Type your learning objectives here.
In this chapter you learned many facts about cells. Specifically, you learned that:

• Cells are the basic units of structure and function of living things.
• The first cells were observed from cork by Hooke in the 1600s. Soon after, van Leeuwenhoek observed other living cells.
• In the early 1800s, Schwann and Schleiden theorized that cells are the basic building blocks of all living things. Around 1850, Virchow saw cells dividing, and added his own theory that living cells arise only from other living cells. These ideas led to cell theory, which states that all organisms are made of cells, all life functions occur in cells, and all cells come from other cells.
• The invention of the electron microscope in the 1950s allowed scientists to see organelles and other structures inside cells for the first time.
• There is variation in cells, but all cells have a plasma membrane, cytoplasm, ribosomes, and DNA.

• • The plasma membrane is composed mainly of a bilayer of phospholipid molecules and forms a barrier between the cytoplasm inside the cell and the environment outside the cell. It allows only certain substances to pass in or out of the cell. Some cells have extensions of their plasma membrane with other functions, such as flagella or cilia.
  • Cytoplasm is a thick solution that fills a cell and is enclosed by the plasma membrane. It helps give the cell shape, holds organelles, and provides a site for many of the biochemical reactions inside the cell. The liquid part of the cytoplasm is called cytosol.
  • Ribosomes are small structures where proteins are made.
• Cells are usually very small, so they have a large enough surface area-to-volume ratio to maintain normal cell processes. Cells with different functions often have different shapes.
• Prokaryotic cells do not have a nucleus. Eukaryotic cells have a nucleus, as well as other organelles. An organelle is a structure within the cytoplasm of a cell that is enclosed within a membrane and performs a specific job.
• The cytoskeleton is a highly organized framework of protein filaments and tubules that criss-cross the cytoplasm of a cell. It gives the cell shape and helps to hold cell structures (such as organelles) in place.
• The nucleus is the largest organelle in a eukaryotic cell. It is considered to be the cell’s control center, and it contains DNA and controls gene expression, including which proteins the cell makes.
• The mitochondrion is an organelle that makes energy available to cells. According to the widely accepted endosymbiotic theory, mitochondria evolved from prokaryotic cells that were once free-living organisms that infected or were engulfed by larger prokaryotic cells.
• The endoplasmic reticulum (ER) is an organelle that helps make and transport proteins and lipids. Rough endoplasmic reticulum (RER) is studded with ribosomes. Smooth endoplasmic reticulum (SER) has no ribosomes.
• The Golgi apparatus is a large organelle that processes proteins and prepares them for use both inside and outside the cell. It is also involved in the transport of lipids around the cell.
• Vesicles and vacuoles are sac-like organelles that may be used to store and transport materials in the cell or as chambers for biochemical reactions. Lysosomes and peroxisomes are vesicles that break down foreign matter, dead cells, or poisons.
• Centrioles are organelles located near the nucleus that help organize the chromosomes before cell division so each daughter cell receives the correct number of chromosomes.
• There are two basic ways that substances can cross the cell’s plasma membrane: passive transport (which requires no energy expenditure by the cell) and active transport (which requires energy).
• No energy is needed from the cell for passive transport because it occurs when substances move naturally from an area of higher concentration to an area of lower concentration. Types of passive transport in cells include:

• • Simple diffusion, which is the movement of a substance due to differences in concentration without any help from other molecules. This is how very small, hydrophobic molecules, such as oxygen and carbon dioxide, enter and leave the cell.
  • Osmosis, which is the diffusion of water molecules across the membrane.
  • Facilitated diffusion, which is the movement of a substance across a membrane due to differences in concentration, but only with the help of transport proteins in the membrane (such as channel proteins or carrier proteins). This is how large or hydrophilic molecules and charged ions enter and leave the cell.
• Active transport requires energy to move substances across the plasma membrane, often because the substances are moving from an area of lower concentration to an area of higher concentration or because of their large size. Two examples of active transport are the sodium-potassium pump and vesicle transport.

• • The sodium-potassium pump moves sodium ions out of the cell and potassium ions into the cell, both against a concentration gradient, in order to maintain the proper concentrations of both ions inside and outside the cell and to thereby control membrane potential.
  • Vesicle transport uses vesicles to move large molecules into or out of cells.
• Energy is the ability to do work. It is needed by every living cell to carry out life processes.
• The form of energy that living things need is chemical energy, and it comes from food. Food consists of organic molecules that store energy in their chemical bonds.
• Autotrophs (producers) make their own food. Think of plants that make food by photosynthesis. Heterotrophs (consumers) obtain food by eating other organisms.
• Organisms mainly use the molecules glucose and ATP for energy. Glucose is the compact, stable form of energy that is carried in the blood and taken up by cells. ATP contains less energy and is used to power cell processes.
• The flow of energy through living things begins with photosynthesis, which creates glucose. The cells of organisms break down glucose and make ATP.

• Cellular respiration is the aerobic process by which living cells break down glucose molecules, release energy, and form molecules of ATP. Overall, this three-stage process involves glucose and oxygen reacting to form carbon dioxide and water.

• • Glycolysis, the first stage of cellular respiration, takes place in the cytoplasm. In this step, enzymes split a molecule of glucose into two molecules of pyruvate, which releases energy that is transferred to ATP.
  • Transition Reaction takes place between glycolysis and Krebs Cycle. It is a very short reaction in which the pyruvate molecules from glycolysis are converted into Acetyl CoA in order to enter the Krebs Cycle.
  • Krebs Cycle, the second stage of cellular respiration, takes place in the matrix of a mitochondrion. During this stage, two turns through the cycle result in all of the carbon atoms from the two pyruvate molecules forming carbon dioxide and the energy from their chemical bonds being stored in a total of 16 energy-carrying molecules (including four from glycolysis).
  • The Electron Transport System, he third stage of cellular respiration, takes place on the inner membrane of the mitochondrion. Electrons are transported from molecule to molecule down an electron-transport chain. Some of the energy from the electrons is used to pump hydrogen ions across the membrane, creating an electrochemical gradient that drives the synthesis of many more molecules of ATP.
  • In all three stages of aerobic cellular respiration combined, as many as 38 molecules of ATP are produced from just one molecule of glucose.
• Some organisms can produce ATP from glucose by anaerobic respiration, which does not require oxygen. Fermentation is an important type of anaerobic process. There are two types: alcoholic fermentation and lactic acid fermentation. Both start with glycolysis.

• • Alcoholic fermentation is carried out by single-celled organisms, including yeasts and some bacteria. We use alcoholic fermentation in these organisms to make biofuels, bread, and wine.
  • Lactic acid fermentation is undertaken by certain bacteria, including the bacteria in yogurt, and also by our muscle cells when they are worked hard and fast.
  • Anaerobic respiration produces far less ATP (typically produces 2 ATP) than does aerobic cellular respiration, but it has the advantage of being much faster.
• The cell cycle is a repeating series of events that includes growth, DNA synthesis, and cell division.
• In a eukaryotic cell, the cell cycle has two major phases: interphase and mitotic phase. During interphase, the cell grows, performs routine life processes, and prepares to divide. During mitotic phase, first the nucleus divides (mitosis) and then the cytoplasm divides (cytokinesis), which produces two daughter cells.

• • Until a eukaryotic cell divides, its nuclear DNA exists as a grainy material called chromatin. After DNA replicates and the cell is about to divide, the DNA condenses and coils into the X-shaped form of a chromosome. Each chromosome consists of two sister chromatids, which are joined together at a centromere.
  • During mitosis, sister chromatids separate from each other and move to opposite poles of the cell. This happens in four phases: prophase, metaphase, anaphase, and telophase.
• The cell cycle is controlled mainly by regulatory proteins that signal the cell to either start or delay the next phase of the cycle at key checkpoints.
• Cancer is a disease that occurs when the cell cycle is no longer regulated, often because the cell’s DNA has become damaged. Cancerous cells grow out of control and may form a mass of abnormal cells called a tumor.

In this chapter, you learned about cells and some of their functions, as well as how they pass genetic material in the form of DNA to their daughter cells. In the next chapter, you will learn how DNA is passed down to offspring, which causes traits to be inherited. These traits may be innocuous (such as eye colour) or detrimental (such as mutations that cause disease). The study of how genes are passed down to offspring is called genetics, and as you will learn in the next chapter, this is an interesting topic that is highly relevant to human health.