Cell

The basic structural and functional unit of a living organisms.

Plasma Membrane

Defines the extent of the cell and is the flexible outer boundary.

The lipid bilayer and proteins constantly changing in fluid mosaic.

Plays a dynamic role in cellular activity.

Separates intracellular fluid from extracellular fluid.

Contains thousands of substances (amino acids, sugars, fatty acids, vitamins, hormones, salts, waste products)

Allows cell to take in what it needs, when it needs it, and keep out what it does not

What are the 3 basic parts of a human cell?

Plasma membrane

Cytoplasm

Nuclues

Cytoplasm

Intracellular fluid

Nucleus

Control Center

Phospholipid (lipid bilayer)

75% of membrane lipids

Phosphate heads are polar and hydrophilic

Fatty acid tails are non-polar and hydrophobic

Glycolipids

5% of membrane lipids

Lipids with polar sugar groups on outer membrane surface

Fatty acid tails are non-polar

Cholesterol

20% of membrane lipids

Increases membrane stability

Hydroxyl group is polar

fused ring system is non-polar

Membrane Proteins

Allow communication with environment

1/2 of the mass of plasma membrane

Most specialized membrane functions

Some float freely, others are "tethered" to intracellular structures that make up the cytoskeleton and are restricted in their movement.

2 types - Integral and peripheral

Integral Proteins

Firmly inserted into the lipid bilayer

Some protrude from one membrane face only, but most are transmembrane that span the entire membrane and protrude on both sides

Have both hydrophobic and hydrophilic regions

Can interact with lipid tails and water

Function as transport proteins (channels and carriers), enzymes, or receptors

Peripheral Proteins

Loosely attached to integral proteins

Include filaments on intracellular surface for membrane support

Function as enzymes; motor proteins for shape change during cell division and muscle contraction; cell-to-cell connections

Six Functions of Membrane Proteins

• Transport
• Receptors for signal transduction
• Attachment to cytoskeleton and extracellular matrix
• Enzymatic activity
• Intercellular joining
• Cell-cell recognition

Lipid Rafts

~ 20% of outer membrane surface

Contain phospholipids, sphingolipids and cholesterol

More stable; less fluid that rest of membrane

May function as stable platforms for cell-signaling molecules, membrane invagination, and other functions

The Glycocalyx

-"Sugar covering" at cell surface

Lipids and proteins with attached carbohydrates (sugar groups)

- Every cell has different patterns of sugars

Specific biological markers for cell to cell recognition

Allows immune system to recognize "self" and "non-self"

Cancerous cells change it continuously

Cell Junctions - Tight Junctions

Adjacent integral proteins fuse to form impermeable junction encircling cell

Prevents fluids and most molecules from moving between cells

Tight junctions can be found between epithelial cells in the GI tract

Cell Junctions - Desmosomes

"Rivets" or "spot-welds" that anchor cells together at plaques (thickening on plasma membrane)

Linker proteins between cells contain plaques

Keratin filaments extend through cytosol to opposite plaque giving stability to cell

Reduces possibility of tearing

They are abundant in tissues subjected to great mechanical stress

Cell Junctions - Gap Junctions

Communication between adjacent cells

Transmembrane proteins form pores (connexons) that allow small molecules to pass cell to cell

For spread of ions, simple sugars, and other small molecules between cardiac or smooth muscle cells

Found in excitable tissues, such as the heart and smooth muscle to synchronize electrical activity and contraction

Passive Process Membrane Transport

Diffusion and filtration

No ATP required

Substances move down its concentration gradient

Diffusion

The tendency of molecules or ions to move from an area where they are in higher concentration to an area where they are in lower concentration.

Three forms -

simple diffusion

carrier and channel mediated facilitated diffusion

Osmossis

Simple Diffusion

Non-polar and lipid soluble (hydrophobic) substances diffuse directly through the lipid bilayer.

E.g. oxygen, carbon dioxide, fat-soluble vitamins

Facilitated Diffusion

Certain molecules and ions are transported passively even though they are unable to pass through the lipid bilayer.

They move through the membrane by binding to protein carriers in the membrane and is ferried across or by moving through water filled protein canals.

E.g. glucose, amino acids, and ions

Channel Mediated Facilitated Diffusion

Aqueous channels formed by transmembrane proteins

Selectively transport ions or water

Two Types -

Leakage Channels - always open

Gated Channels - Controlled by chemical or electrical signals

Osmosis (Passive Process)

Movement of solvent across selectively permeable membrane (water)

Water diffuses through plasma membranes though lipid bilayer/through specific water channels called aquaporins

Occurs when water concentration different on the two sides of a membrane

1. Osmolarity

2. Hydrostatic Pressure

3. Osmotic Pressure

1. Measure of total concentration of solute particles

2. The back pressure exerted by water against the membrane

3. The tendency of water to move into the cell by osmossis

Does osmosis cause the cell to shrink?

Yes, because change in cell volume disrupts cell function, especially in neurons.

Isotonic solutions

Have the same concentrations of non-penetrating solutes as those found in cells. Cells will retain their normal size and shape.

Hypertonic Solutions

Have a higher concentration of non-penetrating solutes as those found in cells. Cells lose water by osmosis and shrink. the solution contains a higher concentration of solutes that are present inside the cell.

Hypotonic Solutions

Solutions are more dilute (contain a lower concentration of non-penetrating solutes) than cells.

Cells take on water by osmosis until they become bloated and burst (lyse).

Lower concentration of solutes that are present inside cells.

Membrane Transport - Active Processes

Two types -

Active Transport

Vesicular Transport

Both require ATP to move solutes across a living plasma membrane because

-Solute is too large for chemicals

-Solute not lipid soluble

-Solute not able to move down concentration gradient

Active Transport-

Primary Active Transport

Requires carrier proteins (solute pumps)

Bind specifically and reversibly with substance

Moves against concentration gradient

Requires energy directly from ATP hydrolysis

Active Transport-

Secondary Active Transport

Requires carrier proteins (solute pumps)

Bind specifically and reversibly with substance

Moves against concentration gradient

Requires energy indirectly from ionic gradients created by primary active transport

Sodium - Potassium Pump

Located in all plasma membranes

Involved in primary and secondary active transport of nutrients and ions

Pumps against Na+ and K+ gradients to maintain high intracellular K+ concentration and high extracellular Na+ concentration

Maintains electrochemical gradients essential for functions of muscle and nerve tissues

It drives Na+ out of the cell against a steep concentration gradient and pumps K+ back into the cell.

It is crucial for cardiac, skeletal muscle, and neuron function.

Secondary Active Transport

Cotransport

Always transports more that one substance at a time

1 - Symport

2 - Antiport

1 - Substances transported in the same direction

2 - Substances transported in the opposite directions

Vesicular Transport

Transport of large particles, macromolecules, and fluids across membrane in membranous sacs called vesicles

Requires cellular energy (ATP)

Vesicular Transport Functions

1) Exocytosis

2) Endocytosis

3) Transcytosis

4) Vesicular Trafficking

1) transport out of cell

2) transport into cell (phagocytosis, pinocytosis, receptor-mediated endocytosis

3) Transcytosis - transport into, across, then out of the cell

4) transport from one area or organelle in cell one cell to another

Phagocytosis (Endocytosis)

Pseudopods engulf solids (often large or solid material, such as bacteria, cell debris, or inanimate particles) and bring them into the cell's interior.

These cells (phagocytes) are experts at ingesting and disposing of bacteria, other foreign substances, and dead tissue cells.

Pinocytosis (Endocytosis)

Fluid phase endocytosis (cell drinking)

Plasma membrane unfolds, bringing extracellular fluid and dissolved solutes inside of cell, then fuses with an endosome.

A routine activity in most cells

Most cells utilize to "sample" environment

Nutrient absorption in the small intestine

Receptor-mediated endocytosis

Allows specific endocytosis and transcytosis

cells use to concentrate materials in limited supply

Clathrin-coated pits provide main route for endocytosis and transcytosis

Uptake of enzymes, low-density lipoproteins, iron, insulin, and viruses, diphtheria, and cholera toxins

Exocytosis

Usually activated by cell surface signal or change in membrane voltage

Substances enclosed in secretory vesicle

v-snares - on vesicle

t-snares - on membrane and bind

Functions - hormone secretion, neurotransmitter release, mucus secretion, ejection of wastes

Generation of Resting Membrane Potential (RMP)

Produced by separation of oppositely charged particles (voltage) across me

membrane in all cells

Cells described as polarized

Typically ranges from -50 to -100 millivolts depending on cell type

Selective Diffusion Establishes RMP

In many cells Na+ affects RMP

Attracted into cell due to negative charge

Potassium's role in Resting Membrane Potential

The resting membrane potential is largely determined by K+ because at rest the membrane is much more permeable to K+ than Na+.

Sodium's role in Resting Membrane Potential

Sodium also contributes to a resting membrane potential because sodium is strongly attracted to the cell interior by its concentration gradient.

Ligands (1st messenger)

Binds to the receptor, changes shape and activates.

G Protein

A regulatory molecule that acts as a middleman or relay to activate (or inactivate) a membrane bound enzyme or ion channel.

Second Messengers

An intracellular chemical signal which connects plasma membrane events to the internal metabolic machinery of the cell.

Cyclic AMP and Ion Calcium

Typically activate protein kinase enzymes, which transfer phosphate group from ATP to other proteins.

Cytoplasm

"Cell forming material"

The cellular material between the plasma membrane and the nucleus.

Made up of 3 elements - the cytosol, organelles, and inclusions

Cytosol

The viscous, semi-transparent fluid in which the other cytoplasmic elements are suspended.

Organelles

The metabolic machinery of the cell. Each carries out a specific function for the cell. Some synthesize proteins, others package those proteins, etc.

Inclusions

Chemical substances that may or may not be present depending on the cell type. Such as stored nutrients, glycogen granules in the liver and muscle cells, lipid droplets in fat cells, melanin in skin and hair cells.

Mitochondria

Threadlike or lozenge-shaped membraneous organelles. They squirm, elongate, and change shape almost continuously. They are the power plants of the cells, providing most of its ATP supply.and ribosomes

Enclosed by 2 membranes.

The outer membrane is smooth and featureless and the inner membrane folds inward, forming shelf like cristae (crests) that protrude into the matrix, the gel-like substance.

They contain their own DNA, RNA, and ribosomes that can reproduce themselves.

Ribosomes

Small, dark staining granules composed of proteins and a variety of RNAs. Each has 2 globular subunits that fit together like the body and cap of an acorn. The are the site of protein synthesis.

Some float freely, some are attached to membranes.

Free Ribosomes

Float freely in the cytoplasm. They make soluble proteins that function in the cytosol, as well as those imported into mitochondria and some other organelles

Membrane Bound Ribosomes

Attached to membranes, forming a complex. They synthesize proteins destined either for incorporation into cell membranes or lysosomes, or for export from cell.

Endoplasmic Reticulum

An extensive system of interconnected tubes and parallel membranes enclosing fluid filled cavities. It coils and twists through the cytosol, with continuous outer nuclear membrane and accounts for about half of the cell's membrane.

Rough Endoplasmic Reticulum

The external surface is studded with ribosomes. Proteins assembled on these ribosomes thread their way into the fluid-filled interior of the ER cisterns. When complete, the newly made proteins are enclosed in vesicles for their journey to the Golgi apparatus where they undergo further processing.

Smooth Endoplasmic Reticulum

Consists of tubules arranged in a looping network. Its network does not play a role in protein synthesis.

Enzyme catalyze reactions involved with the following:

metabolize lipids

synthesize steroid-based hormones

absorb, synthesize, and transport fats

detoxify drugs

breakdown stored glycogen to form free glucose (in liver cells especially)

Skeletal and cardiac muscle cells have an elaborate amount that plays an important role in storing and releasing calcium ions during muscle contraction.

Golgi Apparatus

Consists of stacked and flattened membranous sacs, shaped like hollow dinner plates, associated with timey membranous vesicles.

Acts as the "traffic director" for cellular proteins

Major function is to modify, concentrate, and package the proteins and lipids made at the rough ER and destined for export from the cell. The "finishing touches"

The membranes are shaped like flattened rubber bands.

Peroxisomes

Membranous sacs containing powerful oxidases and catalases

Detoxify harmful or toxic substances

Catalysis and synthesis of fatty acids

Neutralize dangerous free radicals

-Oxidases convert to H2O (also toxic)

-Catalases convert H2O2 to water and oxygen

Lysosomes

Spherical membranous bags containing digestive enzymes

"Safe" site for intracellular digestion

Digest ingested bacteria, viruses, and toxins

Degrade nonfunctional organelles

Destroy cells in injured or non-useful tissue

Break down bone to release Ca2+

Endomembrane System

Overall function

-produce, degrade, store, and export biological molecules

-degrade potentially harmful substances

Includes ER, Golgi apparatus, secretory vesicles, lysosomes, nuclear and plasma membranes

Cytoskeleton

Elaborate series of rods throughout cytosol; proteins to other cell structures, non-membranous, gives it the shape of the cell

3 types

• Microfilaments
• Intermediate filaments
• Microtubules

Microfilaments

Thinnest of cytoskeletal elements

Dynamic stands of protein actin

Each cell unique arrangement of strands

Dense web attached to cytoplasmic side of plasma membrane-terminal web that gives strength, compression resistance

Involved in cell motility, change in shape, endocytosis and exocytosis

Intermediate Filaments

Tough, insoluble, ropelike protein fibers

Composed of tetramer fibrils

Resist pulling forces on cell; attach to desmosomes

(Neurofilaments in nerve cells; keratin filaments in epithelial cells)

Microtubules

Largest of cytoskeletal elements; dynamic hollow tubes; most radiate from centrosome

Composed of tetramer fibrils

Determine overall shape of cell and distribution of organelles

Mitochondria, lysosomes, secretory vesicles attach to microtubules, moved throughout cell by motor proteins