Cell Structure and Function

We have learned that the cell is the fundamental unit of living organisms, and that the interface of the cell with its surroundings is the plasma membrane. We have spent some time discussing how materials move into and out of cells through the plasma membrane. Now it’s time to turn to the cell, the fundamental unit of all living organisms.

Each cell is unique, composed of carbohydrates, proteins, lipids, etc., organized into an orderly structural and functional unit. Just as we saw that macromolecules are remarkable in their structure to function relationship, we shall, in this chapter, see how the structure of cells and, in particular, the structure of cell components, facilitates the functioning of cells.

History of the cell
The study of cells dates back more than three hundred years, coinciding with the development of microscopes. As scientists over the years learned more about cells, a group of common characteristics was developed which we call the Cell Theory. Our use of more sophisticated microscopes and biochemical cell research reinforces these premises.

The Cell Theory

  1. Every living organism is made up of one or more cells.
  2. Cells are the structural and functional unit of living organisms. The smallest living organisms are single cells, and cells comprise the functional units of multicellular organisms.
  3. All cells arise from preexisting cells

Basic Cell Features (Common to All Cells)

Plasma (cell) membrane
Genetic material: nucleus or nucleoid
Each cell contains genetic material (DNA), which stores the instructions for how that cell's structure and function. The DNA may be found within a membrane - bounded nucleus, (eukaryotic organisms – plants, animas, protists and fungi) or simply concentrated in a region of the cytoplasm called the nucleoid (prokaryotic organisms Eubacteria and Archaebacteria)

Cytoplasm
The fluid matrix (called the cytosol) inside the plasma membrane in which everything else in the cell, such as internal membranes, particles and membrane-bounded structures, called organelles, are suspended.

Obtaining Nutrients and Energy
All cells must obtain nutrients and energy resources from their environment. We have seen that these molecules pass through the plasma membrane. We will discuss these processes in some detail later.
Cell Types and Living Organisms
Every organism is composed of one of two fundamental types of cells: prokaryotic or eukaryotic.

Prokaryotic cells do not have their genetic material enclosed within a membrane-bounded structure (no nucleus). Their DNA is concentrated in a region of the cell called the nucleoid. Prokaryotic cells also do not have membrane-bounded organelles within the cytoplasm of their cells.

The DNA of eukaryotic cells is contained within a nucleus. The nucleus is surrounded by the cytoplasm of the cell, much of which is the semi-fluid matrix, the cytosol, in which organelles are suspended.

As you read in chapter one of your text, the world of life is currently organized into three domains and six Kingdoms. Two of the domains, Archaea and Bacteria are prokaryotic. The Domain, Eukarya, is comprised of four Kingdoms: Protista, Fungi, Plantae and Animalia, whose members are composed of Eukaryotic cells.

Brief Review of Domains and Kingdoms
Prokaryotic Organisms
Domain and Kingdom Archaea

Domain and Kingdom Bacteria

Eukaryotic Organisms (Domain Eukarya)
Protista
 Plantae
 Fungi
 Animalia
Most of what we discuss in Biology refers to the eukaryotic cell and eukaryotic organisms. Microbiology focuses extensively on bacteria, the major group of prokaryotic organisms. There is also a chapter in your textbook on bacteria for those interested in more reading.

Cell Organization and Cell Dimensions
While the benefits of a cellular organization seem fairly clear, we must look more closely at how a cell functions to understand why most cells are very small, and why multicellular organisms are comprised of many, many microscopic cells, rather than just a few enormous ones.

Each cell needs to perform a number of functions while maintaining a pretty constant internal environment. Cells must exchange materials with the external environment, and undergo any number of chemical reactions, each with specific chemical requirements, in order to stay alive and do their jobs. The more things needed in a cell, the more exchanges have to occur through the membrane. If the volume of a cell becomes too large, there is not enough membrane surface area to accomplish all that needs to be done.

So the overall limit to cell size seems to be this surface area/volume ratio. As the volume of a cell increases, the cell has proportionally less surface to exchange nutrients, gases and wastes with its environment to sustain the increasing volume. Within the cytoplasm, materials move by diffusion, a physical process that can work only for short distances. A large volume would inhibit the rate of movement too much for cells to function. Cells with minimal metabolic needs can have larger volumes.

Some exceptions are:


Before we spend the bulk of our time looking at the eukaryotic cell, let's take a little time to discuss the distinguishing feature of prokaryotic cells.

Features of Prokaryotic Cells


Features of the Eukaryotic Cell
Let's first list the predominant cell structures for future reference. See Table 5-2 of your text for reference.
Eukaryotic Cell Components
Nucleus
Cytoplasm of
Nucleus
The nucleus is generally the largest or most "conspicuous" (except for when students are trying to find one) structure within the eukaryotic animal cell. The mature plant central vacuole, which you usually cannot see, takes much more of the volume of the plant cell. The nucleus is spherical and quite dense.

Nucleus Functions
Nucleus Structure
Nuclear Envelope
The nucleus is bounded by the nuclear envelope

Chromatin

Nucleolus
 The Cell’s Endomembrane System
Not only do membranes form the boundary of the cell, the plasma membrane, but within the cell we find a membrane system composed of a number of components, each of which may connect to the plasma membrane at some time or another, and to the nuclear envelope as well. In addition, small membrane fragments may be pinched off forming vesicles, used for transport.

Endomembrane Components
Endoplasmic Reticulum
Rough Endoplasmic Reticulum
Smooth Endoplasmic Reticulum
Golgi Complex
The Golgi complex consists of stacks of flattened disk-like membrane sacs that get materials from the endoplasmic reticulum. The Golgi functions as a processing center for materials to be packaged up and distributed in organelles or exported (secreted) from the cell in vesicles pinched off of the tips of the Golgi membranes. Digestive enzymes may be packaged for lysosomes and hormones packaged into vesicles for secretions.

Golgi bodies also modify materials prior to export. The carbohydrate portions of glycoproteins, for example, are added in the Golgi body.

Vesicles formed at ER migrate to the Golgi bodies, merge and pass through the Golgi and are packaged and labeled for export in Golgi vesicles.

Lysosomes
Lysosomes contain hydrolytic enzymes, which can breakdown carbohydrates, proteins, nucleic acids, and many lipids.

Lysosomes are manufactured from enzymes and membranes of the rough ER and packaged in the Golgi complex.

The Lysosome is responsible for disassembly or breakdown of cell components when no longer needed or when damaged or in need of recycling. It is a normal part of cell maintenance and renewal.

Lysosomes can also destroy or degrade bacteria and foreign substances. Macrophages for example, contain large numbers of lysosomes. Amoeba feed by a process of phagocytosis. The food vacuole formed merges with lysosomes for digestion.

During development, lysosomes are important in digestion of parts. Reabsorption of tadpole tails and formation of fingers and toes are two examples of this.

Other Organelles
Vacuoles and Vesicles
We mentioned vesicles earlier, when we discussed the Golgi Complex. Vesicles are membrane-bounded structures that hold something (good definition). Golgi vesicles are used to transport packaged materials from the Golgi complex through the cytoplasm for export. They are generally temporary structures.

Vacuoles are also membrane-bounded sacs that hold something (good definition). Vacuoles contain a variety of substances, such as food, wastes, water, etc. Some of these vacuoles are temporary. Some are permanent structures of cells, such as those involved with water balance. Others are temporary, such as food and waste vacuoles.

We will mention three vacuoles: food vacuoles, contractile vacuoles found in protists, and the central plant vacuole.

Food Vacuoles
Organisms that feed by phagocytosis surround their prey item with a portion of their plasma membrane, and engulf the item by fusing the membrane around it and moving the now "food vacuole" into the cytosol. Once in the cytoplasm of the cell, the food vacuole is merged with lysosomes for digestion. Digested nutrients are moved into the cytosol for use, and non-digested materials are formed into a waste vacuole that is removed from the cell by a more-or-less reverse process to the initial engulfing.

Contractile Vacuoles
Most terrestrial organisms risk dehydration, evaporating water to their surroundings. In contrast, fresh water organisms are in an environment where water tends to move into their cells. Many fresh water protists have contractile vacuoles, structures which collect the water that moves into their cell from the environment, and periodically expel the collected water to the external environment by contracting the vacuole though a pore, hence the name, contractile vacuole.

Central Plant Vacuole
All living, mature plant cells have a large membrane bounded organelle, filled with fluid, called the Central Plant Vacuole. The central vacuole occupies as much as 90 - 95% of the volume of the mature cell. The membrane of the vacuole is called the tonoplast. The tonoplast is poorly permeable to water and water soluble materials.

Functions of the Central Plant Vacuole
Mitochondria
Function of Mitochondria:
Structure of Mitochondria
Peroxisomes
Peroxisomes contain enzymes that transfer hydrogen in biochemical reactions to oxygen, forming hydrogen peroxide as a by-product. Since H2O2 is toxic, peroxisomes also contain an enzyme, catalase, which breaks down the H2O .

Glyoxysomes
Plant cells, especially in seeds, contain glyoxysomes. These cells store oils so that the germinating seed has a fuel supply. During germination, the fatty acids are converted to sugar molecules for the rapid cell respiration needed for successful germination and seedling establishment.

Plastids
Plastids are found in the cells of plants. Animal cells do not contain plastids. In general, a plastid is a membrane bounded organelle that stores something (yes, same as a vacuole). There are three common plastids.

Cytoskeleton
The cytoskeleton is the internal, fibrous framework of cells. Many organelles and some enzymes are organized along this framework.

Components of the cytoskeleton
Microtubules
Microfilaments
Intermediate Filaments


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