Cell Requirements (HSC SSCE Biology): Revision Notes

Cell Requirements

Introduction to cellular needs

All living organisms require both energy and matter to survive and function. Cells must acquire nutrients in two main forms: organic substances and inorganic nutrients.

Organic substances are produced by living organisms and contain carbon and hydrogen atoms. These include molecules such as glucose, amino acids, fatty acids, glycerol, nucleotides and vitamins. In contrast, inorganic nutrients come from the nonliving environment and do not contain carbon and hydrogen in long chains. Examples include gases (oxygen and carbon dioxide), minerals (such as phosphates and sodium ions) and water.

How cells use nutrients

The substances that cells require serve two primary purposes:

1. As building blocks – nutrients form the structural components from which cells and living tissues are constructed
2. As energy stores – nutrients provide stored energy that cells can access when needed

Organic nutrients primarily supply stored energy for living organisms, but they also contribute to cellular structure. Inorganic nutrients function mainly as essential structural components of cells and tissues.

Inorganic nutrients

Inorganic nutrients play vital roles in cellular function despite not being produced by living organisms. The table below summarises the main inorganic nutrients found in cells.

Water

Water forms approximately $90\%$ of the protoplasm (cytoplasm and nucleus). This abundant molecule serves multiple critical functions:

• Acts as the transport medium within cells and throughout organisms
• Functions as an important solvent for many molecules inside cells
• Provides the medium for all chemical reactions in cells, and may participate directly in these reactions

Mineral salts

Various mineral salts dissolve as ions in the cytoplasm and vacuoles of plant cells. These include chlorides, nitrates, phosphates and carbonates of elements such as sodium, magnesium, calcium, potassium and ammonium.

Mineral salts perform several important functions:

• Assist in all chemical reactions occurring within cells
• Provide raw materials for synthesising macromolecules and body tissues (for example, calcium is essential for bones and teeth, whilst iron is crucial for blood cells)
• Help maintain water balance in cells
• Enable proper functioning of cell membranes, nerve cells and muscle cells (particularly sodium ions $Na^+$ and chloride ions $Cl^-$)

Gases

Two key gases dissolve in the protoplasm and are used or produced in chloroplasts and mitochondria:

Carbon dioxide $(CO_2)$:

• Used during photosynthesis in plant cells
• Released as a product of aerobic cellular respiration in both plants and animals

Oxygen $(O_2)$:

• Used by all living organisms during aerobic cellular respiration to release energy for cell function
• Released as a product of photosynthesis

Organic compounds (Biomacromolecules)

Every living cell requires large organic molecules, called biomacromolecules, as part of their structure and to maintain biochemical processes for effective functioning. Four major categories exist based on their chemical composition and structure: carbohydrates, lipids, proteins and nucleic acids.

Carbohydrates

Carbohydrates are a group of organic molecules composed of carbon (C), hydrogen (H) and oxygen (O) atoms in a ratio of $1:2:1$. This gives the general formula for carbohydrates as $(CH_2O)_x$.

Scientists classify carbohydrates based on how many monomers (basic units) link together:

• Monosaccharides – simple sugars consisting of a single monomer
• Disaccharides – complex sugars consisting of two monomers
• Polysaccharides – complex molecules consisting of more than five and up to hundreds of monomers joined together

The product of photosynthesis, glucose, is a monosaccharide. The table below shows the classification and uses of different carbohydrates.

Monosaccharides include glucose, galactose and fructose (all $6$-carbon sugars), as well as ribose and deoxyribose ($5$-carbon sugars). These simple sugars provide a source of quick energy in both plant and animal cells.

Disaccharides form when two monosaccharides join together. Common examples include maltose (formed from two glucose molecules), lactose or milk sugar (glucose plus galactose), and sucrose or cane sugar (glucose plus fructose).

Polysaccharides are large molecules formed when many monomers link together. Important examples include:

• Starch – stores energy in plant cells
• Cellulose – provides structural support in plant cell walls, giving strength and rigidity
• Glycogen (animal starch) – stores energy in animal cells

Lipids

Lipids contain many carbon (C) and hydrogen (H) atoms with only a few oxygen (O) atoms. The fats and oils found in cells typically consist of triglyceride molecules. In these molecules, atoms arrange as a glycerol unit to which three fatty acid chains attach.

Lipids have an oily, greasy or waxy consistency and are relatively insoluble in water.

In cells, lipids perform three important functions:

1. Energy storage – lipids store approximately twice the amount of energy as carbohydrates
2. Membrane structure – lipids form a structural component of cell membranes
3. Hormone production – lipids are essential structural parts of hormones, which act as chemical messengers produced by cells (for example, steroids)

Proteins

Proteins are composed of the elements carbon, hydrogen, oxygen, nitrogen, and sometimes sulphur. These elements combine to form amino acids, which are the building blocks of proteins.

Approximately $20$ different amino acids exist. Cells can join these amino acids in chains of up to $300$ amino acids to form a peptide or polypeptide chain. Proteins consist of one or more of these polypeptide chains twisted together into a particular three-dimensional shape. The DNA in the cell nucleus controls the sequence and arrangement of amino acids, which determines the type of protein formed.

Proteins perform multiple functions in cells:

• Form structural components in cells and tissues
• Act as important structural components of cell membranes
• Serve functional roles, such as enzymes (which control all metabolic or chemical reactions in cells) and hormones (which control the functioning of other cells)
• Combine with other macromolecules to form important structural parts of all membranes within cells

Nucleic acids

Nucleic acids are very large biomacromolecules containing the elements carbon, hydrogen, oxygen, nitrogen and phosphorus. Two types of nucleic acids exist: DNA (deoxyribonucleic acid) and RNA (ribonucleic acid).

DNA consists of two strands that form a double helix. It stores the information controlling cell activities and is the main chemical component of the nucleus. Small amounts of DNA also occur in mitochondria and chloroplasts.

RNA occurs in small amounts in the nucleus and in larger amounts in the cytoplasm, where it assists in protein manufacture.

DNA nucleotides contain:

• Four bases: adenine, guanine, cytosine and thymine
• The sugar deoxyribose
• Cells require DNA nucleotides to make DNA during cell replication

RNA nucleotides contain:

• Four bases: adenine, guanine, cytosine and uracil
• The sugar ribose
• Cells require RNA nucleotides to make ribosomes and RNA so that cells can produce proteins

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