Biology 12 - Cytoskeleton and Cell Membrane

UNIT A: Cell Biology

Chapter 2: The Molecules of Cells

Chapter 3: Cell Structure and Function: Sections 3.3, 3.4

Chapter 4: DNA Structure and Gene Expression

Chapter 5: Metabolism: Energy and Enzymes

Chapter 6: Cellular Respiration

Chapter 7: Photosynthesis

In this chapter, you will learn about how cell structures have critical roles to play in the health of an organism.

UNIT A Chapter 3: Cell Structure and Function

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Chapter 3: Cell Structure and Function

What other cellular organelles have a similar function to the lysosome?

Why doesn’t the cell “clean up” the faulty lysosomes?

3.3 The CytoskeletonThe proteins of the cytoskeleton interconnect

and extend from the nucleus to the plasma membrane. •The cytoskeleton allows eukaryotic cells to maintain their shape and the organelles to move•Three components of the cytoskeleton network are actin filaments, intermediate filaments, and microtubules

UNIT A Chapter 3: Cell Structure and Function Section 3.3

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From Figure 3.12 The Cytoskeleton. Fibroblasts contain (A) actin filaments, (B) intermediate filaments, and (C) microtubules.

A B C

Actin Filaments

• Long, extremely thin (~ 7 nm diameter), flexible fibres that occur as bundles or mesh-like networks

• Each filament contains two chains of globular actin monomers twisted about each other in a helix


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From Figure 3.12 The Cytoskeleton. a. Left to right: Fibroblasts in animal tissue contain actin filaments. The drawing shows that actin filaments are composed of a twisted double chain of actin subunits. The giant cells of the green alga Chara rely on actin filaments to move organelles from one end of the cell to another.

Actin Filaments: Structure and Movement• Actin filaments play a structural role by

forming dense, complex webs under the surface of the plasma membrane. They are also in microvilli, which project from intestinal cells, and pseudopods, which are extensions that allow some cells to move.

• Actin filaments play a role in movement by associating with motor molecules, which are proteins that attach, detach, and reattach farther along the actin filament


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In muscle cells, myosin is a motor molecule. The head interacts with ATP and actin and the tail interacts with the membrane. Myosin pulls actin filaments along, using ATP for energy.

Intermediate Filaments

• Intermediate in size between actin filaments and microtubules (8−10 nm diameter)

• A rope-like assembly of fibrous polypeptides


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From Figure 3.12 The Cytoskeleton. b. Left to right: Fibroblasts in animal tissue contain intermediate filaments. The drawing shows that fibrous proteins account for the ropelike structure of intermediate filaments. Hair is strengthened by the presence of intermediate filaments.

Microtubules

• Small, hollow cylinders about 25 nm in diameter and 0.2 to 25 μm in length

• Composed of 13 rows of alpha and beta tubulin dimers, surrounding an empty core


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From Figure 3.12 The Cytoskeleton. c. Left to right: Fibroblasts in animal tissue contain microtubules. The drawing shows that microtubules are hollow tubes composed of tubulin subunits. The skin cells of a chameleon rely on microtubules to move pigment granules around so they can take on the colour of their environment.

Microtubule AssemblyIn most eukaryotic cells, the centrosome contains a microtubule organizing centre, which regulates microtubule assembly. Microtubules radiate from the centrosome.• Microtubules help maintain the shape of the cell• Microtubules act as tracks that organelles move along, with the aid of motor molecules (kinesin and dynein)

Before cell division, microtubules disassemble then reassemble into a spindle, which attaches to chromosomes for proper distribution and participates in dividing the cell.

After cell division, the spindle disassembles and microtubules reassemble into their former arrangement.


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Centrioles

In animal cells, a centrosome contains two centrioles lying at right angles to each other. •Centrioles are short cylinders of microtubules in a 9 + 0 pattern of triplets ( a ring of nine sets of microtubule triplets)•Centrioles replicate before cell division and each pair becomes part of a centrosome. The centrosomes move apart at cell division.


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Figure 3.13 Centrioles In a nondividing animal cell, a single pair of centrioles lies in the centrosome located just outside the nucleus.

Cilia and Flagella

Cilia and flagella are hair-like extensions that move. •They provide movement for some cells•Both consist of membrane-bound cylinders composed of a 9 + 2 pattern of microtubules (nine microtubule doublets in a circle around two central microtubules)


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From Figure 3.14 Structure of a flagellum or cilium

Cilia and Flagella • Cilia and flagella move when the microtubule doublets move past each other

• The protein dynein acts as a motor molecule


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From Figure 3.14 Side arm motor molecules of dynein are involved in movement of flagella and cilia.


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Check Your Progress

1. Identify the structural makeup of actin filaments, intermediate filaments, and microtubules.

2. Describe the structures of cilia, flagella, and centrioles.

3. Explain how cilia and flagella move.


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3.4 Plasma Membrane Structure and FunctionThe plasma membrane regulates entrance and exit of substances from the cell to help maintain homeostasis.The fluid-mosaic model of the plasma membrane structure:•Phospholipids form a bilayer, with the hydrophilic polar heads facing inside and outside of the cell and hydrophobic tails facing each other•Peripheral proteins are partially embedded on one side of the membrane•Integral proteins span the entire membrane and may protrude into one or both sides; they move laterally•Steroids, glycolipids, and glycoproteins are also present


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Figure 3.15 Fluid-mosaic model of plasma membrane structure.

Functions of the Membrane ProteinsPlasma membranes of different cell types have unique combinations of proteins. Generally, •Peripheral proteins play a structural role by helping to stabilize and shape the plasma membrane•Integral proteins determine a membrane’s specific functions. The following are types of integral proteins:

• channel proteins• carrier proteins• cell recognition proteins• receptor proteins• enzymatic proteins


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Functions of the Membrane Proteins

Channel proteins are involved in the passage of molecules through the cell membrane by forming a channel.


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Carrier proteins are involved in the passage of molecules through the cell membrane by combining with the substance. From Figure 3.16 Examples of membrane

protein diversity.


Cell recognition proteins are glycoproteins involved in cell recognition of pathogens.


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Receptor proteins bind specific molecules, which causes a cell response.

From Figure 3.16 Examples of membrane protein diversity.



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Enzymatic proteins catalyze cell reactions.

From Figure 3.16 Examples of membrane protein diversity.


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Check Your Progress

1. Describe the role of proteins, steroids, and phospholipids in the fluid-mosaic model.

2. Distinguish between the roles of the various integral proteins in the plasma membrane.


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Science

Biology 12 - Cytoskeleton and Cell Membrane