Chapter 3: Cells

Humans have 75 – 100 trillion cells Cells are the structural and functional

units of the body The human body is 65% water The intracellular environment is found

within the cell, separated from the external environment

Cell lie in a supportive bath that surrounds them and provides everything they need,the extracellular environment

The Extracellular Environment Water, dissolved gases, salts, food

particles, cell products Cell products are synthesized by the cell

and released into the extracellular environment by a process called secretion

Cell products include hormones, proteins, vitamins

Water + products + other substances = ECF (extracellular fluid)

The ECF is thick and syrupy

The ECF

2 types:Plasma is found in blood vessels and the chambers of the heart Plasma is the liquid that helps transport and float the cells of blood through the body

Interstitial fluid is found between the cells of the bodyInterstitial fluid provides a pathway between cells

Travel between some areas of the body is restricted by dense strands of protein that form a Matrix

This matrix provides structural support

Intercellular Environment Compartments of the external

cellular environment found between adjacent cells

When cells are close together, these spaces may contain molecular bridges

The bridges connect cells and make rapid chemical communication possible

Intracellular Environment

Protoplasmwater, proteins, fats, carbohydrates, nucleic acids, and electrolytes

When these substances are organized and the life processes are being carried out, protoplasm is called---alive

Intracellular fluid (ICF) is made up of water + proteins + other chemicals

Cell work takes place in the intracellular environment, e.g. energy production, energy storage, product synthesis

Components of Protoplasm The plasma membrane-the barrier that

separates intracellular and extracellular environments

Cytoplasm-a thick, gel-like fluid that contains all the cell’s organelles

Nucleus-Large, oval-shaped structure that regulates the cell’s activities

Cell Structure and Function

Human cells vary in size—all are microscopic

The human egg cell is ~1000 micrometers The human sperm cell is ~ 50

micrometers 25, 400 micrometers = 1 inch Human cells vary widely in shape Variation in size and shape is a reflection

of the various roles different cells play in the body

The Plasma Membrane Envelops the cell completely Regulates movement of materials into and

out of the cell Helps maintain homeostasis Made up of approximately equal weights

of lipids (fats) and proteins plus a very small amount of carbohydrates

Barrier is not solid, it is called selectively permeable

Lipids in the Plasma Membrane Phospholipids and cholesterol Phospholipids have a hydrophilic head (attracted

to water) and hydrophobic tails(repels water)

Due to their opposite interactions with water, phospholipid molecules form a double-layered arrangement called the lipid bilayer

At body temperature, the plasma membrane is liquid (like vegetable oil)

Cholesterol molecules lie in the lipid bilayer and help provide stability

Proteins in the Plasma Membrane Peripheral and Integral Peripheral are attached to the outside and

inside surfaces of the plasma membrane Integral proteins are partially or

completely embedded in the lipid bilayer Integral proteins serve as channels for

transport of materials passing through the membrane

These openings are called pores

Modifications to the Plasma Membrane Modifications in the structure of the plasma membrane

enable some cells to perform special functions

Cells that are close together and cannot move about have 4 Regions:free surface – exposed to a lumen (open space)lateral borders- adjacent to neighboring cellsbasal surface – attached to a matrix of protein called the basement membrane

These cells are in the digestive tract, liver, kidney, walls of the blood vessels

The modifications usually occur at the free surface

Microvilli Some cells have a free surface that is

extensively folded to form many tiny, slender projections called microvilli

Each microvillus is covered by the cell membrane and has a small amount of cytoplasm inside of it

Microvilli are common in cells that absorb materials, such as cells of the small intestine, and in cells that release materials, such as excretory cells in the kidneys

Cilia Cilia are slender projections of the plasma

membrane that extend into a lumen They contain an organized arrangement of

proteins called microtubules Cilia move about in a coordinated, whip-

like manner in order to move body fluids Cilia are common in cells of the respiratory

tract which is layered with mucus

Flagella Another modification of the plasma

membrane A flagellum is normally a single, long

structure Flagella contain microtubules for strength

and stability Flagella whip about and provide propulsion

for cells such as sperm cells

Crossing the Plasma Membrane Different mechanisms for movement of

materials across the cell membrane are classified by the nature of the energy that powers their movement

Passive—powered by kinetic energy ( a force that causes molecules to move randomly)

Active— demand an input of energy from the cell/ATP

Diffusion: A Passive Process Molecules move from areas of high

concentration to areas of low concentration

Molecules move on a concentration gradient by kinetic energy

An equal distribution of molecules is called equilibrium

Facilitated diffusion occurs when some molecules bind with integral proteins in the plasma membrane

Osmosis: A Passive Process The movement of water molecules (only) across a

selectively permeable membrane from an area of higher water concentration to an area of lower water concentration

Isotonic Solutions have equal osmotic pressureCell is at equilibrium

Hypertonic solutions have more solutes outside the cell than in (cell will dump water and shrink, crenation)

Hypotonic solutions have less solutes outside the cell than in (cell will take in water and swell, hypertrophy)