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Chapter 4: Cells - Organelles

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Cell Structure and Organelles

Web Lecture

Cell Structure

A number of major discoveries led to our understanding of cell structure.

herbacious cell

Today the cell is recognized as one of the fundamental characteristics of life, whether the organism involved is composed of one cell or many.

Inside the cell occur all the functions of life we mentioned on the first day of class: conversion and release of energy, growth, reproduction, consumption of nutrients, elimination of wastes, and response to external stimuli. Cell structure is sometimes the crucial difference between what we define as living and things we define as non-living: while they may reproduce and grow, prions and viruses are not considered living things because they lack cell structure.

Cell structure has other implications as well. One of the major difficulties confronting evolution theory is explaining the development of cells from simple molecular components that may spontaneously replicate in lab situations. We'll come back to this problem later on in the text, but in order to understand the magnitude of the difficulty, you need to realize how complex individual cells can be.

All cells share some of the same characteristics and limitations. They are bounded by a membrane, made primarily of the lipids we talked about earlier, but with differences depending on whether the cell is bacteria, archaea, a protist, or a member of a multicellular organisms. They have structural materials that define the shape of the cell and hold substructures called organelles in place. They have strands of genetic material containing DNA and RNA, which direct how they can reproduce themselves and how they make proteins for the enzymes needed to enable cellular functions.

Suface to Volume Ratio

Everything that goes in and out of the cell must cross this membrane, so there is a limit to cell size. As a cell gets bigger, its surface area grows by the square of its side length. But its volume grows as the cube of that length, which means that the as we increase the side (or the ratio) of a cell, the volume of material inside the cell grows faster than the surface are through which all cell nutrients must pass. Eventually, the cell reaches a size where this surface-to-volume ratio is too small: the cell cannot bring nutrients and energy into the cell, or push wastes out fast enough to supply and maintain all the materials inside the cell walls.


Studying cells turns out to be rather tricky. While we have three major types of microscopes we can use, each type of microscope provides a different kind of information about the cell and has drawbacks and advantages. If we want to look at living cells actually performing processes, we are limited to light microscopes, which cannot magnify cells beyond 1000X without distorting the colors of the image. We can increase the magnification possible by using electron microscopes. If we want to look at surfaces in detail, we can use a scanning microscope to see the details of a sample coated in a highly reflective metal. If we want to look at interior structure, we can use a transmitting microscope to send light through a thin slice of organic material. Notice that both scanning and transmitting electron microscopes require us to kill the cell, which means that we cannot study metabolic processes using these tools, only structures.

When you look at pictures of cells and cell structures, be sure to identify the type of microscope that was used, so that you can properly interpret the picture. You should also pay attention to any dyes that have been used to bring out the nucleus, membranes, or other structures.

There is a good summary and introduction to microscope use in biology at the Bellevue Community College Science website. (This is a PDF file).

The Components and Structure of the Cell

There are two major kinds of cells: prokaryotes, which are relatively simple in structure and include Archaea and Bacteria; and eukaryotes, which are much more complex, and which include everything else, from single-celled protists to multi-celled plants, fungi, and animals. There are three major parts to keep track of when looking at cells: