Chapter 3: Cell Structures and Their Functions

Organelles

specialized structures in cells that perform specific

functions

Cell Membrane / Plasma Membrane

the outermost component of a cell.

Cytoplasmic / Intracellular

Materials inside the cell

Extracellular

Materials outside the cell

Cell Membrane Functions

Encloses the cell, Plays a role in communication between cells, Supports cell contents, Acts as a selective barrier for what moves into and out of the cell

Cell Membrane Structure

contains phospholipids, cholesterol, proteins, and carbohydrates.

Phospholipid Structure

has a polar head region that is hydrophilic and a nonpolar tail region that is hydrophobic.

Cell Membrane Components

Phospholipid bilayer, Cholesterol, Proteins, Carbohydrates.

Phospholipid Bilayer

forms a lipid barrier between the inside and outside of the cell; has a fluid quality because the phospholipids are not stationary.

Cholesterol

adds strength and stability by limiting the movement of the phospholipids

Proteins

found on the outer and inner surfaces as well as extending through the membrane; function as membrane channels, receptor molecules, enzymes, and structural support

Carbohydrates

may be bound to protein and phospholipid molecules, modifying their function

The cell membrane is ______

Selectively Permeable

selective permeability

allows only certain substances to pass in and out of the cell.

Substances found in higher concentrations INSIDE the cell

enzymes, glycogen, and potassium

Substances found in higher concentrations OUTSIDE the cell

sodium, calcium, and chloride

Passive Transport

does not require the cell to expend energy (ATP)

passive membrane transport mechanisms

diffusion, osmosis, and facilitated diffusion

Active Transport

does require the cell to expend energy (ATP)

Active membrane transport mechanisms

active transport, secondary active transport, endocytosis, and exocytosis.

Diffusion

• generally involves movement of substances in a solution down a concentration gradient

• occurs through the random movement of the solute through the solvent

• uses the kinetic energy of the solute

• random movement from high concentration to a low concentration

Solution

generally composed of two major parts, solutes and the solvent.

Solute

• substances dissolved in a predominant liquid or gas

• move from an area of higher concentration of a ____ to an area of lower concentration of that same _____ in solution

Solvent

a predominant liquid or gas

Concentration Gradient

• the difference in the concentration of a solute in a solvent between two points/areas

• when one region has a high number of molecules and a nearby one has a lower number.

Materials that can cross the lipid bilayer by diffusion

O2 , CO2 , Cl-, steroids, and urea

oxygen, carbon dioxide, chloride ions, steroids, and urea

Osmosis

the diffusion of water (a solvent) across a selectively permeable membrane from a region of higher water concentration to one of lower water concentration.

Osmosis occurs when ______

the membrane is less permeable or impermeable to the solute and a concentration gradient exists for water across the cell membrane

high solute concentration

less water, so water will move toward it

Osmotic Pressure

• a pressure which is the force required to prevent movement of water across cell membrane.

• depends on the difference of solution concentrations inside a cell relative to outside the cell.

hypotonic

the cytoplasm of a cell has a lower concentration of solutes and a higher concentration of water relative to the cytoplasm of the cell

has less tone, or osmotic pressure, than the cell

cell swelling

lysis

cell swelling to the point of rupture

Isotonic

• the cytoplasm of a cell has the same solute concentrations inside and outside the cell.

• cell neither shrinks nor swells

• IV solutions must be this to the body's cells

Hypertonic

• the cytoplasm of a cell has a lower solute concentration and higher water concentration than the surrounding solution.

• cell shrinkage or crenation

Crenation

cell shrinkage

faciliated diffusion

• carrier-mediated transport process that moves substances across the cell membrane from an area of higher concentration to an area of lower concentration of that substance.

• Because movement is with the concentration gradient, metabolic energy in the form of ATP is not required

lipid soluable substances that can diffuse directly through the phospholipid bilayer

oxygen, carbon dioxide, steroids

Water-soluble substances, such as ions, can diffuse across the cell membrane only by…

membrane channels or through carrier molecules.

membrane channel characteristics

Size – limits what can go through based on the size of the molecule

Shape – some molecules must bond to a receptor that is shape-specific

Charge – since same charges repel and like charges attract, charge can determine what can pass through a particular channel

Two classes of cell membrane channels

Leak and Gated Channels

Leak Channels

Constantly allow ions to pass through

Gated channels

limit the movement of ions across the membrane by opening and closing.

Carrier Molecules

• proteins within the cell membrane involved in facilitated diffusion

• Move water soluble molecules or ions across the membrane

• exhibits specificity

Specificity

only specific molecules are transported by the carriers.

Sodium-Potassium Pump

• A major example of active transport present in cell membranes

• moves Na+ out of cells and K+ into cells.

• The result is a higher concentration of Na+ outside cells and a higher concentration of K+ inside cells.

Secondary Active Transport

• uses the energy provided by a concentration gradient established by the active transport of one substance, such as Na+ to transport other substances.

• No additional energy is required above the energy provided by the initial active transport pump.

Cotransport

the diffusing substance moves in the same direction as the initial active transported substance.

Countertransport

the diffusing substance moves in a direction opposite to that of the initial active transported substance.

Endocytosis

a process that that brings materials into cell by invagination of the cell membrane to form a vesicle containing the material to be taken into the cell.

Receptor-mediated endocytosis

occurs when a specific substance binds to the receptor molecule and is transported into the cell.

phagocytosis

• is often used for endocytosis when solid particles are ingested.

• cell eating

Pinocytosis

• has much smaller vesicles formed, and they contain liquid rather than solid particles.

• cell drinking

Exocytosis

• involves the use of membrane-bound sacs called secretory vesicles that accumulate materials for release from the cell.

• example: secretion of digestive enzymes.

Cell Nucleus

• a large organelle usually located near the center of the cell.

• bounded by a nuclear envelope, which consists of outer and inner membranes with a narrow space between them.

• nuclear membrane contains nuclear pores, through which materials can pass into or out of the ____

Chromosomes

• The nuclei of human cells contain 23 pairs of _______which consist of DNA and proteins.

• the _______ are loosely coiled and collectively called chromatin.

• When a cell prepares to divide, the ______ become tightly coiled and are visible when viewed with a microscope.

Nucleoli / Nucleolus

• diffuse bodies with no surrounding membrane.

• There are usually one to several ____within the nucleus.

• The subunits of ribosomes, a type of cytoplasmic organelle, are formed within a _______

Ribosomes

• the organelles where proteins are produced.

• may be attached to other organelles, such as the endoplasmic reticulum.

Free Ribosomes

ribosomes that are not attached to any other organelle.

Endoplasmic Reticulum (ER)

a series of membranes forming sacs and tubules that extends from the outer nuclear membrane into the cytoplasm.

rough ER

nvolved in protein synthesis and is rough due to attached ribosomes.

smooth ER

has no attached ribosomes and is a site for lipid synthesis, cellular detoxification, and it stores calcium ions in skeletal muscle cells.

Golgi Apparatus / Complex

• consists of closely packed stacks of curved, membrane-bound sacs.

• It collects, modifies, packages, and distributes proteins and lipids manufactured by the ER.

• forms vesicles, some of which are secretory vesicles, lysosomes, and other vesicles.

secretory vesicle

pinch off the Golgi apparatus, then fuse with the cell membrane, and release their contents to the outside of the cell by exocytosis.

vesicle

• a small, membrane-bound sac that transportsor stores materials within the cell.

• Materials such as neurotransmitters and hormones are types of substances manufactured by a cell and then released to communicate with other cells.

Lysosomes

• membrane-bound vesicles formed from the Golgi apparatus.

• contain a variety of enzymes that function as intracellular digestive systems.

• Vesicles formed by endocytosis may fuse with ______ in order to breakdown materials in the endocytotic vesicles.

peroxisomes

• small, membrane-bound vesicles containing enzymes that break down fatty acids, amino acids, and hydrogen peroxide

• The enzymes in ______ break down hydrogen peroxide to water and oxygen.

mitochondria

• organelles responsible for producing considerable amounts of ATP by aerobic (with O2) metabolism.

• The outer membranes have a smooth contour, but the inner membranes have numerous folds, called cristae, which project into the interior of the _________

• Cells with a large energy requirement have more _______ than cells that require less energy.

Cytoskeleton

• gives internal framework to the cell

• consists of protein structures that support the cell, hold organelles in place, and enable the cell to change shape.

Microtubules

• hollow structures formed from protein subunits.

• perform a variety of roles, including helping to support the cytoplasm of cells, assisting in cell division, and forming essential components of certain organelles, such as cilia and flagella.

Microfilaments

• small fibrils formed from protein subunits that structurally support the cytoplasm, determining cell shape

• _____ in muscle cells enable the cells to shorten, or contract.

intermediate filaments

• fibrils formed from protein subunits that are smaller in diameter than microtubules but larger in diameter than microfilaments.

• provide mechanical support to the cell.

• A specific type of ________ is keratin, a protein associated with skin cells.

centrosome

• a specialized area of cytoplasm close to the nucleus where microtubule formation occurs.

• contains two centrioles

centrioles

• each is a small, cylindrical organelle composed of microtubules organized into nine triplets.

• normally oriented perpendicular to each other

• also involved in the process of mitosis or cell division.

Cilia (Cilium, sing)

• responsible for the movement of materials over the top of cells, such as mucus

• project from the surface of certain cells

• are cylindrical structures that extend from the cell and are composed of microtubules.

Flagella

• have a structure similar to that of cilia but are much longer, and they usually occur only one per cell.

• tail

Microvilli

• specialized extensions of the cell membrane that are supported by microfilaments.

• do not actively move as cilia and flagella do.

• are numerous on cells that have them and they increase the surface area of those cells.

• are abundant on the surface of cells that line the intestine, kidney, and other areas in which absorption is an important function.