Cell Structure and Function

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MODULE 3: 
CELL STRUCTURE AND FUNCTION 
SELF-PREPARATION FOR THE 
BIOLOGY ASSESSMENT TEST 
The Cell Theory: Three major principles 
 The cell is the basic unit of life 
 
 All organisms consist of one or more cells 
 (unicellular vs. multicellular) 
 
 New cells are created when existing cells divide 
Common features of cells 
 
Most cells have at least these four components: 
 A flexible plasma membrane that surrounds the cell 
 A thick fluid called cytosol that fills their interior 
 The nucleic acid DNA 
 Protein structures called ribosomes, which are 
responsible for making other proteins important for 
various cell functions 
 
Plasma membranes 
 Consist of a phospholipid bilayer: 
 Two layers of phospholipids arranged so that the hydrophilic 
“heads” in each layer face outwards 
 
 
 
 
 
 
 Plasma membranes are fluid: Phospholipids are able to move, 
usually laterally (side-to-side), within a layer 
 
 
Hydrophilic 
“heads” 
Hydrophobic 
fatty acid “tails” 
Plasma membranes contain 
many other macromolecules 
 
 A variety of proteins, lipids (besides phospholipids), and 
carbohydrates are embedded in a plasma membrane 
 
 
 
 Proteins have many functions in membranes, including: 
 Channels that help the passage of ions or molecules 
 Enzymes, which control chemical reactions 
 Anchors for other cell structures, such as the cytoskeleton 
 Receptors, which bind molecular signals that are 
important for cell-to-cell communication 
 
 
 
 Cholesterol helps to maintain membrane fluidity 
 Makes some space available for phospholipids and other 
macromolecules to move laterally in the membrane 
 
 
 
 Carbohydrates are usually attached to membrane proteins or 
lipids 
 
 They often act as “markers” that aid in cell recognition. 
 Human ABO blood types are determined by carbohydrates 
present on red blood cell membranes. 
 
 
Plasma membranes are selectively permeable 
 Only certain substances can pass through a membrane 
 
 Generally, small and uncharged substances can cross unaided 
(e.g., gases like O2 and CO2) 
 Charge hinders passage regardless of size. The negatively charged 
phospholipid “heads” prevent ions and charged molecules (e.g., 
H2O) from crossing without help. 
 
Hydrophilic 
“heads” 
Substances can diffuse across a 
plasma membrane 
 Atoms and molecules are in 
constant motion 
 
 Diffusion is the net (overall) 
movement of atoms or molecules 
from an area of high to an area of 
low concentration 
 
 Thus, the direction of net 
movement follows the 
concentration gradient 
 
 
 
© JrPol/ Wikimedia Commons / CC-BY-SA-3.0 / GFDL 
 Molecules can easily diffuse 
across a plasma membrane if 
they are small and uncharged 
 
 The direction of net movement 
will follow the concentration 
gradient 
 
 Water (H2O) can also diffuse 
across plasma membranes. 
Their charge hinders diffusion. 
 
 The diffusion of water across a 
selectively permeable 
membrane is called osmosis. 
 
 
 
Two basic cell types: 
Prokaryotic and eukaryotic 
Prokaryotic cells are simply organized because they lack 
internal structures called membrane-bound organelles 
 
Important example: 
A prokaryotic cell lacks a nucleus, which is a membrane-
bound organelle 
 
Prokaryotes are generally unicellular. Bacteria are examples 
of prokaryotic organisms. 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
Prokaryotic cell structures 
Some main structures: 
 
Cell wall: Additional covering for cell besides plasma 
membrane. It is also more rigid than the membrane. 
 
Nucleoid: Region within cell where DNA is located. The 
nucleoid is not a true nucleus because the DNA is not 
enclosed in plasma membrane. 
 
Flagellum (plural = flagella): Protein structure used for cell 
movement. A prokaryotic flagellum has a different 
structure from a eukaryotic flagellum. 
 
 
Two basic cell types: 
Prokaryotic and eukaryotic 
Eukaryote means “true nucleus”! 
 
Eukaryotic cells have a more complex organization than 
prokaryotic ones because they each contain a nucleus and a 
variety of other membrane-bound organelles 
 
 
Eukaryotes can be uni- or multicellular. Animals, plants, and 
fungi are examples of eukaryotic organisms. 
 
In eukaryotic cells, membrane-bound organelles have 
different functions. The next few slides review the 
functions of some of these eukaryotic membrane-bound 
organelles. 
 
Golgi apparatus 
Eukaryotic cells: The Nucleus 
 Function: Store and protect most of the DNA present 
in the cell 
 
 Basic structure 
 Surrounded by a nuclear envelope consisting of two 
layers of plasma membrane 
 Within the nuclear envelope is a layer of protein fibers 
called the nuclear lamina 
 Structures called nuclear pores assist with transport into 
or out of the nucleus 
 
Eukaryotic cells: 
The Endomembrane System 
 A group of membrane-bound organelles that often work 
together to make and transport biological 
macromolecules, like a molecule factory 
 
 Some organelles help to break down macromolecules 
 
 Organelles in the endomembrane system include 
 Smooth endoplasmic reticulum 
 Rough endoplasmic reticulum 
 Golgi apparatus 
 Vesicles and lysosomes 
 
Smooth endoplasmic reticulum (SER) 
 Consists of plasma membrane folds that are smooth 
looking in appearance 
 
 Makes lipids, such as phospholipids and steroids 
 
 Some specialized functions 
 In muscle cells: Stores calcium ions to aid muscle 
contraction 
 In liver cells: Helps to get rid of drugs and toxins 
 
Rough endoplasmic reticulum (RER) 
 Consists of plasma membrane folds with ribosomes 
docked along the membrane (it looks rough!) 
 
 Ribosomes thread proteins into RER 
 
 RER provides a space for these proteins to fold into 
their proper shape. Folded proteins can be modified 
while in the RER. 
 
 RER also produces some lipids and proteins that are 
part of the cell membrane 
 
Rough endoplasmic reticulum (RER) 
 Molecules from RER are shipped by packaging them 
into vesicles, a small bits of RER membrane bud off 
and carry their cargo to another destination. 
 
 
Golgi apparatus 
 Consists of plasma membrane folds that look like a 
stack of pancakes 
 
 Vesicles deliver biological macromolecules (proteins, 
lipids, some carbohydrates) to the side of the Golgi 
apparatus that acts like a receiving dock 
 
 Molecules make their way through the membrane 
stack to the other side of the apparatus. As this occurs, 
they are modified so that they can be shipped to the 
proper final destination. 
 
 
Golgi apparatus 
 Like the rough endoplasmic reticulum, molecules 
leaving the shipping side of the Golgi apparatus are 
packaged into vesicles that bud off and deliver their 
cargo to the final destination. 
 
 The final destination could be another location within 
the cell or the cell membrane for secretion out of the 
cell 
 
 
 
Lysosomes 
 Vesicles that contain enzymes (proteins) that are 
designed to break down biological macromolecules. 
The enzymes are made by the RER, then processed 
and packaged by the Golgi apparatus. 
 
 Substance to be broken down is packaged in plasma 
membrane. This structure fuses with the membrane 
of a lysosome. 
 
 Many unicellular eukaryotes, such as amoebas, rely 
upon lysosomes to help break down food particles thathave been engulfed by the cells 
 
 
Mitochondria 
 Mitochondria (singular = mitochondrion) are where 
most of the process of cellular respiration takes place in 
eukaryotic cells 
 
 Cellular respiration utilizes glucose and O2 (oxygen gas) to 
generate energy in the form of ATP (adenosine 
triphosphate) 
 
 ATP molecules provide energy for a wide variety of cellular 
functions 
 
 A eukaryotic cell can contain up to several hundred 
mitochondria! 
 
The eukaryotic cytoskeleton 
 Cells must maintain their shape. Cell shape is 
regulated by a structure called the cytoskeleton. 
 
 The eukaryotic cell cytoskeleton is a mixture of three 
different protein fibers 
 Microfilaments 
 Intermediate filaments 
 Microtubules 
 
Eukaryotic cilia and flagella 
help some eukaryotic cells move 
 Eukaryotic cells, like single-celled protozoans that live 
in freshwater ponds, move with the help of cilia. 
 
 Other eukaryotic cells, like animal sperm, move with 
the help of flagella. 
 
 Eukaryotic cilia and flagella consist of microtubule 
bundles that are surrounded by the cell’s plasma 
membrane. They beat back and forth to aid cell 
movement. Cilia are much shorter than flagella. 
 
 
Summary of cell structure and function 
 One of three major principles of the cell theory is that the cell is 
the basic unit of life 
 
 Most cells contain four common features, including a plasma 
membrane 
 
 A plasma membrane consists of a phospholipid bilayer with a 
variety of other embedded macromolecules. It is a selectively 
permeable barrier that allows small and uncharged substances to 
diffuse unaided. 
 
 Prokaryotic cells are simply organized 
 
 Eukaryotic cells have a complex internal organization made up 
of various membrane-bound organelles with different 
functions 
Summary of cell structure and function 
 Many eukaryotic membrane-bound organelles are part of 
the endomembrane system, whose main function is the 
production and transport of biological macromolecules 
such as proteins, lipids, and some carbohydrates 
 
 A cell has a cytoskeleton that helps it maintain its shape. 
The cytoskeleton of a eukaryotic cell consists of a mixture 
of three different protein fibers. 
 
 Some eukaryotic cells have cilia or flagella to help with 
cell movement or feeding. These structures consist of 
microtubule bundles covered by plasma membrane. 
END OF MODULE 3: 
CELL STRUCTURE AND FUNCTION 
SELF-PREPARATION FOR THE 
BIOLOGY ASSESSMENT TEST 
	MODULE 3:�CELL STRUCTURE AND FUNCTION
	The Cell Theory: Three major principles
	Common features of cells
	Plasma membranes
	Plasma membranes contain �many other macromolecules
	Slide Number 6
	Slide Number 7
	Slide Number 8
	Plasma membranes are selectively permeable
	Substances can diffuse across a �plasma membrane
	Slide Number 11
	Two basic cell types: �Prokaryotic and eukaryotic
	Slide Number 13
	Prokaryotic cell structures
	Two basic cell types: �Prokaryotic and eukaryotic
	Slide Number 16
	Slide Number 17
	Eukaryotic cells: The Nucleus
	Eukaryotic cells: �The Endomembrane System
	Slide Number 20
	Smooth endoplasmic reticulum (SER)
	Rough endoplasmic reticulum (RER)
	Rough endoplasmic reticulum (RER)
	Golgi apparatus
	Golgi apparatus
	Lysosomes
	Mitochondria
	The eukaryotic cytoskeleton
	Eukaryotic cilia and flagella �help some eukaryotic cells move
	Summary of cell structure and function
	Summary of cell structure and function
	END OF MODULE 3:�CELL STRUCTURE AND FUNCTION