Cell Metabolism

Chapter 4

Lisa Ochs RN, MSN 2008

Metabolism

• The chemical reactions that occur in the body• All energy and material transformations that occur

within living cells• Anabolism

– Reactions that build more complex substances from simpler ones; requires ATP

• Catabolism– Reactions that break down complex substances;

releases ATP

Carbohydrates

• Found in all the best food- bread, potatoes, pasta and sweets

• Organic compounds composed of carbon (C), hydrogen (H), and oxygen (O)

• Classified according to size• Shorter chains are called sugars, longer chains

are called starches

Monosaccharides

• Sugars that contain 3-6 carbons• Glucose, fructose and galactose• Glucose is the most important source of

energy

Disaccharides

• Double sugars• Formed when 2 monosaccharides are joined

together• Include sucrose, maltose and lactose• Must be broken down into monosaccharides

before they can be used by the body

• Sucrose

Polysaccharides

• Many glucose molecules linked together either in straight or branching chains

• Important polysaccharides include plant starch, animal starch and cellulose

• Glycogen- highly branched chains; in humans, glucose is stored as glycogen (in the liver and muscles); it is released and converted into glucose when blood glucose levels drop

Uses of glucose…

• Glucose is used by the body in 3 ways– Burned immediately as fuel for energy– Stored as glycogen and used at a later time– Stored as fat and used at a later time

Breaking down glucose…

• Glucose can be broken down under 2 conditions:

– In the presence of oxygen- aerobic catabolism

– In the absence of oxygen- anaerobic catabolism

Breakdown of glucose…

• Anaerobic– Glucose is broken down in a series of chemical

reactions that result in the production of lactic acid

– Called glycolysis; occurs in cytoplasm– Produces only a small amount of energy (most of

the energy remains in the lactic acid and the body is unable to use it)

Breakdown of glucose…

• Aerobic– Glucose is completely broken down to form CO2,

water and ATP– Occurs in the mitochondria (uses enzymes found

in the mitochondria- remember??)– Since the glucose is completely broken down, a

large amount of ATP is released

Aerobic breakdown of glucose…

• Three important points…• 1. Requires oxygen• 2. Releases a large amount of energy (ATP)• 3. If oxygen is not available, the glucose will

be converted into lactic acid (can be potentially life-threatening)

Figure 4-3 Breakdown of glucose. A, Anaerobic: to lactic acid. B, Aerobic: to carbon dioxide, water, and ATP.

Elsevier items and derived items © 2007, 2003, 2000 by Saunders, an imprint of Elsevier Inc.

Making glucose…

• Cells can not only breakdown glucose, they can also synthesize (make) glucose from non-carbohydrate sources (especially proteins)

• This process is called gluconeogenesis

Diabetic patients

• Glucose breakdown/ synthesis are especially important to diabetics

• Insulin is needed to transport the glucose into the cell (facilitated diffusion); without insulin, the cells cannot use the glucose for energy

• Lack of insulin causes gluconeogenesis of proteins; but the cells are unable to use the glucose, so it accumulates in the blood

Diabetic patients

• Drugs used to treat diabetes work by:– Increasing the uptake of glucose by the cells– Suppressing the gluconeogenesis in the liver

– Both of these work to lower blood glucose levels

Lipids

• Organic compounds commonly called fats and oils– Fats are solid at room temperature– Oils are liquid at room temperature

• Most common are triglycerides, phospholipids and steroids

Lipids

• The building blocks of lipids– Fatty acids– Glycerol

• Triglycerides – 3 long fatty acid chains attached to glycerol

• Phospholipids– Phosphorus group attaches to glycerol

• Steroid – Important steroid in the body- cholesterol; can be

synthesized by the liver; helps synthesize vitamin D and sex hormones

Use of lipids…

• Source of energy for the body• Component of cell membranes and myelin

sheaths• Synthesis of steroids

• Can also be stored long term as adipose tissue and deposited in blood vessels (athersclerosis)

Metabolism of lipids…

• Can be broken down to release stored energy• Long fatty acid chains must be broken into

smaller pieces • Large amount of energy released when lipids

are burned for energy (more than glucose)• Excess fat is stored as adipose tissue

Proteins

• The most abundant organic matter in the body

• Participate in nearly all functions of the body• Found as enzymes, which are in almost all

chemical reactions

Amino Acids

• Building blocks of proteins- about 20 amino acids used in the body

• Can be taken in through food (lean meat, milk, eggs etc.) or synthesized by the body

• Essential amino acids (must be taken in through diet)

• Nonessential amino acids (can be synthesized by the liver)

Amino Acids

•Composed of carbon, hydrogen and oxygen (like carbs and lipids) as well as nitrogen•Amino acids have an amine group (NH2) at one end and an acid group (COOH) at the other end•Peptide bonds are formed when the amine group of one joins with the acid group of another

Peptides

•A peptide is formed when several amino acids are joined together by peptide bonds•A polypeptide is formed when many amino acids are joined together•Proteins are very large polypeptides (most are composed of more than one polypeptide chain)

Uses of proteins…

• Synthesis of several substances including hormones, enzymes, antibodies, plasma, hemoglobin and cell membrane

• Can be broken down and used for energy (not the preferred method)

• Can be broken down and converted to glucose (gluconeogenesis)

Protein Breakdown

•The nitrogen part of the amino acid must be recycled or excreted•Most nitrogen is recycled to produce more amino acids•Some nitrogen is converted to urea by the liver and excreted by the kidneys•Blood Urea Nitrogen (BUN)- blood test used to assess amount of urea in the blood

Protein Breakdown

•Ammonia (NH3) is a by-product of protein breakdown (usually in the intestines)•Usually removed from the blood by the liver and converted to urea, which is excreted by the kidneys•High ammonia levels are toxic to brain cells; results in hepatic encephalopathy

Formation and excretion of urea

Protein Synthesis & DNA

• Proteins are crucial to all body functions• Protein synthesis involves the precise

arrangement of amino acids in specific sequences

• The pattern of assembly is coded and stored within the DNA (deoxyribonucleic acid)- found in the nucleus

• DNA is a code for the structure of proteins

DNA Structure

• DNA is a nucleic acid- composed of smaller units called nucleotides

• Nucleotides are composed of a sugar, a phosphate group and a base

• Joined together in long strands- two strands are arranged in a “twisted ladder” formation (the double helix) to form DNA

DNA Structure

•The sides of the ladder are composed of sugar-phosphate molecules•The rungs of the ladder are composed of bases- one base on each side

DNA Structure

•The bases that make up the rungs include:–Adenine–Cytosine–Guanine–Thymine

•Bases have a specific arrangement, each base combines specifically with another base- this is called base-pairing

The Genetic Code

• The protein-synthesis code for the cell is located along one strand of DNA (one side of the ladder)

• These codes are arranged in units called genes, so it is termed the genetic code

The Genetic Code

Reading the code–Read vertically along the sides of the ladder; read in groups of three, called base-sequencing

Copying the code–Since DNA does not leave the nucleus, ribonucleic acid (RNA) copies the code and delivers it to the ribosomes (where protein synthesis occurs). This process of copying is called transcription

Steps in protein synthesis…• DNA and RNA control protein synthesis• 5 steps involved

1. Base sequences are copied (transcription)2. The mRNA carries it to the ribosomes in the

cytoplasm3. The code on the mRNA tells specific amino

acids where to bind4. The amino acids are now lined up correctly

and peptide bonds form5. The protein chain is finished and a complete

new protein is formed