Unit 3: Cell Biology

Cell Theory

All living things are composed of cells and all cells come from other cells

Robert Hooke

First person to look at a plant through a microscope

• ‘Cellulae’: Tiny compartments

Light Microscopes (LM)

• Earliest type of microscope

• Visible light passes through a sample and glass lenses

  • Objective lens

  • Ocular lens

• Light is bent to magnify the image of the specimen

• Image projected into your eye or a camera

Magnification

The increase in an object’s image size compared with its actual size

• Notation: LM 230X

Resolution

The measure of the clarity of an image. The ability to distinguish two nearby objects as being separate from each other

Electron Microscopes (EM)

• Focuses beams of electrons (instead of light) through a specimen sample

• Electromagnets bend the electron path and magnify the image

• EM images are always black and white

Scanning EM (SEM)

• Study the detailed architecture of a cell

• The sample is coated with a thin film of heavy metal (ex, gold)

• Electrons excite the gold atoms

• Electrons are scattered and detected by a device that projects the image onto a video screen

• 3D Image

Transmission EM (TEM)

• Electron beam passed through a very thin section of a specimen

• Stains containing heavy metals coat certain types of cellular structures

• Electrons scattered by the more densely stained parts

• Scattered electrons are detected, and an image is produced

Prokaryotic cells

The first cells to evolve and lived for about 1.5 billion years before eukaryotic cells evolved

• Bacteria and archaea

• Small, simple cell structure. About 1/10 the size of eukaryotic cells

• No internal membrane-bound structures

• Ribosomes are smaller and have a slightly different structure than in eukaryotes

Eukaryotic cells

Evolved from prokaryotic cells about 1.8 billion years ago

• All higher life forms such as plants, animals, and fungi

• Larger, more complex cells

Structures common to all life

Plasma membrane, ribosomes, cytosol, DNA, cytoplasm

Plasma membrane

Membrane that surrounds the cell. Made mainly out of a phospholipid bilayer

Ribosomes

Machinery for protein synthesis

Cytosol

Aqueous solution that fills the cell

DNA

One or more chromosomes

Cytoplasm

The entire contents of the inside of the cells, excluding the interior of the nucleus (which only Eukaryotes have)

Prokaryotic Cell structure

• Nucleoid

• Cell wall

• (Optional) Capsule

• (Optional) Flagella

Nucleoid

Region of the cell where the chromosome is coiled

Cell Wall

Rigid, chemically complex shell surrounding the plasma membrane. Protects the cell and maintains the cell’s shape

Capsule

Sticky outer coat around the cell wall. Glues the cell to surfaces or to other cells. Not in all prokaryotes

Flagella

Long projection that propels a cell through its environment. Not in all prokaryotes.

Animal cell

• Lysosome and centrosomes

• Flagella and cilia (optional)

  • Very very rare in plant cells

Organelles

Found in eukaryotic cells only. Membrane-bound structures that perform specific tasks. A cell may contain many copies of each organelle

Organelle Function

1. Genetic control of the cell - Nucleus and ribosomes

2. Manufacture, distribution, and breakdown of molecules - Endoplasmic reticulum, Golgi apparatus, lysosomes, vacuoles peroxisomes

3. Energy processing - Mitochondria, chloroplasts

4. Structural support, movement, and communication between cells - Cytoskeleton, plasma membrane, plant cell wall

Cellular metabolism

The chemical activities of the cell

Internal condition of an organelle

Each organelle maintains its own specific internal chemical conditions

• Optimal for enzyme function

• Organelles may have membrane-bound organelles

Plant Cell

• Rigid cell wall (made of cellulose)

• Plasmodesma

• Chloroplasts

• Large central vacuole

Plasmodesma

Cytoplasmic channels that link adjacent cells

Chloroplasts

Location of photosynthesis.

Large central vacuole

Stores water and chemicals

Function of Nucleus

• Contains the cell’s genetic instructions (DNA)

• Controls the cells activities by directing protein synthesis

DNA organization

Organized into chromosomes

• Associates with many proteins

• The proteins help coil the long strands of this to form a chromosome

• A human cell has 46 separate chromosomes

Chromatin

Complex of proteins and DNA. Appears as a diffuse mass within the nucleus

Nucleus Structure

• Nuclear envelope

• Nucleolus

Nuclear envelope

Double membrane enclosing the nucleus. Has pore proteins

Pore proteins

Regulate the flow of large molecules and connect the nucleus to the endoplasmic reticulum

Nucleolus

The location where ribosomal RNA (rRNA) is synthesized. Proteins made in the cytoplasm are brought into the nucleus to assemble with the rRNA to make ribosomes

Ribosomes

The cellular components that use instructions form the nucleus to build proteins (non-membranous organelle). Interact with mRNA to build a protein.

Free Ribosomes

Suspended in the cytosol. Proteins made here generally function in the cytosol.

Bounded ribosomes

Attached to the outside of the endoplasmic reticulum and nucleus. Makes proteins that will be exported from the cell.

Endomembrane System Definition

Internal membrane involved in most cellular functions. Synthesis, storage, distribution, and export of molecules.

Membranes

Divide the cell into functional compartments. May either be physically connected or linked by transport vesicles.

Vesicle

Sac made of membrane

Organelles in the Endomembrane System

Nucleus, nuclear envelope, endoplasmic reticulum (rough and smooth), Golgi apparatus, lysosomes, vesicles, vacuoles, plasma membrane

Endoplasmic Reticulum (ER) - def and function

• Network of flattened sacs and tubules.

• Largest component of the endomembrane system.

• Directly linked to the nuclear envelope (membranes are continuous).

• Vesicles bud from the ER to travel to other organelles

• Function: Major manufacturing site in the cell

Smooth Endoplasmic Reticulum (ER)

• Synthesis of lipids (oils, phospholipids, and steroids)

• Storage of calcium ions

• Detoxification

Rough Endoplasmic Reticulum (ER)

Proteins produced by ribosomes attached to the rough ER are often excreted from the cell (secretory proteins). Synthesizes new membrane fragments.

• Grows its own membrane by adding phospholipids and membrane proteins

• Completed membranes are transported as vesicles to the appropriate area of the cell

Secretory proteins

Proteins excreted from a cell. Produced by ribosomes attached to the rough ER.

Secretory Protein Creation Steps

1. The bound ribosome (attached to the Rough ER) grows a polypeptide inside the Rough ER as specified by the mRNA.

2. Inside the Rough ER the polypeptide is folded and sugar chains are added, turning it into a glycoprotein

3. The membrane around the glycoprotein surrounds it and buds off, turning into a transport vesicle

4. Secretory protein inside the transport vesicle is sent to the Golgi Apparatus for further processing

Golgi Apparatus

Stack of unconnected flattened sacs. A cell may contain hundreds of this organelle (cells active in protein excretion have more)

Function of Golgi Apparatus

Warehouse and processing station for molecules produced by the Endoplasmic Reticulum

→ Carbohydrate portion of a glycoprotein may be modified

→ Molecular identification tag may be added

Once processed, products are transported to their destination

Processing in the Golgi Apparatus

1. Docking station (received as a transport vesicle from the ER)

2. Vesicle adds its membrane and contents to the sac

3. Products are modified as they travel from one sac to the next

4. Shipping side (sent as a transport vesicle)

Lysosome

Membrane-enclosed sac of digestive enzymes. The membranes and enzymes are made in the endomembrane system. Provides an acidic envrioment for its enzymes to function. Protects the rest of the cell from the acidic conditions (compartmentalization)

Digestive functions

Food, pathogens, recycling of damaged cell components

Lysosomal Diseases

Lysosomal enzymes are missing. Lysosomes become engorged because they cannot break down their contents. Interferes with cellular function (Tay-Sachs disease)

Vacuoles

Large vesicles with a variety of functions.

• Food

• Contractile vacuoles (collect water, main wheel expels the water, moving the cell)

• Plant and fungal digestion

• Plant seeds store reserves of proteins for starting growth

• Flower petals hold pigments to attract pollinators

• Plants store compounds that are poisonous or unpalatable to animals

Central vacuole

In plants, absorbs water allowing the cell to grow in size (more rigid). Stores vital chemicals and maybe toxic waste products.

Peroxisomes (def and func)

• Metabolic, membrane-bound compartment that does not originate in the endomembrane system

• Still unknown how they relate to other organelles

Function:

• Break down fatty acids to use as cellular fuel

• Detoxification of harmful compounds in your liver

Mitochondria

Carries out cellular respiration in all eukaryotic cells.

Mitochondria composition

• Enclosed by two membranes

  • Each is a phospholipid bilayer with embedded proteins

  • Inner membrane is called cristae

• The region between the inner and outer membranes is the intermembrane space

• The region inside the inner membrane is the mitochondrial matrix.

Cristae

Inner membrane in mitochondria. Inner membrane is highly folded to increases the membrane surface area to maximize ATP production.

Intermembrane space

Region between the inner and outer mitochondria membranes.

Mitochondrial Matrix

Region inside the inner mitochondria membrane. Contains mitochondrial DNA, ribosomes and enzymes that catalyze reactions of cellular respiration

Chloroplast Structure

• 2 membranes separated by a thin intermembrane space

• Inner membrane contains stroma and thylakoids

Photosynthesis

Coverts solar energy to chemical energy

Stroma

Thick fluid inside the inner membrane. Contains chloroplast DNA, ribosomes, and enzymes

Thylakoid

Network of interconnected membranous sacs. Chlorophyll is embedded in the membranes to trap solar energy.

Granum

Stack of thylakoids. Resembles stack of poker chips

Endosymbiont theory - overall

Mitochondria and chloroplasts were once small prokaryotes that began living inside larger cells:

Grounds for Endosymbiont Theory

Mitochondria and Chloroplasts closely resemble prokaryotic cells
• Single circular DNA molecule
• Ribosomes similar to those of prokaryotes
• Reproduce inside the cells in a similar way to prokaryotes

Endosymbiont theory - mitochondria

• Photosynthetic prokaryotes filled our atmosphere with oxygen

• A large cell engulfed a small cell that could use oxygen to produce large amounts of energy

  • Benefit to host: high levels of ATP production

  • Benefit to small cell: protection, large supply of nutrients

• Host and endosymbiont merge, eventually leading to a eukaryotic cell that contains mitochondria

Endosymbiont theory - chloroplasts

• Early eukaryotic cells engulfed a small photosynthetic prokaryote

• Provided the host with nutrients and energy, got protection and energy in return

• Lead to eukaryotic cells that contain chloroplast

Cytoskeleton

• Networks of protein fibers extending throughout the cells.

• Can interact with motor proteins:

  • Swimming and crawling motility of cells

  • Internal movement of cellular structures (ex, vesicles)

• Three kinds of fibers

• Present in all eukaryotic cells

Kinds of Cytoskeleton Fibers

Microtubules (all cells), microfilaments (most animal), intermediate filaments (all cells)

Microtubules

Straight hollow tubes composed of globular proteins called tubulins. Grow longer or shorter by addition/removal of tubulin proteins.

Tubulins

Globular proteins that make up microtubules. Each protein consists of two subunits.

Centrosome

Region in animal cells that microtubules grow out of

Microtubules function

• Supports and shapes the cell

• Track along which organelles can move with the help of motor proteins

• Guides the movement of chromosomes when cells divide

• Main component of flagella and cilia

Intermediate Filaments

• Fibrous proteins that supercoil into cables

• Permanent fixture in the cell (not made shorter or longer)

• Outer layer of your skin is made of dead cells packed full of intermediate filaments.

Function of Intermediate Filaments

Reinforce cell shape and anchor some organelles that should not move

Microfilaments (Actin Filaments)

• Rods composed of globular actin proteins, arranged as a twisted double chain

• Network in the cell membrane that supports the cell shape

  • Important in animal cells since they do not have a cell wall

Function of Microfilaments (Actin Filaments)

• Supports the cell shape

• Involved in cell movement

• Actin filaments and myosin motor proteins interact to make muscle cells contract and in amoeboid crawling movement

Cilia

Short numerous appendages protruding from the cell. Move together in a sweeping motion

Cilia Function

• Propel single-celled organisms

• Sweep other things, like line our trachea to sweep mucous out of our lungs

• Antennae for signal reception (non-motile)

Flagella

• Long tail-like appendage on cells (one to a few per cell)

• Propels a cell using a undulating whip-like motion

• Commonly found on animal sperm cells

Extracellular Matrix

• Holds cells together in tissues

• Protects and supports the plasma membrane

• Relays signals that affect gene expression across diff cells

• Directs the movement of embryonic cells

Structure of Extracellular Matrix

• Matrix of glycoproteins outside of the plasma membrane

• Most abundant glycoprotein is collagen

  • Collagen fibers are embedded in a network of small glycoproteins and polysaccharides

• Bound to the plasma membrane by proteins called integrins

Integrins

Proteins that bind the extracellular matrix to the plasma membrane

Tight Junction

Plasma membranes of adjacent animal cells are knit tightly together. Used to prevents fluid leakages between layers

Anchoring junction

Intermediate filaments fasten animal cells together into strong sheets. For tissues susceptible to stretching and mechanical stress

Gap Junction

Channels of pores that allow the flow of small molecules from one animal cell to another (ie, nutrients)

Types of Junctions between animal cells

Tight, anchoring, gap.

Plasma Membrane (Cell membrane)

• Flexible boundary between the living cell and its surrounding envrioment. Regulates the flow of material in and out of the cell.

• Proteins embedded in the lipid bilayer

  • Hydrophobic regions embedded in the membrane

  • Hydrophilic regions protrude into the aqueous solutions inside or outside the membrane

• Follows a fluid mosaic model structure and is selectively permeable

Fluid mosaic model

Diverse protein molecules suspended in a fluid phospholipid bilayer

Selective permeability

Some substances can cross the plasma membrane more easily then others

Diffusion

The tendency for particles of any substance to spread out into the available space. Net movement from a high concentration to low concentration until equilibrium is reached due to equal concentrations.Thermal energy allows movement. Particles move through air, water, and across membranes

Passive transport

Diffuse across a membrane with no energy investment. Molecules move down their concentration gradient. Substances move independently of each other

Substances that diffuse through cell membrane using passive transport

• Small non-polar molecules (O2 enters, CO2 leaves ← animals, reverse for plants). Can move directly through phospholipid bilayer

• Ions and polar molecules using simple transport proteins (can’t move through phospholipid bilayer). Must move down their concentration gradient

Osmosis

Diffusion of water across a membrane. Water passes through the membrane until the concentration of a solute is equal on both sides