Cell Anatomy and Physiology

Qualities of Cells

Qualities of Cells

• cells are the basic units of life

• activity of the organism depends on the activities of cells

• activity of cells depend on their shape, form and internal structure

• new cells arise from previously existing cells

• the stage of cancer depends of the health of new cells

Cell Types

• around 200 kinds in the body

• classified on the basis of structure and function

• not all cells have the same components

• most cells have a plasma membrane, cytoplasm and nucleus

Plasma Membrane

• different in different cell types which allows cells to interact differently with the same ECF

Plasma Membrane Structure

1. Double phospholipid layer that is impervious to water

2. Protein molecules that permit the movement of specific chemicals and have cell signaling functions

Phospholipid Bilayer Structure

• hydrophilic head and hydrophobic tail arranged with the tails always pointing towards each other

• cholesterol is arranged within the bilayer

• fluid structure with looser bonds allowing the phospholipids to move within the layer

Cholesterol in Phospholipid Bilayer

• arranged in between phospholipid molecules

• prevent crystallization and provide stability

Function of the Phospholipid Bilayer

1. Form the structure of the plasma membrane

2. Forms a barrier to passage of water soluble substances between ECF and ICF

3. Responsible for fluidity of the membrane as some cells constantly change shape

Proteins

• attached to or inserted within bilipid layer

• plasma membrane contains approximately 50 times as many lipid molecules

• the molecules make up almost half the membranes mass

• two types either integral or peripheral

Integral Proteins

• embedded in the membrane

Peripheral Proteins

• attached to one surface

Protein Function

• structural support

• transport of molecules across the membrane

• enzymatic control of chemical at cell surface (indirect protein control)

• receptors for certain molecules (hormones)

• markers (such as antigens) identify cells (and an individual)

Carbohydrates

• primarily attached to the outer surface of the membrane

• negatively charged

• affects activity of regulatory molecules and interactions between cells such as keeping red blood cells apart

Cilia

• propel fluids across the surface of cells that are firmly attached in place

• example includes the cells of the respiratory tract

Flagella

• similar structure as cilia but generally move the entire cell

• much longer than cilia

• such as sperm cells

Cell to Cell Adhesion

• three kinds of cell junctions assist in binding cells together including tight junctions, desmosomes and gap junctions

Tight Junctions

• integral proteins in adjacent cells fuse together making it difficult for anything to pass between adjacent cells

• found primarily in sheets of epithelial tissue

• highly selective barriers

• seperate compartments having different chemical compositions

• passage across epithelial barrier must place through cells

Desmosomes

• act like a zipper to hold cells together (adhering to junctions)

• important in cells where there is mechanical stress (such as muscle, skin and the uterus)

• keratin filaments insides of cell may extend to desmosome on the opposite side

• provides increased tensile strength

Gap Junctions

• used to communicate between adjacent cells

• permit passage of small signaling molecules

• provides one mechanism of cooperative cell activity

• have connexins made up of six proteins arranged in hollow tube-like structure

• two connexins join end to end

• especially abundant in cardiac and smooth muscle

Membrane Transport

• tissues consist of cells embedded in an extracellular matrix (ECM)

• ECM components differ for each type of tissue

• provide different local environments

• anything that passes between the cell and the surrounding ECM must be able to penetrate the plasma membrane

• the plasma membrane is selectively permeable

Properties of Permeability

• must be highly lipid-soluble particles that can dissolve in the bilipid membrane including uncharged/nonpolar molecules such as oxygen, carbon dioxides and fatty acids

• water soluble particles must be small enough to fit through specific channels

Unassisted Membrane Transport/Passive Transport

• molecules that can penetrate the plasma membrane on their own can be driven by diffusion and follow the concentration gradient

Diffusion

• movement from a high concentration to a low concentration as molecules have a tendency to be evenly spaced out

Factors Affecting Molecule Movement

• temperature is directly proportional to movement speed, substances diffuse faster at higher temperatures

• size is inversely proportional to movement speed, larger molecules diffuse slower

Concentration Gradient

• difference in concentration between adjacent area

Example of Diffusion

• oxygen diffuses out of the lungs, into the bloodstream which then diffuses out of the bloodstream, into tissues

Electrical Gradient

• oppositely charged ions attract each other

Electrochemical Gradient

• if both electrical and concentration gradients act on an ion

Osmosis

• water can readily permeate the plasma membrane because it is small enough to slip between the bilipid

• some cells have aquaporins meaning there is no limit to amount of molecules to facilitate the movement

• about a billion molecules of H2O can pass through the aquaporin every second

• moves by diffusion with the concentration gradient and water flows to areas of higher solute concentration

Concentration of a Solution

• the density of solute in a given volume of water

• in general one molecule of solute will replace one molecule

• as the solute increases the water decreases

What happens if the solute can move through the membrane?

• both water and solute move through the membrane until both are evenly distributed

What happens if the solute cannot pass through the membrane?

• volume of one side increases as a result of water movement

• eventually the solute concentrations on both sides become equal

Tonicity

• depends on concentration of solutes in the extracellular fluid

Isotonic Solution

• same solute/water concentration on inside and outside of the cell

• water moves both into and out of the cell

• no change in cell shape

Hypertonic Solution

• higher concentration of solutes outside of the cell

• water flows out of the cell

• cells shrink

Hypotonic Solution

• lower concentration of solutes outside the cell

• water flows into the cell

• cells expand and burst (lyse)

Assisted Membrane Transport/Active Transport

• sometimes cells need ions to go against the concentration gradient

• poorly lipid soluble molecules and molecules that are very large need help crossing the plasma membrane

• use either carrier mediated transport or vesicular transport

Carrier Mediated Transport

• carrier proteins span the plasma membrane

• can have binding sites at either side (ECF or ICF)

• must display specificity, saturation and competition

• can either be by facilitated diffusion or active transport

Specificity

• carry one or few substances

Saturation

• limited number of binding sites

Competition

• several related compounds may compete for transport

Facilitated Diffusion

• uses a carrier to assist transport of a substance downhill from high to low concentration

• example includes moving glucose into cells because it is an important source of fuel that cannot cross the cell membrane without support

Glucose

• big size and density making it difficult to break down

• polar molecule

• higher concentration in blood than tissues requiring facilitated diffusion

Active Transport

• movement of molecules against the concentration gradient from a low to high concentration

• requires energy in the form of ATP

• example is the uptake of iodine by the thyroid as it must be moved from blood (low concentration) to thyroid (high concentration)

Sodium-Potassium Pump

• carrier protein transports 3 sodium ions out of the cell, concentrating it in the ECF

• 2 potassium ions are picked up outside the cell and transported into the ICF

• ATP is used and converted into ADP+P

Vesicular Transport

• transport of molecules that cannot cross the membrane includes large polar molecules and ingestion of invading bacteria

• molecules are too big for the channels and no carriers exist

• energy is used to wrap the molecules in a membrane enclosed vesicle either by endocytosis or exocytosis

Endocytosis

• moving substances into the cell

• includes phagocytosis, pinocytosis and receptor-mediated endocytosis

Phagocytosis

• large molecules internalized

• can be done by phagocytes and certain types of white blood cells

• destroys tissue debris and bacteria by being engulfed in the vesicle

Pinocytosis

• small droplet of ECF enfolded and sealed by the plasma membrane

• can be done by most body cells

• also a good way to get rid of extra plasma membrane

Receptor-Mediated Endocytosis

• highly selective unlike pinocytosis

• enables cells to import specific large molecules needed by the cell such as cholesterol complexes, vitamin B12, insulin and iron

• can be exploited by certain viruses

Exocytosis

• moving substances out of the cells

• can be used for secreting large highly specific molecules such as hormones and neurotransmitters

• also used to add components to the plasma membrane including carriers, channels and receptors

Cytoplasm

• includes the cytosol and organelles

Cytosol

• semitransparent fluid

• mostly water with proteins, salts and sugars

Organelles

• each carry out specific functions

Mitochondria

• energy production

Smooth Endoplasmic Reticulum

• lipid production and detoxification

Rough Endoplasmic Reticulum

• protein production

Golgi Apparatus

• protein modification and export

Lysosome

• protein break down/destruction

Nucleus

• control center of the cell

• provides instructions, mainly for building proteins

• most cells have one

• skeletal muscle have multiple

• red blood cells have none

Nucleus Membrane

• selectively permeable

• punctuated with nuclear pores

• allows some control over what goes in and out