Comprehensive Biology Revision: Cells, Transport, Enzymes, and Reproduction

Characteristics of Living Things

Living organisms must carry out several fundamental processes to remain alive. These seven characteristics are commonly remembered using the acronym MRS GREN. Movement is defined as an action by an organism or a part of an organism that results in a change of position or place. Respiration involves chemical reactions occurring within cells that release energy from food, with aerobic respiration specifically utilizing oxygen. Sensitivity refers to the ability of an organism to detect and respond to changes in their environment, which are known as stimuli, including factors like light, temperature, or chemicals.

Growth is characterized by a permanent increase in size and dry mass, which is caused by an increase in the number of cells, the size of cells, or both. Reproduction is the production of new individuals, which can occur through sexual or asexual means. Excretion is the process of removing toxic waste products and substances that are present in excess within the organism, such as carbon dioxide and urea. Finally, nutrition is the process of obtaining or making the nutrients required for energy, growth, and repair.

Cell Structure and Organisation

A cell represents the basic structural and functional unit of every living organism. While some organisms are single-celled, multicellular organisms contain many cells that can become specialised for specific roles. There is a hierarchical order to biological organisation: the Cell is the smallest level that can carry out life processes; a Tissue is a group of similar cells working together, such as muscle tissue; an Organ is a group of different tissues working together to perform a function, such as the heart; an Organ system consists of organs working together, such as the digestive system; and an Organism is an individual living thing. This hierarchy follows the sequence: Cell → Tissue → Organ → Organ system → Organism.

Animal and plant cells share several core components, including a cell membrane, cytoplasm, a nucleus, mitochondria, and ribosomes. However, typical plant cells have unique structures that animal cells do not possess. These include a cellulose cell wall, chloroplasts, and a large permanent vacuole. This differentiation allows plant cells to perform specific functions or maintain structural integrity that animal cells do not require.

Structure of a Cell and Its Functions

The cell membrane is a critical component that controls the movement of substances into and out of the cell. The cytoplasm is the jelly-like substance where many chemical reactions take place within the cellular environment. The nucleus contains the genetic material and acts as the control center for all cell activities. Mitochondria are known as the site of aerobic respiration and are responsible for energy release. Ribosomes are the specific sites where protein synthesis occurs.

Specific to plant cells, the cell wall supports and strengthens the cell and is made mainly of cellulose. Chloroplasts are the sites of photosynthesis and contain the green pigment chlorophyll, which captures light energy. The permanent vacuole in plant cells contains cell sap and helps to maintain internal pressure and provide structural support for the cell.

Specialised Cells and Their Adaptations

Red blood cells are specialised to carry oxygen using a protein called haemoglobin. They have a biconcave shape which provides a large surface area for efficient gas exchange. When mature, these cells lack a nucleus to leave more room for haemoglobin, and they are flexible enough to pass through narrow capillaries. Sperm cells are designed to carry male genetic material to the egg. They possess a flagellum for movement and many mitochondria to provide the energy required for that movement. The acrosome at the head of the sperm contains enzymes involved in the fertilisation process.

Egg cells carry the female genetic material and have a large cytoplasm containing nutrients for early development. Following fertilisation, the egg's cell membrane changes to help prevent another sperm from entering. Root hair cells are found in plant roots and are specialised to absorb water and mineral ions from the soil. They have a long projection to provide a large surface area and a thin wall to ensure a short distance for water entry; they also contain many mitochondria to provide the energy needed for active transport.

Palisade cells are located near the upper surface of a leaf and contain many chloroplasts for photosynthesis. Their specific position allows them to receive a plentiful supply of light. Ciliated epithelial cells line parts of the respiratory tract. They possess cilia, which are hair-like structures that move mucus and trapped particles away from the lungs to keep the airways clear.

Diffusion

Diffusion is defined as the net movement of particles from a region of higher concentration to a region of lower concentration, moving down a concentration gradient due to the random movement of particles. This is a passive process, meaning it does not require energy from respiration. Several factors can influence the rate of diffusion. A larger concentration gradient usually increases the rate of movement.

Higher temperatures also increase the rate because particles gain more kinetic energy. A larger surface area allows more particles to cross a membrane at any given time, and a shorter diffusion distance generally increases the rate. Biological examples of this process include oxygen diffusing from the alveoli in the lungs into the blood, and carbon dioxide diffusing from the blood back into the alveoli for exhalation.

Osmosis

Osmosis represents the net movement of water molecules through a partially permeable membrane. This movement occurs from a region of higher water potential to a region of lower water potential. It is important to note that only water molecules are considered in the definition of osmosis. Like diffusion, osmosis is a passive process and requires no energy from respiration.

In plant biology, when a cell gains water via osmosis, it becomes turgid, which helps provide physical support to the plant. Conversely, if a plant cell loses a large amount of water, it can become plasmolysed. Plasmolysis is a state where the cell membrane pulls away from the cell wall. While diffusion refers to the general movement of particles down a concentration gradient, osmosis specifically concerns the movement of water across a partially permeable membrane.

Active Transport

Active transport is the movement of substances from a region of lower concentration to a region of higher concentration, which is against the concentration gradient. Because this movement is energetically unfavorable, it requires energy released by respiration. Specialised transport proteins located in cell membranes help to move specific substances across.

There are vital examples of active transport in nature. Root hair cells use active transport to absorb mineral ions from the soil when the concentration of those ions is lower in the soil than it is inside the root itself. Similarly, cells within the small intestine can utilize active transport to absorb certain nutrients from the gut against a concentration gradient to ensure the body retrieves as much nutrition as possible.

Enzymes

Enzymes are protein molecules that serve as biological catalysts, meaning they speed up chemical reactions without being used up in the process. The substance that an enzyme acts upon is called the substrate. The substrate binds to a specific location on the enzyme called the active site, forming an enzyme-substrate complex. Once the reaction occurs, the products leave the active site, and the enzyme remains unchanged, ready to be used again. Every enzyme has a specific shape for its active site, which leads to enzyme specificity.

Temperature significantly affects enzyme activity. At low temperatures, particles have less kinetic energy, resulting in slower reactions. Raising the temperature increases the rate until the enzyme reaches its optimum temperature. However, above this optimum temperature, the enzyme can denature. During denaturation, the active site changes shape, and the substrate no longer fits properly. Each enzyme also has an optimum pH; extreme pH levels can change the enzyme's shape and reduce its activity.

Substrate concentration also plays a role. Increasing the substrate concentration generally increases the reaction rate until all enzyme active sites become saturated. Once saturation is reached, adding more substrate has little or no further effect on the rate. Specific examples of enzymes include amylase, which breaks starch into smaller sugars; protease, which breaks proteins into amino acids; and lipase, which breaks lipids into fatty acids and glycerol.

Reproduction

Reproduction is the biological process by which organisms produce new individuals, ensuring the continuation of a species. Sexual reproduction usually involves two parents and the formation of male and female gametes. Fertilisation is defined as the fusion of the male and female gamete nuclei. The resulting offspring receive genetic information from both parents, which produces genetic variation. In humans, the male gametes are sperm and the female gametes are eggs.

Asexual reproduction involves only one parent and does not involve the fusion of gametes. Offspring from asexual reproduction are genetically identical or very similar to the parent, with differences only occurring due to mutations. This method can be rapid and does not require the organism to find a mate. Examples include binary fission in microorganisms and vegetative propagation in plants. While sexual reproduction allows populations to adapt to changing environments through variation, asexual reproduction is efficient but leaves populations vulnerable to environmental change due to low genetic variation.

Key terminology in reproduction includes: Gamete, a reproductive cell such as a sperm or egg; Fertilisation, the fusion of the nuclei of male and female gametes; Zygote, the cell formed immediately after fertilisation; and Variation, the differences in characteristics between individuals of the same species.

Practice Questions and Answers

There are several key questions to test understanding of these biological concepts. 1. State the seven characteristics of living organisms. These are movement, respiration, sensitivity, growth, reproduction, excretion, and nutrition. 2. Explain the difference between a tissue and an organ. A tissue is a group of similar cells working together, whereas an organ contains different tissues working together. 3. Name three structures found in both plant and animal cells. These include the cell membrane, cytoplasm, and nucleus (mitochondria and ribosomes are also valid). 4. State two structures found in typical plant cells but not animal cells. These are the cellulose cell wall and chloroplasts; a large permanent vacuole is also characteristic.

5. Explain two adaptations of a red blood cell. Adaptations include a biconcave shape for large surface area, having no nucleus when mature to fit more haemoglobin, and being flexible. 6. Define diffusion. Diffusion is the net movement of particles from higher to lower concentration. 7. State two factors that affect the rate of diffusion. Factors include the concentration gradient, temperature, surface area, and diffusion distance. 8. Define osmosis. Osmosis is the net movement of water molecules through a partially permeable membrane from a region of higher to lower water potential. 9. Explain what happens to a plant cell placed in a dilute solution. Water enters the cell by osmosis, and the cell becomes turgid.

6. Explain why active transport requires energy. Energy is needed because substances are moved against the concentration gradient. 11. Give one example of active transport in plants. An example is root hair cells absorbing mineral ions from the soil. 12. What is an enzyme? An enzyme is a biological catalyst that speeds up a reaction without being used up. 13. Explain what happens to an enzyme above its optimum temperature. The enzyme may denature, which changes the active site and reduces activity. 14. What happens when all active sites are occupied? The enzyme is saturated, and increasing substrate concentration will have little or no further effect on the reaction rate. 15. State two differences between sexual and asexual reproduction. Sexual reproduction involves gametes and fertilisation while creating genetic variation; asexual reproduction involves one parent, no gamete fusion, and produces genetically similar offspring.