Cell Structure and Functions

Every living thing is made up of cells, but not all cells are the same! Animal cells contain a nucleus, cytoplasm, ribosomes, mitochondria, and a cell membrane. Plant cells have all of these plus a cell wall, chloroplasts, and a large vacuole. Bacterial cells are much simpler - they lack a proper nucleus and instead have their DNA floating freely in the cytoplasm.

Each part of a cell has a specific job. The nucleus acts like the cell's control centre, storing DNA and directing all activities. Mitochondria are the powerhouses that provide energy through aerobic respiration, whilst ribosomes build proteins. The cell membrane controls what enters and exits the cell.

Plant cells have some unique features that help them survive. Chloroplasts carry out photosynthesis to make food from sunlight, whilst the cell wall provides extra support and protection that animal cells don't have.

Quick Tip: Remember that bacterial cells are prokaryotic (no nucleus), whilst plant and animal cells are eukaryotic (have a nucleus).

Transport Across Cell Membranes

Your cell membrane isn't just a simple barrier - it's selectively permeable, which means it carefully controls what gets in and out. Think of it like a bouncer at a club who decides who can enter!

Passive transport doesn't need energy and includes diffusion and osmosis. Diffusion moves molecules from high to low concentration (like perfume spreading across a room), whilst osmosis specifically moves water molecules. When plant cells gain water, they become turgid and firm, but when they lose water, they become plasmolysed and wilted.

Active transport is different because it moves substances against the concentration gradient (from low to high concentration) and requires energy. This is like pushing a ball uphill - it takes effort, but sometimes cells need to do this to get essential materials inside.

Animal and plant cells react differently to water changes. Animal cells can burst if they take in too much water, but plant cells are protected by their strong cell wall.

Remember: Passive = no energy needed, Active = energy required!

DNA and Protein Production

DNA is like your cell's instruction manual, containing genes that code for specific proteins. The order of bases (adenine, thymine, guanine, cytosine) in DNA determines which amino acids get linked together to make proteins.

mRNA (messenger RNA) acts as a courier, carrying a complementary copy of the DNA instructions to the ribosomes. At the ribosomes, amino acids are assembled in the correct order to build proteins. This process is crucial because proteins do almost everything in your body!

Proteins have loads of different functions - they can be hormones that send messages, antibodies that fight infections, or enzymes that speed up chemical reactions. Enzymes are particularly important because they have an active site where substrates bind and reactions take place.

Think of enzymes like highly specific tools - each one is designed for a particular job, and the substrate must fit perfectly into the active site for the reaction to work properly.

Key Point: The sequence of DNA bases directly determines the sequence of amino acids in proteins!

Enzymes and Cellular Respiration

Enzymes are biological catalysts that speed up chemical reactions in your cells. They work by providing a specific active site where substrates can bind and react. However, enzymes are quite fussy about their working conditions!

Temperature and pH dramatically affect enzyme activity. Each enzyme has an optimum temperature (usually around 37°C for human enzymes) and pH where it works best. If conditions stray too far from optimal, the enzyme becomes denatured - its shape changes and it can no longer function properly.

Cellular respiration is how your cells release energy from glucose to produce ATP (the cell's energy currency). Aerobic respiration uses oxygen and happens in two main stages. Glycolysis occurs in the cytoplasm, breaking glucose into pyruvate and producing some ATP without oxygen.

When oxygen is available, pyruvate moves into the mitochondria for the second stage, which produces much more ATP (about 36 molecules compared to just 2 from glycolysis). This is why aerobic exercise is so much more efficient than anaerobic activity!

Remember: Denatured enzymes = changed shape = no function = potential health risks!

Fermentation and Genetic Engineering

When oxygen isn't available, cells can still get energy through fermentation, though it's much less efficient. In animals, fermentation produces lactate (causing muscle fatigue), whilst in plants and yeast, it produces ethanol and carbon dioxide. Both processes only yield 2 ATP molecules compared to 38 from aerobic respiration.

Fermentation happens in the cytoplasm and doesn't require oxygen, making it a useful backup system when you're exercising hard and your muscles can't get enough oxygen. The lactate produced can be converted back to pyruvate when oxygen becomes available again.

Genetic engineering allows scientists to modify organisms by moving genes between different species. The process involves extracting a desired gene from one organism, inserting it into a plasmid (circular DNA found in bacteria), and then putting this modified plasmid into a host cell.

This technique has revolutionised medicine and agriculture. Bacteria can be engineered to produce human insulin for diabetics, or crops can be modified to resist pests or improve nutritional content. The modified bacteria multiply rapidly, producing large quantities of the desired product.

Fascinating Fact: Genetic engineering can make bacteria produce human proteins that are identical to those made by our own cells!