Cell Biology: Structure, Function, and Theory

• These notes cover cell structure, function, cell theory, cell size, cell membranes, prokaryotic cells, eukaryotic cells, organelles, and surface specializations.

Cell Theory

• Cell theory is the basis of modern biology.
• The theory states that cells form the fundamental units of life.

Cell Size Measurement

• A micrometer (μm) equals one-thousandth of a millimeter.
• Van Leeuwenhoek first described small organisms visible through a microscope, referring to them as animalcules or beasties.
• Hooke was the first to sketch and name cells.

Development of the Microscope

• Light microscopes in biology labs magnify up to 400x with a resolution of about 200 nanometers.
• Electron microscopes can magnify 10,000x, and have a resolution of 50 picometers.

Origins of Cell Theory

• In 1839, Schleiden and Schwann proposed the core ideas of the modern cell theory.
• All organisms consist of one or more cells.
• The cell is the smallest unit that displays the properties of life.
• Each new cell comes from the division of another, pre-existing cell.
• Each cell transmits its hereditary information to its offspring.

What Is A Cell

• A cell is the smallest unit that demonstrates the properties of life.
• All cells possess: a plasma membrane, cytoplasm, and genetic information in the form of DNA.

Cell Structure

• Eukaryotic cells feature an internal structure divided into functional membrane-bound compartments, including a nucleus.
• Prokaryotic cells are small and simple, lacking a defined nucleus.

All Cells Have

• A plasma membrane regulates the passage of substances into and out of the cell.
• The DNA-containing region may be a nucleus (in eukaryotes) or a nucleoid region (in prokaryotes).
• Cytoplasm is a semifluid mixture containing cell components.

Cell Size

• The surface-to-volume ratio limits cell size because it restricts nutrient and waste transport.

Cell Membranes

• Lipid bilayers are composed of two layers of phospholipids organized with hydrophilic heads facing outward and hydrophobic tails facing inward.
• The bilayer has embedded or attached proteins that carry out membrane functions.

Basic Structure of Cell Membranes

• Cell membranes comprise a double layer of phospholipids and proteins, controlling what enters and exits the cell.

Cells in Review

• Every cell has a plasma membrane that forms a boundary between the cell's interior and the outside environment
• The cell's interior consists of cytoplasm and a central region containing DNA.

Prokaryotic Cells

• Bacteria and archaea, known as prokaryotes ("before the nucleus"), are simple life forms that are small and metabolically diverse.
• Structurally, bacteria and archaea look similar and are similar in size, but they differ in structure and metabolism.

General Prokaryote Body Plan

• The cell wall surrounds the plasma membrane and is made of peptidoglycan (bacteria) or proteins (archaea), and is coated with a sticky capsule.
• A flagellum is used for motion.
• Pili facilitate movement and sexual reproduction.

Microbial Mobs

• Most prokaryotes are single-celled, but some live in groups.
• A biofilm is a layer secreted by single-celled organisms, made of polysaccharides and glycoproteins
• Biofilms can include bacteria, algae, fungi, protists, and archaeans.

Prokaryotic Cells

• Bacteria and archaea are prokaryotic cells, so they have few or no internal membrane-enclosed compartments.
• Prokaryotic cells are the smallest and simplest cells.

Eukaryotic Cells

• Eukaryotic ("true nucleus") cells perform metabolism inside membrane-enclosed organelles.
• An organelle is a structure within a cell that performs a specialized function.

Two Main Types of Eukaryotic Cells

• Plants cells.
• Animal cells.

Cell Membrane in Plants

• Plant cell membranes directly touch the cell wall.
• Membranes press against the cell wall to help the cell maintain its shape.

Cytoplasm

• Cytoplasm is a jelly-like material in cells enclosed by the cell membrane.
• It provides a medium for chemical reactions.
• Organelles that carry out specific jobs are contained within the Cytoplasm.

The Nucleus

• The nucleus controls cell activities and contains DNA in chromosomes
• The nucleus is bounded by a nuclear envelope with nuclear pores that contains chromatin and a nucleolus.
• It is normally the biggest organelle.

The Nuclear Envelope

• Nuclear envelopes consist of two lipid bilayers pressed together into a single membrane.
• The outer bilayer is continuous with the ER.
• Nuclear pores in the envelope allow certain substances to pass through the membrane.
• It protects DNA from degradation.

Inside the Nucleus

• The genetic material (DNA) is located in the nucleus.
• In non-dividing cells, the DNA is in the form of chromatin.
• In dividing cells, DNA is condensed and wrapped as chromosomes made of proteins.

The Endomembrane System

• The endomembrane system is a series of interacting organelles between the nucleus and the plasma membrane.
• It makes lipids, enzymes, and proteins for secretion or insertion into cell membranes.
• Carries out other specialized cell functions.

The Endoplasmic Reticulum

• The endoplasmic reticulum (ER) is an extension of the nuclear envelope that forms a single folded compartment.
• It functions in cell product synthesis and transport.

Two Kinds of Endoplasmic Reticulum

• Rough ER (with ribosomes) folds polypeptides into their tertiary form
• Smooth ER (no ribosomes) creates lipids, breaks down carbohydrates and lipids, and detoxifies poisons.

Vesicles

• Vesicles are small organelles enclosed by a membrane.
• Vesicles hold or transport substances.
• Peroxisomes are vesicles with enzymes to break down hydrogen peroxide, alcohol, and other toxins.
• Vacuoles are vesicles for waste disposal.

Golgi Bodies

• The Golgi bodies has folded membranes for processing polypeptides and lipids from the ER delivered by the ER.
• These bodies package the finished products in vessels that are then moved to the plasma membrane or to lysosomes.

Lysosomes

• Lysosomes contain digestive enzymes that break down food, bacteria, and worn out cell parts through phagocytosis.
• They program for cell death (apoptosis)
• They also lyse and release enzymes to break down and recycle cell parts.

Lysosome Malfunction

• Lysosome malfunction leads to improper recycling with potentially devastating results.
• Different molecules are broken down by different lysosomal enzymes.
• An example is an enzyme breaks down gangliosides, a kind of lipid

Tay Sachs Disease

• Tay Sachs disease arises from genetic mutation of the lysosomal enzyme for breaking down gangliosides that build up in nerve cells.
• Affected children typically die by age five.

Cytoskeleton

• The cytoskeleton maintains cell shape, helps to move organelles around, and is made of proteins.
• Microfilaments consist of threadlike actin.
• Microtubules are tubelike structures made of tubulin.

Mitochondria

• Mitochondria are eukaryotic organelles that use aerobic respiration to create the energy molecule ATP.
• Consists of two membranes, inner and outer.
• Own DNA and ribosomes
• Thought to have evolved through endosymbiosis
• Buildup of hydrogen ions in the outer compartment of the membrane drives ATP synthesis.

Mitochondrion (pl)

• The mitochondria is the cell's "powerhouse" used for generating cellular energy (ATP).
• Active cells like muscle cells have more mitochondria.
• Both plants and animals have mitochondria.
• It is the site of cellular respiration (burning glucose).

Plastids

• Plastids are organelles in plants and algae for photosynthesis or storage.
• These include chromoplasts, amyloplasts, and chloroplasts.
• Chloroplasts are plastids specific to photosynthesis and resemble photosynthetic bacteria, suggesting they evolved through endosymbiosis.

Mitochondria

• Mitochondria originate from the cytoplasm of the egg cell during fertilization.
• You inherit your mitochondria from your mother

Chloroplasts

• Chloroplasts are only existent in producers (organisms containing chlorophyll).
• Chloroplasts utilize sunlight to make their own food (glucose) by storing its energy in the chemical bonds of sugars.

Fluid Filled Vacuoles

• Vacuoles are fluid filled sacs for storage.
• Vacuoles are diminutive or missing in animal cells.
• Plant cells have a big central vacuole.

The Central Vacuole

• A plant organelle, the central vacuole, occupies 50 to 90 percent of a cell’s interior
• It stores amino acids, sugars, ions, wastes, and toxins
• The fluid keeps plant cells firm.

Cell Surface Specializations

• A wall intervenes between a cell’s plasma membrane and its surroundings.

Specialized Structures

• Movement happens by flagellum of human sperm and a predatory amoeba.

Cilia and Flagella

• Cilia are shorter and numerous on cells.
• Flagella is is are are longer and fewer (usually 1-3) on cells.