Cell Organelles and Functions

Microscopes and Cell Theory

• Light microscopes allow scientists to study microorganisms, animal and plant cells, and some structures within cells
• Studies in the 1800s led to cell theory
• Cell theory says all living things are composed of cells
• Cell theory also says the cell is the basic structural and functional unit of life
• Cell theory further mentions that all cells come from other cells

Plasma Membrane

• A barrier that forms a flexible boundary between a living cell and its surroundings
• Phospholipids form a two-layered sheet called a phospholipid bilayer
• Hydrophilic heads face outward, exposed to water
• Hydrophobic tails point inward, and are shielded from water
• Membrane proteins are attached to the membrane surface or embedded in the phospholipid bilayer
• Some membrane proteins form channels or tunnels to shield ions and hydrophilic molecules as they pass through the hydrophobic center of the membrane
• Some proteins serve as pumps, using energy to actively transport molecules into or out of the cell

Prokaryotic vs Eukaryotic Cells

• Bacteria and archaea are prokaryotic cells
• All other forms of life are composed of eukaryotic cells
• Both prokaryotic and eukaryotic cells have these qualities:
• A plasma membrane
• One or more chromosomes
• Ribosomes
• Eukaryotic cells contain these qualities:
• A membrane-bound nucleus
• A variety of organelles
• Prokaryotes have a nucleoid and no true organelles
• The DNA of prokaryotic cells is coiled into a region called the nucleoid, but no membrane surrounds the DNA
• The surface of prokaryotic cells are surrounded by a chemically complex cell wall
• Prokaryotic cells also have these qualities:
• Have a capsule surrounding the cell wall
• Have short projections that help attach to other cells or the substrate
• Have longer projections called flagella that may propel the cell through its liquid environment

Eukaryotic Cells

• Eukaryotic cells have structures and organelles that perform four basic functions
• The nucleus and ribosomes are involved in the genetic control of the cell
• The endoplasmic reticulum, Golgi apparatus, lysosomes, vacuoles, and peroxisomes are involved in the manufacture, distribution, and breakdown of molecules
• Mitochondria in all cells and chloroplasts in plant cells are involved in energy processing
• Structural support, movement, and communication between cells are functions of the cytoskeleton, plasma membrane, and cell wall
• The internal membranes partition a eukaryotic cell into compartments that perform different functions
• Cellular metabolism, the many chemical activities of cells, occurs within organelles
• Almost all organelles and other structures found in animal cells can be in plant cells

Nucleus and Ribosomes

• The nucleus contains most of the cell's DNA
• The nucleus controls the cell's activities by directing protein synthesis by making messenger RNA (mRNA)
• DNA is associated with many proteins in structures called chromosomes
• The nuclear envelope enclosing the nucleus, consisting of a double membrane and has pores that allow material to flow in and out of the nucleus
• The nuclear envelope is attached to a network of cellular membranes called the endoplasmic reticulum
• The nucleolus is a prominent structure in the nucleus
• The site of ribosomal RNA (rRNA)synthesis is also a structural component of ribosomes, and occurs in the nucleolus.

Ribosomes

• Involved in the cell's protein synthesis
• Ribosomes are synthesized from rRNA produced in the nucleolus
• Cells that synthesize large amounts of protein will have a large number of ribosomes
• Free ribosomes are suspended in the cytoplasm
• The role of free ribosomes is to produce proteins that function within the cytoplasm
• Bound ribosomes are attached to the endoplasmic reticulum (ER) associated with the nuclear envelope
• The role of bound ribosomes involves proteins packed in certain organelles or exported from the cell

Endomembrane System

• Many of the membranes within a eukaryotic cell system are part of the endomembrane system
• Some of these membranes are physically connected and some are related by the transfer of membrane segments by tiny vesicles, which are sacs made of membrane
• Many of these organelles work together with synthesis, storage, and export of molecules
• The endomembrane system includes
• The nuclear envelope, endoplasmic reticulum (ER), Golgi apparatus, lysosomes, vacuoles, and the plasma membrane

Endoplasmic Reticulum

• There are two kinds of endoplasmic reticulum
• Smooth and rough
• Smooth ER lacks attached ribosomes
• Rough ER lines the outer surface of membranes
• Even though they are physically interconnected, smooth and rough ER differ in structure and function
• Smooth ER is involved in a variety of diverse metabolic processes
• Smooth ER produces enzymes important in the synthesis of lipids, oils, phospholipids, and steroids
• Other enzymes help process drugs, alcohol, and other potentially harmful substances
• Some smooth ER helps store calcium ions
• Rough ER makes both additional membrane for itself and proteins destined for secretions.

Golgi Apparatus

• Serves as a molecular warehouse and finishing factory for products manufactured by the ER
• Products travel in transport vesicles from the ER to the Golgi apparatus
• One side of the Golgi apparatus functions as a receiving dock for the product and the other as a shipping dock
• Products are modified as they go from one side of the Golgi apparatus to the other and travel in vesicles to other sites

Lysosomes

• A membranous sac containing digestive enzymes
• The membrane serves to safely isolate these potent enzymes from the rest of the cell
• The enzymes and membrane are produced by the ER and transferred to the Golgi apparatus for processing
• Lysosomes help digest food particles engulfed by a cell
• A food vacuole binds with a lysosome and digests the food
• The resulting nutrients are then released into the cell
• Lysosomes also help remove or recycle damaged parts of a cell
• The damaged organelle is first enclosed in a membrane vesicle
• A lysosome fuses with the vesicle, dismantles its contents, and breaks down the damaged organelle

Vacuoles

• Large vesicles that have a variety of functions
• Some protists have contractile vacuoles that help to eliminate water from the protist
• In plants, vacuoles may:
• Have digestive functions
• Contain pigments
• Contain poisons that protect the plant

Energy-Converting Organelles

• Mitochondria are the organelles that carry out cellular respiration in nearly all eukaryotic cells
• Cellular respiration converts the chemical energy in foods to chemical energy in ATP (adenosine triphosphate)
• Mitochondria have two internal compartments:
• The intermembrane space is the narrow region between the inner and outer membranes and the mitochondrial matrix, which contains the mitochondrial DNA, ribosomes, and many enzymes that catalyze some of the reactions of cellular respiration
• Chloroplasts are the photosynthesizing organelles of all photosynthesizing eukaryotes
• Photosynthesis is the conversion of light energy from the sun to the chemical energy of sugar molecules
• Chloroplasts are partitioned into compartments
• Between the outer and inner membrane is a thin intermembrane space
• Inside the inner membrane is a thick fluid called stroma that contains the chloroplast DNA, ribosomes, and many enzymes
• A network of interconnected sacs called thylakoids are also inside the inner membrane
• In some regions, thylakoids are stacked like poker chips
• Each stack is called a granum, where green chlorophyll molecules trap solar energy

Mitochondria, Chloroplasts, and Endosymbiosis

• Mitochondria and chloroplasts have DNA and ribosomes
• The structure of this DNA and these ribosomes is very similar to that found in prokaryotic cells
• The endosymbiont theory proposes that mitochondria and chloroplasts were formerly small prokaryotes
• These prokaryotes began living within larger cells

Cytoskeleton

• Cells contain a network of protein fibers, called the cytoskeleton, which functions in structural support and motility
• Motility and cellular regulation result when the cytoskeleton interacts with proteins called motor proteins
• The cytoskeleton is composed of three kinds of fibers
• Microfilaments (actin filaments) support the cell's shape and involved in motility
• Intermediate filaments reinforce cell shape and anchor organelles
• Microtubules (made of tubulin) give the cell rigidity and act as tracks for organelle movement

Cilia and Flagella

• While some protists have flagella and cilia that are important in locomotion, some cells of multicellular organisms have them for different functions
• Cells that sweep mucus out of our lungs have cilia
• Animal sperm are flagellated

Extracellular Matrix

• Animal cells synthesize and secrete an elaborate extracellular matrix (ECM)
• The extracellular matrix helps hold cells together in tissues and protects and support the plasma membrane

Cell Junctions

• Adjacent cells communicate, interact, and adhere through specialized junctions between them
• Tight junctions prevent leakage of extracellular fluid across a layer of epithelial cells
• Anchoring junctions fasten cells together into sheets
• Gap junctions are channels that allow molecules to flow between cells

Cell Walls

• A plant cell, but not an animal cell, has a rigid cell wall
• A cell wall protects and provides skeletal support that helps keep the plant upright against gravity
• A cell wall is primarily composed of cellulose
• Plant cells have cell junctions called plasmodesmata that serve in communication between cells

This quiz covers the vital roles of cell organelles such as the Golgi apparatus, lysosomes, endoplasmic reticulum, and nucleus. It explores protein synthesis, organelle functions, and cell structures of plant and animal cells. Test your knowledge of cellular biology.