Misconception: lactic acid and lactate are not the same thing!
3.7 Cell structure
The previous parts of this chapter have focused on the chemical level of organization outlining how atoms of elements can bond to make molecules and compounds. Molecules, in turn, combine to make cells, the basic structural and functional unit of an organism. Humans are considered asmulticellular organisms, as there are about 200 different types of cells in our bodies, such as muscle cells, blood cells, skin cells, brain cells and so on.
Cells then combine to maketissues, defined as groups of cells that work together to perform a par- ticular function. There are four basic types of tis- sues in our bodies:epithelial, connective, nervous andmuscle (further classified as skeletal, cardiac and smooth).
When two or more tissues combine, the result is an organ (e.g. the stomach) which, in turn, can also
work together to make asystem(e.g. the digestive system, comprised of the stomach, small and large intestines, liver, etc). Ultimately, when all of the systems (e.g. the digestive, nervous, cardiovascu- lar, musculoskeletal etc) are in place and in opera- tion together, the result is a living organism – for example, the person reading this book!
It is important to note that not all cells are the same. Nevertheless, the average human cell is 10–20µm in diameter and our bodies contain approximately 1014 cells. Most cells are 70–80%
water and have many structural features in common. The basic structure of a cell is shown in Figure 3.11.
3.7.1 The plasma membrane
The boundary of cells is known as the plasma membrane, which is essentially a sturdy but flexi- ble barrier that encloses the cell (see Figure 3.12).
The membrane itself consists of lipids and proteins (to which carbohydrates may be attached), where the basic structural framework is thelipid bilayer. Membrane lipids include phospholipids (lipids with a phosphate group attached), cholesterol (a steroid with a hydroxyl group attached) and glycolipids (lipids with carbohydrate groups attached).
Proteins in the plasma membrane are divided into two categories –integral proteins orperiph- eral proteins. Most integral proteins are also referred to as transmembrane proteins, meaning they span right across the lipid bilayer and are in contact with both the intracellular and extracellular fluid. In contrast, peripheral proteins tend to asso- ciate with membrane lipids or integral proteins at either the inner or outer surface of the membrane.
Membrane proteins have a variety of functions which are essential for maintaining cell viability.
NUCLEUS:
Chromatin Proteasome
Cilium Flagellum
Nuclear pore
Nucleolus Glycogen granules
Rough endoplasmic reticulum Ribosome Golgi complex
Microfilament
Sectional view Microtubule
Mitochondrion Peroxisome Lysosome Secretory vesicle
PLASMA MEMBRANE
Centrioles Pericentriolar material
Intermediate filament Microfilament Microtubule Cytoskeleton:
Centrosome:
Microvilli
Smooth endoplasmic reticulum
CYTOPLASM (cytosol plus organelles except the nucleus)
Nuclear envelope
Figure 3.11 The basic structure of body cells. (From Tortora and Derrickson,Principles of Anatomy and Physiology, Twelfth Edition, 2009, reproduced by permission of John Wiley & Sons Inc.)
Glycoprotein:
Carbohydrate Protein
Extracellular fluid
Pore
Channel protein
Lipid bilayer
Integral (transmembrane) proteins
Peripheral protein
Cholesterol Polar head
(hydrophilic) Polar head (hydrophilic) Phospholipids:
Carbohydrate Lipid Glycolipid:
Peripheral protein
Fatty acid tails (hydrophobic)
Cytosol
Figure 3.12 Basic structure of the plasma membrane. (From Tortora and Derrickson,Principles of Anatomy and Physiology, Twelfth Edition, 2009, reproduced by permission of John Wiley & Sons Inc.)
These include operating asion channels (i.e. pro- teins with pores through which ions can pass into and out of the cell via the process of diffusion), transporter proteins (i.e. proteins which actively transport substances into and out of the cell via facilitated transport) and receptor proteins (i.e.
proteins which recognize an extracellular signal, usually via the binding of hormones, which in turn then alters intracellular functions).
3.7.2 The nucleus
The nucleus is an oval-shaped structure which contains the genetic material within our genes.
Genes are arranged in single file along chromo- somes. Human cells contain 46 chromosomes, of which we inherit 23 from each of our parents.
Each chromosome is a long molecule of deoxyri- bonucleic acid (abbreviated asDNA) that is coiled together with proteins known ashistones. Encoded within the DNA is the essential genetic infor- mation that is required for making new proteins
(protein synthesis). We will cover DNA structure and the process of protein synthesis in Chapter 4.
Similar to the plasma membrane, a lipid bilayer membrane known as the nuclear enve- lope separates the nucleus from the rest of the cell. Embedded within the nuclear envelope are nuclear poreswhich allow selective movement of molecules between the nucleus and cytoplasm.
Inside the nucleus is a structure known as the nucleolus, which is responsible for making ribosomes– organelles which function in making new proteins. It is important to note that, while most cell types contain a single nucleus, some (such as red blood cells) contain none. In contrast, skeletal muscle cells contain many nuclei and are thus referred to asmultinucleated cells.
3.7.3 Cytoplasm and organelles
The cytoplasm is the term used to refer to everything inside the cell except the nucleus. The cytoplasm, in turn, can be further divided as the
cytosol (i.e. the water component of the cell or intracellular fluid, which constitutes about 55% of total cell volume) and theorganelles. The cytosol also contains many components such as ions and the energy sources of glycogen, lipids, proteins, ATP and all of the necessary enzymes needed to maintain important cellular processes. The organelles are structures made up of biomolecules which carry out specific cellular functions.
Let’s look more closely at some of the most important cellular organelles.
Endoplasmic reticulum
The endoplasmic reticulum is an extensive network of membranes which extends from the nuclear envelope (to which it is attached) throughout the cytoplasm. It is divided into two distinctive forms known as therough endoplasmic reticulum and smooth endoplasmic reticulum, which differ in appearance (when examined under a microscope) and function.
The rough endoplasmic reticulum is so called because of its granular-like appearance, which is due to organelles known as ribosomes that are attached to its external surface. Ribosomes are the cellular organelles responsible for protein synthe- sis and consist of proteins and ribosomal ribonu- cleic acid (abbreviated as rRNA).
In contrast, the smooth endoplasmic reticulum has no ribosomes attached to it and is considered to have a ‘smooth’ appearance. It extends from its rough counterpart and is responsible for the synthesis of lipids and various steroids. In certain cells, the smooth endoplasmic reticulum is specialized to perform a variety of functions.
For example, as seen in Chapter 2, calcium ions responsible for muscle contraction are released from thesarcoplasmic reticulum, a form of smooth endoplasmic reticulum unique to skeletal muscle.
Golgi apparatus
Whereas the ribosomes are the sites responsible for protein synthesis, thegolgi apparatus(orgolgi complex) is the organelle responsible for trans- porting these newly made proteins to their correct
intra- or extra- cellular location. It consists of a series of cuplike membranous sacs known asGolgi cisternae. The golgi apparatus is closely associated with the rough endoplasmic reticulum on one side (called thecis face) and with the plasma membrane on the other (called thetrans face).
Proteins made in the ribosomes enter the golgi apparatus on the cis face via transport vesicles.
Once inside the golgi apparatus, the proteins can be further modified (via the addition of carbohy- drates or lipids to form glycoproteins and lipopro- teins, respectively) and are then transported to exit the golgi apparatus on the trans face side viasecre- tory, membrane or transport vesicles. Secretory vesicles deliver proteins to the plasma membrane, where they are then discharged to the extracellu- lar fluid for subsequent transport to other cells.
Membrane vesicles deliver proteins to the plasma membrane for incorporation into the membrane itself. Transport vesicles deliver the proteins to other relevant intracellular locations.
Mitochondria
Mitochondria are oval-shaped structures that are often referred to as theaerobic powerhouse of the cell, as they generate the most ATP. Cells which are highly active, such as muscle, kidney and liver cells, etc. contain many mitochondria. An increase in the number and size of mitochondria in skele- tal muscle that occurs with endurance training is one of the most important adaptations to training which improves endurance performance (Holloszy
& Coyle, 1984).
Mitochondria contain both an outer mito- chondrial membrane and inner mitochondrial membrane, with a small fluid filled space between them. The inner membrane is folded into tubules called cristae, which substantially increase the surface area for the electron transport chain.
The large, fluid-filled cavity enclosed by the inner membrane is known as the mitochondrial matrix. Similar to the nucleus, mitochondria also contain their own special DNA (which we inherit from our mothers), which can make 13 proteins that are needed to make other important proteins for the mitochondria. As mitochondria
can also make their own proteins, ribosomes (i.e.
the protein-making factories) are present in the mitochondrial matrix.
Cytoskeleton
The cytoskeleton is a flexible network of fibrous proteins (referred to asfilaments) which gives the cell structure and support, similar to how the skele- ton provides support within our bodies. However, the cytoskeleton is not a rigid, fixed structure but rather undergoes continual reorganization as it dis- assembles and reassembles when necessary. Three types of protein filaments constitute the cytoskele- ton, and these are named according to their size (increasing diameter): microfilaments, intermedi- ate filaments andmicrotubules.
Microfilaments have the smallest diameter and have two general functions of helping to generate movement and providing mechanical support. One important microfilament is the protein actin, which is involved in the process of muscle contraction.
Intermediate filaments are exceptionally strong proteins that have a diameter between that of microfilaments and microtubules. In skeletal muscle cells, an important intermediate filament is myosin, which works with actin to produce contraction. The intermediate filament desmin is also important as it helps to ‘anchor’ the contractile proteins, and organelles such as the mitochondria and the nucleus in position.
The largest cytoskeletal proteins are known as microtubules and are mainly composed of the protein tubulin. Microtubules help to determine cell shape and also assist in the movement of organelles and chromosomes during cell division.