Biology chapter 4

Cell structure

Organisms are organized in a hierarchy

Cells

⬇

Tissues

⬇

Organ

⬇

Organ system

⬇

Organism

Cells

The basic structural, functional, and biological unit of all known living organisms

• The basic unit

Tissues

An intermediate level of biological organization consisting of a group of structurally and functionally similar cells working together to perform a specific task

• Are made of cells with a common function

Organ

A specialized structure composed of two or more different types of tissues that work together to perform a specific, complex function for the organism

• Formed by several tissues

Organ system

A group of organs that work together to perform vital, complex body functions

• Made up of organs working together

Organism

An individual living system—such as a plant, animal, fungus, or microorganism—that can carry out all basic life processes independently

• Composed of multiple organ systems

Microscopes

Cells are too small to be seen with the naked eye

• These make it possible to see small cells

• Ex: light microscope, electron microscope

Light microscope

An optical instrument that uses visible light and glass lenses to magnify and view small biological specimens like cells and tissues

Magnification

The process of enlarging an object in appearance

• The ratio of an object's image size to its real, actual size, making a specimen appear larger than it is

Resolution/Resolving power

The ability of a microscope to distinguish two adjacent structures as seperate; the higher the resolution, the better the clarity and detail of the the image

Stains

Transparent objects (like cells) are treated with chemical stains to
distinguish different parts

• Chemical: a dye or reagent used to color microscopic structures, cells, or tissues to increase visual contrast and highlight specific components

• Fluorescent: specialized chemical dyes or tagged molecules that bind to specific cellular components or macromolecules and emit visible light when excited by a specific wavelength of light

Electron microscope

An imaging tool that uses a beam of electrons instead of visible light to view ultra-small specimens at a much higher resolution and magnification

Transmission electron microscope

An advanced imaging tool that passes a beam of electrons through an ultra-thinly sliced specimen to view its internal structures at a nanometer or atomic scale

• show fine detail within cells

Scanning electron microscope

An instrument that uses a focused beam of electrons to scan the surface of a specimen, creating detailed, three-dimensional surface images

• provide 3-D exterior views

Cell theory

A significant principle of biology proposed by Schleiden and Schwann in the 1830’s

1. Cells are basic units of life

2. All living organisms are made of cells

3. All cells come from pre-existing cells

Cells have 4 components

1. Plasma membrane: separates the cell’s interior from the outside

2. Cytoplasm: inside of cell (cytosol) where organelles are found

3. DNA: the genetic material

4. Ribosomes: synthesize proteins

Characteristics of prokaryotes

• No nucleus or organelles

• Have a cell wall made of peptidoglycan

• Prokaryotes are similar to the the first cells

• Prokaryotes divided into domains: Archaea & Bacteria

General structure of prokaryotic cells

• One chromosome

• Ribosomes

• The cell membrane surrounded by a cell wall (made of peptidoglycan)

Prokaryotic cells are smaller than eukaryotic cells

Reasons for small size of prokaryotic cells:

1. Surface area to volume ratio is more favorable for moving material in and out of the cell

2. They lack organelles found in eukaryotes

Surface area–to–volume ratio

A mathematical comparison of a cell's outer membrane area to its internal volume, determining how efficiently it can exchange materials with its environment

• As cells get bigger, volume increases faster than surface
  area

• High SA:V Ratio (Small Cells)

• Low SA:V Ratio (Large Cells)

Eukaryotic cells

• Have a nucleus

• More complex than prokaryotic

• Highly compartmentalized (membrane-bound organelles and endomembrane system)

• Have a cytoskeleton for support and to maintain cellular structure

Plasma membrane of Eukaryotes

A selectively permeable phospholipid bilayer that surrounds the cell, separates internal processes from the outside environment, and regulates the movement of substances in and out

• Phospholipid bilayer with embedded proteins

Cytoplasm

The entire region of a cell between the plasma membrane and the nuclear envelope, consisting of the gel-like cytosol, suspended organelles, and the cytoskeleton

• 70% of the cytoplasm is water but it has semi-solid consistency due because of proteins within

Nucleus

The membrane-bound organelle in eukaryotic cells that houses the cell's genetic material (DNA) and serves as the cell's control center

• Only one per cell

• The largest organelle (site of genetic information)

Ribosomes

Non-membrane-bound cellular structures composed of ribosomal RNA (rRNA) and proteins that function as the sites of protein synthesis in all living cells

• Made of two different subunits

• Made of RNA (rRNA) and proteins

• During protein synthesis, ribosomes assemble amino acids into proteins

Mitochondrion

Membrane-bound organelles in eukaryotic cells that produce adenosine triphosphate (ATP) through cellular respiration

• Site where energy is converted to ATP (cell energy)

• Inner membrane is folded: area inside is the mitochondrial matrix

• Extract energy from nutrients; convert energy into ATP

Peroxisomes

A small, single membrane-bound organelles in eukaryotic cells that specialize in lipid metabolism and the detoxification of harmful reactive oxygen species

• Small round organelles

• Break down fatty acids and amino acids

• Peroxisomes detoxify poisons

Comparing animal/plant cells

Animal cells have (plant cells do not):

• A centrosome

• Lysosomes

Plant cells have (animal cells do not):

• Cell wall

• Chloroplasts

• Large central vacuole

Plant cell walls

The protective structure outside the plasma membrane

• A tough, rigid extracellular matrix located outside the plasma membrane that provides structural support, shape, and protection against osmotic lysis

• Plant cell walls different than bacteria because they are made up of cellulose rather than peptidoglycan

Chloroplasts

A double-membrane organelle found in plants and algae that carries out photosynthesis, converting light energy, water, and carbon dioxide into sugars

• Double-membrane organelles; have their own ribosomes
  and DNA like mitochondria

• Site of photosynthesis

Vacuole

Plant cells have a large central vacuole

• Regulates water concentration and contributes to cell
  expansion

• a large central vacuole maintains turgor pressure in plants; in animal cells, vacuoles are smaller and more numerous

• a membrane-bound sac inside eukaryotic cells that handles storage, waste disposal, and structural support

Endosymbiotic theory

Theory states that eukaryotic organelles—specifically mitochondria and chloroplasts—evolved from free-living prokaryotic cells that were engulfed by a larger host cell

• It is hypothesized that mitochondria and chloroplasts originated from prokaryotes

Evidence for endosymbiotic theory

• Mitochondria have their own DNA and ribosomes. The DNA is similar to prokaryotic DNA

• The size of these organelles is similar to that of prokaryotes

• Enzymes and transport mechanisms also similar to prokaryotic cells

The endomembrane system

A network of interacting membranes and organelles in eukaryotic cells that work together to modify, package, and transport lipids and proteins

Includes:

• Nuclear membrane

• Lysosomes

• Vesicles

• Endoplasmic reticulum

• Golgi apparatus

Lysosomes

In animal cells contain digestive enzymes

• The enzymes breakdown biomolecules and old organelles

• Membrane-bound organelles packed with hydrolytic enzymes that function as the cell’s recycling and waste disposal system

Phagocytosis

Aspecific type of endocytosis where a cell engulfs large, solid particles—such as bacteria, cell debris, or whole cells—by extending its plasma membrane around them

Endoplasmic reticulum (ER)

Connected membranous sacs

• Makes proteins (rough ER) and synthesizes lipids (smooth ER)

• The membrane of the ER is continous with the nuclear envelope

• A continuous network of membrane-enclosed sacs and tubules within eukaryotic cells that manufactures, folds, and transports proteins and lipids

Rough endoplasmic reticulum

• Ribosomes in the ER manufacture proteins

• New proteins are modified (folding or adding side chains) in the lumen of the rER

• Modified proteins are incorporated into cellular membranes or secreted from the cell (example: protein hormones, enzymes)

Rough endoplasmic reticulum (rER)

• The rER also makes phospholipids for cells membranes

• Proteins that do stay in the rER travel to their destinations via transport vesicles that bud from the rER’s membrane

• A membrane-bound eukaryotic organelle studded with ribosomes that synthesizes and folds proteins destined for export or membrane insertion

Smooth endoplasmic reticulum (sER)

Is continuous with the rER but doesn’t have any ribosomes on its
surface

• A membrane-bound organelle made of a network of tubular sacs that lacks ribosomes on its surface

• Synthesizes lipids, phospholipids, and steroid hormones, carbohydrates

Functions of the sER

Synthesis of:

• Carbohydrates

• Lipids

• Steroid hormones

• Detoxification of medications and poison

• Storage of Ca++

Golgi apparatus

Lipids or proteins within vesicles are sorted, packaged, and tagged to send to the right place

• A membrane-bound organelle in eukaryotic cells that modifies, sorts, and packages proteins and lipids received from the endoplasmic reticulum (ER) for transport

Golgi apparatus features

• The receiving side of the Golgi apparatus is the cis face; the opposite side is the trans face

• Vesicles from the ER fuse with the cis face and empty their contents into the lumen of the Golgi apparatus

• As the proteins and lipids travel through the Golgi, they are modified and sorted

• This often involves adding short chains of sugar molecules

The cytoskeleteon

A network of protein fibers with several functions:

• Support against tension

• Support against compression

• Support organelles

• Serve as transport highways for some molecules

• Enables cells in organisms to move

a dynamic network of protein fibers extending throughout the cytoplasm that gives a cell its structural support, shape, and ability to move.

Three components of cytoskeleton

Different sizes and functions

• Intermediate filaments: throughout the cell hold organelles in place

• Microfilaments: around the cell's inner edge resist tension

• Microtubules: maintain cell shape resisting compressive forces

Microfilaments

The thinnest components of the cell's cytoskeleton

• Involved in movement
  – Whole cell or internal parts

• Determine & stabilize shape

• Made from actin monomers

Microtubules

The largest hollow, rigid protein tubes of the cell's cytoskeleton

• Form rigid internal skeleton for some cells

• Provide framework for motor proteins to move within cell

• Made of tubulin dimers

Cilia & Flagella (structures for movement)

Microtubule-based cellular projections that extend from the plasma membrane to facilitate cell motility or move fluids across the cell surface

• Made of microtubules (only in eukaryotes)

• Cilia shorter and more numerous

• Cilia only in eukaryotes

• Flagella: a long, whip-like cellular appendage primarily responsible for locomotion in various prokaryotic and eukaryotic cells

Extracellular structures - plant cell wall

A tough, rigid extracellular matrix that surrounds the plasma membrane, providing structural support, shape, and protection against osmotic lysis

• Support

• Barrier to infection

• Plasmodesmata connect cells

• Made of cellulose

Extracellular matrix in animals

complex meshwork of proteins and carbohydrates secreted by animal cells that lies outside the plasma membrane to provide structural support and cell signaling

3 components:

• Collagens & other fibrous
  proteins

• Glycoproteins called
  proteoglycans

• Linking proteins

Intercellular junctions

Specialized contact points between the plasma membranes of adjacent cells that allow them to stick together, form barriers, or communicate

• Provide direct channels of communication between cells

• Plants and animals do this differently

Plasmodesmata

Channels that connect cells and allow materials to move from cell to cell

• Microscopic channels that pass through the cell walls of adjacent plant cells, connecting their cytoplasm to allow direct communication and transport of molecules

Tight junctions

Watertight, plasma-membrane seals between adjacent animal cells that prevent fluid and molecules from leaking through the intercellular space

• Found in epithelial cells of internal organs and cavities

• Only in animal cells

Desmosomes

Specialized cell-to-cell adhesion structures that act like spot welds to hold adjacent cells together under mechanical stress

• Short proteins in the plasma membrane

• Join adjacent cells in tissues that stretch (eg. heart, lungs, muscles)

• Only in animal cells

Gap junctions

Specialized protein channels that directly connect the cytoplasm of adjacent animal cells, allowing for the free passage of ions, water, and small molecules

• Connect animal cells