CHAPTER 31

CELL MEMBRANE2

• Composed primarily of back-to-back phospholipids. Cholesterol is present, contribute to fluidity of the membrane, various proteins are embedded that have a variety of functions.• Hydrophilic molecule – attracted to water.• Hydrophobic molecule – repelled by water.• Amphipathic – contains both hydrophilic and hydrophobic molecules.

Example: soap works to remove oil and grease stains because it has amphipathic properties. Hydrophilic dissolves in water and hydrophobic traps the grease and they get washed away.

CELL MEMBRANE – continued3

• Intracellular fluid – fluid interior of the cell. • Extracellular fluid – fluid environment outside the enclosure of the

cell membrane.• Interstitial fluid – extracellular fluid not contained within blood

vessels.

Transport Across Cell Membrane4

One of the great wonders of the cell membrane is the ability to regulate the concentration of substances inside of the cell. Substances such as Ca++, Na+, K+ and CL-, nutrients including sugars, fatty acids and amino acids; and waste products, particularly CO2, which must leave the cell. • Selective permeability – cell membrane allows only substances

meeting certain criteria to pass through the membrane unaided.

Transport Across Cell Membrane5

All substances that move through the membrane do so by one of two methods.

Passive transport: movement of substances across the membrane without the expenditure of cellular energy.

Active transport: movement of substances across the membrane

using energy from ATP (adenosine triphosphate)

PASSIVE TRANSPORT 6In order to understand passive transport, we need to understand concentration gradients and diffusion.Concentration gradient: difference in concentration of a substance across a

space. Molecules (ions) will spread/diffuse from a higher concentration to a lower concentration until they are equally distributed. When molecules move in this way, they are said to move down their concentration gradient)

Diffusion: movement of particles from an area of higher concentration to an area of lower concentration. (Example – In a closed bathroom and a bottle of perfume is sprayed. The scent molecules would naturally diffuse from where it was sprayed to all corners of the room or until no more concentration gradient remained

PASSIVE TRANSPORT – continued7

• Diffusion – continued. (Example- spoonful of sugar placed in a cup of tea. Eventually the sugar will diffuse throughout the tea until no concentration gradient remains). If the room or tea was hotter, the faster diffusion would take place.

DIFFUSION 8

FACILITATED DIFFUSION9

Facilitated diffusion is the process used for those substances that cannot cross the lipid bilayer due to their size or polarity. This type of diffusion is helped by channel or carrier proteins. Channel proteins: less selective than carrier proteins and usually

mildly discriminate between their cargo based on size and charge. Carrier proteins: more selective, often allowing only one particular

type of molecule to cross. Amino acids must undergo facilitated diffusion to move into a cell and wastes must undergo facilitated diffusion to move out of a cell. Since this is a passive process, it does not require energy expenditure.

FACILITATED DIFFUSION – continued10

WATER CAN MOVE FREELY ACROSS THE CELL MEMBRANE OF ALL CELLS, EITHER THROUGH THE PROTEIN CHANNELS OR BY SLIPPING BETWEEN THE LIPID TAILS OF THE

MEMBRANE ITSELF

OSMOSIS 11Osmosis is the diffusion of water through a semipermeable membrane down its concentration gradient. Occurs when there is an imbalance of solutes outside of the cell versus inside the cell.Isotonic solution: (equal tension) – two solutions that have the same concentration of solutes. Concentration of water molecules is the same outside and inside the cells. Cells maintain normal shape. (solute concentration equal to another solution)Hypertonic solution: a solution that has a higher concentration of solutes than another solution. Cells in a hypertonic solution will shrivel as water leaves the cell via osmosis. (solute concentration higher than another solution)Hypotonic solution: a solution that has a lower concentration of solutes than another solution. Cells in a hypotonic solution will take on too much water and swell with the risk of eventually bursting. (solute concentration lower than another solution)

Osmosis – continued 12Homeostasis in living things work to create an environment in which all of the body’s cells are in an isotonic solution. The kidneys work to maintain this homeostasis.

Passive transport – continued13

Filtration: uses hydrostatic pressure gradient to push fluids and the solutes within, from a higher pressure area to a lower pressure area. (Example: Filtration pressure in the kidneys provides the mechanism to remove wastes from the bloodstream)

In all of the passive transport mechanisms, the cell expends no energy. They do not use ATP.

ACTIVE TRANSPORT14

During Active Transport, ATP is required to move a substance across a membrane, often with the help of protein carriers and usually against the concentration gradient (from an area of low concentration to an area of high concentration). Pumps are one of the most common types of active transport. Sodium-potassium pump: transport sodium out of cell while moving potassium into the cell. Active transport pumps can also work together with other active or passive transport systems to move substances across the membrane.

ACTIVE TRANSPORT – continued 15

Endocytosis (bringing “into the cell”): process of a cell ingesting material by enveloping it in a port of its cell membrane and then pinching off that portion of membrane. Once it is pinched off it becomes an independent, intracellular vesicle.Phagocytosis (“cell eating”): endocytosis of large particles that must be broken down or digested.Pinocytosis (“cell drinking”): brings fluid containing dissolved substances into a cell through membrane vesicles. Exocytosis (taking “out of the cell”): process of the cell exporting material using vesicular transport.

Active Transport – continued16

PROTOTYPICAL HUMAN CELL17

Cytoplasm and Cellular Organelles18

Cytosol: jelly-like substance within the cell, provides the fluid medium necessary for biochemical reactions.

Organelle (“little organ”): one of several different types of membrane-enclosed bodies in the cell, each performing a unique function.

Cytoplasm: composed of organelles and cytosol Nucleus: cell’s central organelle, contains the cell’s DNA

Endomembrane System of the Cell19

Set of three major organelles that form a system within the cell to work together to perform various cellular jobs, including the task of producing, packaging, and exporting certain cellular products. Endoplasmic Reticulum (ER): series of winding thoroughfares similar to

waterway canals in Venice. The ER provides passages throughout much of the cell that function in transporting, synthesizing and storing materials. It supports many functions.

There are two forms of ER:• Rough ER: so-called because its membrane is dotted with embedded

organelles called ribosomes, giving the RER, a bumpy appearance. Primary job of rough ER is the production of proteins.

Endomembrane System of the Cell20

• Ribosomes: organelle that serves as the site of protein synthesis. • Smooth ER: contains no ribosomes. Synthesis of lipids. Cells that

produce large quantities of steroid hormones (female ovaries and male testes), contain large amounts of smooth ER.

Golgi apparatus: looks like stacked flattened disks, almost like oddly shaped pancake stakes. Responsible for sorting, modifying, and shipping off products that come from the rough ER, (like the post-office). It has 2 sides. One side receives products in vesicles. The other side releases the product after repacking into new vesicles.

Endomembrane System of the Cell21

Lysosomes: organelle that contains enzymes that break down and digest unneeded cellular components. Similar to a wrecking crew that takes down old and unsound buildings. They also break down foreign material. • Autophagy: “self-eating”- process of a cell digesting its own

structures.• Autolysis: “self-destruct mechanism”- in the case of damaged or

unhealthy cells, the lysosomes can be triggered to open up and release their digestive enzymes into the cytoplasm of the cell thus killing the cell. Apoptosis – process of cell death controlled.

Organelles for Energy Production and Detoxification 22• Mitochondria (plural): membranous, bean-shaped organelle that is

the “energy transformer” of the cell. The inner membrane of the mitochondria is where a series of proteins, enzymes, and other molecules perform the biochemical reactions of cellular respiration. These reactions convert energy stored in nutrient molecules (Glucose) into adenosine triphosphate (ATP), providing usable cellular energy to the cell. Oxygen molecules are also required during cellular respiration. Cells use ATP constantly and so the mitochondria are constantly working.

Cytoskeleton 23Cytoskeleton – group of fibrous proteins that provide structural support for cells, but this is only one of the functions. These components are critical for cell motility, cell reproduction and transportation of substances within the cell.• Microtubules – thickest of the protein-based filaments. Maintain the

cells shape and structure, helps resist compression of the cell and play a role in positioning the organelles within the cell. They have two cellular appendages: cilia(lines the airways of the respiratory system and flagella(specialized for cell locomotion, sperm cell). • Microfilament – thinner type of cytoskeletal filament.

Cytoskeleton 24• Intermediate filament – intermediate thickness between

microtubules and microfilaments.

Nucleus 25• Nucleus is the largest and most prominent of the cell’s organelles.

Considered the control center of the cell because it stores all of the genetic instructions for manufacturing proteins. Nucleus of living cells contains the genetic material that determines the entire structure and function of the cell. • Inside the nucleus lies the blueprint that dictates everything a cell will

do and all of the products it will make. This information is stored within DNA. • The following structures are in order from least to most complex in

organization: DNA, nucleosome, chromatin, chromosome.

Cellular Differentiation 26• Stem Cells: unspecialized cell that can divide without limit as needed

and can, under specific conditions, differentiate into specialized cells. The first embryonic cells that arise from the division of the zygote are the ultimate stem cells.

oTotipotent: stem cells that have the potential to differentiate into any of the cells needed to enable an organism to grow and develop.

oPluripotent: develop from totipotent stem cells and are the precursors to the fundamental tissue layers of the embryo. It has the potential to differentiate into any type of human tissue but cannot support the full development of an organism.

Cellular Differentiation – continued27

oMultipotent: stem cell that are slightly more specialized and has the potential to differentiate into different types of cells within a given cell lineage or small number of lineages, such as red blood cell or white blood cells. In the process of hematopoiesis, it involves the differentiation of multipotent cells into blood and immune cells. These hematopoietic stem cells give rise to many different cell types including red and white blood cells and platelets.

oOligopotent: limited to becoming one of a few different cell types. oUnipotent: fully specialized cell and can only reproduce to generate

more of its own specific cell type.