Chapter 5: Cell Structure and Function

5-1 The Cell Theory

The Cell Theory

All living things are made of cells Cells are the basic units of structure and

function in living things Anton van Leeuwenhoek, a Dutch

biologist, was the inventor of the microscope – Enabled him to see things that no one had

ever seen before

The Cell Theory

Van Leeuwenhoek’s work interested other people– Robert Hooke examined slices of cork– Saw that it was composed of thousands of tiny chambers– Called these chambers cells

Over the next 200 years, other scientists began to discover that cells were not only found in plants but in other living things too– Robert Brown—1833—discovered the nucleus– Matthias Schleiden—1838—stated that all plants are made

of cells– Theodore Schwann—1839—all animals are made of cells– Rudolf Virchow—1855—all cells arise from preexisting cells

The Cell Theory

The observations of these scientists are summarized into the cell theory– All living things are composed of cells.– Cells are the basic units of structure

and function in living things.– All cells come from preexisting cells.

Chapter 5: Cell Structure and Function

5-2: Cell Structure

Cell Structure

Enormous variety in the size and shape of cells Despite differences in size and shape, there

are certain structures that are common to most cells

The cells of animals, plants, and related organisms have three basic structures: the cell membrane, or boundary of the cell; the nucleus, or control center; and the cytoplasm, or material between the cell membrane and the nucleus

Cell Membrane

All cells are separated from their surroundings by a cell membrane– Regulates what enters and leaves the cell– Aids in protection and support of the cell

It must communicate with other cells, take in food and water, and eliminate wastes

Composed of several kinds of molecules– Lipids (bilayer)– Proteins– Carbohydrates

Cell Wall

In organisms such as plants, algae, and some bacteria, the cell membrane is surrounded by a cell wall– Helps protect and support the cell– Very porous—allows water, oxygen,

carbon dioxide, and other substances to pass through

– Made of cellulose

Nucleus

In many cells we can see a large, dark structure, called the nucleus– Contains DNA

Prokaryotes are organisms whose cells lack nuclei– Bacteria– Small and unicellular

Eukaryotes are organisms whose cells contain nuclei– Mostly multicellular

Nuclear Envelope

Surrounding the nucleus are two membranes that form the nuclear envelope– Form the boundary around the nucleus

• Contains dozens of nuclear pores allowing substances to flow in and out of the nucleus

Nucleolus

Most nuclei contain a small region called the nucleolus that is made up of RNA and proteins

– The structure in which ribosomes are made

• Aid in the production of proteins within the cell

Chromosomes

The DNA in the nucleus of eukaryotic cells is attached to special proteins and forms large structures called chromosomes– Contain the genetic information that must

be passed to each new generation of cells

Cytoplasm

The cytoplasm is the area between the nucleus and the cell membrane

Contains many important structures

Chapter 5:Cell Structure and Function

5-3: Cytoplasmic Organelles

Cytoplasmic Organelles

The structures inside the cytoplasm are generally called organelles

An organelle is a tiny structure that performs a specialized function in the cell.– Each organelle has a special job

Mitochondria and Chloroplasts: Power Stations All living things require energy The mitochondrion and the

chloroplast are key organelles that change energy from one form to another– Mitochondria change chemical energy in

food into compounds that the cell can use– Chloroplasts trap solar energy and convert

it into chemical energy

Mitochondria and Chloroplasts: Power Stations Mitochondrion

– Contains two special membranes• Outer membrane surrounds the organelle• Inner membrane has many folds that increase the

surface area of the mitochondrion Chloroplast

– Found only in plant cells and algae– Three membranes

• Two envelope-like membranes• Third membrane where the sun’s energy is

changed into chemical energy

Ribosomes: Protein Factories

Ribosomes are the structures in which proteins are made– Made of RNA and protein– Some are attached to membranes; some

are found free in the cytoplasm– Among the smallest organelles

Endoplasmic Reticulum and Golgi Apparatus: Manufacturers and Shippers

Many cells are filled with a complex network of sacs known as the endoplasmic reticulum, or ER

– Transports materials through the inside of the cell

– Two types:

• Smooth—no ribosomes attached

• Rough—ribosomes attached—involved in the synthesis of proteins

Endoplasmic Reticulum and Golgi Apparatus: Manufacturers and Shippers

Proteins are often modified by special enzymes that attach carbs and lipids to them

In most cases, the proteins are first moved into special compartments known as the Golgi apparatus– Flattened stack of membranes– After modification, the proteins may then be

released from the cell**The Golgi apparatus modifies, collects,

packages, and distributes molecules made at one location of the cell and used at another**

Lysosomes: Cleanup Crews

When a cell encircles a particle, the cell membrane forms a pocket around the foreign material

The foreign material must now be digested,or broken down– Lysosome

• Contain chemicals and enzymes necessary for digestion

• Formed by the Golgi apparatus• Plants cells do not have lysosomes

Vacuoles and Plastids: Storage Tanks Vacuoles store materials such as water,

salts, proteins, and carbohydrates Plastids are plant organelles that may take

many forms, one of which is the chloroplast– Involved in the storage of food and

pigments– Examples:

• Leukoplasts—store starch• Chromoplasts—store pigments

Cytoskeleton: Framework

Most cells are capable of some type of movement

Cytoskeleton – framework of filaments and fibers that support cell structure and drive cell movement– Microfilaments-long, thin fibers that function in

the movement and support of the cell– Microtubules-hollow tubes made out of

proteins• Cilia and flagella

Chapter 5:Cell Structure and Function

5 – 4

Movement of Materials Through the Cell Membrane

Diffusion

The driving force behind the movement of many substances across the cell membrane is called diffusion

Diffusion is the process by which molecules of a substance move from areas of higher concentration of that substance to areas of lower concentration

Several factors determine whether diffusion occurs across a membrane– Equilibrium point is reached– Permeability

• Most membranes are known as selectively permeable

Osmosis

Water molecules pass through most cell membranes very rapidly

The diffusion of water molecules through a selectively permeable membrane is called osmosis

The force exerted by osmosis, or osmotic pressure, tends to move water across membranes from a more dilute solution into a more concentrated solution

If water moves in too freely, the volume of a cell will increase until the cell becomes swollen and bursts like an over inflated balloon

Osmosis

Cells deal with the problem of osmotic pressure in a variety of ways– Cells of many organisms do not come into

contact with fresh water– Plant cells and bacteria have cell walls that

prevent the cells from expanding– Some cells pump out the water with a

contractile vacuole

Facilitated Diffusion

Osmosis and diffusion are forms of passive transport across the cell membrane because energy is not needed

But, many molecules are transported across a membrane in the direction of lowest concentration by a carrier protein

– Facilitated diffusion

• Is fast, specific, and does not require energy…it is still driven by diffusion

• Can only occur if a concentration distance exists cross that membrane

Active Transport Active transport is an energy-requiring process that enables material to

move across a cell membrane against a concentration difference Two types

– Individual molecules are carried through membrane-associated pumps

– Large amounts of materials are transported through movements of the cell membrane

• Endocytosis– Process of taking material into the cell by means of

infoldings, or pockets, of the cell membrane• Phagocytosis

– Process that occurs when large particles are taken in by endocytosis

• Pinocytosis– Process in which cells take in liquid and pinch off to form

vacuoles within the cell• Exocytosis

– Process that occurs when large materials are removed from the cell

Chapter 5:Cell Structure and Function

5 – 5

Cell Specialization

Cell Specialization

Cell specialization is one of the key characteristics of cells in a multicellular organisms

By cell specialization, we mean that cells are often uniquely suited to perform a particular function within the organism– Movement– Make products

Chapter 5:Cell Structure and Function

5 – 6

Levels of Organization

Levels of Organization

The levels of organization in a multicellular organism include cells, tissues, organs, and organ systems that make up the organism

Tissues

In multicellular organisms, cells – the first level of organization – are organized in specialized groups called tissues

– A group of similar cells that perform similar functions

– Most animals, which are multicellular organisms, have four main types of tissues

• Muscle, epithelial, nerve, and connective tissue

Organs

An organ is a group of tissues that work together to perform a specific function

Many types of tissue may be used to form a particular organ

Each tissue performs an essential task to help the organ function successfully

Organ Systems

An organ system, or a group of organs, works together to perform a certain function

There are 10 organ systems in the body including the muscular system, circulatory system, and digestive system