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Unit 1:
Ch. 1
Ch. 2
Ch. 3
Interlude A
Unit 2:
Ch. 4
Ch. 5
Ch. 6
Ch. 7
Ch. 8
Ch. 9
Interlude B
Unit 3:
Ch. 10
Ch. 11
Ch. 12
Ch. 13
Ch. 14
Ch. 15
Interlude C
Unit 4:
Ch. 16
Ch. 17
Ch. 18
Ch. 19
Interlude D
Unit 5:
Ch. 20
Ch. 21
Ch. 22
Ch. 23
Ch. 24
Ch. 25
Ch. 26
Ch. 27
Ch. 28
Ch. 29
Ch. 30
Interlude E
Unit 6:
Ch. 31
Ch. 32
Interlude F
Unit 7:
Ch. 33
Ch. 34
Ch. 35
Ch. 36
Ch. 37
Ch. 38
Interlude G

» Getting Started » A Guide to the Reading » Tying it all together

Getting Started

Below are a few questions to consider prior to reading Chapter 27. These questions will help guide your exploration and assist you in identifying some of the key concepts presented in this chapter.

  1. What is the role of the human skeleton and how does its composition help with this role?
  2. How do endoskeletons and exoskeletons differ? 
  3. What is the importance of the joints in our skeletal system?
  4. How do actin and myosin interact within a sarcomere to produce a contraction?
  5. What is the difference between a slow muscle fiber and a fast muscle fiber?
  6. How do our skeleton and muscles work together to produce a lever system?
  7. What are the two types of drag that animals must deal with when they move in their environment?
  8. How do the energetic costs of swimming, running, and flying differ?

A Guide to the Reading

When exploring the content in Chapter 27 for the first time, the following concepts typically give students the most difficulty. For each concept, one or more references have been identified which may help you gain a better understanding of these potentially problematic areas.

Support Systems of Organisms

Organisms get their support and shape from external skeletons, internal skeletons, and hydrostats.  Humans have an internal or endoskeletal skeleton with an axial skeleton that supports and protects the trunk of the body and an appendicular skeleton composed of our arms, legs, and pelvis.  The human skeleton is composed of osteocytes, specialized living bone cells, surrounded by phosphate and calcium compounds that provide hardness.  The outer portion of bone is known as the hard compact bone and the inner region is a spongy bone containing marrow that is involved in the production of red and white blood cells.   Cartilage, composed of a matrix collagen protein, forms other softer parts of the human skeleton.  Skeletons can provide either internal or external forms of support.  The human skeleton is an internal endoskeleton and the hard covering of an insect is an external exoskeleton.  Increased body size in terrestrial organisms requires that skeletons be proportionately weaker so they do not overburden animals with their excess weight. This helps explain why vertebrates such as cats can withstand high-impact falls much better than can larger animals such as humans and elephants.  Many organisms, especially those living in aquatic environments, rely on hydrostats for support. Hydrostats provide support through an elastic membrane stretched over a pressurized fluid. Contraction and relaxation of muscles help to provide the pressure on the fluid to maintain support and provide stiffness. The human tongue and a caterpillar’s body are examples of hydrostats.

For more information on this concept, be sure to focus on:

  • In Section 27.1, The human skeleton is composed of bone tissue
  • In Section 27.1, Cartilage and hydrostats provide additional support
  • Figure 27.5, Nematodes Use Hydrostats for Support
  • Figure 27.6, Endoskeletons and Exoskeletons

Muscles Make Us Move

Contraction of muscle tissue powers animal movement.  The basic contractile unit of muscle is the sarcomere.  Sarcomeres are composed of two main proteins, actin and myosin.  Actin filaments are attached to the two ends of a sarcomere at the Z discs.  The myosin filaments lie between the actin filaments, but do not attach to the Z discs.  During contraction the sarcomere shortens as the two Z discs are pulled towards each other.  The contraction of the sarcomere occurs because heads on the myosin filaments attach to the actin and then pull the actin toward the center of the sarcomere.  This process can be thought of as “walking” the myosin filaments down the actin filaments.  The interaction between actin and myosin during contraction occurs with the help of ATP.  A series of sarcomeres attached end-to-end makes up a single muscle fibril, so a muscle contraction occurs when the multiple sarcomeres all shorten together.  Multiple muscle fibrils are bundled to form a muscle fiber, which is further bundled to constitute a functional body muscle (e.g., the human biceps).  Muscles produce force only when contracting and they must work as opposing pairs to give the full range of possible motion.  When a person exercises, muscle strength initially increases because the muscle fibers become more efficient and more of them contract simultaneously. With continued weight training, increasing numbers of muscle cells develop.  In contrast, muscle speed is influenced by muscle length and the percentage of fast or slow muscle fibers.  Most of our muscles are composed of two different types of muscle fibers.  Fast muscle fibers, such as those used by sprinters, contract quickly and use up their energy supply quickly.  Slow muscle fibers are involved in slower, longer contractions such as when running a marathon.  Slow muscle fibers have large amounts of myoglobin that help supply oxygen to the muscle. 

For more information on this concept, be sure to focus on:

  • In Section 27.4, Muscle tissue is built for contraction
  • In Section 27.5, The speed of muscle contraction depends on the length and type of muscle
  • Table 27.2, Fast and Slow Muscle Fibers
  • Figure 27.9, The Microscopic Structure of Muscle

Movement of Animals Within Their Environment

Muscles and skeletons work together to control the strength and speed of animal movement.  The physical model of a lever system with a fulcrum, power arm, and load arm describes how vertebrate skeletons and muscles interact to achieve this movement.  Animals move by producing thrust when swimming, running, or flying to overcome the two types of drag that act upon the motion of an animal.  Pressure drag results from the difference in air pressure in front of and behind a moving object, resulting in a backward force on the object. Friction drag is caused by friction with the surrounding molecules and the pressure of those molecules sticking to the body. This is apparent especially when an animal moves through a viscous or heavy fluid such as water. Natural selection has favored organisms that minimize drag.  There are significant differences in the amount of energy expended on each of the three types of movement.  Organisms that run tend to use the most energy to move a given distance. In addition to moving forward or backward, they must move their bodies up and down as well.  Swimmers are the most efficient organisms because the water helps support their bodies and they lose only a little energy to changes in momentum.

For more information on this concept, be sure to focus on:

  • In Section 27.6, Natural selection favors bodies that minimize drag
  • In Section 27.6, Different modes of locomotion have different energetic costs
  • Figure 27.12, Adaptations of the Lever System in Mammalian Legs
  • Figure 27.14, Modes of Locomotion in Animals
  • Figure 27.16, The Relative Costs of Locomotion

Tying it all together

Several concepts presented in this chapter build upon concepts presented in previous chapters and are also revisited and discussed in greater detail in subsequent chapters, including:

Proteins as the Structural Basis of Muscle

  • Chapter 4 – in Section 4.5, Amino acids are the building blocks of proteins

Organs Using Smooth Muscle

  • Chapter 21 – Section 21.2, The Human Digestive System

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