtubule doublets anchored to a central pair of microtubules.
• Dynein side arms, composed of motor molecules, cause bending and movement of the flagellum in the presence of ATP.