7.1 Life is Cellular

The Discovery of the Cell

• In 1665, Robert Hooke used a microscope to look at thin slices of cork from plants that seemed to be made of many tiny, empty boxes; Hooke called these boxes “cells,” and they are the basic units of life

• The cell theory states that all living things are made up of cells, that cells are the basic units of structure and function in living things, and that new cells come from existing cells

Exploring the Cell

• A microscope gives you a larger view of something very small, such as a cell

  • Light microscopes use glass lenses to focus light and magnify the object

    • Light microscopes can make clear images of objects only to a magnification of about 1000 times

  • Electron microscopes produce even higher magnifications by using electrons instead of light

    • Electron microscopes can show things that are 1 billionth of a meter in size

    • Transmission electron microscopes send beams of electrons through thin slices of cells and tissues

    • In scanning electron microscopes, a beam of electrons is scanned over the surface of an object to give a three-dimensional image of the object’s surface

Prokaryotes and Eukaryotes

• Despite their differences, at some point in their lives, all cells have DNA - the molecule that carries genetic information; all cells are also surrounded by a thin, flexible barrier called a cell membrane

• There are two main kinds of cells: one kind has a nucleus, and the other does not

  • The nucleus is a structure that contains the cell’s genetic material in the form of DNA and controls many of the cell’s activities

  • Eukaryotes are cells that enclose their DNA in nuclei

  • Prokaryotes are cells that do not have nuclei and their DNA is not separated from the rest of the cell

• Eukaryotic cells are generally larger and more complex than prokaryotic cells

  • Eukaryotes can be very different from one another; some, like “protists,” live as single cells, while others make up large organisms with many cells (plants, animals, and fungi)

• Prokaryotic cells are generally smaller and simpler than eukaryotic cells

  • Even though they are simpler than eukaryotes, prokaryotes do all the activities that living things must do to be called “alive:” they grow, reproduce, respond to the environment, and, in some cases, glide along surfaces or swim through liquids

  • The living things that we call bacteria are prokaryotes

7.2 Cell Structure

Cell Organization

• It’s easy to divide each eukaryotic cell into two major parts: the nucleus and the cytoplasm (the fluid portion of the cell outside the nucleus)

  • Prokaryotic cells have cytoplasm, too, even though they do not have a nucleus

• Many structures in plant and animal cells act like specialized organs and are known as organelles - specialized structures that perform important cellular functions within a eukaryotic cell

• The nucleus holds DNA and controls most of what goes on in the cell; the small, dense area in the nucleus is the nucleolus, where ribosomes are first put together

  • Ribosomes are used by the cell to build proteins

  • The nucleus is surrounded by a nuclear envelope made up of two membranes

  • The nuclear envelope is dotted with thousands of holes that allow material to move into and out of the nucleus

Organelles That Store, Clean Up, and Support

• Many cells have large, membrane sacs called vacuoles that store materials like water, salts, proteins, and sugars

  • Plant cells carry a singular, large central vacuole filled with liquid

  • Some single-celled organisms have vacuoles; a paramecium, for example, has an organelle called a contractile vacuole that pumps extra water out of the cell by contracting over and over

  • Nearly all eukaryotic cells have much smaller membrane sacs called vesicles that store and move materials between organelles as well as to and from the outside of the cell

• The lysosome is the cell organelle that breaks down lipids, carbohydrates, and proteins into small molecules that can be used by the rest of the cell

  • They remove “junk” that might otherwise pile up in the cell

  • Animal cells have lysosomes, and a few special kinds of plant cells also have them

• A cytoskeleton is the network of protein filaments in a eukaryotic cell that gives the cell its shape and internal organization and is involved in movement

  • The cytoskeleton includes two types of protein filaments called microfilaments and microtubules

    • Microfilaments are threadlike structures made up of a protein called actin that supports the cell and helps some cells move

    • Microtubules are like thin, hollow pipes made up of proteins known as tubulins

      • Microtubules help the cell keep its shape and are also found in hairlike organelles called cilia and flagella that help some cells swim

      • In cell division, microtubules also form a structure known as the mitotic spindle that helps to separate the different sets of DNA that each daughter cell will get; in animal cells, organelles called centrioles are also formed from tubulins; plant cells do not have centrioles

Organelles That Build Proteins

• Protein synthesis occurs on ribosomes - cell organelles consisting of RNA and proteins found throughout the cytoplasm in a cell

  • Many ribosomes are free in the cytoplasm, while others are attached to the endoplasmic reticulum

• The endoplasmic reticulum (ER) is the internal membrane system found in eukaryotic cells and the place where lipid components of the cell membrane are assembled

  • Rough ER: Proteins are made on the rough ER; it is called “rough” because the ribosomes on its surface make it bumpy

  • Smooth ER: The smooth ER is called “smooth” because there are no ribosomes on its surface; in many cells, the smooth ER has groups of enzymes that make membrane lipids and get rid of toxins, such as drugs

• The Golgi apparatus looks like a stack of flat membrane sacs and is an organelle in cells that modifies, sorts, and packages proteins and other materials from the endoplasmic reticulum for storage in the cell or release outside the cell

Organelles That Capture and Release Energy

• Most cells are powered by food molecules that are built using energy from the sun

• Plants and some other living things have chloroplasts - organelles found in cells of plants and some other organisms that capture the energy from sunlight and convert it into chemical energy in a process called photosynthesis

  • Inside the chloroplasts are large stacks of other membranes that hold the green pigment chlorophyll

• Nearly all eukaryotic cells, including plant cells, have mitochondria - cell organelles that convert the chemical energy stored in food into compounds that are more convenient for the cell to use

  • Like chloroplasts, mitochondria are surrounded by two membranes—an outer membrane and an inner membrane

• Both chloroplasts and mitochondria have their own genetic information in the form of small DNA molecules

Cellular Boundaries

• All cells are surrounded by a barrier known as the cell membrane, and many cells also have a stiff layer around the membrane known as a cell wall

• The main job of the cell wall is to support, shape, and protect the cell

  • Most prokaryotes and many eukaryotes, such as plant cells, have cell walls, though animal cells do not have cell walls

• The cell membrane keeps track of what enters and leaves the cell and also protects and supports the cell

  • Cell membranes are made up of a double-layered sheet called a lipid bilayer that makes membranes flexible and lets them form a strong barrier between the cell and its surroundings

  • The special lipids in the cell membrane have two parts: a head and a tail

    • The head is a chemical group that mixes well with water; it is called hydrophilic, or water-loving

    • The tail is made up of fatty acid chains that mix well with oil; it is called hydrophobic, or water-hating

    • When two of these layers come together, they are like a sandwich, with the water-loving parts of the lipids form the outside layer; the oily parts of the lipids stick together to form the inner layer, and a lipid bilayer is the result

  • The lipid bilayers of most cell membranes contain many different proteins, some of which form channels and pumps that help to move material across the cell membrane

  • If a substance is able to cross a membrane, the membrane is said to be permeable to it

    • A membrane is not permeable to substances that cannot pass across it

    • Most cell membranes are selectively permeable, which means that some substances can pass across them and others cannot

7.3 Cell Transport

Passive Transport

• One of the most important jobs of the cell membrane is to keep up the right balance between the liquid in the cell and the liquid around the cell

• Diffusion is the process by which molecules of a substance move from an area of higher concentration (where they are closer together) to an area of lower concentration (where they are more spread out)

  • Diffusion does not need energy from the cell; the movement of materials across the cell membrane without using cellular energy is called passive transport

• Facilitated diffusion is the process of diffusion in which molecules pass across the membrane through cell membrane channels

  • This action does not use energy; therefore, facilitated diffusion is a form of passive transport

• Osmosis is the diffusion of water through a selectively permeable membrane

  • In osmosis, as in all diffusion, molecules move from higher concentrations to lower concentrations—lower concentrations of water

• Sometimes cell membranes have solutions that are the same on both sides, but sometimes the solutions are different

  • When the solutions inside and outside of the cell are the same, the solutions are said to be isotonic

  • When the solution outside of the cell has a higher concentration than the inside solution, the outside solution is hypertonic

  • When the solution outside of the cell has a lower concentration, it is hypotonic

• Sometimes there are differences in concentration of salts, sugars, proteins, and other dissolved molecules on one side of the cell membrane that produce a force known as osmotic pressure - the pressure that must be applied to prevent osmotic movement across a selectively permeable membrane

Active Transport

• Active transport is the movement of particles from an area of lower concentration to an area of higher concentration across the cell membrane using energy

  • Small molecules or ions move across a cell membrane through the work of protein pumps found in the membrane

  • Larger molecules and big clumps of material can also be moved across the cell membrane through vesicles by processes known as endocytosis and exocytosis

    • Endocytosis is the process of taking material into the cell through a pocket of the cell membrane that folds into the cell; the pocket pinches off and breaks loose from the cell membrane before forming a vesicle or vacuole in the cytoplasm

    • Many cells release large amounts of material through a process known as exocytosis

      • Exocytosis is the opposite of endocytosis—materials leave the cell instead of coming into it

      • During exocytosis, the membrane of a vesicle sticks to and becomes part of the cell membrane; as the membranes fuse, the contents of the vesicle are forced out of the cell

      • Cells use exocytosis to get rid of wastes and to give off chemical signals

7.4 Homeostasis and Cells

The Cell as an Organism

• To maintain homeostasis (relatively constant internal physical and chemical conditions), unicellular organisms grow, respond to the environment, change food or sunlight into useful energy, and reproduce

• Single-celled organisms include both prokaryotes and eukaryotes

Multicellular Life

• The cells of a many-celled organism are specialized for different jobs

• From simplest to most complex, the levels of organization in a many-celled organism are cells, tissues, organs, and organ systems

  • A tissue is a group of similar cells that perform a particular function

  • An organ is a group of tissues that work together to perform closely related functions

  • An organ system is a group of organs that work together to perform a specific function

• Cells in a large organism use chemical signals passed from one cell to another to communicate

  • In order to communicate, certain cells form connections, or cellular junctions, to neighboring cells some of which hold cells together firmly, and others that allow small molecules carrying chemical messages or signals to pass from one cell to the next

  • To “understand” one of these chemical signals, a cell must have a receptor that sticks to the chemical signal; some receptors are on the cell membrane, while other kinds of receptors are inside the cytoplasm