Cell Structure and Basic Biology Concepts

Every living thing is made of cells, and understanding their structure is your foundation for all biology. Animal cells contain a nucleus, cytoplasm, cell membrane, mitochondria, and ribosomes. Plant cells have all of these plus a cell wall, vacuole, and chloroplasts for photosynthesis.

Prokaryotic cells (like bacteria) are much simpler and smaller than the cells in your body. They don't have a proper nucleus - instead, their DNA floats freely in the cytoplasm. These bacterial cells are typically 0.2-2.0µm in size, whilst your cells are much larger at 10-100µm.

DNA has a famous double helix structure discovered by Watson and Crick. It's made of four bases: adenine, thymine, guanine, and cytosine. Remember that A always pairs with T, and G always pairs with C - this is crucial for DNA replication.

Quick Tip: All enzyme names end in "-ase" - carbohydrase breaks down carbohydrates, protease breaks down proteins, and lipase breaks down fats!

Enzymes and Key Life Processes

Enzymes are biological catalysts that speed up reactions in your body using the lock and key theory. Each enzyme has a specific active site that perfectly matches its substrate molecule, making enzymes highly specific. Temperature and pH changes can affect how well enzymes work.

Photosynthesis happens in chloroplasts and uses chlorophyll to capture light energy. The equation is: carbon dioxide + water → glucose + oxygen. This endothermic reaction requires light energy and is affected by light intensity, CO₂ concentration, and temperature.

Respiration occurs in mitochondria and releases energy from glucose. Aerobic respiration uses oxygen: glucose + oxygen → carbon dioxide + water. Anaerobic respiration happens without oxygen - in muscles it produces lactic acid, whilst in yeast it produces ethanol and CO₂.

Using microscopes properly is essential for observing cells. Light microscopes use visible light and are great for basic cell observation, whilst electron microscopes provide much higher magnification for detailed structures.

Remember: Photosynthesis makes food using light energy, whilst respiration breaks down food to release energy - they're opposite processes!

DNA Replication and Cell Division

Before any cell divides, it must copy its entire genome through DNA replication. This process has seven key steps: initiation, primer synthesis, leading strand synthesis, lagging strand synthesis, primer removal, ligation, and termination. Each daughter cell receives an identical copy of the parent's DNA.

Mitosis is how your body grows and repairs damage. The cell cycle includes growth phases, DNA synthesis, error-checking, and finally mitosis itself. Two identical daughter cells are produced, each containing the same genetic information as the parent cell.

Understanding factors affecting photosynthesis is crucial for your exams. Without sufficient light intensity, plants cannot photosynthesise quickly even with plenty of water and CO₂. Similarly, inadequate carbon dioxide concentration limits the process, and temperatures that are too cold slow down the rate significantly.

Stomata are tiny pores that let carbon dioxide enter and exit leaves. They're essential for gas exchange during photosynthesis, allowing CO₂ to diffuse from the air into the plant.

Exam Focus: Learn the photosynthesis equation by heart - it appears in most biology papers and understanding it helps with limiting factors questions.

Transport Across Membranes

Three main processes move substances across cell membranes: diffusion, osmosis, and active transport. Diffusion is the movement of particles from high to low concentration - no energy needed. Think of how a scent spreads across a room.

Osmosis is specifically the movement of water molecules from high water potential to low water potential through a partially permeable membrane. When plant cells gain water by osmosis, they become turgid (swollen). When they lose water, they shrivel.

Active transport is different because it moves substances against the concentration gradient from low to high concentration. This requires energy from respiration and uses special carrier proteins to transport specific molecules across membranes.

Water potential measures the tendency of water molecules to move. Pure water has high water potential, whilst salty solutions have low water potential. Water always moves from high to low water potential via osmosis.

Memory Trick: Active transport goes "uphill" against the gradient, so it needs energy like climbing a mountain needs effort!

Circulatory System and Plant Transport

Your heart pumps blood through a double circulatory system with two separate circuits. The right side pumps deoxygenated blood to the lungs, whilst the left side pumps oxygenated blood around your body. The heart has four chambers with valves preventing backflow.

Arteries carry blood away from the heart under high pressure and have thick muscular walls. Veins return blood to the heart under low pressure and contain valves. Capillaries are microscopic vessels where gas exchange occurs between blood and tissues.

Plants have their own transport systems. Xylem transports water and mineral salts from roots to other plant parts. These vessels are made of dead cells with tough walls containing lignin for strength. Phloem consists of living cells that transport sugars like sucrose around the plant.

Translocation is the movement of dissolved sugars through phloem vessels. In spring, sucrose moves from storage areas in roots to growing areas like leaves and shoots where it's needed for growth.

Quick Check: Remember that xylem transports water UP from roots, whilst phloem transports sugars in ALL directions depending on the plant's needs.

Gas Exchange and Heart Function

Gas exchange happens through diffusion - oxygen moves from areas of high concentration in your lungs to low concentration in your blood. Carbon dioxide moves in the opposite direction. This process is affected by temperature, distance, and surface area.

Understanding how your heart works is essential. Deoxygenated blood enters the right atrium through the vena cava, passes through the tricuspid valve to the right ventricle, then gets pumped to the lungs via the pulmonary artery. Oxygenated blood returns through pulmonary veins to the left atrium, passes through the bicuspid valve to the left ventricle, then gets pumped around your body through the aorta.

Active transport requires energy because molecules move against their concentration gradient - from low to high concentration. Carrier proteins embedded in cell membranes facilitate this process, using energy from respiration to transport specific molecules.

The heart's cardiac muscle contracts powerfully to maintain blood circulation. Semi-lunar valves prevent blood flowing backwards into the ventricles after each heartbeat.

Exam Tip: Trace blood flow through the heart step-by-step - right atrium → right ventricle → lungs → left atrium → left ventricle → body.

Nervous System and Hormonal Control

Your nervous system consists of the central nervous system (brain and spinal cord) and peripheral nervous system (all other nerves). Neurons are specially adapted cells with dendrites to receive signals, a long axon to carry impulses, and a fatty sheath for insulation.

Reflex actions protect you from harm by providing automatic, rapid responses. The pathway goes: stimulus → receptor → sensory neuron → relay neuron → motor neuron → effector → response. By bypassing conscious thought, reflexes are much faster than normal reactions.

Hormones are chemical messengers produced by glands and carried in your bloodstream to target organs. Adrenaline from adrenal glands increases heart rate and breathing during stress. Thyroxine from the thyroid controls your metabolic rate.

Thermoregulation maintains your body temperature at 37°C because human enzymes work best at this temperature. Your brain monitors temperature and controls responses like sweating, shivering, and blood flow to skin. High temperatures cause heat stroke, whilst low temperatures cause hypothermia.

The contraceptive pill contains oestrogen and progesterone to prevent pregnancy by inhibiting FSH production, stopping egg maturation.

Body Fact: Your nervous system can send signals at speeds up to 120 metres per second - faster than most cars drive through towns!

Hormones and Reproduction

The endocrine system produces hormones that control many body functions. Key glands include the pancreas (insulin and glucagon), adrenal glands (adrenaline), pituitary gland (FSH and LH), testes (testosterone), and ovaries (oestrogen and progesterone).

The menstrual cycle is controlled by four main hormones. FSH causes eggs to mature, LH triggers ovulation (egg release), whilst oestrogen and progesterone maintain the uterus lining. This 28-day cycle repeats from puberty until menopause.

Testosterone is the main male reproductive hormone produced in testes. It stimulates sperm production and controls male secondary sexual characteristics. Oestrogen is the main female reproductive hormone that triggers egg maturation and controls the menstrual cycle.

Your eye is perfectly designed for vision. The cornea and lens focus light onto the retina, which contains light-sensitive cells. The iris controls pupil size to regulate light entry, whilst ciliary muscles change lens thickness for focusing on near or distant objects.

Contraception methods include hormonal options (pills, injections, implants) and barrier methods (condoms, intrauterine devices). Hormonal methods are over 99% effective when used correctly.

Hormone Hub: The pituitary gland is called the "master gland" because it produces hormones that control other glands throughout your body.